Keywords: Électrochimie bipolaire, Asymétrie, Particules chirales, Particules Janus, Bipolar electrochemistry, Asymmetry, Chiral particles, Janus particles, Electrodeposition

Bipolar Electrochemistry for Generating Complex Asymmetric Objects at Different Length Scales

Bipolar electrochemistry is a technique used for the asymmetric modification of conducting particles. When placing a conducting object in an electric field produced by a pair of feeder electrodes, the potential distribution along the object polarizes it. If the potential difference between the extremities of the object is large enough, electrochemical reactions can occur at its surface and the symmetry of the system is broken. This symmetry breaking electrochemical technique has already been used for the deposition of patches or deposits of various materials on the extremities of of spherical conductive particles, modifying them into Janus particles. Bipolar electrochemistry also has different analytical applications.

The main goal of this thesis is the development of new bipolar electrochemical techniques for the synthesis of complex asymmetric and chiral objects from isotropic objects. First, techniques that involve asymmetric AC electric fields were developed. By using these techniques, it is possible to deposit two patches of the same metal, but of different sizes at both extremities of spherical conductive particles in a single experimental step. Then, a new bipolar set-up was developed in order to be able to iteratively deposit multiple patches with different compositions at any position on the surface of isotropic objects. This new set-up permits the bulk synthesis of chiral objects, similar to an asymmetric carbon atom with four different substituents and with a defined handedness. Finally, preliminary results about a bipolar electrochemical approach for the modification of core-shell microparticles with an insulating core using bipolar electrochemistry are discussed.

Résumé

Électrochimie bipolaire pour la synthèse d'objets asymétriques complexes à différentes échelles de longueurs L'électrochimie bipolaire est une méthode utilisée pour la modification asymétrique de particules conductrices. Quand un objet conducteur est placé dans un champ électrique produit par une paire d'électrodes génératrices, ce dernier est polarisé par la distribution de potentiel le long de sa surface. Si la différence de potentiel entre les deux extrémités de l'objet est assez grande, des réactions électrochimiques peuvent avoir lieu à sa surface, causant une rupture de la symétrie du système. Cette technique électrochimique, fondamentalement asymétrique, a déjà été utilisée pour déposer des patchs ou dépôts de différents matériaux aux extrémités de particules conductrices sphériques, les modifiant en particules Janus. L'électrochimie bipolaire a aussi de nombreuses applications analytiques.

L'objectif principal de cette thèse est la mise au point de nouvelles méthodes d'électrochimie bipolaire pour la synthèse d'objets asymétriques et chiraux complexes à partir d'objets isotropes. Premièrement, des techniques utilisant des champs électriques alternatifs asymétriques ont été développées. Ces techniques permettent le dépôt de deux patchs de même composition mais de tailles différentes aux deux extrémités d'une particule conductrice sphérique en une seule étape. Ensuite, un nouveau protocole expérimental a été développé afin de pouvoir déposer successivement plusieurs patchs de différentes compositions à n'importe quelle position à la surface d'objets isotropes. Ce protocole rend possible la synthèse de nombreux objets chiraux, similaires à un atome de carbone avec quatre substituants différents. Enfin, des résultats préliminaires concernant une méthode permettant la modification de microparticules avec un coeur isolant par électrochimie bipolaire sont discutés.

Résumé en français

L'électrochimie bipolaire (ou « électrochimie sans fils ») est une technique électrochimique qui permet à un objet conducteur appelé électrode bipolaire de se comporter simultanément comme une cathode et comme une anode. De plus, une électrode bipolaire n'a pas besoin d'être connectée à une source de courant pour que des réactions électrochimiques se déroulent à sa surface. Cette technique permet donc d'utiliser de nombreuses électrodes bipolaires dispersées en solutions dans une seule cellule électrochimique. Il est donc possible d'utiliser cette méthode pour synthétiser des particules Janus en solution sans avoir besoin d'une interface pour briser la symétrie du système.

Les particules Janus sont des particules dont les deux hémisphères possèdent des propriétés différentes (hydrophile/hydrophobe, blanc/noir, conducteur/isolant, etc.). De nombreuses techniques physico-chimiques existent pour la synthèse de ces particules. Cependant, il existe peu de méthodes permettant la synthèse d'objets plus complexes avec plus de deux dépôts qui ont des compositions différentes. L'objectif principal de cette thèse est le développement de nouvelles techniques d'électrochimie bipolaire pour synthétiser des particules asymétriques complexes (y compris des particules chirales) à partir de particules sphériques et isotropes.

Les deux premiers chapitres de cette thèse présentent l'état de l'art dans les domaines des particules asymétriques ainsi que de l'électrochimie bipolaire. Les chapitres suivants présentent différentes nouvelles méthodes d'électrochimie bipolaire pour la synthèse d'objets asymétriques complexes.

R.1 Les particules Janus et asymétriques

Le premier chapitre de la thèse (p. 23) présente différentes méthodes de synthèse de particules Janus et asymétriques précédemment publiées ainsi que différentes applications pour ces particules.

Les particules Janus sont des particules dont les deux hémisphères possèdent des propriétés différentes (Fig. R.1a). Elles sont nommées d'après Janus, le dieu romain des transitions et des portes qui possède deux visages. Les particules asymétriques sont une catégorie plus large de particules non-isotropes qui peuvent avoir plus de deux propriétés ou compositions à leur surface.

La plupart des techniques publiées pour la synthèse de particules Janus reposent sur l'immobilisation de particules homogènes à une interface afin de pouvoir modifier seulement l'un des deux hémisphères. Parmi ces méthodes, on trouve des méthodes de dépôt en phase vapeur dans lesquels seulement une partie de la surface des particules est exposée à la vapeur et un dépôt asymétrique se forme. [START_REF] Nie | Janus and Ternary Particles Generated by Microfluidic Synthesis: Design, Synthesis, and Self-Assembly[END_REF]. (b) Schéma d'une particule Janus propulsée par la production de bulles. Adapté de [START_REF] Gao | Multi-Fuel Driven Janus Micromotors[END_REF]. (c) Double hélice auto-assemblée à partir de particules asymétriques avec trois patchs. Adapté de [START_REF] Song | Patchy Particle Packing under Electric Fields[END_REF].

Les méthodes reposant sur une interface ont généralement des rendements assez faibles car les particules sont dispersées sur une surface en deux dimensions et non dans la totalité du volume trois dimensionnel.

Il existe également des méthodes qui permettent la synthèse de particules asymétriques directement dans le volume de la solution. L'une d'entre elles est l'auto-assemblage de copolymères. Un copolymère contenant des blocs hydrophiles et des blocs hydrophobes peut s'auto-assembler spontanément en particules possédant un coeur hydrophobe et des patchs hydrophiles à sa surface. La structure finale des particules asymétriques obtenues est contrôlée en jouant sur les intersections entre le solvant et les différents blocs du copolymère.

Les particules Janus et asymétriques possèdent de nombreuses applications. De la même manière que des molécules amphiphiles peuvent s'auto-assembler pour former des structures plus complexes (micelles, bicouches lipidiques...), les particules asymétriques peuvent s'auto-assembler pour former de plus grandes structures tel que des chaines (Fig. R.1c) ou des couches deux dimensionnelles.

Les particules Janus peuvent aussi être utilisées comme micromoteurs. Une particule isotrope en suspension dans un liquide se déplace dans des directions aléatoires (mouvement brownien). Pour pouvoir diriger une particule dans une direction particulière, la symétrie du système doit être rompue afin de favoriser une direction particulière. Les particules Janus permettent cette rupture de symétrie. Par example, si l'un des hémisphères d'une particule Janus catalyse une réaction produisant un gaz (par exemple R.2. L'électrochimie bipolaire 2 H 2 O 2 --→ 2 H 2 O + O 2 ), des bulles se forment à la surface de cet hémisphère et propulsent la particule dans la direction opposée (Fig. R.1b).

R.2 L'électrochimie bipolaire

Le second chapitre (p. 43) présente les principes de base de l'électrochimie bipolaire (ou « électrochimie sans fil ») ainsi que les principales applications de cette technique très versatile. Lorsqu'un objet conducteur est placé dans un champ électrique généré par deux électrodes génératrices, il se polarise et des surtensions apparaissent à ses extrémités. Si la différence de potentiel entre les deux extrémités est assez grande, des réactions électrochimiques peuvent se produire (Fig. R.2a) ; l'objet conducteur devient alors une électrode bipolaire. La différence de potentiel ∆V entre les deux pôles d'une électrode bipolaire est donnée par la relation :

∆V = U • l L (R.1)
avec U la tension appliquée entre les deux électrode génératrices, l la taille de l'électrode bipolaire et L la distance entre les deux électrodes génératrices. Pour que des réactions électrochimiques se déroulent aux pôles de l'électrode bipolaire, ∆V doit être supérieure à ∆V min , la différence entre les deux potentiels standard E • Ox et E • Réd des demi-équations d'oxydation et de réduction utilisées :

∆V > ∆V min = E • Ox -E • Réd (R.2)
L'électrochimie bipolaire peut être utilisée pour modifier asymétriquement de nombreuses particules conductrices simultanément (Fig. R.2b). Si l'une des demi-réactions utilisées a pour produit un solide, un dépôt de ce matériau se forme à la surface du pôle de l'électrode où la réaction se déroule. Une des réactions les plus couramment utilisées pour la synthèse de particules Janus est la réduction de chlorures d'or en or métallique à la surface du pôle cathodique d'une électrode bipolaire :

AuCl 4 -+ 3 e ---→ Au 0 + 4 Cl - (R.3)
De nombreux autres matériaux ont également été déposés électrochimiquement à la surface d'électrodes bipolaires : d'autres métaux (Pt, Ni, Cu...), des polymères (PEDOT, PPy...) ainsi que des matériaux hybrides. Il est aussi possible d'indirectement déposer des matériaux en utilisant des réactions électrochimiques pour déclencher une réaction différente. Par exemple, il est possible d'utiliser l'électrolyse de l'eau pour modifier le pH localement et acidifier la solution près des pôles cathodiques des électrode bipolaires et déclencher la formation d'un hydrogel à la surface des pôles.

Puisque l'intensité des réactions ne dépend que de la différence de potentiel entre les deux extrémités des électrodes bipolaire et non de la valeur absolue du potentiel de la solution, la position exacte des électrodes bipolaires dans le champ électrique n'a pas d'effet sur le dépôt final. Il est donc possible de modifier de nombreux objets conducteurs dispersés dans la solution simultanément. L'électrochimie bipolaire permet donc la synthèse de particule Janus avec un rendement élevé en comparaison avec d'autres techniques.

R.3 Utilisation de potentiels périodiques

Dans le troisième chapitre (p. 57), une nouvelle méthode permettant la bifonctionnalisation de particules isotropes avec deux dépôts métalliques de même composition mais de tailles différentes est présentée.

Premièrement, afin de pouvoir modifier des particules isotropes en particules Janus par électrochimie bipolaire, leur rotation doit être complètement arrêtée. Si les électrodes bipolaires sont libres de tourner pendant une déposition, elles seront modifiées uniformément en particules noyau-enveloppe avec une couche métallique recouvrant la totalité de leur surface. Pour bloquer leurs rotations durant une électrodéposition bipolaire, les électrodes sont dispersées dans un hydrogel qui contient le précurseur utilisé pour la déposition.

Nous avons donc bifonctionnalisé des particules sphériques en carbone vitreux avec des diamètres allant de 630 µm à 1000 µm en une seule étape. Pour cela, le gel dans lequel les électrodes bipolaires sont dispersées contient un précurseur métallique (AuCl 4 -, PdCl 4 2 -, Ni 2+ ...) qui peut être réduit en une couche de métal solide au pôle cathodique des électrodes bipolaires. Un signal carré périodique est appliqué entre les électrodes génératrice. Puisque la direction du champ électrique s'inverse périodiquement, les positions des pôles anodiques et cathodiques des électrodes bipolaires s'inverse également périodiquement. À la fin de l'électrodéposition, les deux extrémités des électrodes bipolaires auront passé un temps égal en tant que pôle cathodique et seront donc modifiées de manière identique (Fig. R.3a).

Une étude quantitative de la taille des dépôts métallique en fonction de la tension appliquée entre les deux électrodes génératrices a été réalisée. L'angle α formé entre le centre de d'une électrode bipolaire et la limite d'un dépôt métallique suis la relation : Un moyen de synthétiser des particules asymétriques plus complexes est l'utilisation de signaux périodiques mais non symétriques. Si les tensions positives et négatives du signal périodique sont différentes, les deux dépôts obtenus à la fin de la déposition seront de tailles différentes (Fig. R.3b).

Les précurseurs métalliques utilisés dans cette thèse pour la synthèse de particules complexes sont ioniques. Quand un ion est placé dans un champ électrique, il migre vers l'électrode génératrice de charge opposée avec une vitesse proportionnelle à l'intensité du champ électrique. Si un signal périodique symétrique est utilisé pour générer le champ électrique, les migrations dans les deux directions se compensent. Quand un signal asymétrique est utilisé, les deux migrations ne se compensent plus et on observe une déplétion des précurseurs ioniques au voisinage des électrodes bipolaires. Les durées pendant lesquelles les deux potentiels différents sont appliqués sont donc ajustées pour compenser cette migration : les tensions les plus basses sont appliquées pour une durée plus longue.

Il est aussi possible de déposer un seul patch en utilisant une tension périodique entre les électrodes génératrices. En effet, si une des deux tensions appliquées est assez basse pour que ∆V < ∆V min , les précurseurs ioniques migrerons toujours vers une électrode génératrice mais aucune réaction électrochimique ne se déroulera aux pôles des électrodes bipolaires. Cette méthode permet de déposer un seul dépôt sur une des deux extrémités des électrodes bipolaires sans les inconvénients causés par la migration des précurseurs ioniques présente lors de l'utilisation de tensions continues entre les électrodes génératrices.

R.4 Synthèse de particules avec plusieurs dépôts métalliques différents

Dans le Chapitre 3, nous avons mis au point une méthode qui permet la bifonctionnalisation de particules sphériques avec deux dépôts face à face de composition identique mais de tailles différentes. Dans le Chapitre 4 (p. 73), deux méthodes qui permettent la synthèse de particules plus complexes sont présentées.

Pour obtenir des particules plus complexes que les particules bifonctionnalisées présentées précédemment, il est nécessaire de pouvoir déposer un dépôt métallique à n'importe quelle position à la surface des électrodes bipolaires. De plus, afin de pouvoir synthétiser des particules chirales, il est nécessaire de pouvoir déposer des patchs hors du plan horizontal.

Pour cela, nous avons d'abord mis au point plusieurs cellules bipolaires possédant plus d'une paires d'électrodes génératrices (Fig. R.4a,b) afin de pouvoir choisir la direction du champ électrique. Cette approche a finalement été abandonnée pour plusieurs raisons :

• Ce type de design est toujours limité dans le choix des emplacements possible pour les dépôts. Il est seulement possible de déposer des patchs le long des axes définis par la position des paires d'électrodes génératrices ;

• Les électrodes bipolaires ne peuvent être placées qu'à l'intersection des différents champs électriques, ce qui réduit le volume disponible pour les dépositions et donc le rendement ;

• Pour pouvoir déposer des patchs hors du plan horizontal -et donc synthétiser des particules chirales -des électrodes génératrices doivent être placées au-dessus et en dessous du gel contenant les électrodes bipolaires. Le gel bipolaires ne peut donc plus être posé sur le fond de la cellule électrochimique et doit être suspendu. Cette configuration complique également l'évacuation des bulles de gaz produites à la surface des électrodes génératrices ;

• Quand elles ne sont pas utilisées pour produire un champ électrique, les électrodes génératrices sont toujours présentes en solution et se comportent comme de très grandes électrodes bipolaires : elles détournent la quasi-totalité du courant passant entre les deux électrodes génératrices polarisées à travers elles et loin du gel contenant les particules conductrices à modifier.

Une approche différente a donc été utilisée. Au lieu de changer la direction du champ électrique autour d'électrodes bipolaires fixes, ce sont les électrodes qui tournent à l'intérieur d'un champ électrique fixe. Pour cela, un nouveau type de cellule bipolaire à été mis au point. Le gel est placé dans un moule cubique composé d'une membrane poreuse. Ce cube est ensuite placé entre les deux électrodes génératrices. Le cube peut être réorienté indépendamment de la cellule bipolaire entre deux électrodépositions. Avec ce type de cellule, il est donc possible de déposer plusieurs patchs métalliques à n'importe quelle position dans le plan horizontal (Fig. Afin de pouvoir synthétiser des particules chirales, une particule doit avoir au moins trois dépôts répartis dans les trois dimensions et ces dépôts doivent avoir des compositions différentes. Pour cela, le précurseur présent avec les électrodes bipolaires dans le gel doit pouvoir être changé entre deux dépositions. Cependant, il n'est pas possible de dissoudre le gel et de redisperser les électrodes bipolaires dans un nouveau gel qui possèderait la nouvelle composition. En effet, si les électrodes bipolaires ne sont plus dispersées dans un gel, elles sont à nouveau libres de leurs mouvements et elles seront redispersées dans des orientations aléatoires dans le second gel. Il est donc nécessaire de pouvoir changer le précurseur métallique présent dans le gel tout en maintenant la position et l'orientation des électrodes bipolaires.

Pour cela, nous avons mis au point une technique reposant sur la diffusion des précurseurs dans et hors du gel. Après la première déposition, le gel contenant les électrodes bipolaires est placé dans un bécher d'eau distillée afin que les ions précurseurs toujours présents diffusent hors du gel. Après cette étape de lavage, le gel est placé dans une solution du précurseur utilisé pour la seconde déposition afin que celui-ci puisse diffuser dans le gel : c'est l'étape de chargement. Une deuxième déposition peut ensuite être réalisée pour déposer un patch avec le nouveau précurseur.

R.5 Modification de microparticules par électrochimie bipolaire

Dans le dernier chapitre (p. 95), plusieurs techniques qui permettent la synthèse de microparticules conductrices plus petites sont explorées.

La diminution de la taille des électrodes bipolaires causes différents problèmes qui ne sont pas présents pour la modification d'électrodes bipolaires de plus grandes tailles. La différence de potentiel nécessaire entre les électrodes génératrices pour déclencher des réactions bipolaires est inversement proportionnelle à la taille des électrodes bipolaires. Cette augmentation de la tension cause une augmentation de la température de la solution par effet Joule. Si la solution atteint une température trop élevée, cela peut causer la dissolution du gel contenant les électrodes bipolaires et, dans certains cas, l'ébullition de la solution dans la cellule bipolaire. La production de bulles d'oxygène et d'hydrogène produites par l'électrolyse de l'eau à la surface des électrodes génératrices augmente R.5. Modification de microparticules par électrochimie bipolaire également ; cela peut générer une couche de gaz qui isole les électrodes génératrices de la solution. Des solutions doivent être trouvées soit pour dissiper rapidement la chaleur générée par effet Joule soit pour diminuer les tension nécessaires pour déclencher les réactions électrochimiques aux pôles des électrodes bipolaires.

Une méthode utilisée pour la modification de microparticules est l'électrodéposition bipolaire assistée par capillaires (Capillary-assisted bipolar electrodeposition). Dans cette méthode, les deux électrodes génératrices sont placées dans deux récipients différents reliés par un tube capillaire. Les particules non-modifiées sont placées dans un des récipients. Le champ électrique créé un flux électroosmotique qui transporte les particules vers l'autre récipient. Les particules s'alignent dans la direction du champ électrique et sont modifiées par électrochimie bipolaire lors de leur passage dans le capillaire.

Une autre méthode pour modifier des particules plus petites est de diminuer le ∆V min nécessaire pour déclencher les réactions aux pôles des électrodes bipolaires. Si ∆V min = 0 V, l'intensité des réactions électrochimiques ne dépend plus de la taille des particules. Nous avons donc essayé de modifier des particules core-shell (noyau-enveloppe). Ces particules sont composées d'un noyau de verre enveloppé d'une enveloppe de cuivre. Aux pôles anodiques, le cuivre est oxydé en ions Cu 2+ tandis ce qu'aux pôles cathodiques, des ions Cu 2+ présents en solution sont réduits en cuivre métallique (Fig. R.6). Puisque les deux demi-réactions sont identiques, ∆V min = 0 V et il est possible de modifier des microparticules de petite taille sans utiliser de tensions élevées entre les électrodes génératrices. Une tension U > 0 V est quand même nécessaire afin de polariser les électrodes bipolaires et briser la symétrie du système. Cette méthode permet aussi la synthèse de particules Janus avec des matériaux non conducteurs, ce qui n'est pas possible avec les méthodes par électrodéposition présentées dans les chapitres précédents. Ici l'asymétrie est rompue, non pas en déposant un matériau à la surface, mais en révélant le noyau de verre de la particule via l'électrodissolution de l'enveloppe conductrice.

R.6 Conclusion

Dans cette thèse, plusieurs méthodes d'électrochimie bipolaire ont été mises au point afin de synthétiser des particules asymétriques complexes à partir de particules sphériques isotropes.

Premièrement, l'utilisation de tensions périodiques asymétriques au lieu de tensions continues entre les deux électrodes génératrices permet de déposer deux dépôts de tailles différentes aux deux extrémités d'une électrode bipolaire en une seule étape. Cette technique permet également de diminuer les effets de déplétion dans le gel causés par la migration des précurseurs ioniques dans le champ électrique.

Ensuite, une méthode qui repose sur la réorientation des électrodes bipolaires de manière contrôlée entre deux electrodépositions a été développée, permettant ainsi la déposition de patchs métalliques à n'importe quelle position à la surface de particules sphériques.

Une méthode reposant sur la diffusion de précurseurs dans et hors d'un gel a été mise au point afin de pouvoir changer le précurseur entre deux dépositions et ainsi choisir indépendamment les compositions pour chaque électrodéposition successive à la surface des électrodes bipolaires. Ces trois méthodes permettent la synthèse de particules asymétriques complexes en brisant la symétrie de particules isotropes dans les trois dimensions. Ces méthodes permettent donc la synthèse de particules chirales.

Enfin, des résultats préliminaires pour la synthèse de microparticules Janus plus petites sont présentés. Il est possible de modifier des particules noyaux-enveloppes par électrochimie bipolaire en utilisant des tension très basses entre les électrodes génératrices si l'oxydation et la réduction aux pôles des électrodes bipolaires utilisent la même demiréaction.

Les particules asymétriques synthétisées dans cette thèse pourraient avoir de nombreuses applications. Ces particules pourraient s'auto-assembler en des molécules colloïdales ou d'autres structures complexes de manière contrôlée. De la même manière que les molécules chirales, les particules chirales font tourner le plan de polarisation de la lumière polarisée. Une méthode permettant la synthèse d'objets chiraux complexes ouvre donc également de nouvelles perspectives dans le domaine de la chiroptique.

Introduction

Bipolar electrochemistry is an electrochemical technique that allows a conducting object called a bipolar electrode to act simultaneously both as a cathode and as an anode. Moreover, the bipolar electrode does not have to be electrically connected to a power source in order to trigger electrochemical reactions. This allows the use of numerous bipolar electrodes dispersed in solution in a single bipolar cell. Bipolar electrochemistry has numerous applications, both for the synthesis of micro and nano objects as well as for analytical purposes.

At the molecular scale, chirality is an important property that can have a great influence on the biological properties of a molecule. Numerous organic chemistry techniques have been developed in order to control the chirality of synthesized molecules or to separate the two enantiomers of a chiral molecule. The study of chirality at the microparticle scale is more recent and has been less extensively studied. Nowadays, few synthesis techniques exist for the production of intrinsically asymmetric or chiral nano and microparticles.

Different electrochemical as well as non-electrochemical techniques for the synthesis of Janus particles (i.e., particles with two different hemispheres) have been previously developed. However, these techniques only break the symmetry of isotropic particles along a single plane of symmetry. The aim of this thesis is to develop new bipolar techniques for the synthesis of more complex particles that have multiple patches and a broken symmetry along multiple planes. The final goal of the thesis is the synthesis of chiral objects (i.e., objects without any planes of symmetry) from isotropic, spherical particles through bipolar electrochemistry techniques.

The first chapter of the thesis reviews several non-electrochemical methods for the synthesis of anisotropic micro-and nanoparticles and their different applications; e.g. the controlled self-assembly of asymmetric particles into more complex structures and the synthesis of micromotors. The second chapter introduces the theory of bipolar electrochemistry and also presents the different families of bipolar set-ups and techniques previously developed as well as the main applications of bipolar electrochemistry.

The following chapters present the different experiments and set-ups developed during my thesis. Chapter 3 focuses on the use of AC electric signals for breaking the symmetry along one axis and the synthesis of spherical Janus particles with one or two patches. It also presents a qualitative as well as a quantitative analysis of the obtained patches. Chapter 4 presents novel set-ups and techniques that permits the synthesis of more complex asymmetric particles with metallic patches along different axes. These new techniques allow for the synthesis of chiral particles with up to four different patches at any position on the particle's surface. Chapter 5 presents preliminary results for the bipolar modification of microparticles and of particles with an insulating core.

Chapter 1

Anisotropy at the microand nanoscale: Janus, patchy and multicompartment particles Anisotropic particles (or asymmetric particles) are particles which are not centrosymmetric and possess several properties at different controlled points of their surfaces. Patchy particles are particles with one or several patches on their surface with different physical or chemical properties. Multicompartment particles are particles composed of several compartments of different composition and which are different not only at the surface, but also in their cores. Janus particles are particles on which two hemispheres have different physical or chemical properties. They are named after the two-faced Roman god Janus, god of transitions, gates and duality. The first Janus particles were synthetized in 1988 by Casagrande and Veyssié [START_REF] Casagrande | Grains Janus : réalisation et premières observations des propriétés interfaciales[END_REF][START_REF] Casagrande | Janus Beads: Realization and Behaviour at Water/Oil Interfaces[END_REF]: by treating only one hemisphere of glass beads, they obtained amphiphilic particles with one hydrophilic hemisphere and one hydrophobic hemisphere. The concept of Janus particles was popularised further by de Gennes in his 1991 Nobel lecture. [START_REF] De Gennes | Soft Matter (Nobel Lecture)[END_REF] Janus particles can be either patchy or multicompartment particles depending on their composition and the method of synthesis used (Fig. 1.1).

In this chapter, the main synthesis techniques used to produce Janus, patchy and multicompartment particles and the applications of such particles will be discussed. Applications for anisotropic particles cover a large variety of fields, ranging from colloidal science to microswimmers and e-paper. [START_REF] Loget | Bulk synthesis of Janus objects and asymmetric patchy particles[END_REF][START_REF] Du | Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application[END_REF][START_REF] Walther | Janus Particles: Synthesis, Self-Assembly, Physical Properties, and Applications[END_REF][START_REF] Perro | Design and synthesis of Janus micro-and nanoparticles[END_REF] Since Casagrande and Veyssié's first synthesis, numerous other synthesis methods for Janus and patchy particles have been discovered.

Modification of isotropic particles at an interface

Immobilisation at an interface

Most techniques used to transform isotropic particles into asymmetric particles rely on the immobilisation of the particles on an interface. When on an interface, it is easy to modify only one hemisphere of a particles while the other hemisphere is protected from modification by the interface. Several methods are used to immobilise isotropic particles at an interface.

One method used is to partially embed particles into a polymer layer (e.g., polystyrene, PS). By controlling the thickness of the layer, it is possible to control the proportion of the particles masked from the outside medium. [START_REF] Anderson | Robust Plasma Polymerized-Titania/Silica Janus Microparticles[END_REF] Then only the exposed areas of the particles are physically or chemically modified; then, the particles are released, revealing the unmodified hemispheres.

Pickering emulsions, i.e. emulsions stabilised by solid particles instead of surfactants, have been extensively studied and their properties are well known. [START_REF] Chevalier | Emulsions stabilized with solid nanoparticles: Pickering emulsions[END_REF] A Pickering emulsion is stabilised by the adsorption of solid isotropic particles at the water/oil interface. In order to stabilize such an emulsion, partial wetting of the particles by water and oil is necessary.

Pickering emulsions can be used for the synthesis of Janus patchy particles. Only the part of the particle that is in contact with the correct phase is modified. The main advantage of using emulsions for the production of Janus particles is the much higher surface area of the interface in an emulsion compared to a classical flat interface.

Perro and co-workers prepared large amounts of Janus silica nanoparticles by using a Pickering emulsion. [START_REF] Perro | Production of large quantities of "Janus" nanoparticles using wax-in-water emulsions[END_REF] They used silica nanoparticles to produce a Pickering emulsion of paraffin wax in water. Once the particles were at the water/wax interface, the wax was solidified in order to block the rotation of the particles. The parts on the particles exposed to water were then selectively functionalized with amine groups by mixing the emulsion with an aqueous solution of aminopropyltriethoxysilane. The Janus particles were then collected by centrifugation (Fig. 1.2).

Vapour deposition

Vapour deposition is a technique usually used to deposit uniform layers of materials on a surface. A solid material is vaporized and the created vapour is ejected towards the surface. The vapour then deposits on the surface creating a new material layer. Vapour deposition techniques can be split into two categories: physical and chemical vapour deposition (PVD and CVD). In PVD, the vapour will simply deposit on the targeted surface to form a new layer whereas in CVD, the vapour will react with the targeted surface to produce a new material layer.

These techniques can be used to create Janus particles. The vapour will only deposit on the exposed hemisphere of particles immobilised on a surface, coating them asymmetrically. Takei and Shimizu prepared Janus latex spheres with one hemisphere coated with a gold layer by using this method. [START_REF] Takei | Gradient Sensitive Microscopic Probes Prepared by Gold Evaporation and Chemisorption on Latex Spheres[END_REF] Then, they functionalized the particles by modifying the gold hemisphere with thiols. Shepard et al. were able to deposit poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO) layers on one hemisphere of silica and PS particles by PVD, producing Janus particles. [START_REF] Shepard | Patchy Janus particles with tunable roughness and composition via vapor-assisted deposition of macromolecules[END_REF] A frozen solution of the polymer to be deposited was irradiated by a laser. This produced nanodroplets of the solution that were ejected towards the particles. The solvent evaporates and the resulting polymer droplets deposited on the exposed hemispheres of the particles.

Glancing angle deposition (GLAD) is a variation of vapour deposition where the vapour arrives at an angle with the particles to be modified. If the particles are densely packed, they will screen each other and deposits of different shapes can be observed instead of circular ones observed in classical vapour deposition (Fig. 1.3). Two angles can be controlled to determine the shape of the deposited layer: the glancing angle θ which is the angle between the vapour flow and the surface on which the particles are deposited and the crystal orientation α which determines how the particles will cast their shadows on each other. [START_REF] Pawar | Patchy Particles by Glancing Angle Deposition[END_REF] Figure 1.3 -(a) On a densely packed colloidal crystal, the particles cast shadows on each other that will modify the shape of the deposits. (b) On a single particle, GLAD is not useful since the deposit will remain hemispherical. (c) The deposit shapes formed by GLAD depend on the angles θ and α at which the deposition occurs. Reprinted from Ref. [START_REF] Archer | Glancing angle metal evaporation synthesis of catalytic swimming Janus colloids with well defined angular velocity[END_REF].

In 2008, Pawar and Kretzschmar produced PS microparticles with gold patches of various sizes and shapes using this technique. [START_REF] Pawar | Patchy Particles by Glancing Angle Deposition[END_REF] More recently, Archer and co-workers deposited asymmetric platinum patches on a colloidal crystal composed of PS particles. [START_REF] Archer | Glancing angle metal evaporation synthesis of catalytic swimming Janus colloids with well defined angular velocity[END_REF] These particles were then used as microswimmers.

Microcontact printing

Once particles are immobilized on a surface, they can be partially modified by microcontact printing. In this technique, a chemical ink is adsorbed on a polymer (usually polydimethylsiloxane (PDMS)) which is used as a stamp to deposit the ink on the top part of immobilized particles. With this technique only the part of the particle in contact with the polymer stamp is modified (Fig. 1.4). [START_REF] Cayre | Fabrication of asymmetrically coated colloid particles by microcontact printing techniques[END_REF] Ravoo's team used a variation of this technique to bi-functionalize particles. They "sandwiched" particles between two PDMS stamps loaded with different chemical inks. This method permits the deposition of two different patches on the opposite hemispheres of the particles in one single step. [START_REF] Kaufmann | Chemically orthogonal trifunctional Janus beads by photochemical "sandwich" microcontact printing[END_REF][START_REF] Seidel | Preparation of Microscale Polymer Janus Particles by Sandwich Microcontact Printing[END_REF][START_REF] Kaufmann | Sandwich" Microcontact Printing as a Mild Route Towards Monodisperse Janus Particles with Tailored Bifunctionality[END_REF][START_REF] Kaufmann | Bifunctional Janus beads made by "sandwich" microcontact printing using click chemistry[END_REF] Polymer stamps can also be used to "catch" immobilized particles and flip them in order to modify their bottom half. Zhang and co-workers immobilized graphene sheets on a surface to asymmetrically modify them and then used a PMMA film to catch the The stamp is applied on the particles which are modified. Reprinted from Ref. [START_REF] Cayre | Fabrication of asymmetrically coated colloid particles by microcontact printing techniques[END_REF].

graphene and flip it to modify the other side. [START_REF] Zhang | Janus graphene from asymmetric two-dimensional chemistry[END_REF] 1.2 Bulk synthesis of asymmetric particles Bulk synthesis techniques allow for larger yields of particles since they are not limited by the surface of an interface.

A bulk technique that allows for the asymmetrical modification of conductive particles is bipolar electrochemistry and will be developed in details in the other chapters of this thesis.

Self-assembly of copolymers

One method for producing polymer Janus and patchy particles is to assemble multicompartmented micelles from the self-assembly of block copolymers. The micelles can then be stabilized through the crosslinking of the polymers. Ye et al. obtained patchy particles from polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) copolymers. [START_REF] Ye | Template-Free Bottom-Up Method for Fabricating Diblock Copolymer Patchy Particles[END_REF] The polymer chains were dissolved in dimethylformamide (DMF). Since DMF is a better solvent for P4VP than for PS, the copolymer self-assembled into micelles with a PS core. Water was then added to the solution. Since water does not solvate PS and is a worse solvent for P4VP than DMF, the PS shrunk into a denser core and the P4VP chains aggregated progressively into patches (Fig. 1.5). The number of patches can be tuned by changing the length of the P4VP blocks. Deng and co-workers produced similar particles by solvent evaporation rather than solvent mixing. [START_REF] Deng | Diblock Copolymer Based Janus Nanoparticles[END_REF] The polymer chains can then be crosslinked to obtain more stable particles. [START_REF] Gröschel | Facile, Solution-Based Synthesis of Soft, Nanoscale Janus Particles with Tunable Janus Balance[END_REF] More complex patchy particles have been obtained by Gröschel et al. by using block terpolymers instead of copolymers. [START_REF] Gröschel | Facile, Solution-Based Synthesis of Soft, Nanoscale Janus Particles with Tunable Janus Balance[END_REF][START_REF] Gröschel | Guided hierarchical co-assembly of soft patchy nanoparticles[END_REF] They used triblock terpolymers to synthesize Janus particles with a core composed of polybutadiene and two patches of polystyrene and poly(methyl methacrylate) or other combinations of polymers. This method allows for the synthesis of a wide variety of Janus and patchy particles with highly tunable properties.

Asymmetric growth of materials

A method to produce Janus particles is to asymmetrically grow materials on the surface of isotropic particles. The asymmetry can be generated through several methods. Competition between polymers or ligands can create several distinct areas on the surface of a particle that can then be asymmetrically modified. It can also be generated by precisely controlling the nucleation in order to control the number of seeds and therefore patches on a particle. [START_REF] Bao | One-Pot Colloidal Synthesis of Plasmonic Patchy Particles[END_REF][START_REF] Rouet | Colloidal Molecules from Valence-Endowed Nanoparticles by Covalent Chemistry[END_REF] Polymers growing on an isotropic particle can also form a single sphere on one side of the particle instead of forming a shell thanks to surface tension. [START_REF] Ohnuma | A Facile Synthesis of Asymmetric Hybrid Colloidal Particles[END_REF] Chen and co-workers produced Janus particles by controlling the deposition of silica on a gold nanoparticle through competitive ligand coordination. [START_REF] Chen | Scalable Routes to Janus Au-SiO 2 and Ternary Ag-Au-SiO 2 Nanoparticles[END_REF] Gold nanoparticles (AuNPs) were exposed to a mixture of 4-mercaptophenylacetic acid (4-MPAA) and poly(acrylic acid) (PAA). The competition between the two ligands leads to the creation of two zones on the surface of the particle (Fig. 1.6). Then, tetraethyl orthosilicate (TEOS) was used as an SiO 2 precursor. After hydrolysis, only the part of the AuNP, not covered with PAA is functionalized with silica since the PAA chains block the diffusion of TEOS to the surface of the AuNP. Chen et al. also further modified the obtained particles by adding a silver layer on the exposed gold part of the new Janus particles.

Hollow sphere crushing

Janus sheets can be produced by the crushing of hollow spheres with different composition on the outside and in the inside. Once crushed, the resulting pieces will have one side with the outside composition and the other side with the inside composition, resulting in Janus sheets (Fig. 1.7). Grüning et al. patented this method in 1987 to produce amphiphilic plates from commercial hollow microspheres whose outside was treated to become hydrophobic. [START_REF] Grüning | Particles, modified at their surface by hydrophilic and hydrophobic groups[END_REF] Liang and co-workers used this technique on silica spheres synthetized by a self-assembled sol-gel process. It allowed them to precisely tune the properties and composition of both sides of the nanosheets. [START_REF] Liang | Inorganic Janus Nanosheets[END_REF]

Flow techniques

In these techniques, a continuous multiphasic flow is used to produce Janus droplets which can then be solidified to form Janus particles. These techniques allow for a precise control of the geometry of the particles by controlling the size and composition of the flowing solution. They also permit a continuous synthesis of Janus particles which allows high production yields.

Microfluidics

Microfluidics methods are commonly used for the controlled production of droplets. [START_REF] Anna | Formation of dispersions using "flow focusing" in microchannels[END_REF][START_REF] Yang | Microfluidic synthesis of multifunctional Janus particles for biomedical applications[END_REF] By using a flow-focusing geometry in a microchannel, it is possible to produce droplets of an immiscible liquid inside another. If two or more solutions are put in contact just before the droplets are produced, multiphasic droplets can be obtained. This method is only possible in microfluidic designs since the microscopic size of the channels allows for very low Reynolds numbers and purely laminar flow without turbulences, which permits the co-flow of different fluids with very limited mixing. The produced compartmentalized droplets can then be quickly photopolymerized by UV irradiation into Janus particles (Fig. 1.8). [START_REF] Nie | Janus and Ternary Particles Generated by Microfluidic Synthesis: Design, Synthesis, and Self-Assembly[END_REF] This microfluidic co-flow technique was used for the synthesis of numerous different types of Janus and multi-compartmented particles (Fig. 1.9). [START_REF] Nie | Janus and Ternary Particles Generated by Microfluidic Synthesis: Design, Synthesis, and Self-Assembly[END_REF][START_REF] Nisisako | Synthesis of Monodisperse Bicolored Janus Particles with Electrical Anisotropy Using a Microfluidic Co-Flow System[END_REF][START_REF] Nisisako | Recent advances in microfluidic production of Janus droplets and particles[END_REF] Yuet et al. used this method to synthetize superparamagnetic particles. [START_REF] Yuet | Multifunctional Superparamagnetic Janus Particles[END_REF] Janus droplets of two UV curable poly(ethylene glycol)-diacrylate solutions were synthesized, one of them containing magnetite nanoparticles. The droplets were then irradiated with UV to polymerise them into Janus particles (Fig. 1.9c).

Flow techniques

Electrohydrodynamic co-jetting

Electrospinning is a technique commonly used to produce thin fibres. A thin jet of an electrically charged compound is produced by applying a strong electric field between the solution and a counter electrode. After solvent evaporation, the liquid jet solidifies into a fibre.

A variation of this method called electrohydrodynamic co-jetting can be used to produce Janus fibres. [START_REF] Liu | An Efficient Bicomponent TiO 2 /SnO 2 Nanofiber Photocatalyst Fabricated by Electrospinning with a Side-by-Side Dual Spinneret Method[END_REF][START_REF] Srivastava | Microfluidic electrospinning of biphasic nanofibers with Janus morphology[END_REF][START_REF] Wang | Electrospun Hydrophilic Janus Nanocomposites for the Rapid Onset of Therapeutic Action of Helicid[END_REF] In electrohydrodynamic co-jetting, two or more parallel flows of different solutions are jetted at the same time in order to produce a multiphasic jet that can be solidified into Janus fibres (Fig. 1.10). When using a high enough voltage, the continuous jet can split into individual droplets. The droplets can then solidify and lead to Janus particles. [START_REF] Du | Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application[END_REF][START_REF] Roh | Biphasic Janus particles with nanoscale anisotropy[END_REF][START_REF] Bhaskar | Towards Designer Microparticles: Simultaneous Control of Anisotropy, Shape, and Size[END_REF] Figure 1.10 -Principle of electrospinning. A biphasic jet is created by submitting solutions containing charged species to an intense electric field. If the field's strength is high enough, the jet splits into Janus droplets that transform into Janus particles through solvent evaporation. Adapted from Ref. [START_REF] Walther | Janus particles[END_REF].

Self-assembly of asymmetric particles

The self-assembly of isotropic particles is mainly governed by van der Waals forces or other non-directional forces. This results in the formation of spherical or amorphous clusters. Due to their asymmetrical structure, Janus particles will tend to assemble in specific and directional ways and be able to form complex structures like chains or 2D latices.

Asymmetric particles can also self-assemble into colloidal molecules. Colloidal molecules are assemblies of colloidal particles that emulate the chemical bonds found in regular molecules. In the same way that molecules are composed of several atoms of different sorts assembled in a specific structure with specific angles between chemical bonds, colloidal molecules are assemblies of several simpler spherical particles in a specific arrangement. [START_REF] Duguet | Design and elaboration of colloidal molecules: an overview[END_REF][START_REF] Van Blaaderen | Colloidal Molecules and Beyond[END_REF] 1.4.1 Stabilisation of emulsions Amphiphilicity, i.e. the property of an object to have both hydrophilic and hydrophobic parts, is fundamental in colloid science. Indeed, amphiphilic molecules can self-assemble into more complex structures (e.g. micelles or the bilayer lipidic membranes of biological cells) or stabilise emulsions. The amphiphilicity of a molecule can be quantified by the hydrophilic-lipophilic balance (HLB). Knowing the HLB of a surfactant is very useful to predict how it will act in the presence of a biphasic water/oil mixture.

Many synthetized Janus particles (including the first ones by Casagrande and Veyssié [START_REF] Casagrande | Grains Janus : réalisation et premières observations des propriétés interfaciales[END_REF][START_REF] Casagrande | Janus Beads: Realization and Behaviour at Water/Oil Interfaces[END_REF]) are amphiphilic with an hydrophobic hemisphere and an hydrophilic one. In 2007, Jiang and Granick introduced the concept of Janus balance to better quantify the amphiphilicity of Janus particles. [START_REF] Jiang | Janus balance of amphiphilic colloidal particles[END_REF] The definition of Janus balance is similar to the one of HLB: it is defined as the ratio between the work needed to transfer a particle from an oil/water interface into the oil phase and the work needed to transfer the same particle into the aqueous phase. [START_REF] Jiang | Janus balance of amphiphilic colloidal particles[END_REF] The main way used to tune the Janus balance of a Janus particle is by controlling the relative surface areas of the hydrophilic and hydrophobic sides. [START_REF] Gröschel | Facile, Solution-Based Synthesis of Soft, Nanoscale Janus Particles with Tunable Janus Balance[END_REF][START_REF]Controlling the Geometry (Janus Balance) of Amphiphilic Colloidal Particles[END_REF] Amphiphilic Janus particles have been used for the emulsification of biphasic solutions in order to combine the advantages of both Pickering and classical emulsions. [START_REF] Binks | Particles Adsorbed at the Oil-Water Interface:.1667em A Theoretical Comparison between Spheres of Uniform Wettability and "Janus" Particles[END_REF][START_REF] Walther | Emulsion Polymerization Using Janus Particles as Stabilizers[END_REF] Hou and co-workers synthetized Janus silica particles that can be used for the cleaning of oil spills in sea water. [START_REF] Hou | Amphiphilic Janus particles for efficient dispersion of oil contaminants in seawater[END_REF] The amphiphilic particles can emulsify the oil phase into small droplets which can be biodegraded more easily. This method could allow an environmentally friendly dispersion of oil spill without needing molecular surfactants that can themselves be pollutants.

Physical self-assembly

Similarly to amphiphilic molecules, Janus and patchy particles can self-assemble into more complex structures without any chemical bonds between the particles; only bonding through physical interaction like solvent-particle interactions or electrostatic forces.

Wolters and co-workers produced PS "Mickey Mouse" particles composed of a smooth core and two rough lobes at approximately 90°of each other, resembling the silhouette of the famous cartoon mouse (Fig. 1.11a). [START_REF] Wolters | Self-assembly of "Mickey Mouse" shaped colloids into tube-like structures: experiments and simulations[END_REF] The smooth cores of the particles attract each other through depletion and electrostatic interactions while the rough lobes act as steric constraints and orient the assembly in one favoured direction to form tubular structures (Fig. 1.11b,c).

Wang et al. assembled colloidal molecules using depletion interactions. [START_REF] Wang | Three-Dimensional Lock and Key Colloids[END_REF] They produced particles with a precise number of spherical cavities by partial encapsulation of silica sphere clusters with 3-(trimethoxysilyl)propyl methacrylate (TPM) and then dissolving the SiO 2 spheres with hydrofluoric acid (Fig. 1.11d). The TPM particles are then suspended in solution with PS spheres with a similar diameter than the cavities. The PS spheres will bind themselves inside the cavities, forming colloidal molecules with controlled valence and geometry (Fig. 1.11e). (a-c) Adapted from Ref. [START_REF] Wolters | Self-assembly of "Mickey Mouse" shaped colloids into tube-like structures: experiments and simulations[END_REF]. (d) Synthesis of TPM particles with multiples cavities. TPM is grown partially encapsulating a silica particle cluster that is then dissolved by HF. (e) The resulting particles are mixed with PS spheres that assemble themselves into the cavities through depletion interactions. (d-e) Adapted from Ref. [START_REF] Wang | Three-Dimensional Lock and Key Colloids[END_REF].

The interaction between the materials of the different patches and a solvent can trigger the assembly of the particles in non-isotropic ways with solvophobic patches clustering together. Zhao et al. produced colloidal vesicles composed of charged patchy micelles. [START_REF] Zhao | Self-assembly of positively charged polymer patchy micelles in organic solutions and the reversible ultrasound responsivity of the assemblies[END_REF] Nanomicelles were created from a copolymer made of poly(2-(dimethylamino)ethyl methacrylate) and poly(oligo(ethylene glycol) monomethylether methacrylate) blocks (PDMA-EMA-b-POEGMA). The PDMAEMA blocks were selectively crosslinked to form a solid core with a POEGMA patch on its surface. The resulting patchy micelles were then suspended in water and tetrahydrofuran (THF) was added. THF is a good solvent for POEGMA, but does not solubilize PDMAEMA. Therefore, in order to minimize the interaction energy between the THF and the PDMAEMA, the micelles self-assembled into vesicles with the POEGMA patches in contact with the solution.

Assembly under external stimuli

The use of an external stimulus is useful to trigger the assembly of particles into bigger structures. The use of electric or magnetic fields can also give a directionality to the assembly.

Chains are the most common structures created when placing patchy or Janus particles in an electric field [START_REF] Song | Patchy Particle Packing under Electric Fields[END_REF][START_REF] Yan | Reconfiguring active particles by electrostatic imbalance[END_REF][START_REF] Shah | Actuation of shape-memory colloidal fibres of Janus ellipsoids[END_REF][START_REF] Chaudhary | Reconfigurable assemblies of Janus rods in AC electric fields[END_REF], but other structures can be formed such as clusters [START_REF] Yan | Reconfiguring active particles by electrostatic imbalance[END_REF][START_REF] Zhang | Directed Self-Assembly Pathways of Active Colloidal Clusters[END_REF] or 2D layers [START_REF] Song | Patchy Particle Packing under Electric Fields[END_REF].

Song et al. successfully synthetized PS patchy particles that could self-assemble into various structures when polarised by an electric field. [START_REF] Song | Patchy Particle Packing under Electric Fields[END_REF] Two-patch particles were assembled into 1D chains, 2D layers or 3D packings depending on the strength of an AC electric field (Fig. 1.12a-b). Three-patch particles were also assembled into chains but since it is impossible for all three patches to align with the field, these particles formed double helix structures (Fig. 1.12c).

Chemical self-assembly

Chemical bonding can also be used to join particles together. DNA is a powerful molecule for this kind of applications. Two single-stranded DNA molecules will only match if their nucleotide sequences are complementary. It is therefore possible to precisely control how particles functionalised with DNA will self-assemble. Schreiber and co-workers functionalized gold nanoparticles with a single DNA strand which folds into a DNA origami covering the whole surface of a nanoparticle. [START_REF] Schreiber | Ordering Gold Nanoparticles with DNA Origami Nanoflowers[END_REF] Since the nucleotide sequence of the surface DNA is known at each point on the surface of the particle, it is possible to further functionalize the particles with single-stranded DNA in a site selective manner. Those DNA strands can then selectively hybridize with their complementary strands on other particles, forming chains or 2D lattices of nanoparticles (Fig. 1.13).

Synthesis of chiral particles

A chiral object is an object which cannot be superposed on its mirror image. The word chiral comes from the Ancient Greek word χ ίρ meaning "hand". Indeed, a left hand and a right hand are mirror images of each other. Two objects that are images of each other are called enantiomorphs (enantiomers in the case of molecules). Two enantiomers of a single molecule will have the same physical and chemical properties (density, melting point, refractive index, etc.) except that they will rotate the polarization plane of polarized light in opposite directions. This makes the synthesis of a specific enantiomer of a molecule or the separation of a racemic mixture (i.e. a mixture with equal proportion of two enantiomers) very difficult. Most organic synthesis techniques either yield a racemic mixture of enantiomers from achiral precursors or simply transfer chirality from one compound to another. Despite similar physicochemical properties, two enantiomers can have very different biochemical properties. For example, one enantiomer can have beneficial effects and be used as a drug while the other enantiomer can have no effects or even harmful effects on the organism. In the 1950s, the chiral molecule thalidomide was released as a painkiller 1.5. Synthesis of chiral particles drug. Unfortunately, it was later discovered that only one of the two enantiomers had the desired therapeutic effects while the other enantiomer could cause severe birth defects on babies if the mother used the drug during pregnancy. This tragic example shows the importance of understanding the properties of chirality and of controlling the chiral purity of newly synthesized molecules.

The study of chiral micro-and nanoparticles is more recent. Chiral micro-and nanoparticles can have strong chiroptical effects due to the presence of plasmons on chiral surfaces. [START_REF] Valev | Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals, Recent Progress, and Outlook[END_REF] Several techniques exist for synthesizing chiral objects. The two main types of chiral objects are helices and particles with a chiral centre similar to a tetravalent carbon atom in organic molecules.

Helix chirality

Helices are chiral structures; one helix turning clockwise and one helix turning anticlockwise are mirror images of each other and cannot be superposed.

Schamel and co-workers produced micro helices by using the GLAD technique. [START_REF] Schamel | Chiral Colloidal Molecules And Observation of The Propeller Effect[END_REF] The helices are synthesized by depositing material on silica seeds immobilized on a substrate rotating at an angle with the material source (Fig. 1.14). The chirality and pitch of the helices can be chosen by tuning the direction and rotation speed of the substrate on which they grow.

Tetravalent-carbon-like chirality

Another form of chirality is the presence of a chirality centre. The most common chirality centre observed in organic chemistry is a tetravalent carbon atom with four different substituents (e.g. lactic acid, Fig. 1.15).

Rouet and co-workers synthesized silica-based chiral colloidal molecules with a similar geometry. [START_REF] Rouet | Colloidal Molecules from Valence-Endowed Nanoparticles by Covalent Chemistry[END_REF] They synthesized silica particles with four cavities with aminated PS at their bottom in a tetrahedral geometry. Subsequently they exposed these particles to silica nanospheres of four different diameters. The nanosphere and the tetrahedral particles self-assembled into colloidal molecules. Statistically a part of the self-assembled colloidal molecules contained one nanosphere of each size and were chiral. This method has a low yield to produce chiral assemblies since only a small fraction of the self-assembled colloidal molecules will be chiral. Moreover, this technique will only produce racemic mixtures of chiral objects.

Ben Zion et al. published in 2017 a method for the synthesis of chiral colloidal particles in a more controlled manner. [START_REF] Ben Zion | Self-assembled three-dimensional chiral colloidal architecture[END_REF] They used DNA origamis to control the positions and angles of the self-assembly of several polymer particles. An L-shaped DNA origami is designed with four ends that consist of a single-stranded DNA with specific nucleotide sequences. These sticky ends will then selectively bind to particles modified with the matching nucleotide sequences (Fig. 1.16). By controlling the angles of the origami and the nucleotide sequences at its sticky ends, it is possible to control the angle between each particle forming the colloidal molecule. This technique permits the bulk self-assembly of enantiomerically pure colloidal molecules.

Microswimmers

When an isotropic colloidal particle is suspended in a homogenous solution, it will move on a random path known as Brownian motion. In order for swimmers to move in a controlled direction, some sort of asymmetry must be established. The asymmetry can come from a gradient in the solution or from an asymmetric feature of the particles. Therefore, anisotropic and Janus particles are very useful for the design of microswimmers. Several mechanisms are used to trigger motion of microparticles. [START_REF] Guix | Self-Propelled Micro/Nanoparticle Motors[END_REF]

Bubble propulsion

In bubble propulsion, a gas generating reaction is catalysed on only one side of a Janus particle in solution. The asymmetrical production of bubbles propels the particle in the opposite direction. [START_REF] Gibbs | Autonomously motile catalytic nanomotors by bubble propulsion[END_REF] One commonly used reaction is the catalytic decomposition of hydrogen peroxide into water and oxygen gas at a platinum surface (Eq. (1.1)). [START_REF] Guix | Self-Propelled Micro/Nanoparticle Motors[END_REF]

H 2 O 2 Pt/Pd ---→ H 2 O + 1 2 O 2 (1.1)
Gao and co-workers prepared Al/Pd Janus particles that can be propelled in two different directions depending on the type of fuel used. [START_REF] Gao | Multi-Fuel Driven Janus Micromotors[END_REF] In the presence of H 2 O 2 , the latter decomposes on the palladium according to Equation (1.1), propelling the particles. However, in strong alkaline (pH > 11) or acidic (pH < 3) conditions, OH -or H + will react with the aluminium side and produce hydrogen bubbles, propelling the particles in the opposite direction (Fig. 1.17). The same team also produced water driven micromotors that do not require an addition of other species to be used as fuel to generate motion. [START_REF] Gao | Water-Driven Micromotors[END_REF] They synthesized Janus particles with one hemisphere composed of an Al-Ga alloy. This allows for water to be reduced by the aluminium producing H 2 gas following Equation (1.2).

Self-phoretic swimmers

When a colloidal particle is placed in a concentration gradient, an osmotic flow of the solvent from the low concentration area to the high concentration area will happen near the surface of the particle and the particle will move in the opposite direction towards the low concentration region. [START_REF] Anderson | Colloid Transport by Interfacial Forces[END_REF] This phenomenon is called diffusiophoresis. [START_REF] Ebel | Diffusiophoresis of latex particles in electrolyte gradients[END_REF][START_REF] Velegol | Origins of concentration gradients for diffusiophoresis[END_REF] It is possible for some asymmetric particles to create their own chemical gradient through asymmetrical reactivity. They are then known as self-phoretic swimmers. [START_REF] Aubret | Eppur si muove, and yet it moves: Patchy (phoretic) swimmers[END_REF] Gao et al. synthesized silica Janus microswimmers with an iridium patch. The micromotors use the catalysed decomposition of hydrazine (N 2 H 4 ) on iridium to produce gradients of H 2 , N 2 and NH 3 in the solution [START_REF] Gao | Catalytic Iridium-Based Janus Micromotors Powered by Ultralow Levels of Chemical Fuels[END_REF], triggering its movement (Fig. 1.18). Movement of the particles were observed even with only trace amounts of hydrazine in solution (down to 10 -7 %). Similar phenomena can be observed with other kinds of gradients like (self-)electrophoresis [START_REF] Pavel | Nanorods with Biocatalytically Induced Self-Electrophoresis[END_REF] or (self-)thermophoresis. [START_REF] Jiang | Active Motion of a Janus Particle by Self-Thermophoresis in a Defocused Laser Beam[END_REF] The self-phoretic effects can also be triggered with an outside stimulus like light. [START_REF] Jiang | Active Motion of a Janus Particle by Self-Thermophoresis in a Defocused Laser Beam[END_REF][START_REF] Ibele | Schooling Behavior of Light-Powered Autonomous Micromotors in Water[END_REF]

E-paper and electronic displays

Electronic papers (e-papers) are a family of passive electronic displays. Contrary to other active display systems (e.g., LCD or OLED), e-paper does not produce its own light but, akin to regular paper, only reflects light from an outside source. Moreover, epaper only consumes energy when its pixels need to be switched between different states. These properties make e-paper an electronic display with a very low energy consumption compared to more classical displays.

Charged or magnetic bicoloured Janus particles can be used as pixels for e-paper. Such particles can be oriented with one coloured hemisphere up or down by using magnetic or electric fields (Fig. 1.19c). Once set into the right orientation, the field can be switched off, increasing the energy efficiency of this kind of display compared to active ones that have to constantly spend energy to be able to display anything. Janus particles with one magnetic half and one fluorescent half were synthetized by Yin 1.7. E-paper and electronic displays and colleagues by a microfluidic method. [START_REF] Yin | Versatile Bifunctional Magnetic-Fluorescent Responsive Janus Supraballs Towards the Flexible Bead Display[END_REF] They used these particles to build a display system with fluorescent pixels that can be controlled by magnetic fields (Fig. 1.19c).

Crowley et al. produced e-paper by placing bicolored black and white wax particles between two glass plates covered with transparent electrodes forming pixels. [START_REF] Crowley | Dipole moments of gyricon balls[END_REF] By controlling the potential applied between each pair of electrodes, they could control the colour of each pixel independently (Fig. 1.19d). Reprinted from Ref. [START_REF] Komazaki | Electrically and magnetically dual-driven Janus particles for handwriting-enabled electronic paper[END_REF]. (c) Principle of a display using Janus particles. Each bicoloured particle can be independently rotated by an external stimulus. Adapted from Ref. [START_REF] Yin | Versatile Bifunctional Magnetic-Fluorescent Responsive Janus Supraballs Towards the Flexible Bead Display[END_REF]. (d) Image of the border between a white and a black area of an e-paper display using bicolored Janus particles. Reprinted from Ref. [START_REF] Crowley | Dipole moments of gyricon balls[END_REF]. (e) Handwritten letter using a magnet on a surface of Janus beads with the mechanism shown in (a). Reprinted from Ref. [START_REF] Komazaki | Electrically and magnetically dual-driven Janus particles for handwriting-enabled electronic paper[END_REF].

Other less conventional methods for displaying patterns using Janus objects have been proposed. Komazaki et al. designed a surface containing black and white Janus particle that can be written on by dragging a magnet along it (Fig. 1.19e). [START_REF] Komazaki | Electrically and magnetically dual-driven Janus particles for handwriting-enabled electronic paper[END_REF] Sahoo and coworkers presented a display composed of levitating bicolored expanded PS beads that can be remotely rotated mid-air. [START_REF] Sahoo | JOLED: A Mid-air Display Based on Electrostatic Rotation of Levitated Janus Objects[END_REF] Chapter 2

Introduction to bipolar electrochemistry

Bipolar electrochemistry -or "wireless electrochemistry" -is an electrochemical technique allowing for an oxidation and a reduction to happen on two poles of a same electrode without the need for the electrode to be connected to an external circuit. [START_REF] Backhurst | A Preliminary Investigation of Fluidized Bed Electrodes[END_REF][START_REF] Fleischmann | Electrochemical behavior of dispersions of spherical ultramicroelectrodes[END_REF][START_REF] Crooks | Principles of Bipolar Electrochemistry[END_REF][START_REF] Kuhn | A Compelling Case for Bipolar Electrochemistry[END_REF] In a classical, non-bipolar, electrochemical set-up (e.g., a three-electrode set-up), the electrochemical reactions are driven by the potential difference between the working electrode and a reference electrode. In a bipolar set-up however, the electrochemical reactions are driven by the potential difference between two different points of the solution and the interface of a single electrode: the bipolar electrode (BPE). Since a BPE is composed of a conducting material, its potential remains uniform along its length. Therefore, overpotentials are created on each of its sides. [START_REF] Li | Visualizing the Zero-Potential Line of Bipolar Electrodes with Arbitrary Geometry[END_REF] If the potential difference between the two extremities of the BPE is high enough, cathodic and anodic poles are created and electrochemical reactions can occur (Fig. 2

.1).

It is therefore necessary for bipolar electrochemistry to happen that a potential gradient exists inside the solution. To generate such a gradient, two feeder electrodes (or driving electrodes) connected to a power supply are placed parallel to each other in the solution to create a linear electric field. Other methods exist to generate electrical gradients in solution such as streaming potentials [START_REF] Delgado | Measurement and interpretation of electrokinetic phenomena[END_REF][START_REF] Dumitrescu | Pressure-Driven Bipolar Electrochemistry[END_REF] or differential aeration [START_REF] Gruyter | Corrosion due to differential aeration reconsidered[END_REF] but those methods will not be discussed in this thesis. This technique has multiple applications ranging from microswimmers [START_REF] Sentic | Light-Emitting Electrochemical "Swimmers[END_REF][START_REF] Jurado-Sánchez | Janus Micromotors for Electrochemical Sensing and Biosensing Applications: A Review[END_REF] to material synthesis [START_REF] Bradley | Pulsed Bipolar Electrodeposition of Palladium onto Graphite Powder[END_REF][START_REF] Shida | Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional Material Synthesis[END_REF][START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF][START_REF] Koefoed | Functionalizing Arrays of Transferred Monolayer Graphene on Insulating Surfaces by Bipolar Electrochemistry[END_REF][START_REF] Mayorga-Martinez | Bipolar Electrochemical Synthesis of WS 2 Nanoparticles and Their Application in Magneto-Immunosandwich Assay[END_REF][START_REF] Wang | Bipolar Electrochemistry as a Simple Synthetic Route toward Nanoscale Transition of Mo 2 B 5 and W 2 B 5 for Enhanced Hydrogen Evolution Reaction[END_REF][START_REF] Bjerglund | Efficient Graphene Production by Combined Bipolar Electrochemical Intercalation and High-Shear Exfoliation[END_REF], separation and concentration of analytes [START_REF] Dhopeshwarkar | Electrokinetics in Microfluidic Channels Containing a Floating Electrode[END_REF][START_REF] Perdue | Bipolar Electrode Focusing: The Effect of Current and Electric Field on Concentration Enrichment[END_REF][START_REF] Laws | Bipolar Electrode Focusing: Simultaneous Concentration Enrichment and Separation in a Microfluidic Channel Containing a Bipolar Electrode[END_REF][START_REF] Mavré | Bipolar Electrodes: A Useful Tool for Concentration, Separation, and Detection of Analytes in Microelectrochemical Systems[END_REF], sensors [START_REF] Jurado-Sánchez | Janus Micromotors for Electrochemical Sensing and Biosensing Applications: A Review[END_REF][START_REF] Mavré | Bipolar Electrodes: A Useful Tool for Concentration, Separation, and Detection of Analytes in Microelectrochemical Systems[END_REF][START_REF] Bouffier | Advances in bipolar electrochemiluminescence for the detection of biorelevant molecular targets[END_REF][START_REF] Dauphin | Tracking Magnetic Rotating Objects by Bipolar Electrochemiluminescence[END_REF][START_REF] Rafatmah | Colorimetric and visual determination of hydrogen peroxide and glucose by applying paper-based closed bipolar electrochemistry[END_REF][START_REF] Rahn | Recent Advancements in Bipolar Electrochemical Methods of Analysis[END_REF] and circuit wiring [START_REF] Ohira | Synthesis of linear PEDOT fibers by AC-bipolar electropolymerization in a micro-space[END_REF][START_REF] Bradley | Creating electrical contacts between metal particles using directed electrochemical growth[END_REF][START_REF] Bradley | Wire formation on circuit boards using spatially coupled bipolar electrochemistry[END_REF][START_REF] Bradley | Programmable Hard-Wiring of Circuitry Using Spatially Coupled Bipolar Electrochemistry[END_REF][START_REF] Zhang | Bipolar Electrochemistry Regulation for Dynamic Meniscus Confined Electrodeposition of Copper Micro-Structures by a Double-Anode System[END_REF]. In this chapter, we will mainly discuss the applications of bipolar electrochemistry for the asymmetric modification of conducting objects.

Open bipolar electrochemistry

In a classical "open" bipolar electrochemical set-up, one or several BPEs are placed in a solution between two feeder electrodes, which generate an electric field driving the electrochemical reactions at the poles of the BPE (Fig. 2.1). The electric field polarizes the conducting object, which acts as a BPE, allowing electrochemical reactions to occur at its extremities.

∆V

Ox 1 Red 1 Red 2 Ox 2 δ - δ +
In the bulk of the solution between the feeder electrodes, the potential gradient is, in a first order approximation, linear. This means that since the force driving the electrochemical reaction is the potential difference between the poles of the BPE and not the absolute value of the potential, the exact position of the electrode in the electric field does not matter from an electrochemical point of view. This property of bipolar electrochemistry allows for numerous BPEs to be placed in solution at the same time and react in an identical way. A reference electrode is therefore not needed in this set-up.

The electric field strength E depends on U , the voltage applied between the feeder electrodes and L, the distance between the feeder electrodes:

E = U L (2.1)
Since the electric field is uniform in the bulk of the solution, the potential difference ∆V between the extremities of a BPE is given by:

∆V = E • l ⇒ ∆V = U • l L (2.2)
where l is the length of the BPE along the axis perpendicular to the feeder electrodes' plane. In order for electrochemical reactions to occur on the poles of the BPE, ∆V needs to be higher than ∆V min which depends on the standard potentials (E • ) of the two half-reactions involved:

Red 1 -→ Ox 1 + n 1 e -E • Ox (2.3) Ox 2 + n 2 e --→ Red 2 E • Red (2.4) ∆V min = E • Ox -E • Red (2.5)
Since ∆V min depends only on the nature of the electrochemical half-reactions involved, Equation (2.2) shows that the voltage between the feeder electrodes needed in order for reactions to happen at the BPE's poles is inversely proportional to the length of the BPE. This physical limitation makes the use of small BPE difficult.

A solution to avoid this scaling issue is the use of photocatalyst semiconductors (e.g., TiO 2 ) as BPEs. When a semiconductor is irradiated with photons with an energy higher than its bang gap (E g ), electrons jump from the valance band (VB) to the conduction band (CB), leaving holes (h + ) in the valance band (Fig. 2.2). If the semiconductor is polarised by an electric field, the holes and electrons will migrate towards the poles and will electrochemically react there: the semiconductor acts as a BPE. [START_REF] Ongaro | Asymmetric Modification of TiO 2 Nanofibers with Gold by Electric-Field-Assisted Photochemistry[END_REF][START_REF] Tiewcharoen | Anisotropic Metal Deposition on TiO 2 Particles by Electric-Field-Induced Charge Separation[END_REF] In this configuration, the reaction is now driven by the energy of the incident light and the electric field simply directs the electrons and holes to the poles. The ∆V > ∆V min condition is therefore switched to E g > ∆V min , removing the dependence on the length of the BPE. Another solution to avoid the scaling issue is discussed in Chapter 5. -When irradiated by photons with a high enough energy, electrons will jump from the VB to the CB, leaving a hole in the VB. Then, in the presence of an electric field the holes and the electrons will move to the poles of the BPE where they will electrochemically react. Adapted from Ref. [START_REF] Ongaro | Asymmetric Modification of TiO 2 Nanofibers with Gold by Electric-Field-Assisted Photochemistry[END_REF].

In an open set-up, the current flowing between the feeder electrodes is split between the faradic current going through the BPE and the ionic current flowing through the solution. Techniques used to maximize the faradic current are developed further in Section 4.1.

Asymmetric modification of conductive objects using bipolar electrochemistry

One of the main applications of bipolar electrochemistry in its open configuration is the site-selective modification of BPEs to produce asymmetric objects (e.g., Janus particles). If one of the products of the electrochemical reactions happening at the poles of a BPE is solid, a deposit will form on one side of the electrode, thus breaking its symmetry and producing a Janus object.

The size of a deposit created via bipolar electrochemistry can be theoretically determined. In order for electrochemistry to happen on a BPE, ∆V must be greater than ∆V min . It is possible to modify Equation (2.2) to calculate a maximal polarisation ∆V (x) for each point at the surface of the BPE:

∆V (x) = E × x + l 2 (2.6)
where x is the distance from the centre of the BPE along the axis parallel to the electric field E (Fig. 2.3a). In the case of a spherical BPE of diameter d, Equation (2.6) can be rewritten as:

∆V (α) = E d 2 (1 + cos α) (2.7)
where α is the angle between the centre and the point on the surface of the BPE (Fig. 2.3b). The limit of the deposit will be situated at the positions on the BPE where ∆V = ∆V min .

Since BPEs can be dispersed in the bulk of a solution, this technique allows for the bulk synthesis of Janus particles [START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF][START_REF] Loget | Single point electrodeposition of nickel for the dissymmetric decoration of carbon tubes[END_REF], permitting higher yields than techniques requiring an interface.

When placed in an electric field, non-isotropic conductive objects will tend to align along the field. This means that it is possible to disperse non-isotropic particles (e.g. carbon micro-or nanotubes) in a solution and modify them identically since they will align with the field and be parallel to each other. [START_REF] Loget | Single point electrodeposition of nickel for the dissymmetric decoration of carbon tubes[END_REF][START_REF] Warakulwit | Dissymmetric Carbon Nanotubes by Bipolar Electrochemistry[END_REF][START_REF] Fattah | Straightforward single-step generation of microswimmers by bipolar electrochemistry[END_REF] Numerous types of electrochemical reactions have been used to produce asymmetric objects via bipolar electrochemistry such as the reduction of metallic precursors [START_REF] Loget | True Bulk Synthesis of Janus Objects by Bipolar Electrochemistry[END_REF][START_REF] Koizumi | Bifunctional Modification of Conductive Particles by Iterative Bipolar Electrodeposition of Metals[END_REF][START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF], electropolymerisation [START_REF] Koizumi | Electropolymerization on wireless electrodes towards conducting polymer microfibre networks[END_REF], electrografting of organic molecules [START_REF] Koefoed | Functionalizing Arrays of Transferred Monolayer Graphene on Insulating Surfaces by Bipolar Electrochemistry[END_REF][START_REF] Madsen | Twophase bipolar electrografting[END_REF][START_REF] Kumsapaya | Wireless Electrografting of Molecular Layers for Janus Particle Synthesis[END_REF] or oxidation of metals [START_REF] Sopha | Fabrication of TiO 2 nanotubes on Ti spheres using bipolar electrochemistry[END_REF][START_REF] Asoh | AC-Bipolar Anodization of Aluminum: Effects of Frequency on Thickness of Porous Alumina Films[END_REF].

One of the main ways to synthetize Janus particles is by the reduction of ionic metal precursors into metallic patches. Noble metals are preferred for such deposits since their standard potentials are higher than those of other metal, lowering the ∆V min necessary to trigger the deposition. One precursor very often used is gold chloride (Eq. (2.8)). [START_REF] Loget | True Bulk Synthesis of Janus Objects by Bipolar Electrochemistry[END_REF][START_REF] Warakulwit | Dissymmetric Carbon Nanotubes by Bipolar Electrochemistry[END_REF][START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF] The counter reaction on the anodic pole is usually water oxidation.

AuCl 4 -+ 3 e ---→ Au 0 + 4 Cl -E • AuCl4 -/Au = +0.99 V vs. SHE (2.8)
Other metals have been deposited on conducting objects by bipolar electrochemistry such as platinum [START_REF] Koizumi | Bifunctional Modification of Conductive Particles by Iterative Bipolar Electrodeposition of Metals[END_REF][START_REF] Fattah | Straightforward single-step generation of microswimmers by bipolar electrochemistry[END_REF], nickel [START_REF] Loget | Single point electrodeposition of nickel for the dissymmetric decoration of carbon tubes[END_REF][START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF], copper [START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF] or palladium [START_REF] Bradley | Pulsed Bipolar Electrodeposition of Palladium onto Graphite Powder[END_REF] (Fig. 2.4). Bipolar electropolymerisation is a method which allows the synthesis of polymer/metal hybrid asymmetric particles. The polymer is synthetized by the oxidation of a monomer (e.g., EDOT or pyrrole) at the BPE's anode to produce a conducting polymer (e.g. PE-DOT [START_REF] Inagi | Site-selective anisotropic modification of conductive objects by bipolar electropolymerization[END_REF][START_REF] Watanabe | In-Plane Growth of Poly(3,4-ethylenedioxythiophene) Films on a Substrate Surface by Bipolar Electropolymerization[END_REF] or PPy [START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF][START_REF] Babu | Guiding water into carbon nanopipes with the aid of bipolar electrochemistry[END_REF]).

It is possible to tune the properties of the deposit by doping the polymer during the polymerisation. Koizumi et al. successfully incorporated poly(styrene sulfonate) (PSS) and hexachloroplatinate (PtCl 6 2 -) in synthetized PEDOT deposits, resulting in hybrid PEDOT-PSS and PEDOT-Pt fibres on the poles of a gold BPE. [START_REF] Koizumi | Synthesis of Poly(3,4-ethylenedioxythiophene)-Platinum and Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) Hybrid Fibers by Alternating Current Bipolar Electropolymerization[END_REF] The morphology of the resulting polymer can be tuned by controlling the design of the bipolar cell. Ohira and co-workers used AC potentials and careful cell design to synthetize PEDOT fibres on a BPE by limiting the diffusion of the monomer at the poles of the BPE, forcing a linear growth instead of a dendritic one (Fig. 2

.5). [107]

Gradient synthesis using bipolar electrochemistry

Material gradients are useful tools for material science as they allow for the simultaneous screening of a wide variety of compositions in a single experiment.

As Equation (2.6) shows, the potential difference ∆V (x) increases as a function of the distance from the centre of the BPE (Fig. 2.6). Since electrochemical reaction kinetics depend on the potential difference between the surface of the electrode and the solution, a deposit on the surface of a BPE will not be uniform along its length but will form a gradient.

Several types of metallic gradients have been realised. Composition gradients use the competition between the deposition of several metals to obtain a gradient along the length of the BPE. Xu and co-workers developed a one-step method to synthetize a Cu-Ni gradient with a composition ranging from 0 % to 89 % of nickel atoms. [START_REF] Xu | One-step deposition of Ni x Cu 1 -x alloys with both composition gradient and morphology evolution by bipolar electrochemistry[END_REF] Other composition gradients of various metals have also been produced. [START_REF] Tisserant | Generation of metal composition gradients by means of bipolar electrodeposition[END_REF][START_REF] Termebaf | Two-Step Bipolar Electrochemistry: Generation of Composition Gradient and Visual Screening of Electrocatalytic Activity[END_REF] Bipolar electrochemistry has been used for the grafting of organic gradients or the fabrication of morphological gradients of nanoclusters. [START_REF] Najafabadi | Wireless electrochemical preparation of gradient nanoclusters consisting of copper(II), stearic acid and montmorillonite on a copper wire for headspace in-tube microextraction of chlorobenzenes[END_REF] Bouffier and co-workers designed a wetting gradient on a gold surface by using the reductive desorption of a thiol molecule. [START_REF] Bouffier | Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry[END_REF] A self-assembled monolayer was generated by immersing the BPE in a thiol solution. During the subsequent bipolar reductive desorption, the thiol molecules were partially removed, resulting in a wetting gradient that follows the polarization potential difference along the BPE (Fig. 2

.7b).

Polymer gradients have also been synthesized. Shida et al. combined bipolar electrochemistry and electrochemical atom transfer radical polymerization to prepare BPE with a thickness gradient of polymers (Fig. 2.7a). [START_REF] Shida | Electrochemically Mediated Atom Transfer Radical Polymerization from a Substrate Surface Manipulated by Bipolar Electrolysis: Fabrication of Gradient and Patterned Polymer Brushes[END_REF][START_REF] Shida | Layer-by-layer Multilayered Film Formation on Gradient Polyelectrolyte Brush Prepared by Bipolar Electrochemistry[END_REF] Composition gradients of several polymers [START_REF] Inagi | Fabrication of gradient polymer surfaces using bipolar electrochemistry[END_REF] or doping gradients of polymers [START_REF] Shida | Electro-Click Modification of Conducting Polymer Surface Using Cu(I) Species Generated on a Bipolar Electrode in a Gradient Manner[END_REF] have been synthesized.

The potential gradient along a BPE can also be used for analytical purposes. A wide range of potentials can be screened simultaneously since the potential is varying spatially along the BPE. [START_REF] Chang | Snapshot Voltammetry Using a Triangular Bipolar Microelectrode[END_REF] This allows for fast voltammetry since the potential is varying over the length of the BPE and not over time.

Material gradients synthetized by bipolar electrochemistry or other methods have also been used themselves as BPEs to screen the properties of a wide variety of alloy compositions in a single experiment. By placing a BPE with an alloy gradient, synthetized by magneton sputtering, perpendicularly to an electric field, Latyshev et al. could use the alloy gradient to screen hydrogen evolution reaction (HER) efficiency on a wide variety of compositions in a single bipolar experiment. [START_REF] Latyshev | Screening of electrocatalysts for hydrogen evolution reaction using bipolar electrodes fabricated by composition gradient magnetron sputtering[END_REF] The HER takes place on the cathodic pole of the BPE, while the oxidation of a molybdenum film is used on the anodic side as a reporting reaction. The amount of oxidized molybdenum is directly proportional to the efficiency of the HER for a particular composition of the alloy (Fig. 2.8a). They used this method to test HER efficiency on Ni-Mo, Au-Mo and Au-Ni alloy gradients. Termebaf and co-workers used a similar set-up to screen a Ni-Cu alloy gradient for HER (Fig. 2 Another possible configuration for bipolar electrochemistry is the "closed" set-up where the anodic and cathodic sides of the BPE are located in two different solutions. In this configuration the only way for the current to go from one feeder electrode to the other is to go through the BPE (Fig. 2.9). In this case, the driving force is not a gradient inside 2.2. Closed bipolar electrochemistry the solution around the BPE, but the potential difference between the solutions separated by the BPE. Therefore, the potential does not need to linearly decrease in solution and almost all the potential drop can happen directly near the poles of the BPE: ∆V ≈ U . Therefore, closed bipolar electrochemistry needs usually lower voltages between the feeder electrodes, since the whole voltage drop is experienced by the BPE and not just a small fraction that depends on its size.

.8b). [130]

Closed bipolar electrochemistry

Since the bipolar oxidation and reduction do not happen in the same solution, it is possible to separate in each cell the reactants and products of each half-reaction. In a closed configuration, it is also possible to split the BPE and link its anodic and cathodic poles with an electrical wire. This allows to physically separate the two cells. It also allows to combine bipolar electrochemistry with other electrochemical concepts like the use of micro-and nanoelectrodes. [START_REF] Guerrette | Coupled Electrochemical Reactions at Bipolar Microelectrodes and Nanoelectrodes[END_REF] Ongaro and co-workers used a closed bipolar set-up to functionalize carbon microwires and gold nanowires with platinum and semiconductor oxides. [START_REF] Ongaro | Closed Bipolar Electrochemistry for the Low-Potential Asymmetrical Functionalization of Micro-and Nanowires[END_REF] Using the set-up shown in Figure 2.10, they simultaneously reduced Pt(NO 3 ) 2 to Pt 0 at the cathodic side and oxidized TiCl 3 to TiO 2 at the anodic side. Such a deposition is not possible in an open set-up since Pt(NO 3 ) 2 and TiCl 3 would spontaneously react with each other in the bulk if they were both present in the same solution as it would have been the case in an open set-up.

Since the two poles of the BPE are separated in a closed bipolar set-up, but are still directly electrically linked, it is useful for the design of sensors. One side of the electrode is in contact with the analyte and is the detection side and the other side is the reporting side. Since the number of electrons reacting at the analyte side is equal to the number of electrons reacting at the reporting side, the reporting reaction intensity is directly proportional to the quantity of analyte reacting on the other side of the electrode. Several chemical systems can be used as the reporting reaction.

Electrochemiluminescence (ECL) is one of the most used concepts for the reporting reaction. [START_REF] Zhang | Graphite paper-based bipolar electrode electrochemiluminescence sensing platform[END_REF][START_REF] Bouffier | Generation of electrochemiluminescence at bipolar electrodes: concepts and applications[END_REF] The intensity of the generated light is directly proportional to the reaction rate of the analyte (Fig. 2.11). The electro-dissolution or deposition [START_REF] Hadjixenis | Bipolar electrochemical detection of reducing compounds based on visual observation of a metal electrodeposited track at the onset driving voltage[END_REF] of a metal layer have also been used as reporting events.

Indirect bipolar electrochemistry

One of the usual drawbacks of (bipolar) electrochemistry for object modification is the limitation to deposits made from electroactive compounds. Numerous interesting coating compounds can only be deposited through non-electrochemical methods like pHtriggered precipitations or polymerisations. Indirect bipolar electrochemistry is a method which permits the combination of these non-electrochemical techniques and bipolar electrochemistry.

In indirect bipolar electrochemistry, the reaction of interest is not the electrochemical reaction happening at the pole of the BPE, but another reaction triggered by the electrochemical one. A straightforward example is an acid/base reaction triggered by the generation of a pH gradient due to the oxidation and reduction of water (Eq. (2.9) and Eq. (2.10)) by bipolar electrochemistry (Fig. 2.12a). Ino et al. used this technique to deposit hydrogel on a BPE by using water electrolysis to acidify the solution near the anodic pole of the electrode, triggering the formation of a Ca-alginate gel (Fig. 2.12b). [START_REF] Ino | Hydrogel electrodeposition based on bipolar electrochemistry[END_REF] Bipolar water electrolysis was also used for the deposition of electrophoretic paint. [START_REF] Loget | Indirect Bipolar Electrodeposition[END_REF]

H 2 O -→ 1 2 O 2 + 2 H + + 2 e -E • O2/H2O = +1.23 V vs. SHE (2.9) 2 H + + 2 e --→ H 2 E • H2/H + = +0.00 V vs. SHE (2.10)
Water electrolysis is not the only electrochemical reaction used to trigger a deposition. Fattah et al. used the bipolar dissolution of a zinc BPE to trigger the precipitation of electrophoretic paint [START_REF] Fattah | Indirect bipolar electrodeposition of polymers for the controlled design of zinc microswimmers[END_REF]. The Zn 2+ ions produced by the oxidation lead to a complexation of the polymer chains, triggering the precipitation on the electrophoretic paint.

Qi and co-workers used indirect bipolar electrochemistry for water treatment by electrocoagulation. By using sacrificial iron BPEs, they produced iron cations, inducing the precipitation and coagulation of numerous pollutants. [START_REF] Qi | Wireless Electrocoagulation in Water Treatment Based on Bipolar Electrochemistry[END_REF]

Generating movement with bipolar electrochemistry

As previously discussed in Section 1.6 (page 39), to generate movement, some sort of asymmetry must be established. Bipolar electrochemistry is an inherently symmetrybreaking technique. It is therefore particularly well-suited for the generation of movement and the synthesis of swimmers [START_REF] Loget | Electric field-induced chemical locomotion of conducting objects[END_REF]. Numerous Janus objects synthesized by bipolar electrochemistry have been used as swimmers. [START_REF] Fattah | Straightforward single-step generation of microswimmers by bipolar electrochemistry[END_REF][START_REF] Moo | Nano/Microrobots Meet Electrochemistry[END_REF] Even isotropic BPEs can be put into movement by exploiting the potential gradient along their surfaces. Loget and Kuhn were able to move a GC bead in a tube by using a DC potential to trigger the reduction of protons into H 2 at the bottom of the bead. [START_REF] Sentic | Light-Emitting Electrochemical "Swimmers[END_REF][START_REF]Bipolar electrochemistry for cargo-lifting in fluid channels[END_REF] The hydrogen bubbles' buoyancy lifts the bead (Fig. 2.13a). In order to avoid water oxidation and oxygen bubble formation at the top of the bead, hydroquinone (HQ) was introduced in the solution. Since the standard potential of HQ oxidation into benzoquinone (BQ) (Eq. (2.11)) is lower than the H 2 O/O 2 standard potential (Eq. (2.9)), HQ is favourably oxidized and no O 2 bubbles are produced.

-→ + 2 H + + 2 e -E • HQ/BQ = +0.70 V vs. SHE (2.11)
Loget and Kuhn also produced motion via bipolar self-regeneration of a zinc dendrite. [START_REF] Loget | Propulsion of Microobjects by Dynamic Bipolar Self-Regeneration[END_REF] The dendrite was placed in a capillary containing a zinc sulfate solution. When an electric field was applied, the zinc at the anodic pole started to get oxidized into Zn 2+ , dissolving the pole. At the cathodic pole, Zn 2+ reduced into Zn 0 and deposited, thickening the pole. This results in a net movement of the dendrite along the electric field (Fig. 2

.13c).

Bipolar electrochemistry has also been used to produce other kinds of motion apart from swimmers. Eßmann and co-workers created a cross shaped BPE which acts as a rotor when a strong enough electric field is applied. [START_REF] Eßmann | Wireless Light-Emitting Electrochemical Rotors[END_REF] One of the arms of the BPE acts as the cathodic pole and produces hydrogen bubbles through water reduction, which lift the arm thanks to their buoyancy, thus triggering a rotation of the whole BPE. The bubbles are released when the arm reaches the top and the next arm starts to act as the cathodic pole, maintaining the rotational movement (Fig. 2.13b).

Gupta et al. used bipolar electrochemistry to create swelling gradients in polypyrrole films through differential reduction of the conducting polymer. This differential swelling causes the polymer film to bend, creating movement (Fig. 2.13d). [START_REF] Gupta | Wireless Electrochemical Actuation of Conducting Polymers[END_REF] They used this bending to develop chemically selective actuators [START_REF] Zhang | Wireless Electromechanical Readout of Chemical Information[END_REF][START_REF] Assavapanumat | Chiral platinum-polypyrrole hybrid films as efficient enantioselective actuators[END_REF] and to produce electrochemical crawlers able to "walk" on a surface [START_REF] Gupta | Bipolar Conducting Polymer Crawlers Based on Triple Symmetry Breaking[END_REF].

Spot-bipolar electrochemistry for the patterning of conductive surfaces

So far in this chapter, we have discussed bipolar techniques using linear fields which spanned the whole length of the BPE. In this section, we will discuss a bipolar electrochemical technique using local, non-linear fields for the patterning of conducting surfaces. This spot-bipolar configuration was developed by Ishiguro and colleagues in 2012 [START_REF] Ishiguro | Site-Controlled Application of Electric Potential on a Conducting Polymer "Canvas[END_REF]. In this case, the feeder electrodes are not plane electrodes on each side of the solution producing a linear potential gradient in the whole bulk, but two concentric circular electrodes producing a local electric field near the surface of the BPE (Fig. 2.14a). Due to the unique shape and the local nature of the electric field, the anodic and cathodic areas are not positioned at the extremities of the BPE but they form concentric discs directly below the circular feeder electrodes (Fig. 2.14a). Since the positions of the electroactive areas depend only on the position of the feeder electrodes, they will follow the feeder electrodes if the latter are moved. It is therefore possible to draw patterns on a BPE simply by moving the pair of feeder electrodes along its surface: only the portion of the BPE that has been exposed to the electric field (i.e. which has had the feeder electrodes directly on top of it) will be electrochemically modified (Fig. 2.14b-d). [START_REF] Shida | Electrochemically Mediated Atom Transfer Radical Polymerization from a Substrate Surface Manipulated by Bipolar Electrolysis: Fabrication of Gradient and Patterned Polymer Brushes[END_REF][START_REF] Ishiguro | Site-Controlled Application of Electric Potential on a Conducting Polymer "Canvas[END_REF][START_REF] Braun | Localized Electrodeposition and Patterning Using Bipolar Electrochemistry[END_REF]

Conclusion

In these first two chapters, we have seen that numerous non-electrochemical techniques exist in order to produce Janus and asymmetric particles and also that bipolar electrochemistry is a very versatile technique for the asymmetric modification of conductive objects. However, most of the non-electrochemical methods need the presence of an interface in order to break the symmetry of the system and create asymmetric particles. This greatly limits their yields since the reaction space is confined near a 2D surface instead of occupying a truly 3D volume. On the other hand, the bipolar techniques previously developed for the modification of isotropic objects do permit a true bulk synthesis of Janus objects. Nevertheless, they only allow for the breaking of a single axis of symmetry to produce Janus objects. Furthermore, they do not allow for the modification of particles with multiple patches with different compositions.

In the next chapters, we will present new bipolar techniques that allow for the production of more complex asymmetric particles. These techniques rely on the use of asymmetric electric signals as well as a precise control of the orientation of the BPEs relative to the electric field generated by the feeder electrodes. These techniques allow for the modification of isotropic objects into complex asymmetric objects with multiples patches of different compositions positioned at any location on the surface of the objects. This allows for the bulk synthesis of chiral objects with a controlled handedness.

Chapter 3 AC-bipolar electrochemistry for the synthesis of asymmetric particles

The methods for the synthesis of asymmetric objects presented in the previous chapter only allow for the deposition of a single patch or deposit on the surface of conducting objects. They do not allow the synthesis of more complex asymmetric objects. In order to synthesize complex particles with multiple deposits distributed in a predefined way over their surface, like for example chiral particles, new bipolar electrochemistry approaches must be developed. In this chapter, methods that can simultaneously deposit two patches on a single conducting particle in a single step by applying more complex signals than simple DC voltages at the feeder electrodes during a bipolar electrodeposition will be explored.

Pulsed bipolar electrochemistry

When an anisotropic conducting object is placed in an electric field, it will spontaneously align with the field. In 2011, Loget et al. bifunctionalized carbon tubes (CTs) with a deposit at each end in a single step using pulsed bipolar electrochemistry. [START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF] When placed in a solution of metallic precursors in an electric field, a CT will align with the field and become a BPE with its ends acting as the poles. If a pulsed signal is applied between the feeder electrodes instead of a DC voltage, during the pulses, the tube will be aligned with the field and the reduction of the metallic precursors will occur at its cathodic pole. During the relaxation period, the reactions stops and the CT will lose its alignment and orient itself randomly in the solution. At the beginning of the next pulse, all the CTs will realign themselves with the electric field but, statistically, half of them will have switched their orientation and their extremities will have inverted anodic and cathodic poles. After numerous pulses, both extremities of a CT will have spent equal periods of time as cathodic poles and as anodic poles and the CT will be modified with two identical patches on its extremities. This method relies on the spontaneous orientation along the electric field of nonisotropic BPEs. Hence, it cannot be used to modify isotropic objects (e.g., spherical particles) and create asymmetry. Since they do not have a preferential axis, spherical particles will not orient themselves along the electric field. The BPEs will keep rotating randomly in the solution during the pulse period. Therefore, the cathodic and anodic poles of the BPEs will continuously move on their surface [START_REF] Roche | Physico-chemical milling for controlled size reduction of metal beads[END_REF]. After a while, the poles will have explored the whole surface of a BPE and thus a uniform deposit covering the entire surface is obtained.

Rotation of spherical BPEs

To selectively deposit patches at the surface of a spherical particle using bipolar electrochemistry, the rotation of the particles in the solution must be stopped or at least slowed down significantly with respect to the timescale of the experiment. Two methods are mainly used.

Blocking the BPEs on an interface

One possibility to fix the orientation of spherical BPEs is to deposit them on a horizontal support that blocks their movements (Fig. 3.1a). This method is the easiest to implement and is often used for designing proof-of-concept experiments where a high yield is not necessary. We first modified particles by using such a set-up. Double-sided adhesive tape has been used to fix GC spheres at the bottom of a bipolar cell. The precursor solution was then poured in the cell and the bipolar electrodeposition was started. At the end of the experiment, the functionalized particles were removed from the tape using tweezers.

Limiting the possible locations of the BPEs to a single plane significantly decreases the quantity of BPEs that can be modified per batch. Consequently, one of the main advantages of bipolar electrochemistry for the modification of particles is lost: the possibility to asymmetrically modify particles dispersed in the bulk of the cell without relying on an interface. Moreover, the part of the BPE's surface in contact with the adhesive is not exposed to the solution and cannot be modified. Finally, the support with the adhesive tape alters the electric field lines around the particle, inducing inhomogeneous deposits. Since the goal is to selectively deposit patches at any position on the surface of BPEs, this method was not explored further except for some preliminary experiments. Instead, another method, not relying on an interface was explored.

Blocking the BPEs with a gel

Another commonly used approach to stop the movement of BPEs during a bipolar experiment is to disperse them in a hydrogel containing the precursor solution. The highly viscous medium blocks any movement of the dispersed particles. The gel is then placed in the bipolar cell between the feeder electrodes (Fig. 3.1b). The main advantage of this technique is that the BPEs are dispersed in the bulk of the solution and not solely on a 2D plane. This configuration allows modifying numerous particles in a single bipolar experiment. Moreover, the entire surface of the BPEs is in contact with the solution. This enables the generation of patches at any location on the BPEs' surface.

In this chapter, the particles used as BPEs have a diameter of around 1000 µm and the distance between the feeder electrodes is 30 mm. With such values, the potential difference between the feeder electrodes must be in the 10 V -100 V range, depending on the involved electrochemical half-reactions, in order to generate deposits at the poles of the BPEs. Since the deposition occurs in an aqueous environment and uses high voltages, water electrolysis will occur at the feeder electrodes accompanied by hydrogen and oxygen bubble formation. If the gel is directly in contact with the feeder electrodes, it will trap the gas bubbles at the interface with the electrode and a gas layer will form, insulating the feeder electrodes from the bulk of the gel. Consequently, a potassium chloride (KCl) solution is placed between the gel and the feeder electrodes to allow the bubbles to rise to the air/water interface and for the ions to move between the feeder electrodes and the gel containing the BPEs. In a classic three-electrode set-up, a high concentration of supporting electrolyte (e.g., 1 mol•L -1 ) is typically used to decrease the potential drop in the bulk of the solution. In contrast, in a bipolar set-up, reactions are driven by the potential difference between two points of the solution. Thus, it is necessary to maximize the potential drop in the solution and therefore its resistivity. Thus, a very low concentration of only 1 mM of KCl is used.

The gel most commonly used to disperse BPEs is an agarose gel. A solution containing agarose, the precursor used for the bipolar deposition and the BPEs is heated until the agarose is completely dissolved and is then poured into the bipolar cell. While cooling, the agarose polymer chains will link themselves with hydrogen bonds and form a 3D network that causes the gelation of the solution. This gelation process is reversible. After the deposition, the gel is heated, which allows collecting the functionalized particles from the liquefied phase.

Agarose is a polysaccharide and can be oxidized. [START_REF] Zhang | Preparation of oxidized agar and characterization of its properties[END_REF] As the precursors used to deposit metallic patches on BPEs are oxidants, they eventually can be reduced by the agarose, causing the formation of small metal particles in the bulk of the gel. The presence of these particles can change the distribution of the electric field, and thus disturb the functionalization of the BPEs, resulting in less reproducible experiments and less homogenously functionalized particles. However, this reduction process is slow and for short bipolar experiments, agarose gel can still be used. For longer experiments, an elec-trochemically inert gelling agent is necessary.

Fumed silica is a powder composed of microparticles of amorphous silica with a very high surface area commonly used as a thickening agent. Since silica is electrochemically inert, a gel based on fumed silica as a gelling agent can be used for longer experiments without the risk of the parasitic chemical reduction of the metallic precursors. Fumed silica powder is mixed with an aqueous solution of the precursor containing the BPEs. The mixture is then stirred until the formation of a homogenous gel which is poured into the bipolar cell. At the end of the deposition, the gel is removed from the bipolar cell, immersed in distilled water at room temperature and then either stirred using a vortex mixer or slowly heated to around 75 ℃. This causes the gel to lose its cohesion and to dissolve in water. The particles can then be collected and washed. The heating method is preferred since the strong mixing of the gel can damage the patches at the surface of the particles.

One drawback of the use of hydrogels in bipolar cells is that in a gel the absence of convection does not allow maintaining a homogenous precursor concentration in the bulk. This can cause local depletions of the precursor and inhomogeneous deposits. In the method described previously [START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF], Loget and colleagues used the random rotation of the CTs in solution combined with their spontaneous alignment with an electric field to statistically rotate half of the CTs by 180°at the beginning of each pulse. This technique cannot be used with spherical BPEs. In order to deposit patches on opposite sides of a spherical BPE, either the orientation of the BPE must be changed by another mean, or the direction of the electric field itself must be switched. Thus, the use of AC voltages instead of pulses is an interesting concept to explore as it allows imposing electric 3.3. Symmetric AC voltages fields of various orientations and amplitudes. Koizumi et al. published a method [START_REF] Koizumi | Bifunctional Modification of Conductive Particles by Iterative Bipolar Electrodeposition of Metals[END_REF] for synthesizing particles with two metallic patches on opposite sides using bipolar electrochemistry in a single step (Fig. 3.2a). By applying a square wave potential difference between the feeder electrodes instead of a DC voltage, they periodically reversed the direction of the electric field. The cathodic pole on which the reduction of the metallic precursor occurred was therefore periodically switched from one extremity of the BPE to the other. This process results in two identical metallic patches on opposite ends of a particle. Using this method, they bifunctionalized GC particles with two patches by reducing gold tetrachloride (AuCl 4 -) in order to generate metallic gold on both ends of the particle (Fig. 3.2b-d). The size of the patches can be tuned by controlling the amplitude of the applied square wave, similar to DC-bipolar electrochemistry (see Section 2.1.1).

Symmetric AC voltages

We synthetized similar particles by using AC-bipolar electrochemistry. Glassy carbon particles with a size ranging from 630 µm to 1000 µm were immobilized on a double-sided adhesive tape at the bottom of a bipolar cell in a solution of 1 mM of HAuCl 4 between two feeder electrodes located 30 mm from each other. An AC voltage is then applied between the feeder electrodes in order to reduce the precursor at both poles of the BPEs. After the experiment was finished, the modified particles were removed from the tape using tweezers, washed and observed by SEM (Fig. 3.3). A large sample of frequencies ranging from 0.1 Hz to 10 kHz was tested in order to determine the optimal range for the deposition of gold. It was found that particles bifunctionalized with two uniform patches can be obtained for frequencies ranging from 10 Hz to 1 kHz. At lower frequencies, depletion effects similar to the ones observed in DCbipolar experiments lead to inhomogeneous and ring-like deposits. At higher frequencies, the capacitive currents dominate the faradic currents and no electrochemical reactions occur at the poles of the BPEs.

As mentioned previously, this method where the BPEs are fixed in a plane only allows modification of approximately five or six particles per batch. Moreover, the particles must be manually placed in the cell and collected after the experiments using tweezers that can damage the deposits. In order to functionalize more particles per experiment, the set-up was changed and the particles were dispersed in a gel.

Glassy carbon particles are mixed with an aqueous fumed silica gel containing 1 mM of a metallic precursor (either HAuCl 4 or (NH 4 ) 2 PdCl 4 ). The gel is then placed between two 30 × 30 mm 2 GC feeder electrodes, oriented parallel to each other at a distance of 30 mm.

The absence of convection forces in the gel and the difference in conductivity between the solution and the gel leads to lower optimal frequencies for the AC-bipolar functionalization. Signals with longer periods were tried out in order to find the optimized polarization periods for this set-up.

For an optimized deposition of gold from HAuCl 4 , each deposition step should last for 135 s, which means that for a symmetric deposition, the square wave must have a period of 270 s (Fig. 3.4a). For the deposition of palladium, shorter deposition periods are possible and 20 s has been used for a symmetric deposition (Fig. 3.4d). The optimized amplitudes for a successful generation of deposits were 30 V (1 kV•m -1 ) for gold and 40 V (1.3 kV•m -1 ) for palladium. The counter reaction is the oxidation of water to O 2 gas. In both cases, the AC voltage was applied for a total time of 30 min. Since the applied AC voltage is symmetric, the overall deposition times for each side of the BPEs are the same and equal to 15 min.

At the end of the deposition, the gel was dissolved in distilled water and the modified particles were collected, washed again with distilled water and observed by SEM. A representative set of modified particles is shown in Figure 3.4. The palladium patches (Fig. 3.4e-f) are smaller than the gold patches (Fig. 3.4bc) despite a higher applied voltage, because the standard potential for the reduction of palladium (Eq. (3.1)) is lower than the standard potential for the reduction of gold (Eq. (2.8)).

PdCl 4 2-+ 2 e ---→ Pd 0 + 4 Cl -E • PdCl4 2-/Pd = +0.59 V vs. SHE (3.1)
The size of a patch depends on ∆V (Eq. (2.7)), which itself depends on the size of the BPE (Eq. (2.2)). Since the particles have a size ranging from 630 µm to 1000 µm, the potential difference between the poles of the biggest particles will be approximately 1.6 times bigger than the potential difference between the poles of the smallest particles. This size distribution can decrease the reproducibility of the bipolar electrodeposition. If the size distribution of the particles used in a specific experiment is skewed compared to the distribution of the commercial, unmodified particles, the actual ∆V between the extremities of the BPEs can be different than the calculated ∆V . This causes the resulting patches to be bigger or smaller than expected. This effect is especially noticeable when modifying only few particles per experiment where it is statistically more likely to obtain a skewed size distribution. Furthermore, some of the commercial particles used are not perfect spheres and are elongated along one axis to form an oval shape (e.g., the modified BPE at the top left of Figure 3.4b). For these non-isotropic objects, the size of the deposits depends on the orientation of the objects in the electric field. This lack of reproducibility can be improved by manually choosing particles with a more homogenous size. However, this is only possible when modifying a few BPEs at a time.

For experiments using many particles per batch, their size distribution is more likely to remain consistent between experiments, but this leads to inhomogeneities in the size of the patches obtained on the functionalized particles of a single experiment. This results in a more reproducible but less accurate modification of the BPEs. The inhomogeneous diameter can even lead to completely unmodified particles when using low voltages, because ∆V > ∆V min for the bigger BPEs but ∆V < ∆V min for the smaller ones. When choosing a certain deposition voltage, one should therefore always calculate the ∆V for the smallest BPEs present in order to be sure that all the BPEs will be functionalized. These inhomogeneities in size and ∆V make the deposition of small patches difficult: either it is possible to deposit small patches on the bigger BPEs, but no patches will be formed on the smaller ones, or generate small patches even on the BPEs with a small diameter, but leading to rather big deposits on the larger particles.

Asymmetric AC voltages

The method described in the previous section only allows for the deposition of two identical patches on each side of a BPE. In this section, a variation of this method that allows for the deposition of two patches with different size on the poles of the BPEs will be presented. Instead of a simple symmetrical square wave, an asymmetrical signal with different positive and negative voltage amplitudes is applied. Since the positive and negative voltages are independently adjustable, the size of the two patches can be tuned at will to be different.

Compensation of the migration of ionic precursors

During bipolar electrodeposition, an electric field is applied to the gel containing the BPEs. The charged species present in the gel will therefore migrate towards the feeder electrode of opposite polarity. Since most of the precursors used for the deposition experiments are ionic, they will have migrated after a while close to a feeder electrode and away from the BPEs. Consequently, the deposition will stop. Since the deposition takes place inside a gel, there are no convection forces to counter this migration effect.

The use of a symmetric AC field cancels this migration effect, since the potential difference and thus the direction of migration switches periodically, resulting in no net migration during a full period. However, in the case of an asymmetric signal, for which the positive and negative potentials are not equal in absolute values, the migration effects in both directions will not compensate each other anymore, and a net migration in one direction will be observed. To compensate for this effect and cancel the net migration with different positive and negative potentials, the time during which each potential is applied must be adjusted. The formula for the electrophoretic velocity v e of an ion migrating in an electric field E can be used to determine how the duration of each potential can be adjusted:

v e = µ e • E (3.2)
The distance d e travelled by the ion during a time t is therefore:

d e = µ e • E • t (3.3)
with µ e being the electrophoretic mobility of the ion. We want no net migration i.e., the distances d + and d -that the ions travel in both directions during the positive (T + ) and negative (T -) fractions of a period T must be equal:

|d + | = |d -| (3.4) ⇒ µ e • |E + | • T + = µ e • |E -| • T - (3.5) ⇒ T + T - = E - E + (3.6)
The period T of the AC voltage used for the deposition is thus:

T = T + + T - (3.7)
⇒ T = T + 1 + E + E - (3.8)
and the duty cycle D of the signal is:

D = T + T = |E -| E + + |E -| (3.9)
The proportions of the total period T for every field orientation can be adjusted using Equations (3.8) and (3.9) in such a way that no net migration effect is observed during the experiment. Since E ∝ U (Eq. (2.1)), E + and E -can be substituted in the equations (3.6) to (3.9) by U + and U -respectively, the potentials applied between the feeder electrodes.

t total T = t + T + (3.10) ⇒ t total = t + 1 + U + U - (3.11)
Equation (3.11) illustrates that when keeping U + constant, the lower |U -| is, the longer the total deposition time will be.

GC particles were suspended in a fumed silica gel and placed in a KCl solution between the two feeder electrodes in the same way as described previously for symmetric particles. The deposition times and periods were determined using Equations (3.8), (3.9) and (3.11). For the deposition of gold patches, the applied voltages are shown in Figures 3.5a and 3.5d. Even though U + and U -are different for the two experiments, Equations (3.8) and (3.11) show that T and t total only depend on the ratio between the two voltages and not their values. Since this ratio is identical for both experiments, T and t total are also identical. As previously shown, 135 s is an optimised length for T + to deposit a gold patch. By using Equation (3.8), it was determined that for this U + /U -ratio, T must be equal to 405 s. The shortest cumulated deposition time during an experiment t + must be equal to 15 min. Thus, using Equation (3.11), the duration t total of the experiments needs to be equal to 45 min. Representative examples of the asymmetric particles with two gold patches of different size are shown in Figures 3.5b, 3.5c, 3.5e and 3.5f.

For the deposition of palladium, the applied voltage signal is shown in Figure 3.5g. The period was shorter (40 s), since the minimum deposition time for palladium is only 10 s. To keep the shorter deposition time t + equal to 15 min, the total deposition time was in this case equal to 60 min.

Size of the metal deposits as a function of U

On a BPE, the cathodic and anodic poles both cover a specific portion of each extremity of the BPE. On a spherical BPE, the surface covered by the poles can be measured by the angle between the centre of the BPE, the centre of the pole and the circle delimiting the end of the deposit (Fig. 3.6a). This angle is α for the cathodic pole and β for the anodic pole. When depositing a metal, the precursor will be reduced on the entire cathodic pole surface which experiences a sufficient polarization. Thus, the angle α will also correspond to the size of the resulting metal deposit and can be measured by observing the modified particles by SEM.

In order to observe electrochemical reactions on a BPE, the potential difference between the poles ∆V must be higher than ∆V min , the difference between the standard potentials of the two half-reactions involved (see Section 2.1). This potential difference depends on the electric field E and the distance between the poles (Fig. 3.6a). The poles' limits will be the positions at the surface of the BPE where ∆V = ∆V min . Hence:

∆V min = E d 2 (cos α + cos β) (3.12)
Assuming that the areas covered by the cathodic and anodic poles are equal, α = β and:

∆V min = Ed cos α = U d L cos α (3.13) ⇒ α = arccos ∆V min L U d (3.14)
with L the distance between the feeder electrodes and U the potential difference applied between them. The value for α given by Equation (3.14) can be compared with the angle measured on particles modified with gold patches using AC-bipolar electrochemistry at different voltages (Fig. 3.6b). The values predicted by Equation (3.14) are consistently higher that the values measured on particles observed by SEM. This discrepancy can be due to potential drops at the liquid/gel interfaces causing the value of the electric field to be lower inside the gel than if it remained perfectly linear between the feeder electrodes. Small potential drops are also present at the surface of the feeder electrodes. Moreover, the theoretical value determined for ∆V min assumes there is no overpotentials for the redox reactions at the poles of the BPEs. This causes an underestimation of ∆V min and thus, an overestimation in the predicted sizes of the patches since the arccosine is a decreasing function.

The variability in the measured angles is due to two main factors. First of all, as already discussed previously, the sizes of the particles used as BPEs range from 630 µm to 1000 µm causing a variation in the size of the patches. Additionally, when observed by SEM, the modified particles are randomly oriented (e.g., see Fig. 3.5b). This variation in orientation can hide some of the patches or change the apparent size of the visible patches, making a precise measure of α more difficult.

Relative thickness of the metal deposits

When depositing two patches using asymmetrical AC-bipolar electrochemistry, the two patches have different sizes, but it is also observed that the patches have a different thickness (e.g., see the patches on the particle shown in Figure 3.5c). The smaller patch is much thinner than the larger one. The thickness of a deposit depends on the length of the deposition time, but also on the current density at the cathodic pole during the deposition. The current density at the cathodic pole of a BPE is given by the Butler-Volmer equation [START_REF] Duval | Bipolar electrode behaviour of the aluminium surface in a lateral electric field[END_REF]:

j c = -j 0 exp - α c nFη c RT (3.15)
with:

• j c , the current density at the cathodic pole of the BPE;

• j 0 , the exchange current density;

• α c , the cathodic charge transfer coefficient;

• n, the number of electrons involved in the electrochemical half-reaction;

• F, the Faraday constant;

• R, the gas constant;

• T, the absolute temperature;

• η c , the cathodic overpotential.

The Butler-Volmer equation assumes that the mass transfer rate is much larger than the electron transfer rate and that the precursor concentration near the BPE remains similar to the concentration in the bulk. This is not true when the BPE is in a gel, but the Butler-Volmer equation can still allow a qualitative analysis of the change in current density as a function of potential. The overpotential η c (α) at a point on the surface positioned at an angle α on the cathodic pole is the difference between E c (α), the potential at that point and E • , the standard electrode potential of the half-reaction:

η c (α) = E c (α) -E • (3.16)
By assuming that the line of zero overpotential remains close to the centre of the BPE [START_REF] Li | Visualizing the Zero-Potential Line of Bipolar Electrodes with Arbitrary Geometry[END_REF] (see Fig. 2.1) and using Equation (3.13), E c (α) can be approximated by:

E c (α) ≈ -∆V min 2 = - U d 2L cos α (3.17)
with d the diameter of the BPE and L the distance between the feeder electrodes. The current density at a point of the cathodic pole positioned at an angle α is therefore:

j c (α) = -j 0 exp α c nF RT U d 2L cos α + E • (3.18)
Since the cosine function is decreasing between 0°and 90°, the farther away the point is from the extremity of the cathodic poles, the lower the current density is and the less thick the deposit will be at this point. This effect can be observed on some of the particles presented previously but it is partially balanced by another effect that tends to decrease the thickness of the centre of a deposit when using anionic precursors. This other effect will be discussed in more details in Section 3.5.

The ratio between the current densities for the two deposition processes occurring during an asymmetric AC-bipolar experiment is:

j c+ (α) j c-(α) = exp - α c nF RT (η c+ (α) -η c-(α)) (3.19)
with j c+ and j c-the current densities and η c+ and η c-the cathodic overpotentials during the positive and negative fractions of a period respectively. The difference in overpotentials is:

η c+ (α) -η c-(α) = E c+ (α) -|E c-(α)| (3.20) ⇒ η c+ (α) -η c-(α) = - d 2L (U + -|U -|) cos α (3.21)
Therefore, by using Equations (3.21) and (2.2), Equation (3.19) can be rewritten as:

j c+ (α) j c-(α) = exp α c nF RT d 2L (U + -|U -|) cos α (3.22)
The ratio between the current densities varies exponentially with the difference between the two deposition voltages. For a symmetric AC signal, j c+ /j c-= 1 and the two patches will have the same thickness but for an asymmetric signal, when |U -| decreases and U + is kept constant, the current density ratio increases exponentially while the period ratio (T + /T -) only decreases linearly (Eq. (3.6)). This linear decrease of the period ratio cannot compensate for the exponential increase of the current density ratio. This explains that on a particle modified by asymmetric AC-bipolar electrochemistry, despite having a longer deposition time, the patch deposited with a lower potential is much thinner than the patch deposited with a higher potential.

Deposition of a single patch using asymmetric AC bipolar electrochemistry

As described in Section 2.1, if the potential gradient present in solution results in a potential difference at the poles of the BPEs lower than ∆V min , no deposition will occur. This can be exploited to use AC-bipolar electrochemistry to deposit a single patch instead of two. If |E -| is low enough, deposition will occur only during the positive cycle, but the migration of the ionic precursor will still be compensated by the low negative potential. With this method, it is possible to use longer deposition times since the precursor will globally not migrate away from the BPEs. Using Equations (2.2) and (2.5), the ∆V min and U min at which no deposition will occur can be determined. In the case of gold deposition combined with water oxidation as the counter reaction, ∆V min = 0.24 V was obtained. For the case of 1000 µm particles, placed between feeder electrodes which are separated by 30 mm, a value of U min = 7.2 V can be estimated. If a potential U < U min is applied, no deposition will be possible at the cathodic poles of the BPEs, but the ionic precursor will still migrate in the electric field created by the feeder electrodes.

Particles were modified with a single metallic patch by using this method. A high positive voltage (e.g., 40 V) was applied while the negative voltage was limited to -5 V which is lower than ∆V min for all the metallic precursor used. The signal used and the resulting particles can be observed in Figure 3.7.

Synthesis of ring deposits with anionic precursors

As discussed previously, during a DC-bipolar electrodeposition, the ionic precursors present in solution will migrate in the electric field towards the feeder electrode of opposite polarity. Cationic precursors (e.g. Ag + ) will migrate towards the cathodic feeder electrode. Near a BPE, they will therefore also migrate towards the cathodic pole of the BPE where the deposition is taking place (Fig. 3.8a). In the opposite case of anionic precursors (e.g. AuCl 4

-or PtCl 6 2 -), they will migrate towards the anodic feeder electrode and also away from the cathodic pole of the BPE. This creates a low concentration near the cathodic pole and no electrodeposition will take place. The only precursors that migrate to this zone are anions coming from the backside of the BPE on their way from the cathodic to the anodic feeder electrode. These ions need to move around the BPE and thus arrive first at a moderately polarized cathodic zone, located close to the equator of the particle, where they can be reduced (Fig. 3.8b). This effect has been used by Roche et al. to deposit ring shaped deposits on spherical particles (Fig. 3.8c,d). [START_REF] Roche | Straight-forward synthesis of ringed particles[END_REF] Figure 3.8 -The effect of precursor migration on the shape of the metallic deposits. (a) When the metallic precursor is a cation, it will migrate towards the feeder cathode and therefore towards the cathodic pole of the BPE where electrodeposition is possible. This results in a uniform spherical deposit. (b) When the precursor is an anion, it will migrate towards the feeder anode and therefore away from the cathodic pole of the BPE. This depletion effect results in the deposition of a ring at the border of the cathodic pole of the BPE. (c) SEM image of GC particles modified with a platinum ring. Inset: detailed view of a single modified particle (d) SEM images of GC particles modified with a gold ring. Inset: detailed view of a single modified particle. Adapted from Ref. [START_REF] Roche | Straight-forward synthesis of ringed particles[END_REF].

While this effect can be used on purpose, it is an issue when the goal is the deposition of uniform patches. Asymmetric AC-bipolar electrochemistry can be used to counter this effect. Since the AC signal can be adjusted in such a way that there is no net migration during the deposition, no overall depletion of the precursor should occur near the cathodic pole and a uniform deposit will form instead of a ring. Hence, in the rest of this thesis, all homogeneous deposits involving anionic precursors will be generated by using AC-bipolar electrochemistry and not the simpler DC method, in order to avoid the formation of rings. The method for depositing a single patch with an AC voltage, described in the previous section, will be used.

Conclusion

In this chapter, a versatile method for the modification of conducting particles allowing a control of the shape and position of the deposit was presented. This method allows generating up to two deposits of the same chemical composition along a single axis in a single step. It also enables the deposition of metal patches from anionic precursors without the formation of rings, which are normally observed due to a depletion effect near the cathodic poles of the BPEs.

While allowing for the synthesis of more complex particles with asymmetric patch sizes, this method is restricted to the modification of particles along a single axis and with patches of a single chemical composition. In order to synthesize more complex particles like chiral particles, it is mandatory to deposit patches at different angles with respect to the initial axis. It is also necessary to generate patches with a different chemical composition. Therefore, in the next chapter a combination of the approach presented here with new bipolar electrochemical set-ups will be described, in order to enable the deposition of patches along different angles. The possibility to deposit patches of different chemical compositions on the same particle will also be demonstrated.

Chapter 4

Site-selective deposition of different metals at the surface of a spherical BPE In the previous chapter, it was shown that, by using more complex voltages than simple DC voltages, it was possible to deposit patches of different sizes, but with the same chemical composition at each end of a BPE. However, in order to be able to synthesize chiral particles by using bipolar electrochemistry, even more complex deposition techniques must be developed. For a particle to be chiral, it must not have any plane of symmetry. To achieve this, the particles need to have patches in the three spatial dimensions and not just along a single axis. Moreover, it is mandatory to deposit at least three patches with different chemical compositions to break all symmetry axes. If two identical patches are present at the surface of a spherical particle, the plane defined by the centre of the particle and the centres of the two patches is a plane of symmetry and the particle is not chiral.

In this chapter, two different methods are developed. The first method permits for the deposition of patches at any position of BPEs by using bipolar electrochemistry. The second method permits a change of the precursor present in a bipolar cell in between individual deposition steps and thus, allows for the control of the composition of different patches deposited sequentially at the surface of BPEs. By combining these two techniques it is possible to transform isotropic spherical conductive particles into chiral particles with four different patches arranged in a tetrahedral geometry.

Repartition of the current flowing between the feeder electrodes

When performing bipolar electrochemistry experiments in an open configuration, the charges will flow through the solution from one feeder electrode to the other, but when they reach the vicinity of a BPE, they can either keep moving through the solution or react at a pole, where ionic current is temporarily converted into an electron flow through the BPE, which is then again transformed back into an ion flow via a redox reaction at the other end of the BPE before travelling to the second feeder electrode (Fig. 4.1a). In bipolar electrodeposition, one wants to maximize the fraction of the charge that reacts at the poles of the BPEs. Therefore, i BPE must be maximized. Whatever the path taken by a charge is, the potential difference between both ends of a BPE is the same and equal to ∆V (see Section 2.1). Hence:

∆V = R BPE • i BPE = R sol,2 • i ion (4.1) ⇒ i BPE = R sol,2 R BPE • i ion (4.2)
To maximize i BPE , either i total , the total current flowing between the feeder electrodes, can be increased or the fraction of the current that goes through the BPEs instead of crossing the solution can be maximized by maximizing the R sol,2 /R BPE ratio. Mavré et al. showed that in an open configuration, less than 1 % of the total current flows through a BPE. [START_REF] Mavré | A Theoretical and Experimental Framework for Understanding Electrogenerated Chemiluminescence (ECL) Emission at Bipolar Electrodes[END_REF] Therefore, it can be considered that i ion ≈ i total and that the nature of the BPEs has negligible effects on i total which is completely determined by the voltage applied between the feeder electrodes, the resistivity of the solution, its cross section and the distance between the feeder electrodes.

The R sol,2 /R BPE ratio depends on the compositions of the BPE and the solution, as well as the two half-reactions occurring at the poles of the BPE. R BPE can be decomposed into three different resistances: R mat , the electrical resistance of the bulk material composing the BPE; R Ox and R Red , the charge transfer resistances of the oxidation and reduction reactions happening at the poles of the BPE. The latter two resistances do not depend on the diameter d of the BPE. However, both R mat and R sol,2 depend on d:

Shape of the electric field

R mat = ρ mat d A BPE = 4ρ mat πd (4.3) R sol,2 = ρ sol d A sol (4.4)
with ρ mat and ρ sol the resistivities of the material composing the bulk of the BPE and of the solution; and with A BPE = πd 2 /4 and A sol the cross sections of the spherical BPE and of the solution respectively. Since the volume of the solution is much larger than the volume of the BPE, it can be assumed that A sol does not depend on d. Hence:

R sol,2 R BPE = ρ sol d A sol 4ρ mat πd + R Ox + R Red (4.5) ⇒ R sol,2 R BPE = ρ sol A sol • πd 2 4ρ mat + πd(R Ox + R Red ) (4.6)
R Ox , R Red , d and ρ mat directly depend on the nature of the desired modified particles. They cannot be changed without changing the final product. To increase the R sol,2 /R BPE ratio, ρ sol and thus, R sol,2 can be increased by decreasing the concentration of the ionic species in solution. However, this also increases R sol,1 and thus, decreases i total and therefore, the absolute value of the current flowing through the BPE despite a lower ratio. Hence, an optimal concentration that maximizes i BPE must be determined.

It was determined that a concentration of ionic precursor in the range of 1 mM is optimal for the metal electrodepositions used in this thesis. With a higher concentration, the ionic charges would prefer crossing the solution rather than reacting at the poles of the BPEs. Whereas, with lower concentrations, the total current is too low to obtain a significant deposit at the cathodic poles of the BPEs.

Shape of the electric field

As shown in Equation (2.7), the size of a deposit on a BPE is directly linked to the BPE's size and to the potential difference between its poles. In order to modify numerous BPEs in a uniform way, the electric field must be uniform in the volume of fluid between the two feeder electrodes.

In order to generate such a uniform field, large planar feeder electrodes are used. The electric field between two charged plates is approximately uniform (Fig. 4.2a). On the opposite, smaller electrodes will generate non-linear electric fields (Fig. 4.2b), resulting in BPEs with an inhomogeneous modification.

To obtain a linear electric field, 30 × 30 mm 2 glassy carbon electrodes were used. The electrodes were placed 30 mm from each other. This configuration creates a 30 × 30 × 30 mm 3 cube with an approximately uniform electric field. The BPEs were dispersed in a smaller cube placed in the middle of the electric field to avoid the non-linear effects present near the electrode plates.

Bipolar set-ups using multiple pairs of feeder electrodes

One straightforward method to deposit patches by bipolar electrodeposition outside of a single axis is to use several pairs of feeder electrodes in order to generate multiple electric fields, oriented along several directions. By sequentially applying voltages between different pairs of feeder electrodes, the direction of the electric field can be modified and therefore, the positions of the bipolar poles at the surface of the BPEs will change accordingly. The feeder electrodes in a set-up with more than one pair can be categorised into the active feeder electrodes (aFE), between which a voltage is applied and the passive feeder electrodes (pFE) which are present in solution during a bipolar experiment but between which no voltage is applied by an external source. Since the electrodes are made out of conductive materials, if a passive feeder electrode is in contact with the electric field produced by the active feeder electrodes, it will act as a bipolar electrode. This creates a new path for the current flowing between the two active electrodes (Fig. 4.3). Since the passive electrodes are much more conductive than the solution and much larger than the BPEs dispersed between the active electrodes, this new path will be greatly favoured and most of the current will travel through the passive feeder electrodes rather than through the solution and the BPEs to be modified. This current diversion results in the absence of any deposit at the surface of the BPEs dispersed in solution.

Cross-shaped bipolar cells

Despite the effect described previously, several groups have developed methods that use two perpendicular pairs of feeder electrodes to deposit patches with a 90°orientation with respect to each other. Koizumi et al. realized orthogonal depositions on the surface of spherical particles using two pairs of feeder electrodes. [START_REF] Koizumi | Bifunctional Modification of Conductive Particles by Iterative Bipolar Electrodeposition of Metals[END_REF] To avoid the effect described earlier, they used a cross-shaped bipolar cell and placed the BPEs at the intersection of the two channels (Fig. 4.4a). By doing so, they isolated each pair from the other pair's electric field. Fosdick et al. used a cross-shaped cell made of two microfluidic channels crossing each other at a 90°angle. [START_REF] Fosdick | Two-Dimensional Bipolar Electrochemistry[END_REF] By applying different voltages between the two pairs of feeder electrodes they could control the position of an ECL reaction on the perimeter of a square BPE placed at the intersection of the channels. The electric field at the intersection of the channels is the sum of the electric fields generated by the two pairs of feeder electrodes (Fig. 4.4b). By simultaneously and independently controlling both voltages, they could selectively control the orientation of the field near the BPE and thus the position of the cathodic and anodic poles at the surface of the BPE.

One of the main disadvantages of this technique is that the BPEs can only be placed at the intersection of the two channels. This reduces the available volume to disperse the BPEs and thus the quantity of BPEs that can be modified in a single experiment. Moreover, the four feeder electrodes must be placed farther away from the BPEs than when a single pair is used. Hence, to obtain an electric field of similar intensity near the BPEs, the voltage applied between a pair of feeder electrodes is higher than if the feeder electrodes were not placed in separate channels.

Bipolar electrodeposition in three dimensions

All the set-ups presented so far only allow deposition of patches in the horizontal plane. Since the patches are always deposited along the axis of the electric field and the direction of the field is determined by the position of the feeder electrodes, to be able to deposit patches outside of the horizontal plane, at least one pair of feeder electrodes must be placed outside of the horizontal plane. This new constraint creates several problems that are not present when designing planar bipolar cells:

• The feeder electrodes must be placed at specific angles in 3D in order to obtain the desired directions for the electric field;

• A large feeder electrode placed on top of the cell can trap bubbles generated by water electrolysis at the surface of the other feeder electrodes;

• At least one feeder electrode must be placed below the BPEs in order to generate electric fields with a vertical component. Therefore, the BPEs or the gel containing them cannot simply be supported by the bottom of the cell and must be suspended in the middle of the bipolar cell.

Despite these constraints, a bipolar cell with four pairs of feeder electrode was designed. The cell is composed of two hemispheres with screw threads that can be joined to form a watertight sphere with a diameter of 60 mm. Eight electrodes (∅ = 2 mm) are positioned inside the surface of the sphere. Each electrode is paired with the electrode facing it to form four pairs of feeder electrodes. The electrodes are placed in such a way that the cathodic and anodic feeder electrodes form two tetrahedra opposite of each other (Fig. 4.5). This configuration of electrodes permits the generation of electric fields in four different directions to deposit up to four patches at the surface of BPEs in a tetrahedral geometry. To generate an electric field in a desired direction, the two feeder electrodes along this direction were connected to a voltage source. A hole was made at the top of the cell to allow the gas bubbles generated at the feeder electrodes to leave the cell.

Bipolar experiments were tried with this cell by using glass spheres coated with a thin layer of gold on their surface. The experiments were carried out with a single BPE suspended in the middle of the cell at the intersection of the four possible electric fields directions. These experiments were unsuccessful. The feeder electrodes were too small and therefore generated non-uniform fields that are not suitable for bipolar depositions (Fig. 4.2b). Furthermore, the current i passing through an electrode is equal to j • A with j the current density and A the area of the electrode. The current density is determined using the Butler-Volmer equation (Eq. (3.15)) and does not depend on the geometry of the electrode. Hence, at a constant potential, a smaller electrode produces smaller currents in the solution than a larger one. This 3D design was not explored further because of its lack in versatility. The patches can only be deposited at the positions determined by the axes defined by the feeder electrode pairs. A new cell with feeder electrodes placed at different positions would have to be designed each time one wants to deposit metals at new positions on the surface of BPEs. A 3D cross-shaped cell similar to the one described in Section 4.3.1, but with three channels perpendicular to each other, two horizontal and one vertical, could generate electric fields in an arbitrary direction at the intersection of the channels by adding three perpendicular electric fields. This possibility was not explored either since only the small volume located at the intersection of the three channels could be used to modify BPEs. Moreover, such a geometry would generate problems with respect to the evacuation of the gas bubbles produced at the surfaces of the feeder electrodes.

Using arrays of small electrodes as feeder electrodes

It is possible to combine the advantages of large and small feeder electrodes by replacing a large feeder electrode by an array of smaller electrodes that share the same electrical potential. Far from the array, the generated electric field is very similar to the field generated by planar electrodes (Fig. 4.6a). When an array acts as an active feeder electrode, the individual electrodes must be electrically connected to the generator and to each other in order to share the same electric potential. On the opposite, when an array acts as a passive feeder electrode, the individual electrodes must not be electrically connected to each other. If two electrodes located at different points of an electric field are electrically connected to each other they will act as a single bipolar electrode called a split bipolar electrode. A split BPE experiences the same polarisation than a BPE with a length equal to the distance between the two connected electrodes. A method able to electrically connect all the electrodes of an array together when the array is used as an active feeder electrode but to disconnect them when the array is used as a passive feeder electrode (Fig. 4.6b) must be developed.

In order to deposit metallic patches 90°from each other at the surface of spherical GC particles, a cell with four perpendicular arrays was developed. The arrays are composed of a grid of five cylindrical electrodes (∅ = 2 mm) arranged in a quincunx with a side of 10 mm. Each electrode is individually connected to a different wire. When an array is used as an active FE, the five wires leading to the array's electrodes are connected to each other and to the voltage generator with the help of a crocodile clip. The wires of the passive arrays' electrodes are left disconnected from each other so that each small passive electrode remains electrically independent from the others.

The deposition of metal patches using this set-up was not successful. The electrodes used to form the arrays were still bigger than the BPEs dispersed in solution and still diverted a significant part of the current. Electrodes with even smaller diameter (e.g., ∅ = 500 µm) would be necessary to avoid this deviation of the current. Moreover, since the current passing through the solution is proportional to the area of the feeder electrodes, an array still generates a lower current than a plate electrode of similar size. Finally, to obtain a uniform field in a large volume of solution, larger arrays are necessary. To build a 30×30 mm 2 array -the size of the planar electrodes of the bipolar cell employed previously -using electrodes with a diameter of 500 µm, each of them separated by a distance of 1 mm from their neighbours, approximately 400 individual electrodes would be necessary per array. Each of those electrodes would have to be individually able to be connected or disconnected as a function of the role of the array during a bipolar experiment. While not theoretically impossible, this was determined to be impractical.

Reorienting BPEs in a static electric field

The methods presented earlier that rely on multiple pairs of electrodes have numerous potential drawbacks. The bipolar cell designs can become very complex while only permitting the modification of BPE in small volumes and for the most part, only along the direction determined by the positions of the feeder electrodes. Therefore, in this section a method that relies only on a single pair of feeder electrode, but still allows the deposition at any location on a spherical BPE's surface is developed. This method uses a much simpler bipolar cell while allowing a greater versatility in the type of possible bipolar depositions.

Instead of using multiples pairs of electrodes to orient the electric field around the BPEs, it is possible to orient the BPEs in a static electric field. It allows the use of simpler bipolar cells with a single pair of feeder electrodes.

As mentioned previously, during bipolar electrodeposition, spherical objects must be immobilized in a gel to block their rotation during the deposition and maintain their electrochemical poles in a specific orientation. After the deposition(s), the gel is dissolved and the BPEs lose their orientation. Hence, it is not possible to remove the BPEs from the gel between multiple deposition steps. Therefore, in order to uniformly reorient the BPEs for a second deposition, the whole gel should change its orientation and not just the BPEs. This can be achieved by confining the gel in a mould being independent from the bipolar cell. In order to allow a reorientation of the gel between two bipolar deposition steps, it must be independent from the bipolar cell and not directly poured in a compartment of the cell. The gel containing the precursor and the BPEs is therefore poured into a 16 × 16 × 16 mm 3 cubic mould made of filter paper. Filter paper was chosen for its high permeability, low cost and because it can be easily cut and folded into various shapes. Once the cubic mould has been filled with gel, it is placed in the bipolar cell and oriented at a specific angle θ (Fig. 4.7). The bipolar cell is then filled with a 1 mM KCl solution and a voltage is applied between the feeder electrodes in order to trigger the deposition. At the end of the first deposition the mould can be easily reoriented inside the cell at a different angle, followed by a second deposition.

Movable gel independent from the bipolar cell

Several examples of particles with multiples patches deposited by using this method are shown in Figure 4.8. The particles were modified with multiple gold or palladium patches placed at 90°or 120°from each other.

Deposition of patches outside the horizontal plane

In order to deposit patches at any position on a BPE's surface, the mould containing the gel must also be able to be oriented vertically with an angle ϕ. This angle ϕ must be variable between +90°and -90°in order to cover the total surface. Since the mould containing the gel is a cube, it is easy to tilt it by ±90°simply by choosing which of its faces sits on the bottom of the cell.

To deposit patches at angles that are not multiples of 90°, inclines are used to fix a certain orientation of the cube (Fig. 4.9). By using an incline with a 30°angle, it is possible to orient the cubic mould at any angle that is a multiple of 30°. The incline is used to tilt the mould by an angle of ϕ = ±30°, depending on the orientation of the incline. This tilt can be combined with the tilts of ±90°described previously to position the mould with any angle that is a multiple of 30°. To tilt the mould at angles that are not multiples of 30°, inclines with different angles (e.g., 45°) may be used.

Particles with three or four patches that are not coplanar were synthetized with this method. For the particles with three patches positioned at an angle of 90°with respect to each other, the two first patches were deposited in the horizontal plane using the method described previously (ϕ = 0°and θ = 0°or 90°). Then, the gel containing the BPEs was turned on its side to obtain a rotation of ϕ = 90°and the third deposition was carried out. For the particles with four patches in a tetrahedral geometry, two deposits were realized in the horizontal plane (ϕ = 0°and θ = 120°or 240°). Then, the gel was placed on a 30°i ncline and tilted on its side to obtain an angle ϕ = +90°-30°= 60°, and a third deposit with θ = 0°was made. Finally, the incline was placed in the opposite direction and the gel was tilted in the opposite way to obtain an angle ϕ = -90°+ 30°= -60°followed by the fourth and final deposition. The BPEs are then removed from the gel, washed and observed by SEM (Fig. 4.10).

Continuously rotating BPEs

Previously, we modified spherical BPEs with patches by maintaining their position constant during a bipolar deposition. If the BPEs are free to move during a deposition, they will rotate randomly and a homogeneous deposit on their entire surface will form. Roche et al. exploited this random rotation of BPEs dispersed in a liquid in order to produce spherical and monodisperse conductive particles. [START_REF] Roche | Physico-chemical milling for controlled size reduction of metal beads[END_REF] At the anodic pole, the metal of the BPE was electrodissolved while protons were reduced at the cathodic pole. This caused a homogeneous milling of the surface of the particles, reducing their size. The milling stops as soon as the BPEs are small enough so that ∆V < ∆V min . The size of the final particles can therefore be tuned by controlling the voltage U applied between the feeder electrodes. However, if the rotation of the BPEs during a bipolar deposition is controlled on purpose, it is possible to obtain metal deposits with different geometries at the surface of spherical BPEs.

Rotating BPEs have previously been used for analytical applications. Dauphin et al. used rotating BPEs to track changes in the chemical composition of a solution. [START_REF] Dauphin | Tracking Magnetic Rotating Objects by Bipolar Electrochemiluminescence[END_REF][START_REF] Dauphin | Remote Actuation of a Light-Emitting Device Based on Magnetic Stirring and Wireless Electrochemistry[END_REF] A bipolar cell was placed on top a magnetic stirrer. An LED was placed in the cell to serve as a BPE. Since the branches of the LED are ferromagnetic, it will start spinning when the magnetic stirrer is turned on. The potential difference between the branches of the LED will vary as a function of the angle between the branches and the electric field. The luminosity of an LED is directly proportional to the current that flows through it. Hence, the luminosity will also vary as a function of the angle between the LED's branches and the field and as a function of the reaction rate at the BPE's poles. This set-up has been used to track changes in chemical composition of a solution. The rotation of the LED ensures a continuous mixing of the solution in the vicinity of the BPE, therefore ensuring a more stable signal. This method that uses a magnetic stirrer to continuously rotate BPEs during a bipolar experiment can also be used to modify spherical particles with rings instead of patches. Since GC particles are not ferromagnetic, they cannot be directly moved by the magnetic field of the stirrer. To ensure their uniform rotation, the set-up shown in Figure 4.11 is used. The BPEs are immobilized in a gel placed in the same cubic mould as described previously. The mould is then placed in a holder that has a magnetic bar encased in its silicone base. The holder with the mould is placed in the bipolar cell and when the magnetic stirrer is switched on, the magnetic bar starts rotating, together with the holder and the mould containing the BPEs. A voltage is then applied between the feeder electrodes to trigger the bipolar deposition.

Once a BPE has rotated by 180°, its cathodic and anodic poles will have switched place. If the deposition reaction is reversible (e.g., the electrodeposition of a non-noble metal), the precursor that was reduced at the cathodic pole will be oxidized back at the anodic pole, and the newly deposited metal will dissolve back into the solution. Therefore, only non-reversible electrochemical depositions (e.g., the reduction of a noble metal precursor) can be used for the deposition of rings using this technique.

When performing an AC-bipolar deposition, the period of the AC voltage must be optimized. When a BPE is rotating, even though an DC signal is used, a given point at the surface of a rotating BPE experiences an electric field with a periodically oscillating amplitude, with the period T being equal to the time it takes for the BPE to complete a full rotation. (T = 60/ω with ω, the rotation speed, measured in rpm.) Therefore, the rotation speed chosen for depositing a specific metal as a ring must match as closely as possible the optimal period for the deposition of patches with symmetrical AC signals determined in Section 3.3. The optimal period used for depositing palladium patches was determined to be T Pd = 90 s (equivalent to ω Pd = 0.67 rpm) and the optimal period for depositing gold was determined to be T Au = 270 s (equivalent to ω Au = 0.22 rpm). The slowest rotation speed possible with the magnetic stirrer used for this experiment is 1 rpm (equivalent to T = 60 s). This rotation speed is close enough to the optimal period for the deposition of palladium and a uniform ring deposit is observed (Fig. 4.12a). However, 1 rpm is too fast to deposit uniform gold rings on GC particles. When trying to deposit a gold ring on spherical gold BPEs at this speed, only a small quantity of gold is deposited (Fig. 4.12b). A method to modify spherical BPEs with ring shaped deposits was already published in 2014 by Roche et al. [START_REF] Roche | Straight-forward synthesis of ringed particles[END_REF] (see Section 3.5). However, the method presented here has several advantages that the already published one does not have. Most importantly it does not rely on the charge of the metal precursor used to generate the ring. Therefore, this method can be used to create ring deposits from any precursor and not only anionic ones, as long as the deposition reaction used is not reversible. Moreover, in Roche's method, the rings are deposited at the extremity of the cathodic pole of the BPE and not on the equator of the particle.

Depositing multiple patches of different compositions at the surface of a BPE

By using the methods described previously, it is possible to deposit patches at any position at the surface of a spherical BPE. However, to be able to synthesize chiral particles, these patches must have different chemical compositions, in analogy to the different substituents attached to a chiral carbon atom. Multiple precursors must therefore be reduced sequentially to deposit patches with controlled composition at different positions at the surface of the BPEs. Several approaches have been explored in order to be able to selectively deposit a specific metal.

Use of multiple precursor solutions with different compositions

In a set-up where the BPEs are immobilized at a surface, it is possible to change the solution present in the bipolar cell in between different deposition steps. A first deposition is performed and then, the solution is removed from the cell. The surface with the BPEs is turned by the desired angle and a new solution of a different precursor is poured into the cell. A second bipolar deposition of a different metal can then be carried out at a different position on the surface of the BPEs. This method, while easy to set up, still has some drawbacks described previously in this thesis that are inherent to the use of an interface to immobilize spherical BPEs: low yields, the shielding of parts of the BPEs' surface from the solution and the impossibility to deposit patches along the vertical axis. A method that permits the use of multiple precursors while the BPEs are dispersed in a gel should therefore be developed.

Use of a gel containing multiple precursors

When using gels to immobilize the BPEs, it is not possible to move the BPEs from a gel containing the first precursor to a gel containing the second precursor without losing the orientation of the BPEs and therefore losing the ability to control the relative positions of the different patches deposited. A possible way around this problem is to directly prepare a gel that contains the precursors for all the planned depositions. By carefully choosing the voltages applied between the feeder electrodes, it should be possible to only deposit the metal whose half-reaction has the less demanding standard potential (Fig. 4.13). Once all the precursor ions of this first metal are depleted from the gel, either by reacting at the BPEs' cathodic poles or by diffusing/migrating out of the gel, the gel is reoriented at a new angle and the voltage between the feeder electrodes is raised to deposit the next metal. With this method, the voltage window for the deposition of a metal is much lower than in the case where only one precursor is present in solution. The minimal voltage needed to start reducing a precursor becomes the higher bound for the voltage window that can be used for the deposition of the metals with higher standard potentials (Fig. 4.13). This limits the possible size range of the metal patches that can be deposited on the BPEs. Moreover, when depositing a metal with a low standard potential, if there are still precursors of the metals with higher standard potentials present in solution, these precursors will also be reduced and the patch will not be composed of only the target metal, but will be an alloy of the target metal and of all the metals with higher standard potentials previously deposited. Since the leftover concentration of the previous precursors is not known, the proportion of the different metals in the patch will also be unknown.

Furthermore, as mentioned previously, the particles used as BPEs are not uniform in size and when using voltages close to the minimum voltages needed to observe a deposition, some of the smaller BPEs may not be modified at all. Usually, higher voltages are used to ensure that all the BPEs are modified. However, when several precursors are present in solution, the range of available voltages is low and it cannot be guaranteed that a voltage high enough to modify all the BPEs can be applied between the feeder electrodes.

A solution where a single precursor is present in the gel during each successive deposition would allow for more freedom when choosing the voltages applied between the feeder electrodes and therefore the size of the resulting patches. It would also ensure that the patches are made of a single metal and not of an alloy with an unknown composition. Thus, a method to change the precursor present in the gel between two depositions without losing the orientation of the BPEs should be developed.

Changing the precursor present in the gel by using diffusion

In order to deposit patches composed of a single metal at the surface of the BPEs, only the precursor of the target must be present in the gel containing the BPEs. It is not possible to simply extract the BPEs from a gel to disperse them again in a second gel with a different composition, since it would allow them to rotate freely and it would be impossible to control the position of the subsequent patches. Therefore, a method to change the precursor present in the gel without destroying it, and losing the orientation of the BPEs, would be an attractive alternative. A method relying on the diffusion of precursors in and out of the gel was developed. A gel containing the precursor used for the first deposition is prepared and patches of the first metal are deposited by using the method described earlier (Fig. 4.14a). Then, once this first deposition is accomplished, the gel is removed from the bipolar cell and immersed in a bath of distilled water overnight (Fig. 4.14b). The next day, most of the precursor that remained in the gel after the first bipolar experiment will have diffused out of the gel into the distilled water. The gel is then placed in a bath containing 1 mM of the precursor used for the second deposition (Fig. 4.14c). In this bath, the precursor will diffuse into the gel until the concentration gradient present between the solution and the gel equilibrates. Since the volume of the precursor solution is much larger than the volume of the gel, it is assumed that the equilibrium concentration of precursor in the gel is equal to the original concentration of precursor in solution. The gel is then placed again in the bipolar cell with a different orientation and a second deposition can be started (Fig. 4.14d). This washing-loading-depositing sequence can then be repeated multiple times to deposit more than two different metals at the surface of BPEs.

As explained in Section 3.2, these depositions are too long to be able to use agarose gels without the agarose polymer chains reacting with the various precursors. Only fumed silica gels were used for the modification of particles with multiple patches by this method. In theory, this approach allows for the deposition of any number of patches at the surface of a BPE. However, in practice, after around four depositions the gel starts to decompose and loses most of its cohesion; this can allow the BPEs to start to rotate freely again.

If during the loading step, a patch of a metal with a low standard potential is in contact with the precursor of a metal with a higher standard potential (e.g., a nickel patch in contact with a solution of gold precursor), the metal can be oxidized by the precursor, changing the composition of the patch. To avoid this effect, when depositing several metals using this method, deposits of multiple metals are still performed in a sequence from the metal with the highest standard potential to the metal with the lowest standard potential. This method was used to modify spherical GC particles with patches of different metals positioned at specific locations on their surfaces (Fig. 4.15). First, a fumed silica gel containing 1 mM of tetrachloroauric acid (HAuCl 4 ) and the BPEs was prepared. The gel was then poured into a cubic mould and the mould was positioned in the bipolar cell with the desired orientation for the first patch. The cell was then filled with a solution of 1 mM of KCl and a first AC-bipolar deposition was carried out. After the end of this first deposition, the gel was removed from the bipolar cell and left immersed overnight in distilled water. The next day, most of the gold precursor that was initially present in the gel had diffused out of the gel into the distilled water. The gel was then immersed overnight into a 1 mM solution of the precursor used for the second deposition: ammonium tetrachloropalladate ((NH 4 ) 2 PdCl 4 ). In the bath, the palladium precursor is able to diffuse inside the gel thanks to the concentration gradient between the solution and the gel. The next day, the gel was put back in the bipolar cell with a different orientation and a second AC-bipolar deposition is started. With this method, particles with two patches, one composed of gold and a second one composed of palladium were obtained (Fig. 4.15a). However, it is possible to repeat the washing/diffusion steps with a third precursor (e.g., nickel acetate, Ni(CH 3 COO) 2 ) to obtain particles modified with three different metal deposits (Fig. 4.15b). The deposition of nickel was carried by using DC-bipolar electrochemistry since the used precursor is a cation (Ni 2+ ) and therefore, there is no risk of the formation of ring-shaped deposits. The modified particles were observed by SEM and the composition of the patches was analysed by energy-dispersive X-ray spectroscopy (EDX).

The loading step of this method (Fig. 4.14c) can also be used when depositing multiple patches of a single metal. When depositing a metal patch by bipolar electrochemistry, some of the precursor will diffuse and migrate out of the gel and into the KCl solution. In order to have a consistent concentration of precursor during multiple depositions, it is possible to immerse the gel into a bath of the same precursor in between the sequential deposition steps so that the concentration gradient between the gel and the solution will equilibrate back to the original concentration.

Synthesis of chiral particles

With the ability to deposit patches at any position on the surface of a spherical BPE and to change the precursor between deposition steps, it is possible to synthesize particles with four patches arranged in a tetrahedral geometry composed of four different metals. A carbon atom with four different substituents is chiral. Similarly, a spherical particle with four different patches arranged in a tetrahedral geometry is also chiral.

When the positions of two substituents of an asymmetric carbon atom are swapped, the resulting molecule will be the mirror image of the original molecule, resulting in a so-called enantiomer. Similarly, by swapping the position of two patches, the mirror enantiomorph of the original particle is obtained. It is therefore possible to obtain a specific enantiomorph of a modified particle.

To differentiate the two enantiomers of a chiral molecule, the R and S descriptors are used. These descriptors are assigned to the enantiomers by using the Cahn-Ingold-Prelog (CIP) rules. [START_REF] Cahn | Specification of Molecular Chirality[END_REF][START_REF] Prelog | Basic Principles of the CIP-System and Proposals for a Revision[END_REF] It is also possible to assign a descriptor to the enantiomorphs of a chiral particle by following rules similar to the CIP rules. The patches at the surface of the particle are ranked using the atomic weights of the metals that compose them; the heavier metals are ranked with a higher priority than the lighter ones. Once a priority order has been assigned to each patch, the same rules as for asymmetric carbon atoms apply. The particle is oriented with the patch with the lowest priority facing away from the observer and the sense of rotation of the curve passing through the three remaining patches in decreasing order of priority determines the descriptor: R for clockwise and S for anticlockwise.

The two enantiomorphs of a particle with four different metal deposits were synthesized by using the methods described previously. Gold, platinum, palladium and nickel were deposited in that order in a tetrahedral geometry at the surface of glassy carbon particles (Fig. 4.16). A fumed silica gel containing 1 mM of HAuCl 4 was prepared and placed in a cube made out of filter paper that was then placed between two 30 × 30 mm 2 GC feeder electrodes in the bipolar cell. An asymmetric AC-bipolar electrodeposition with a voltage oscillating between +30 V and -5 V with a period of 945 s was performed for 90 min. The gel was then washed overnight in distilled water and placed in a bath of 1 mM of hexachloroplatinic acid (H 2 PtCl 6 ) overnight. The AC-bipolar deposition of the platinum patch was carried out the next day with a voltage oscillating between +40 V and -5 V with a period of 1338 s for 135 min. The gel was again washed overnight and then put in a bath of 1 mM of (NH 4 ) 2 PdCl 4 overnight. The palladium was deposited by AC-bipolar electrochemistry with a voltage oscillating between +40 V and -5 V with a period of 90 s for 135 min. Finally, the gel was washed overnight, placed in a bath of nickel acetate overnight and the nickel patch was deposited by DC-bipolar electrochemistry at a voltage of +70 V for 90 min. The optimal deposition periods and times were previously determined in Sections 3.3 and 3.4.

To obtain the second enantiomorph, the same procedure was used but the angles used for the depositions of gold and platinum were inverted. The modified particles were observed by SEM and the composition of the patches was analysed by EDX (Fig. 4.16). Secondary and tertiary amines with three different substituents are chiral molecules since the bonds between the nitrogen atom and the substituents are not coplanar. However, since the energetic barrier for the inversion of an amine into its mirror image is very low, the amine will constantly oscillate between its two enantiomeric forms and it is usually not possible to isolate a specific enantiomer. This low energetic barrier of inversion is not present for particles with three different non-coplanar patches. It is thus possible to synthesize chiral particles with only three patches. Such a particle is shown in Figure 4.17 For a particle to be chiral, the deposited patches do not need to have necessarily different compositions, they only need to be different. It is therefore theoretically possible to synthesise chiral particles with three or four patches with the same composition but different sizes. However, as discussed earlier, it is difficult to precisely control the size of a patch deposited via bipolar electrochemistry, due to the variation in diameter of the unmodified spherical GC particles that are used (see Section 3.4.3). This lack of precise control of the size of the patches might lead to both enantiomorphs being synthesized at the same time or to the synthesis of achiral particles with two patches of the same size.

In theory, the method presented in this chapter allows for the deposition of any number of patches at the surface of BPEs. However, in practice, after around four depositions the gel has lost a lot of its cohesion and starts to decompose and to dissolve in the solution.

Conclusion

The methods developed in this chapter allow the modification of isotropic BPEs and transforming them into chiral objects in the bulk of a solution, with a perfect control of the chirality of the resulting objects. Most other methods for the synthesis of chiral objects either only allow for the synthesis of the racemic mixture of two enantiomorphs [START_REF] Rouet | Colloidal Molecules from Valence-Endowed Nanoparticles by Covalent Chemistry[END_REF][START_REF] Liu | Customized Chiral Colloids[END_REF] or rely on an interface [START_REF] Lin | All-optical reconfigurable chiral meta-molecules[END_REF][START_REF] Ni | Programmable colloidal molecules from sequential capillarity-assisted particle assembly[END_REF] that limits the available volume for the reaction.

All the techniques developed so far, were used for the modification of rather big particles, with diameters ranging from 630 µm to 1000 µm. In the next chapter, techniques for the modification of smaller particles will be discussed.

Chapter 5

Modification of microparticles with bipolar electrochemistry

Most of the BPEs that we used so far were rather big objects (glassy carbon spheres with sizes ranging from 630 µm to 1000 µm). Larger BPEs obviously have several advantages: relatively low voltages can be used for depositions and these BPEs can be manipulated and observed more easily.

However, microparticles have specific properties that their more macroscopic equivalents do not have. Microparticles can be dispersed in colloidal suspensions that have unique properties. They can also combine into larger, self-assembled structures like colloidal molecules or colloidal crystals (see Section 1.4). Therefore, it would be interesting to try applying bipolar electrochemistry concepts also to this category of particles.

However, the modification of microparticles using bipolar electrochemistry exacerbates several of the issues that were already encountered in the previous chapters, as well as introduces new issues. In this chapter, multiple techniques used for the bipolar modification of microparticles will be discussed.

Bipolar modification of spherical GC microparticles

As already detailed earlier in this thesis, when keeping the distance L between the feeder electrodes constant, the minimum voltage U min needed to trigger electrochemical reactions at the poles of BPE is inversely proportional to the diameter d of the BPE:

U min = L d ∆V min (5.1)
It was also determined earlier (Section 3.4.5) that for the bipolar deposition of Au 0 from AuCl 4 -, ∆V min = 0.24 V. Therefore, to modify 100 µm particles using feeder electrodes positioned 30 mm from each other, U min = 72 V. In the previous chapter, the least noble metal that was deposited was nickel and a voltage of 70 V was necessary. Here, the deposition of gold -one of the easiest metals to deposit -requires voltages that are higher. Furthermore, to be certain to obtain large patches on all the BPEs, voltages higher than U min must be used.

When high voltages are applied to a solution, Joule heating becomes an issue. If the Joule heating is too high, the solution's temperature can rise quickly until it reaches its boiling point, in which case any bipolar experiment becomes impossible. Even if the boiling point is not reached, agarose and fused silica gels lose their cohesion and disintegrate at high temperatures. (Fused silica gels liquefy when reaching a temperature of around 75 ℃.) Thus, a system to cool the cell during bipolar experiments using high voltages must be developed. An easy solution is to place the bipolar cell in an ice bath. Ice absorbs most of the heat produced in the cell, maintaining the solutions at relatively low temperatures.

The thickness of a metal deposit directly depends on j BPE , the current density at the cathodic pole of a BPE. The relation between j BPE and the strength of the applied electric field has already been studied in Section 3.4.4. However, j BPE also depends on the size of the BPE, independently of the voltage applied. As already explained, j BPE is not uniform on the BPE's poles surface. However, for a qualitative analysis of the thickness of the patches we can approximate that, on average, j BPE ≈ i BPE /A BPE . (With A BPE = πd 2 /4, the cross section of a spherical BPE of diameter d.) By using the Equations (4.2) and (4.6), we have:

j BPE = i total A BPE • R sol,2 R BPE (5.2) ⇒ j BPE = i total • 4 πd 2 • ρ sol A sol • πd 2 4ρ mat + πd(R Ox + R Red ) (5.3) ⇒ j BPE = i total • ρ sol A sol • 1 ρ mat + π 4 d(R Ox + R Red ) (5.4)
i BPE (d) is an increasing function (Eq. (4.6)) and thus, the smaller the BPE is, the smaller the current flowing through it is as well. However, Equation (5.4) shows that j BPE (d) is a decreasing function. Therefore, the smaller BPEs should have higher current densities and thus, thicker deposits.

Glassy carbon microparticles with sizes ranging from 80 µm to 200 µm were dispersed in an agarose gel containing 1 mM of HAuCl 4 . The agarose gel was placed in a bipolar cell between two feeder electrodes and a solution of 0.1 mM KCl was poured in the bipolar cell. The cell was placed into an ice bath to avoid excessive heating of the solution and of the gel due to the Joule effect. A DC voltage of 300 V was applied between the feeder electrodes for 5 min. The gel was then removed from the bipolar cell and dissolved. The modified microparticles were gathered, washed and observed under SEM (Fig. 5.1a).

The resulting particles are inhomogeneously modified. When using particles in the 630 µm to 1000 µm range, the largest particles are approximately 1.6 times larger than the smallest ones. When using particles in the 80 µm to 200 µm range, the largest particles are 2.5 times bigger than the smallest ones. This increase in size ratio makes it harder to modify all the BPEs homogeneously. Moreover, some of the microparticles shown in Figure 5.1a are oddly shaped. They are composed of several aggregated GC microspheres, forming non isotropic particles of various sizes. The presence of these odd-shaped BPEs makes the production of homogeneously modified BPEs almost impossible. Finally, since a DC voltage was applied between the two feeder electrodes, the AuCl 4 -ions migrated towards the feeder anode. This causes a precursor depletion on the side of the gel further away from the feeder anode. This concentration gradient also causes inhomogeneities between the different BPEs dispersed in the gel. This effect is amplified by the high voltage since the migration speed of the ions is proportional to the strength of the electric field (Eq. (3.2)). Because of these effects, it is hard to observe the influence of the size d of the BPEs on j BPE that was determined earlier.

AC-bipolar electrodepositions were also performed on microparticles. The GC particles were dispersed into an agarose gel containing 0.2 mM of HAuCl 4 . The gel was placed in the bipolar cell, which was filled with a 0.1 mM solution of KCl. The cell was positioned in an ice bath in order to evacuate the heat generated through Joule effect and an AC voltage with a frequency of 1 Hz and an amplitude of 200 V was applied for 8 min. The modified particles were then collected, washed and observed by SEM (Fig. 5.1b).

Potential differences ranging in the hundreds of volts are needed to deposit golda very noble metal -on BPEs with sizes ranging from 80 µm to 200 µm. To deposit other, less noble metals on BPEs of similar size or smaller, even higher voltages would be necessary. Since the heat transferred to the solution by Joule effect is proportional to U • i sol , an ice bath would not be able to cool the bipolar cell fast enough to avoid a decomposition of the gel and boiling of the solution. Completely different designs must be developed for the modification of micro-or even nanosized BPEs. Liquid nitrogen has been used to cool bipolar cells when applying high voltages. [START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF] Another drawback of high voltage bipolar electrodeposition is the large amount of gas bubble produced by the electrolysis of water at the feeder electrodes. Non aqueous solutions and gels have also been used to decrease this bubble production. [START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF]

Capillary-assisted bipolar electrodeposition

In 2008, Warakulwit et al. developed CABED (capillary-assisted bipolar electrodeposition), a method that allows for the bulk modification of microsized objects. [START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF][START_REF] Warakulwit | Dissymmetric Carbon Nanotubes by Bipolar Electrochemistry[END_REF][START_REF] Fattah | Applications of bipolar electrochemistry: from materials science to biological systems[END_REF][START_REF] Bouffier | Capillary-assisted bipolar electrochemistry: A focused mini review[END_REF] In this set-up, the two feeder electrodes are placed in two different vials that are linked by a capillary (Fig. 5.2). The vials and the capillary are filled with a solution containing the precursor that will be used for the bipolar deposition. Unmodified BPEs are introduced into the vial containing the feeder anode and a high voltage is applied between the two feeder electrodes. The potential gradient generated between the two vials generates an electroosmotic flow (EOF) through the capillary towards the feeder cathode. This flow carries the BPEs from one vial to the other through the capillary. While being in the capillary, the BPEs experience a high potential gradient that triggers electrochemical reactions at their poles. The modified BPEs can then be collected in the vial containing the cathode. A voltage is applied between the two feeder electrodes (Yellow). The potential difference between the two vials creates an EOF that moves the particles toward the negative feeder electrode through the silica capillary. During their travel through the capillary, the CTs act as BPE and can be asymmetrically modified. A detector is placed along the capillary to monitor the flow of the BPEs. Adapted from Ref. [START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF].

The use of a capillary has several advantages compared to a more classical bipolar set-up. The high surface to volume ratio of a capillary permits a better dissipation of the heat produced by the Joule effect. The solution travelling through the capillary is characterised by a very low Reynolds number and the flow is purely laminar without turbulences; the orientation of the BPEs is therefore better preserved than in a bulk solution. The O 2 and H 2 bubbles produced by water electrolysis at the feeder electrodes do not enter the capillary and thus do not disturb the modification of the BPEs. Finally, the small diameter of the capillary increases the current density in the vicinity of the BPEs and therefore the relative number of charges that reacts at the poles of the BPEs. These advantages allow the use of much higher electric fields (up to hundreds of kV•m -1 ) than what would be possible with a classic bulk bipolar set-up.

Since the BPEs are modified while they are travelling between the two vials, CABED is a bipolar technique that can produce modified objects in a continuous manner instead of the usual batch method. Since one vial only contains the unmodified objects while the other one is progressively filled with the modified ones, it is possible to continuously add more unmodified particles in the first vial while collecting the modified particles from the second vial.

CABED has been used for the modification of carbon micro- [START_REF] Loget | Versatile Procedure for Synthesis of Janus-Type Carbon Tubes[END_REF][START_REF] Fattah | Capillary electrophoresis as a production tool for asymmetric microhybrids[END_REF] and nanotubes [START_REF] Loget | Single point electrodeposition of nickel for the dissymmetric decoration of carbon tubes[END_REF][START_REF] Warakulwit | Dissymmetric Carbon Nanotubes by Bipolar Electrochemistry[END_REF], as well as less common materials such as DNA strands [START_REF] Malytska | Exploring bipolar electrochemistry for the modification of unusual conducting substrates[END_REF]. It has also been used to control the growth of gold nanowires. [START_REF] Wood | Bipolar Electrochemical Method for Dynamic In Situ Control of Single Metal Nanowire Growth[END_REF] However, CABED relies on the selfalignment of the BPEs along the electric field to stop their rotations. Thus, it can only be used with anisotropic objects and it is not suitable for the modification of spherical particles.

Controlled peeling of a metallic layer at the surface of a BPE

Bipolar electrochemistry relies on the conductivity of the BPE to transfer the electrons from the anodic pole to the cathodic pole. Hence, it is not possible to deposit materials on objects made from non-conductive materials (e.g., glass) with classic bipolar methods.

To break the symmetry of an isotropic object with a non-conducting core, an alternative to bipolar deposition of a material is the peeling of a uniform shell from the surface of a core-shell particle. Core-shell particles are particles composed of a core made from one material that is totally surrounded by a shell composed of a different material.

A core-shell particle with a shell that can be electrochemically peeled and a core that is not electrochemically active is placed in a bipolar cell. The generated electrical field triggers the oxidation and peeling of the shell material at the anodic pole of the BPE, slowly revealing the core of the particle. At the end of the experiment, a Janus particle is obtained with a portion of the particle being still covered by the initial shell material and the other section exposing the naked core of the particle. Since all the electrochemical reactions happen at the shell of the particle, the faradic current i BPE remains in the shell, at the surface of the BPE and thus, the core of the BPE does not have to be conductive. Therefore, by using this technique it is possible to synthesise Janus particles made from non-conductive materials that cannot be obtained by using the more classic bipolar techniques. However, for this concept to work, the shell material must be able to be oxidized in order to be peeled from the surface. Hence, noble metals cannot be used as shell materials.

This method also has another major advantage compared to classic bipolar electrodeposition. At the anodic pole of the core-shell BPE, the metal composing the shell is oxidized into its ionic precursor (e.g., Cu 0 --→ Cu 2+ + 2 e -), but if this precursor is also introduced into the solution at the beginning of the experiment, it can be simultaneously reduced at the cathodic pole (Cu 2+ + 2 e ---→ Cu 0 ). In this case, the two half-reactions involved in the bipolar experiment are reverse reactions of the same redox couple and therefore theoretically ∆V min = 0 V (Eq. (2.5)). By using Equation (5.1), it follows that U min = 0 V and that core-shell particles can be modified by using very low voltages. Furthermore, since in this case, U min does not depend on the size of the BPE in a first order approximation, very small particles can be modified by applying only relatively small voltages. A voltage U > 0 V is still needed to polarize the BPEs in order to localize the half-reactions at the two opposite poles of the BPE, otherwise symmetry will not be broken.

Loget and Kuhn used a similar method based on reverse half-reactions occurring at both poles of a BPE to trigger the movement of a zinc dendrite through bipolar selfregeneration. [START_REF] Loget | Propulsion of Microobjects by Dynamic Bipolar Self-Regeneration[END_REF] Zinc was oxidized into zinc(II) ions at the anodic pole of the dendrite while Zn 2+ ions were reduced into Zn 0 on the cathodic pole, at the other end of the dendrite, generating an apparent net movement of the dendrite towards the anodic feeder electrode. (See Section 2.4.) Gurumoorthy and Juvekar used this method to synthesize Janus spheres with a nonconductive glass core and a hemispherical copper patch. [START_REF] Gurumoorthy | Experiments on the Peeling of Copper Coating Deposited Via the Electroless Method on Glass Beads[END_REF] They first deposited a 1 µm thick layer of metallic copper on the surface of glass beads with diameters ranging from 3.5 mm to 7.5 mm by using an electroless method to obtain core-shell particles. Then, the particles were immobilized in an agarose gel containing 100 mM copper(II) sulfate (CuSO 4 ) and placed between two feeder electrodes in a bipolar cell. An electric field was applied and the copper present at the anodic pole of the BPE gets oxidized into Cu 2+ ions while Cu 2+ ions are reduced into metallic copper at the cathodic pole (Fig. 5.3a). After a while, enough copper is removed to expose the glass core of the particle. From that point onwards, the hole in the copper shell grows and reveals more and more the bare glass core until half of the copper is removed (Fig. 5.3b-d) and a glass-copper Janus sphere is obtained. In contrast to classic bipolar electrodeposition, for which the size of a patch is controlled by the voltage applied between the feeder electrodes, in a bipolar peeling experiment, the size of the patch is mainly controlled by the duration of the experiment. In bipolar electrodeposition, the material deposits simultaneously on the entire part of the cathodic pole having the right polarization. The duration of the experiment allows controlling the thickness of the patch. In the case of bipolar peeling, the size of the exposed core section grows with time (see Fig. 5.3b-d). Theoretically, since the metal shell never reaches a size where ∆V < ∆V min , the shell's size will decrease until all of the metal has been oxidized. However, in practice, the position of the dissolution front slows down and stops near the equator of the BPE. Once the peeling front has passed the equator and the copper covers less than half of the glass core, the insulating core starts shielding the metallic layer from the electric field. The metallic layer thus enters an electric shadow region, and the polarization of the metallic layer -and therefore the peeling of the copper layer -slows down and then stops.

Modification of core-shell microparticles

Prof. Gurumoorthy from the Vellore Institute of Technology (India) provided us with glass microspheres having a size distribution ranging between 60 µm and 100 µm, coated with a layer of metallic copper (Fig. 5.4a). If the same half-reaction is used at both poles of the BPE then ∆V min = 0 V, and it should be possible to modify these microparticles by bipolar electrochemistry by applying only relatively small voltages at the feeder electrodes in comparison to the case of electrodeposition of a metal. The core-shell microparticles were dispersed into an agarose gel containing 1 mM of CuSO 4 . The gel was placed in a bipolar cell between two feeder electrodes separated by 30 mm. A voltage of 5 V was applied between the feeder electrodes. At the end of the bipolar experiments, the particles were removed from the gel, washed and observed under SEM and their chemical composition was analysed by EDX (Fig. 5.4b, c).

In Figure 5.4a, it can be observed that some particles already had some of their glass core exposed before any bipolar experiment was carried out. This is mainly due to mechanical damage to the copper shell during transport, storage and handling of the particles. Therefore, when observing the modified core-shell particles by SEM and EDX, it is difficult to determine which particles were modified by the bipolar peeling and which particles were mechanically damaged before and/or after the experiment. Nevertheless, it can be seen that particles that have been exposed for a longer period of time to an electric field tend to have a bigger fraction of their glass core exposed (Fig. 5.4b, c). Hence, the bipolar experiment does have an effect on the peeling of the copper layer, even if it is hard to quantify the effect.

If copper had been deposited more classically by reducing Cu 2+ ions instead of using bipolar peeling, the counter half-reaction would have been the reduction of protons and ∆V min = E • Cu 2+ /Cu -E • H + /H2 = 340 mV. With L = 30 mm and d = 60 µm, U min would have had to be equal to 170 V (Eq. (5.1)). The voltage actually applied between the feeder electrodes would have been even higher in order to ensure a homogeneous modification of all BPEs. By using the bipolar peeling technique instead, it was possible to modify microparticles by applying only voltages as low as 5 V between the feeder electrodes, a decrease by more than 97 % of the required driving force. Furthermore, it was possible to use bipolar electrochemistry to produce asymmetric particles with non-conductive cores, a challenge that cannot be addressed with classic bipolar electrodeposition techniques.

Conclusion

The bipolar modification of small microparticles exacerbates most of the issues already discussed and introduces new ones. In order to solve these problems, either alternative bipolar set-ups must be designed, such as the CABED approach, or new bipolar techniques which do not rely on the electrodeposition of metals can be used.

The bipolar peeling of core-shell particles permits the modification of smaller particles while only applying low voltages between the feeder electrodes. Moreover, it enables the synthesis of asymmetric or Janus particles with non-conductive cores like glass. Thus, this technique expands the range of possible objects that can be synthesized by bipolar electrochemistry.

However, since the metallic shell of the microparticles is very fragile, new methods that minimizes the mechanical stress applied on the BPEs before and after the bipolar experiment need to be developed. Moreover, the modification of core-shell particles with different compositions should be explored in order to synthesize Janus particles with a wider range of composition and properties than simple bipolar electrodeposition would allow.

Chapter 6 Conclusion and perspectives

Conclusion

The first two chapters presented an overview of the state of the art in the fields of asymmetric particles and bipolar electrochemistry. There are numerous non-electrochemical methods to synthesize Janus and asymmetric particles. These particles also have many applications ranging from the study of colloidal molecules to micromotors. Bipolar electrochemistry also offers numerous opportunities for the synthesis of Janus objects as well as for analytic applications.

In Chapter 3, a new bipolar technique based on the use of asymmetric AC electrical signals, that permits the synthesis of Janus particles with two patches with the same composition but different sizes in a single experimental step, was developed. This ACbipolar technique also allows the synthesis of a single patch without the issues caused by the migration of ionic metallic precursors (the migration of the precursors towards the feeder electrodes and away from the BPEs and the ring effect that appears when using anionic precursors [START_REF] Roche | Straight-forward synthesis of ringed particles[END_REF]). Models based on the theory of bipolar electrochemistry and the Butler-Volmer equation were developed in order to both qualitatively and quantitatively analyse the size and thickness of the patches obtained at the surface of the modified particles.

In Chapter 4, possible set-ups for depositing metallic patches at different positions on the surface of spherical particles were examined. Finally, a set-up where the BPEs are dispersed in a gel that can be independently moved in the bipolar cell was developed. This gel is cube-shaped and can be freely rotated in order to reorient the BPEs in between two bipolar depositions, thus changing the position of the cathodic and anodic poles on the BPEs' surface. This method allows for the sequential deposition of multiple patches at any position on the surface of spherical BPEs. A variation of this method, where the gel containing the BPEs is continuously turning during the bipolar deposition, permits the synthesis of ringed particles without relying on the diffusion of anionic precursors. This method was then modified by adding a new step between two depositions where the composition of the gel is modified by letting the previous precursor diffuse out of it and allowing a new precursor to diffuse in. This new step permits the deposition of patches with different compositions on the surface of the BPE. This modified technique enables the deposition of up to four patches of different compositions at the surface of conducting particles. Chiral particles were synthesised using this method.

In Chapter 5, preliminary results for the bipolar modification of smaller conductive particles as well as non-conductive particles were explored. Core-shell particles with a glass core covered by a copper layer were submitted to an electric field, the copper was progressively oxidized into Cu 2+ ions, revealing the inner glass cores. This method permits the synthesis of Janus particles with non-conductive cores. Theoretically this method also does not depend on the strength of the applied electric field. Hence, it can be used to modify smaller microparticles.

Perspectives

Possible improvements of the developed techniques

The different bipolar methods developed in this thesis could be combined with each other or with some of the methods described in the first chapter to produce even more complex composite objects. For example, polymer Janus particles synthesized through the self-assembly of copolymers could be coated with a metallic layer by vapour deposition to form complex core-shell particles that could be modified further either by the bipolar deposition of metallic patches or by removing sections of the metallic shells of the particles by using the bipolar peeling method outlined in Chapter 5.

The different patches at the surface of microparticles could also be further selectively modified. One could add self-assembled monolayers at the surface of the metallic patches (e.g., a monolayer of thiol molecules on top of a gold patch) in order to obtain patches composed of materials that cannot directly be deposited by bipolar electrochemistry. This would allow for a greater variety of possible functional patches at the surface of asymmetric particles. These molecular monolayers could be tailored in order to grant specific properties to the patch. [START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF][START_REF] Koefoed | One-step preparation of bifunctionalized surfaces by bipolar electrografting[END_REF] Moreover, the monolayers could be used as seeds for the growth of various polymers. If enough polymer grows at the surface of the particles, the patches would support polymer spheres. This would allow the synthesis of non-spherical composite particles with a highly tunable geometry.

The preliminary results presented in Chapter 5 could be investigated further in order to use the other results presented in the rest of the thesis to synthesize smaller asymmetric or chiral particles.

The combination of the various bipolar techniques with other non-electrochemical methods would allow for the synthesis of an almost infinite variety of complex composite particles with tunable size, shape and composition.

Applications of complex asymmetric particles

Numerous possible applications exist for complex asymmetric particles synthesized by bipolar electrochemistry.

As already explained in Section 1.4, asymmetric particles are used to form colloidal molecules through self-assembly. Colloidal molecules are clusters of colloidal particles that emulate the three-dimensional structure of molecules with microparticles instead of 6.2. Perspectives individual atoms. The complex particles synthesized in this thesis could be used for the assembly of more complex structures with a precise tailoring of the colloidal molecules' geometry. By controlling the angles between the patches of different composition at the surface of the particles, it would be possible to control the angle at which different particles bind to each other during self-assembly. The patches could also be modified with molecular monolayers in order for them to specifically bind to another patch. By depositing a monolayer of single-stranded DNA molecules with a specific sequence at the surface of gold patches and single-stranded DNA molecules with the complementary sequence at the surface of palladium patches, particles with palladium and gold patches would selectively assemble to form Au-Pd "bonds". By controlling the angles between the patches and the number of patches, it should be possible to precisely control the size and shape of the resulting colloidal molecules or crystals.

In Section 1.6, different propulsion mechanisms for microswimmers were reviewed. An asymmetric particle with patches of different materials could be used to trigger different propulsion mechanisms at different locations on its surface. E.g., a particle with a patch producing a self-phoretic effect when exposed to a light source and another patch that triggers bubble propulsion through bipolar electrochemistry when exposed to an electric field. Different forces could therefore be selectively applied at different points on the surface of the particle, controlling the direction toward which it is moving and thus creating steerable microswimmers.

Chiral particles, like chiral molecules, rotate the polarization plane of polarized light. [START_REF] Shemer | Chirality of Silver Nanoparticles Synthesized on DNA[END_REF] The methods developed in this thesis, allowing the bulk synthesis of chiral particles with a controlled handedness, open also new perspectives for the study of the chiroptical properties of nano-and microparticles.

Recently, Kuznetsova et al. developed the concept of "lab-on-a-microsphere". [START_REF] Kuznetsova | Lab-on-Microsphere-FRET-Based Multiplex Sensor Platform[END_REF] By analogy with "lab-on-a-chip" devices -in which, several different analytical functions are placed on a single chip -a "lab-on-a-microsphere" is a spherical microparticle with multiples functions placed at its surface. They developed polymer microspheres covered with quantum dots sensitive to different dyes. Asymmetric microparticles might be functionalized with several patches of different compositions, each sensitive to a different analyte (e.g., multiple patches each functionalised with a different fluorescent marker). These particles could be used for multiplexing the analysis of complex media. The screening of several biomarkers in biological samples could be carried out in a single experiment.

To conclude, this thesis opens up several new opportunities for further research, either as a direct follow-up work in the field of bipolar electrochemistry or in the frame of applications in other fields of physical chemistry, like for the design of microswimmers or in colloidal science with respect to the self-assembly of asymmetric particles.

A.2.2 Preparation of the gels

Agarose gel 12 mg of agarose are added to 10 mL of an aqueous solution of the precursor used in the (first) deposition of the bipolar experiment (usually at a concentration of 1 mM). The solution is then heated until it reaches its boiling point. While the solution starts to cool, the BPEs are added in. When the solution starts to gel, it is poured into the cubic mould used for the bipolar experiment. The solution then finishes to cool in the mould and forms a cube of gel with the BPEs dispersed in it.

Fumed silica gel 25 g of fumed silica powder and 30 mL of an aqueous solution of the precursor are mixed in a container until a homogeneous gel is obtained. Once the gel is homogeneous, the BPEs are added and the gel is stirred until the BPEs are homogeneously dispersed. Part of the gel is then poured into the mould used for the bipolar experiment.

A.2.3 Bipolar electrodepositions

Once the mould has been filled with the gel, it is placed in a beaker between two 30 × 30 mm 2 plate feeder electrodes. The cube of gel is oriented with the correct angles to deposit the metal at the desired position at the surface of the BPEs. The beaker is then filled with a 1 mM KCl solution and the desired voltage is then applied between the two feeder electrodes.

A.2.4 Diffusion of a new precursor into a gel

After a first bipolar electrodeposition, the mould containing the gel is first washed to remove the remaining precursor by being placing it in 50 mL of distilled water overnight. The next day, the mould is removed from the water and is again immersed overnight, this time in 50 mL of a solution of the new precursor at the desired concentration for the next bipolar electrodeposition (usually a concentration of 1 mM). The next day, the mould is removed from the precursor bath and can be used for another bipolar electrodeposition.

Contents 5 . 1

 51 Bipolar modification of spherical GC microparticles . . . . . . . . . . . . . 5.2 Capillary-assisted bipolar electrodeposition . . . . . . . . . . . . . . . . . . 5.3 Controlled peeling of a metallic layer at the surface of a BPE . . . . . . . . 5.3.1 Modification of core-shell microparticles . . . . . . . . . . . . . . . 5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Conclusion and perspectives 6.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Possible improvements of the developed techniques . . . . . . . . . 6.2.2 Applications of complex asymmetric particles . . . . . . . . . . . . A Materials and methods A.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2.1 Fabrication of the filter paper moulds . . . . . . . . . . . . . . . . . A.2.2 Preparation of the gels . . . . . . . . . . . . . . . . . . . . . . . . . A.2.3 Bipolar electrodepositions . . . . . . . . . . . . . . . . . . . . . . . A.2.4 Diffusion of a new precursor into a gel . . . . . . . . . . . . . . . . A.2.5 Recovery of the modified particles . . . . . . . . . . . . . . . . . . . B Symbols and abbreviations B.1 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure R. 1 -

 1 Figure R.1 -(a) Exemple de particules Janus avec un hémisphère noir et un hémisphère blanc. Las barres d'échelle font 100 µm. Adapté de [1]. (b) Schéma d'une particule Janus propulsée par la production de bulles. Adapté de [2]. (c) Double hélice auto-assemblée à partir de particules asymétriques avec trois patchs. Adapté de [3].

Figure R. 2 -

 2 Figure R.2 -(a) Schéma du principe de l'électrochimie bipolaire. Le champ électrique polarise l'objet conducteur qui se comporte comme une électrode bipolaire ; des réactions électrochimiques (une réduction et une oxydation) se déroulent à ses extrémités. (b) Particules en carbone vitreux avec des dépôts d'or à l'une de leurs extrémités. Adapté de [4].

Figure R. 3 -

 3 Figure R.3 -Images par microscopie électronique à balayage de particules sphériques en carbone vitreux modifiées avec des dépôts d'or par électrochimie bipolaire en utilisant des signaux périodiques entre les électrodes génératrices. (a) Particule modifiée en utilisant un signal symétrique. (b) Particule modifiée en utilisant un signal asymétrique.

  R.5a). Pour obtenir des dépôts hors du plan horizontal, le gel contenant les électrodes bipolaires peut être penché à l'aide de plans inclinés (Fig. R.4c). Il est donc possible de déposer plusieurs patchs métalliques à n'importe quelle position à la surface de particules conductrices sphériques (Fig. R.5b).

Figure R. 4 -

 4 Figure R.4 -Plusieurs cellules bipolaires testées pour la déposition de patchs à la surface de particules sphériques le long de plusieurs axes différents. (a) Schéma d'une cellule en forme de croix mise au point par Koizumi et al. Adapté de [5]. (b) Schéma de cellule sphérique avec quatre paires d'électrodes génératrices. L'électrode bipolaire (sphère noire) est placée au centre de la cellule, à l'intersection des quatre champs électriques (jaune). (c) Schéma de la cellule utilisée pour la déposition de patchs métalliques à n'importe quelle position à la surface d'électrodes bipolaires. Ici, c'est le gel contenant les électrodes bipolaires et non le champ électrique qui est réorienté entre les dépositions.

Figure R. 5 -

 5 Figure R.5 -Particules en carbone vitreux modifiés avec plusieurs dépôts métalliques le long de différents axes. (a) Particule avec deux dépôts d'or placés à 90°l'un de l'autre. (b) Particule avec trois dépôts d'or placés le long de trois axes perpendiculaires les uns aux autres. (c) Particules avec deux dépôts, l'un en or et l'autre en palladium, placés à 90°l'un de l'autre. (d) Particule chirale avec quatre dépôts (or, platine, palladium et nickel) placés aux sommets d'un tétraèdre. (Jaune : Au ; rouge : Pt ; bleu : Pd ; vert : Ni.)

Figure R. 6 -

 6 Figure R.6 -Synthèse d'une particule Janus à partir d'une particule noyau-enveloppe par électrochimie bipolaire. Au pôle anodique de l'électrode bipolaire (gauche), l'enveloppe de cuivre est oxydée en ions Cu 2+. Au pôle cathodique (droite), les ions Cu 2+ présents en solution sont réduits en cuivre métallique à la surface de l'électrode bipolaire. Puisque la même demi-réaction est utilisée aux deux pôles, ∆V min = 0 V. Après un certain temps, toute l'épaisseur de l'enveloppe de cuivre du pôle anodique est oxydée et le noyau de verre de la particule est révélé, créant une particule Janus cuivre-verre.

Figure 1 . 1 -

 11 Figure 1.1 -Euler diagram of different schematic representations of patchy, multicompartment and Janus particles. Janus particles can be patchy, multicompartment or both at the same time.

Figure 1 . 2 -

 12 Figure 1.2 -Method used by Perro et al. to synthesize Janus particles. Silica nanoparticles are placed at the interfaces of wax droplets in water. The surface of the particles in contact with water are functionalized, producing Janus particles. The particles are then collected by dissolution of the wax and centrifugation. Reprinted from Ref. [15].

Figure 1 . 4 -

 14 Figure 1.4 -Scheme of the protocol used for the first Janus particle synthesis by microcontact printing. A monolayer of surfactant (octadecyltrimethylammonium bromide, ODTAB) is formed on a PDMS stamp.The stamp is applied on the particles which are modified. Reprinted from Ref.[START_REF] Cayre | Fabrication of asymmetrically coated colloid particles by microcontact printing techniques[END_REF].

Figure 1 . 5 -

 15 Figure 1.5 -Polymer based formation of patchy particles. When water is added to the DMF solution of PS-b-P4VP, the polymer micelles transition into patchy particles. Reprinted from Ref. [26].

Figure 1 . 6 -

 16 Figure 1.6 -Synthesis of Janus particles by competitive coordination of ligands. (a) principle of the synthesis. 4-MPAA and PAA forms two areas of ligands on the surface of an AuNP. When TEOS is added, it only reacts on the surface not covered by PAA to form a SiO 2 layer. (b) TEM image of the resulting Janus nanoparticles and a photograph of a solution of the particles (inset). Reprinted from Ref. [33].

Figure 1 . 7 -

 17 Figure 1.7 -A hollow silica sphere with different properties on the outside and inside is crushed. This results in the production of Janus nanosheets with different properties on their two sides. Adapted from Ref. [34].

Figure 1 . 8 -

 18 Figure 1.8 -Immiscible monomers M 1 and M 2 are emulsified into droplets in a water solution W and photopolymerized with UV light into (a) Janus and (b) ternary particles. Adapted from Ref. [1].

Figure 1 . 9 -

 19 Figure 1.9 -Janus and ternary particles synthesized through the microfluidic co-flow method. (a-b) Optical microscopy and fluorescence microscopy (inserts) images of Janus and ternary particles photopolymerized from two immiscible polymers. The scale bars are 100 µm. Adapted from Ref. [1]. (c) Differential interference contrast microscopy and fluorescence microscopy images (insert) of Janus hydrogel particles. One compartment contains magnetic nanoparticles, the other one contains rhodamine B. The scale bars are 100 µm for the main image and 25 µm for the insert. Adapted from Ref. [40]. (d) Bicoloured Janus particles. The black compartments are loaded with carbon black and the white compartments are loaded with titanium oxide. Adapted from Ref. [38].

Figure 1 .

 1 Figure 1.11 -(a) SEM picture of Wolters and co-workers' "Mickey Mouse" particles. (b) Scheme of a tubular structure formed by "Mickey Mouse" particles. (c) Optical micrograph of the tubular structures. (a-c) Adapted from Ref. [54]. (d) Synthesis of TPM particles with multiples cavities. TPM is grown partially encapsulating a silica particle cluster that is then dissolved by HF. (e) The resulting particles are mixed with PS spheres that assemble themselves into the cavities through depletion interactions. (d-e) Adapted from Ref. [55].

Figure 1 .

 1 Figure 1.12 -(a) Chains of two-patch particles assembled under an electric field. (b) Layer of assembled two-patch particles in the presence of an electric field. Scales bars are 6 µm. (c) Double helix assembly of three-patch particles in the presence of an electric field. Adapted from Ref. [3].

Figure 1 .

 1 Figure 1.13 -Directed assembly of particles (a) Two NPs are linked by the hybridization of two complementary strands of DNA. (b) Scheme and TEM image of a chain obtained by two DNA hybridizations oriented 180°from each other. (c) Scheme and TEM image of a square lattice obtained by four DNA hybridizations oriented 90°from each other. (d) Scheme and TEM image of a hexagonal lattice obtained by four DNA hybridizations oriented 60°and 120°from each other. Reprinted from Ref. [61].

Figure 1 .

 1 Figure 1.14 -(a) Principle of GLAD technique used by Schamel and co-workers. The vapour deposits on the silica seeds at an angle α. By rotating the substrate, the deposition angle continuously changes, forming helices. (b) SEM image of the resulting helices. The scale bar is 2 µm. Adapted from Ref. [65].

Figure 1 .

 1 Figure 1.15 -(S )-lactic acid (left) and (R)-lactic acid (right) are not superposable since the central carbon atom has four different substituents.

Figure 1 . 16 -

 116 Figure 1.16 -Method proposed by Ben Zion et al. for the assembly of chiral colloidal particles. A DNA origami is bound to a PMMA particle. Then, three other particles are sequentially added and selectively stick to the DNA origami. By controlling the size and composition of the added particles, it is possible to control the chirality of the formed colloidal molecules. Adapted from Ref. [66].

Figure 1 . 17 -

 117 Figure 1.17 -Scheme of a multi-fuel Janus micromotor driven by bubble propulsion. H 2 O 2 , H + or OH - can react on the metallic patches of the particles, producing O 2 or H 2 bubbles and propelling the particle. Reprinted from Ref. [2].

Figure 1 . 18 -

 118 Figure 1.18 -Scheme of Gao and co-workers' SiO 2 /Ir micromotor. N 2 H 4 catalytically reacts on the iridium surface, creating chemical gradients which trigger an osmotic flow, moving the particle. Adapted from Ref. [74].

Figure 1 . 19 -

 119 Figure 1.19 -Example of electronic papers. (a-b) When submitted to an electric or magnetic field, paramagnetic Janus particles will orient with the field exposing either their light or dark hemispheres.Reprinted from Ref.[START_REF] Komazaki | Electrically and magnetically dual-driven Janus particles for handwriting-enabled electronic paper[END_REF]. (c) Principle of a display using Janus particles. Each bicoloured particle can be independently rotated by an external stimulus. Adapted from Ref.[START_REF] Yin | Versatile Bifunctional Magnetic-Fluorescent Responsive Janus Supraballs Towards the Flexible Bead Display[END_REF]. (d) Image of the border between a white and a black area of an e-paper display using bicolored Janus particles. Reprinted from Ref.[START_REF] Crowley | Dipole moments of gyricon balls[END_REF]. (e) Handwritten letter using a magnet on a surface of Janus beads with the mechanism shown in (a). Reprinted from Ref.[START_REF] Komazaki | Electrically and magnetically dual-driven Janus particles for handwriting-enabled electronic paper[END_REF].

Figure 2 . 1 -

 21 Figure 2.1 -Schematic representation of the principle of bipolar electrochemistry in an open configuration.The electric field polarizes the conducting object, which acts as a BPE, allowing electrochemical reactions to occur at its extremities.

Figure 2 .

 2 Figure 2.2 -When irradiated by photons with a high enough energy, electrons will jump from the VB to the CB, leaving a hole in the VB. Then, in the presence of an electric field the holes and the electrons will move to the poles of the BPE where they will electrochemically react. Adapted from Ref.[START_REF] Ongaro | Asymmetric Modification of TiO 2 Nanofibers with Gold by Electric-Field-Assisted Photochemistry[END_REF].

  Figure 2.2 -When irradiated by photons with a high enough energy, electrons will jump from the VB to the CB, leaving a hole in the VB. Then, in the presence of an electric field the holes and the electrons will move to the poles of the BPE where they will electrochemically react. Adapted from Ref.[START_REF] Ongaro | Asymmetric Modification of TiO 2 Nanofibers with Gold by Electric-Field-Assisted Photochemistry[END_REF].

Figure 2 . 3 -

 23 Figure 2.3 -Coordinate system used for determining the distance of a point at the interface from the centre of (a) a linear BPE and (b) a spherical BPE. The potential difference at the interface of a BPE depends of its position from the centre of the BPE along the axis parallel to the field E.

Figure 2 .

 2 Figure 2.4 -(a) GC particles with a gold patch deposited by bipolar electrochemistry. (b-d) Carbon tubes modified with copper, platinum and nickel respectively by bipolar electrochemistry. (a) and (c) are adapted from Ref. [4]. (b) and (d) are adapted from Ref. [122].

Figure 2 . 5 -

 25 Figure 2.5 -Synthesis of PEDOT fibres by bipolar electrochemistry. A glass plate is placed on top of the BPE, limiting the supply in monomer and forcing the PEDOT to grow linearly instead of forming a dendritic structure. (a) Optical microscope picture and (b) SEM image of the resulting fibre. Reprinted from Ref. [107].

Figure 2 . 6 -

 26 Figure 2.6 -Principle of the synthesis of a gradient on a BPE. The potential difference increases as a function of the distance from the centre of the BPE.

Figure 2 .

 2 Figure 2.7 -(a) Scheme of a polymer gradient synthesized by bipolar electrochemistry. Adapted from Ref. [132]. (b) Wetting gradient fabricated by the bipolar reductive desorption of thiol molecules on a gold surface. Adapted from Ref. [133].

Figure 2 . 8 -

 28 Figure 2.8 -Screening of catalyst gradients for HER. The oxidation of the molybdenum strips and the silver wires ((a) and (b) respectively) are directly proportional to the quantity of H 2 produced through HER on the alloy composition in front of it. (a) Adapted from Ref. [138]. (b) Adapted from Ref. [130].

Figure 2 . 9 -

 29 Figure 2.9 -Principle of closed bipolar electrochemistry. In a closed set-up, the anodic and cathodic poles of the BPE are in two different compartments. The current cannot bypass the BPE.

Figure 2 .

 2 Figure 2.10 -Closed bipolar set-up used for the simultaneous deposition of platinum and TiO 2 on carbon microwires. Adapted from Ref. [139].

Figure 2 .

 2 Figure 2.11 -Closed bipolar sensor for the detection of hydrogen peroxide. (a) On the sensing side of the electrode, H 2 O 2 is reduced and triggers the ECL reaction on the reporting side. (b) ECL read-out at the reporting side for several concentrations of H 2 O 2 . Adapted from Ref. [141].

Figure 2 .

 2 Figure 2.12 -(a) Visualisation of the pH gradient created around a BPE. A BPE is immersed in a solution of universal pH indicator. The indicator turns blue in basic conditions at the feeder cathode and the cathodic pole of the BPE. On the opposite, it turns red in an acidic environment at the feeder anode and the anodic pole of the BPE. Adapted from Ref. [145]. (b) Array of BPEs with hydrogel deposited on their anodic poles. Adapted from Ref. [144].

Figure 2 . 13 -

 213 Figure 2.13 -Examples of motion generated by bipolar electrochemistry. (a) Lifting of a particle by bubble generation. Adapted from Ref. [149]. (b) Circular motion created by a bipolar rotor. Adapted from Ref. [150]. (c) Linear motion by self-regeneration of a metal dendrite. Adapted from Ref. [151]. (d) Deformation of a conducting polymer by differential swelling. Adapted from Ref. [152].

Figure 2 .

 2 Figure 2.14 -Patterning of conductive surfaces by spot-bipolar electrochemistry. (a) Principle of spotbipolar electrochemistry. The electric field between the two circular feeder electrodes locally polarizes the surface, permitting electrochemical reactions. Adapted from Ref. [157]. (b) Pattern drawn by the bipolar chlorination of a poly(3-methylthiophene) film. Adapted from Ref. [157]. (c-d) Bipolar patterning by respectively deposition and dissolution of a copper film on a gold surface. Adapted from Ref. [158].

Figure 3 . 1 -

 31 Figure 3.1 -Two bipolar set-ups where the BPEs' movements are blocked. (a) The BPEs are immobilized on a surface and immersed in a solution of the precursor. (b) The BPEs are dispersed in a hydrogel and cannot move. The gel is separated from the feeder electrodes by a low concentration solution of supporting electrolyte to allow the bubbles generated at the feeder electrodes to rise to the air/water interface. This method allows for the modification of particles in the bulk.

Figure 3 . 2 -

 32 Figure 3.2 -(a) Set-up used by Koizumi et al. for the AC-bipolar bifunctionalization of GC particles. The particles are suspended in an agarose gel between two planar electrodes. A square wave potential is applied between the electrodes resulting in an alternating electric field surrounding the BPEs. (b-d) GC particles bifunctionalized with gold patches of different sizes. The two patches on every particle have the same size. Adapted from Ref. [5].

Figure 3 . 3 -

 33 Figure 3.3 -SEM images of GC particles after an AC-bipolar electrodeposition of gold at various frequencies (a: 1 Hz; b: 100 Hz; c: 10 kHz).At low frequencies, the deposits are not homogeneous. At high frequencies no deposit is observed at the poles of the particles. Only depositions using frequencies ranging from 10 Hz to 1 kHz produced particles bifunctionalized with two uniform gold patches.

Figure 3 . 4 -

 34 Figure 3.4 -Particles bifunctionalized by AC-bipolar electrochemistry. (a, d) AC voltage signals used for the deposition of gold and palladium, respectively. (b, c) SEM images of GC particles modified with two gold patches using AC-bipolar electrochemistry (e, f) SEM images of GC particles modified with two palladium patches using AC-bipolar electrochemistry.

Figure 3 . 6 -

 36 Figure 3.6 -(a) The cathodic (right) and anodic (left) poles on a spherical BPE of diameter d cover the areas delimited by the angles α and β respectively. (b) Comparison between the measured α of the patches at the surface of particles modified with gold by AC-bipolar electrochemistry and the theoretical value given by Equation (3.14) as a function of the voltage U applied between the feeder electrodes. The error bars show one standard deviation.

Figure 3 . 7 -

 37 Figure 3.7 -GC particles modified with a single metallic deposit using AC-bipolar electrochemistry. (a, c) Voltage signals used for the modification of the gold and palladium particles respectively. The signals oscillate between the deposition potentials (30 V for gold and 40 V for palladium) and a potential (-5 V), which is low enough so that no electrochemical reaction occurs at the poles of the BPEs, but the anionic precursors sufficiently migrate in the opposite direction to compensate for the migration that occurred during the deposition cycle. (b) SEM image of particles modified with a single gold patch. (d) SEM image of particles modified with a single palladium patch.

Figure 4 .

 4 Figure 4.1 -(a) Scheme illustrating the different paths that the current can take between the feeder electrodes. First, the charges flow through the solution to reach the vicinity of a BPE. Then, they can either flow directly through the solution by ionic conduction or react at one pole of the BPE and cross the particle via an electron flow before reacting again at the other pole. (b) Equivalent electrical circuit of a bipolar cell in an open configuration. The total current is split between the faradic current going through the BPEs and the ionic current remaining in the solution. The repartition between these two currents is determined by the ratio between R sol and R BPE .

Figure 4 . 2 -

 42 Figure 4.2 -Simulations of the electric field between (a) two charged plates and (b) two charged points. The two plates create a much more uniform field between them compared to the point charges. The black lines are the flow lines of the electric field and the dotted lines are equipotential lines. The red objects are positively charged and the blue objects are negatively charged.

Figure 4 . 3 -

 43 Figure 4.3 -(a) Scheme illustrating the different paths the current can take when passive feeder electrodes are present in the bipolar cell. The pFEs act as very large BPEs and most of the current flows through them instead of flowing through the solution and the BPEs dispersed in the solution. (b) Equivalent circuit of a bipolar cell with passive feeder electrodes. Since R pFE is much smaller than R sol or R BPE , most of the solution and the BPEs are short-circuited by the passive feeder electrodes.

Figure 4 . 4 -

 44 Figure 4.4 -(a) Cross-shaped bipolar cell used for the bifunctionalization of conducting particles. The particles are iteratively modified by using each pair of FEs sequentially. Each pair is in its own channel to avoid the influence of the other pair's electric field. Adapted from Ref. [5]. (b) Simulation of the electric field created by applying the same voltage simultaneously on both pairs of FEs. At the intersection of the two channels, the electric fields add to each other to form an electric field with a 45°angle. Adapted from Ref. [164].

Figure 4 . 5 -

 45 Figure 4.5 -Scheme of a spherical bipolar cell with four pairs of feeder electrodes (blue and red dots) arranged into a tetrahedral geometry. The BPE (black sphere) is placed in the centre of the cell at the intersection of all four possible electric field directions (yellow).

Figure 4 . 6 -

 46 Figure 4.6 -(a) Simulation of the electric field produced by two arrays of small charged electrodes. Despite using small electrodes, the created field is very similar to the field created by two parallel plates (Fig. 4.2a). The black lines are flow lines of the electric field and the dotted lines are equipotential lines. The red objects are positively charged and the blue objects are negatively charged. (b) Electrical circuits of a bipolar set-up that uses four arrays of electrodes as feeder electrodes. When the arrays are used as active feeder electrodes (left and right arrays) all the individual electrodes (black dots) are electrically connected to each other. When used as passive feeder electrodes (top and bottom arrays), the individual electrodes are disconnected from each other.

Figure 4 . 7 -

 47 Figure 4.7 -Set-up for the modification of particles with several patches placed at several angles. After a first deposition, the gel is turned by an angle θ and a second deposition can be carried out. This rotationdeposition sequence can be repeated multiple times to deposit more than two patches at arbitrary angles in the horizontal plane.

Figure 4 . 8 -

 48 Figure 4.8 -SEM images of GC particles modified with multiple metal patches. (a) Particle with two gold patches located at an angle of 90°with respect to each other. (b) Particles with two palladium patches located at an angle of 90°with respect to each other. (c) Particle with three gold patches located at an angle of 120°with respect to each other.

Figure 4 . 9 -

 49 Figure 4.9 -Set-up for the deposition of patches at an arbitrary position on the surface of BPEs. The gel containing the BPEs and the precursor is poured into a cubic mould. The mould is placed between two planar feeder electrodes and on an incline forming an angle ϕ with the horizontal plane. The gel can be further rotated by an angle θ to determine the exact position of the patches on the surface of the BPEs. The incline is made from a non-conductive polymer mesh in order to avoid disturbing the electric field.

Figure 4 .

 4 Figure 4.10 -SEM images of GC particles modified with multiples gold patches in the three spatial dimensions. (a) Particle with three gold patches placed along three axes orthogonal to each other. (b) Particles with four gold patches that form a tetrahedron similar to the positions of the hydrogen atoms in a methane molecule.

Figure 4 .

 4 Figure 4.11 -Set-up for the deposition of rings on spherical BPEs. The bipolar cell is placed on top of a magnetic stirrer. The gel cube containing the BPEs is placed on a support containing a magnetic stir bar. When the stirrer is switched on with an angular velocity ω, the support starts rotating, causing the gel to rotate. A DC voltage is then applied between the feeder electrodes and the deposition of metal starts at the equators of the BPEs.

Figure 4 .

 4 Figure 4.12 -SEM images of GC particles after a rotating bipolar deposition of palladium and gold respectively with an angular velocity of 1 rpm. (a) The palladium ring has a uniform thickness. (b) Very little gold was deposited because the BPEs' rotation speed was too fast compared to the optimal deposition period.

Figure 4 . 13 -

 413 Figure 4.13 -Minimal voltages needed between two feeder electrodes placed 30 mm apart in order to trigger the depositions of different metals at the cathodic pole of a 1000 µm BPE. The minimal voltages for the depositions of platinum and palladium are particularly close to each other.

Figure 4 . 14 -

 414 Figure 4.14 -Protocol for changing the precursor present inside a gel. A gel containing the precursor of the first metal to deposit is prepared and (a) the first bipolar deposition is carried out. Then, (b) the gel is immersed into distilled water overnight to let the remaining precursor diffuse out of the gel. (c) The gel is then immersed into a solution of a second precursor which diffuses into the gel overnight. Finally, the gel is placed back into the bipolar cell with a different orientation and (d) the bipolar deposition of the second metal can be performed. Steps (b) trough (d) can be repeated multiple times with different precursors in order to deposit more than two metals on the same BPEs.

Figure 4 .

 4 Figure 4.15 -Superposed SEM images and EDX mappings of GC particles with patches of different compositions. (a) Particles with a gold patch and a palladium patch with a 90°orientation with respect to each other. (b) Particle with patches of gold, palladium and nickel with a 120°orientation with respect to each other. Image (b) has been resized to compensate a stretching effect caused by a bad configuration of the EDX detector's software. (Yellow: Au; blue: Pd; green: Ni.)

Figure 4 .

 4 Figure 4.16 -Superposed SEM images and EDX mappings of the (a) R and (b) S enantiomorphs of a chiral particle with four patches of gold, platinum, palladium and nickel arranged in a tetrahedral geometry. Image (b) has been resized to compensate for a stretching effect caused by a bad configuration of the EDX detector's software, the unmodified particles was already oval shaped. (Yellow: Au; red: Pt; blue: Pd; green: Ni.)

  .

Figure 4 .

 4 Figure 4.17 -EDX mapping of the R enantiomorph of a chiral particle with three different patches arranged with the same geometry as the substituents of a tertiary amine. The image has been resized to compensate for a stretching effect caused by a bad configuration of the EDX detector's software, the unmodified particles was already slightly oval shaped. (Yellow: Au; green: Pd; magenta: Pt.)

Figure 5 . 1 -

 51 Figure 5.1 -SEM images of GC microparticles modified by bipolar electrochemistry. (a) Microparticles modified by applying a DC voltage of 300 V for 5 min. Some GC sphere are fused together and form larger, odd-shaped BPEs. (b) Microparticle modified by applying an AC voltage with an amplitude of 200 V for 8 min.

Figure 5 . 2 -

 52 Figure 5.2 -Set-up used for capillary-assisted bipolar electrodeposition. The right vial is filled with unmodified carbon tubes (CTs).A voltage is applied between the two feeder electrodes (Yellow). The potential difference between the two vials creates an EOF that moves the particles toward the negative feeder electrode through the silica capillary. During their travel through the capillary, the CTs act as BPE and can be asymmetrically modified. A detector is placed along the capillary to monitor the flow of the BPEs. Adapted from Ref.[START_REF] Loget | Electric field-generated asymmetric reactivity: from materials science to dynamic systems[END_REF].

Figure 5 . 3 -

 53 Figure 5.3 -(a) Bipolar peeling of a core-shell particle. At the anodic pole of the BPE (left), the shell of metallic copper is oxidized into Cu 2+ . At the cathodic pole (right) the Cu 2+ ions in solution are reduced into metallic copper and the copper shell thickens. Since the same half-reaction is used at both poles, theoretically ∆V min = 0 V. (b-d) Pictures of the BPE at the beginning of the experiment and after 30 min and 90 min respectively. The exposed glass area grows as a function of time. The bead has a diameter of 7.26 mm. Adapted from Ref. [176].

Figure 5 . 4 -

 54 Figure 5.4 -Superposed SEM images and EDX mappings of glass microparticles coated with a layer of metallic copper. (a) Unmodified core-shell microparticles. On some of the particles, the copper layer does not cover the whole glass core. This is due to mechanical damage occurring during transport, storage and handling of the particles. (b, c) Microparticles after a bipolar experiment with U = 5 V during 15 min and 30 min respectively. (Blue: Cu.)

Figure A. 1 -

 1 Figure A.1 -Cut-and-fold template used for the production of the cubic filter paper moulds used for the bipolar electrodepositions. The full lines have to be cut and the dotted lines have to be folded in order to form a cube. (Template to scale if printed on A4 paper.)

  Distance travelled by an ion during an electrophoretic migration mm d + Distance travelled by an ion during the positive part of the period of an AC electric field mm d - Distance travelled by an ion during the negative part of the period of an AC electric field mm Value of the electric field during the positive part of an AC signal V•m -1 E - Value of the electric field during the negative part of an AC signal V•m -1

  Distance along the axis of the electric field between the centre of a BPE and a point on its surface µm α Angle between the axis of the electric field and a line from the centre of a BPE to a point on its surface °αc Cathodic charge transfer coefficient βAngle between the axis of the electric field and a line from the centre of a BPE to a point on its surface

				B.2. Abbreviations
	Symbol Abbreviation Meaning Meaning	Unit
	n BPE	Number of electrons exchanged in an electrochemical Bipolar electrode	-
		BQ	reaction Benzoquinone
	R CABED	Gas constant Capillary-assisted bipolar electrodeposition	J•K -1 •mol -1
	R BPE CB	Resistance of a bipolar electrode Conduction band	Ω
	R Ox CIP	Charge transfer resistance of an oxidation Cahn-Ingold-Prelog	Ω
	R pFE CT	Resistance of a passive feeder electrode Carbon tube	Ω
	R Red CVD	Charge transfer resistance of a reduction Chemical vapour deposition	Ω
	R sol DC	Resistance of a solution Direct current	Ω
	t	DMF	Time	Dimethylformamide	s
	t total DNA	Total length of a bipolar deposition Deoxyribonucleic acid	min
	T e -	Absolute temperature Electron	K
	T ECL	Period Electrochemiluminescence	s
	T + EDOT	Positive part of the period of an AC signal 3,4-Ethylenedioxythiophene	s
	T -EDX	Negative part of the period of an AC signal Energy-dispersive X-ray spectroscopy	s
	U EOF	Voltage applied between the feeder electrodes Electroosmotic flow	V
	U + FE	Voltage applied during the positive part of an AC sig-Feeder electrode	V
		GC	nal	Glassy carbon
	U -GLAD	Voltage applied during the negative part of an AC Glancing angle deposition	V
		h +	signal Electron hole
	x hν		Photon
		HER		Hydrogen evolution reaction
		HLB		Hydrophilic-lipophilic balance
		HQ		Hydroquinone
		LCD		Liquid-crystal display
		LED		Light-emitting diode	°δ+
		4-MPAA		4-Mercaptophenylacetic acid
		NP	Positive partial charge Nanoparticle	-
	δ -ODTAB	Negative partial charge Octadecyltrimethylammonium bromide	-
	∆V OLED	Potential difference Organic light-emitting diode	mV
	∆V min Ox	Minimum potential difference between the two poles Oxidant	mV
		P4VP	of a BPE for bipolar electrochemistry Poly(4-vinylpyridine)
	η a PAA	Anodic overpotential Poly(acrylic acid)	mV
	η c PDMAEMA	Cathodic overpotential Poly(2-(dimethylamino)ethyl methacrylate)	mV
	µ e PDMS	Electrophoretic mobility Polydimethylsiloxane	m 2 •V -1 •s -1
	θ	PEDOT	Longitude of a patch on an spherical particle Poly(3,4-ethylenedioxythiophene)	°ρmat
	E • PEO f ρ sol pFE F ϕ PMMA i POEGMA i BPE ∅ PPy i ion PS i pFE PSS i total j PVD B.2 Abbreviations Standard electrode potential Resistivity of the material composing a BPE Poly(ethylene oxide) Frequency Resistivity of a solution Passive feeder electrode Faraday constant Latitude of a patch on an spherical particle Poly(methyl methacrylate) Electrical current Rotation speed Poly(oligo(ethylene glycol) monomethylether methacrylate) Ω•m V Ω•m Hz °ω C•mol -1 rpm mA Current going through a BPE Diameter mm Polypyrrole mA Ionic current Polystyrene mA Current flowing through a passive feeder electrode Poly(styrene sulfonate) mA Total current mA Current density Physical vapour deposition R Rectus mA•mm -2 j BPE Current density flowing through a BPE Red Reductant mA•mm -2 j c Current density at a cathodic pole mA•mm -2 j 0 Exchange current density Abbreviation rpm Revolutions per minute Meaning S Sinister mA•mm -2 l Length of a BPE µm L Distance between the feeder electrodes mm AC SEM Scanning electron microscopy Alternating current aFE SHE Standard hydrogen electrode Active feeder electrode b Block TEM Transmission electron microscopy

1.2. Bulk synthesis of asymmetric particles

Al + 6 H 2 O --→ 2 Al(OH)

+ 3 H 2 (1.2)

Acknowledgements

First and foremost, I would like to thank my thesis supervisor, Prof. Alexander Kuhn, for his help, mentorship and advice all along my doctoral work. I would like to thank all the members of my PhD defence committee, especially Prof. Shinsuke Inagi and Dr. Gabriel Loget for accepting to be the referees of my thesis, I also thank Prof. Étienne Duguet, Prof. Robbyn Anand and Prof. Serge Ravaine for evaluating my work.

3.4.2 Synthesis of particles with two differently sized patches of the same material The time ratio between the positive and negative voltages inside a period is given by Equation (3.9). The total time of the deposition is determined so that the duration t + of the positive deposition (i.e., using the highest absolute voltage and the shortest part of the period) is always equal to 15 min to obtain similar deposits as for the case of a symmetric AC-bipolar electrodeposition. Hence, it is possible to calculate the total duration t total of the AC-bipolar deposition:

Appendix A Materials and methods

A.1 Materials

The chemicals used were bought from commercial providers and used without any further purifications. The 630 µm -1000 µm glassy carbon spheres used throughout this thesis were purchased from Alfa Aesar. The feeder electrode plates were made of SIGRADUR G glassy carbon, bought from HTW Hochtemperatur-Werkstoffe GmbH.

The glass/copper core-shell microparticles used in Chapter 5 were provided by Prof. Anand V. P. Gurumoorthy from the Vellore Institute of Technology, India.

The voltages applied between the feeder electrodes were generated using a DG1000Z arbitrary waveform generator from RIGOL Technologies, Inc. and amplified using a A800DI linear voltage amplifier from FLC Electronics AB.

The SEM images shown were made either with a SEM model VEGA 3-SBH with a tungsten filament electron source from TESCAN or with a TM-1000 tabletop SEM from Hitachi, each time using a backscattering electron detector. The EDX mappings were made with an XFlash 630M detector from Bruker installed on the TESCAN SEM.

The electric field simulations shown in

A.2 Methods

A.2.1 Fabrication of the filter paper moulds

The template shown in Figure A.1 is used to form the filter paper moulds used in the thesis. The template is composed of nine 16 × 16 mm 2 squares. The template is cut from a sheet of filter paper and folded into a 16 × 16 × 16 mm 3 cube with the top face open. The gel is then poured into the mould and the mould is placed in the centre of the bipolar cell. The mould is held in shape either with a small amount of adhesive tape or a stitch of insulated thin electrical wire through a hole made with a needle. Each mould holds around 4 mL of gel.

A.2. Methods

A.2.5 Recovery of the modified particles

Once all the bipolar electrodepositions are finished, the gel containing the modified particles is removed from its mould and immersed into distilled water. The water is then slowly heated until the gel decomposes and the particles are released. The particles are then gathered with a plastic Pasteur pipette and washed with distilled water. The particles can then be observed by SEM or stored in a small amount of water.

During the recovery of the particles, stirring or any movement of the particles are kept to a minimum in order to avoid any shock of the particles between them or with the sides of the beaker. These frictions can cause scratches on the deposits, removing part of the deposited metal.

Abbreviation Meaning

TEOS