ALD -Atomic Layer Deposition APTES -(3-Aminopropyl)triethoxysilane CD -Circular dichroism D 2 O -Deuterium oxide DMSO-d6 -Dimethyl sulfoxide (Deuterated) EA -Ethanolamine EtOH -Ethanol FRAP -Fluorescence recovery after photobleaching FT-IR -Fourier transform infrared spectra GMP -Guanosine 5'-monophosphate LbL -Layer-by-layer deposition

-Acknowledgement -

This work was performed in the collaboration between the laboratory of Chimie & Biologie des Membranes et des Nanoobjets (CBMN) and Institut de Chimie de la matière condensée de Bordeaux (ICMCB). I would like to thank their directors Erick DUFOURC and Claude DELMAS as well as Jean-Jacques TOULMÉ director of Institut Européen de Chimie et Biologie (IECB) for the provided opportunity to work in their laboratories.

I am very grateful to all the members of the jury. I would like to thank David AMABILINO and Michel WONG-CHI-MAN who kindly agreed to be reviewers for this manuscript. I am also grateful to Christian BERGAUD who took over the duties of the president of the jury.

I would like express gratitude to my supervisors Reiko ODA and Marie-Helene DELVILLE for being the driving force in my study, for all the advices and of cause for reading of this manuscript. Thank you a lot; it was real pleasure to work with you. You showed me how the world of science looks like. I really appreciate the help of Emilie POUGET with the scientific and bureaucratic questions.

There is the number of people whom I was collaborating with during this project. I would like to thank them all and in particular: Satyabrata SI who worked on the part of TiO 2 and ZnO mineralization of silica helices; Christian BERGAUD in whose lab mechanical properties of silica nanohelices and nanostructures were measured; Said HOUMADI and Naima Sabrina HABTOUN who performed these measurements; and Nicola PINNA (University of Berlin) who synthesized the core-shell nanohelices by ALD technique. I am very grateful to all the engineers of IECB and CBMN who helped me with the equipment and personally to the Axelle GRELARD for deepening my knowledge and improving my skills of the NMR experiments. I would like express gratitude to Elisabeth SELLIER from CREMEM for the help with HRTEM and SEM.

-Acknowledgement -4 I would like to thank all the people whom I was working with during these three years: Zhe Annie CHENG, Jiaji CHENG, Alexandre CUNHA, Rajat Kumar DAS, Marie-Christine DURRIEU, Gregor KEMPER, Guillaume LE SAUX, Alla MALINENKO, Sylvain NLATE, Yutaka OKAZAKI, Laurent PLAWINSKI. Thank you for all the suggestion you gave to me, for all the problems I was able to share with you, for all the jokes, candies and beer. You really were making my day better.

And, of course I am grateful to my family and my friends who believed in me and supported me every day. Thank you my dears.

-Résumé -Ce travail décrit la fabrication de matériaux composites pour les applications en nanosystèmes électromécaniques (NEMS). Une approche ascendante basée sur l'auto-assemblage de molécules organiques a permis d'atteindre un niveau de miniaturisation des structures avec une morphologie complexe et contrôlable généralement inaccessible aux techniques classiques de fabrication. La chimie sol-gel, d'autre part est connue pour la synthèse dans des conditions douces de matériaux robustes dont les propriétés mécaniques et électriques spécifiques sont requises. La combinaison de ces deux méthodes permet la synthèse de nanomatériaux composites fonctionnels avec une morphologie contrôlée et les propriétés électromécaniques spécifiques.

L'auto-assemblage organique a été utilisé pour la synthèse des fibres hélicoïdales. La forme hélicoïdale a été choisie pour l'élément fonctionnel du fait des propriétés physiques uniques d'une telle morphologie. Ces structures organiques sont ensuite revêtues d'une couche mince de silice afin d'obtenir la solidité requise du système. Enfin une deuxième minéralisation avec du TiO 2 ainsi que du ZnO a été appliqué en vue de fournir les propriétés électriques nécessaires au matériau.

Dans le premier chapitre, nous avons fait une étude bibliographique sur l'auto-assemblage organique chiral de petites molécules, tout d'abord en décrivant la classification des molécules amphiphiles, et les bases du mécanisme d'auto-assemblage. Ceci est suivi par la description des principales techniques de caractérisation : tels que les techniques spectroscopiques, diffusion de rayons X, la microscopie et modélisation utilisée pour l'étude des agrégats supramoléculaires tant au niveau moléculaire que macroscopique. Dans la dernière partie du chapitre, nous avons donné un aperçu de la littérature sur les études récentes d'auto-assemblages chiraux en milieu aqueux. La formation d'agrégats à fort ratio d'aspect et forte torsion par assemblage de petites molécules chirales, de molécules chirales et par le co-assemblage de gélifiants et d'ions métalliques y est détaillée. La structure chimique de l'amphiphile et sa concentration, le pH, la température, la force ionique, la composition du solvant ainsi que la présence d'additifs sont les principaux facteurs qui influent sur la morphologie des assemblages chiraux. La morphologie des agrégats peut également être modifiée par des stimuli externes (par exemple par irradiation UV) ou évoluer sur le vieillissement du gel.

Dans le deuxième chapitre, nous avons introduit un nouveau système d'auto-assemblage basé sur la complexation d'amphiphile cationiques jumeaux non-chiraux avec la guanosine 5'-monophosphate chirale et anionique . Nous avons constaté que ce système s'auto--Résumé -6 assemble en structures semblables à des longues aiguilles en milieu aqueux. La forte interaction entre les molécules de GMP qui stabilise les aiguilles peut être perturbée par la présence d'autres anions comme le bromure. Dans ce cas les GMP deviennent plus labiles et sont capables de tordre la bicouche de molécules amphiphiles transférant de fait leur chiralité à une échelle supramoléculaire.

Des structures micrométriques en fibres avec un diamètre d'environ 500 nm sont ainsi formées, ces fibres étant elles-mêmes constituées de fibres d'un diamètre de 20 à 70nm. Nous avons constaté que le rapport GMP/bromure dans les agrégats est l'un des facteurs déterminants sur la morphologie des fibres auto-assemblées. Si la concentration de GMP est trop élevée, leur interaction est forte, et rend donc la bicouche très robuste et donc incapable de s'incurver. D'autre part, si la quantité de GMP à l'interface est trop faible, ceux-ci ne sont pas en mesure de déformer le ruban. Nous avons montré que des structures hélicoïdales peuvent aussi être formées lorsque d'autres anions tels que le chlorure ou le nitrate sont utilisés à la place du bromure. La température de Krafft et le pKa de l'anion on un effet direct sur le rapport amphiphile/GMP optimal pour la formation d'hélices. Le pH du système détermine l'état de protonation de la guanosine monophosphate et par conséquent, la longueur et le niveau d'agrégation des fibres chirales. Nous montrons également que la structure de la molécule amphiphile (c.à.d. l'espaceur et la longueur de la chaîne alkyle) a un effet sur la morphologie des agrégats auto-assemblés. Le subtil équilibre entre la solubilité et la cristallinité de l'échantillon déterminé par ces paramètres est requis pour la formation de longues structures à morphologie chirale. D'après les résultats décrits ci-dessus les conditions de la formation des micro-hélices individualisées ont été optimisées. Nous avons utilisé la microscopie optique et la technique de FRAP, afin d'étudier la croissance des hélices et il a été trouvé qu'elles croissent à partir de leur base et non par leur partie distale. Enfin, un mécanisme de la formation des hélices en fonction de la dynamique d'échange ionique est proposé.

Dans le troisième chapitre, nous avons utilisé la transcription inorganique de la matrice organique conférant ainsi la solidité nécessaire à la structure chirale. La première partie du chapitre est consacrée à l'étude bibliographique du procédé sol-gel de la silice. L'essentiel du procédé y est décrit ainsi qu'une introduction à la modification de surface de la silice. Cette section se termine par une vue d'ensemble des principales approches de la fabrication de nanostructures chirales en silice.

Dans la deuxième partie du chapitre, nous avons continué le travail de Carole AIME et Rumi Tamoto consacré à la synthèse de nano-hélices en silice à partir du gel 16-2-16 tartrate comme structure organique chirale. La cinétique de condensation du précurseur de silice est le principal -Résumé -7 facteur qui affecte la morphologie des hélices nanométriques de silice. Dans le protocole de transcription de la silice rapporté précédemment par notre groupe, une maturation de trois semaines était nécessaire à la formation des hélices de silice. En ajustant le taux d'hydrolyse de TEOS par addition d'acide tartrique au mélange réactionnel ce temps à été réduit à un seul jour. De plus, l'utilisation d'ultrasons à permis d'obtenir des hélices de silice plus courtes et individualisées. Les propriétés du solvant dans lequel est effectué le clivage par ultrasons, telles que la densité, la viscosité et la tension superficielle jouent un rôle important dans l'efficacité du clivage et de la dispersion, mais également les paramètres de sonication -temps, puissance et volume de l'échantillon.

Enfin, nous rapportons la synthèse des hélices de silice de taille micrométrique à partir de fibres hélicoïdales organiques décrites au chapitre 2. La réactivité des précurseurs est un facteur déterminant de la morphologie des structures inorganiques. Le TEOS comme précurseur le plus courant dans le procédé sol-gel de la silice n'a été jugé approprié pour la transcription des hélices micrométriques organiques en raison de sa faible cinétique de condensation dans les conditions expérimentales. Le métasilicate de sodium acidifié par de l'acide acétique a été choisi à la place comme précurseur de la silice en raison de sa solubilité et de sa réactivité élevée. Les conditions de réaction telles que la concentration en précurseur, le pH et le temps de réaction ont été ajustées pour les meilleures conditions de transcription. Les fibres minérales obtenues ont été étudiées par microscopie optique et microscopie électronique à transmission et balayage. Nous avons constaté que les hélices micrométriques sont formées de fibres doubles, maillées en parallèle avec un diamètre d'environ 500 nm.

Dans le chapitre 4, nous avons montré la possibilité de synthétiser des structures hélicoïdales d'oxyde de titane à partir d'une matrice organique. Deux précurseurs du TiO2 ; le bis (ammonium lactato) dihydroxyde de titane (IV) (TiBALDH) et le diisopropoxyde bis(acétylacétonate) de titane (TDAA) étaient les plus appropriés pour la fabrication de rubans hélicoïdaux inorganiques. Dans une approche biomimétique du procédé, le TiBALDH a été hydrolysé par une diamine dans un milieu aqueux à température ambiante, l'hydroxyde de sodium pouvant aussi être utilisé. Le TDAA a été dissous dans l'eau en présence d'une petite quantité d'alcool et ensuite hydrolysé en l'absence de base. Nous avons constaté que le mode de mélange du gel organique et le précurseur de TiO 2 a un effet significatif sur la morphologie de la réplique inorganique. La sonication améliore la conservation des structures fibreuses de gel organique tandis que l'agitation provoque l'effondrement -Résumé -8 de la structure fibreuse/chirale. Le principal inconvénient des fibres de TiO 2 ainsi obtenues est leur faible résistance mécanique.

Il est possible de surmonter le problème de la faible résistance mécanique des fibres de TiO 2 en utilisant des structures noyau-enveloppe SiO 2 @ TiO 2 . L'utilisation de butoxide de titane (IV) (TBOT) est la méthode la plus simple pour la synthèse de composites silice @ titane. Le contrôle précis de la teneur en eau est extrêmement important dans cette approche, mais est difficile car les hélices de silice sont synthétisées en milieux aqueux et retiennent une couche d'eau en surface. Utilisation de TiBALDH et TDAA en tant que précurseurs de TiO 2 permet de réaliser la synthèse dans des solvants aqueux ou mixtes sans nécessairement avoir à contrôler précisément la teneur en eau. Trois approches différentes ont été utilisées pour la formation d'une enveloppe de TiO 2 ces précurseurs. La première est l'adsorption successive d'une couche d'un sel de chlorhydrate d'oligoamine puis de TiBALDH . Cette approche a été jugée trop longue et pose un problème avec le lavage du précurseur en raison de la forme complexe de la matrice. La seconde approche consiste en l'hydrolyse du TiBALDH ou du TDAA par une oligoamine adsorbée en surface des hélices de silice. Dans ce cas, l'attraction électrostatique du précurseur de l'amine et de la valeur élevée de pH local à la surface des hélices favorise la formation préférentielle de la couche de TiO 2 sur les hélices.

Dans une troisième approche, les hélices sont fonctionnalisées de manière covalente avec de l'aminopropyl triethoxysilane (APTES). De cette façon, les hélices présentent des groupes amine en surface. Le mécanisme de la formation de la l'enveloppe TiO 2 dans ce cas est similaire à celui décrit précédemment. Cette méthode a pour avantage d'éviter la présence d'amines libres en solution et permet un bien meilleur contrôle sur le dépôt d'oxyde de titane. La concentration en précurseur, la concentration en APTES utilisés pour la modification de la silice et la composition du solvant sont les principaux facteurs déterminant la morphologie de la couche de TiO 2 . Enfin, une couche de TiO 2 cristallin avec une structure anatase a été obtenue par calcination des structures noyau-enveloppe SiO 2 @ TiO 2 à 450 ° C pendant 3 heures.

Deux procédures ont été étudiées pour la synthèse de structures noyau-enveloppe SiO 2 @ ZnO: une libération lente de la base et une libération lente du précurseur. Dans la première approche, la base est formée in situ par l'hydrolyse thermique de l'urée en milieu aqueux. L'effet de la concentration en précurseur, de la température et du temps de réaction sur la formation de ZnO a été étudié. Pour toutes les conditions expérimentales, il a été trouvé que la dissolution des hélices de silice pendant le chauffage avec l'urée est plus rapide que la vitesse de formation de la couche de ZnO. Dans la seconde approche, la synthèse a été effectuée dans l'éthanol. Le précurseur du zinc et -Résumé -9 l'amine ont été mélangés avec la base pour former une solution stable à la température ambiante. Ce complexe se décompose par chaleur résultant en la libération lente du précurseur, son hydrolyse et ainsi la formation de ZnO. Plusieurs types d'agents de complexation ont été étudiés et la triéthylènetétramine ainsi que l'éthanolamine se sont révélés être les plus appropriées pour la synthèse de composites SiO 2 @ ZnO nanométriques. Modifier la surface de silice a aussi un impact positif sur le dépôt de ZnO.

L'Atomic Layer Deposition (ALD) a été utilisée comme alternative au procédé solgel pour la fabrication de nano-hélices noyau-enveloppe. Cependant, cette technique fonctionne sur des poudre et abouti à des objets enchevêtrés. Ces échantillons étaient donc difficiles à séparer et à positionner pour les mesures par AFM.

Enfin, en vue du développement de NEMS, les propriétés mécaniques de nano-hélices de silice et des nanotubes ont été mesurées. Les nanostructures ont été déposées sur un substrat de silicium ad hoc et serties avec du platine déposé par faisceau ionique induit. Les propriétés mécaniques des nano-objets en suspension ont été sondées par l'AFM dans l'air. Le régime linéaire des courbes de force en fonction du déplacement vertical obtenu pour les nano-hélices est plus grand que celui obtenu pour les nanotubes. La raideur des nano-hélices est inférieure à celle mesurée pour les nanotubes (0,28 N / m vs 0,36 N / m pour un échantillon à de 1,5 µm de longueur). Ceci démontre que les hélices sont plus souples que les tubes et confirme l'idée principale du projet où l'utilisation d'hélices en place de tubes ou de fils d'une plus grande confère une plus grande sensibilité pour une même force appliquée.

Mots-clés: auto-assemblage, chiralité, hélices, tensioactif gémini, guanosine monophosphate, tartrate, sol-gel, silice, core-shell, oxyde de titane, oxyde de zinc, NEMS, AFM, raideur, régime linéaire.

-Résumé -

-Abstract -

In this work we focused on the fabrication of composite materials for NEMS applications. A bottom-up approach based on the use of organic self-assembly of small molecules allowed achieving a level of miniaturization of the structures with a complex and tunable morphology typically unattainable with conventional top-down fabrication techniques. Sol-gel chemistry on the other hand is known to be efficient for the synthesis of robust materials with required mechanical and electrical properties in soft conditions. The combination of these two methods allows us synthesizing functional composite nanomaterials with a well-controlled morphologies and specific electromechanical properties.

In this work, organic self-assembly was used for the synthesis of the helical fibers. Helical shape was chosen for the functional element due to the unique characteristic physical properties of such a morphology. These organic structures were coated with a thin silica layer in order to achieve the required robustness of the system. Finally a second mineralization with TiO 2 as well as ZnO was applied in order to provide the necessary electrical properties to the material.

In the first Chapter of the work, we made a bibliographic study on the chiral organic selfassembly of the small molecules. We started from the classification of the amphiphile molecules, and basics of the self-assembly mechanism. It was followed by the description of the main techniques of characterization: such as spectroscopical techniques, X-Ray scattering, microscopy and modeling used for the studying of supramolecular aggregates on both molecular and macroscopical level. In the last part of the Chapter we made a literature overview on the recent studies of chiral self-assemblies in aqueous medium. The formation of high aspect ratio aggregates with the prominent twist was reported through the assembly of the small chiral molecules, achiral molecules and through the co-assembly of the gelating molecules and metal ions. The chemical structure of the amphiphile and its concentration, pH, temperature, ionic strength, solvent composition as well as presence of additives were shown to be the main factors affecting the morphology of the chiral assemblies. The morphology of the aggregates can also be changed by external stimuli (for example by UV irradiation) or evolve upon the aging of the gel.

In the second Chapter we introduced new self-assembly system based on non-chiral cationic gemini amphiphile complexed with the chiral Guanosine 5'-monophosphate anion .

We found that this system self-assembles into long needle-like structures in aqueous medium. The -Abstract -12 packing of GMPs in the needle can be disturbed by other anions, like bromide. Guanosine monophosphates in this case become more mobile and able to twist bilayer of amphiphile molecules transferring chirality from molecular to supramolecular level. Micrometric structures made of smaller fibers with a diameter of around 500 nm are formed in this case. These fibers themselves consist of smaller fibers with diameters of 20-70nm. We found that the ratio of GMP to Bromide in the aggregates is one of the crucial factors determining the morphology of the self-assembled fibers.

If the concentration of GMPs is too high they start to stack and this makes the bilayer very robust and so unable to twist. On the other hand if there is a very low number of GMPs at the interface they are unable to twist the ribbon. We showed that helical structures can be formed as well when other anions such as chloride or nitrate are used instead of bromide. Krafft temperature and pKa value of the anion affect the optimal GMP/gemini ratio at which helices are formed. The pH of the system determines the protonation state of the Guanosine 5'-monophosphate and therefore the length and level of aggregation of the chiral fibers. We also show that the structure of the amphiphile molecule i.e. spacer and alkyl chain length has an effect on the morphology of the self-assemble aggregates.

The subtle balance between solubility and crystallinity of the sample determined by those parameters are required for the formation of the long aspect ratio structure with the chiral morphology. Based on the results described above the conditions for the formation of the individualized microhelices were optimized. We applied optical microscopy and FRAP technique in order to study the growth of the helices and found that they were formed from the base. Finally a mechanism of the helices formation based on the dynamic ion-exchange was proposed.

In the third Chapter, we used inorganic transcription of the organic template in order to achieve the necessary robustness of the chiral structure. The first part of the Chapter is devoted to the bibliographic study of the silica sol-gel process. Basics of the method are described following by the introduction into the silica surface modification. Section finished by the overview of the main approaches to the fabrication of chiral silica nanostructures. In the second part of the Chapter we continued the work by Carole AIME and Rumi TAMOTO devoted to the synthesis of silica nanohelices based on 16-2-16 Tartrate gel used as a template. The condensation kinetics of silica precursor is the main factor affecting the morphology of the nanometric silica helices. According to the silica transcription protocol reported earlier by our group three weeks of organic gel aging required for the silica helices formation. By tuning the hydrolysis rate of TEOS via the addition of tartaric acid to the reaction mixture we succeeded in reducing to only one single day the synthesis of -Abstract -13 the silica helices from organic gels. We report also we can control the silica helices length and obtain shorter and individualized helices by ultrasonication. Not only the solvent properties such as density, viscosity and surface tension played an important role in the cutting and dispersion efficiency but also the sonication parameterstime, power and volume of the sample.

Finally, we report the synthesis of the micrometric size silica helices based on organic helical fiber from the self-assembly of gemini nucleoamphiphile as reported in Chapter 2 as a template.

The reactivity of the precursors appears to be a crucial factor determining the morphology of the inorganic structures. TEOS as the most common silica precursor for sol-gel process was found to be inappropriate for the transcription of organic micrometric helices due to its slow condensation kinetics under the experimental conditions. Sodium metasilicate acidified by acetic acid was chosen instead as a silica precursor due to its solubility and high reactivity. Reaction conditions such as precursor concentration, pH and reaction time were tuned for the best transcription conditions.

Obtained inorganic fibers were studied by optical, scanning and transmission electron microscopy.

We found that micrometric helices are formed of double fibers with a diameter of around 500 nm twisting in parallel.

In the Chapter 4 we showed the possibility of synthesis of titanium oxide helical structures based on the organic template. Two TiO 2 precursors -Titanium(IV) bis(ammonium lactato)dihydroxide (TiBALDH) and Titanium diisopropoxide bis(acetylacetonate) (TDAA) were most adequate for the fabrication of inorganic helical ribbons. In the biomimetic process approach, TiBALDH was hydrolyzed by a diamine in aqueous medium at room temperature, though sodium hydroxide could be used as well. TDAA was dissolved in water in the presence of small amount of alcohol and then hydrolyzed in the absence of base. We found that the way of mixing of the organic gel and the TiO 2 precursor has a significant effect on the morphology of inorganic replica.

Sonication results in improvement of the conservation of the fibrous structures of organic gel whereas stirring and vortexing cause the collapse of the fiber/chral structure. The main drawback of the obtained TiO 2 fibers is their low mechanical strength.

The SiO 2 @TiO 2 core-shell structure is able to overcome this problem of the mechanical strength characteristic for the pure TiO 2 fibers. Usage of Titanium(IV) butoxide (TBOT) is the most straightforward method for the synthesis of silica@titania composite. The precise control of the water content is extremely important in this approach; however it is difficult to achieve this control correctly due to the water-born silica. Usage of TiBALDH and TDAA as TiO 2 precursors allows -Abstract -14 performing the synthesis in aqueous or mixed solvents eliminating in such a way the necessity of a precise control over the water content. Three different approaches were used for the TiO 2 shell formation with these precursors. The first one is the LbL adsorption of oligoamine hydrochloride salt and TiBALDH. This approach was found to be time consuming and having an issue with the washing out of the precursor due to the complex shape of the template. The second approach was to hydrolyze TiBALDH or TDAA by oligoamine electrostatically adsorbed on the surface of the silica helices. In this case both the electrostatic attraction of the precursor to the amine and the high local pH value at the surface of the helices promoted the preferential formation of the TiO 2 layer on the template. In the third approach amine groups were covalently grafted onto the silica helices. The mechanism of the TiO 2 shell formation in this case was similar to the one described above. The advantage of this method is the absence of any free amine in the solution and then a much better control over the titanium oxide deposition. The precursor concentration, the concentration of APTES used for the silica modification and the composition of the solvent are the main factors determining the morphology of the TiO 2 layer. Crystalline TiO 2 layer with anatase phase crystal structure was obtained by calcination of SiO 2 @TiO 2 core-shell nanohelices at 450°C for 3 hours. Two procedures were studied for the synthesis of SiO 2 @ZnO core-shell: a slow release of the base and a slow release of the precursor. In the first approach, the base was formed in situ by the thermal hydrolysis of urea in aqueous medium. Effect of the precursor concentration, reaction temperature and time on the formation of ZnO was studied. For all the experimental conditions, we found that dissolution of the silica helices upon the heating with urea is faster than the speed of ZnO layer formation. In the second approach, the synthesis was performed in ethanol. The complex of zinc precursor and the amine were mixed with the base forming a stable solution at room temperature. This complex was decomposed upon heating resulting in a slow release of the precursor, its hydrolysis and the formation of ZnO. Several types of complexation agents were studied and triethylenetetramine along with ethanolamine were found to be the most suitable for the synthesis of nanoscale SiO 2 @ZnO composites. The surface modification was shown to have a positive impact on the ZnO deposition.

As an alternative to the sol-gel process Atomic Layer Deposition (ALD) was used for the fabrication of core-shell nanohelices. However, this technique works on powder samples which resulted on rather entangled core-shell objects. These samples were difficult to separate and position for AFM measurements.

-Abstract -Finally mechanical properties of silica nanohelices and nanotubes were measured as a first step of NEMS development. The nanostructures were deposited on patterned silicon substrate and clamped with platinum by the Ion Beam-Induced Deposition technique. Mechanical properties of suspended nanoobjects were probed by AFM in air. The linear regime of force vs. vertical displacement curves obtained for the nanohelices is larger than the one obtained for nanotubes. The stiffness of nanohelices is lower than that measured for nanotubes (0.28 N/m vs. 0.36 N/m at the suspended length of 1.5 µm). This demonstrates that the helices are softer than the tubes confirming the main idea of the project that by using helices instead of tubes or wires a higher sensitivity can be obtained for the same applied force.

Keywords: self-assembly, chirality, helices, gemini surfactant, Guanosine monophosphate, tartrate, sol-gel, silica, core-shell, titanium oxide, zinc oxide, NEMS, AFM, stiffness, linear regime

-General introduction -

NanoElectroMechanical Systems (NEMS) are devices integrating electrical and mechanical functionalities at the nanoscale. They usually consist of multiple transducers and a mechanical element whose response is well characterized [1].

Recent advances in resonant NEMS based on the use of 1D nanostructures such as carbon nanotubes, silicon nanowires, etc. open up an important variety of applications, including ultrasensitive mass and force sensing, ultra-low power radio frequency (RF) signal generation and timing, and fundamental studies in quantum mechanics [2]. When the electromechanical system is utilized as a sensor, the mechanical response (whether static or dynamic) is constantly monitored. Changes in the transducer signal are used as indication of variations in the environment. The sensitivity of the electromechanical system to environmental changes relies heavily on the size of the device. As device dimensions are reduced to nanometric scale, previously unattainable characteristic properties are reached [3].

Among a number of nanostructures, which have attracted great attention in nanotechnology and nanoscience over the past decade, helical nanosprings represent a new variety of nanoobjects.

One decisive advantage of helices or twisted ribbons, compared to traditional 1D nanostructures such as nanotubes and nanowires, lies in the fact that 3D morphologies with extremely high surface to volume ratio exhibit unconventional physical properties and can be advantageously used as building blocks in functional nanodevices [4,5] due to:

-Their helicity and periodicity that can be varied to tune the spring constant.

-Their electrostatic polarization, with charged surfaces across the thickness of the nanoobjects.

-Their structural flexibility, the helical shape is also ideal for inducing polarization effects under mechanical stress [6].

-Their conversion between electric energy and mechanical energy, which is favored since a large deformation can be induced under a small mechanical perturbation (or vice-versa) [7].

-The enhanced electromechanical performances, which can be obtained using nanosprings with high aspect ratio leading to an increased free boundary for volume expansion and contraction. This is particularly true for piezoresistive or piezoelectric nanosprings [8]. Referencespage 26

In spite of the proven interest on the 3D nanohelices, they are hardly achievable using topdown nanofabrication techniques since they rely on fabrication processes that are inherently two dimensional [8].

On the other hand, using bottom-up approaches, twisted piezoelectric nanobelts, or nanohelices have been synthesized from different materials such as carbon [9], silicon carbide (SiC) [10], SiGe/Si and SiGe/Si/Cr [11], or zinc oxide (ZnO) [12]. However, it is still very challenging to achieve high yields and controlled synthesis of uniform helical nanostructures. Less than 10% of the synthesized structures exhibit coiled shapes leaving the samples as mixtures of nanohelices and unscrolled nanobelts. Additionally, it is very difficult to control the nanohelices size, leading to a broad range of dimensions (diameters from 300 to 700 nm and widths from 100 to 500 nm, and lengths of up to 10 µm). There are few examples of these nanohelices with diameters below a hundred nanometers.

We propose an alternative solution: a bottom-up approach based on an organic nanohelix structure (diameter = 30-41 nm) used as a template for metal oxide growth. Self-assemblies from chiral molecules are indeed known to form spontaneously various chiral nanostructures such as twisted or helical ribbons and tubules with tunable morphologies and dimensions under specific conditions. These structures give access to well-defined inorganic nano-structures (helices and tubules) [13][14][15][16][17][18][19], hardly obtainable with conventional techniques.

The originality of our project consists in the synergy between organic and sol-gel chemistries that do not require expensive environment such as vapor deposition techniques or clean rooms.

In the first Chapter, we will make a literature overview on the topic of organic self-assembly.

Structure of the main types of amphiphile molecules and basics of the process will be given. This will be followed by the description of main characterization techniques used for the study of selfassembled systems. At the end we will give some examples of very recent studies of the chiral selfassembly. Main stress will be put on the systems that are able to express supramolecular chirality in aqueous medium.

With this knowledge in mind we will develop new self-assembly system in the second Chapter. Previously our group already reported the formation of micrometric scale helical aggregates from monocationic nucleoamphiphile [20,21]. In this Chapter we will extend the Referencespage 26 approach used in that work to the family of gemini amphiphiles. We will study which parameters are important for the transferring of the molecular chirality of the anion to the macroscopic level and how we can tune the morphology of the aggregates at a larger temperature range, as well as concentration range. Using various techniques we will explore this system on different scales.

Finally a mechanism of the helices formation will be proposed.

While molecular self-assembly gives access to structures with rich morphology diversity on both the micro-and nanometric scale, the aggregates formed by the assemblies of the small organic molecules based on weak interaction such as hydrogen bonds hydrophobic effect, π-stacking are too fragile to be incorporated into NEMS system. In the third Chapter of this work we will use organic chiral assembly as a template for the synthesis of chiral inorganic material. These materials are able to provide a hard enough basis for further modifications by semiconducting metal oxides to exhibit piezoresistive or piezoelectric transduction phenomena usable in functional nanodevices. Gemini amphiphile with tartrate counteranions are able to form chiral nanoobjects with the diameters of ~36 nm and were previously reported in our group [22][23][24]. A fine control on the shape of the final inorganic nanoobjects obtained from this organic template was possible upon tuning parameters such as the temperature, the reaction time for the processes and the concentration of the different reactants [25]. In Chapter 3 we will continue the optimization of the synthesis parameters in order to obtain individualized inorganic nanostructures with controlled length and with a high yield.

Different approaches to the fabrication of the chiral silica replica will be shown on the example of the micrometric helical structures described in Chapter 2.

Silica nanoobjects do not have exploitable electrical properties. In the fourth Chapter we will focus on the further modification of the silica nanomaterial with the semiconducting metal oxides such as titanium dioxide and zinc oxide. These materials are known to exhibit piezoresistive/piezoelectric transduction phenomena that can be used in functional nanodevices.

In the last Chapter we will describe the measurement of the mechanical properties of the silica nanotubes and nanohelices by AFM as the first step of the NEMS development.

Obtained results show that the stiffness of nanohelices is lower than that of nanotubes. This demonstrates that the helices are softer and so by using them as the mechanical elements of NEMS a Referencespage 26 higher sensitivity and resonance frequencies can be obtained compared to the conventionally used nanowires or nanotubes.

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Introduction

Self-assembly forms the basis for many natural processes including protein folding, formation of double helical DNA through hydrogen bonding of the individual strands, and the formation of cell membranes. The process of self-assembly is governed by inter-and intramolecular forces that drive the molecules into a stable, low energy state. These forces include hydrophobic interactions, hydrogen bonding, electrostatic interactions, metal coordination, π-π interactions and van der Waals forces.

Most of biological molecules are chiral 1 . Due to the role of chiral recognitions between drug and biomolecules, the enantioselective synthesis of chemical compounds is of critical significance for pharmaceutical industry [1]. It is also well recognised that chirality is intimately associated with the growth and stability of many self-assembled structures. Chiral nanostructures have therefore attracted the attention of the scientific community in order to better understand the biological processes as well asand for the development of chiral nanomaterials which find a wide range of applications in chemistry, physics and biology.

In this Chapter, the bibliographic study on the self-assembly of small molecules is presented. For this thesis we will concentrate on the self-assembly in water. At first amphiphilic moleculesbuilding blocks of most of the chiral aggregates formed in aqueous mediumwill be briefly introduced. Their molecular structures, mechanism of self-assembly and structures will be discussed. Then we will give an overview of the very recent achievements in the field of chiral selfassembly of the small molecules. Finally we will describe the main techniques used for the characterization of these chiral micro-/nanostructures. 1 i.e. such type of molecules that cannot be superimposed with its mirror image Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

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Self-assembly in water

Most of the self-assembled aggregates in water are formed by amphiphilic molecules. The word amphiphile comes from the Greek roots: αμφις, amphis, which means ''both'', ''double'' and φιλíα, philia, which expresses affinity. Amphiphilic compounds exhibit a dual affinity, i.e. possess both hydrophilic (water-loving) and lipophilic (fat-loving) properties.

A typical amphiphilic molecule consists of a polar hydrophilic group, usually called the head, attached to a nonpolar hydrophobic moiety called the tail (Figure 1.1). The head consists of a charged or uncharged polar group, like phosphate, sulfate, ammonium, amino acid, peptide or sugar.

The hydrophobic tail usually consists of a hydrocarbon chain, which may be saturated or unsaturated, linear or branched [2]. Due to their double affinity, amphiphilic molecules self-organize in water to form supramolecular assemblies. It was proposed by Tanford [3] that the hydrophobic effect drives the segregation of the hydrocarbon chains in water, thus providing the stimulus for self-organization. On the other hand, repulsive forces (electrostatic and/or hydratation) and steric effects between the polar heads prevent the formation of large three-dimensional crystals and hence phase separation.

Depending on the molecular structure of the amphiphile and on experimental conditions (concentration, temperature, pH and ionic strength etc.), the self-aggregation of amphiphilic molecules yields a rich variety of assemblies (Figure 1.2) [2,4].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

33 Referencespage 58 Beyond the more common micelles, vesicles and liquid crystalline mesomorphic phases, amphiphilic molecules can hierarchically self-assemble in a rich variety of more organized nanostructures such as fibers, ribbons, helices, super helices and tubes [2].

Self-assembled nanofibers are generally obtained by dispersing the amphiphiles into aqueous media by heating and/or sonication. Then the solution is allowed to stand at the designated temperature for a few minutes to several days, yielding the formation of the supramolecular assemblies [5].

Two common (but not essential) features usually encountered in self-assembled nanostructures with high aspect ratios are the presence of stereogenic centers [6] and of moieties suitable for intermolecular directional interactions, such as hydrogen bonds [2] or ππ stacking [6].

Thereby, small molecules that assemble into chiral structures often possess hydrogen-bonding functions and may combine amino acid or amide functions, sugar groups, or both as in gluconamides, as well as urea functions [6].

Thus, formation of the high aspect ratio chiral structure results from the combination of hydrophobic effect with directional interactions such as hydrogen-bonding, aromatic ππ stacking or, in some cases, metal coordination.

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Chirality: Why is it important?

A chiral molecule /ˈkaɪərəl/ is one that has a non-superposable mirror image. Since the identification of chirality in tartaric acid in 1848, it has fascinated the world of chemistry. It was later found that the vast majority of biological molecules are chiral. For instance, 19 of the 20 common amino acids that form proteins are themselves chiral, as are a host of other biologically important molecules, together with the higher structures of proteins, DNA and RNA.

Because of the chiral recognition between drug and biomolecules, the enantioselective synthesis of chemical compounds is of critical significance for pharmaceutical industry [1]. This is where chiral nanostructures find their application in chemistry as a platform for asymmetric catalysis [7,8].

Chiral molecules are able to rotate the plane of the polarization of light. Recently, chiral plasmonic nanostructures have been shown to have similar abilities which allow using them for various applications in optics. Collective plasmonic circular dichroic response was induced by chiral self-assembly of gold nanorods, surfactants, and phospholipids film [9]. Chiral assembly of gold nanoparticles was as well prepared through the peptide self-assembly and peptide-based nucleation of discrete nanoparticles [10]. DNA-origami based self-assembly of chiral plasmonic nanostructures with tailored optical response was reported recently [11]. Self-assembled chiral liquid crystals were developed for laser applications [12].

A new oligothiophene-based organogelator bearing two crown ethers at both ends was reported to be applicable as a reversible switching memory device, featuring memory creation by a heat mode and memory erasing by a mechanical mode [13].

Chiral self-assembly finds more and more applications in biological functional materials. As an example they are used for the mimicking of the collagen triple-helix motif found in biological systems [14], for the synthesis of responsive biomimetic networks [15] and for the directing the commitment of human mesenchymal stem cells (hMSCs) into osteoblast lineage in vitro, free of osteogenic-inducing media [16]. Chiral supergelator based on the bis-urea motif was used for the fabrication of stimuli-responsive supramolecular gel which was afterwards applied for the enzymatic biocatalysis in organic media [17]. It has to be mentioned that introduction of a stereogenic center improved the molecule gelation ability significantly relative to its achiral analogue. Recently, nanofiber scaffold for the 3-D cell culture formed by the chiral self-assembly of the D-form peptide system was reported [18]. Cells incubated in such 3-D microenvironment exhibit high viability and low-level of apoptosis. [26]. Spectroscopic observations, such as UV-visible and circular dichroism (CD) spectroscopies indicate that the aggregates are constructed on the basis of S-and R-chirally ordered structures through interamide interactions in water and organic media, respectively, and that their chiroptical properties can be controlled thermotropically and lyotropically. It is also reported that the chiral assemblies provide specific binding sites for achiral molecules (NK-2012) and then induce chirality for the bonded molecules [26]. Left-and right-handed chiral PPy (L-CPPy and R-CPPy) nanotubes can be prepared by using surfactants derived from l-and d-glutamic acid, respectively [27].

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Referencespage 58 dependent on the choice of their amino acid spacers [29].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

39 Referencespage 58 Unwinding transition, leading to twisted tapes, is observed on heating. Nanotubes and ribbons reform on cooling [30]. Lior Ziserman et al. presented experimental evidence for the morphological transition from twisted to helical ribbons in amphiphilic aggregates of short lipo-amino acid, N-lauryl-lysylaminolauryl-lysineamide, referred to as C 12 -β 12 (Figure 1.8). This transition, from structures possessing negative Gaussian curvature to helically curved structures, is shown to be directly linked Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study. 40 Referencespage 58

to the ribbon width. Time-resolved cryo-transmission electron microscopy images of a peptidomimetic amphiphile further capture the dynamic transformation between the two geometries along a single ribbon unit. Quantitative analysis indicates that both ribbon width and pitch grow with ribbon maturation, maintaining a constant ratio. The study confirmed the expectation from theory and simulations that the ribbon shape is controlled by its width: narrow chiral ribbons favor Gaussian saddle-like twisted curvature, while for wider ribbons (for the majority of the observed structures when w > 35 nm) the cylindrical curvature becomes favorable [31]. Four dipeptide sodium salts with long alkyl chains derived from L-and D-alanines were synthesized by Yi Li et al. (Figure 1.9) [32]. The dipeptide salts organized into twisted nanoribbons, whose handedness was controlled by the terminal alanine chirality. The handedness of nanoribbons formed in water was opposite to that of those formed in THF. The dipeptide salts self-assembled into similar inter-digitated bilayer structures in water and THF, but CD spectra of the gels indicated that the stacking of carbonyl groups was opposite. The formation of this handedness inversion is proposed to arise from the difference in interlayer distance and chiral stacking of carbonyl groups near the C-terminals. Referencespage 58

Self-assembly of small achiral molecules

Consistent stacking of achiral matchsticklike molecules in a staggered configuration could break the mirror and inversion symmetries, leading to chiral superstructures. Normally, one would expect the product to be a racemic mixture, because there are always two ways to stack achiral blocks. An exception is when the molecular self-assembly is extremely self-promoting (in a way similar to the nucleation process in crystal growth), so that a few initial superstructures could

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

43 Referencespage 58 ''seed'' the assembly in a large space/area. There were relatively few reports on the chiral assembly from achiral units, presumably because it is difficult to detect chiral supramolecular organization unless the product is large enough to be directly observed by electron microscopy [1].

Qingchao Hu et al. reported formation of chiral nanotubes from achiral amphiphilic azobenzene in acidic conditions (Figure 1.12) [36]. Tubular structures were formed from the helically coiled ribbon structures. It was revealed that tubes are consisted of 4 to 8 bilayer membranes formed by trans-azobenzene derivatives through the hydrogen bonding and π-π stacking. This is an interesting example of chiral self-assembly from an achiral molecular building block. It was further revealed that the morphology and chirality of the self-assembled nanostructures change in response to external stimuli such as light and heat. The straight nanobelts formed by cisazobenzene derivatives were observed in this case. The following mechanism of nanostructure formation was proposed. Two amphiphilic molecules are connected in a dimer by the intermolecular H-bonds; two dimers connected together by carboxyl groups form glide packing along the long molecular axis, resulting in J-aggregate.

Nanoribbons are formed by stacking of multiple bilayers via the hydrogen-bonding and π-π stacking. During the formation of nanoribbons, the symmetry was broken through the side-to-side H-bonding, resulting in the molecular tilt between the glide lamellas. The steric hindrance of the neighboring intermolecular H-bonds (head-to-head and side-by-side) leads to a small distortion dihedral angle between the benzene ring and the cyclic H-bonds, resulting in a twisting angle A B

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Referencespage 58 between the two connected dimer building blocks. Starting from such a unit with a helical sense, a tubular architecture with macroscopic chirality is formed [36].

Chiral films formation from achiral rigid-flexible molecules at the air/water interface upon compression in pure water and in presence of AgNO 3 was reported by Libin Liu et al. (Figure 1.13) [37]. The chirality of the Langmuir-Blodgett films deposited from pure water sub-phase was assumed to appear due to hydrogen bonding between the molecules and π-π stacking between the aromatic rod segments. The intermolecular hydrogen bonding is in a linear fashion and such an aggregation mode is favored for aromatic rod segments with larger steric constraints to stack in a cooperative way to form chirality. When the molecules were spread on an aqueous AgNO 3 subphase, the adjacent imidazole groups in the molecule are coordinated with Ag ions. Because the aromatic rod segments are very large, the molecules are sterically overcrowded when aligned in the same plane during coordination. Therefore, the aromatic rod segments must tilt to some extent along the polymer backbone (Figure 1.13). Once a molecule starts to distort in a certain direction, the neighboring molecules would follow to form a semi-helical structure at the air/water interface [37].

The self-assembly and supramolecular chirality of a dianionic tetrakis(4-sulfonatophenyl) porphyrin (TPPS) in the presence of ionic liquids, 1-alkyl-3-methylimidazolium tetrafluoroborate (alkyl = C 2 , C 4 or C 6 ), have been investigated by Li Zhang et al. [38]. It has been confirmed that mimBF 4 ionic liquids (Figure 1.14) significantly promoted the J-aggregation of TPPS and the alkyl chain length in the imidazolium cation was closely related to the TPPS aggregation. This ability decreased in the order of C 2 , C 4 or C 6 in side chain. Interestingly, the formed TPPS assemblies with A B Referencespage 58 the ionic liquids showed supramolecular chirality although both TPPS and ionic liquids were achiral. It was found that the supramolecular chirality of the TPPS/IL system always appeared after the formation of the J aggregate. The dynamic process of the emergence of the handedness in the initial achiral system was monitored by the time-dependent CD spectra. A proposed mechanism for the transformation of the conventional J-aggregate to the chiral J-aggregate is shown on Figure 1.14.

At the beginning, the cation from the ionic liquid interacted with the TPPS through ionic interaction and promoted the J-aggregation of TPPS. During incubation, more cations were attached to the sulfonate group of TPPS replacing the small ions, which caused an overcrowded stacking of the neighboring groups to form a helical aggregate and thus produced the chirality. Illustration of the ionic liquid promoted J-aggregation and the transformation of non-chiral J-aggregate to chiral one during the self-assembly of TPPS with ionic liquid (B) [38].

Detailed studies of the self-assembly of other porphyrin based systems forming chiral Jaggregates in aqueous solution were also performed in the groups of Roberto Purrello [39] and Josep M. Ribo [40].

Co-assembly of small molecules and metal ions.

In the previously discussed self-assembly of lipids and lipid analogues, the intermolecular forces were mainly the stacking interactions among the hydrophobic ''tails'' or ''plates''. In comparison, it was found that a certain combination of molecules, bound together by a network of coordination and hydrogen bonds, could also give chiral wires [1].

A B Referencespage 58

The Tang group carried out a detailed study of the self-assembly process of Ag(I)/cysteine (Cys), with different chiralities of the amino acid. The self-assembly of Ag(I)/L-Cys leads to the production of pure right-handed helical nanobelts, whereas Ag(I)/D-Cys gives rise to their "mirror image", i.e., pure left-handed helical nanobelts. As a comparison, racemic Ag(I)/DL-Cys forms a totally different product: two-dimensional achiral nanosheets. Density functional theory (DFT) simulation revealed that the consistently staggered arrangement of Cys in the lattice was the origin of the chirality in the superstructure. Thus, the transcription of chirality from molecules to superstructure was achieved because of the crystalline lattice order within the fibers [41].

Self-assembly of several low-molecular-weight L-glutamic acid-based gelators (Figure 1.15), which individually formed helical nanotube or nanofiber structures, was investigated in the presence of Cu 2+ ion by Qingxian Jin et al. [42]. The addition of Cu 2+ ion caused a transition from monolayer nanotube of N,N'-hexadecanedioyl-di-L-glutamic acid (L-HDGA) to a multilayer nanotube with a thickness of the tubular wall about 10 nm. For the other amphiphiles, the gel was destroyed and nanofiber structures were mainly formed. The Cu 2+ -mediated nanotube structure showed not only accelerated reaction rate, but enhanced enantiomeric selectivity in asymmetric catalyst for Diels -Alder cycloaddition between cyclopentadiene and aza-chalcone. It was suggested that, through the Cu 2+ mediated nanotube formation, the substrate molecules could be anchored on the nanotube surfaces and produced a stereochemically favored alignment. Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

Referencespage 58

In the work by Yan Qiao et al. [43] well-defined right-handed helical nanoribbons in calcium-cholate systems are systematically investigated and a series of metal ions are exploited to drive metal-cholate supramolecular helix. It is anticipated that the incorporation of metal ions may endow versatile functionalities and merits to the self-assembled nanohelices. The cooperative effect of multiple weak interactions (i.e., hydrogen bond, coordinating interaction and hydrophobic effect) is proposed to account for the hierarchical nanohelices. Later on, the self-templating approach in which metal ions can serve as both the inorganic precursor and constituent of the template was used to synthesize a series of 1D inorganic nanotubes including ZnS, CuS, NiS, CdS, CoS, ZnSe, and ZnTe nanotubes. Moreover, the structural diversity and dynamic nature of the organic architectures allow the preparation of size and shape-adjustable 1D nanotubes through conveniently temporal and thermal controls [44].

Special examples of the chiral self-assemble systems

In this section we will overview self-assembled systems with properties that make them somehow unique and different from the examples discussed above.

Normally, gelation occurs upon the cooling of amphiphile containing solution and at elevated temperatures stiffness of the gel decreases. Temperature-controlled, one-dimensional hierarchical architectures including nanotubes; coiled-coil, ropelike structures; nanohelices; and nanoribbons were fabricated by supramolecular self-assembly in the lanthanum-cholate system by Yan Qiao et al. [45]. These supramolecular hydrogels display "heating-enhanced stiffness" and "heatingpromoted gelation" behavior. Multiple weak molecular interactions, such as hydrophobic effect, metal-ligand cooperation, and hydrogen bonds, are supposed to account for supramolecular selfassemblies in the lanthanum-cholate system. The temperature-mediated hydrogen bonds between water and cholate are believed to be the structural origin of temperature-dependent lanthanumcholate self-assembly. Jan H. van Esch et al. have demonstrated that the morphological transitions of a multisegment amphiphile can be programmed in the design of its segments [46,47]. A multi-segment amphiphile that consists of two covalently linked but orthogonally self-assembling building blocks (Figure 1.16) was used. Through the addition of a chaperone, analogue authors were able to selectively switch off the self-assembly of each of these segments individually, resulting in architectures of the other segment. Such an approach can also be applied to other segmented self- By Silvia Marchesan et al. it was reported that a change of chirality of the first N-terminal amino acid of tripeptides VFF and FFV from L to D resulted in self-assembled hydrogels at physiological pH from non-assembling L-analogues [48]. Interestingly, changing the chirality of F yields very different nanostructures; nanotapes are observed for D-VFF, twisted fibers for D-FFV. Sangita Roy et al. studied the effect of Hoffmeister series ions on the self-assembling structure of aromatic dipeptide amphiphiles in aqueous media [49]. Hydrophobic interactions are greatly affected, resulting in differential order and chirality which is expressed as variable mechanical properties in the resulting gel-phase materials. The authors demonstrate that it is possible to regulate the supramolecular functionality, such as materials stiffness, presentation of bioactive groups, electroconductivity charge-transfer properties by a simple addition of organic or inorganic salts instead of the chemical redesign of the gelator molecules. Thus, it indicates that the ionic composition represents an important parameter to be taken into account when developing supramolecular hydrogels.

Construction of supramolecular self-assemblies whose self-assembling process and selfassembled architectures can be controlled by external stimuli is a fascinating and challenging topic Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study. 49 Referencespage 58 for supramolecular chemists. The amphiphilic diarylethene derivatives introduced in the work by Takashi Hirose and Kenji Matsuda exhibited interesting photoresponsive supramolecular functions in aqueous media [50]. For instance, photo-switching of supramolecular chiral environments and photo-induced isothermal lowest critical solution temperature (LCST) transition at room temperature were shown [50]. Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study. 50 Referencespage 58

Techniques

A wide variety of characterization techniques are used for evaluating the self-assembly at the molecular and supramolecular levels. Spectroscopic techniques are able to provide information on the molecular organization of the aggregate. X-Ray diffraction is used to determine the packing pattern in the supramolecular assembly. Dimensions of the chiral self-assembly vary from the nanometer up to the micrometer range. Depending on the size of the aggregate, various microscopy techniques such as atomic force microscopy, transmission and scanning electron microscopy as well as optical microscopy are used to characterize its morphology. Finally microscopic parameters of the self-assembly can by probed by rheology and differential scanning calorimetry.

The ultimate aim of any characterization study is to develop a better understanding of the structure and formation mechanism of the chiral aggregate, in order to get a better control over its morphology and rationally design new families of self-assembling molecules.

Methods that retain the solvated state of the samples during characterization are preferred over the techniques that require their drying.

Spectroscopy

Various spectroscopic techniques have been applied to organic self-assembly systems, including NMR, fluorescence, UV, CD, and IR spectroscopies, all of which can provide information on the molecular organization of the self-assembly [51].

Solution-state NMR and FT-IR can be used to identify the formation of hydrogen bonds during self-assembly [22]. In the case of NMR, relaxation time measurements (T1 and T2) can be used to identify those parts of the molecule whose conformational and translational motions slow down during the aggregation [4]. Solid-state magic angle spinning NMR has been used to examine the aggregated state and identify chemical shift changes from the solution spectrum, indicating a change in aggregation [4]. Chirality transfer in the self-assembly was detected by NMR as well.

Selective interaction between chiral tartrate anions and achiral dicationic surfactant head groups results in diastereotopic NMR patterns in the signal of the protons of the head groups [24].

UV/vis. and fluorescence techniques are used to detect a change in the hydrophobicity of the surroundings of the reporter group (it can be a part of the self-assembling molecule or an added probe) and identify π-π stacking or metal coordination that result in aggregation [4].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

51 Referencespage 58 IR is useful for confirming the presence of H-bonding and determining the protonation state of carboxylic acids. Intense signal of OH stretching band of water can overlay with the peaks of interest (such as the NH stretch), the water can be replaced by D 2 O. Sometimes, these studies are done with dehydrated samples. These results must be interpreted cautiously, since structural changes may have occurred during removal of the solvent [4].

The most versatile tool for studying assemblies of chiral molecules is circular dichroism (CD) spectroscopy. CD measures the difference of absorption in the UV/Vis. range for right-and left-handed circularly polarized light. Since a CD spectrum results from the subtraction of two slightly different absorption spectra, CD bands are very weak compared to those of the corresponding absorption spectra. However, the intensity and sign of CD bands are extremely sensitive to changes in the conformation of the molecules in the vicinity of the chromophores to which they are allied. Induced CD may be observed in chromophores remote from any stereogenic centers when the overall packing has a chiral twist [6].

One of the typical uses of CD is to monitor self-assembly kinetics during cooling of the sample from the temperature above gelation temperature T gel to below T gel . A sharp change in the CD spectrum does not coincide necessarily with gelation since the latter monitor the macroscopic assembly formation. Using this method, transition temperatures, critical aggregation concentrations, and the influence of added ions or of the solvent have been determined [6].

CD spectroscopy may be useful to monitor changes in the aggregate morphology, for example, for the transformation from nanotubes to twisted ribbons Slight changes in the CD spectrum of tubules of the diacetylenic phospholipid have also been used to determine the number of lipid bilayers in the tubule walls [6].

CD spectroscopy may give insights into whether some chiral molecules mix in the same fibers or, on the contrary, phase separate into different aggregates. If a complete phase separation of the enantiomers into aggregates of opposite handedness occurs, the CD intensity is expected to vary linearly with the enantiomeric excess (ee) of the mixture. Similarly, if the two enantiomers co-crystallize to form a racemic crystal, CD will arise from the remaining single enantiomer and a linear dependence of CD intensity on ee is expected. However, if the two enantiomers mix, at least partially, regardless of their proportions, co-aggregation may occur with negative or positive cooperativity and it is likely that the CD intensity would deviate from a linear dependence on the ee [6].

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It is difficult to conclude from a CD spectrum about the conformations of molecules and their relative positions in a fiber. However, when assignment can be made and when artifacts due to potential linear dichroism can be eliminated, the appearance of exciton coupling in the CD spectra may be interpreted to determine the sign (clockwise or counterclockwise) of a chiral twist of the molecular arrangement [6].

Vibrational circular dichroism (VCD) may provide extra information compared to CD in the UV/Vis. range. Contrary to conventional CD in this technique absorption in the region of vibrators is measured, and so the study of the chromophores-free molecules becomes possible [6].

An example of the use of VCD was provided in a study of 16-2-16Tartrate gemini surfactants consisted of nonchiral cationic amphiphile with chiral tartrate anion by Berthier et al.

from Oda's group [24]. These compounds aggregate into chirally twisted ribbons that express the chirality of the counterions despite the fact that the surfactants are not chiral. However, VCD measurements show induced CD bands in the symmetric and antisymmetric stretching modes of the CH 2 groups of the alkyl chains of the achiral cations. This demonstrates that chirality of the tartrate anions is translated to the cations which adopt chiral conformations resulting in chiral ribbons [6].

Diffraction

Two diffraction techniques, SAXS (small-angle X-ray scattering) and SANS (small-angle neutron scattering) are able to provide structural information about the self-assembly. Typically SAXS is carried out to acquire information about the packing pattern in the supramolecular assembly [29,32,52]. In addition, because native fibrous samples can be examined using SAXS, several authors have used the technique to model the role of solvent molecules in fiber formation [4].

Wide-angle X-ray powder diffraction (XRD) has great potential for elucidating the molecular structure of aggregate. One of the most useful pieces of information from a powder pattern is the long d spacing which corresponds to the longest repeat distance in the crystalline structure. By comparing the d spacings to molecular dimensions, factors such as the packing of molecules in an extended or bent conformation or the possible interdigitation of alkyl chains can be determined [29,42,53]. The number of the layers in the high aspect ratio structure and the transition from a monolayer to a multilayer chiral nanotube can be evaluated by combination of XRD and TEM techniques [42]. X-ray powder diffraction investigations together with molecular dynamics

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

53 Referencespage 58 simulations have allowed to elucidate the structure of the pseudo-crystalline racemic mixture of 16-2-16 tartrate multi-bilayer self-assembled ribbons [54].

Single-crystal X-ray diffraction gives the most complete and accurate description of the conformations and packing of molecules in the solid state. However, crystals of self-assembling molecule suitable for X-ray diffraction are very difficult to obtain, precisely because these molecules tend to aggregate into fibers, or at best into thin needles instead of crystals of sizable dimensions. This is why crystallographic studies are often performed on analogous molecules that possess poor or no gelling abilities [6]. In these cases, however, caution should be exercised, since the very fact that one molecule form sfibers and the other crystallizes points to a difference in their molecular assembly [4].

Microscopy

Microscopy, depending on the technique, provides insight into the micro-and nanostructure of a given self-assembly.

Optical microscopy with polarized light has a relatively low resolution, (0.2 µm is the theoretical limit due to the wavelength of visible light) however it does not require manipulation of the sample, allowing the native structure of the aggregates to be observed in real time [55,56]. In addition, the extinction pattern provides information about the degree of order in the aggregates.

Atomic Force Microscopy (AFM) is a high-resolution technique that has the potential to image hydrated samples in situ under high humidity conditions and without dehydration [57]. Direct observation of native plasmid DNA in water becomes possible by using an ultra-low-noise AFM with the highly sensitive force detection method (frequency modulation AFM: FM-AFM) (Figure 1.18) [58]. The sub-nanometer resolution imaging of peptide nanotubes in water was demonstrated using improved frequency modulation atomic force microscopy [59]. The drying of the samples facilitates the imaging, but can also introduce artifacts [60].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study. 54 Referencespage 58 Electron microscopy techniques, both scanning (SEM) and transmission (TEM), allow imaging of features with subnanometer resolution, and provide valuable information about the morphology of the aggregates. However sample preparation procedures such as drying and metallization for SEM or drying and staining for the TEM (both the microscopes operate under high vacuum) can result in artifacts when observing the self-assembling systems that inherently exist in a solvated state [61]. The stains sometimes can interact with the assemblies changing their morphology [62].

Cryogenic transmission electron microscopy (cryo-TEM) is a powerful tool to image selfassembled structures, via cryovitrication of the aqueous medium [30]. For cryo-TEM imaging, aqueous samples are flash-frozen in liquid ethane, creating thin ice films. Contrast created by the ice is sufficient for the visualization of organic assemblies in their native state without staining [63][64][65][66].

Figure 1.19 shows schematic of a cryo-TEM specimen of possible chiral aggregates and the corresponding image. Thermo-reversible helical unwinding transition of a self-assembling peptide amphiphile was recently revealed by this technique assisted by small-angle X-ray scattering [30].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

55 Referencespage 58 Other cryo-technique is called freeze-fracture TEM. Afrozen sample is fractured, usually splitting bilayers in half. Then a replica of the fracture surface is made and imaged under standard TEM operating conditions [67]. It is also possible to perform cryo-SEM imaging of organic assemblies [68]. It is more difficult to prepare completely vitrified sample for SEM observation, however, a potential advantage of this technique is that SEM images secondary electrons emitted from the sample's surface, can provide pseudo-three-dimensional images [4].

Modeling

Since atomic resolution is difficult to reach it is possible to use modeling, based on data collected from several of the complementary techniques discussed above, to obtain the possible molecular organization in the self-assembly [69]. Higher level energy minimization and molecular dynamics calculations have also been used to model the primary structure of aggregate. In this way, possible modes of aggregation of the molecules in aqueous solution have been identified [4].

Rheology and DSC

The macroscopic properties of a self-assembly can be determined by rheology and DSC.

Such measurements can provide values of the gelation temperature that correspond to the breaking of cross-links or molecular rearrangements. In a basic DSC experiment, energy is introduced both cells are raised identically over time. The difference in the input energy required to match the temperature of the sample to that of the reference would be the amount of excess heat absorbed or released due to the processes occurring in the sample [70]. This data allows to evaluate the enthalpy of formation of the nanofiber and provide a real time analysis of the self-assembly process [71].

Chapter 1: Chiral self-assembly of small amphiphilic molecules. Bibliographic study.

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Referencespage 58

Conclusion

Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. This process is governed by weak interactions such as hydrophobic interactions, hydrogen bonding, electrostatic interactions, metal coordination, π-π interactions and van der Waals forces.

In this Chapter we have described the classification of the amphiphile molecules, bases of the self-assembly mechanism and the main structures that can be formed by these molecules. These structures can be probed by a variety of techniques which allow studying them at the molecular and macroscopic levels. Spectroscopic techniques are used for the investigation of the intermolecular interactions in the self-assembly. Diffraction techniques provide information about the packing of the molecules in the aggregates. CD and VCD are used to monitor the presence and changes in the chiral organization of the molecules within the fibers. Microscopy techniques provide insight into the micro-and nanostructures of a given self-assembly. Finally, modeling can be used in order to complete the data collected from the techniques described above and describe possible molecular organization in the fiber.

In the last part of the Chapter we made a literature overview on the very last study of the chiral self-assembly in aqueous medium. Formation of the high aspect ratio aggregates with the prominent twist was reported through the assembly of small chiral molecules, achiral molecules and through the co-assembly of gelating molecules and metal ions. Reported chiral self-assembled systems involve nearly all the types of amphiphile molecules and a variety of the head groups. The chemical structure of the amphiphile, its concentration, the pH, the temperature, the ionic strength, the solvent composition, and the presence of additives were shown to be the main factors affecting the morphology of these chiral assemblies. The morphology of the aggregates can also be changed by external stimuli (for example by UV irradiation) or evolve upon the aging of the sample.

One has to keep all this in mind in order to study and control the chiral self-assembly. In this chapter we will describe a new system based on cationic gemini surfactants complexed with Guanosine 5'-monophosphate which, under certain conditions, self-assemble into micrometer size helical structures in the presence of Hofmeister series anions. The effect of pH, temperature as well as nature and amount of additives on the morphology of aggregates will be discussed. TEM, SEM and spectroscopic techniques were used in order to investigate the structure of the helices and their mechanism of formation.

An undergraduate student Barbara MARTIN worked on this project for two month (2013) as a part of her internship.

Micrometrical chiral structures based on organic self-assembly

While there are a number of examples found in the literature for the fabrication of nanometric chiral structures based on bottom-up self-assembled systems, there are much fewer reports on the formation of micrometer size chiral structures by organic self-assembled ones.

Ayyappanpillai Ajayaghosh et al. reported the formation of micrometric helical structures based on oligo(p-phenylenevinylene) (Figure 2.1) [1]. Two types of chiral and achiral molecules were used in their study. They found that the mixture of the two exhibited a CD signal at a much lower concentration than the chiral molecule alone. Such a system, in which a chiral molecule transfers its chirality to the supramolecular structure mainly formed by achiral molecules, can be described as a system following the "sergeants and soldiers" principle. George B. Benedek with his group showed that a variety of multicomponent systems consisting of a surfactant, a phosphatidylcholine or a fatty acid, and a sterol can produce large yields of high and low pitch helices [2]. A commercially available system from Gibco/BRL company called "Chemically defined lipid concentrate", for example, self-assembles into helical ribbons (Figure 2.2) with high yields and with high reproducibility [3]. The authors showed that the direct investigations of the elastic properties of these helices could be performed by tethering them, applying well defined stresses, and measuring the associated strain. Aurelie Brizard from the group of Reiko Oda studied anionic oligopeptides complexed with cationic surfactant [6]. Among various systems investigated, the formation of helical structures was observed for the systems based on oligoalanine-cationic surfactant systems (Figure 2.5) [7,8]. Later on, the in situ structural transition from vesicles of nonchiral C 12 or C 14 cationic amphiphiles with acetate counterions to helices of nucleoamphiphiles in aqueous solution was demonstrated as well [10]. In the reported system, anionic nucleotides such as adenosine monophosphate or guanosine monophosphate interact with cationic vesicles of didodecyle (C 12 ) or ditetradecyle (C 14 ) dimethylammonium acetate, exchange with the counteranions of the Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

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Referencespage 115 amphiphiles, and organize themselves at the membrane surfaces through weak interactions such as H-bonds and π-π stacking. This reorganization and the directional intermolecular interactions induce the formation of micrometric helices on the time scale of hours. The amphiphile to nucleotide ratio is an important factor that determines the helix dimensions. The main difference between AMP and GMP systems is the kinetics of helix formation and the helix dimensions as well as the solution properties (precipitate vs. gel). This could be attributed to the nature of the intermolecular interactions between GMP or AMP.

Chemistry of guanosine monophosphate

Guanosine monophosphate, also known as 5'-guanidylic acid or guanylic acid and abbreviated GMP, is a nucleotide that is used as a monomer in RNA. GMP consists of a phosphate group, a pentose sugar ribose, and a nucleobase guanine ( The surface region of micelles and vesicles was shown to provide an effective microenvironment for both base-base stacking and hydrogen bonding interactions. The nature of the aromatic stacking, the important non-covalent interaction which stabilizes the double helical structure, are not well understood. Electrostatic interactions, dispersion forces, and solvophobic effects are the potentially important factors in stabilizing the face-to-face base-base interactions [11].

The multiple hydrogen-bond donor and acceptor sites within the guanine units allowed the guanosine derivatives to form the hydrogen-bonded G-quartet or continuous helix in the presence of

A B C

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

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Referencespage 115 alkali metal ions as a template. Na + and K + ions were shown to be effective in inducing the Gquartet formation contrary to Li + and Cs + . The assemblies in the 5'-GMP gel were shown to be continuous helices with 15 bases [11].

Intermolecular interaction in self-assembled systems is directed through non-covalent interactions such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, and/or electrostatic ones. Therefore, the protonation of nucleotide governs this interaction, and determines the molecular conformation and so morphology of supramolecular aggregate. Protonation states of Guanosine 5'-monophosphate as a function of the pH are shown in the Figure 2.9 [12][13][14].

In this chapter we will describe new systems based on cationic gemini surfactant complexed with Guanosine 5'-monophosphate as anion which self-assemble into micrometer size helical structures under certain conditions. The effect of pH, temperature, nature and amount of additives on the morphology of the resulting aggregates will be discussed. TEM, SEM and spectroscopic studies will be applied in order to understand the structure of the helices and the mechanism of their formation.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 71 Referencespage 115 72 Referencespage 115

Kinetics of self-assembly of 12-2-12GMP alone

In the Introduction we saw that the formation of nano-to micrometric left-handed helices in aqueous solution may be induced by complexing a Guanosine 5'-monophospate (GMP) or an Adenosine 5'-monophospate (AMP) with a nonchiral monocationic amphiphile. Here we are extending this approach to the family of gemini surfactants.

Molecule consisted of nonchiral cationic gemini surfactant and 2 chiral Guanosine 5'monophosphate moieties was synthesized according to the procedure described in the "Materials and methods" section and denoted as 12-2-12GMP (Figure 2.10). Since Guanosine 5'-monophosphate has several protonation states, different morphologies may be obtained by self-assembly of 12-2-12GMP at different pH values. In this experiment 12-2-12GMP was formed in situ by the reaction of 12-2-12Ac with Guanosine 5′-monophosphate (pKa1 = 0.7; pKa2 = 6.1 [15]) disodium salt in the presence of acetic acid (pKa = 4.76 [15]) for the pH control. Typically 500 μL of 20 mM aqueous solution of 12-2-12Ac was mixed with 500 μL of 40 mM GMP-Na 2 solution. Acetic acid (1 M) was added to this mixture for the pH adjustment. The sample was then treated in the same way as one of 12-2-12GMP.

If no acetic acid was added, the pH of the sample was 7.6 and it didn't completely dissolve even after a long heating. Just after cooling it formed spherical particles similar to thoses of 12-2-12GMP. After 24 hours these particles aggregated (Figure 2.12A and B). No further structural changes were observed with time. No chiral structure formation was observed therefore for the mixture of Guanosine 5'monophospate, nonchiral gemini amphiphile, and acetic acid, contrary to the results obtained in our group with GMP and monocationic amphiphile.

A B C 5 µm 1 µm 200 nm

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

76 Referencespage 115

Self-assembly of 12-2-12GMP in the presence of bromide anions

It is known that chiral self-assemblies based on chiral molecules are quite sensitive to additives [16][17][18].

Bromide was chosen as an additive for this study as it is the anion of a strong acid which can compete with GMP as a counter-ion, also the properties and behaviors of gemini bromide are well studied in the literature [19] and in our group [20].

In this section, we will study the self-assembly of 12-2-12GMP/12-2-12Br mixtures with different ratios obtained by reaction of 12-2-12Br with GMP-Na 2 in the presence of acetic acid (Figure 2.14).

Figure 2.14 -Schematic of the experiment

First we will show the influence of the pH, temperature and GMP to bromide ratio on the morphology of the system. The effect of the structure of the amphiphile molecule, i.e. spacer and chain length will be mentioned. Then we will show the details of the internal structure of the aggregates by electron microscopy and spectroscopic techniques. Finally we will try to understand the mechanism of the helices formation.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 77 Referencespage 115

Kinetics of 12-2-12GMP self-assembly in the presence of bromide anion

Effect of the pH

The number of negative charges on the phosphate group of GMP determines its interaction with the cationic gemini surfactant making the system pH sensitive.

In order to study the effect of the pH on the self-assembly of 12-2-12GMP/12-2-12Br mixture, we fixed the ratio of GMP to bromide to 1:1 for all the samples. 12-2-12Br was dissolved in water at 50 °C to get a 20 mM solution. 500 μL of this solution were mixed with 500 μL of a 40 mM Guanosine monophosphate disodium salt solution (GMP-Na 2 ) and the pH was adjusted with a 1 M acetic acid solution 2 . When mixed, white precipitates were formed. The samples were heated at 70 °C for 5 minutes and stored 20 °C for aging.

When equal amount of 12-2-12Br and GMP-Na 2 are mixed; the solution exhibits a pH of 7.4; however the compounds cannot be completely dissolved at 70 °C. Unstructured precipitate was formed after the cooling. No further changes of morphology were observed in this case (Figure 2.15A).

The addition of acetic acid drastically changes the behavior of the system. For all the samples with a pH lower than 7, a complete dissolution was obtained upon heating and a white precipitate or a gel were formed after cooling. A further increase of acetic acid concentration leads to the elongation of the helices and their aggregation (Figure 2.15D-F). All the samples with pH lower than 5.5 after cooling form weak gels in the 1.5 mL Eppendorf test tube. Those gels are stable when the sample tube is flipped upside down, but may be easily broken by shaking. 2 Although the acetic acid being a weak acid, it limits the range of pH that can be studied, the stronger acids may compete with GMP and bromide making interpretation of the results more sophisticated. The highest ratio of individualized helices was observed at pH = 5.9 (Figure 2.15C). At this pH, ~60% of GMP have one negative charge on the phosphate group and ~40% have two negative charges.

Effect of the temperature

The temperature is another important factor which influences the kinetics and morphology of self-assembly of gemini GMP/Br. In this study we fixed the ratio of GMP/gemini to be 2:1 (GMP/Br = 1:1) and pH = 5.9. The concentration of amphiphile was 10 mM. Krafft temperature for 12-2-12GMP and 12-2-12Br is 48°C and 17°C respectively. If the ratio in the mixture is GMP/gemini = 2:1 (GMP/Br = 1:1) at temperature lower than 35°C all the amphiphile precipitates from the solution (see section 3. °C also exhibits an increase of the diameter of the helices, but a higher degree of aggregation of the ribbons. The sample aged at 1 °C didn't undergo significant changes during one week: mainly aggregates with no obvious structure were observed along with limited number of the small helices and twisted ribbons.

The speed of the cooling is also known to have a significant impact on the morphology of self-assembled structures [21,22]. To separately analyze the effect of these factors, the following experiment was performed. The sample dissolved at 70 °C was cooled at 1 °C for 10 minutes (this time is enough to precipitate all the amphiphile present in solution) and then stored at 20 °C for aging. From Figure 2.18 it can be seen that after one week of aging there is no difference in morphology between the samples first cooled down to 1 °C and then aged at 20 °C (fast cooling)

and one stored at 20 °C immediately after the heating (Figure 2.17D). This fact suggests that fast cooling does not have any influence on the shape of helical aggregates.

Effect of GMP to Bromide ratio

Another important factor affecting the self-assembly of 12-2-12Br + GMP-Na 2 system is GMP to Bromide ratio. In this study (Figure 2.14) 500 μL of 20 mM 12-2-12Br solution were mixed with 500 μL of GMP-Na 2 solution of different concentrations to give the molecular ratios of GMP-Na 2 /12-2-2Br -0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, 3.2, 3.6, 4.0 (from 0.2 to 2.0 for GMP/Br respectively) and a final concentration of gemini of 10 mM. The pH was adjusted to 5.9 by 1 M acetic acid. Then samples were heated at 70 °C for 5 minutes and stored at 20 °C for aging.

The sample with the GMP-Na 2 /12-2-12Br ratio of 0.4:1 remains transparent after 1 week of aging whereas all the others with higher ratios form precipitates within several minutes.

The samples with the GMP-Na 2 /12-2-12Br ratios from 0.8 to 1.6 form clusters of small ribbons within 1 hour. These ribbons continue to grow, and in 24 hours create a network of fibers without obvious chirality (Figure 2.20B). After 1 week of aging, no helices were formed (Figure In all the cases above, we studied the dynamic equilibrium of 12-2-12Br and 12-2-12GMP in the presence of free GMP-Na 2 and NaBr (Figure 2.14). In order to evaluate the effect of the presence of free GMP-Na 2 we performed the following experiment. 12-2-12Br was mixed with 12-2-12GMP in a 1 to 1 ratio in absence of free GMP-Na 2 . The total concentration of the gemini amphiphiles was 10 mM. Although the ratio between GMP and Br in this case is the same as in the case of 2 to 1 GMP-Na 2 /12-2-12Br mixture, the behavior of the system is drastically different. When free GMP-Na 2 was presented in the mixture we observed helix formation. In absence of free GMP-Na 2 in the solution and after 24 hours of aging, the first needle-like structures started to form and within a week all the precipitates transformed into the "needles" (Figure 2.21). Results of this experiment suggest that not only GMP-Na 2 /12-2-12Br ratio by itself, but also the ion strength and the dynamic of ion exchange at the interface of the aggregates determine the morphology of self-assembly.

Effect of spacer and chain length

A set of gemini bromides with different alkyl chain lengths n and spacers m was used to study the effect of these factors on the self-assembly of the system n-m-nBr + GMP-Na 2 (Figure 2.22). The chain length was varied from 10 to 16 and the spacer length from 2 to 6. The concentration of amphiphile was 10 mM and GMP to gemini ratio was 2:1 in all the experiments. However we observed helices formation for the amphiphile with the spacer length of 4 carbons. Small helical fibers form already after 1 hour and during a week become bigger and better defined (Figure 2.25D-F).

Increasing of the spacer to 6 leads to even more pronounced helical shape of aggregates (Figure 2.25G and H). The number of helices as well as their length and average diameter increase. But higher level of aggregation of fibers was observed in this case. For Gemini surfactants, Krafft temperature at 3 mM increases with increasing alkyl chain length: 14 °C for 12-2-12Br, 32 °C for 14-2-14Br and 45 °C for 16-2-16Br. Extrapolated value for 10-2-10Br is 1°C. The effect of the spacer length is more complex. Krafft temperature of gemini amphiphile decreases for 16-4-16Br (T k = 34 °C) and then increases again for 16-6-16Br (T k = 41°C) still remaining lower than one of 16-2-16Br [23].

Figure 2.26 summarizes data about the structure of the self-assembly as a function of spacer and chain length. It can be seen that the balance between the solubility and crystallinity of the sample is important for the formation of chiral structures. High solubility for 10-2-10 Bromide results in small amount of the formed structures as well as a slow kinetics of helices growthprecipitation of 10-2-10GMP is the prevailing process. 16-2-16Br is the other extreme case. It readily precipitates at 20 °C, decreasing the possibility of double salt formation which seems to be a prerequisite for the helical structures (see section "3.3.2. NMR study" of this chapter). Due to the high crystallinity of the sample no chiral structure is formed. In the intermediate cases where solubility and crystallinity are well balanced, the formation of the helical fibers was observed.

In this part of the work we studied kinetics of 12-2-12GMP self-assembly in the presence of Bromide anions. We showed that this system is able to form chiral fibers with diameters in the micrometer range. GMP to 12-2-12Br ratio along with the total GMP concentration at the interface of aggregates are the main factors determining morphology of the self-assembly. The sensitivity of Guanosine 5'-monophosphate to pH affects the interaction of amphiphile with GMP and governs the

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Referencespage 115 self-assembly of GeminiBr/GMP. For the12-2-12Br/GMP system, helical structures were observed only at pH lower than 6; they grow with decreasing pH and fibers entanglement accompanied by gel formation was observed for the pH lower than 5.5. The temperature also affects the morphology of the fibers. Helices are more aggregated at 30 °C than those formed at 20°C. We observed a low number of chiral structures and very slow kinetics of their formation when aging was performed at 1 °C.

Organic helical structures are often used as a template for composite material manufacturing [24][25][26]. In this case individualized helices are needed for most of the applications such as microsensors and actuators, high elastic electric conductors, micro-robots, etc. The best conditions for the formation of individualized microhelices were found to be: In the next part of this study we will investigate the internal structure of the chiral fibers and the possible mechanism of their formation will be proposed.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 89 Referencespage 115

Structural characterization

TEM and SEM

The structure of micrometric helices formed in the mixture of 12-2-12Br and GMP-Na 2 (pH = 5.9; amphiphile concentration -10 mM) with a GMP/12-2-12Br ratio of 3 was studied more in detail by electron microscopy: TEM and SEM.

The organic helices have high enough electron density for the visualization in TEM without additional staining (Figure 2.28). It can be seen that helical structures are formed mainly from two fibers with a diameter of around 500 nm. The space between the helices is filled with dense network of much smaller nanofibers with a width of 20-70 nm. It is worth to remind, that same nanofibers we observed in the structure formed by 12-2-12GMP in absence of bromide. This fact points out the similarity of the self-assemble mechanism for the two studied systems.

NMR Study

NMR measurements allow to reveal the composition of self-assemble aggregates as well as to understand some details of their formation. In this section we will describe results of two types of NMR experiments. First, we will monitor sample composition upon controlled cooling. That allows us to answer the question if 12-2-12Br and 12-2-12GMP co-precipitate during the cooling process or if the precipitation of 12-2-12Br follows that of 12-2-12GMP and a mixed structure is formed during the aging. In the second experiment we will study the composition of aggregates formed at different GMP-Na 2 /12-2-12Br ratio. This study will give us an answer about actual GMP/gemini ratio in the self-assembly.

NMR experiment with a controlled cooling was performed according to the procedure described in Materials and methods. Another conclusion we can make from this experiment is that not all the GMP presented in the mixture incorporates into the structure of the aggregates, part of it remains in a free form in the solution. In other words, GMP/gemini ratio used for the mixture preparation is different from the GMP/gemini ratio in the aggregates.

In order estimate the GMP/gemini ratio in the aggregates as well as to probe its interrelation with the mixture composition and structural aspect of self-assembly we performed the following NMR experiment. Kinetics of self-assembly for mixtures of 12-2-12Br and GMP-Na 2 with the GMP/gemini ratios from 0.4 to 4 for a final concentration of gemini amphiphile of 10mM were studied with optical microscope observation (see section 3.1.3 of this chapter). After one week of aging, these samples were centrifuged and the precipitates were dissolved in 500 µL of DMSO-d 6 in the presence of 2 µL of 48 % HBr. The integration of NMR spectra was calibrated by the amphiphile part of compound. We used the integration of protons H8, H1' and H4' (L, K and H on Here we plotted GMP/gemini ratio in the fibers as a function of GMP/gemini ratio in the original mixture. The GMP/gemini incorporated into the self-assembled structure naturally increases with increase of original GMP/ Gemini ratio, reaching a maximum of 1.6 at the GMP/gemini ratio in the mixture of 3.2. Helical structures were observed for the content from 1.1 to 1.6 GMP/Gemini ratio in the aggregates obtained for the GMP/gemini ratios in the mixture from 2.0 to 3.2.

Helical ribbons Achiral fibers Unstructured precipitate

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

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It can be assumed, that if there are less than 1 GMP per gemini in the self-assembled structure, the molecular chirality cannot be transferred to the microscopic level due to the lack of chiral molecules in the system. On the other hand, when the GMP to gemini ratio in the aggregate exceeds a value of 1.6, the interaction between GMPs become strong enough to cause rigidity of the system and so lead to the absence of microscopic chirality.

It is interesting to note that even though the amount of guanosine 5'-monophosphate in the aggregates is nearly the same (~1.6) for the mixture compositions GMP/gemini = 3.2 and 3.6 , the morphology of these structures is entirely different (Figure 2.20I and Figure 2.20L). This indicates that not only the composition of the self-assembled structure, but also the composition of the solution in equilibrium is also important for determining the morphology of the assembly structure.

Infrared study

Infrared study was performed in order to get additional information about intermolecular interactions in the studied system. Fourier transform infrared spectra (FT-IR) of a 12-2-12Ac + GMP-Na 2 + acetic acid (gemini/GMP ratio = 1:2, concentration of amphiphile -10 mM; pH 5.9) mixture which self-assembles into needle-like structures and 12-2-12Br + GMP-Na 2 + acetic acid (gemini/GMP ratio = 1:3, concentration of amphiphile -10mM, pH = 5.9) mixture which selfassembles into helical structures were recorded in D 2 O (Figure 2.33). Deuterated solvent was used in order to avoid overlapping of the peaks of interest with the peak of the solvent. Samples were aged for 1 week prior to the FT-IR measurement. Similarity of the spectra suggests that there are no interactions that are specific for one of the systems and which can be detected by infrared measurement. Absence of the shifts between the corresponding peaks in the spectrum of GMP-Na 2 and the spectra of the two studied self-assemblies indicates that there is no hydrogen binding within the samples. Obtained results suggest that π-π stacking which is invisible by the FT-IR technique can be the interaction that determines the difference in the morphology between the studied systems.

Helices growth

Until now we focused on the morphology of the helical aggregates. However, the formation of these structures is a dynamic process. We will now try to answer the question: "How do the helices grow?" There are two possibilities for the growth of the helical ribbons. In the first case, molecules, constituents of the core (i.e. aggregates formed just after the cooling), are rearranged to form the helix which is expanded from the core (growth from the base mechanism). In the other case, the core is slowly dissolving and its molecules diffuse and attach to the tip of the helix (growth from the tip mechanism). 96 Referencespage 115

Optical microscopy

To study the formation of the helices, we recorded optical microscope images every 5 minutes during the first 12 hours of the helix growth. In this experiment, we used the best sample composition found before: GMP-Na 2 + 12-2-12Br (2:1, pH = 5.9) (see section "3.1 Kinetics of 12-2-12GMP self-assembly in presence of bromide anion"). Several stages of the helix growth are shown in Figure 2.34. There is a kink formed at the end of the fiber (shown with the red arrows). The fact that we observe its rotation upon the helix growth brings us to the conclusion that the fiber is extending from the base.

Fluorescent labeling

In order to confirm the results obtained by optical microscopy and check which growth mechanism corresponds to our system we designed the following experiment based on the fluorescence recovery after photobleaching (FRAP) technique. 98 Referencespage 115

The second option is the reaction of amine groups of guanosine monophosphate with fluorescein isothiocyanate (often abbreviated as FITC -Figure 2.36B) which is the original fluorescein molecule with an isothiocyanate group (-N=C=S), attached on the bottom ring of the structure. This derivative is reactive towards primary amine groups via the formation a thiourea linkage.

A third approach is to use 5-dodecanoylaminofluorescein ( For the fluorescent staining, we used the same best mixture composition as found before (see section "3.1 Kinetics of 12-2-12GMP self-assembly in the presence of bromide anion"). 500 µL of 20 mM 12-2-12Br solution were mixed with 50 0µL of 60 mM GMP-Na 2 and 10 µL of 1 mM sodium fluorescein aqueous solution (ratio of dye to gemini is 1:1000). The pH of the mixture was adjusted to 5.9 with 1 M acetic acid. The sample was heated at 70 °C for 5 min in order to obtain a homogeneous solution and was then stored at 20 °C for the helices formation.

A similar procedure was applied for the staining with 5-dodecanoylaminofluorescein. It is impossible to dissolve this fluorophore at 1 mM in water, so 1 mM ethanolic solution was used for the experiment.

NMR experiments (section 3.2.2) have shown that not all the GMP is incorporated into the structure of the helices (Figure 2.32). This fact made usage of fluorescein isothiocyanate (FITC) an As it can be seen from Figure 2.37, fluorescein based dyes incorporated into the structure of self-assembled helical aggregates seem to show good contrast between helices and background. The high hydrophobicity of the compound promotes its coupling with the amphiphile resulting in a good staining over the all period of the helices formation. For the FRAP experiment sample was sealed between the two microscope slides and observed with PicoQuant MT200 optical microscope, 100x objective (Numerical aperture N=1.40; oil immersion). For the epifluoerscence, a LED at 380nm has been used for excitation. the 432 nm Unfortunately, we were not able to determine the mechanism of helices growth in this experiment due to the technical limitation (presence of free fluorophore in solution). Usage of highly hydrophobic fluorophores is one of the solutions for this problem.

Even though the ultimate goal of this experiment wasn't achieved, the obtained results are interested on their own. The possibility to incorporate fluorescent moieties into the structure of the helical aggregates without disturbing their morphology can be used for future studies of this system and other applications.

Study of 12-2-12GMP self-assembly in the presence of Hofmeister series ions

In order to determine the ion effect on the morphology of 12-2-12GMP aggregates we studied its self-assembly in presence of several Hofmeister series anions [27]. We chose iodide, nitrate and chloride for this study. Hydrophobicity of these anions is decreasing in the order I -> NO 3

-~ Br -> Cl -. Aggregation properties such as critical micelle concentration (CMC), aggregation number, micellization energy and ionization degree of gemini amphiphiles with these anions follow the same trend [28].

In this study we apply the same procedure as we have used for the GMP-Na 2 + 12-2-12Br system (see section 3.1.3 of this chapter).

12-2-12I is not soluble at a concentration of 20 mM at room temperature (T k = 58 °C) so it was dissolved at 100 °C in boiling water prior to the mixing with guanosine 5'-monophosphate disodium salt.

At the ratio GMP/12-2-12I = 0.4, only needle-like crystals of gemini iodide were observed For the ratio GMP/12-2-12I = 4.0, chiral aggregates were observed after one week of observations (Figure 2.39C).

It can be assumed that the high Krafft temperature of 12-2-12I is the main factor determining the self-assembly of 12-2-12I + GMP-Na 2 mixture by shifting the equilibrium towards the formation of 12-2-12I as most insoluble product.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

102 Referencespage 115 For the system, 12-2-12Cl + GMP-Na 2 , the sample remained soluble after 1 week of aging for the GMP/12-2-12Cl ratio of 0.4 whilst with the ratio of 0.8, the sample formed very small amount of precipitate after 1 day. For higher ratios, a precipitate was formed after 10 minutes. No structured precipitate was observed in all the samples with GMP to 12-2-12Cl ratio of 2 and lower (Figure 2.39D) as well as for the samples with GMP/12-2-12Cl = 3.2 and higher. At the ratio of 2.4, the first helices were observed after 6 hours of aging. With further aging, the number and the length of the helices increased (Figure 2.39E). A similar behavior was observed for the case of GMP to 12-2-12Cl ratio of 2.8 (Figure 2.39F).

It has to be mentioned that samples with chloride have much stronger tendency to form aggregates rather than individualized fibers compared to the ones with bromide. Kinetics as well is much slower in the case of chloride: after 24 hours only a small number of helices was formed and after 1 week their formation was not complete whereas already after 1 day almost all the sample was represented by the helical structures in the case of 12-2-12Br and GMP-Na 2 mixture.

The formation of the first helices for the ratio GMP/12-2-12NO 3 = 2.0 was observed after 3 hours and after 6 hours they become better defined, after 24 hours, the helices transformed into long fibers without obvious chirality. Only sporadic helices were observed in this case after 2 and 3 days. From the Table 2.1 and Figure 2.40 it can be seen, that nature of the anion has significant influence on the range of GMP/gemini ratios in which formation of helical structures is possible.

Both high Krafft temperature of 12-2-12I (T k ≈ 58 °C versus T k ≈ 48 °C for 12-2-12GMP)

and low pKa of Iodide (pKa = -10 [29]) may be the main parameters determining self-assembly of 12-2-12I/GMP. Due to these factors, the equilibrium in the 12-2-12I/GMP system is shifted towards the formation of gemini iodide. This explains the fact that it is only at high GMP concentration that the helices formation was observed.

Gemini with bromide (pKa = -9 [29]) as the anion has the widest range of the gemini/GMP ratios for which we observe the formation of helical structures. With the chloride (pKa = -7 [30]), this range is smaller and for nitrate (pKa = -1.4 [29]) there was only one condition at which the helices were formed.

The results described above seem to indicate the pKa of the counter-anions is related to the range of GMP/12-2-12X ratios at which the formation of helical ribbons is observed. Usually, at a particular GMP/12-2-12X ratio, anions X -with higher pKa are more easily replaced by GMP. Thus the optimal GMP/X -ratio at the interface for the helices formation is exceeded at the lower GMP/12-2-12X ratio in the mixture in the case of the anions X -with higher pKa. This explains the rise of the upper limit of the helices existence in the GMP/gemini ratio with the decrease of the pKa of the anion. However the lower limit doesn't follow same tendency, as it could be expected. On the contrary, we observe a widening of the range of the GMP/12-2-12X ratios in which helices are formed with the decrease of the pKa of the anion X -. This fact suggests that there are other parameters of the anion controlling the self-assembly of the system.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

106 Referencespage 115

Conclusion

We have shown that guanosine 5'-monophosphate complexed with gemini amphiphile (12-2-12GMP) self-assembles into long needle-like structures. This is in contrast with micrometric helices observed with monocationic amphiphile-GMP complex. The packing of GMPs in the needle can be disturbed by other anions, like bromide. Guanosine monophosphates, in this case, become more mobile and able to twist the bilayer of amphiphile molecules transferring the chirality from molecular to supramolecular level. Micrometric structures made of smaller fibers are formed in this case; they exhibit a diameter of around 500 nm.

Schematically, the process of helices formation may be described in a following way. Upon cooling, amphiphiles form aggregates with a certain ratio and distribution of GMP and bromide anions in the aggregates. However, the mixture of 12-2-12Br and 12-2-12GMP in the absence of GMP sodium salt in solution doesn't form helical structures suggesting that a dynamic equilibrium of anions between the precipitate and the solution is necessary for the formation of chiral aggregates. The ion exchange between the gemini and sodium salts in solution redistributes the anions at the interface of the bilayer resulting into its twist.

The ratio of GMP and Bromide in the aggregates is one of the crucial factors determining the morphology of the self-assembled aggregates. If the concentration of GMPs is too high, they start to

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 107 Referencespage 115 stack and this makes the bilayer very robust and so unable to twist. On the other hand, if there is very low number of GMPs at the interface they are unable to twist the ribbon.

The pH of the system determines protonation state of guanosine 5'-monophosphate which thendetermines the length and level of aggregation of the chiral fibers.

The Krafft temperature and the pKa value of the chosen anion affect the optimal GMP/gemini ratio at which the helices can form.

Conditions for the formation of the individualized microhelices were optimized.

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

108 Referencespage 115

Materials and methods

Materials

Guanosine monophosphate disodium salt was purchased from Acros Organics. Other chemicals were purchased from Sigma Aldrich and used without further purification.

Ultrapure water (18.2 MΩ cm) was obtained from Purelab ultra ELGA system.

Synthesis

Balance Sartorius BP211D with an accuracy of 0.01 mg was used for the weighting.

Gilson Pipetman micropipets were used to measure and move small amounts of liquid reagents.

Liebherr AquaLytic Thermostatic cabinet adjusted to 20°C was used for the aging of organic gels.

Synthesis of 12-2-12Br

12-2-12Br was synthesized by alkylation of tetramethylethylenediamine (TMEDA) with 1bromododecane in acetonitrile.

In a typical synthesis, 5 mL of TMEDA (1 eq.) was mixed with 23.85 mL of 1bromododecane (3 eq.) in 50 mL of acetonitrile in a 250 mL round-bottom flask. Alkyl bromide was added in excess in order to eliminate the formation of monoalkylated byproduct. Reaction was carried out for 48 hours under the reflux at 85 °C with continuous intensive stirring at 400 rpm.

Then reaction mixture was cooled down to room temperature and stored in a refrigerator for 1 hour.

White precipitate of gemini bromide was formed at the bottom of the flask.

Product was filtered and purified by crystallization from acetonitrile in the following manner.

Powder was mixed in a 250 mL round bottom flask with small quantity of acetonitrile and heated to 70 °C under continuous stirring. Acetonitrile was added with small portions until complete dissolution of gemini bromide. Then stirring and heating was stopped and solution was let to cool down to room temperature. Small white needle-like crystals of 12-2-12Br were formed within one hour. They were filtered, rinsed with acetone and dried under vacuum.

Absence of starting material and monoalkylated byproduct in final product was checked by NMR analysis.

Synthesis of gemini bromides with other spacer and chain length

The same synthesis procedure was used for gemini with other chain lengths such as 10, 14 and 16 as well as tetramethyldiamines with chain of 2, 3, 4 and 6 carbons.

Synthesis of 12-2-12Ac

12-2-12Ac was synthesized from 12-2-12Br by an ion exchange reaction with silver acetate.

In a typical synthesis, 2 g of 12-2-12Br (1 eq.) and 1.63 g of silver acetate (3 eq.) were mixed in a 250 mL round-bottom flask with 50 mL of MeOH. The flask was covered with aluminum foil and the reaction mixture was stirred at 50 °C for 24 hours. Then the suspension was filtered on celite to remove the formed precipitate of silver bromide and unreacted silver acetate. A transparent solution was obtained after filtration. MeOH was evaporated on Rotary evaporator; the product was then dissolved in a minimum of MeOH at 60 °C and precipitated by addition of ~100 mL of acetone. recombination is sensitive to their optical path difference (i.e. the product of refractive index and geometric path length). Adding an adjustable offset phase determining the interference at zero optical path difference in the sample, the contrast is proportional to the path length gradient along the shear direction, giving the appearance of a three-dimensional physical relief corresponding to the variation of optical density of the sample, emphasizing lines and edges though not providing a topographically accurate image.

For fluorescent microscopy analysis, a Leica DM5500B microscope was used.

Electron microscopy

400-mesh carbon-coated copper grids were used for Transmission Electron Microscope (TEM) observations. Organic samples were prepared by the negative staining technique. 5 µL of sample were drop casted on the carbon grid. After 5 minutes, the solvent was blotted with filter paper. 5 µL of 0.2 wt% uranyl acetate aqueous solution were deposited on the grid for the staining and removed with filter paper after 1 minute. Sample was allowed to dry on air.

TEM observations were performed using a Philips EM 120 electron microscope operating at 120 kV and the images were collected by 2k × 2k Gatan ssCCD camera.

Scanning electron microscope (SEM) observations were performed on SEM-FEG JEOL 6700F microscope operating at 5 kV.

NMR experiment with controlled cooling

Annex

In this chapter we described new systems based on guanosine 5'-monophosphate complexed with gemini amphiphiles which in the presence of some anions such as bromide, chloride and nitrate self-assembled into micrometric helical structures.

Previously from our group we reported formation of chiral micrometric helical aggregates from GMP complexed with monocationic amphiphile dimethylditetradecylammonium acetate -(C 12 ) 2 Ac [10] as well as with dimethylditetradecylammonium chloride -(C 12 ) 2 Cl [31].

As we showed above (see section 4 of this chapter) self-assembly of gemini nucleotide is significantly affected by presence of counter-ions. In the presence of acetate, only flat needle-like ribbons were observed, whereas chloride and nitrate induced helices formation. Individualized and more defined helices were obtained in a reproducible manner when gemini amphiphile with bromide anion was complexed with GMP-Na 2 .

In this annex, we show the self-assembly of (C 12 ) 2 Br + GMP-Na 2 and compare the two systems: gemini and monocationic nucleoamphiphiles. 100 µL of 40 mM (C 12 ) 2 Br solution were mixed with 100 µL of 40 mM GMP-Na 2 . The pH of the system was adjusted to 5.9 (best value found for 12-2-12Br/GMP system) with 1M acetic acid. Sample was heated at 70°C for 5 minutes and then stored at 20°C for aging.

Kinetics of self-assembly was followed by optical microscope observation (Figure 2.41). As we showed in section 3.1.1 pH of the sample has a significant influence on the aggregation of the self-assembled fibers. In order to differentiate effect of pH and type of anion samples with (C 12 ) 2 Ac and GMP-Na 2 were prepared under the same conditions as one with (C 12 ) 2 Br.

After 1 day of aging twisted fibers and helical ribbons with high level of aggregation were observed (Figure 2.43). Such a result suggests that not only the pH but also the nature of the counter-ion has an impact on the interaction between the self-assembled structures. Introduction part of this Chapter will highlight basics of sol-gel process of silica, methods for its chemical modification and show main approaches to the fabrication of helical silica structures.

Among which are the systems demonstrated by our group 5 years ago based on nanometrical organic self-assembled helices. It was shown how nanometrical helices with controllable morphologies and pitches can be transcribed to inorganic structures. The next part of the Chapter will be devoted to the optimization of parameters such as kinetics (speeding up) and quantity (scaling up) for the silica nanohelices fabrication process. In the last section we will discuss the silica transcription of organic micrometric helices described in Chapter 2.

Chemistry of silica

The most widely used silica-gel precursors are alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). According to the Brinker and Scherer [11] the sol-gel process of silicon alkoxides is described by the three following reactions:

(

(

where R is alkyl group C x H 2x+1 .

Process starts from hydrolysis of alkoxide group of the precursor (Eq. 1 and Figure 3.1) with formation of silanol group and corresponding alcohol as a by-product. Steric effects such as branching and increasing of the chain length lower the hydrolysis rate:

Si(OMe) 4 > Si(OEt) 4 > Si(O n Pr ) 4 > Si(O i Pr) 4 > Si(O n Bu) 4 > Si(OHex) 4
Subsequent condensation reaction leads to formation of siloxane bonds between silanol and alkoxide group with an alcohol as a by-product (alcohol condensation -Eq. 2) or between two silanol groups with a water molecule as a by-product (water condensation -Eq. 3). Under most of the experimental conditions condensation reaction (Eq. 2 and Eq. 3) starts before hydrolysis is completed. However, conditions such as, pH, H 2 O/Si molar ratio (R), and catalyst can force completion of hydrolysis before condensation begins [12]. Properties of a particular sol-gel inorganic network are related to a number of factors that affect the rate of hydrolysis and condensation reactions, such as, pH, temperature and time of reaction, reagent concentrations, catalyst nature and concentration, H 2 O/Si molar ratio (R), aging temperature and time, and drying [15,16]. Hereon we will focus mainly on the role of the pH since other parameters can't be varied in a wide range due to specificity of the template used for the silica formation.

Hydrolysis occurs by the nucleophilic attack of the oxygen from water molecules on the silicon atom as evidenced by the reaction of isotopically labeled water with TEOS which produces only unlabeled alcohol both in acidic and basic media [15]: Hydrolysis is more rapid and complete when catalysts such as mineral acids or ammonia are employed. Acetic acid, KOH, amines, KF, HF, titanium alkoxides, and vanadium alkoxides and oxides also were reported to be used as a catalyst in sol-gel processing. The hydrolysis is catalyzed both by specific acid (hydronium ion) and specific base (hydroxyl ion) (Figure 3.3) [11]. It has been observed that the rate and extent of the hydrolysis reaction is most influenced by the strength and concentration of the acid-or base catalyst.

Aelion et al. found that under acidic conditions, all strong acids behave similarly, whereas weaker acids require longer reaction times to achieve the same extent of reaction. Hydrolysis was shown to be first-order in acid concentration [17].

Under basic conditions, the hydrolysis reaction was also found to be first-order in base concentration. However, as the TEOS concentration was increased the reaction deviated from a simple first-order to a more complicated second-order reaction. Compared to acidic conditions, base hydrolysis kinetics is more strongly affected by the nature of the solvent [17]. In our study we are focused on the sol-gel process performed in aqueous medium in absence of alcohol. At the high H 2 O/TEOS ratio polymerization reactions of silicon alkoxides are similar to the ones of aqueous silicates [11].

For the polymerizations below pH 2, the condensation rates are proportional to the [H + ] concentration. Because the solubility (see Figure 3.4) of silica is quite low below pH 2, formation and aggregation of primary silica particles occur together and ripening (i.e., growth of a network)

Chapter 3: Silicon oxide helical structures based on organic template 123 Referencespage 165 contributes little to growth after particles exceed 2 nm in diameter. Thus, developing gel networks are composed of exceedingly small primary particles [11].

It is generally agreed that between pH 2 and pH 6 condensation rates are proportional to [OH -] concentrations. Condensation preferentially occurs between more highly condensed species and those less highly condensed. This suggests that the rate of dimerization is low, however, once dimers form, they react preferentially with monomers to form trimers, which in turn react with monomers to form tetramers. Cyclization occurs because of the proximity of the chain ends and the substantial depletion of the monomer population. Further growth occurs by addition of lower molecular weight species to more highly condensed species and aggregation of the condensed species to form chains and networks. The solubility of silica in this pH range is again low and particle growth stops when the particles reach 2-4 nm in diameter. [12] Above pH 7, polymerization occurs in the same way as in the pH 2 to pH 6 range, i.e., condensation rates are proportional to [OH -] concentrations. However, in this pH range, condensed species are ionized and therefore, mutually repulsive. Growth occurs primarily through the addition of monomers to the more highly condensed particles rather than by particle aggregation. Due to the greater solubility of silica and the greater size dependence of solubility above pH 7, particles grow in size and decrease in number as highly soluble small particles dissolve and reprecipitate on larger, less soluble particles. Growth stops when the difference in solubility between the smallest and largest particles becomes indistinguishable. This process is referred to as Ostwald ripening. Particle size, is therefore, mainly temperature dependent, in that higher temperatures produce larger particles. Additionally, in this pH range, the growth rate depends upon the particle size distribution [11]. Water soluble silicates, sodium metasilicates in particular, are other types of precursors for silica-gel production. Kraut prepared monosilisic acid by dissolving sodium metasilicate hexahydrate in various acidic solutions at low temperature [19]. Weitz, Franck, and Schuchard demonstrated that when Na 2 SiO 3 was reacted with acetic acid it liberated Si(OH) 4 [20]. Initially, alkoxide groups hydrolyzed and corresponding silanols are formed. Monomers condense to oligomers via siloxane bond formation. The degree of hydrolysis and polymerization of the silanes is determined by the amount of water present in the reaction media as well as by the number and the nature of the organic substituents. As the next step oligomers form hydrogen bonds with the -OH groups of the substrate. Finally, during the drying or thermal treatment covalent bonds with the substrate are formed [22] with the concomitant loss of water. The formation of covalent bonds to the surface proceeds with a certain degree of reversibility. Bonds may form, break, and reform to relieve internal stress during the drying or heating. All the reactions mentioned above may occur simultaneously.

Silanes can modify surfaces under anhydrous conditions consistent with monolayer and vapor phase deposition requirements (Figure 3.6B). Extended reaction times (4-12 hours) at elevated temperatures (50°-120°C) are typical in this case.

-Hydrolytic (A) [22] and anhydrous (B) deposition of silanes

More in details we will discuss main aspects of silica functionalization on example of (3-Aminopropyl) triethoxysilane (APTES) which has the chemical structure shown below. This alkoxysilane molecule is widely used for creation of positive charge on silica surface, as an intermediate for addition of carboxylic acid functionality [23][24][25][26], covalent attachment of catalyst [27] and biologically active molecules [28][29][30].

Even though the schematic of surface silanization described above may appear pretty simple, the actual mechanism is extremely complicated and affected by the range of external factors.

APTES concentration, reaction time and temperature affect both the internal structure of aminosilane layer as well as the morphologies of aminosilane films (Figure 3.7) [31].

Deposition at elevated temperature (∼70 °C) leads to denser layer where Si-O-Si bonds between the molecules and the SiO 2 surface are more structured and ordered with only a limited number of remaining unreacted ethoxy groups. In contrast, post-annealing of the functionalized SiO 2 samples after room temperature reaction with APTES does not lead to the improvement in the film structure [32]. Stability tests of APS films in deionized water confirm the idea about higher deposition temperature leading to better organized film. Immersion of the saturated APS film prepared at room temperature in deionized water (pH∼6) results in the degradation of the films within 1 to 2 hours. Saturated APS films prepared in situ at 70 °C also exhibit the same trend in water, but the process is much slower. The areas of the Si-O-Si and SiO-C modes in IR spectra remain essentially unchanged for the first 10 min. Then, the area of all the modes decreases following an exponential decay. After 6 hours in deionized water, approximately 50% of the spectral SiO-X region area has been removed. In comparison, the APS film obtained at room temperature is almost completely removed from the Si surface after the same time (i.e., 6 h) [32].

High pH (pH = 11) also have to be shown to have positive impact on the grafting of aminosilanes [33]. Comparison of the aminopropylsilane (APS) films grown under non-anhydrous conditions and anhydrous conditions for identical reaction parameters showed that APS films grown under nonanhydrous conditions are thicker because of extensive vertical polymerization, i.e., formation of Si-O-Si bonds in the solution among neighboring molecules that are loosely bound to the surface [32].

Different techniques such as conductometric titration [25,33], XPS [23], 2,4,6trinitrobenzene sulfonic acid solution (TNBS) test [34], 4-cya-nopentanoic acid dithiobenzoate (CPDB) test [24], back titration and Rhodamine B isothiocyanate (RITC) test [35] as well as ninhydrin test [36] are used for the quantification of amino groups. Density of 4.5-5 molecules/nm 2 is reported for the well packed monolayer [37].

Synthesis of chiral silica structures

Usage of organic structures as a template for the sol-gel process is a powerful tool that allows obtaining of silicas with large variety of morphologies on the scale of nanometers. During the last decade, chiral architectures attracted much attention, due to the fact that they are potentially applied in asymmetric catalysis and optics. Here we do not put us a goal to overview all the existing literature devoted to the synthesis of chiral silica structures, but rather to highlight main approaches used in the field. Therefore several reviews can be recommended as an additional reading [10,[38][39][40].

Several groups investigate spontaneous helicity expression of mesoporous silica formed in presence of achiral surfactants.

In 2006 Tiehong Chen et al. reported the observation of hierarchically helical mesostructured silica nanofibers templated by an achiral cationic surfactant (CTAB) under acidic conditions [41].

The mesoporous silica was synthesized by a two-phase route at room temperature under quiescent conditions: surfactant and HCl were mixed in water and tetraethylsiloxane was dissolved in hexane.

Obtained nano-fibers possess two levels of helical structures (Figure 3.8): 1) the twisted hexagonal morphology with chiral mesochannels winding around the fiber axis, and 2) these twisted hexagonal nanofibers could further curve spirally to form higher-order helical morphology. As no chiral molecules are used in the synthesis, the hierarchically helical morphology of nanofibers could be explained by the different kinds of topological defects existing in the silicate liquid crystal seeds formed in a diffusion-controlled process, and these defects could initiate and direct the growth of particular forms of mesostructured silica. Yang et al. as well reported formation of helical silica structures in presence of achiral surfactant [42,43]. Helical mesoporous materials with hexagonal structures and tunable pitch size were synthesized under basic conditions by using Perfluorooctanoic acid (PFOA) and cetyltrimethylammonium bromide (CTAB) as co-templates with different PFOA/CTAB weight ratios (R). Inorganic structures were obtained by addition of TEOS and continuous stirring for 2 Referencespage 165 hours at 80°C. The templates were removed by calcination at 550 °C for 5h. Authors suggest a purely interfacial interaction mechanism to explain the formation of helical mesostructures. With this model, the reduction in surface free energy is the driving force for the spontaneous formation of the spiral morphology, while the increase of bending energy and the derivation from a perfect hexagonal mesostructure after the helical morphology is formed may limit the curvature of the helices.

The silica helices (Figure 3. Seung-Man Yang et al. used the flow-induced microstructure of the CTAB/NaSal wormlike micelles as a template for the preparation of the helical silica tubules of the mesopores [45]. They found that the wormlike micelles form well-defined helical structures under the coil-spring flow generated by Taylor instability in between of the two moving coaxial cylinders. This method is very promising, since a helical structure is easily formed by the coil-spring flow and the orientational conformation of the mesopores can be controlled by the flow intensity. In their study, tetraethylorthosilane (TEOS) in hydrochloric acid was used as a precursor of the silica substrate used to prepare the silica-micelle meso-phases via sol-gel chemistry. The helical structures were Another example of morphology control of the obtained silicas by changing the solvent composition was reported from the same group [48]. Compound L-18Val4PyBr (Figure 3.13) was dissolved in a mixture of aqueous NH 3 and ethanol. TEOS was used as a silica precursor. With increasing the ethanol/water volume ratio from 4∶6 to 8∶2, the morphological shapes of silica changed from coiled nanoflake to left-handed double twisted nanoribbon. Referencespage 165 Fabrication of silica structures with the controlled helicity and chirality is an attractive goal because of their potential application in enantiomeric purification, high surface area catalysis, and chemical sensing.

Shinkai's group reported a highly enantioselective synthesis of chiral pyrimidyl alkanol 2 using right-and left-handed helical silica, a chiral amorphous material [50]. Silica helices (Figure 3.1.2B, C) were synthesized by sol-gel transcription in chiral diaminocyclo-hexane-based organogel systems (Figure 3.15A) [51]. TEOS was used as a silica precursor according to the procedure reported previously for the templated preparation of hollow silica fibers and tubules [52,53]. To the best of our knowledge, this is the first example of the creation of both "rightand left-handed helical silica structures" by a sol-gel transcription method. Moreover, single-handed helical 1,4-phenylene-silica, ethenylene-silica and methylene-silica nanotubes were also prepared [54]. Silica nanotubes with spring-like pore channels in the walls were prepared by using the selfassemblies of L-16Val6PyBr (variation of a molecule Figure 3.13A with a shorter alkyl chain) as templates through a single-templating approach [55] by the same group. Nanoworms with concentric circular pore channels and nanoflakes with vertical pore channels were prepared by changing the concentration of the catalyst.

Recently Yonggang Yang et al. reported silica twisted ribbons with controlled handedness [56]. They used two enantiomers of the chiral compound shown in the Figure 3.17 which are able to gel in water and DMSO at 25°C forming respectively left and right handed twisted ribbons. Their diameters were about 20-100 nm. Those ribbons were used as a template for silica transcription with APTMS and tetraethyl orthosilicate (TEOS) as silica precursors in water. The handedness of the mesoporous silicas was controlled by the chirality of gelators. Takashi Tatsumi showed a templating route for preparing well-ordered mesoporous silicas with the chiral channels based on the self-assembly of chiral anionic surfactants and inorganic precursors by using aminosilane or quaternized aminosilane as a co-structure-directing agent [58].

Formation of a chiral mesostructure has been achieved by using N-miristoyl-L-alanine sodium salt (C14-L-AlaS) partially neutralized with HCl, and N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (the cation denoted here as TMAPS) or 3-aminopropyltrimethoxysilane (APTMS). The presence of C14-L-AlaA induces changes in the conformation of the amphiphile in the micelle and distorts the micellar shape, leading to chiral micelles. The positively charged ammonium site of TMAPS or APTMS interacts electrostatically with the negatively charged head

showing the microscopic features of this sample (A). Schematic drawing of a structural model of the chiral mesoporous material for TEM image simulation (B). Cross-section (C). One of the chiral channels in the material (D) [58].

Later on chiral mesoporous silicas with helical particles were successfully synthesized in the same group by using the chiral anionic surfactants N-miristoyl-D-alanine as well as N-miristoyl-Lalanine [59]. The left-/right-handed ratios for the helical particles synthesized with C14-L-AlaA and C14-D-AlaA proved to be 75/25 and 25/75, respectively, suggesting that the predominant handedness of helices was directed by the handedness of the surfactants.

Reiko Oda et al. demonstrated the fine-tuning of the shape, size, and morphology transitions of twisted and helical membranes formed by non-chiral dicationic n-2-n gemini amphiphiles complexed with chiral tartrate anions [60]. Such systems express the chirality of the tartrate components at a supramolecular level resulting in the right-handed (L-Tartrate) or left-handed (D-Tartrate) nanostructures formation. Diverse chiral nanometric ribbons and tubules formed by selfassembly of organic amphiphilic molecules were transcribed to inorganic nanostructures using a novel sol-gel transcription protocol with Tetraethyl orthosilicate (TEOS) in the absence of catalyst or cosolvent [61]. By controlling parameters such as temperature or the concentration of the different reactants, it was possible to finely tune the morphology of the inorganic nanostructures In all the examples described above chirality of the silica resulted from the chirality of the template. Chirality of the template, in turn, was induced by the chiral self-assembly, spontaneous symmetry braking or mechanical stress. Different approach was proposed by Michel Wong Chi Man et al. [62][63][64]. Instead of using the chiral template with achiral precursor they incorporate the chiral properties directly into the structure of the precursor. Series of the silylated diureido derivatives of trans(1,2)-diaminocyclohexane (Figure 3.21A) with the different length of the alkylene chain spacer between the urea groups and the silicon atoms was synthesized. These compounds were hydrolyzed in aqueous acidic conditions in order to examine the role of the carbon chain spacer on the resulting hybrid bridged silsesquioxanes. In the case of shorter alkylene chain lengths (n =3 and 4), the chiral property of the main bridging diaminocyclohexyl group prevails resulting in the chiral selforganized hybrid materials (Figure 3.21B and C). These results show the first example of selforganized chiral hybrid material. Onward we will discuss optimization of some reaction conditions of this process.

-Scheme of the synthesis of the molecular precursors and the corresponding hybrids (A). SEM images of (B) HS4(R,R) and (C) HS4(S,S). Adapted from [62].

A C B

Transcription in presence of Tartaric acid

It was previously shown by our group, that morphology of silica fibers obtained by the transcription of 16-2-16 Tartrate gels is strongly dependent on the aging time of organic gel [61]. Unwinding of the organic fibers during the transcription procedure may be explained by replacing of chiral tartrate, which induces supramolecular chirality in the system, with achiral silicate anion. If silica condensation occurs faster than the significant ion exchange the original morphology of organic fibers may be preserved (Figure 3.23A), otherwise unwound silica structures will be obtained (Figure 3.23B). Rate of silica condensation depends on the pH (Figure 3.4), temperature and concentration of hydrolyzed TEOS species that as well depends on the pH of the system (Figure 3.3).

Because of the ionic nature of the gelating molecule and the fact that supramolecular chirality of the system is induced by the anion it is impossible to use strong mineral acids to control the pH. An organic acid type would be appropriate and tartaric acid would be the most appropriate since, its anionic form is equivalent to the one of 16-2-16 Tartrate gel can be utilized without the risk to disturb the chiral morphology of organic fibers.

According to the procedure of silica transcription [61] pre-hydrolysis of TEOS is performed in pure MilliQ water, with a pH close to 6. By performing hydrolysis in a 0.1 mM tartaric acid with a pH of 3.8 the rate of hydrolysis may be increased up to two orders of magnitude compared to the conventional procedure (Figure 3.3). At the same time lower pH decreases self-condensation rate and giving rise to a greater amount of reactive silica precursor in the system at the moment of the introduction of organic template.

In order to compare the effect of tartaric acid addition to the TEOS emulsion at the prehydrolysis stage with the effect of tartaric acid presented in a sample at the stage of silica condensation two sets of experiments were performed. In first experiment, 5 wt. % TEOS emulsion (0.5 mL) was prepared in MilliQ water by sonication for 5 minutes and aged for 15 hours at 20 °C for the pre-hydrolysis. The pH of these samples was 6.2. For the second set of experiments, 5 wt. % TEOS emulsion was prepared in 0.1 mM tartaric acid solution with the pH of 3.8 (same enantiomer as for 16-2-16 Tartrate was used) and aged for 7 or 15 hours prior to the mixing with the organic gel.

Gels of 16-2-16 Tartrate were prepared in MiliQ water (0.0 mM tartaric acid) as well as in 0.1, 0.5 and 1 mM tartaric acid and aged at 20°C for 1 day. The pH of 1 mM 16-2-16 Tartrate gels prepared in MilliQ water was typically in the range of 4 to 5. Helical ribbons were observed in all the samples after 1 day of aging. Pre-hydrolyzed TEOS was sonicated for 5 minutes and mixed with 0.5 mL of organic gel by sonication for another 5 minutes. Reaction mixture was kept at 20°C for 15 hours for the transcription. Finally samples were washed with ethanol by the repetitive centrifugation and redispersion by sonication in the fresh portion of the solvent. Comparing two sets of the experiments, we can see that the most drastic difference is observed for the organic gels prepared without tartaric acid (Figure 3.24A, E). In this case the conventional transcription procedure (TEOS pre-hydrolyzed in 0.0 mM tartaric acid) resulted in the twisted ribbons without any helices (Figure 3.24A). Samples prepared with the TEOS pre-hydrolyzed at lower pH (in 0.1 mM tartaric acid) mostly showed silica helices with a small number of twisted ribbons (Figure 3.24E). Similar results were obtained for the organic gels prepared in the presence of excess of tartaric acid. Only difference was higher number of helices observed in the samples prepared with the TEOS pre-hydrolyzed in absence of tartaric acid (Figure 3.24F-H vs. Figure 3.24B-E); probably due to faster hydrolysis of precursor after the mixing with the template. More homogeneous morphology of the inorganic samples prepared with the TEOS pre-hydrolyzed at lower pH (Figure 3.24F-H) may be explained in a way that higher amount of hydrolyzed (and so soluble in water) precursor at the moment of mixing with the organic gel allows obtaining of more homogeneous transcription conditions. In the mixture of 16-2-16 Tartrate gel prepared in 1 mM tartaric acid and TEOS hydrolyzed in the presence of 0.1 mM tartaric acid (Figure 3.24H), condensation reaction was significantly slowed down due to the low pH (3.0 -3.2) (Figure 3.4) which resulted in a thin, unstable silica layer and so deformed fibers morphology.

A fine tuning of the pH during the TEOS hydrolysis allows decreasing of the organic gel aging time required for the fabrication of silica helices from several weeks to several days.

Scaling up

Transcription procedure established by our group [61] is optimized for the volume of organic gel of 0.5 mL and typically results in 1 mg of silica helices. Since significantly larger amounts of silica nanofibers are required for further chracterizations and applications, we investigated how it is possible to scale up the transcription procedure.

System under investigation is presented by three immiscible phases: water, partially hydrolyzed TEOS and organic fibers of 16-2-16 Tartrate. Once hydrolyzed, TEOS becomes soluble in water and may diffuse to the surface of helices where it undergoes polycondensation and creation of silica layer. The ratio between the time needed for the diffusion of hydrolyzed TEOS and the rate of condensation determines the concentration of the silica precursor near the interface of organic helices. As it was discussed in the previous section, the rate of silica layer formation which, among the other factors, is determined by the concentration of precursor and is a main factor that affects the morphology of the transcribed fibers. So, it can be concluded that a short time of the precursor diffusion to the surface of the organic helices is prerequisite for the successful transcription.

The density of TEOS is 0.933 g/mL which makes it to flow and create a layer on top of the water phase. Since diffusion coefficient is constant under the experimental conditions, time of the diffusion of hydrolyzed TEOS to the bottom of the sample increases with the increasing of the height of container used for the transcription.

Mixing of the sample by stirring or shaking may overcome this limitation. However, as it was shown by the experiments both stirring and shaking lead to the aggregation of organic and, as a result, of silica fibers.

Keeping the container height as small as possible appears to be the best option for the scaling up without affecting the morphology of SiO 2 fibers. As a test, we performed the transcription in a 50 Different amounts of 16-2-16 Tartrate organic gels (aged for 1-3 days): 1 mL, 2.5 mL, 5 mL, 10 mL and 25 mL were mixed with pre-hydrolyzed TEOS (5 wt. % in 0.1 mM tartaric acid; 7 hours of pre-hydrolysis) in 1:1 v/v ratio in the 50 mL centrifugation tube by the sonication for 5 minutes.

Pre-hydrolyzed TEOS was sonicated for 5 minutes prior the mixing in order to achieve its best Results obtained in this section show the possibility of helix to helix silica transcription with a quantity of up to 50 mg of silica per sample.

Fragmentation and dispersion of the silica nanohelices

The template structure used for silica transcription consists of dense and entangled network of organic helices. This leads to a high level of entanglement and aggregation of silica fibers which becomes a problem for practical application of these nanostructures for which individualized objects are typically required.

Level of bridging between individual helices is relatively low under the transcription conditions and stability of aggregates is provided by physical entanglement of helices. Short nanostructures have lower chances to be mechanically aggregated and so are more suitable for the applications.

Ultrasonication is widely used for preparing dispersions of the high aspect ratio structures, such as Single-Walled Carbon Nanotube (SWNT) in different solvent media. It has been shown to play a critical role in dispersing and debundling SWNTs [66,67]. At the same time sonication induces the scission of the tubes that are near imploding cavitation bubbles. Nanotube scission arises from the fluid friction at the surface of the nanotubes in the radial elongational flow field that forms around a cavitation bubble [66]. The efficiency of the cavitation process is closely correlated to many solvent parameters, including vapor pressure, viscosity, and surface tension, as well as the sonication frequency, intensity, and time [67].

High intensity ultrasonic processor "Vibra cell 75186" equipped with 6 mm microtip and operating at 130 W (maximum power), 20 kHz in the pulse mode was used for the fragmentation of silica nanohelices.

Sonication was performed in the pulse regime: 1 second sonication pulse and 1 second pause. The helices sonicated in water show poorly conserved helical morphologies. Similar but less pronounced effect may be observed for the ribbons sonicated in EtOH. In trifluoroethanol (TFE), on the other hand, after the sonication helices become shorter but their overall morphology remains unchanged. This can be explained by the electron-negativity of fluorines which reduces the negative charge at the oxygen at the silica surfaces and decreases its reactivity.

It was also observed by Lucas et al. that the scission rate of Single Wall Carbon Nanotubes

(SWNT) is governed by the acoustic energy supplied by collapsing bubble [66]. According to the theory of ultrasonic processes, the cavitation effect is strongly dependent on solvent parameters, notably the solvent vapor pressure, viscosity, and surface tension. Initiation of the cavitation process requires more energy in viscous solvents and therefore more energy is released upon collapse. These conclusions are in good agreement with the results of Cheng et al. [67] reported ultrasound assisted dispersion of SWNT. They showed that physical parameters of the solvent, including vapor pressure, viscosity, surface tension, density, and molecular weight, rather than solubility parameters, govern the dispersion process. Helices were successfully cut and dispersed in mixed solvent without surface damage at 65W output power (Figure 3.28D-F). 26W (20% max power) appears not to be enough for fragmentation

(Figure 3.28B).
The main drawback of the usage of trifluoroethanol is its high toxicity. 2-propanol (isopropyl alcohol) has properties close to those of TFE (Table 3.1) whereas it is significantly less toxic, so it was decided to use 2-propanol instead of TFE. In order to check the effect of the volume of the sample on the cutting procedure we increased its volume to 10 mL (50 mL sample tube was used). From the results of ultrasonication at 65 W during 15 minutes (1 second pulse, 1 second pause; total time -30 minutes) in ethanol/isopropanol (1:1 v/v) it can be seen that most of the helices remain not fragmented and big aggregates of the fibers are still present in the sample (Figure 3

Micrometric helices

In the previous chapter we discussed the formation of micrometric helices by the selfassembly of cationic gemini surfactant in presence of bromide and Guanosine monophosphate anions. Sensitivity of this organic template to the pH, temperature, presence of anions, changing of concentration or solvent composition limiting number of its possible applications. Protecting the organic fiber with inorganic shell or fabrication of inorganic replica of organic helical structures allows overcome limitations mentioned above. However, restricted range of conditions compatible with the particular morphology of organic template makes inorganic transcription of micrometric helices formed by 12-2-12GMP/Br not trivial and challenging task.

Transcription with TEOS as silica precursor

As it was discussed in the Introduction section alkoxysilanes, such as tetramethyl oxysilicate (TMOS) and tetraethyl orthosilicate (TEOS) are the most commonly used silica precursors. Due to their immiscibility with water alcoholic solutions are typically prepared. Helices of 12-2-12GMP/Br are relatively stable in water/ethanol mixture. However when an alcoholic solution (methanol, ethanol and 2-propanol are tested) of TEOS is added (1:1 v/v) a fast degradation of organic helices is observed. The process speeds up with the increasing of silica precursor concentration (Figure 3.32). Similar behavior was observed for alcoholic solutions of TMOS. If the water emulsion of TEOS was used, changes in the morphology of organic fibers were slower compared to the alcoholic solution. 0.5 mL of organic gel were mixed with 0.5 mL of TEOS Two methods can be used for the removal of organic template and testing of silica transcription efficiency. First one is the washing with the hot water. 12-2-12GMP/Br fibers have very low solubility in a wide range of the solvents, but readily solubilize in hot water (~70°C).

Another method implies the addition of HBr (100 mM, 1:1 v/v) to the 12-2-12GMP/Br fibers that leads to the replacement of Guanosine 5'-monophosphate in gemini amphiphile with bromide and formation of soluble 12-2-12Br and GMP. Other strong acids like HCl can be used instead of HBr, however usage of HBr is preferable since smaller amount of it is needed for the complete dissolution of organic fibers. suggest that rate of TEOS condensation is not high enough to preserve original morphology of the organic fibers.

According to Iler [18] (Figure 3.4) silica condensation may be accelerated by increasing the pH. However phosphate groups of Guanosine monophosphate has pKa of 6.1 and increasing of the pH beyond this value leads to the destruction of helical organisation of organic fibers (see Chapter 2). At the pH 8 (adjusted with NaOH), corresponding to the maximum rate of silica condensation,

Na 2 SiO 3 /acetic acid mixture as silica precursor

Another type of silica precursor is water soluble silicates, sodium metasilicate in particular.

We chose this precursor for its well-known high reactivity. Being acidified by acids, even weak ones like acetic acid, sodium metasilicate produces silicic acid (see Introduction section).

For the silica transcription of organic micrometric helices we acidified sodium metasilicate aqueous solution with acetic acid down to pH 5.9-6. Acidity was chosen to be identical to that of the organic template since its morphology is pH sensitive as we showed in previous chapter. 100 µL of precursor were added in one shot to 100 µL of organic helices without any stirring or sonication and the mixture was stored at 20°C for the silica condensation. After the end of transcription samples were washed with water and HBr (100 mM, 1:1 v/v) was added for the template dissolution. We observed complete gelation of the sample with Na 2 SiO 3 concentration of 200mM after 15 hours. It shows that shorter reaction times are needed for the silica gel formation from sodium metasilicate due to it solubility and high reactivity compared to TEOS. Fast condensation kinetics of precursor is a crucial factor to guarantee the formation of the silica replica without notable morphological differences from the organic template.

Well defined helices were obtained after 1 hour for concentrations of precursor higher than 100 mM (Figure 3.34). At 50 mM only part of the fibers were mineralized and after the washing most of the sample was presented in a form of unstructured precipitate. Extended reaction time in Referencespage 165 this case didn't improve the transcription indicating that an amount of precursor of at least 100 mM should be used to achieve the complete covering of the organic template.

One hour was found to be the optimal time for the silica deposition. Reduced reaction time (30 minutes) led to a thin silica layer and fragile inorganic structures which were easily to break by sonication. For the longer reaction times silica condensation in a bulk was observed.

In order to study the effect of the silicate to acetic acid ratio, we adjusted it to 1:2 (pH = 6),

Conclusion

In this Chapter we described the silica helices formation.

Basics of the silica sol-gel process were discussed in the introduction. It follows by the literature overview of the silica surface modification; stress was made on the amine functionalization. We finish introduction section with the bibliographic study of the main approaches to the fabrication of chiral silica structures.

The kinetics of silica precursor is the main factor affecting the morphology of the nanometric silica helices based on 16-2-16 Tartrate gel used as a template. According to the silica transcription protocol reported by our group three weeks of organic gel aging required for the silica helices formation. By tuning the hydrolysis rate of TEOS via the addition of tartaric acid to the mixture we succeeded in reducing to only one day the synthesis of the silica helices from organic gels.

We obtain shorter and individualized helices by ultrasonication. Properties of the solvent such as density, viscosity and surface tension play an important role in the cutting and dispersion efficiency. Sonication parameterstime, power and volume of the sample also have significant impact on the cutting and dispersion of the silica helices.

Micrometric organic helical fiber formed by self-assembly of gemini nucleoamphiphile in water were then used as a template for the silica transcription. The reactivity of the precursors appears to be a crucial factor determining the morphology of the inorganic structures. TEOS as the most common silica precursor for sol-gel process was found to be inappropriate in this case due to its slow condensation kinetics under the experimental conditions. Sodium metasilicate was chosen instead due to its solubility and high reactivity.

Reaction conditions such as precursor concentration, pH and reaction time must be finely tuned for the best transcription conditions. Micrometric silica helices were successfully prepared at 20°C by using 200 mM Na 2 SiO 3 solution acidified by acetic acid. Reaction time was 1 hour.

Obtained inorganic fibers were studied by optical, scanning and transmission electron microscopy.

We found that micrometric helices are formed by double fibers with the diameter of around 500 nm twisting in parallel. These fibers themselves consist of smaller fibers with diameters of 50-70nm.

.

Introduction

This chapter will be devoted to the fabrication of functional nanohelices for the NanoElectroMechanical System (NEMS) oriented towards sensing and actuating applications. In the previous Chapter we have discussed the preparation of silica based chiral nanostructures through the sol-gel transcription of the organic nanohelices. However these silica nanostructures have limited utility in terms of electronic properties. Metal oxides such as ZnO and TiO 2 on other hand are particularly interesting as materials for NEMS as they can exhibit very high piezoresistive or piezoelectric coefficients when patterned at submicrometer scale dimensions [1][2][3][4].

In the first part of the introduction we will make an overview of the literature devoted to the synthesis of Titanium dioxide nanostructures. It will be followed by Zinc oxide and Zinc oxide based composite nanostructures.

In the main part of this chapter we will show our approach to the fabrication of core-shell SiO 2 @TiO 2 and SiO 2 @ZnO nanohelices where SiO 2 is the core of the system and TiO 2 or ZnO its shell. Sol-gel method, layer-by-layer deposition as well as Atomic Layer Deposition techniques will be described.

Study on core-shell SiO 2 @TiO 2 and SiO 2 @ZnO structures was performed in collaboration with Dr. Satyabrata Si from ICMCB, Pessac, France. Great efforts have been made in the past decades in the research of TiO 2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and sensors. [5] The performance of TiO 2 -based devices is largely influenced by the sizes of the TiO 2 building units, apparently at the nanometer scale [5]. Closely packed first-order particles are yielded via a three-dimensionally developed gel skeleton [5].

Due to a much slower hydrolysis rate than for conventional precursors such as titanium tetrabutoxide, titanium diisopropoxide bis(acetylacetonate) (Figure 4.1B) is used as a precursor for the geometry-controlled synthesis of uniform TiO 2 nanostructures [6]. Thus titanium complexes like Titanium(IV) bis(ammonium lactato)dihydroxide (Figure 4.1C) meet the requirements of prospective raw materials for synthesis of titanium-containing ceramics in aqueous solution via conventional processes [7][8][9] as well as by protein-and peptide-directed syntheses [10][11][12][13].

Direct organic templating

Hanabusa et al. showed formation of tubes [14] and helical structures [15] Toshimi Shimizu with his colleague Qingmin Jia have used peptidic lipid (Figure 4.3A) for the formation of nanotubes with average outer diameter of 80 nm in MilliQ water [16]. These nanotubes were used as a template for the TiO 2 replication through an unconventional procedure.

The aqueous suspension of nanotubes was frozen in liquid nitrogen. The ethanol solution of Ti(OPr) 4 was then added to the iced lipid nanotubes at -20 °C and the whole reaction system was then kept at 0 °C for 2 weeks. Finally, the sample turned to a gel state and was lyophilized. The Nanocomposites with high electron mobility may be used to efficiently collect photogenerated electrons and so improve the performances of photovoltaic devices. Angela M. Belcher et al. reported the synthesis of single-walled carbon nanotube@TiO 2 nanocrystal core-shell nanocomposites using a genetically engineered M13 virus as a template [17]. In the first step virus/SWNT complexes were fabricated through non-covalent interactions. An aqueous solution of each different virus/SWNT complex, pre-cooled at 4 °C, was poured into the ethanol solution of titanium n-butoxide (pre-cooled at -20 °C) under vigorously stirring. The final solution typically comprised 25% water and 75% ethanol. After 1 h of stirring, the precipitates were washed with ethanol and water, and then dried under vacuum. The yield of biomineralized TiO 2 was higher than 90%. By using the nanocomposites as photoanodes in dye-sensitized solar cells, authors have

(A) (C) B References -page 227
demonstrated that small fractions of nanotubes improve the power conversion efficiency up to 10.6% by increasing the electron collection efficiency.

Recently Raffaele Mezzenga reported β-lactoglobulin amyloid protein fibrils directed synthesis of titanium dioxide hybrid nanowires [18]. β-lactoglobulin is a globular milk protein which forms semiflexible long linear amyloid fibril structures by a heat treatment at high temperature and in low pH conditions (Figure 4.4A). These fibrils are polydisperse in size with an average contour length of the order of several μm and a diameter less than 8 nm.

Layer-by-layer (LbL) deposition

The layer-by-layer (LbL) deposition procedure includes repetitive deposition of positively charged polyelectrolyte and negatively charged water-soluble titania precursor (typicallytitanium(IV) bis(ammonium lactato) dihydroxide (TiBALDH)) layers followed by washing out of the unattached polyelectrolyte and precursor. An advantage of the LbL colloid-templating approach over other methods is the nanoscale control that can be exerted over the wall thickness by varying the number of deposition cycles. Finally samples are calcined in order to obtain crystalline TiO 2 .

Frank Caruso et al. reported generation of monodisperse hollow titania spheres with defined diameter and wall thickness by LbL colloid-templating method [20,21]. Fabrication procedure involved the construction of polyelectrolyte/ TiBALDH multilayers on colloid particles. Aqueous solutions of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrenesulfonate) (PSS) were used as polyelectrolytes. Calcination of the TALH/ PDADMAC-coated particles resulted in the formation of hollow titania spheres (Figure 4.6B-C). Layer-by-layer film growth of TiBALDH and various positively charged polyelectrolytes was studied in details by UV-vis spectrophotometry, ellipsometry, and by a quartz crystal microbalance (QCM) [22]. Kiyofumi Katagiri et al. reported the core@shell particles formation by the alternate

deposition of titanium-glycolate complex ((NH 4 ) 6 [Ti 4 (C 2 H 2 O 3 ) 4 -(C 2 H 3 O 3 ) 2 (O 2 ) 4 O 2 ]
) and positively charged polyelectrolyte, poly(allylamine hydrochloride) on spherical polystyrene (PS) templates [23]. Hollow spheres were obtained by removal of the PS templates ( Fang et al. demonstrated a simple protein-mediated approach for preparing freestanding (silica free) microscale titania structures with morphologies inherited from complex-shaped biosilica templates (diatom frustules) [25]. The silica diatom frustules were exposed in a repetitive alternating fashion to a silica-binding, titania-forming protein (protamine) and then to an aqueous titania precursor to build up a conformal titania-bearing coating (Figure 4.8A). After organic pyrolysis at 500°C, the conformal, continuous nature of the resulting crystalline anatase titania coating was obtained. Selectively removing the silica template yields freestanding titania structures that retained the 3-D diatom frustule shape (Figure 4.8). Titania-coated frustule was demonstrated to be acting as a sensitive electrochemical hydrogen detector.

SiO 2 /TiO 2 composites

The difficulty of obtaining very small SiO 2 @TiO 2 core-shell nanoparticles is probably due to the fact that hydrolysis and condensation of the TiO 2 precursorsusually titanium tetraalkoxides Ti(OR) 4is very fast and difficult to control. A possibility consists in preparing core-shell colloids by hydrolysis and condensation of Ti(OR) 4 around seed nanoparticles in an ethanolic solution with a small amount of water [26]. This method was used by H. Tang et al. to prepare the outermost coating with catalytically active Ti-O-Si sites on the SiO 2 @FePt particles [27]. SiO 2 @FePt in ethanol, water, titanium nbutoxide (TBOT), and ethanol were mixed with stirring for 48 h. The Fe 3 O 4 /SiO 2 /TiO 2 composite microspheres were synthesized by X. Huang et al. through a drop-wise addition of water/ethanol mixture (1:5 v/v) into the suspension of Fe 3 O 4 @SiO 2 microspheres in an ethanolic TBOT solution [28]. Suspension was stirred for 2h at room temperature. After being oven-dried, the powders obtained were calcined at 450 °C for 2 h to obtain anatase TiO 2 .

SiO 2 @TiO 2 core-shell spheres were synthesized by H.L. Meng et al. via addition of tetrabutylorthotitanate (TBOT) in ethanol and aqueous ammonia as a catalyst to ethanolic SiO 2 @TiO 2 core-shell spheres were annealed at 550°C for 1 h in air to crystallize TiO 2 [29].

Mechanism of TiO 2 deposition in presence of ammonia is described in a work by Fang-Qiong Tang et al. who reported a mixed-solvent method to coat polystyrene (PS) spheres with shells of amorphous titania by ammonia catalysis [30]. The thickness of titania shells was controlled in the range of 8-65 nm by varying the concentration of titanium tetrabutoxide (TBOT) in the ethanol/acetonitrile mixed solvent. After the calcination of core-shell particles for 2 h at 500 °C, spherical hollow titania shells with quite smooth and homogeneous surfaces were obtained ( Abhaya K. Datye et al. showed the possibility of TiO 2 deposition on the silicon oxide substrate from both TBOT under the reflux conditions and water soluble titanium complex (titanium (IV) bis (ammonium lactato) dihydroxide -TALH) via its reaction with urea at 100 °C in water [33].

It was shown that if the precursor is allowed to hydrolyze and condense only to the extent of forming oligomeric clusters which are then allowed to nucleate on the surface of the support, smooth coatings will be formed. If the precursor is allowed to hydrolyze and condense till colloidal for the 24 hours in water [34].

Nitrogen-doped SiO 2 @TiO 2 core@shell nanoparticles with uniform diameter were fabricated by C. Kim et al. using SiO 2 template sol-gel method [35]. A colloidal solution containing SiO 2 nanoparticles was prepared via Stöber method [36]. Then, titanium (IV) isopropoxide (TTIP), ethanol, and various volume of triethylamine were mixed and added to the colloidal SiO 2 solution pre-cooled to 5°C. TTIP selectively reacts on the surface of SiO 2 due to ammonium ions used in Stöber method on their surface. Low temperature synthesis provides uniform TiO 2 shell formation with controlling sol-gel reaction of TTIP. The mixture was vigorously stirred for 6 h. The final product was obtained after calcining dried white powder at various temperatures from 150 to 550°C.

Ahmet Faik Demirörs et al. described a general one-pot method for coating colloidal particles with amorphous titania [37]. Various colloidal particles such as silica particles, large silver colloids, gibbsite platelets, and polystyrene spheres were successfully coated with a titania shell by the following procedure. Aqueous solution of the nonionic surfactant Lutensol ON50 was dissolved in a dispersion of core particles in ethanol. The solution of TBOT in ethanol was then added to the core particles dispersion, mixture was sonicated for 20 min and kept for 2 h to allow the reaction to go to completion. The absolute ethanol used in this work was cooled to 7°C prior to use. This slows down the condensation of the titania precursor. Anatase titania shell was obtained after HF etching of Silica@Titania particles calcined at 500°C for 0.5 h.

SiO 2 @TiO 2 composite microspheres with microporous SiO 2 core/mesoporous TiO 2 shell structures were prepared by L. Zhao et al. through the hydrolysis of titanium tetrabutoxide (TBOT) in the presence of microporous silica microspheres using hydroxypropyl cellulose (HPC) as a surface esterification agent and porous template [38]. The HPC concentration was shown to be the crucial parameter for a uniform mesoporous titania shell formation (Figure 4.11). Recently a similar approach was used by J. Wu et al. [39]. Sébastien Abramson et al., who studied the TiO 2 layer condensation onto the Fe 2 O 3 @SiO 2 nanoparticles, show that the thickness of the external TiO 2 layer, with a molar ratio of 1:1 (thiol:alkene) were added to the ethanol solution of vinyl-functionalized silica microspheres. After bubbling with N 2 at 45°C for 30 min, the solution was heated at 80°C for additional 24 h for complete reaction. Afterward, the SiO 2 @TiO 2 core@shell microspheres were prepared by hydrolysis of TBOT via sol-gel process in mixed ethanol/acetonitrile (3/1 v/v) solvent, in which the activity of TBOT could be easily controlled. Reaction was performed at room temperature with ammonia solution as catalyst. The carboxyl groups on the surface of silica particles promote the formation of a dense and smooth titania layer thickness of which could be tuned from 12 to 100 nm. After calcination at 500 °C for 2 h, the amorphous TiO 2 layer turned into anatase titania.

A B D C

ZnO nanostructures

Currently there are several methods available for synthesizing ZnO nanostructures. For example, Zn 1-x Fe x O nanotubes [41] and yttrium doped ZnO nanowires [42] were fabricated via electrochemical deposition. ZnO nanocandles and ZnO nanomultitips have been synthesized by chemical vapor deposition (CVD) method on a silicon substrate [43]. ZnO nanohelices with diameters between 300 to 700 nm and lengths up to 100 µm were grown by a vapor-solid process [44,45]. Arrays of aligned ZnO nanorods were fabricated on flat silicon substrates covered with a thin ZnO film by decomposing zinc acetate at ∼ 250 °C in air [46]. ZnO microtubes were synthesized by microwave assisted [47] and NH 4 + directed [48] methods in aqueous medium.

Core-shell arrays were prepared by Vinayak S. Kale et al. through thin, controlled, highly crystalline and conformal coating of zinc oxide as shell using the plasma assisted-atomic layer deposition (PA-ALD) route on vertically aligned silicon nanowire arrays core [49]. ZnO is deposited by using easily pyrolisable diethyl zinc (DEZn) as a Zn precursor and oxygen radicals (O • ). 10 nm ZnO shell thickness is achieved by 100 cycles of PA-ALD. Deposition process is shown in Figure In this overview we will focus more into the details on the sol-gel route of ZnO synthesis as the most suitable one for the fabrication of core-shell composite nanostructures.

Zinc acetate dihydrate is one of the most suitable molecular precursors of ZnO due to its low cost, ease of use and commercial availability. As a matter of fact, the acetate group, as contaminant of the gel, is easily removable during the post-treatment of annealing, and, at the same time, it acts as a stabilizer of the colloidal sol. Finally, the hydration water molecules of Zinc acetate dihydrate allow hydrolyzing Zn 2+ ions without further addition of water [50].

Hongxia Li et al. used spin coating technique for ZnO deposition on glass substrates via the sol-gel method [51]. Zinc acetate dihydrate, 2-methoxyethanol and monoethanolamine (MEA) were used as starting material, solvent and stabilizer. All components were stirred at 60°C for 2 hours to yield a clear and homogeneous coating solution. The molar ratio of MEA to zinc acetate was maintained at 1.0 and the concentration of zinc acetate was 0.75 M. After spin coating, the films were dried at different temperatures for 10 min to evaporate the solvent and to remove the organic residuals. The procedures from coating to drying were repeated seven times. The films were then heated at 600 °C for 1 h as the second heat treatment. Young Hwan Hwang et al. applied this protocol for the fabrication of thin-film transistors (TFTs) with zinc oxide channel [52]. The same procedure with minor changes was used by Toshinobu Yoko et al. for the preparation of aluminumdoped zinc oxide films on silica-glass substrates by dip coating [53]. Referencespage 227 Orlando Tari et al. described the main aspects of the sol-gel synthesis of ZnO thin films [50]. The control of the Zn 2+ reactivity in a hydro-alcoholic environment was shown to be the key aspect of the preparation procedure. The use of triethanolamine (TEA), thanks to its high chelating efficiency towards Zn 2+ , has allowed to prepare stable zinc acetate solutions with [Zn 2+ ] > 0.50 M without to resort to high hydrolysis temperatures. This result, together with the optimization of the substrate wettability, allowed setting a sol-gel route, wholly performed at room temperature, from which ZnO thin films could be prepared by dip-coating. The influence of the annealing time and/or temperature on the nucleation and crystals growth rates of ZnO nanocrystals was shown. The electrical resistivity of the resulting films was shown to be strongly influenced by the environment of the post-annealing treatment.

ZnO nanowire arrays were synthesized by Shr-Nan Bai from hydrothermal solutions of zinc nitrate hexahydrate and hexamethylenetetramine on silicon substrates that were covered with a thin ZnO seed-layer film [54]. The textured ZnO seed layer was produced by the sol-gel method, with a sol concentration of 1 M, and subsequent annealing at 750 °C for 1 h. The silicon substrates were first spin-coated with 1 M zinc acetate in ethanol and ethanolamine and then heated at 300 °C for 10 min. This step was performed six times to ensure complete coverage. Then, the thin ZnO film coated silicon substrates were placed in the aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine at 95 °C with stable stirring in a thermostatically controlled bath for 2 h. It was shown that the ZnO nanowires became larger and denser as the concentration of zinc nitrate increased. Similar procedure was used by Pipat Ruankham et al. for the fabrication of hybrid solar cells based on ZnO nanorods [55]. Gururaj V. Naik et al. demonstrated that the structural and optical properties of a sol-gel deposited zinc oxide thin film can be tuned by varying the composition of the sol, consisting of ethylene glycol and glycerol [56]. The solution of zinc acetate dihydrate in a solvent mixture of glycerol and ethylene glycol was spin-coated onto oxidized silicon and glass substrates. The sol coated onto the substrates was hydrolyzed in humid air at 100°C, followed by baking at 150°C for 30 min to evaporate excess solvents. Pyrolysis/annealing of the hydrolyzed sol films, was carried out in oxygen ambient for 30 min at 550°C. was added drop-wise into this suspension keeping the magnetite/Zn molar ratio at 1:2 and reacted for 1h under vigorous stirring at room temperature. After the pH value was increased up to 9.0 at 80 °C, the resulting suspension was stirred for 3 additional hours to promote the dehydration and atomic rearrangement involved with the formation of ZnO on the surface of magnetite.

CdSe@ZnO core-shell nanomaterials were prepared by B.P. Rakgalakane and M.J. Moloto [60]. Zinc nitrate precursor was slowly added to CdSe colloidal solution under vigorous magnetic stirring and inert atmosphere. The resulting solution was slowly adjusted to pH 12 with 0.1M NaOH solution. The reaction temperature was maintained at 40 °C for 5 minutes. The spherical core@shell structures were observed when 5 mL of zinc nitrate solution (0.05M) were added while more star shapes formed as more zinc oxide precursor were introduced.

A B Referencespage 227

2. Fabrication of the TiO 2 and SiO 2 @TiO 2 core-shell nanohelices.

Titanium oxide deposition on organic template

In the Chapter 3, we discussed fabrication of silica replica from organic self-assembling nano/micro helices. As a development of this line, we tried to use titanium alkoxides precursor with a similar sol-gel process for the preparation of TiO 2 nanohelices. This idea was supported by examples of TiO 2 nanostructures based on organic template formed from cationic surfactants already shown in the section1.1.1 of this Chapter.

For the first experiment, we added 0.5 mL of aqueous 1 mM 16-2-16L-Tartrate gel to the same amount of ethanolic Titanium(IV) butoxide (TBOT) solution (1 or 10 mM) in one shot without any stirring. Opposite order of mixing was studied as well. The reaction mixture was allowed to stand 24 hours at room temperature and then the sample was washed several times with ethanol to remove organic and unreacted precursor.

As a result for all the experiments we obtained branched networks of TiO 2 fibers without evidence of chiral organization (Figure 4.16). The reactivity of TBOT in presence of water is known to be extremely high, that can results in the titanium oxide formation before the precursor will reach the template which remains uncovered with the TiO 2 shell and dissolves in ethanol. Another option is that the kinetics of the organic template dissolution in alcohol is even faster than that of TiO 2 condensation. Variation of the precursor concentration and order of mixing didn't have any positive impact on the results of the experiment. Since organic helices are soluble and get quickly destroyed in alcohol, water soluble titania precursors have to be used. For this purpose we have chosen commercially available water soluble titanium precursor: Titanium(IV) bis(ammonium lactato)dihydroxide. All the experimental procedures based on this precursor may be roughly devided in two categories: hydrothermal methods where the precursor is hydrolysed in an autoclave at temperatures higher than 100 °C and methods based on the usage of amine rich proteins, peptide sequences or polyelectrolytes under mild conditions. Due to the specificity of the considered template we focused on the latter methods.

Whereas the mechanism of the hydrothermal synthesis of TiO 2 is relatively well studied, the nature of the biomolecules directed titanium oxide formation remains an object of speculations.

Nevertheless, it is typically agreed that the ability of a protein or a peptide to hydrolyze the TiBALDH molecule is tied to the net positive charge carried by the biomolecule [10].

The elementary building block used in our study for the self-assembling system is rich in quaternary ammonium cations that ensure the good affinity of TiBALDH for the surface of the organic nanohelices. In order to achieve a homogeneous mixing of organic helices and titanium precursor we tried several procedures (Figure 4.17). We didn't obtain good mixing in the case when no mixing procedures were applied. Stirring with magnetic stirrer or vortex mixer is able to provide decent mixing conditions however leads to the aggregation of the organic gel fibers. Sonication was found to be only the technique able to ensure good mixing of organic helices and TiBALDH without the colapse of the gel.

191 Referencespage 227 High density of the quaternary ammonium cations at the surface of organic helices ensure the good affinity of TiBALDH to it, however it is not the case for the formation of titanium oxide.

We observed complete dissolution of the helices when ethanol was added (organic template is well soluble in ethanol). This indicates that the net positive charge of the nitrogen containing groups are not enough for inducing hydrolysis of the precursor. Indeed addition of ammonium hydroxide results in the precipitation of TiO 2 whereas addition of ammonium chloride has no effect as we saw in blank experiment (performed in absence of organic helices). This indicates that hydroxyl groups generated upon the protonation of primary amines in biomolecules are initiators of the TiO 2 formation.

Cole and Valentine [11,61] reported, that presence of at least 3 amines (2 primary and 1 or more secondary amines) in the structure of the peptide is required to initiate TiO 2 precipitation in biomimetic process. This is why we used primary diamine -Ethylenediamine to hydrolyze TiBALDH in the presence of organic helices. For this we mixed 0.5 mL of 1 mM 16-2-16L-Tartrate gel with equal amount of 10 mM TiBALDH aqueous solution. Ethanol amine was added in order to The most straightforward way for the fabrication of TiO 2 replica of organic helices turned out to be use of TDAA (titanium diisopropoxide bis(acetylacetonate)) as a precursor. As purchased, TDAA (75% in isopropanol) is not soluble in water. However, we found that 100 mM solution of TDAA in ethanol may be mixed with water in any proportions. Even though basic conditions are typically used for the hydrolysis of TDAA [6,62] it may also be hydrolyzed and in the absence of base.

For the TiO 2 deposition, we mixed 900 µL of water with 100 µL of 100 mM TDAA ethanolic solution. This resulted in the formation of yellowish solution which became cloudy with time. We proceeded as for the silica transcription of organic helices in which we used prehydrolyzed precursor. It was indeed shown in Chapter 3, that the pre-hydrolysis of the precursor has a significant impact on the results of transcription. After the preparation of ethanolic TDAA/water mixture, it was kept at room temperature from 0 to 60 minutes, then mixed with the organic gel in 1:1 v/v ratio. After 4 hours of reaction at room temperature, the samples were washed several times with EtOH in order to remove the organic and unreacted precursor. Significant differences were observed for the cases when not pre-hydrolyzed precursor and pre-hydrolyzed precursor were used (Figure 4.20). While non pre-hydrolyzed precursor was used A), inorganic fibers were less defined with considerable amount of TiO 2 formed between them, the organic helices with pre-hydrolysed precursor induced better defined inorganic fibers. This can be explained in a following way. Similarly to what happens for the silica transcription, during the titania transcription, three processes occurs in parallel: the hydrolysis of precursor, the complexation of the hydrolyzed precursor on the gemini surfactant, the condensation of the hydrolyzed precursor and the formation of a TiO 2 layer. If the amount of hydrolyzed TDAA molecules is small the condensation is slow and the reaction of the precursor with the gemini amphiphiles is the predominant process. This process results in the destabilization and destruction of the organic self-assembly. On the contrary, when the concentration of the hydrolyzed species is high the condensation becomes the predominant process.

Formed TiO 2 layer stabilizes the structure and allows preserving the original morphology of organic fibers.

It seems that a pre-hydrolysis time of 15 minutes is the right time to get the best transcription. Helices are better defined in this case compared to these obtained with 0 and 5 minutes of pre-hydrolysis. 60 minutes of pre-hydrolysis, on other hand, doesn't improve the result compared to the 15 minutes. The main drawback of the direct deposition of TiO 2 on an organic template is the low mechanical stability of the resulting structures. As prepared, inorganic helical structures obtained with TDAA as a precursor by the procedure described above were subjected to sonication bath for 1 minute. Results (Figure 4.22) show that long fibers were broken into shorter pieces (shown by red In this section we demonstrated the possibility of a direct deposition of TiO 2 on chiral organic template which resulted into helical inorganic fibers. Two of the three studied titanium oxide precursors (TiBALDH and TDAA) were suitable for the successful transcription of organic helices. Both precursors have good affinity for the template. Basic conditions are required for the hydrolysis of TiBALDH (mineral acids alongside with primary amines were found to be suitable for this purpose) whereas TDAA can be hydrolyzed in the absence of the base. Concentration of precursor and pre-hydrolysis are main factors determining the morphology of the inorganic fibers in this case.

Silica helices, on other hand, are robust and so SiO 2 @TiO 2 composite can satisfy both requirements for mechanical and electrical properties for the active element of NEMS devices.

Layer-by-layer (LbL) deposition of TiO 2 on SiO 2 template

Range of core@shell TiO 2 structures were fabricated by layer-by-layer (LbL) deposition technique (see Introduction section). This is a promising approach allowing the nanoscale control over the wall thickness by varying the number of deposition cycles. Silica surface and TiBALDH have the same charge and so precursor cannot be directly deposited on the silica helices. This is why positively charged electrolytes like poly(ethylenimine) are used to create positively charged surface. For LbL adsorption of of the titania precursor 100 µL of silica helices suspended in water were mixed with 400 µL of tris(2-aminoethyl)amine hydrochloride with three different concentrations: 1, 10 and 100 mM. After 15 minutes (this time is reported in [22]) unreacted salt was washed out by centrifugation and redispersing with sonication in fresh portion of water. Washed silica helices in 100 µL of water were mixed with TiBALDH at the concentration equal to that of the oligoamine salt. It was allowed to adsorb during 15 minutes and then excess precursor was washed out.

There is no apparent difference between samples prepared at different concentrations of TiBALDH after 1 deposition cycle (Figure 4.24), suggesting the formation of a single monolayer of titanium precursor. Such thin layer is not suitable for the fabrication of the crystalline TiO 2 film with the percolation pathway. At least 5 nm thick titanium oxide layer is required.

In order to increase the amount of deposited TiBALDH the procedure was repeated for 3 times. We found that precursor was adsorbing inhomogeneously, preferentially between the pitches of the helices (Figure 4.24F). With triethylenetetramine hydrochloride we obtain results similar to those of tris(2aminoethyl)amine hydrochloride.

Since LbL approach didn't allow us to obtain the desired thickness of TiO 2 layer we move on to the classical sol-gel methods of core@shell structure fabrication.

SiO 2 @TiO 2 core@shell from TBOT

A common way for SiO 2 @TiO 2 core-shell structure fabrication is to react silica particles with titanium tetraalkoxides Ti(OR) 4 in ethanol with controlled amount of water.

In a typical synthesis we used 500 µL of ethanolic suspension of silica helices. 5 µL of TBOT were dissolved in 500 µL of absolute ethanol. Then this solution was added to the silica solution and the mixture was homogenized by sonication for 5 minutes. Since SiO 2 helices were prepared in aqueous medium and then washed with ethanol we expected that some water would remain adsorbed on the hydrophilic surface of the fibers after the washing and so no additional water was added to the mixture. The deposition of the TiO 2 layer was carried out for 24 hours at room temperature. Finally the unreacted precursor was washed out with ethanol.

A TiO 2 layer of around 6 nm was formed under these experimental conditions (Figure 4.25A-C). The thickness of the titania layer strongly depends on the concentration of used precursor.

When 3 µL of TBOT was added instead of 5 µL, only very thin layer of titanium oxide was deposited on silica helices (Figure 4.25F) indicating that this is the minimum amount of precursor required for TiO 2 deposition. The thickness of the shell can be increased by increasing the amount of used TBOT beyond 5 µL as well, however, as the thickness of TiO 2 film increases, they form continuous film which looks more like tubules, and the helical morphology of the fibers are not anymore obvious.

The main issue with usage of TBOT for the synthesis of core-shell helical structures is the non-uniform growth of the TiO 2 layer due to an unequal distribution of water within the sample (Figure 4.25D and E). These silica helices are structures with a high aspect ratio which tend to form entangled aggregates. Water in this case can be entrapped in the confined space formed by the fibers. Since the kinetics of the precursor hydrolysis is determined by the water concentration it then become impossible to control the TiO 2 growth. Another issue is that the amount of water adsorbed on the helices can vary from one sample to sample and is difficult to regulate. Therefore, a procedure which is not dependent on the water content in reaction mixture is required for the SiO 2 @TiO 2 core-shell structure fabrication.

SiO 2 @TiO 2 core@shell from TiBALDH

Due to the presence of two chelating ligands, TiBALDH is a titanium oxide precursor which remains stable in water. This allows us to perform synthesis of SiO 2 @TiO 2 in aqueous medium and in this way eliminates the problem of the control of water content at the silica surface we faced with the TBOT as a TiO 2 precursor.

TiO 2 formation by TiBALDH hydrolysis with urea at 95°C in aqueous medium

Hanprasopwattana et al. in their work [33] reported the synthesis of SiO 2 @TiO 2 composite by the reaction of silica, TiBALDH, and urea under reflux condition in aqueous medium.

In hot water, urea decomposes forming ammonia and CO 2 according to the scheme:

Following this reference, we performed a synthesis of the core-shell structures as follows. In a typical experiment, 500 µL of silica helices suspended in water were mixed with aqueous solution of TiBALDH and urea. Ratio of TiBALDH to urea was kept 1:4. Reaction mixture was heated to 95 °C and kept at this temperature for 24 hours. The white precipitate at the bottom of sample tube was thoroughly washed with MilliQ water and examined with TEM (Figure 4.26). The formation of an unstructured TiO 2 precipitate and the absence of the silica helices were observed for the studied experimental conditions. A control experiment in which an aqueous mixture of SiO 2 helices and urea were heated at 95°C for 24 hours confirmed the complete dissolution of the silica fibers under these conditions. Obtained results suggest that the rate of silica dissolution upon the heating in basic medium is much higher than the rate of the precursor hydrolysis and TiO 2 condensation under the same conditions.

These results demonstrate that experimental procedure requiring the heating of silica helices in the presence of a base is not suitable for the fabrication of core-shell nanohelices.

TiO 2 deposition on silica helices from TiBALDH precursor in the presence of oligoamines

The experiment with the direct deposition of TiO 2 on organic nanohelices had shown the possibility of using diamines for the biomimetic synthesis of titanium oxide from TiBALDH in aqueous medium (see corresponding section of this Chapter). We applied the same approach for the fabrication of SiO 2 @TiO 2 composite.

In a typical experiment, 500 µL of aqueous suspension of silica helices (pH~6) were mixed with 500 µL of TiBALDH solution and 10 µL of 100 mM Tris (2-aminoethyl)amine (pKa1 = 8.43, pKa2 = 9.43, pKa3 = 10.13 [63]). The concentration of the precursor was varied from 2 to 10 mM. As the zero charge point of silica is at the pH 2-3, the surface is negatively charged at higher pH .

The protonated amine electrostatically adsorbs onto the surface of the silica helices making it positively charged. The negatively charged precursor then electrostatically adsorbs on this surface then hydrolyzes and condenses due to the elevated pH.

As it can be seen from the results, the morphology of the TiO 2 shell is a function of the precursor concentration. The helical nature of the composite fibers is clearly seen when 2 mM TiBALDH were used, it is a less obvious in a sample with 5 mM precursor and was almost completely masked when 10 mM TiBALDH were used.

Presence of the free amines in the solutions causes TiO 2 formation in the bulk along with the shell growth.

TiO 2 deposition on amine modified silica helices

An extension of the method described above is to covalently attach amino groups onto the surface of the silica helices.

Amine groups were grafted on the silicon oxide nanohelices by the surface modification involving (3-Aminopropyl)triethoxysilane (APTES) in ethanol as described in Materials and methods section.

For the TiO 2 deposition, 500 µL of amine modified silica helices suspended in MilliQ water (~1mg/mL) were mixed by sonication with 500 µL of aqueous solution of TiBALDH. The As it can be seen from the results (Figure 4.28A-C) that 1 mM precursor is not enough to completely cover the silica helices. Titanium oxide formation begins on the helix and then TiO 2 layer tends to grow in all the directions. With 10 mM precursor overall covering of the helices was achieved. Finally with 100 mM precursor very dense layer of TiO 2 was formed. concentration of hydroxyl groups, this approach could generate a smart way to independently vary amount of the precursor and its hydrolysis rate.

SiO 2 @TiO 2 core@shell from TDAA

We also manage to deposit TiO 2 layer on the amine modified silica helices from the TDAA (titanium diisopropoxide bis(acetylacetonate)) as a precursor. For this 100 µL of amine modified silica helices in ethanol (see Materials and methods) were mixed with 100 µL of ethanolic solution of TDAA by 5 minutes sonication. Then 200 µL of water were added and mixture was sonicated for another 10 minutes. After 15 hours the sample was washed 5 times by centrifugation and subsequent redispersing in ethanol.

In this procedure the surface modification is used for 2 purposes: it creates electrostatic attraction of negatively charged precursor to the silica surface and at the same time forms a basic environment near the surface of the helices causing preferential hydrolysis of TDAA and condensation onto the silica helices.

The rate of TDAA hydrolysis depends on the precursor to OH -ratio. The amount of hydroxyl groups directly correlated with the amount of amine groups on the functionalized silica surface for a given amount of used silica helices is constant. Then, when a 100 mM TDAA solution was used (Figure 4.29A and B) the ratio of precursor to OH -was 10 times lower and the corresponding kinetics was slower compared to the case with 10 mM TDAA solution (Figure 4.29C). This explains why the TiO 2 thickness is smaller (~2 nm vs ~5 nm) for the same reaction time despite a higher amount of precursor. In order to determine the effect of amine modification on the formation of TiO 2 layer we performed experiments using different amounts of APTES at the same deposition conditions (Figure 4.31). We prepared amino modified helices by reaction of the bare silica with increasing amounts of APTES (0.5 mM, 1 mM, 2 mM and 5 mM). We observed that there is no obvious difference for the samples with APTES concentration of 1 mM and higher. In the case of 0.5 mM APTES, the morphology of the formed titanium oxide is different. This result suggests that 1 mM APTES is sufficient to saturate the surface with amino groups. At 0.5mM, the amount of amine groups may be too low to compensate the natural negative charge on the surface of the silica helices. Most of the amino groups probably interact with the negative Si-O -of the surface. Based on this results we used 2 mM APTES for the silica helices modification in all the following experiments. For the NEMS applications individualized and well dispersed core-shell nanohelices are required. In previous chapter we showed that the silica helices could be dispersed by ultra-sonication in ethanol/isopropanol mixture. In fact, there are two pathways for the synthesis of individualized SiO 2 @TiO 2 nanohelices. In the first one, silica helices can be fragmented/individualized first and then modified with amine groups; and then TiO 2 shell is grown on this template. The second one is to first modify the surface of the silica helices and then fragment them. The disadvantage of the first approach is that the silica helices suspended and modified with amine must be centrifuged in order to wash out unreacted APTES which can result in their aggregation. On the other hand there is the chance to remove part of attached amine groups during the ultra-sonication when second approach is used.

To compare these two approaches, silica helices were fragmented and dispersed by ultrasonication according to the procedure developed in Chapter 3 (details are described in Materials and methods section). For the first set of the experiments the amine modification was performed before the fragmentation of silica helices and for the second set after the fragmentation. 100 µL of amine Results (Figure 4.32A and B) show that it is possible to synthesize core@shell SiO 2 @TiO 2 structures using both approaches. However the centrifugation after the fragmentation (the first method) results into the aggregation of the fibers (Figure 4.32C) whereas with the second method, we obtain more individualized nanostructures (Figure 4.32D) with the length from 100 to 2000 nm and mean value of 500 nm. In all the experiments described in this section, the obtained TiO 2 layer was amorphous. In order to be used as an element of NEMS, the helices have to exhibit semi-conductive properties which requires the titanium oxide shell to be crystalline with anatase type crystal structure.

Fabrication of SiO 2 @ZnO core-shell nanohelices

The silica helical skeletons can also be used as templates for the further replication of piezoelectric metal oxides such as ZnO. Zinc oxide is a well-known II-VI semiconductor with a reasonable band gap of 3.37 eV and a large exciton binding energy of 60 meV. Its electrical properties make ZnO a promising candidate for the NEMS fabrication.

As we showed in the Introduction, two procedures can be used for the synthesis of SiO 2 @ZnO nanohelices: (i) the classical sol-gel formation of core-shell particles [58], (ii) ZnO nanoparticles with a carboxylate-modified surface can be coated onto SiO 2 helices modified with amine groups [57]. The control of kinetics of zinc acetate hydrolysis for ZnO deposition by sol-gel method can be achieved either by 1) slow release of the precursor by thermal decomposition of Zinc acetate/amine complexes [50,51] or by 2) in situ formation of ammonia via thermal hydrolysis of urea in water. Control over the temperature and the precursor to base ratio are necessary for both cases. Atomic layer deposition of ZnO will be used as an alternative synthesis of these core-shell systems.

Slow base release approach

For the experiment of in situ base formation, we used 100 µL of silica helices dispersion in water mixed with aqueous solution zinc acetate and urea. The ratio of Zn(OAc) 2 to urea was 1:4.

The volume of the sample was adjusted with water to 1 mL. The reaction mixture was heated at 95°C for 15 hours. After the end of reaction, the samples were vigorously washed with MilliQ water through centrifugation and redispersion in fresh portions of solvent by sonication. Jeevanandam [65] and Ruzimuradov [66] showed possibility to hydrolyse urea at lower temperatures: 80°C and 60°C respectively. So we performed a ZnO deposition under milder conditions in order to prevent the silica dissolution. Since stability of the used silica template is limited in basic conditions and elevated temperatures in water we focused on the methods of ZnO synthesis in non-aqueous medium.

Slow release of precursor.

As discussed in the Introduction, the formation of zinc oxide from the complexes of zinc acetate with ethanolamine in ethanol is widely used for the fabrication of ZnO nanostructures. This approach allowed the authors to considerably increase the concentrations of the precursor (complex of zinc acetate with ethanolamine) in the alcoholic medium at room temperature as compared to the very low solubility of the zinc acetate itself. This zinc acetate/amine complex is then heated up in the presence of a base; it slowly decomposes releasing the Zn 2+ ions which readily reacts with the base to form ZnO. The nature of the base, the precursor to base ratio, the temperature and the reaction time are the main factors determining the deposition of Zinc oxide.

Sodium hydroxide is the most common base used for the hydrolysis of Zinc oxide precursors. In our work, we found that the silica template readily dissolves in NaOH solution upon heating (80 °C), this is why weaker bases such as lithium hydroxide (LiOH) [67][68][69] and tetramethylammonium hydroxide (TMAH) [70] were chosen for the study.

In order to study the effect of the nature of the base, we prepared two precursor solutions.

We mixed 50 µL of 100 mM Zn(OAc) 2 methanolic solution and 100 µL of 100 mM ethanolamine (EA) in EtOH, (MeOH was chosen to dissolve Zn(OAc) 2 because it revealed impossible to obtain a 100 mM solution of Zn(OAc) 2 in ethanol). EA forms a stable complex with zinc acetate at RT. Higher temperature results in the formation of bigger amount of ZnO. Increase of the amount of used base as well results in the increase of ZnO formed at constant temperature. At both temperatures we observed formation of bigger particles of Zinc oxide when more base was used.

Effect of substrate surface modification. Comparison of the stabilizers.

The measured value of the association constant K A for the zinc acetate in MeOH solution suggesting that 99.99 at% of Zn 2+ is bound to Ac - [71]. It means that in the alcoholic solution there can be no electrostatic interaction between the silica surface and zinc precursor which can direct the ZnO layer formation.

Alternative to electrostatic interaction is the covalent bonding of ZnO seeds to the substrate.

It was shown [START_REF] Singh | Adsorption of mercaptosilanes on nanocrystalline and single crystal zinc oxide surfaces[END_REF][START_REF] Najari | Adsorption of γmercaptopropyltrimethoxysilane on zinc: A study of the competition between thiol and silanol functions related to the age of the siloxane solution, its pH and the oxidation state of the surface[END_REF] that mercapto-silanes can covalently attach to zinc oxide substrate (through both the formation of the Si-O-ZnO and Si-(CH 2 ) 3 S-ZnO bonds). Zinc acetate readily forms complexes with the amines [50,51], therefore the silica surface functionalization with aminosilanes can be used as well for the directing of the ZnO growths.

In order to check if the ZnO would have a better affinity towards thiol and amine terminated silica helices we compared them with bare silica helices under the same deposition conditions.

Initially 200 µL of the modified silica helices in ethanol were reacted with 50 µL of 100 mM methanolic solution of zinc acetate for 30 minutes. Then ethanolamine and LiOH in the ratio of 2:1 with respect to zinc acetate were added and the sample was adjusted to 1 mL with ethanol. Reaction mixture was stirred at 80 °C for 15 hours. After the end of the reaction the samples were cooled down to room temperature and thoroughly washed with absolute ethanol. Results obtained for all the substrates are shown on Figure 4.39. It can be seen that the surface modification has a significant effect on the ZnO deposition.

Whereas for the bare silica we observed formation of zinc oxide particles mainly in the bulk (Figure 4.39A-B). In the case of silica helices functionalized with thiol terminated silanes, most of the ZnO was formed on the surface of the nanofibers (Figure 4.39C-D). A more smooth and homogeneous ZnO layer was obtained with the amine terminated silica helices (Figure 4.39E-F). These results suggest that by varying the surface chemistry of the substrate the deposition efficiency can be controlled.

As we discussed in the Introduction, monoethanolamine and triethanolamine can be used as complexing agents (stabilizers) for zinc acetate ( In order to check the effect of the stabilizer on the zinc oxide shell formation we compared monoethanolamine with triethanolamine as well as with two oligoamines: tris(2-aminoethyl) amine and tryethylenetetramine.

In this experiment we used thiol modified silica helices as a substrate. Ethanolamine acts as a bidentate ligand for the zinc ion whereas triethanolamine can act as a tetradentate ligand [50].

According to this the zinc acetate to ethanolamine ratio was fixed to 1:2 and for the rest of amines -1:1. The complex to LiOH ratio was 1:2 for all the samples. The reaction mixture was stirred at 80 °C for 15 hours, after which samples were washed with absolute ethanol.

We found, that tris(2-aminoethyl) amine forms an extremely strong complex with zinc acetate which doesn't decompose even upon heating in aqueous base solution at 95 °C overnight.

We can assume that this happens due to the presence of four reactive amine groups in this molecule.

Results obtained with other stabilizers are shown in As we can see from the results the highest amount of ZnO is formed when zinc acetateethanolamine complex is used. This indicates highest rate of the complex decomposition under the experimental conditions. Significantly smaller amount of Zinc oxide were formed from Zinc acetatetriethylenetetramine complex. Finally, with triethanolamine we only observed a slight staining of the silica helices after 15 hours of reaction.

A shorter reaction time (5 hours) resulted in the formation of a smaller number of ZnO particles from ethanolamine complex and no zinc oxide formed from the complex with triethylenetetramine. Comparison of the complexing agents suggests that triethylenetetramine is the most promising stabilizer for the formation of core-shell nanostructures with the complex morphology due to the high efficiency of the control over the reaction kinetics (Figure 4.42).

Stabilizers: Ethanolamine (A), Triethylenetetramine (B).

In this section we showed that two different approaches could be used for the sol-gel formation of zinc oxide. In the first case the base was slowly formed in situ by the thermal

Conclusions

In this Chapter we showed the possibility of synthesis of titanium oxide helical structures based on the organic template. Two TiO 2 precursors -Titanium(IV) bis(ammonium lactato)dihydroxide (TiBALDH) and Titanium diisopropoxide bis(acetylacetonate) (TDAA) were used for the fabrication of inorganic helical ribbons. In the biomimetic process approach, TiBALDH was hydrolyzed by diamine in aqueous medium at room temperature, though sodium hydroxide can be used as well. TDAA was dissolved in water in presence of small amount of alcohol and then hydrolyzed in absence of any base. We found that the way of mixing of the organic gel and the TiO 2 precursor has significant effect on the morphology of inorganic replica. Sonication was preferred to stirring and vortexing in order to preserve the helical structures during the secondary transcription . The main drawback of the obtained TiO 2 fibers is their low mechanical strength.

The SiO 2 @TiO 2 core-shell structure is able to overcome this problem of the mechanical strength characteristic for the pure TiO 2 fibers. Usage of Titanium(IV) butoxide (TBOT) is the most straightforward method for the synthesis of silica@titania composite. The precise control of the water content is extremely important in this approach, however it is difficult to achieve correctly with the water-born silica. Usage of TiBALDH and TDAA as TiO 2 precursors allows performing the synthesis in aqueous or mixed solvent eliminating in such a way the necessity of a precise control over the water content. Three different approaches were used for the TiO 2 shell formation with these precursors. The first one is the LbL adsorption of oligoamine hydrochloride salt and TiBALDH. This approach was found to be time consuming and having an issue with the washing out of the precursor due to the complex shape of the template. The second approach was to hydrolyze TiBALDH or TDAA by oligoamine electrostatically adsorbed on the surface of the silica helices. In this case both the electrostatic attraction of the precursor to the amine and the high local pH value at the surface of the helices promoted the preferential formation of the TiO 2 layer on the template. In the third approach, amine groups were covalently grafted onto the silica helices. The mechanism of the TiO 2 shell formation in this case was similar to the one described above. The advantage of this method is the absence of any free amine in the solution and so much better control over the titanium oxide deposition. The precursor concentration, the concentration of APTES used for the silica modification and the composition of the solvent are the main factors determining the morphology of the TiO 2 layer. Crystalline TiO 2 layer with anatase phase crystal structure was obtained by calcination of SiO 2 @TiO 2 core-shell nanohelices at 450°C for 3 hours. Referencespage 227

Two procedures were studied for the synthesis of SiO 2 @ZnO core-shell: a slow release of the base and a slow release of the precursor. In the first approach the base was formed in situ by the thermal hydrolysis of urea in aqueous medium. Effect of the precursor concentration, reaction temperature and time on the formation of ZnO was studied. For all the experimental conditions we found that dissolution of the silica helices upon the heating with urea is faster than the speed of ZnO layer formation. In the second approach the synthesis was performed in ethanol. The complex of Zinc precursor and the amine were mixed with the base forming a stable solution at room temperature. This complex was decomposed upon heating resulting in a slow release of the precursor, its hydrolysis and the formation of ZnO. Several types of complexation agents were studied and triethylenetetramine along with ethanolamine were found to be the most suitable for the synthesis of nanoscale SiO 2 @ZnO composite. The surface modification was shown to have a positive impact on the ZnO deposition.

As an alternative to the sol-gel process Atomic Layer Deposition (ALD) was used for the fabrication of core-shell nanohelices. However, this technique works on powder samples which resulted on rather entangled core-shell objects. These samples were difficult to separate and position for AFM measurements.

Materials and methods

Materials

All the chemicals were purchased from Sigma Aldrich and used without further purification.

Ultrapure water (18.2 MΩ cm) was obtained from Purelab ultra ELGA system.

Synthesis

Balance Sartorius BP211D with an accuracy of 0.01 mg was used for the weighting.

Gilson Pipetman micropipets were used to measure and move small amounts of liquid reagents.

Amine modification of silica helices

Solution of (3-Aminopropyl)triethoxysilane (APTES) in absolute ethanol with the concentration of 100 mM was prepared. Typically 10 mL of silica helices (1mg/mL) suspended in absolute ethanol were used. 20 µL of APTES solution per 1 mL of ethanolic silica helices suspension was added, mixture was sonicated for 5 minutes and immersed in preheated to 70 °C oil bath. After 15 hours the sample was washed several times with absolute ethanol.

Cutting of silica helices

High intensity ultrasonic processor "Vibra cell 75186" equipped with 6 mm microtip and operating at 130 W (maximum power), 20 kHz in the pulse mode was used for the cutting of silica nanohelices. 2 mL of silica helices in ethanol/2-propanol (1:1 v/v) were sonicated at 65 W during 15 minutes in a pulse regime (1 second pulse, 1 second pause; total time -30 minutes) in ice cooled conditions.

Electron microscopy

400-mesh carbon-coated copper grids were used for Transmission Electron Microscope (TEM) observations. For the observation of the samples synthesized in ethanol 5 µL of the sample were dropcasted on the grid and allowed to dry on the air. For the observation of the samples synthesized in water or in water/ethanol mixture hydrophilic grids were prepared. For this carbon-coated copper grids were treated in BioForce Nanosciences UV/Ozone ProCleaner 220 for 10 minutes. Then 5 µL Referencespage 227 of the sample were drop-casted on the grid and solvent was removed after 5 minutes with filter paper. The thin layer of the liquid remained on the grid was allowed to dry on air.

TEM observations were performed using a Philips EM 120 electron microscope operating at 120 kV and the images were collected by 2k × 2k Gatan ssCCD camera.

Introduction

This chapter will be devoted to the testing of mechanical properties of silica nanostructures as the first step of the NanoElectroMechanical System (NEMS) fabrication.

In the introduction we will explain what NEMS are and show the main techniques for their fabrication.

In the main part of this chapter we will describe basics of the Atomic Force Microscopy (AFM) technique used for the measurement of the mechanical properties of the silica nanostructures: nanohelices and nanotubes. Here we used silica nanoobjects as a model for the optimization of the measurement procedure. The data on the mechanical properties of the silica nanostructures obtained in this study are important as a basis for the future study of the core-shell nanohelices. Results of the measurements and importance of the experimental parameters, such as clumping conditions, will be discussed into the details. NanoElectroMechanical Systems (NEMS) are devices integrating electrical and mechanical functionalities at the nanoscale. The electromechanical system usually consists of multiple transducers and a mechanical element whose response is well characterized. The transducers enable the actuation and detection of mechanical motion, through optical or electrical methods. The NEMS can be employed either in its static or dynamic mode. In static systems, the device experiences only slight changes in deflection. In contrast, in dynamic mode the mechanical structure is under constant oscillation, often in its resonant mode where the response is significantly amplified [1]. When the electromechanical system is utilized as a sensor, the mechanical response (whether static or dynamic) is constantly monitored. Changes in the transducer signal are used as indication of variations in the environment. The sensitivity of the electromechanical system to environmental changes relies heavily on the size of the device. As device dimensions are reduced to nanometric scale, previously unattainable characteristic properties are reached [2]. The difference in behavior of electromechanical systems at the nanoscale is largely due to their ability to mechanically respond with high frequency (or speed) and the improvement in the quality factor (Q-factor) as the size of the system is reduced. The mechanical performance of the system can also be enhanced by using mechanically strong but light-weight nanomaterials [2].

Study of mechanical and electrical properties of chiral nanostructures were performed by

NEMS are used as nanometer scale sensors, actuators, devices and systems with critical feature sizes ranging from 100 to a few nanometers. The effective masses, heat capacities and power consumption of NEMS are proportional to the critical feature size, either linearly or nonlinearly, while the fundamental frequencies, mass/force sensitivities and mechanical quality factors are inversely proportional to the critical feature size. The extremely small size of NEMS provides fundamental frequencies in the microwave range, mechanical quality factors in the tens of thousands, force sensitivities at the zeptonewton level [3], active masses in the yoctogram range [4],

mass sensitivity at the level of individual molecules and heat capacities far below a yoctocalorie (4.18400 × 10 -24 joule) [5]. The detection of monolayers of molecules, of single virus and of attograms of gold or carbon particles are outstanding results that have been demonstrated recently [6].

As devices decrease in size they become more sensitive to small changes in mechanical, electrical and magnetic forces. Therefore NEMS are used in the set-up of new characterization techniques, including atomic force microscopes and scanning tunneling microscopes [7].

enable the testing and manipulation of individual biomolecules, which is not possible at the macroscale. In addition, NEMS can operate where larger machines will generally not fit, such as inside the human body or in other instrumentation. This is important because it can enable real-time diagnostics of biological, mechanical and chemical systems [8].

Materials and methods for the NEMS fabrication.

There are two different general approaches for NEMS fabrication: (1) traditional top-down microfabrication methods, such as electron beam lithography, physical and chemical vapor deposition followed by etching, etc., that are adaptations of the existing MEMS fabrication techniques, and (2) building such devices based on the assembly of molecules i.e. "bottom-up approach" [7].

Two common micromachining processes are depicted in As in the case of surface micromachining, the etching reactions rely on the formation of compounds that can be physically removed from the substrate. In addition to bulk and surface Because of its widely tunable electrical properties, polysilicon is extensively used in MEMS/NEMS fabrication. The electrical conductivity of polysilicon can be extensively varied by doping, and may also be increased by depositing a metal or metal silicide film on top of it. It is a natural material for the fabrication of MEMS structures, as the integration of mechanical and electrical components is straightforward. These structures range in complexity from basic actuators and simple gears to complex mechanical structures [7]. Although diamond films are generally employed as insulating layers, devices have also been successfully fabricated entirely from diamond. Diamond nanowires from single-crystal diamond were used as cantilevers for RF switches. The novelty of such a switch lies on the fact that diamond is employed as both the conductor for the beam of the switch and the insulator for the switch substrate, replacing silicon-on insulator technology by making use of unique electrical conductivity properties of diamond [7].

A carbon nanotube (CNT) is a one-dimensional nanostructure which can be thought of as a graphene sheet rolled up into a cylinder. The electrical properties of CNTs are either metallic or semiconducting according to the wrapping angle of the carbon atoms around the nanotube axis.

CNTs are unique one-dimensional structures in which high flexibility and high strength coexist; they can sustain their structure under strain leading to a 20 % elongation and have a Young's modulus of around 1 TPa. Since CNTs have outstanding mechanical and electrical properties, they are considered to be among the most promising candidates as building-blocks for NEMS [2]. PMMA area is replaced by metal. Using this method, it is possible to avoid the materials being exposed to strong acid so that the choice of low-dimensional nanomaterials for NEMS can be extended; any materials that are not damaged by organic solvents can be used. However, since the CNTs are randomly dispersed on the sacrificial layer, usually only a single NEMS device can be fabricated at a time [2].

Production of suspended and aligned individual SWNTs was achieved by using a combination of the ac dielectrophoresis technique and electron beam lithography (Figure 5.2C).

PMMA was selectively patterned so that it was used as a support between two Au electrodes to prevent the CNT from falling down onto the substrate. Ac dielectrophoresis was applied for positioning the CNTs (or other one-dimensional nanostructure) between pre-patterned Au electrodes. Besides controlling the position of the CNT, one can also control the number of aligned CNTs by controlling the density of the CNT suspension and by adjusting the applied ac field parameters. The suspended CNT structures are finally produced after evaporating upper electrodes to clamp the CNT in place, followed by a lift-off process to remove the PMMA and critical point drying to avoid the collapse of the suspended structure [2]. The method has several advantages.

Firstly, it can be applied not only to CNTs but also to other low-dimensional nanostructures.

Secondly, the dielectrophoresis technique makes it possible to position the nanostructures in predefined place. Moreover, it is possible to make many electromechanical devices in a single experiment. The CNTs (or other nanostructures) can be aligned on each pair of electrodes simultaneously when an ac voltage is applied to one side of the electrode pairs in an array of devices. Finally, this method makes it possible to pre-define the electrode structures under the suspended CNTs so that a variety of device geometries can be realized. Besides these fabrication methods, CNT-based NEMS structures can be realized using nanomanipulation techniques. A nano-manipulator can pick-up an individual CNT. The CNT can be attached to the end of the manipulator tip with conductive glue. The attached CNT can be freely suspended so that it is mechanically movable as well as electrically connected through the manipulator. However, it is not possible to make complicated structures, it is time-consuming and it is not a technique that can be easily up-scaled [2].

It is also possible to synthesize the CNTs after fabrication of other components. Steele et al.

have grown suspended SWNT using chemical vapor deposition (CVD) to realize resonator structures as the last step of the device fabrication. In this case the suspended SWNTs are not influenced by etching or affected by any resist or other residues which might be attached to CNTs during the fabrication process. Therefore, very clean CNTs with few defects can be used for the devices so that more intrinsic electromechanical properties of the CNTs can be investigated [2].

Jinhui Song [9] et al. reported the experimental observation of piezoelectric generation from a single ZnO wire/belt for illustrating a fundamental process of converting mechanical energy into electricity at nanoscale. By deflecting a wire/belt using a conductive atomic force microscope tip in A C B

Mechanical properties of silica nanostructures

When Nanohelices (NHs) or Nanotubes (NTs) are used in NEMS, they dictate the overall behavior of the nanodevice in both static and dynamic regimes. Whilst a large number of systems with nanowires or nanotubes were investigated for their potential use in nanoelectronics nothing has ever been reported concerning the mechanical characteristics of inorganic NTs or NHs templated from organic self-assemblies, to the best of our knowledge. It is then crucial to study their mechanical properties. For this purpose, three point bending tests were performed using atomic force microscopy (AFM) technique as illustrated in Figure 5.3. These measurements were performed in the LAAS in the group of Christian Bergaud. The AFM consists of a cantilever with a sharp tip (probe) at its end which scans the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature of the order of a few nanometers. When the tip is brought into proximity of a sample surface, interaction forces between the tip and the sample lead to a deflection of the cantilever. The deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. The sample mounted on the piezoelectric stage that can move it in the x, y, and z directions.

The AFM can be operated in different modes, depending on the application. In general, the possible imaging modes are divided into static (also called contact) modes and a variety of dynamic (non-contact or "tapping") modes where the cantilever is vibrated [20]. In the static mode operation, the static tip deflection is used as a feedback signal. AFM static mode is almost always done in contact where the overall force is repulsive: the force between the tip and the surface is kept Referencespage 253 constant during scanning by adjusting the tip-to-sample distance with the feedback mechanism. In tapping mode, the cantilever is driven to oscillate up and down at near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder. The amplitude of this oscillation is typically 100 to 200 nm. The interaction of forces acting on the cantilever when the tip comes close to the surface causes the amplitude of this oscillation to decrease as the tip gets closer to the sample.

An electronic system uses the piezoelectric actuator to control the height of the cantilever above the sample. It adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample. A tapping AFM image is therefore produced by imaging the force of the intermittent contacts of the tip with the sample surface [21]. This method of "tapping" lessens the damage done to the surface and the tip compared to the amount done in contact mode.

All the experimental procedures were established for the silica nanostructures, as a model system, with the possibility of their further extension on composite core-shell structures. Silica nanohelices and nanotubes were synthesized accordingly to the procedure reported from our group in [22] and in the Chapter 3 of this manuscript. The NTs and NHs were dispersed in ethanol by sonication for 10 minutes and then casted onto oxidized Si wafers with inverted pyramids shaped micro-cavities obtained after The elastic modulus E of the NTs was determined using the Euler-Bernoulli beam bending theory of double clamped beam [24], that gives the relationship between E and the elastic deformation of a suspended and clamped cylindrical beam or tube (only for f applied at the middle of the beam) according to the equation ( 1): (1) where I is the moment of inertia given by ( ) for a tube with outer and inner diameters D out and D in respectively; Δz is the nanotube deflection for a given applied force f. The deflection of the nanostructure (Figure 5.9) is given by the difference between the total deflection ( ( )) and the deflection on the substrate ( ( )), which is the deflection of the cantilever [25]:

( ) ( ) (2)
This is done in order to remove the influence of the stiffness of the cantilever on the measurement. The term in (1) is the stiffness of the suspended nano-object [25]. The determination of this value gives us the spring constant of the nanotubes as well as of the nanohelices.

For all the NTs investigated through bending tests, the average value of the elastic modulus E was determined to be 74 ± 7 GPa, which is comparable to that of bulk SiO 2 [26] or amorphous SiO 2 nanowires obtained using chemical vapor deposition [27].

The bending of SiO 2 NT was also estimated by Finite Element Method (FEM) calculations using the COMSOL MULTIPHYSICS Software. The NT was modeled as a perfect tube with the same dimensions as those used in the experiments (i.e., 1 µm-long with outer and inner diameters of 36 nm and 10 nm, respectively). The extremities of the NT are clamped in position so as to correspond to the experimental clamping, and a normal force of F = 10 nN is applied in the midpoint in order to assure symmetrical bending of the NT. As shown in Table 5.1, the NT bending obtained from FEM calculations for different lengths using the E value extracted from experimental measurements is in very good agreement with the one measured from F-d curves. To further investigate the effect of the boundary conditions on the measured Young's modulus, different clamping conditions were simulated and examined in detail. NT with the length of 1.5 µm with outer and inner diameters of 36 nm and 10 nm, respectively was modeled. The extremities of the NT are clamped so that only 1 µm of the NT is suspended over the cavity. As in the previous experiments, the NT was bent by applying a normal force of F = 10 nN in the midpoint.

In Table 5.2 are given the bending calculated from FEM and the corresponding elastic moduli compared to those extracted from F-d curves for a NT of 1.5 µm in length with different clamping conditions.

For the NT clamped far from the edge of the cavity, bending is about 14 nm. This value is much smaller than the one obtained for clamping conditions corresponding to Our synthesis method enables the control of helicity of the NHs and hence their resonance frequency. This will facilitate their design for NEMS with high frequency resonators usable for mass sensing and RF signal transduction. 3 Pitch of 55 nm corresponds to the silica helices used in this study As a perspective of this work the technique developed for the measurement of mechanical properties of the silica fibers will be used for the composite helical structures. Four point measurement of the resistivity of the nanohelice will be performed (Figure 5.11) as well. This approach compared to the classical two point measurement allows removing the influence of the contact resistance on the results of the measurement.

Conclusions

In this Chapter we discussed the measurements of the mechanical properties of silica nanohelices and nanotubes as a first step of NEMS development. The nanostructures were deposited on patterned silicon substrate and clamped with platinum by the Ion Beam-Induced Deposition technique. Their mechanical properties as well as resonance frequency were accurately determined using experimental (AFM measurement) and numerical (Finite Element Method) approaches.

Clamping conditions of the nanostructures were found to play a crucial role in the extraction of the elastic modulus. The linear regime of force vs. vertical displacement curves obtained on the nanohelices is larger than the one obtained for nanotubes. The stiffness of nanohelices is lower than that of nanotubes. This demonstrates that the helices are softer than the tubes validating the main idea of the project that by using helices instead of tubes or wires a higher sensitivity can be obtained from the same applied force.

and therefore the length and level of aggregation of the chiral fibers. We also show that the structure of the amphiphile molecule i.e. spacer and alkyl chain length has an effect on the morphology of the self-assemble aggregates. Based on the results described above the conditions for the synthesis of the individualized microhelices were optimized and a mechanism of the helices formation based on the dynamic ion-exchange was proposed.

In the Chapter 3 we used inorganic transcription of the organic template in order to achieve the necessary robustness of the chiral structure. The reactivity of the precursors appears to be a crucial factor determining the morphology of the inorganic structures for both nanometric and micrometric helices. By tuning the hydrolysis rate of TEOS via the addition of tartaric acid to the reaction mixture we succeeded in reducing to only one single day the synthesis of the silica helices from 16-2-16 Tartrate organic gels. We can control the silica helices length and obtain shorter and individualized helices by ultrasonication. Not only the solvent properties such as density, viscosity and surface tension played an important role in the fragmentation and dispersion efficiency but also the sonication parameterstime, power and volume of the sample. Moreover, we successfully synthesized silica replica of organic micrometric helices by exploiting the specific properties of sodium metasilicate acidified by acetic acid such as the solubility and high reactivity. Studying obtained silica replica we found that micrometric helices are formed of double fibers with a diameter of around 500 nm twisting in parallel.

In the Chapter 4 we developed composite nanohelices based on titanium oxide as well as zinc oxide in order to obtain nanomaterial with the required electrical properties.

TiO 2 nanohelices can be synthesized by inorganic transcription of organic helices with the Titanium(IV) bis(ammonium lactato)dihydroxide (TiBALDH) hydrolyzed by diamine or with the Titanium diisopropoxide bis(acetylacetonate) (TDAA) hydrolyzed by water in presence of small amount of ethanol. The main drawback of the obtained TiO 2 fibers is their low mechanical strength.

The SiO 2 @TiO 2 core-shell structures on other hand are robust enough to be used in NEMS.

Commonly used approaches of such structures synthesis were found to have significant limitation once applied to the nanometric fibers with the complex morphology like the silica helices:

 It is difficult to precisely control water content and so Titanium(IV) butoxide (TBOT) hydrolysis with the water-born silica;

-General conclusion -

257

 The LbL deposition of oligoamine hydrochloride salt and TiBALDH is time consuming and have an issue with the washing out of the precursor due to the complex morphology of the template.

 If oligoamine used instead of its salt the SiO 2 @TiO 2 core-shell structures can be synthesized, however presence of free amines in the solution causes TiO 2 formation in the bulk.

The best results we obtain by titanium oxide formation on amine modified silica helices from TiBALDH or TDAA precursors. The advantage of this method is the absence of any free amine in the solution and then a much better control over the titanium oxide deposition. As a result TiO 2 shell fully repeating the morphology of the silica helices can be synthesized. The precursor concentration, the concentration of APTES used for the silica modification and the composition of the solvent are the main factors determining the morphology of the TiO 2 layer. Crystalline TiO 2 layer with anatase phase crystal structure was obtained by calcination of SiO 2 @TiO 2 core-shell nanohelices at 450°C for 3 hours.

Two procedures were studied for the synthesis of SiO 2 @ZnO core-shell: a slow release of the base and a slow release of the precursor. In the first approach the base was formed in situ by the thermal hydrolysis of urea in aqueous medium. However, we found that dissolution of the silica helices upon the heating with urea is faster than the speed of ZnO layer formation. In the second approach the synthesis was performed from the zinc acetate/amine complex in ethanol under the heating in presence of base. Several types of complexation agents were studied and triethylenetetramine along with ethanolamine were found to be the most suitable for the synthesis of nanoscale SiO 2 @ZnO composites. The surface modification was shown to have a positive impact on the ZnO deposition.

As an alternative to the sol-gel process Atomic Layer Deposition (ALD) was used for the fabrication of core-shell nanohelices. However, this technique works on powder samples which resulted on rather entangled core-shell objects. These samples were difficult to separate and position for AFM measurements.

In the Chapter 5 we discussed measurement of the mechanical properties of silica nanohelices and nanotubes by AFM as a first step of NEMS development. The linear regime of force vs. vertical displacement curves obtained for the nanohelices is larger than the one obtained -General conclusion -258 for nanotubes. The stiffness of nanohelices is lower than that measured for nanotubes (0.28 N/m vs. 0.36 N/m at the suspended length of 1.5 µm). This demonstrates that the helices are softer than the tubes confirming the main idea of the project that by using helices instead of tubes or wires a higher sensitivity of the NEMS can be obtained. The finite element calculations of the resonant frequency of the nonohelices showed that it can be tuned by varying the helical pitch.

-Chapter 1 :Chapter 2 : 18 3. 2 .Chapter 3 :Chapter 4 :

 1218234 General introduction -..................................................................................................................... Chiral self-assembly of small amphiphilic molecules. Bibliographic study. ............ 1. Introduction ......................................................................................................................... 2. Self-assembly in water ........................................................................................................ 3. Chirality: Why is it important? ........................................................................................... 4. Examples of chiral self-assembly .......................................................................................4.1. Self-assembly of small chiral molecules .................................................................. 4.2. Self-assembly of small achiral molecules ................................................................ 4.3. Co-assembly of small molecules and metal ions. .................................................... 4.4. Special examples of the chiral self-assemble systems ............................................. 5. Techniques .......................................................................................................................... 5.1. Spectroscopy ............................................................................................................ 5.2. Diffraction ................................................................................................................5.3. Microscopy .............................................................................................................. 5.4. Modeling .................................................................................................................. 5.5. Rheology and DSC ................................................................................................... Conclusion .............................................................................................................................. References ............................................................................................................................... Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion ......................................................................................................... 1. Introduction ......................................................................................................................... 1.1. Micrometrical chiral structures based on organic self-assembly ........................... 1.2. Chemistry of guanosine monophosphate ................................................................. 2. Kinetics of self-assembly of 12-2-12GMP alone ............................................................... 3. Self-assembly of 12-2-12GMP in the presence of bromide anions .................................... 3.1. Kinetics of 12-2-12GMP self-assembly in the presence of bromide anion.............. -Outline -Structural characterization ...................................................................................... 3.3. Helices growth ......................................................................................................... 4. Study of 12-2-12GMP self-assembly in the presence of Hofmeister series ions ............. Conclusion ............................................................................................................................ Materials and methods .......................................................................................................... Annex .................................................................................................................................... References ............................................................................................................................. Silicon oxide helical structures based on organic template ..................................... 1 Introduction ........................................................................................................................ 1.1 Chemistry of silica .................................................................................................. 1.2 Silica surface modification ..................................................................................... 1.3 Synthesis of chiral silica structures ........................................................................ 2 Nanometric helices ............................................................................................................. 2.1 Transcription in presence of Tartaric acid ............................................................. 2.2 Scaling up ................................................................................................................ 2.3 Fragmentation and dispersion of the silica nanohelices ........................................ 3 Micrometric helices ........................................................................................................... 3.1 Transcription with TEOS as silica precursor ......................................................... 3.2 Na 2 SiO 3 /acetic acid mixture as silica precursor .................................................... Conclusion ............................................................................................................................ Materials and methods .......................................................................................................... References ............................................................................................................................. Core-Shell nanohelices ................................................................................................ 1. Introduction ....................................................................................................................... 1.1. TiO 2 nanostructures ............................................................................................... 1.2. ZnO nanostructures ............................................................................................... 2. Fabrication of the TiO 2 and SiO 2 @TiO 2 core-shell nanohelices. .....................................

4 .Chapter 5 :

 45 ALD deposition of TiO 2 and ZnO on silica nanohelices .................................................. Conclusions ........................................................................................................................... Materials and methods .......................................................................................................... References ............................................................................................................................. Towards NEMS ........................................................................................................... 1. Introduction ....................................................................................................................... 1.1. What is NEMS? ...................................................................................................... 1.2. Materials and methods for the NEMS fabrication. ................................................ 2. Mechanical properties of silica nanostructures ................................................................. Conclusions ........................................................................................................................... References ............................................................................................................................. -General conclusion -.................................................................................................................... -Perspectives -................................................................................................................................ -Outline -Referencespage 26

Figure 1 . 1 -

 11 Figure 1.1 -Classification of amphiphilic molecules [2]

Figure 1 . 2 -

 12 Figure 1.2 -Schematic illustrations of possible aggregates of amphiphilic molecules in aqueous solution. The arrows represent possible inter-conversion pathways, dependent on concentration and other solution variables (pH, ionic strength, etc.) [4].

Figure 1 . 3 -Chapter 1 :

 131 Figure 1.3 -Phase-contrast micrographs for selected samples. The scale bar represents 10 µm. The temperature at which the images were recorded is indicated in the figure. (A and B) In the presence of 13 mM HCl, (C) 16.7 mM NaOH; (D) 23% ethanol; (E) 41% ethanol; and (F) 50% stearic acid [25].

Figure 1 . 4 -

 14 Figure 1.4 -Molecular structure of L-glutamic acid derivatives and TEM images of self-assemble aggregates formed in water [26].

Figure 1 . 5 -

 15 Figure 1.5 -Illustration of the synthesis of CPPy nanotubes and CCNTs [27].

Figure 1 . 6 -

 16 Figure 1.6 -Molecular structures of various amino acid based gelators derived from 2,4,6-trichloropyrimidine-5carbaldehyde.

Figure 1 . 7 -

 17 Figure 1.7 -Schematic of the thermo-reversible transition, and (bottom right) structure of the peptide amphiphile.

Figure 1 . 8 -

 18 Figure 1.8 -Molecular structure of C 12 -β 12 (A). Qualitative description of the time-dependent evolution of structures (width and morphology) (B). Cryo-TEM images of twisted ribbons (C) and helices (D). Bar = 100 nm [31].

Figure 1 . 9 -

 19 Figure 1.9 -Molecular structures of the gelators and morphology of the gels obtained from (L,L)-2 in water and THF [32].

Figure 1 . 10 -Figure 1 . 11 -

 110111 Figure 1.10 -Molecular structure of glycolipids (A) and corresponding twisted nanoribbons: Malt-Tz-C 16 (0.1 wt % water) (B), Cell-Tz-C 16 (0.5 wt % water) (C) and Lact-Tz-C 16 (1 wt % water) (D) [33, 34].

Figure 1 . 12 -

 112 Figure 1.12 -Molecular structure of bolaamphiphilic azobenzene (A). TEM and SEM images of the selfassembled chiral nanostructures obtained in acidic conditions (B) [36].

Figure 1 . 13 -

 113 Figure 1.13 -Structure of the rigid-flexible molecule (A) and schematics of the chiral assemblies formed from the molecules (B) (PPO chain omitted for clarity). [37]

Figure 1 . 14 -

 114 Figure 1.14 -The structure and abbreviation of TPPS and ionic liquids used in the work by Li Zhang et al. (A).

Figure 1 .

 1 Figure 1.15 -L-Glutamic acid-based derivatives used in the work of Qingxian Jin et al. L-HDGA, L-HGA, and L-BECA can work as hydrogelator [42].

Figure 1 . 16 -

 116 Figure 1.16 -Structure of the multi-segment amphiphile (MA) (A). MA assembles into architectures with properties of both parental building blocks; tapes of hydrogen-bonded fibrils that are held together by hydrophobic interactions (B). By selectively switching off one segment, a morphological transition is brought about, resulting in morphologies of the other segment (C). Adapted from [46].

Figure 1 . 17 -

 117 Figure 1.17 -CD spectral change of gelator upon photo-irradiation in water. Only the structure composed of the closed-ring isomers exhibited a chiral supramolecular environment [50].

Figure 1 .

 1 Figure 1.18 -FM-AFM images of plasmid DNA (2686 base pairs) in aqueous solution [58]

Figure 1 . 19 -

 119 Figure 1.19 -Schematic of a typical cryo-TEM specimen and the corresponding image. The right panel displays twisted ribbons (a), helical ribbons (b), and nanotubes (c), together with their appearance in the projected images, and the corresponding Gaussian (left) and bending (right) curvatures [31].

Chapter 2 :

 2 Micrometric helical structures based on cationic

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 65 Referencespage 115

Figure 2 . 1 -

 21 Figure 2.1 -Structures of the achiral and chiral molecules used in the work (A); co-assembled structure showing the left-handed M (minus) and right-handed P (plus) supercoiled segments (B); a right-handed coiled-coil super structure formed from left-handed coiled tapes (C) [1].

Figure 2 . 2

 22 Figure 2.2 Helical ribbons self-assembled from Gibco/BRL "Chemically defined lipid concentrate" from ref. [2].

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion. 66 Referencespage 115 nm. Nanoscale fibers themselves are only loosely twisted or helically wound up. The chiral rope formation of T-10-T is triggered by the photodimerization of trace amounts of the thymine moieties in the T-10-T assemblies. Hydrogen bond formation between the complementary hetero-base pairs prevented such a photoreaction and resulted in no chiral rope formation.

Figure 2 . 3 -

 23 Figure 2.3 -Molecular structure of homoditopic T-n-T bolaamphiphiles (A). Dark-field light micrograph of double-helical ropes formed from the homoditopic T-10-T (at 25 °C in 10% ethanolic/aqueous solutions) (B). The arrows denote representative interwoven portions of the helical ropes. EF-TEM image of the double-helical rope (C) [4].

Figure 2 . 4 -Chapter 2 :

 242 Figure 2.4 -Phase-contrast micrograph of the self-assembly of 12-hydroxystearic acid in the presence of 50% stearic acid [5].

Figure 2 . 5 -

 25 Figure 2.5 -Molecular structure of cationic gemini surfactants complexed with anionic acetylated oligoalanine, m-2-m(AcAla p ) 2 (A); TEM images of chiral ribbons obtained from 10-2-10(AcAla 5 ) 2 (B) and 12-2-12(AcAla 5 ) 2 (C) [7].

Figure 2 . 6 -

 26 Figure 2.6 -Molecular structure of C 12 GMP surfactant (A), C 14 AMP surfactant (B). Corresponding helical aggregates obtained from C 12 GMP and C 14 AMP amphiphiles (C, D) [9]

Figure 2 . 7 )

 27 . There are four chiral centers presented in the molecule (IUPAC name: [(2R,3S,4R,5R)-5-(2-Amino-6-oxo-3H-purin-9-yl)-3,4dihydroxyoxolan-2-yl]methyl dihydrogen phosphate).

Figure 2 . 7 -

 27 Figure 2.7 -Chemical structure of Guanosine 5'-monophosphate indicating numbering of carbon atoms.

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 69 Referencespage 115

Figure 2 . 8 -

 28 Figure 2.8 -Typical hydrogen bonding patterns for the principal nucleobases: base pairs (A), cyclic oligomer (B) and infinite linear polymers (C)

Figure 2 . 9 -

 29 Figure 2.9 -Protonation states of Guanosine 5'-monophosphate as a function of the pH

Figure 2 . 10 -

 210 Figure 2.10 -Structure of 12-2-12GMP

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 73 Referencespage 115

Figure 2 . 11 -

 211 Figure 2.11 -OM images of 12-2-12GMP (10 mM) just after dissolution at 70°C and cooling to 20°C (A); after 24 hours (B) and after 2 days (C).

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

Figure 2 . 12 -Figure 2

 2122 Figure 2.12 -OM images of 12-2-12GMP (10 mM) formed by reaction of 12-2-12Ac with GMP-Na 2 at different pHs and aging time.

Figure 2 . 13 -

 213 Figure 2.13 -TEM images of 12-2-12GMP (10 mM) formed by reaction of 12-2-12Ac with GMP-Na 2 at the pH 5.4 after 10 minutes of aging.

1

  equivalent (in regards to gemini) of acetic acid decreases the pH of the mixture to 6.4. The sample completely precipitates overnight. Small individualized twisted fibers were observed after 24 hours (Figure 2.15B). No changes were observed even after 1 week. When 1.5 equivalent of acetic acid are added (pH = 5.9) the formation of micrometer size helices are observed after only 1 hour. Within 24 hours, the formation of the helices is completed (Figure 2.15C).

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.78Referencespage 115

Figure 2 . 15 -

 215 Figure 2.15 -OM images of the mixture 12-2-12Br + GMP-Na 2 acidified with different amounts of acetic acid. Aging time -24 hours. Arrows show helical structures.

Figure 2 .

 2 Figure 2.16. shows the fraction of various protonation states of GMP as a function of the pH of the solution as calculated by Henderson-Hasselbalch equation:

Figure 2 . 16 -Chapter 2 :

 2162 Figure 2.16 -Protonation curves for Guanosine 5'-monophosphate

  2.2. NMR study). Nevertheless the structures of the samples from 12-2-12Br/GMP mixture aged at 20°C and 30°C are significantly different (Figure 2.17A and B).

Figure 2 .

 2 Figure 2.17 -OM images of the mixture of 12-2-12Br + GMP-Na 2 (pH = 5.9) aged for 1 day at 20 °C (A), 30 °C (B) and 1 °C (C); for 7 days at 20 °C (D), 30 °C (E) and 1 °C (F).

Figure 2 . 18 -

 218 Figure 2.18 -OM image of 12-2-12Br + GMP-Na 2 mixture (pH = 5.9) aged for 10 minutes at 1 °C and then for 7 days at 20 °C.

50 µm Chapter 2 :

 502 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 81 Referencespage 115

2 .

 2 20C).Once the GMP-Na 2 /12-2-12Br ratio reaches 2.0, helices formation are observed. Helices start to form already after 1 hour of aging (Figure2.20D) and within 24 hours, mostly helical structures are observed (Figure2.20E).Most well-structured helices are observed for a GMP-Na 2 /12-2-12Br ratio of 3.2. Helices begin to be observed after 1 hour of aging. After 1 day, helices are the major morphologies which continue to grow in length and diameter (Figure2.20H and I). Size distribution of the 100 structures clearly identified as the helices after 1 week of aging presented in Figure2.19. It worth to be mentioned, that some twisted fibers with the length of more than 200 µm were also presented in the sample.

Figure 2 . 19 -Chapter 2 :

 2192 Figure 2.19 -Length and diameter distribution of the helical structures formed at GMP-Na 2 /12-2-12Br ratio of 3.2 after 1 week of aging

Figure 2 .Chapter 2 :

 22 Figure 2.20 -OM image of 12-2-12Br + GMP-Na 2 mixture (pH = 5.9) with different GMP-Na 2 to 12-2-12Br ratios at various aging times

Figure 2 .

 2 Figure 2.21 -OM images of 12-2-12Br + 12-2-12GMP (1:1) mixture with gemini concentration of 10 mM after 1 hour (A), 24 hours (B) and 1 week (C)

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.84Referencespage 115

Figure 2 . 22 -

 222 Figure 2.22 -Schematic representation of reaction of n-m-nBr with GMP-Na 2

Figure 2 . 23 -Chapter 2 :

 2232 Figure 2.23 -OM images of mixture 10-2-10Br + GMP-Na 2 after 1 hour (A), 1day (B), 1 week (C)

Figure 2 .

 2 Figure 2.24 -OM images of mixture 14-2-14Br + GMP-Na 2 after 1 hour (A), 1day (B), 1 week (C). Same for 14-4-14Br + GMP-Na 2 mixture (D, E, F)

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.86Referencespage 115

Figure 2 . 25 -Chapter 2 :

 2252 Figure 2.25 -OM images of mixture 16-m-16Br + GMP-Na 2 at different aging times

Figure 2 . 26 -

 226 Figure 2.26 -Structure of the self-assembly after one week aging as a function of the spacer and the chain length. Best conditions are marked with red.

  20mM 12-2-12Br solution (100 µL) mixed with one equivalent volume of 60 mM GMP-Na 2 . i.e. 10 mM 12-2-12 Br/30 mM GNP-Na 2 solution. The pH of the mixture was adjusted to 5.9 with 1M acetic acid. The sample dissolved at 70 °C during 5 min and then stored at 20°C for the helices formation. Well defined individualized helical structures were observed under these conditions after 2-3 days of aging (Figure 2.27).

Figure 2 .

 2 Figure 2.27 -OM images of mixture 12-2-12Br + GMP-Na 2 (pH = 5.9) with GMP/12-2-12Br ratio of 3 after 1 week of aging

Figure 2 .Chapter 2 :

 22 Figure 2.28 -TEM images of organic micrometric helical structures formed in the mixture 12-2-12Br + GMP-Na 2 (pH = 5.9) with GMP/12-2-12Br ratio of 3. Arrows show the two nanofiber components of micrometric helical ribbon.

Figure 2 . 29 -

 229 Figure 2.29 -SEM images of organic micrometric chiral aggregates. Right handed helices marked as P (plus) and left handed as M (minus)

Figure 2 .Chapter 2 :

 22 Figure 2.30 shows the 1 H NMR spectrum of the GMP-Na 2 /12-2-12Br 1:1 mixture (gemini concentration -10 mM; mixture acidified with acetic acid) obtained at 80 °C with the

Figure 2 .Chapter 2 :

 22 Figure 2.30 -1 H NMR spectrum of 12-2-12Br + GMP-Na 2 + acetic acid mixture (D 2 O, 80 °C) with the correspondence of the peaks

Figure 2 . 31 -

 231 Figure 2.31 -NMR spectra obtained for 12-2-12Br + GMP-Na 2 (1:2) mixture upon the controlled cooling. Arrows indicate positions of the peaks corresponding to the gemini amphiphile

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 93 Referencespage 115

Figure 2 .

 2 30) of guanosine 5'-monophosphate for the quantification of amount of GMP presented in the aggregates.Each signal corresponds to a single proton and so one proton in this case corresponds to one GMP molecule per amphiphile. Results of experiment are summarized and combined with the data of optical microscope observations in Figure2.32.

Figure 2 .

 2 Figure 2.32 -GMP/gemini ratios in the aggregates found from 1 H NMR experiment as a function of GMP togemini ratio in the mixture (pH 5.9).

  1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 95 Referencespage 115

Figure 2 .

 2 Figure 2.33 -FT-IR spectra of 12-2-12Ac+GMP-Na 2 and 12-2-12Br+GMP-Na 2 mixtures acidified with acetic acid, as well as of GMP-Na 2 alone. Solvent -D 2 O

12- 2 -Chapter 2 :

 22 12Ac + GMP-Na 2 12-2-12Br + GMP-Na 2 GMP-Na 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

Figure 2 .

 2 Figure 2.34 -OM images of the helix growth process. Arrows show the bundle at the tip of the helix rotating during the helix growth and direction of its rotation

Figure 2 .

 2 Figure 2.35 Scheme of the experiment

Figure 2 .Figure 2 . 36 -

 2236 Figure 2.36 -Fluorescent markers: Fluorescein (A), Fluorescein isothiocyanate (FITC) (B), 5-Dodecanoylaminofluorescein (C)

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 99 Referencespage 115 unacceptable option, due to possibility of its reaction with free GMP in the solution resulting in strong background fluorescence.

Figure 2 . 37 -

 237 Figure 2.37 -Fluorescent microscope images of micrometric helices labeled with sodium fluorescein after 1 hour (A) and 7 days (B) (artificial coloring). Same for 5-dodecanoylaminofluorescein (C and D).

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion. 100 Referencespage 115 wavelength as the light source. For the FRAP the 375nm pulsed diode laser from PicoQuant was used at 5MHz repetition rate. Direct measurement of the background fluorescence during the fluorescent microscopy experiments (Figure 2.37C and D) for the helical fibers labeled with 5-dodecanoylaminofluorescein showed the presence of the free fluorophore in solution (Figure 2.38). This indicates that fast fluorescence recovery at the bleached point is due to the fast exchange between the dye in free solution and the dye incorporated in the helices, which makes long time scale experiment worthless.

Figure 2 .

 2 Figure 2.38 Confocal microscope image of the sample (A). Green contour shows the area for which measurement of background fluorescence was performed. Total fluorescence decay at this area (B).

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 101 Referencespage 115

  after 1 hour and after one week they form flat plates. Fiber formation was observed during the first hour of sample aging at the ratio 1.6, however, after 6 hours crystal structures were the main component of the sample (Figure2.39A). At the ratio GMP/12-2-12I = 2.8, helices were observed after 24 hours of aging. But after 3 days only flat plates were present in the sample (Figure2.39B).

Figure 2 .

 2 Figure 2.39 -OM images of the samples obtained from the GMP-Na 2 + 12-2-12X (X = I -, Cl -, NO 3 -) mixtures with different GMP/gemini ratios after one week of aging. Arrow shows helical fiber.

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 103 Referencespage 115

After 1

 1 week of aging no helices were observed for these conditions (Figure2.39G). Stable helices were observed only for the ratio of GMP/12-2-12NO 3 = 2.4 (Figure2.39H). Samples with the ratio of GMP to 12-2-12NO 3 from 2.8 to 4 exhibit similar behavior: unstructured precipitate formed after 10 minutes and only individual helices could be observed after 1 week of aging in the sample with the ratio 2.8 (Figure2.39I).Results of observations of self-assembly of 12-2-12 gemini amphiphile with Hofmeister series anions (X = I -, NO 3 -, Br -, Cl -) in presence of guanosine 5'-monophosphate are summarized in the Table2.1.

Figure 2 . 40 -

 240 Figure 2.40 -Morphology of 12-2-12X + GMP-Na 2 self-assembly as a function GMP/gemini ratio. Anions are ranged by their pKa values.

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 109 Referencespage 115 Typical example of 12-2-12Br 1 H NMR (300 MHz, MeOD, 25 °C, δ ppm, TMS):4.00 (4H, s), 3.47 (4H, m), 3.25 (12H, s), 1.84 (4H, s), 1.30 (36H, m), 0.90 (6H, t, 3 J = 6.74 Hz)12-2-12Br13 C NMR (75MHz, MeOD, 25 °C, δ ppm, TMS):65.60, 50.50, 31.68, 29.36, 29.28, 29.21, 29.08, 28.91, 25.92, 22.34, 13.03

A

  fine white powder of gemini acetate was formed. If the presence of residual silver compounds was observed: formation of black particles of grey color; then the powder was dissolved in MeOH and filtered on Celite once more. Finally, 12-2-12Ac was filtered, rinsed with acetone and dried under vacuum.Completion of the reaction and absence of bromide in the final product was probed by reaction of small amount of 12-2-12Ac with silver acetate at room temperature in MeOH overnight.Black or brown coloring of silver acetate powder indicate the presence of residual bromide.Typical example of 12-2-12Ac1 H NMR (300MHz, MeOD, 25 °C, δ ppm, TMS): 3.92 (4H, s), 3.42 (4H, m), 3.21 (12H, s), 1.90 (6H, s), 1.82 (4H, s), 1.30 (36H, m), 0.90 (6H, t, 3 J = 6.74 Hz) 12-2-12Ac 13 C NMR (75MHz, MeOD, 25°C, δ ppm, TMS): 65.57, 55.62, 50.33, 31.67, 29.35, 29.28, 29.21, 29.07, 28.93, 25.96, 22.34, 22.29, 13.03 Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion. of guanosine 5'monophosphate was obtained from the corresponding disodium salt on an ion exchange column. 200 mL of 2 M HCl were passed through the cationic ionexchange column filled with 60 g of DOWEX resin. Then the acid was washed out from the column with MilliQ H 2 O until neutral pH. 250 mg of Guanosine 5'monophosphate disodium salt in 5 mL of H 2 O were passed through the column. The solution was collected in a 1 L flask until neutral pH. The solution of GMP was concentrated down to ~30 mL and finally freeze-dried. Column was recuperated by 50 mL of 2 M NaOH and then washed with H 2 O until neutral pH. 12-2-12GMP was synthesized by slow addition of 12-2-12Ac dissolved in MeOH to 2.2 eq. of GMP under stirring. White precipitate of 12-2-12GMP was filtered out, washed with acetone and freeze dried from water solution.Proton NMR spectra were used to check the absence of the peak of acetate (6H at 1.90 ppm)and the 2 to 1 stoichiometry of GMP to gemini.Synthesis of 12-2-12 Gemini with anions of strong acids12-2-12 Gemini with anions of strong acids was synthesized by reaction of 12-2-12Ac with 2 eq. of the corresponding acid. The reaction was carried out under vigorous stirring in MeOH at 40 °C for 1 hour. The final product was crystallized from acetonitrile (12-2-12I) or acetone (12-2-12Cl, 12-2-12NO 3 ) by a procedure similar to the one used for 12-2-12Br. After the crystallization, compounds were filtered, rinsed with acetone (12-2-12I) or diethyl ether (12-2-12Cl and 12-2-12NO 3 ) and dried under vacuum. If NMR analysis showed no peak corresponding to the acetate anion, (6H at 1.90 ppm) the reaction was considered as completed. Transmission optical microscopy (OM) 20 µL of sample were sealed between microscope slide and cover slip. Observations were performed with a Nikon Eclipse PhysioStation E600FN with adequate condenser and prism for Differential interference contrast (DIC) observations. Differential interference contrast microscopy, also known as Nomarski Interference Contrast (NIC) or Nomarski microscopy, is an optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples. DIC works by separating a polarized light source into Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 111 Referencespage 115 two orthogonally polarized mutually coherent parts which are spatially displaced (sheared) at the sample plane, and recombined before observation. The interference of the two parts at

  Temperature controlled NMR experiments were performed on Bruker Avance 400MHz NMR spectrometer. The sample were prepared in standard 5 mm glass NMR tube. D 2 O was used as deuterated solvent. GMP-Na 2 (40mM, 250µL), 12-2-12Br (20 mM, 250 µL) and acetic acid (1 M) were dissolved and mixed at 70 °C prior the NMR experiment. Spectra were acquired every 5 °C in the range from 80 °C to 20 °C. At each temperature, the mixture was let to equilibrate for 10 minutes.

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion. performed on a Bruker IFS 55 FT-IR spectrometer equipped with germanium crystal. The samples were prepared in a D 2 O. The spectra were registered during 400 scans with the resolution of 4 cm -1 and 45° polarization.

Chapter 2 :

 2 Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion 113 Referencespage 115

Figure 2 . 41 -Chapter 2 :

 2412 Figure 2.41 -OM images of (C 12 ) 2 Br + GMP-Na 2 (1:1, pH = 5.9) after 1 day (A) and 1 week (B). Amphiphile concentration -20 mM.

Figure 2 . 42 -

 242 Figure 2.42 -OM images of helix formation observed with (C 12 ) 2 Ac + GMP (A) [10] and (C 12 ) 2 Cl + GMP (B) [31]

Figure 2 . 43 -

 243 Figure 2.43 -OM image of (C 12 ) 2 Ac + GMP-Na 2 (1:1, pH = 5.9) after 1 day of aging. Amphiphile concentration -20mM.

Figure 3 . 1 -Chapter 3 :

 313 Figure 3.1 -Silicon-oxygen groups relevant to sol-gel processing (R = C x H 2x+1 ).

Figure 3 . 2 -

 32 Figure 3.2 -TEOS, H 2 O, Synasol (95% EtOH, 5% H 2 O) ternary-phase diagram at 25°C [14].

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 122 Referencespage 165

Figure 3 . 3 -

 33 Figure 3.3 -pH rate profile for the hydrolysis of aqueous silicates [11].

Figure 3 . 4 -

 34 Figure 3.4 -pH dependence of dissolution rate and gel times for aqueous silicates [18].

Figure 3 .

 3 Figure 3.5 summarizes data about silica behavior in aqueous medium. Condensation occurs in such a way to maximize the number of Si-O-Si bonds and minimize number of terminal -OH groups through internal condensation. Cyclic structures are quickly formed from dimers according to this rule. Particles formed by reaction of cyclic silicates with free monomers serve as the nuclei.Growth occurs via an Ostwald ripening mechanism when highly soluble small particles dissolve and reprecipitate on larger, less soluble particles. Growth stops when difference in solubility of smallest and largest particles becomes negligible. At pH>7 formation of the large particles is predominant process (especially at elevated temperatures) due to the high surface charge and so repulsion between the particles. At pH<7 and in presence of salts aggregation starts to occur soon after polymeric particles are formed. Particles stop to growth after they reach diameter of 2-3 nm.

Figure 3 . 5 -Chapter 3 :

 353 Figure 3.5 -Polymerization behavior of silica in aqueous medium. A = in presence of salts / acidic medium, B = alkaline medium. (Reproduced from [18]).

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 126 Referencespage 165

Figure 3 .

 3 Figure 3.6 -Hydrolytic (A) [22] and anhydrous (B) deposition of silanes

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 127 Referencespage 165

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 128 Referencespage 165

Figure 3 . 7 -

 37 Figure 3.7 -Three-by-three matrix of atomic force microscopy (AFM) scans of APTES films. Each individual image is 2 µm x 2 µm with a height scale of 200 nm (displayed left) [31].

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 129 Referencespage 165

Figure 3 . 8 -

 38 Figure 3.8 -Schemes of: a hexagonal mesoporous silica fiber (A); a twisted hexagonal mesoporous fiber (B); a spirally-curved twisted hexagonal mesoporous fiber (hierarchically helical structure) (C); SEM (D) and TEM images (E) of a hierarchically helical nanofiber.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 130

9 )

 9 were synthesized by Yan Li et al. via a cetyltrimethylammonium bromide (CTAB)-assisted solution approach in acidic conditions [44]. The procedure involves the formation of a mixed solution of HNO 3 and surfactant CTAB and the subsequent growth of silica helix by the hydrolysis of tetraethylorthosilicate (TEOS) in the above solution. It was shown that the reaction time and temperature, molar ratio of TEOS and CTAB and types of acids are crucial to the formation of helices.

Figure 3 . 9 -

 39 Figure 3.9 -TEM images of silica helices with various morphologies obtained under quiescent conditions after 2 h [44].

Figure 3 . 10 -Chapter 3 :

 3103 Figure 3.10 -SEM images taken from the samples prepared in the coil-spring flow at different Reynolds numbers. a) For Re=1072, b) for Re=640, c) for Re=320 [45].

Figure 3 . 11 -

 311 Figure 3.11 -TEM images of chiral mesoporous silica synthesized at various stirring rates 200 rpm (a), 400 rpm (b), 600 rpm (c), and 800 rpm (d)[46].

Figure 3 . 12 -

 312 Figure 3.12 -Molecular structure of chiral gelator 12K (A), FESEM (B) and TEM (C-E) images of left-handed coiled tubular nanoribbons and tube-in-tube nanostructures.

Figure 3 .

 3 Figure 3.13 Molecular structure of L-18Val4PyBr compound (A). FESEM (B) and TEM (F) images of the calcined silicas prepared at different ethanol/water volume ratios: 4∶6 (B) and 8∶2 (F).

Figure 3 . 14 -Chapter 3 :

 3143 Figure 3.14 -Schematic illustration of the formation of multi-helical DNA-silica fibers (MHDSFs) and nonhelical DNA-silica fibers (NHDSFs) [49].

Figure 3 . 15 -

 315 Figure 3.15 -Molecular structures of gelators used in the study (A). SEM pictures of left-handed (B) and righthanded (C) helical silica prepared by a sol-gel method. Diameters of the helical silica are 100-110 nm [50, 51].

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 135 Referencespage 165

Figure 3 .

 3 Figure 3.16 Molecular structures of L-C12ValC10COONa and D-C12ValC10COONa (A). FESEM (B, C) and TEM (D) images of left-handed helical 4,4'-biphenylene-silica nanotubes prepared from D-C12ValC10COONa at the water/ethanol volume ratio of 2.2:1.8; FESEM (E) and TEM (F and G) images of right-handed helical 4,4'biphenylene-silica nanotubes prepared from L-C12ValC10COONa at the water/ethanol volume ratio of 2.2:1.8.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 136 Referencespage 165

Figure 3 . 17 -

 317 Figure 3.17 -Molecular structure of compounds RR-1 and SS-1 and TEM images of the helical mesoporous silicas prepared using correspondently RR-1 and SS-1 molecules.

Figure 3 . 18 -

 318 Figure 3.18 -Right-and left-handed helical silica nanotubes [57].

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 137 Referencespage 165 group of the C14-L-AlaS or C14-L-AlaA. The methoxy-silane sites of TMAPS and APTMS are cocondensed with tetraethyl orthosilicate (TEOS), to be assembled subsequently to form the silica framework (Figure 3.19). Such chiral materials have a clear advantage in their physical strength over chiral liquid-crystal assemblies, yielding a permanent chiral space. Natural chiral imprinting in inorganic structures probably occurs via a similar inorganic/organic chiral interface.

Figure 3 . 19 -

 319 Figure 3.19 -SEM image and schematic drawings of a structural model of chiral mesoporous silica. SEM image,

Figure 3 . 20 -

 320 Figure 3.20 -Effect of organic gel concentration (A), gel aging (B) and transcription temperature (C) on the morphology of transcribed silica structures.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 139 Referencespage 165

Figure 3 .

 3 Figure 3.21 -Scheme of the synthesis of the molecular precursors and the corresponding hybrids (A). SEM

Figure 3 .

 3 Figure 3.22 demonstrates structures of silica fibers obtained from 5 mM 16-2-16 L-Tartrate at different aging times [65]: twisted ribbons with Gaussian curvature, helical ribbons with cylindrical curvature, and tubules, which are the grown from helical ribbons.

Figure 3 . 22 -

 322 Figure 3.22 -TEM images of silica fibers obtained from 5 mM 16-2-16 L-Tartrate at different aging times [65]

Figure 3 . 23 -Chapter 3 :

 3233 Figure 3.23 -Schematic representation of dependence of the silica structure morphology on the replication conditions. Adapted from [61].

Figure 3 .

 3 Figure 3.24 summarizes results of the experiments. Hydrolysis at pH 3.8 is significantlyfaster compared to the one at pH 6.2 so seven hours of pre-hydrolysis in presence of 0.1 mM tartaric acid is enough to obtain sufficient amount of hydrolyzed precursor for the condensation stage.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 143 Referencespage 165

Figure 3 .

 3 Figure 3.24 -TEM images of silica fibers obtained by SiO 2 transcription of 1-day aged 1 mM 16-2-16L-Tartaric gel.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 144 Referencespage 165

mL polypropylene centrifuge tube (Figure 3 .

 3 25A) with the diameter of 3 cm. The tube was kept horizontally during the transcription. Level of the 50 mL sample in this tube is roughly equal to the level of the 1 mL sample in 1.5 mL standard Micro Test Tube (eppendorf tube) (Figure 3.25B) used in our previous experiments (Figure 3.25C).

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 145 Referencespage 165 homogeneous distribution inside the sample. Then the tube with the sample was kept horizontally during the transcription procedure. Transcription of 0.5 mL organic gel + 0.5 mL pre-hydrolyzed TEOS in 1.5 mL Micro Test Tube was used as a control experiment. After the 15 hours of transcription samples were washed with ethanol in order to remove organic and unreacted precursor.

Figure 3 .

 3 Figure 3.25 -50 mL polypropylene centrifuge tube (A) and 1.5 mL polypropylene Micro Test Tubes (B); comparison of the tubes sizes (C)

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 146 Referencespage 165

Figure 3 . 26 -

 326 Figure 3.26 -TEM images of helical SiO 2 obtained from mixture of different volumes of 16-2-16L-tartrate gel and prehydrolyzed TEOS: 0.5 mL + 0.5 mL (A), 1 mL + 1 mL (B), 2.5 mL + 2.5 mL (C), 5 mL + 5 mL (D), 10 mL + 10 mL (E), 25 mL + 25 mL (F). Sample (A) prepared in 1.5 mL Micro Test Tube, others in 50 mL centrifuge tube.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 147 Referencespage 165

  Time of sonication was fixed to be 15 minutes (total time: 15 minutes of sonication + 15 minutes of pauses) and volume of the sample was 2 mL (15 mL sample tube was used) for all the experiment. Sample was immersed in the ice bath in order to prevent solvent evaporation. Results of sonication at 65 W (50% power) in three different solvents are shown at Figure 3.27.

Figure 3 . 27 -

 327 Figure 3.27 -TEM images of helical SiO 2 cut by ultrasonication at 65 W in water (H 2 O), ethanol (EtOH) and trifluoroethanol (TFE)

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 148 Referencespage 165

Figure 3 . 28 -

 328 Figure 3.28 -TEM images of helical SiO 2 cut by ultrasonication in the EtOH/TFE (1:1 v/v) mixture at 26 W (A-C) and 65 W (D-F).

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 150 Referencespage 165

Figure 3 . 29 - 65 W for 15 minutes

 3296515 Figure 3.29 -TEM images of helical SiO 2 cut by ultrasonication in the EtOH/isopropanol (1:1 v/v) mixture at 65 W for 15 minutes

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 151 Referencespage 165

Figure 3 . 30 -

 330 Figure 3.30 -TEM images of helical SiO 2 cut by ultrasonication in the EtOH/isopropanol (1:1 v/v) mixture during 5 minutes (1 second sonication + 1 second pause; total time 10 minutes) at 65 W (A, D), 97 W (B, E) and 130 W (C, F).

Chapter 3 :

 3 .31). It is clearly seen by the comparison of the low magnification images (Figure 3.29A and Figure 3.31B). Silicon oxide helical structures based on organic template 152 Referencespage 165

Figure 3 . 31 -

 331 Figure 3.31 -TEM images of helical SiO 2 cut by ultrasonication in the EtOH/isopropanol (1:1 v/v) mixture at 65 W for 15 minutes. Volume of the sample is 10 mL.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 153 Referencespage 165

Figure 3 . 32 -

 332 Figure 3.32 -Optical microscope (OM) images of SiO 2 structures obtained at different concentrations of ethanolic TEOS solutions: 250 mM (A) and 500mM (B)

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 154 Referencespage 165 pre-hydrolysed for 15 hours in water and transcription reaction was carried out for 24 hours. Sample was washed for several times with EtOH and MilliQ water.

Figure 3 . 33 -

 333 Figure 3.33 -OM images of SiO 2 structures obtained at different concentrations of aqueous TEOS emulsions: 100 mM (A), 250 mM (B) and 500 mM (C). Arrows are indicating helical structures.

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 155 Referencespage 165 no helices were observed 10 minutes after the sample preparation independently of the precursor concentration.

Figure 3 . 34 -

 334 Figure 3.34 -OM images of SiO 2 helical structures obtained after 1 hour of transcription at different Na 2 SiO 3 concentrations: 50 mM (A), 100 mM (B), 200 mM (C), 500 mM (D).

Chapter 3 :

 3 Silicon oxide helical structures based on organic template 156

1: 3

 3 and 1:4 (pH = 4.5). The concentration of metasilicate was fixed at 200 mM for all the samples, and reaction time at 1 hour. Stoichiometric ratio of 1 to 2 gave the best transcription conditions, whereas an excess of acid, and a lower pH resulted in slower condensation kinetics and led to a higher number of fibers with distorted morphology (Figure 3.35).

Figure 3 . 35 -

 335 Figure 3.35 -OM images of SiO 2 helical structures obtained at different Na 2 SiO 3 /acetic acid ratio: 1:2 (A), 1:3 (B), 1:4 (C)

Figure 3 . 36 -

 336 Figure 3.36 -SEM images of SiO 2 helical structures obtained by silica deposition on organic template

Figure 3 . 37 -

 337 Figure 3.37 -TEM images of SiO 2 helical structures obtained by mineralization of organic template

Figure 3 . 38 -

 338 Figure 3.38 -TEM images of SiO 2 helical structures with the directions of composing ribbons

TiO 2

 2 nanomaterials have been synthesized with the sol-gel method from hydrolysis of a titanium precursor. This process normally proceeds via an acid-catalyzed hydrolysis step of titanium (IV) alkoxide, for example Titanium (IV) butoxide (Figure 4.1A), followed by condensation. The development of Ti-O-Ti chains is favored with low content of water, low hydrolysis rates, and excess titanium alkoxide in the reaction mixture. Three-dimensional polymeric skeletons with close packing result from the development of Ti-O-Ti chains. With high hydrolysis rates for a medium amount of water, the formation of Ti(OH) 4 is favored. The presence of a large quantity of Ti-OH and insufficient development of three-dimensional polymeric skeletons lead to loosely packed firstorder particles. In the presence of a large excess of water polymeric Ti-O-Ti chains are developed.

Figure 4 . 1 -

 41 Figure 4.1 -TiO 2 precursors: Titanium(IV) butoxide (A), Titanium diisopropoxide bis(acetylacetonate) (B), Titanium(IV) bis(ammonium lactato)dihydroxide (C)

  of trans-(1R,2R)and trans-(1S,2S)-1,2-diamino-cyclohexane derivatives containing PF 6 -and ClO 4 -as counterions in ethanol. The corresponding titanium oxide structures (Figure 4.2) were obtained by sol-gel polymerization of titanium isopropoxide in water/ethanol mixture under ice cooled condition with NH 4 OH playing a role of catalyzer. The organic template was then removed by calcination at 450 °C.

Figure 4 . 2 -

 42 Figure 4.2 -Molecular structure of the organogelators (A) and the corresponding SEM images of tantalum oxide tubes (B, C) [14] and helical fibers (C, D) [15] obtained from organic template

  organic template was removed by calcination at 500 °C under air and open-ended tubular titania structures with outer diameters of about 80 nm, and 20-30 nm wall thickness were obtained as final product (Figure 4.3B).

Figure 4 . 3 -

 43 Figure 4.3 -Structure of peptidic lipid (A). STEM image of titania tubular structures after calcination and the inset picture is a SEM image showing an open-ended titania nanotube (B). Possible sol-gel transcription mechanism (C) [16].

  Fibrils were mixed with Ti-precursor compound -titanium (IV) bis (ammonium lactato) dihydroxide (TiBALDH) in an aqueous solution. Reaction mixture was slowly agitated at room temperature resulting into TiO 2 /Protein composite nanowires (Figure 4.4B).

Figure 4 . 4 -

 44 Figure 4.4 -Atomic force microscope (AFM) images of the β -lactoglobulin amyloid fibrils showing long linear semi-flexible structures with polydisperse contour lengths (A). AFM image of titanium dioxide decorating the surface of the amyloid fibrils. The inset of the figure describes the height variation prior and after TiO 2 hybrid nanowire formation (B) [18].

Figure 4 . 5 -

 45 Figure 4.5 -Microscopy and schematic drawings of the as-prepared organic lipid-TiO 2 hybrid fibers [19]. SEM images of the right-handed double-helices (A, B); TEM image that shows a tube in the center of the fiber (C); Schematic drawing of a structural model of the chiral TiO 2 tubular fiber (D).

Figure 4 . 6 -

 46 Figure 4.6 -TEM (A) and SEM (B and C) images of hollow anatase titania spheres prepared by calcination of composite particles consisting of a PS core coated with seven layer pairs of TiBALDH and PDADMAC at 450°C (A and B) or 950°C (C) [20]. TEM images of PS spheres coated with seven PAH/titanium-glycolate layer pairs (D), hollow capsules obtained by removal of the PS cores with THF (E), hollow TiO 2 spheres prepared by hydrothermal treatment of the hollow capsules formed at pH 4 (F) and 10 (G) at 200 °C for 24 h [23].

Figure 4 .Figure 4

 44 6D, E). Formation of the polyionic complex affected the crystallization of TiO 2 from titanium-glycolate by hydrothermal treatment. Hollow spheres of brookite and anatase were obtained by hydrothermal treatment at pH 4 and 10, respectively (Figure4.6F and G).Nanostructured titania-polyelectrolyte composite and pure anatase and rutile titania tubes were successfully prepared in the group of Ian R. Gentle by layer-by-layer (LbL) deposition of a TiBALDH and the oppositely charged poly(ethylenimine) (PEI) to form multilayer films (Figure4.7A)[24]. The tube structure was produced by deposition inside the cylindrical pores of a polycarbonate (PC) membrane template, followed by calcination at various temperatures (

Figure 4 . 7 -

 47 Figure 4.7 -The procedure used for preparing titania tubes by templating with a PC membrane (A). SEM (B) and TEM (C) images of titania nanotube obtained from (PEI/ TiBALDH) 12 PEI coated PC membrane with 0.4 µm diameter pores after heat treatment [24].

Figure 4 . 8 -

 48 Figure 4.8 -Illustration of the Protein-Enabled Layer-by-Layer Deposition of a Conformal Titania Coating on a diatom frustule (A). Secondary electron images of a complete (B) and a partial (C) frustule-derived titania structure, respectively, after 12 cycles of protamine/ TiBALDH exposure, 500°C/2 h treatment, and selective silica dissolution. The specimen in (C) after partial ion milling (D) [25]. Scale bars: 2 μm.

  2 nanospheres. Mixture was vigorously stirred at 60°C for 3 h. Resulted

Figure 4 .

 4 9B and C). Proposed mechanism of the titania coating formation shown on Figure 4.9A. First, the NH 4 + species are formed according to the eqs. 1 and 3. Then the NH 4 + species can be held by the counter-charged surface of PS spheres. Finally, with the accumulation of NH 4 + species on the surface of the PS spheres, the negatively charged ≡TiO -species are adsorbed by the NH 4 + species onto the surface of the PS spheres, and then the condensation process by the catalysis of NH 4 + species will occur on the surface of PS spheres, resulting in core-shell particles.

Figure 4 . 9 -Figure 4 . 10 -

 49410 Figure 4.9 -Scheme for depositing the titania coating onto the PS surface by the surface catalysis of ammonia (A). SEM micrograph of the calcined titania-coated PS sample containing some artificially crushed spheres (B) and TEM image of titania-coated spheres calcined at 500 °C (C). [30].

  formed a rougher coating will result. If the hydrolysis-condensation proceeds very fast, precipitation of titania would be expected.Hui Zhang et al. reported a procedure in which anatase TiO 2 was directly coated on silica modified CoFe 2 O 4 particles via hydrolysis of TALH by urea at 65°C for 4 hours and then at 97°C

Figure 4 . 11 -

 411 Figure 4.11 -SEM micrographs of the SiO 2 @TiO 2 composite microspheres prepared at different concentrations of hydroxypropyl cellulose: 0.32 (A), 0.96 (B), 3.2 (C), and 6.4 (D) mmol•L -1 and calcined at 700°C for 2 h, respectively [38].

Chapter 4 :

 4 Core-Shell nanohelices 184 Referencespage 227

Figure 4 .

 4 Figure 4.12 -A scheme illustrates the synthetic approach for the preparation of SiO 2 @TiO 2 core-shell particles (A). SiO 2 @TiO 2 core@shell microspheres before calcination with titania shell thickness of 35 nm (B); 61 nm (C) and 73 nm (D) [40].

Figure 4 . 13 -

 413 Figure 4.13 -Schematic representation of synthesis of ZnO shell on silicon nanowire arrays [49].

Figure 4 . 14 -

 414 Figure 4.14 -Schematic diagram of the assembly of SiO 2 @ZnO core@shell microspheres (A). A TEM image of SiO 2 @ZnO core-shell microspheres formed by coating an SiO 2 core with a layer of ZnO nanoparticles (B) [57].

Figure 4 . 15 -

 415 Figure 4.15 -TEM images of Fe 3 O 4 cores (A) and Fe 3 O 4 @ZnO core-shell nanoparticles (B), the inset shows the higher resolution images [58].

Figure 4 . 16 -

 416 Figure 4.16 -TEM images of structures obtained from TiO 2 deposition on organic nanohelices from Titanium(IV) butoxide (TBOT) precursor

Figure 4 . 17 -

 417 Figure 4.17 -TEM images of 16-2-16L-Tartrate gel/TiBALDH mixture formed without special mixing procedures (A), by stirring (B), vortex mixing (C) and sonication (D)

Figure 4 . 18 -

 418 Figure 4.18 -Nanostructures obtained by reaction on TiBALDH and ethylenediamine in presence of organic template

Figure 4 . 19 -

 419 Figure 4.19 -Nanostructures obtained by TiO 2 deposition on organic template from TiBALDH hydrolyzed with NaOH.

Figure 4 . 20 -

 420 Figure 4.20 -TiO 2 Nanostructures obtained by TDAA polycondensation on organic template. TDAA concentration 10 mM, transcription time -4 hours. Pre-hydrolysis time: 0 min (A), 5 min (B), 15 min (C), 60 min (D)

Figure 4 . 21 -

 421 Figure 4.21 -TiO 2 Nanostructures obtained by TDAA polycondensation on organic template (1 mM). Prehydrolysis time -15 min, transcription time -4 hours. TDAA concentrations: 2.5 mM (A), 5 mM (B), 7.5 mM (C), 10 mM (D)

Figure 4 . 22 -

 422 Figure 4.22 -TEM images of TiO 2 structures obtained by TDAA polycondensation on organic template sonicated for 1 minute after the transcription. Arrows show pieces of TiO 2 fibers.

  for the LbL deposition are too big compared to the dimensions of the used silica helices. This is why we prepared small electrolyte molecule by reaction of 1 molar aqueous solution of tris(2-aminoethyl)amine with 3 equivalents of HCl resulting in the formation of the corresponding salt (Figure4.23). The pH of the mixture was checked to be around 6.

Figure 4 . 23 -

 423 Figure 4.23 -Reaction of tris (2-aminoethyl) amine with hydrochloric acid

Chapter 4 :

 4 Core-Shell nanohelices 198 Referencespage 227

Figure 4 . 24 -

 424 Figure 4.24 -Silica helices with TiO 2 precursor electrostatically adsorbed on the surface. TiBALDH concentration 100 mM, 1 cycle (A, D), 10 mM, 1 cycle (B, E), 1 mM, 3 cycles (C, F).

Figure 4 . 25 -

 425 Figure 4.25 -TEM images of SiO 2 /TiO 2 composite nanostructures obtained at different concentrations of TBOT. Arrows show inhomogeneity of TiO 2 deposition.

Chapter 4 :

 4 Core-Shell nanohelices 201 Referencespage 227

Figure 4 . 26 -

 426 Figure 4.26 -TEM images of samples obtained by hydrolysis of TiBALDH in presence of urea at 95°C. TiBALDH concentration 5 mM (A) and 2 mM (B)

  The final pH of the sample was 9.2 (2 mM precursor), 8.8 (5 mM precursor) and 8.3 (10 mM precursor). Mixture was homogenized with sonication for 5 minutes. After 24 hours of reaction the samples were washed several times with MilliQ water. Results are shown in Figure4.27.

Figure 4 . 27 -

 427 Figure 4.27 -SiO 2 @TiO 2 core-shells obtained by Tris(2-aminoethyl)amine directed deposition of TiBALDH

  titanium oxide precursor was varied from 1 to 100 mM. The reaction mixture was kept at room temperature for 24 hours and then washed with MilliQ water to remove unreacted precursor.

Figure 4 . 28 -

 428 Figure 4.28 -TiO 2 deposited from TiBALDH on amine modified silica helices in aqueous medium (A-C) and in mixed Ethanol/Water solvent (D and E). Amine functionalized silica helices.

Figure 4 . 29 -

 429 Figure 4.29 -TEM images of TiO 2 deposited on amine modified silica helices from 100 mM (A, B) and 10 mM solution (C) of TDAA.

Figure 4 . 30 -

 430 Figure 4.30 -TEM images of TiO 2 deposited on amine modified silica helices from 10 mM solution of TDAA. Solvent composition: EtOH+H 2 O (1:1 v/v) (A), EtOH+H 2 O (3:1 v/v) (B)

Figure 4 . 31 -

 431 Figure 4.31 -TEM images of SiO 2 @TiO 2 nanohelices obtained by reaction of TDAA with silica helices previously modified with different concentrations of APTES: 0.5 mM (A), 1 mM (B), 2 mM (C) and 5 mM (D)

  in ethanol (or ethanol/isopropanol mixture (1 mg/mL) were mixed with 100 µL of 10 mM ethanolic solution of TDAA by 5 minutes sonication. Then 200 µL of water was added and the mixture was sonicated for another 10 minutes. After 15 hours of reaction, individualized sample was washed 5 times by centrifugation and subsequent redispersing in ethanol.

Figure 4 . 32 -

 432 Figure 4.32 -TEM images of SiO 2 @TiO 2 nanohelices obtained by Cutting→Amine modification→TiO 2 deposition (A and C) and by Amine modification→Cutting→TiO 2 deposition protocols (B and D)

Figure 4 .

 4 Figure 4.33 -X-Ray spectra of the core-shell SiO 2 /TiO 2 helices calcined at 450°C for 3 hours in air

Chapter 4 :

 4 Core-Shell nanohelices 212 Referencespage 227

Figure 4 . 34 -

 434 Figure 4.34 -TEM images of ZnO formed in the presence of silica helices via hydrolysis by urea at 95 °C. Zinc acetate concentration: 10 mM (A, D), 5 mM (B, E), 2 mM (C, F).

Figure 4 . 35 -

 435 Figure 4.35 -TEM images of ZnO formed in presence of silica helices via hydrolysis by urea at 80°C (A, B) and 60°C (C, D). Zinc acetate concentrations: 5 mM (A, C), 2 mM (B, D).

Figure 4 . 36 -

 436 Figure 4.36 -TEM images of ZnO formed in the presence of silica helices via hydrolysis by urea at 95 °C (A, B) and 60 °C (C). Zinc acetate concentration: 5 mM. Reaction time -3 hours.

  Lithium hydroxide and tetramethylammonium hydroxide (TMAH) -100 µL of 100 mM solution in methanol (precursor/base ratio = 1:2) were used for the hydrolysis of the zinc precursor. 200 µL of silica helices dispersed in EtOH were added to the above solutions and the volume of the samples was adjusted to 1 mL with EtOH. Then mixture was immersed into a preheated to 80 °C oil bath and stirred at this temperature for 3 hours. After the end of the reaction samples were thoroughly washed with absolute ethanol.With LiOH only small nanoparticles are formed after 3 hours of reaction (Figure 4.37A), whereas bigger aggregates are observed with TMAH (Figure 4.37B). Similar results to the one of TMAH were obtained after longer reaction time with LiOH (Figure 4.37C). It have to be mentioned that big amount of ZnO nanoparticles was formed in a bulk solution, especially in the case where LiOH was used as a base (Figure 4.37A). Referencespage 227

Figure 4 . 37 -

 437 Figure 4.37 -TEM images of ZnO formed from Zn(OAc) 2 -EA complex in the presence of LiOH (A) and TMAH (B) at 80 °C. Precursor/base ratio = 1:2, reaction time -3 hours. With LiOH during 15 hours (C).

Figure 4 . 38 -

 438 Figure 4.38 -TEM images of ZnO deposited from Zn(OAc) 2 -EA complex in presence of LiOH at 70 °C (A, B) and 60 °C (C, D). Precursor to base ratio is 1:2 (A, C) and 1:4 (B, D).

Chapter 4 :

 4 Core-Shell nanohelices 217 Referencespage 227

Figure 4 . 39 -

 439 Figure 4.39 -TEM images of ZnO deposited from Zn(OAc) 2 -EA complex hydrolyzed by LiOH at 80 °C. Substratebare silica helices (A-B); Thiol modified silica helices (C-D); Amine modified silica helices (E-F).

Figure 4

 4

Figure 4 . 40 -

 440 Figure 4.40 -Amines used in this work as complexing agents for Zinc acetate

Figure 4

 4

Figure 4 . 41 -

 441 Figure 4.41 -TEM images of ZnO deposited on SiO 2 -SH helices in presence of LiOH during 15 hours at 80 °C. Stabilizers: Ethanolamine (A), Triethylenetetramine (B), Triethanolamine (C).

Figure 4 . 42 -

 442 Figure 4.42 -TEM images of ZnO deposited on SiO 2 -SH helices in presence of LiOH at 80 °C during 5 hours.

Chapter 4 :

 4 Core-Shell nanohelices 221 Referencespage 227 ZnO shell growth whereas usage of stabilizers allows tuning the kinetics of the deposition process. Comments LiOH is preferred over NaOH (silica dissolves upon the heating) and TMAH (very fast reaction). ZnO formation speeds up with the increase of the reaction temperature and concentration of the base. Triethylenetetramine and ethanolamine are more preferable complexing agents than triethanolamine and tris(2aminoethyl) amine since the last two form very stable complexes with zinc acetate which barely decomposes upon heating. Surface functionalization allows formation of the ZnO seeds layer and so directing the ZnO shell formation through the complexation (aminosilane) or covalent bond formation (mercaptosilane) with the precursor.

Figure 4 . 43 -

 443 Figure 4.43 -SEM images of the core-shell SiO 2 @TiO 2 nanohelices fabricated by 60 cycles of ALD (A and B). Target TiO 2 shell thickness is 5 nm. TEM images of SiO 2 helices before (C) and after 60 cycles of ALD (D and E)

Chapter 4 :

 4 Core-Shell nanohelices 225 Referencespage 227

Chapter 4 :

 4 Core-Shell nanohelices 226

Figure 5 . 1 .

 51 Surface micromachining (Figure5.1, left) involves deposition, photo-lithographic patterning, and selective etching of multilayered thin films to form mechanical structures. A sacrificial oxide layer is first deposited as a supporting platform for subsequent layers, and then removed by an etching process to release a freestanding device. Polycrystalline silicon is typically employed as the structural material, silicon dioxide as the sacrificial material, and silicon nitride as an electrical isolation layer. Removal of the sacrificial layer can be achieved by means of both wet (immersion of the device in a highly reactive acid or base solution) and dry (exposure of the device to reactive gases) etching methods. Plasma or Reactive Ion Etching (RIE) is also a very common etching method, employing Radio Frequency energy to drive chemical reactions through reactions with a surrounding plasma. The bulk micromachining process (Figure5.1, right) involves selective etching of the substrate to form combinations of holes, membranes, and trenches to obtain mechanical structures. Etching methods that are employed allow the structures to be fabricated alongside the electronic components.

Figure 5 . 1 -

 51 Figure 5.1 -Schematic of surface (left) and bulk (right) micromachining processes [7].

  Metal based MEMS provide an attractive alternative to silicon, particularly when high mechanical strength, low power consumption and/or low electrical resistance for contact switches are required. They are usually surface micromachined onto either silicon or glass substrates. One of the most common applications for metal-based MEMS is the radio frequency MEMS switch (RF MEMS) -electrical component that have moving parts to provide radio frequency functionality for interrupting the electrical current flow in the circuits. Their wide bandwidth range, high speed switching, inexpensive fabrication, low signal loss, low power consumption, low on-state resistance (1-2 Ohm), and high off-state resistance (infinite), compare favorably with existing semiconductor Chapter 5: Towards NEMS 239 Referencespage 253 technology devices. RF MEMS switching devices are used in the area of wireless RF communications, tuners and amplifiers. Metals such as gold, ruthenium, platinum, nickel, and their alloys are commonly used as switch contact materials given their low electrical resistivity, low chemical reactivity and good wear resistance [7].

Figure 5 .

 5 Figure 5.2A shows suspended Single Wall Carbon Nanotube (SWNT) structures which were fabricated by a wet etching process. Carbon nanotubes are first dispersed or deposited on the substrate. The metal anchor structures are defined by electron beam evaporation on top of the SWNTs after an electron-beam or photo-lithography process. Then the chip is immersed in a buffered wet etch (typically HF). Critical point drying is used to avoid the CNT being pulled down due to surface tension effects during the drying process [2]. G.T. Kim et al. introduced a simple method to suspend CNTs and other nanowires without using wet etching (Figure 5.2B). One-dimensional nanostructures are separated in advance from the substrate by a poly(methylmethacrylate) (PMMA) sacrificial layer. An additional PMMA layer is spin-coated after deposition of SWNTs, resulting in the SWNTs being sandwiched by the two PMMA layers. The anchor structures are patterned by e-beam lithography and the developed

Figure 5 . 2 -

 52 Figure 5.2 -Schemes of suspended CNT structures fabrication by wet etching (A), with sacrificial poly(methylmethacrylate) layer (B) and by combination of the ac dielectrophoresis technique and electron beam lithography (C) [2].

Figure 5 . 3 -

 53 Figure 5.3 -Schematic representation of the AFM bending test for mechanical measurements of a suspended NT or NH [19].

Chapter 5 :

 5 Towards NEMS 244

Figure 5 .

 5 4 give an example of SiO 2 nanotubes (NTs) and nanohelices (NHs) with outer and inner diameters of 35±4 and 10±4 nm respectively.

Figure 5 .

 5 Figure 5.4 a) AFM image of an isolated SiO 2 NT on the Si substrate. (Inset) its corresponding cross section giving the outer diameter about 35 nm; b) AFM image of an isolated SiO 2 nanospring on the Si substrate. (Inset) its corresponding cross section giving the outer diameter about 35 nm

Chapter 5 :

 5 photolithography and KOH etching. The width of the cavities ranges from 0.8 to 1.75 μm with a depth from 0.5 to 1.5 μm. The samples were then dried at ambient temperature.The suspended NTs and NHs were first located and imaged by SEM (Hitachi S-4800 high resolution SEM). The chosen ones were clamped by the Ion Beam-Induced Deposition technique using Pt pads. AFM images of clamped structures were recorded in tapping mode (Figure5.5)[23].

Figure 5 . 5 -

 55 Figure 5.5 -Tapping mode AFM image of a SiO 2 NT suspended above a microcavity of a Si substrate and clamped at its extremities (A). With the numbers are shown: (1) Si substrate; (2) middle point of the suspended part of the NT; (3) part of the NT in the contact with the substrate. SEM image of the same NT (B) [23].

Figure 5 . 6 -

 56 Figure 5.6 -Typical F-d curves of a doubly clamped NH (A) and a doubly clamped NT (B) [19].

Figure 5 . 7 -

 57 Figure 5.7 -AFM image of a NT before bending test (A), AFM image of the same NT broken after bending test (B). Same for NH (C, D)

Figure 5 . 8 -

 58 Figure 5.8 -Stiffness of NTs and NHs as a function of the suspended length [19].

Figure 5 . 9 -

 59 Figure 5.9 -Schematic diagram of a force-deflection (F-d) curve (linear part) adapted from [25].

Figure 5 . 10 -

 510 Figure 5.10 -AFM (A) and SEM (B) images of a suspended NT with non-ideal clamping conditions [23].

Figure 5 .Table 5 . 2 -

 552 5 (i.e., 31 nm). This discrepancy can be explained by a partial loss of the mechanical energy likely due to stretching in the portion of the NT positioned on the substrate before the clamped ends (lengths C and D on Figure 5.10). This gives rise to a smaller deflection of the suspended part of the NTs for an identical applied load and therefore leads to an over-estimation of the elastic modulus E. Summary of NT bendings calculated from FEM and their corresponding elastic modulus compared to those extracted from F-d curves obtained with a normal force of 10 nN applied at the midpoint of a NT with a length of 1.5 µm with different clamping conditions These results demonstrate the importance of the clamping conditions on the bending test measurements and their direct impact on the extraction of the elastic properties of 1D nanostructures. Under conditions where the total length of the NT is equal to its suspended part, provided the extremities of the NT are clamped, a good estimation of the elastic modulus E can be obtained using the beam bending theory of clamped-clamped beam. Otherwise, as shown by FEM calculations, the elastic modulus is always overestimated. The extracted value of E was then used for numerical finite element calculations and the resonance frequency of the NTs and the helical NHs was determined. Calculated value of resonance frequency for the NTs is 173 MHz. The values were obtained for the NHs with the different helix pitch and are summarized in the Table 5.3.

Figure 5 . 11 -

 511 Figure 5.11 -Four probe STM measurement of the resistivity of the composite nanofibers

  Titanium oxide deposition on organic template .................................................... 2.2. Layer-by-layer (LbL) deposition of TiO 2 on SiO 2 template ................................... 2.3. SiO 2 @TiO 2 core@shell from TBOT ...................................................................... 2.4. SiO 2 @TiO 2 core@shell from TiBALDH ................................................................ 2.5. SiO 2 @TiO 2 core@shell from TDAA ...................................................................... 3. Fabrication of SiO 2 @ZnO core-shell nanohelices ............................................................ 3.1. Slow base release approach .................................................................................. 3.2. Slow release of precursor. ..................................................................................... 3.3. Effect of substrate surface modification. Comparison of the stabilizers. ..............

	-Outline -
	2.1.

  The samples aged both at 20° C and 30 °C exhibit helices formation already after 1 hour. But if at 20 °C the helices are more individualized, at 30 °C they substantially tend to aggregate. For the sample aged at 1 °C the kinetics is much slower and after 1 hour only small number of helical ribbons was observed. A similar tendency was observed after one week of aging. The helices in the sample aged at 20 °C became longer and bigger in diameter upon the aging. The sample aged at 30

	20°C; 1 day	30°C; 1 day	1°C; 1 day
	A	B	C
	50 µm	50 µm	50 µm
	20°C; 7 days	30°C; 7 days	1°C; 7 days
	D	E	F
	50 µm	50 µm	50 µm

Chapter 2: Micrometric helical structures based on cationic gemini surfactant with guanosine monophosphate counteranion.

80

Referencespage 115

Table 2 .1 -Morphology of 12-2-12X + GMP-Na 2 self-assembly as a function GMP/gemini ratio Anion Fibers with no chirality Helices Unstructured precipitate

 2 Figure 2.40 is a graphic representation of the data from the Table 2.1.

	Iodide	≤ 3.6	4.0	-
	Bromide	≤ 1.6	2.0 -3.2	≥ 3.6
	Nitrate	≤ 2.0	2.4	≥ 2.8
	Chloride	≤ 2.0	2.4 -2.8	≥ 3.2

Table 3 .1 -Some physical properties of solvents at 25°C [68-70]

 3

	Solvent	Molecular weight,	Density, g/ml	Viscosity, cP	Surface tension,
		g/mol			dynes/cm
	Water	18.02	0.999	0.89	72.7
	Ethanol	46.07	0.787	1.07	22.0
	Trifluoroethanol	100.04	1.390	1.73	21.1(20°C)
	2-Propanol	60.10	0.783	2.04	23.3
	From table 3.1 it can be seen that water has lower viscosity and much higher surface tension
	compared to ethanol and trifluoroethanol. While these two alcohols have similar surface tension,
	they have significantly different viscosities. Results suggest that both low surface tension and high
	viscosity of the solvent are required for the successful cutting of silica helices without destroying of
	their morphology.
	Since surface tension of ethanol and trifluoroethanol are similar we decided to perform
	ultrasonication in 1:1 v/v EtOH/TFE mixture. Results obtained at two different output powers are
	shown in Figure 3.28.

Chapter 3: Silicon oxide helical structures based on organic template 149 Referencespage 165

Table 4 .1 -Summary on the TiO 2 and SiO 2 @TiO 2 core-shell nanohelices

 4

	Template	Solvent	Precursor	Hydrolyzing agent	Results and comments
	Organic helices	H 2 O	TBOT in EtOH	H 2 O	No helices. Organic template dissolved in EtOH
	Organic helices	H 2 O	TiBALDH in H 2 O	ethylene-diamine +H 2 O	Helices. Fragile. sonication	Destroyed	by
	Organic helices	H 2 O	TDAA in EtOH/H 2 O	H 2 O	Helices. Fragile. sonication	Destroyed	by
					Smooth TiO 2 shell and
					some	TiO 2	formed
	Silica helices	EtOH	TBOT	H 2 O	between the helices. It is difficult to control
					water content with water-
					born silica
					Very	thin	layer	of
			LbL deposition:	No	TiBALDH		after	3
					deposition cycles.
	Silica helices	H 2 O	TiBALDH +	TiO 2 is formed	Time consuming. Have an issue with the
			oligoamine	during the	washing out of the precursor
			hydrochloride	calcination	due to the complex shape of
					the template
				Ammonia	No helices.
				produced	Silica	dissolved	upon
	Silica helices	H 2 O	TiBALDH	from urea	heating in presence of
				upon the	ammonia.
				heating

Table 4 .2 -Summary on the SiO 2 @ZnO core-shell nanohelices Slow base release Slow precursor release

 4

	Precursor	Zinc acetate	Zinc acetate/amine complex
	Base	Ammonia produced by the thermal hydrolysis of urea	LiOH (NaOH, TMAH)
	Solvent	H 2 O	EtOH/MeOH
	Temperature 95 °C (80 °C, 60 °C)	Reflux at 80 °C (70 °C, 60 °C)
	Results	Silica helices dissolved upon heating	SiO 2 @ZnO core-shell nanohelices were
		with ammonia before the ZnO shell is	synthesized from the aminosilane and
		formed under all the experimental	mercaptosilane functionalized silica
		conditions.	helices with triethylenetetramine and
			ethanolamine as stabilizers and LiOH
			as a base.
		Effect of the precursor concentration,	This process is preferred over the slow
		reaction temperature and time on the	base release approach.
		formation of ZnO was studied.	Silica	surface	functionalization
			provides required directionality to the

Table 5 .1 -Summary of NT bendings calculated from FEM and from AFM measurements with a normal force of 10 nN applied at the midpoint of the NT.

 5

	Tube length (µm)	Bending Δz (nm)	Bending Δz (nm)
		from FEM	from F-d curves
	1	9	11±1
	1.2	15	17±2
	1.4	24	26±2

Table 5 .3 -Frequency of the first resonance mode as a function of a pitch 3 calculated using finite element method.

 5

	Pitch (nm)	10	25	55	105
	Frequency Helix (MHz)	4	28	74	112

Referencespage 26

Referencespage 58

Referencespage 165

Materials and methods

Materials

Guanosine monophosphate disodium salt was purchased from Acros Organics. Other chemicals were purchased from Sigma Aldrich and used without further purification.

Ultrapure water (18.2 MΩ cm) was obtained from Purelab ultra ELGA system.

Balance Sartorius BP211D with an accuracy of 0.01 mg was used for the weighting.

Gilson Pipetman micropipets were used to measure and move small amounts of liquid reagents.

Synthesis of organic micrometric helical structures

12-2-12Br was synthesized accordingly to the protocol described in Chapter 2. Aqueous solution of amphiphile was prepared by dissolution of the powder in MilliQ water at 50°C. 100μL of 20mM 12-2-12Br was mixed with 100μL of 60mM GMP-Na 2 solution. The pH of the mixture was adjusted to 5.9 by 1M acetic acid. Sample was heated at 70°C for 5 minutes for dissolution, than cooled down and stored 20°C in Thermostatic cabinet for aging. Samples with aging time of 3 days were used for silica transcription.

Synthesis of 16-2-16Br

16-2-16Br was synthesized according to the procedure developed by R. Zana et al., by alkylation of tetramethylethylenediamine (TMEDA) with 1-bromohexadecane in acetonitrile.

In typical synthesis 1 mL of TMEDA (1 eq.) was mixed with 16.5 mL of 1bromohexadecane (3 eq.) in 50mL of acetonitrile in 250 mL round-bottom flask. Alkyl bromide was taken in excess in order to eliminate formation of monoalkylated byproduct. Reaction was carried out for 48 hours under the reflux at 85°C with continuous intensive stirring at 400rpm. Then reaction Product was filtered and purified by crystallization from acetonitrile in a follow manner.

Powder was deposited in a 250 mL round bottom flask together with small quantity of acetonitrile and heated to 70°C under continuous stirring. Acetonitrile was added with small portions until complete dissolution of Gemini Bromide. Then stirring and heating was stopped and solution was let to cool down to room temperature. Small white needle-like crystals of 16-2-16Br were formed within one hour. They were filtered, rinsed with acetone and dried under the vacuum.

Absence of start material and monoalkylated byproduct in final product was proved by NMR analysis.

Typical example of 16-2-16Br

Transmission electron microscope (TEM)

400-mesh carbon-coated copper grids were used for TEM observations. Organic samples were prepared by negative staining technique. 5 µL of sample were drop casted on the carbon grid. After 5 minutes solvent was blotted with filter paper. 5 µL of 0.2 wt% uranyl acetate aqueous solution were deposited on the grid for the staining and removed with filter paper after 1 minute. Sample was allowed to dry on air.

Due to the high contrast of silicon on the carbon coated cupper grids, no metallization or staining was needed to observe inorganic samples.

-General conclusion -

In this work we focused on the fabrication of composite materials for NEMS applications. A bottom-up approach based on the organic self-assembly of small molecules allowed achieving a level of miniaturization of the structures with a complex morphology typically unreachable with conventional top-down fabrication techniques. The combination of bottom-up approach with sol-gel chemistry allows us synthesizing functional composite nanomaterials with a well-controlled morphology and specific electro-mechanical properties.

In this work, organic self-assembly was used for the synthesis of the helical fibers. Helical shape was chosen for the functional element due to the unique characteristic physical properties of such a morphology. These organic structures were coated with a thin silica layer in order to achieve the required robustness of the system. Finally a second mineralization with TiO 2 as well as ZnO was applied in order to provide the necessary electrical properties to the material.

In the first Chapter of the work we made a bibliographic study on the chiral organic selfassembly of the small molecules in aqueous medium. We made an introduction into the basics of the process, discussed recent examples of the chiral self-assembly as well as characterization techniques used for the study of such systems. Most of the chiral assemblies in water are formed by the amphiphilic molecules contain chiral units through the combination of the hydrophobic effect with the directional interactions such as hydrogen bonding or π-π stacking. The chemical structure of the amphiphile and its concentration, pH, temperature, ionic strength, solvent composition as well as presence of additives were shown to be the main factors affecting the morphology of the chiral assemblies. The morphology of the aggregates can also be changed by external stimuli (for example by UV irradiation) or evolve upon the aging of the gel.

In the Chapter 2 we introduced new self-assembly system based on non-chiral cationic gemini amphiphile complexed with the chiral Guanosine 5'-monophosphate anion (12-2-12GMP).

We found that this system can self-assembly into the chiral fibers in presence of bromide (as well as nitrate or chloride) anion. These micrometric fibers are made of smaller ribbons with a diameter of around 500 nm. These fibers themselves consist of smaller fibers with diameters of 20-70nm. Krafft temperature and pKa value of the anion affect the optimal GMP/gemini ratio at which helices are formed. The pH of the system determines the protonation state of the Guanosine 5'-monophosphate

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In this work we showed the possibility to obtain nanomaterials with well controlled morphology and required mechanical and (in progress) electrical properties.

Results of the measurements of the mechanical characteristics of synthesized nanohelices confirm that these 3D structures exhibit higher level of flexibility and extended linear regime compared to the traditional 1D nanomaterials such as nanowires or nanotubes. These unique properties of the nanosprings open new perspectives in the field of flexible electronics due to its importance for the industry [1][2][3].

Here we focused on the homogeneous growth of the inorganic layer. However, recent studies on the Janus structures [4][5][6][7][8][9] showed the possibility to modify only one side of the structure or deposit two different types of materials on opposite sides. Application of this approach to the helical nanostructures is the way to provide them with new functionalities. For example, it was shown that Janus particles with one metallic hemisphere and one dielectric hemisphere self-propel in lowfrequency alternating current (AC) electric field [10]. In this way swimming Nanopropellers [11] can be synthesized from the Janus nanohelices. Recently nanohelices metalized on the one side were synthesized in our consortium through the Ion Beam Induced deposition of platinum. They were positioned onto metallic pads that allowed their electrical characterization with a 4-probe STM located at IEMN in Lille, showing a piezoresistive behavior. These first results are proof of the proposed concept. Moreover, positioning the chemically different components directly opposite each other can provide amphiphile properties to the material allowing additional possibilities for self-assembly [12,13].

It was shown that via the co-assembly of amphiphile molecules different functions can be presented on the surface of the aggregate [14]. Incorporation of, for example, alkylamine to the structure of the self-assembly of gemini tartrate can give rise to new types of composite materials. Such co-assembly can be used for the synthesis of gold-nanoparticles or zinc oxide shell directly on the surface of helical aggregates.

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Single-and double-walled magnetic nanotubes were reported from the co-self-assembly of chiral amphiphiles N-dodecanoyl-L-serine and Fe 3 O 4 nanoparticles in toluene. Chiral magnetic nanostructures, such as helical coils, were obtained by subsequent calcination [15]. Similar materials can be obtained by the decoration of the silica nanohelices with the magnetic nanoparticles. Results obtained in our group on the immobilization of gold nanoparticles on the silica helices [16] can be extended to iron oxide nanoparicles for this proposal.

Recently Micro bimorph coils that can function as powerful torsional muscles driven by a metal-insulator phase transition in VO 2 were reported [17]. Since vanadium oxide can be obtained via sol-gel route [18], bulk synthesis of such micro-/nanomuscles can be performed via the method proposed in this thesis.

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