Andreia S Monteiro

Marcos De Oliveira

Silvia Santagneli

Carole Carcel

Torsten Gutmann

Gerd Buntkowsky

Michel Wong

Chi Man

Hernane S Barud

Sidney Ribeiro

Marcos De Oliveira

Modification of Bacterial Cellulose Membrane with 1,4bis(triethoxysilyl)benzene: a thorough physical-chemical characterization study

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The field of pharmaceutical technology is expanding rapidly because of the increasing number of drug delivery options. Successful drug delivery is influenced by multiple factors, one of which is the appropriate identification of materials for research and engineering of new drug delivery systems. Bacterial cellulose (BC) is one such biopolymer that fulfills the criteria for consideration as a drug delivery material. [START_REF] Gorgieva | Bacterial Cellulose: Production, Modification and Perspectives in Biomedical Applications[END_REF][START_REF] Wasim | An Overview of Synthesized Bacterial Cellulose Nanocomposites for Biomedical Applications[END_REF] This biopolymer is a hydrogel formed of a threedimensional structure of fibrils with an exclusive combination of properties such as high surface area (high aspect ratio of fibers with diameter 20-100 nm), hydrophilic nature of BC (due to hydroxyl groups), resulting in high water-holding capacity (over 100 times of its own weight), better adherence, and increased moisture content, distinct physical and mechanical properties, including insolubility, biocompatibility, high crystallinity index (84%-89%) and polymerization degree, high mechanical strength, with a tensile strength of 200 -300 MPa, and a Young's modulus of up to 15 GPa as well as high thermal stability (with a decomposition temperature ranging between 340 and 370 °C). Due to its high purity (absence of lignin and hemicellulose), BC is considered as a non-cytotoxic, non-genotoxic, non-allergenic and highly biocompatible material.1 - 6 These excellent and unique properties of BC, show a high potential as material in a wide range of high-tech domains including biomedical applications, and most notably in controlled drug-delivery systems. [START_REF] Wasim | An Overview of Synthesized Bacterial Cellulose Nanocomposites for Biomedical Applications[END_REF][START_REF] Silvestre | Do bacterial cellulose membranes have potential in drug-delivery systems?[END_REF][START_REF] Almeida | Bacterial cellulose membranes as drug delivery systems: an in vivo skin compatibility study[END_REF] BC has been used in several systems for drug delivery (see refs. [START_REF] Wasim | An Overview of Synthesized Bacterial Cellulose Nanocomposites for Biomedical Applications[END_REF][START_REF] Silvestre | Do bacterial cellulose membranes have potential in drug-delivery systems?[END_REF][START_REF] Almeida | Bacterial cellulose membranes as drug delivery systems: an in vivo skin compatibility study[END_REF][START_REF] Trovatti | Biocellulose membrane as supports for dermal release of lidocaine[END_REF][START_REF] Trovatti | Bacterial cellulose membranes applied in topical and transdermal delivery of lidocaine hydrochloride and ibuprofen: in vitro diffusion studies[END_REF][START_REF] Silva | Topical caffeine delivery using biocellulose membranes: a potential innovative system for cellulite treatment[END_REF] ) or after some sort of physical or chemical modification in the form of nanocomposite materials with diverse polymeric matrices. [START_REF] Stoica-Guzun | Effect of electron beam irradiation on bacterial cellulose membranes used as transdermal drug delivery systems[END_REF][START_REF] Jipa | Controlled release of sorbic acid from bacterial cellulose based mono and multilayer antimicrobial films[END_REF][START_REF] Halib | Swelling of bacterial celluloseacrylic acid hydrogels: sensitivity towards external stimuli[END_REF] Several BC products for drug delivery systems have been developed and marketed. [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF][START_REF] Serafica | Microbial cellulose materials for use in transdermal drug delivery systems, methods of manufacture and use[END_REF][START_REF] Bayon | Medical device including bacterial cellulose reinforced by resorbable or non-resorbable materials[END_REF][START_REF] Lin | Bacterial cellulose film and uses thereof[END_REF][START_REF] Trexler | [END_REF] These systems have been mainly tested in vitro in transdermal drug delivery, used as a temporary skin substitute and wound dressing in treatment of burns and chronic wounds, oral drug delivery and tissue engineering/regeneration. The BC can be modified with different nanomaterials opening new perspectives for the design of tailored drug-delivery systems based on this natural biopolymer.

Mesoporous silica nanoparticles (MSNs) have been applied as carriers of active drugs to develop drug delivery systems. [START_REF] Carvalho | Highlights in Mesoporous Silica Nanoparticles as a Multifunctional Controlled Drug Delivery Nanoplatform for Infectious Diseases Treatment[END_REF][START_REF] Chen | Nanomachines and other caps on mesoporous silica nanoparticles for drug delivery[END_REF][START_REF] Manzano | Mesoporous silica nanoparticles for drug delivery[END_REF][START_REF] Baeza | Recent advances in porous nanoparticles for drug delivery in antitumoral applications: inorganic nanoparticles and nanoscale metal-organic frameworks[END_REF][START_REF] Lu | Biocompatibility, biodistribution, and drug-delivery efficiency of mesoporous silica nanoparticles for cancer therapy in animals[END_REF][START_REF] Narayan | Mesoporous silica nanoparticles: a comprehensive review on synthesis and recent advances[END_REF][START_REF] Giret | Mesoporous-silica-functionalized nanoparticles for drug delivery[END_REF] Their outstanding structural and physicochemical properties, such as small particle size, high specific surface area and chemical and physical stabilities, ordered structure, tunable pore size, high chemical and physical stabilities, and hydroxyl groups available on their surface (exterior and interior of the pores), are amazing and can be tuned making them emerging nanocarriers for specific and controlled drug release, with low toxicity and biological behavior. The pores structure can promote drugs confinement, controlled loading and release processes avoiding drug degradation or premature release before achieving their target. In addition, the hydroxyl groups available in MSNs can be easily functionalize by functional groups favored by drug molecules in order to further enhance drug loading and releasing ability. [START_REF] Carvalho | Highlights in Mesoporous Silica Nanoparticles as a Multifunctional Controlled Drug Delivery Nanoplatform for Infectious Diseases Treatment[END_REF][START_REF] Narayan | Mesoporous silica nanoparticles: a comprehensive review on synthesis and recent advances[END_REF][START_REF] Flodström | Mechanism of mesoporous silica formation. A time-resolved NMR and TEM study of silica-block copolymer aggregation[END_REF] In view of the potential of these two materials, the combination of BC with MSNs for the design of drug-delivery systems, was the motivation for the development of this work.

The incorporation of suitable bridged organic groups endows the formed organo bridged MSNs properties such as improved hydrothermal and hydrolytic stability, adjustable biodegradability, high loading capacity towards specific drugs owing to high affinity, framework-induced stimuli-responsive release behavior and higher biocompatibility. Organo-bridged MSNs have recently been synthesized by hydrolysis and condensation reactions of 100% bridged organosilica precursors, without using any classical silica source such as tetraethoxysilane (TEOS). [START_REF] Flodström | Mechanism of mesoporous silica formation. A time-resolved NMR and TEM study of silica-block copolymer aggregation[END_REF][START_REF] Poscher | Trends in Degradable Mesoporous Organosilica-Based Nanomaterials for Controlling Drug Delivery: A Mini Review[END_REF][START_REF] Du | Mesoporous silica nanoparticles with organo-bridged silsesquioxane framework as innovative platforms for bioimaging and therapeutic agent delivery[END_REF][START_REF] Cho | Preparation of mesoporous benzene-silica nanoparticles[END_REF][START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] 1,4-bis(triethoxysilyl)benzene (BTEB) is extensively used as bridged precursor to form benzene-bridged mesoporous silica nanoparticles with a variety of morphological and textural properties. [START_REF] Cho | Preparation of mesoporous benzene-silica nanoparticles[END_REF][START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF][START_REF] Shi | Heterogeneous organocatalysis at work: functionalization of hollow periodic mesoporous organosilica spheres with MacMillan catalyst[END_REF][START_REF] Croissant | Mixed periodic mesoporous organosilica nanoparticles and core-shell systems, application to in vitro twophoton imaging, therapy and drug delivery[END_REF][START_REF] Haffer | Periodic mesoporous organosilica (PMO) materials with uniform spherical core-shell structure[END_REF][START_REF] Cho | Direct Synthesis of sulfonic acid-functionalized periodic mesoporous benzenesilicas with large pores[END_REF][START_REF] Morell | Synthesis and characterization of highly ordered biofunctional aromatic periodic mesoporous organosilicas with different pore sizes[END_REF] The incorporation of organosilicates on the Bacterial Cellulose Membrane (BCM) with the formation of the silica nanoparticles on the fibers of the BC, has been reported by some works. [START_REF] Barud | Bacterial cellulose-silica organic-inorganic hybrids[END_REF][START_REF] Li | High permeation and antifouling polysulfone ultrafiltration membranes with in situ synthesized silica nanoparticles[END_REF][START_REF] Hou | Functional bacterial cellulose membranes with 3D porous architectures: Conventional drying, tunable wettability and water/oil separation[END_REF][START_REF] Shen | Engineering Sustainable Antimicrobial Release in Silica-Cellulose Membrane with CaCO3-Aided Processing for Wound Dressing Application[END_REF] To the best of our knowledge, no report on the incorporation of organo-bridged precursor in the BC has been published so far, as well as the preparation of porous organo-silica nanoparticles on BC.

In this context, this work reports the preparation of BCM and their modification by in-situ methodology with the organo-bridged precursor BTEB. The modification of BCM with BTEB emerged as a promising strategy to the design of high-performance BC as a potential strategy for drug-delivery systems. The BCM-BTEB was studied by several characterization techniques, namely, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), Solid state nuclear magnetic resonance (RMN) and Thermogravimetric analysis (TGA) to obtain information on morphological, structural and chemical composition. Solid state nuclear magnetic resonance is a powerful characterization tool for amorphous compounds. However, this technique suffers from low sensitivity, especially in the case of low abundant nuclei, such as [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si and [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C or species with very low concentration, as is the case for functional species in cellulose surfaces. To overcome this issue, we combined solid-state NMR with Dynamic Nuclear Polarization (DNP) to achieve sensitivity enhancement. [START_REF] Barnes | High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR[END_REF][START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF][START_REF] Ni | High frequency dynamic nuclear polarization[END_REF][START_REF] Griffin | Clear signals from surfaces[END_REF][START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF] In the DNP technique, typically paramagnetic radicals are introduced within the sample using post-synthesis impregnation with a glass forming solution (DNP matrix). [START_REF] Tanaka | DNP NMR spectroscopy of cross-linked organic polymers: rational guidelines towards optimal sample preparation[END_REF][START_REF] Le | Brute-force solvent suppression for DNP studies of powders at natural isotopic abundance[END_REF][START_REF] Sauvée | Highly efficient, water-soluble polarizing agents for dynamic nuclear polarization at high frequency[END_REF][START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperature up to 200 K[END_REF] The sample is then pumped with microwave (mw) radiation, saturating electron paramagnetic resonance transitions while the NMR experiment is performed and a part of the strong polarization of electronic spins (about 660 times greater than the polarization of 1 H nuclear spins) is transferred to the NMR-active nuclei (details can be found in several recent reviews). [START_REF] Overhauser | Polarization of Nuclei in Metals[END_REF][START_REF] Thurber | Recent Advances in Solid-State White Ligght-Emitting Electrochemical Cells[END_REF] This combination of high resolution high field NMR techniques with the increase in sensitivity provided by the DNP technique is an essential tool for the characterization of species on the surface of nanostructured materials, [START_REF] Hooper | Dynamic Nuclear Polarization (DNP) 101: A New Era for Materials[END_REF][START_REF] Rankin | Recent development in MAS DNP-NMR of materials[END_REF][START_REF] Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF] including cellulose based materials. 53-56 In a very recent work, Gutmann et al. have demonstrated that DNP can be used to selectively enhance those groups on the surface of cellulose fibers by an appropriate choice of the DNP-matrix. [START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF] They have shown that compatibility between the solvation properties of both DNP-matrix and target molecule is determinant for the DNP efficiency. Following their conclusions, we have selected organic solvents for our DNPmatrix in order to selectively enhance the NMR signal of the hydrophobic BTEB moieties on BCM surface.

The BTEB-based nanoparticles incorporated in BCM are obtained from an in situ methodology, which may lead to a final structure for the nanoparticles which differs from the structure reported in the literature for BTEB-based nanoparticles alone. For this reason, we report here not only the successful incorporation of organosilane nanoparticles on cellulose, but also a detailed structural characterization of the surface species by techniques sensitive to the local-range order, such as DNP-NMR. As explained above, due to the sensitivity enhancement, the DNP technique provides extremely valuable information about species on BCM surface, which could not be obtained by other techniques.

EXPERIMENTAL

Material and Reagents.

Tetraethylorthosilicate, TEOS (≥99%), 1,2-dibromoethane (99%), 1,4-dibromobenzene (98%), tetrahydrofuran, THF (99%, analytical grade), hexane (99%, analytical grade), sodium hydroxide (analytical grade), and Agar were supplied from Sigma-Aldrich. Yeast extract was from Kasvi, peptone G was from Acumedia and d-glucose anhydrous (analytical grade), disodium hydrogen phosphate (analytical grade), citric acid (analytical grade), magnesium sulphate heptahydrate (analytical grade), and monopotassium phosphate (analytical grade) were purchased from Synth. Ethanol (99,8%, analytical grade) was purchased from Lac. Ultrapure water (Millipore, specific resistivity: 18 M cm) was used throughout the experiments. The strain Komagataeibacter Xylinus (ATCC53524) was supplied by André Tobello Foundation, Campinas-SP, Brazil.

Preparation of Organosilica precursor 1,4-bis(triethoxysilyl)benzene.

Organosilica source, 1,4-bis(triethoxysilyl)benzene (BTEB), was prepared by the method reported in the literature. [START_REF] Shea | Arylsilsesquioxane gels and related materials. New hybrids of organic and inorganic networks[END_REF] The reaction (Scheme 1) was carried out under nitrogen by use of a vacuum line and dried solvents. To a mixture of magnesium turnings (15.0 g) in 50.0 mL of THF (0.62 mol) under nitrogen 20 drops of 1,2-dibromoethane were added, under vigorous stirring during 5 min. To this solution, 450 mL of TEOS (2.02 mol) in 250 mL of THF (3.09 mol) were added and the mixture was brought to reflux to activate the magnesium turnings. This can be evidenced by ethylene gas evolving (formed from the reaction of 1,2-dibromoethane with metallic magnesium. When no more gas bubbles are formed, the solution was cooled to 20°C. Afterward, a solution of 26.1 mL of 1,4-dibromobenzene (20.4 mmol) in 1100.0 mL of THF (1.23 mol) was added dropwise for 2 h. Within 30 min of initiating the addition, the reaction became mildly exothermic. Care should be taken so that the temperature of the reacting mixture never exceed 40°C After the completion of the BTEB addition, the grey-green mixture was refluxed for 2 h at 60 °C and then left at room temperature for 12 h. 200.0 mL of hexane was added to precipitate any remaining magnesium salts in solution, and the mixture was quickly filtered under nitrogen to yield a clear, light brown solution. Hexane and THF were removed in vacuum, and the remaining TEOS was distilled off under vacuum leaving a brown oil. The oil was distilled (135 °C/0.05 mmHg) to give a clear colorless oil (1.72 g, 67% yield). 1H NMR (CDCl3) 7.67 (s, 4 H, ArH), 3.84 (q, 12 H, OCH2CH3), 1.22 (t, 18 H, OCH2CH3). The strain used was Komagataeibacter Xylinus (ATCC53524). BCM were prepared following the procedures adapted from literature. [START_REF] Pecorano | Polymers and Composites from Renewable Resources[END_REF] An aliquot of the bacterium Komagataeibacter Xylinus (ATCC53524) was replicated to a sterile solid medium plate HS (Hestrim-Schramm) containing 12 g of glucose (0.067 mol), 3 g of yeast extract, 3 g of peptone G, 1.62 g of disodium hydrogen phosphate (0.011 mol), 0.69 g of citric acid (3.59 mmol), 15 g of agar and 585 mL of milipore water (32.5 mol). After growth time, the microorganism was inoculated into sterile liquid culture medium consisting of 50 g of glucose (0.277 mol), 4 g of yeast extract, 0.73 g of magnesium sulphate heptahydrate (2.96 mmol), 2 g of monopotassium phosphate (0.015 mol), 20 mL of ethanol (0.342 mol) and 980 mL of millipore water (54.4 mol) and maintained for 72 h in an oven with air circulation and with constant temperature at 28 °C. Subsequently, the BCM produced were purified following the subsequent procedure: washing the BCM with water 5 times per day during 3 days; washing the BCM with water at 80 °C during 50 min; immersion of the BCM in 100 mL of an ethanolic solution of sodium hydroxide (0.1 M) at 80 °C during 50 min; and washing again with millipore water until pH 7.0. Finally the BCM was dried at 80 °C for 24 h. The resulting sample will be called in the following 194 BCM.

b. Preparation of Bacterial Cellulose Membrane functionalized with BTEB.

The organosilane BTEB were incorporated into BCM by in situ methodology, following similar procedure for the preparation of the BCM, with the exception the addition of 1% (w/w) of organosilane BTEB (with respect to the amount of glucose of the liquid culture medium), in the liquid culture medium, at the same time as the microorganism (Scheme 2). The resulting sample will be called in the following BCM-BTEB. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed at the European Membrane Institute (EMI), Montpellier, France, using a high-resolution environmental scanning electron microscope (Hitachi S4800 model) and an energy-dispersive X-ray spectrometer (Oxford Instruments X-Max N SDD), respectively. The samples were analyzed with coating of 1 nm thick carbon layer.

Transmission electron microscopy (TEM) images were performed at the Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM), France, using a Philips CM30T electron microscope with a LaB6 filament as the source of electrons, operated at 80 kV to minimize beam damage. The BCM-BTEB sample was dispersed in high purity ethanol under sonication and a drop of the colloidal suspension was placed on a holey carbon TEM grid and left to dry at ambient conditions. The average particle size and standard deviations of nanomaterials incorporated into the BCM were calculated from the diameters of 50 particles randomly selected from SEM micrographs, and using the ImageJ software.

The Fourier transform infrared (FTIR) spectra were collected with a Bruker Vertex 70 spectrophotometer equipped with an attenuated total reflectance (ATR) accessory in the range of 650-4000 cm -1 , with 4 cm -1 resolution and 16 scans at the Institut Charles Gerhardt 224 Montpellier (ICGM) at the ENSCM, Montpellier, France.

Thermogravimetric analysis (TGA) were carried out on a Q50 V6.7 Build 203 from TA-Instruments at the ENSCM. The experiments were performed in the temperature range of 50 to 900 °C at a heating rate of 10 °C min -1 under an oxygen atmosphere, 60 mL min -1 .

Standard [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si and [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C solid state NMR experiments were collected on a Bruker Avance III HD 400WB spectrometer (9.4 T), using a 4 mm double resonance probe. The spectra were acquired with a MAS frequency of 10 kHz. [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si{ 1 H} and 13 C{ 1 H} cross polarization magic angle spinning (CPMAS) spectra were recorded using a contact time of 4.5 and 2.0 ms, and a typical repetition delay of 4 and 3 s, respectively. All spectra were acquired with TPPM [START_REF] Bennett | Heteronuclear decoupling in rotating solids[END_REF] proton decoupling during the data acquisition. Chemical shifts are reported relative to tetramethylsilane (TMS).

All solid-state DNP experiments were carried out on a Bruker Avance III 400 MHz NMR spectrometer equipped with an Ascend 400 DNP magnet and a 3.2 mm triple resonance 1H/X/Y low-temperature MAS probe. The microwave (mw) irradiation was provided by a 9.7 T Bruker gyrotron system operating at 263 GHz. The samples were ground in a ball mill apparatus (400 rpm during 2 min). The powdered samples were prepared before measurements by mixing ca.

10 mg of the sample with ca. 20 μL of the polarizing solution, which consisted of a hydrophobic 15 mM solution of TEKPol radical in 1,1,2,2-Tetrachloroethane (TCE). [START_REF] Zagdoun | A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF] Also, samples prepared with a second polarizing solution, consisting of 15 mM solution of bis-TEMPO-bis-ketal (bTbk) [START_REF] Kobayashi; Lafon | Analysis of Sensitivity Enhancement by Dynamic Nuclear Polarization in Solid-State NMR: A Case Study of Functionalized Mesoporous Materials[END_REF] in Dimethyl sulfoxide (DMSO), were measured to confirm or exclude the 244 presence of [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C lines overlapping with the TCE signal. The 13 C{ 1 H} and 29 Si{ 1 H} CPMAS 245 spectra with and without mw irradiation were acquired with a MAS frequency of 8-10 kHz. 246 The sample temperature was nominally 117 K (without mw) and 128 K (with mw), and was 247 stabilized by a Bruker BioSpin low temperature MAS cooling system. [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C{ 1 H} and 29 Si{ 1 H} 248 CPMAS NMR spectra were recorded with 1 H excitation pulses of 2.5 μs, a contact time of 2 249 ms, a typical repetition delay of 3.6 s. Dipolar interactions with protons were decoupled 250 employing SPINAL-64. [START_REF] Fung | An Improved Broadband Decoupling Sequence for Liquid Crystals and Solids[END_REF] DNP enhanced [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C{ 1 H} and 29 Si{ 1 H} heteronuclear correlation 251 (HETCOR) experiments were performed using the pulse sequence proposed by van Rossum et 252 al., [START_REF] Van Rossum | High-Field and High-Speed CP-MAS 13 C NMR Heteronuclear Dipolar-Correlation Spectroscopy of Solids with Frequency-Switched Lee-Goldburg Homonuclear Decoupling[END_REF] which employs FSLG (frequency-switched Lee-Goldburg) homonuclear decoupling 253 during 1 H evolution. A decoupling field of 90 kHz was used. Short contact times of 200 μs and 254 500 μs were chosen respectively for 13 C{ 1 H} and 29 Si{ 1 H} CP transfer, in order to select the 255 correlations between nearest 13 C/ [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si and 1 H neighbors.

RESULTS AND DISCUSSION

The purification of the BCM-BTEB after their preparation includes several washings, namely with water, hot water and ethanolic solution of sodium hydroxide (0.1 M). As such, this procedure can be considered in order to evaluate the stability of the BTEB on the BCM. This assessment is an important criterion for the validation of the obtained product.

The SEM images of pristine BCM show smooth longitudinal fiber-like structures. After the incorporation of the organosilane BTEB, the micrographs reveal the formation of spherical organosilica nanoparticles with heterogeneous particle size (30-100 nm), imparting nanoscale surface roughness (Figure 1). In the SEM images of each sample is identified (red square) where the X-ray elemental mappings (Figure 3) were performed. The TEM micrographs of the BCM-BTEB, Figure 2, indicate that these nanoparticles might probably present porous structure. The X-ray elemental mappings (Figure 3), obtained during SEM experiment, evidence the successful incorporation of the BTEB on the BCM through the identification of element Si. Indeed, the C and O elements are homogeneously distributed throughout the BCM surface as expected for both samples. However while BCM (Figure 3A) does not present any Si element, BCM-BTEB sample demonstrates that Si containing nanoparticles are homogeneously distributed on the BCM surface on a microscopic scale (Figure 3B). The presence of element C in the samples in part, are due to the carbon characteristic of cellulose and organosilane BTEB and also of the carbon layer deposited on the samples which was used to increase conductivity during the SEM analysis. The FTIR-ATR spectrum of pristine BCM (Figure 4b) displays the characteristic bands of cellulose: stretching (3500-3300 cm -1 ) and symmetric bending (1645 cm -1 ) vibrations of H-bonded hydroxyl groups and adsorbed water, respectively; CH and CH2 (CHOH; CH2OH) stretching (3000-2870 cm -1 ); C-OH and CH bending (1430-1330 cm -1 ); C-O (-C-O-H) stretching (1200-1000 cm -1 ); C-O (C-O-C) stretching (1150-1000 cm -1 ); CH2 asymmetric bending in plane (900-700 cm -1 ) and OH bending out of the plane (700-650 cm -1 ) vibrations. [START_REF] Pecorano | Polymers and Composites from Renewable Resources[END_REF][START_REF] Osei-Prempeh | Fluorocarbon and hydrocarbon functional group incorporation into nanoporous silica employing fluorinated and hydrocarbon surfactants as templates[END_REF] In the spectrum of the BCM-BTEB (Figure 4a), additional vibrational bands are detected, when compared with BCM, that are assigned to the incorporated organo-bridged silica sources BTEB (red squares): C-H symmetric stretching of -CH3 (2974 cm -1 ), C-H antisymmetric stretching of -CH2 (2932 cm -1 ), C=C stretching and C-H deformation in plane of Si-Phenyl (1481 cm -1 ), C=C stretching and C-H deformation in plane 2 stretching Si-Phenyl of Si-Phenyl (1481 cm -1 ), C-H symmetric deformation of -CH2 (1390 cm -1 ), antisymmetric stretching of Si-O-Si (1145 cm -1 ), in-plane stretching Si-O of ≡Si-O (958 and 811 cm -1 ) and C-H outof-plane ring deformation of Si-Phenyl (702 cm -1 ) vibrations. [START_REF] Osei-Prempeh | Fluorocarbon and hydrocarbon functional group incorporation into nanoporous silica employing fluorinated and hydrocarbon surfactants as templates[END_REF][START_REF] Socrates | Infrared and Raman Characteristic Group Frequencies: Tables and Charts[END_REF][START_REF] Ou | Near-and mid-infrared spectroscopy of sol-gel derived ormosils: vinyl and phenyl silicates[END_REF][START_REF] Pereira | Designing Novel Materials by One-Pot Co-condensation: From Hydrophobic Mesoporous Silica Nanoparticles to Superamphiphobic Cotton Textiles[END_REF] Although solid state NMR experiments confirm the presence of BTEB moieties in the final functionalized material BCM-BTEB, the low concentration of these species limits the structural information available from NMR. To obtain more detailed information about the BCM functionalization, DNP enhanced solid state NMR experiments were performed. Figure 7a shows solid state [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C{ 1 H} CPMAS NMR spectra for the BCM-BTEB sample obtained under DNP conditions, with (black curves) and without (grey curves) mw irradiation. The 13 C attributions are summarized in Table 1. A 15 mM solution of TEKPol radical in TCE (TEKPol@TCE) was used as polarization matrix, to selectively enhance the [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C NMR signal of the hydrophobic BTEB moieties, as previously suggested by Gutmann and co-workers. [START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF][START_REF] Schäfer | Selective DNP Signal Amplification to Probe Structures of Core-Shell Polymer Hybrid Nanoparticles[END_REF] Due to the presence of a strong [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C resonance around 77 ppm, corresponding to the TCE matrix, we have also performed an experiment using a 15 mM solution of bTbk in DMSO (bTbk@DMSO) as polarization source. This second experiment was used, to discard the presence of extra [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C peaks overlapping with the TCE signal. All peaks observed in this region (around 65 to 80 ppm) correspond solely to cellulose species C2, C3 and C5, as shown in the NMR results above.

Table 1 -13 C, 1 H NMR chemical shift attributions for the BCM and BTEB moieties from DNP NMR experiments. Carbon numbers refers to the labeling in Figure 6. [START_REF] Gorgieva | Bacterial Cellulose: Production, Modification and Perspectives in Biomedical Applications[END_REF] H chemical shifts refering to the carbon labeling corresponds to protons directly bonded to the corresponding carbon atom.

For the spectra obtained with TEKPol@TCE matrix, as previously observed for other cellulose-containing materials, [START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF] the hydrophobic character of the employed DNP-matrix preferentially enhances the signal from the hydrophobic BTEB moieties (C7-10). Enhancements of around ε = 35 to 100 are observed for the [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C lines attributed to BTEB (Enhancements for each line are indicated in Figure 7), while a moderate enhancement of around ε = 6 is observed for the [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C lines corresponding to cellulose (C1,4,6). Unlike standard solid-state NMR experiments described above, for the TEKPol@TCE DNP spectrum the chemical shifts for BTEB phenyl carbons 9 and 10 (around 137 and 131 ppm respectively) can be clearly resolved from each other, confirming the integrity of the BTEB structure. Besides the lines corresponding to carbon groups in cellulose and BTEB, two additional lines are observed in the [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C DNP spectrum of BCM-BTEB with isotropic shifts around 43 ppm and 35 ppm (i and ii) in Figure 7a (upper spectrum)). Both signals are not observed for the spectra measured without mw irradiation and can be attributed to unidentified impurities from the synthetic route. [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C spectra were measured using two different polarizing matrices, a solution of 15 mM TEKPol in TCE (TEKPol@TCE) and a solution of 15 mM bTbK in DMSO (bTbK@DMSO). [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si spectra were measured using TEKPol@TCE as polarizing matrix. The DNP 391 enhancements, ε, for each line are indicated in the spectrum. Enhancement for the line around -32 ppm could not be estimated. The enhancement for the spectrum measured with bTbK@DMSO is the same for all lines. Asterisks indicate spinning sidebands. MAS rate was 8 kHz for all spectra.

Figure 7b shows DNP NMR [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si{ 1 H} CPMAS spectra measured for the BCM-BTEB compound. The [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si CPMAS spectrum shows signals in the region expected for Si species of T n type coordinated with phenyl groups (PhSi(OSi)nRn-1), with n = 1,2,3 being the number of Si-O-Si linkages per Si unit and R being either CH2-CH3 or OH groups. [START_REF] Kono | 13C and 1H Resonance Assignment of Mercerized Cellulose II by Two-Dimensional MAS NMR Spectroscopies[END_REF] The lines around -180, -71 and -57 are attributed respectively to T 3 , T 2 , T 1 while the small hump at -32 ppm is 1attributed to impurities. T 3 , T 2 , T 1 species show DNP enhancements of about 24, 10 and 59, 1respectively. It is well-known that due to its bi-functionality, BTEB tends to form porous 1covalent network structures where phenyl species are interconnected by Si-O-Si linkages. [START_REF] Saito | Hydrolysis behavior of a precursor for a bridged polysilsesquioxane 1,4-bis(triethoxysilyl)benzene: a 29Si NMR study[END_REF] The differences in enhancements for the various species suggests preferential location of the groups within the nanoparticles. T 1 and T 0 groups show the highest enhancements, letting us conclude that these species are mostly located on the surface of the nanoparticles, being more accessible to the polarizing matrix. On the other hand, the relatively high enhancement (ε = 24) for T 3 let us assume that these groups are close to the surface of the material, probably cross-linked via T 1 groups. The small enhancement for T 2 groups indicates that these groups probably belong to polymerized BTEB in the bulk of the material. Due to the bi-functional character of BTEB, any clear attribution to the coordination of T 2 and T 3 species would require additional experiments, which are beyond the scope of the present work. Figure 8a shows a 2D 13 C{ 1 H} FSLG HETCOR spectrum for BCM-BTEB. Table shows the 1H chemical shifts for all observed species. Carbons C1, 4 and 6 from BCM moieties show the expected 1 H correlations around 4.3, 3.7 and 4 ppm, respectively, in agreement with two dimensional MAS NMR results reported by Kono et al. [START_REF] Kono | 13C and 1H Resonance Assignment of Mercerized Cellulose II by Two-Dimensional MAS NMR Spectroscopies[END_REF] The resonance peak for methyl carbon C7 from BTEB shows correlations around 1.5 ppm, corresponding to CH3 and around 4 ppm, corresponding to the OCH2 protons from the neighbouring C8 carbon, as confirmed by the strong correlation observed between these protons and C8. Also, carbon C8 shows correlation with CH3 protons (1.5 ppm), corresponding to the neighbouring C7 protons, and an additional correlation with 1H resonating around 7.8 ppm, which can be attributed to protons from aromatic groups (C10). Carbons C9 and C10 show correlation with aromatic protons, resonating around 7.8 ppm. Also, a second correlation peak is observed for C10 around 3.7 ppm, probably corresponding to interaction with OCH2 from alkoxysilane fragments or alternatively, with protons from C6 species in BCM. The 13 C lines around 35 and 43 ppm show correlations with protons resonating at 1.5 ppm, leading to the attribution of these impurity species to CH3 or CH2 in aliphatic groups. The origin of these species is still under investigation.

Figure 8b shows the DNP enhanced [START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si{ 1 H} FSLG HETCOR NMR spectrum for the BCM-BTEB sample. The lines corresponding to T 3 and T 1 groups give the strongest contribution to the HETCOR spectrum, while T 2 groups shows only very weak correlation peaks. The T3 line shows only one correlation, with 1 H species resonating around 7.8 ppm, corresponding to protons in the phenyl group from BTEB, as discussed above. On the other hand, besides the correlation with phenyl protons, the T 1 line shows correlations with 1 H species resonating around 4.3 ppm and 1.5 ppm. The line around 1.5 ppm corresponds to CH2/CH3 groups from the non-hydrolysed ethoxy silyl groups of the organosilane BTEB, as confirmed by [START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C NMR results shown above, indicating that the hydrolysis reaction is not complete. The line around 4.3 ppm corresponds to SiOH species in fully hydrolysed BTEB. For T 0 species only a correlation with 1 H around 3.7 ppm is observed, which can correspond to hydroxyl groups or protons bonded to carbon C8, indicating once more that the hydrolysis of BTEB is not complete. As derived above from the strong DNP enhancement for T 0 and T 1 groups, they correspond to BTEB moieties at the surface of the silicon containing nanoparticles. Adding to this information the incomplete hydrolysis observed in the 13 C{ 1 H} HETCOR experiments, we assume that BTEB moieties are covalently attached to the surface of the nanoparticles. Furthermore, the T 0 groups are only visible by DNP, which means that they occur in very minor concentration, which underlines that they are most probably not present in the bulk of the particles. To sum up, the tentative binding model of the benzene-silica nanoparticles shown in Figure 9 can be derived. Further investigations are however required to clearly prove this model. The amount of the silica nanoparticles formed after the incorporation of BTEB in the BCM was quantified by TGA. The TGA curves of the BCM and the BCM-BTEB display two significant weight losses: from ambient temperature to 150 °C and from 150 to 500 °C. The first weight loss is due to membrane dehydration. Physically adsorbed and hydrogen-bonded linked water molecules are removed at this first stage. The second weight is associated to the decomposition of the BCM. [START_REF] Barud | Bacterial cellulose-silica organic-inorganic hybrids[END_REF][START_REF] Monteiro | Bacterial cellulose-SiO2@TiO2 organic-inorganic hybrid membranes with self-cleaning properties[END_REF] From 500 °C, in the BCM-BTEB, the residue of 3.2% is observed which confirms BCM-BTEB mainly consists of organics with a low amount of BTEB.

CONCLUSION

The incorporation of BTEB on the surface of BCM has been successfully achieved as demonstrated by the physicochemical characterization. SEM and TEM microscopy show the formation of spherical silica nanoparticles with heterogeneous particle size (30-100 nm), and probably porous structure, which are homogeneously distributed throughout the BCM surface. Solid state DNP-NMR experiments demonstrate the integrity of the BTEB functionalities and its polycondensation (presence of Si-O-Si linkages). They also show the presence of ethoxy silyl groups (SiOCH2CH3) in the final material, indicating that the hydrolysis reaction of BTEB is incomplete. From these experiments a first tentative model of the benzene-silica nanoparticles could be derived which has to be further confirmed by advanced spectroscopic techniques. Altogether, we have demonstrated a simple strategy for the incorporation of porous silica nanoparticles on BCM opening new perspectives for the design of tailored drug-delivery systems which will find application in biomedical area.

Scheme 1 .

 1 Scheme 1. Synthesis of 1,4-bis(triethoxysilyl)benzene (BTEB) 2.3. Preparation of Bacterial Cellulose Organic-Inorganic Hybrid Membrane. a. Preparation of Bacterial Cellulose Membrane.The strain used was Komagataeibacter Xylinus (ATCC53524). BCM were prepared following the procedures adapted from literature.[START_REF] Pecorano | Polymers and Composites from Renewable Resources[END_REF] An aliquot of the bacterium Komagataeibacter Xylinus (ATCC53524) was replicated to a sterile solid medium plate HS (Hestrim-Schramm) containing 12 g of glucose (0.067 mol), 3 g of yeast extract, 3 g of peptone G, 1.62 g of disodium hydrogen phosphate (0.011 mol), 0.69 g of citric acid (3.59 mmol), 15 g of agar and 585 mL of milipore water (32.5 mol). After growth time, the microorganism was inoculated into sterile liquid culture medium consisting of 50 g of glucose (0.277 mol), 4 g of yeast extract, 0.73 g of magnesium sulphate heptahydrate (2.96 mmol), 2 g of monopotassium phosphate (0.015 mol), 20 mL of ethanol (0.342 mol) and 980 mL of millipore water (54.4 mol) and maintained for 72 h in an oven with air circulation and with constant temperature at 28 °C. Subsequently, the BCM produced were purified following the subsequent procedure: washing the BCM with water 5 times per day during 3 days; washing the BCM with water at 80 °C during 50 min; immersion of the BCM in 100 mL of an ethanolic solution of sodium hydroxide (0.1 M) at 80 °C during 50 min; and washing again with millipore water until pH 7.0. Finally the BCM was dried at 80 °C for 24 h. The resulting sample will be called in the following 194 BCM.

Scheme 2 .

 2 Scheme 2. Schematic Representation of the Preparation of Bacterial Cellulose Membrane 205 modified with BTEB. 2.4. Physicochemical Characterization.

Figure 1 .

 1 Figure 1. SEM micrographs with different magnifications of (A) BCM and (B) BCM-BTEB samples. The red square is the region where the X-ray elemental mappings was performed.

Figure 2 .Figure 3 .

 23 Figure 2. TEM micrographs with different magnifications of BCM-BTEB sample.

Figure 4 .

 4 Figure 4. FTIR-ATR spectra of (a) BCM-BTEB, (b) BCM and (c) BTEB in the 3600-2600 cm -1 (left) and 1550-650 cm -1 (right) range. The red squares highlight the characteristic peaks of BTEB.

  The[START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si{ 1 H} CP MAS NMR spectra of BCM-BTEB (Figure5), present peaks at -59, -70 and -78 ppm, which are respectively associated to T 1 [CSi(OR)2(OSi)], T 2 [CSi(OR)(OSi)2] and T 3 [CSi(OSi)3] silicon environments, corresponding to the condensation rate of organosilane BTEB through 1, 2 and 3 Si-O-Si linkages,[START_REF] Small | Arylene-and alkylene-bridged polysilsesquioxanes[END_REF] indicating the incorporation of BTEB in the BCM successfully. The successful incorporation of BTEB in the material is further confirmed by[START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C{ 1 H} CPMAS NMR spectra, displaying the characteristic chemical shifts of BTEB (Figure6): at 134 ppm which correspond to C-Phenyl and at 58 and 18 ppm, attributed to remaining Si-OCH2CH3 groups.

Figure 5 .

 5 Figure 5. 29 Si{ 1 H} CPMAS NMR spectra of BCM-BTEB sample and schemes showing the attributions to T n Si groups. R is either an ethoxy group or hydrogen. The asterisk indicates a background signal from the NMR probe used in the experiment.

Figure 6 .

 6 Figure 6. 13 C{ 1 H} CPMAS NMR spectra of the BCM and BCM-BTEB and signal assignments.

Figure 7 -

 7 Figure7-13 C{ 1 H} (a) and[START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si{ 1 H} (b) CPMAS NMR spectra for the BCM-BTEB sample measured under DNP conditions with (black curves) and without (grey curves) mw irradiation.[START_REF] Chen | Bacterial cellulose composite with capsules embedded therein and preparation thereof[END_REF] C spectra were measured using two different polarizing matrices, a solution of 15 mM TEKPol in TCE (TEKPol@TCE) and a solution of 15 mM bTbK in DMSO (bTbK@DMSO).[START_REF] Croissant | Syntheses and applications of periodic mesoporous organosilica nanoparticles[END_REF] Si spectra were measured using TEKPol@TCE as polarizing matrix. The DNP 391 enhancements, ε, for each line are indicated in the spectrum. Enhancement for the line around -32 ppm could not be estimated. The enhancement for the spectrum measured with bTbK@DMSO is the same for all lines. Asterisks indicate spinning sidebands. MAS rate was 8 kHz for all spectra.

Figure 8 -

 8 Figure 8 -DNP enhanced 13 C{ 1 H} (a) and 29 Si{ 1 H} (b) FSLG HETCOR NMR spectra for BCM-BTEB. The numbers are attribution to carbon species labelled in Figure 6. Asterisks indicate spinning sidebands.

Figure 9 -

 9 Figure 9 -Illustration of the possible species occurring at the surface of BTEB nanoparticles. The color code refers to the types of Si species (T 0 , T 1 , T 2 and T 3 ).

Figure 10 .

 10 Figure 10. Thermogravimetric curves of (a) BCM and (b) BCM-BTEB, under O2 atmosphere.

ACKNOWLEDGMENTS

This work has been financial supported by Fundação de Amparo à pesquisa do estado de São Paulo (FAPESP), through project 2015/12908-2 and 2017/21005-1. ASM thanks FAPESP for a grant. MOJr is grateful for a CAPES-Humboldt research fellowship. GB thanks the Deutsche Forschungsgemeinschaft, DFG, for financial support under contract Bu-911-24/2. TG thanks the DFG under contract GU 1650/3-1 for financial support. H. S. Barud thanks CNPq (grant 407822/2018-6), FAPESP (grant 2018/25512-8 and 2013/07793-6). We thank Dr. Olivier Ouari (Aix-Marseille University) for providing the bTbk radical and André Tobello Foundation for offering the Strain Gluconacetobacter Xylinum (ATCC23760).


Mathilde Laird

Philippe Gaveau

Philippe Trens

Carole Carcel

Masafumi Unno

John R Bartlett
email: j.bartlett@westernsydney.edu.au

Michel Wong

Chi Man

Post-synthesis modification of functionalised polyhedral oligomeric silsesquioxanes with encapsulated fluorideenhancing reactivity of T 8 -F POSS for materials synthesis

Introduction

Since their discovery in the 1940s and 1950s, 1,2 polyhedral oligomeric silsesquioxanes (POSS) have commanded significant interest within the chemistry, biomedical and materials communities. The Tn POSS family, also referred as Tn cage silsesquioxanes, are composed of a polyhedral silsesquioxane cage, (SiO1.5)n, with functional organic branches covalently bonded to the vertices of the cage. The value of n can vary from n = 8, 10, 12, etc., with T8 compounds (featuring a central void of 0.3 to 0.5 nm [3][4][5] ) being the most commonly described. The organic moieties direct the functionalisation and reactivity of Tn compounds, and enable the cages to be modified by a palette of reaction including metathesis, [6][7][8] Heck coupling, 9,10 hydrosilylation, 11 thiol-ene click [12][13][14] or polymerisation. [15][16][17][18] A diverse range of applications for Tn cage systems have now been investigated, including bio/nanomedicine, 19- 25 catalysis, [26][27][28][29] optoelectronics, 30 polymer chemistry, [31][32][33] and optical and sensing applications. [33][34][35][36] Postfunctionalised Tn compounds have also been incorporated into self-organised systems, [37][38][39][40][41] organic polymer-based nanohybrids, 31,33,[42][43][44][45] POSS micelles and templated POSS systems leading to mesoporous materials with embedded T8 cages. [46][47][48] In 2003, Bassindale's group reported a new T8 cage silsesquioxane, in which a fluoride anion was entrapped within the central T8 void, leading to the emergence of the T8-F POSS family. 49 It was subsequently demonstrated that such compounds can be obtained when the T8 cage is functionalised with electron-withdrawing organic groups, but not with electron-donating species. 50 For example, in the case of electron-donating sp 3 -hybridised organic groups, T8 cages without encapsulated fluoride were obtained while with electronwithdrawing sp 3 or sp 2 hybridised groups, T8-F compound were formed. In addition, attempts to modify the organic substituents linked to the T8-F cage via reactions typically used in organic synthesis were found to be unsuccessful. 51 To the best of our knowledge only one report of the modification of organic moieties linked to T8-F cages, using electrochemical techniques, has appeared. 52 However, the integrity of the T8-F cages was not demonstrated, 52 and no additional examples of successful modification of intact T8-F cages could be found. A consequence of the difficulties encountered with post-functionalisation of the T8-F family has been limited progress in exploring potential applications for such compounds over the past two decades. In a previous study, we reported the synthesis of new styryl-based cage silsesquioxanes (Tn with n = 8, 10, 12 53 or 18 54 ), where the styryl group is bonded to all n vertices of the cages via the phenylene moiety. Subsequently, we described the preparation of the corresponding styryl-functionalised T8-F compound, 55 whose formation was facilitated by the presence of the electron-withdrawing group linked to the vertices of the T8 host. Herein, we investigate different potential approaches to functionalise the T8-F cage and describe the successful modification of the styryl-bearing T8-F molecule via grafting of a triethoxysilyl moiety using a thiol-ene click reaction, 56,57 without loss of fluoride from the T8-F cage. The styryl/thiol-ene platform enables a much wider variety of reactive functional groups to be grafted onto the T8-F cage, including electron-donating groups that cannot usually be covalently bound to T8-F. We then demonstrate an example of potential approaches 57 for converting the reactive modified T8-F precursor into a material, using sol-gel processing to produce an organic-inorganic hybrid in which the T8-F cages remain intact. To the best of our knowledge, this constitutes the first report of a material incorporating intact T8-F cage silsesquioxanes.

Experimental

Chemicals and purification methods

Tetrabutylammonium fluoride (TBAF, 1 M solution in THF (containing 5% H2O)), (4vinylphenyl)trimethoxysilane and dichloromethane (DCM, Analytical Grade) were obtained from Sigma Aldrich, TCI and VWR, respectively, and were used without further purification. Azobisisobutyronitrile (AIBN) was purchased from Sigma-Aldrich and was recrystallised. 3-Thiopropyltriethoxysilane was purchased from Fluorochem and was distilled prior to use. All syntheses were undertaken using HPLC-quality water.

Synthesis of triethoxysilylated functionalised T8-F from styryl-functionalised T8-F

The synthesis of styryl-functionalised T8-F has been previously described. 55 In a flame dried Schlenk flask equipped with a Teflon rotaflos tap, styryl-functionalised T8-F (250 mg, 167 mmol, 1 eq.) and AIBN (10.5 mg, 64.0 mmol, 0.5 mol%) were dissolved in dry chloroform (2.5 mL) under an inert atmosphere. The mixture was heated to 40 °C. 3-Thiopropyltriethoxysilane (320 mL, 1.33 mmol, 8 eq.) was then added dropwise and the temperature was increased to 60 °C. The mixture was stirred at 60 °C overnight and the solvent was then evaporated. The excess of 3-thiopropyltriethoxysilane was removed by washing the oil product with dry pentane. The excess of solvent was finally evaporated to yield a yellowish oil in nearly quantitative yield. 1,3,5,7,9,11,13,15- 4,6,8,10,12,14,16,17,18,19,20-dodecaoxa-1,3,5,7,9,11,13,15-octasilapentacyclo[9.5.1.13,9.15,15.17,13]icosane fluoride, tetrabutylammonium salt (C152H268O36Si16S8NF, yellowish oil): 1 HNMR (400 MHz, CDCl3, , ppm): 7.73-7.71 (, CHar, 16H, J = 7.8 Hz), 7.10-7.08 (, CHar, 16H, J = 7.8 Hz), 3.83-3.78 (q, Si-O-CH2-CH3, 48H, J = 7.0 Hz), 2.82-2.80 (t, Ph-CH2-CH2-S, 16H, J = 4.5 Hz), 2.73-2.69 (t, Ph-CH2-CH2-S, 16H, J = 4.5 Hz), 2.59-2.55 (t, S-CH2-CH2-CH2-Si, 16H, J = 7.4 Hz), 2.27 (m, NBu4, 8H), 1.75-1.67 (quint, S-CH2-CH2-CH2-Si, 16H, J = 7.4 Hz), 1.23-1.19 (q, Si-O-CH2-CH3, 72H, J = 7.0 Hz), 0.83 (m, NBu4, 8H), 0.76-0.72 (t, S-CH2-CH2-CH2-Si, 16H, J = 7.4 Hz), 0.69 (m, NBu4, 12H). 13 19 F NMR (380 MHz, CDCl3, , ppm): -25.16. 29 Si NMR (80 MHz, CDCl3, , ppm): -45.79 (Si(OEt)3), -81.39 (cage, , J = 2.4Hz). IR(, cm -1 ): 2973-2884 (CH2 and CH3), 1605 (C-C ring), 1072 (Si-O-Si).

Octakis(4-(2-((3- (triethoxysilyl)propyl)thio)ethyl)-phenyl)-2,
MALDI-MS: m/z: calculated for (M + Na + H): 3188.1; found: 3188.0 (N.B.: during acquisition of the MALDI-MS spectrum, a cation exchange process occurs, resulting in substitution of the TBA + ion by a sodium ion. The observed m/z is thus characteristic of (M + Na + H) rather than the expected (M + TBA)).

Synthesis of T8-F gel

In a flame dried Schlenk flask, the solution of triethoxysilylated T8-F cage in chloroform (568.0 mg, 0.166 mmol, 1 eq.) was evaporated and then dissolved in dry THF (2.0 mL). Water (24 mL, 1.3 mmol, 8 eq.) and TBAF (27 mL, 27 mmol, 0.16 eq.) were then successively added under stirring. After 3 min, the agitation ceased due to the rapid gelation of the mixture. After three days of ageing, the solid was crushed and washed three times with water and then acetone. The product was dried overnight under vacuum and recovered as a yellowish powder (350 mg). T8-F Gel (yellowish powder): 13 C NMR (75 MHz, CP-MAS, , ppm): 140.5 (Car), 134.8 (CHar), 127.3 (CHar), 58.6 (residual ethoxy), 35.4, 24.0, 13.0. 19F NMR (565 MHz, , ppm): -21.9. 29 Si NMR (60 MHz, , ppm): -60.0 (alkylene T 2 ), -66.4 (alkylene T 3 ), -79.3 (cage). IR (, cm -1 ): 1606 (C-C ring), 1066 (Si-O-Si).

Characterisation methods

FTIR spectra were recorded from 650 to 4000 cm -1 using a PerkinElmer Spectrum 100 equipped with a Gladia-ATR accessory. MALDI-MS spectra were acquired using a Bruker RapifleX spectrometer with dithranol/sodium trifluoroacetate as desorption matrix. Liquid 1 H, 13 C, 19 F and 29 Si NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl3 at room temperature and at concentrations of around 10 mg mL -1 . 29 Si[ 1 H] and 13 C[ 1 H] solid-state NMR spectra were obtained on a VARIAN VNMRS 300 MHz spectrometer. 29 Si[ 1 H] CP-MAS (cross polarisation magic angle spinning) spectra were recorded with a 5 ms contact time, a 5 s recycle delay and a 6 kHz spinning rate. For 13 C[ 1 H] CP-MAS spectra, a contact time of 0.5 ms, a recycle delay of 3 s and spinning rate of 12 kHz were used. 19 F and 29 Si[ 19 F] solid-state NMR spectra were measured on a VARIAN VNMRS 600 MHz spectrometer. 19 F spectra were acquired using SP-MAS (single pulse magic angle spinning) techniques with a 3 s /2 pulse, a 60 s recycle delay and a 25 kHz spinning rate. For 29 Si[ 19 F] CP-MAS spectra, a contact time of 15 ms, a recycle delay of 10 s and spinning rate of 8 kHz were used. On both spectrometers, spectra were acquired using a 3.2 mm VARIAN T3 MAS probe with ZrO2 rotors and were calibrated using secondary references: Q8M8H (octa(dimethylsiloxy)-octasilsesquioxane) for 29 Si (left line at -2.25 ppm); adamantane for 13 C (left line at 38.5 ppm); and PTFE (polytetrafluoroethylene) for 19 F (line at -122.3 ppm).

Thermogravimetric analyses (TGA) were carried out between 25 and 900 °C, using a heating rate of 10 °C min -1 . The gel samples were analysed using a TA Instruments Q50 apparatus with an air flow of 60 mL min -1 (T8 and T8-F gels), while the styryl-functionalised T8-F POSS was analysed using a Netzsch STA 409 Luxx ® instrument in static air. A Micromeritics ASAP 2020 instrument was used for the determination of nitrogen physisorption isotherms at -196 °C. The samples were activated at 100 °C for 3 days under secondary vacuum prior to analysis. Transmission electron microscopy (TEM) images were recorded on a JEOL 1400 + microscope operating at 120 kV with a LaB6 source. Samples were prepared by deposition on Formvar/ Carbon 300 MeshTEM grids (FCF300-Cu-50).

Results and discussion

Functionalisation of octastyryl T8-F

As indicated above, there has been only one prior report addressing the post-functionalisation of the T8-F system via electrochemical techniques, although the integrity of the T8-F cage was not demonstrated in that earlier work. 52 Indeed, in allreports to date, reactions that have been successfully used to modify organic ligands on organo-functionalised Tn POSS compounds have led to the degradation of the POSS cage in the case of T8-F compounds. For example, Taylor et al. Investigated the use of bromination, hydrosilylation, cycloaddition and cross metathesis reactions of the vinyl group on a vinyl-functionalised T8-F compound. 51 The former two reactions resulted in the degradation of the cage, while the fluoride was found to deactivate the alkene dienophile of the addition reaction. In the case of the metathesis reaction, it was assumed that the TBA + cation led to deactivation of the Grubbs catalyst, thus preventing the reaction from proceeding. 51 In our work, preliminary scoping experiments using hydrosilylation and Heck coupling reactions led to a mixture of impure compounds, and hence these approaches were not pursued further (ESI, † Fig. S1-S3). Similarly, although the metathesis reaction yielded a modified product, characterisation studies indicated that the structure of the T8-F cage had been modified (ESI, † Fig. S4 andS5), and hence this strategy was also abandoned. In contrast, preliminary experiments involving the use of a thiol-ene click reaction suggested that this strategy could be used to modify the vinyl group without degrading the T8-F cage. Indeed, the fluoride anion remains entrapped within the cage due to the presence of the electron-withdrawing phenylenemoiety. Accordingly, we investigated the functionalisation of the styryl-functionalised T8-F cage with 3thiopropyltriethoxysilane using a thiol-ene click reaction, as shown in Fig. 1, to produce a triethoxysilylated derivative. Our initial approach followed a procedure described by Bivona et al., 28 using toluene as a solvent. However, due to the low solubility of styryl-functionalised T8-F in toluene, the reaction was carried out in dry chloroform, with AIBN as the radical initiator. The MALDI mass spectrum of the product obtained, Fig. S6 (ESI †), exhibits a peak at 3188.0 m/z, which is attributed to an adduct formed between the target molecule (3164.1 m/z), Na and H (M + Na + H, expected m/z = 3188.1). The 1 H solution NMR spectrum of the product obtained (Fig. 2), illustrates the loss of the signal arising from the vinylic protons at around 6.7 ppm (Fig. S7, ESI, † 55 ), together with the appearance of new signals at 2.81 and 2.72 ppm arising from the -S-CH2-CH2-moiety, indicating completion of the thiol-ene click reaction and functionalisation of all eight styryl groups. The signals arising from the aromatic protons are also shifted from 7.73 and 7.31 ppm for the styryl-functionalised T8-F cage to 7.72 and 7.09 ppm, respectively, for the triethoxysilylated product. Signals arising from the propyl moiety are observed at 2.57, 1.71 and 0.74 ppm together with those of the ethoxy groups at 3.80 and 1.22 ppm. It should be noted that a singlet observed at around 1.71 ppm, which overlaps the signals associated with the propyl group, arises from AIBN. In addition, the signals associated with the TBA + counter ion are still seen at 2.26, 0.80 and 0.68 ppm. The integration of the various signals is consistent with the proposed product (Fig. 2). Fig. 2 Solution 1 H NMR spectrum of the triethoxysilylated functionalised T8-F cage. For clarity, the functional group is shown on only one vertex of the cages. 19 F solution NMR spectroscopy confirms the retention of the fluoride in the cage (Fig. 3 and Fig. S8, ESI †), with the signal observed at -25.16 ppm being characteristic of the T8-F cage. 49,51,55,58 In particular, styryl-functionalised T8-F exhibits a signal at-25.17 ppm 55 prior to thiol-ene grafting. Additional signals are also evident on the base of the main peak, with the strongest of these being observed at -25.43 ppm. The complete conversion of the vinyl groups initially present on the styrylfunctionalised T8-F confirms that the latter signal does not arise from a partially functionalised cage (i.e. all eight styryl groups have been functionalised). Although these weak signals cannot be unambiguously attributed on the basis of the available data, their chemical shifts confirm that they still correspond to an encapsulated fluoride anion, and they are tentatively assigned to oligomeric species arising from styryl oligomerisation catalysed by AIBN (see Section 1.5 in the ESI †). The 29 Si solution NMR spectrum (Fig. 4 and Fig. S9,ESI †) is also consistent with the formation of the expected product. A signal arising from the triethoxysilylated moiety is observed at -45.79 ppm, while that associated with the silsesquioxane cage is seen as a doublet at -81.39 ppm (JSi-F = 2.4 Hz). The latter chemical shift and coupling constant are similar to those observed for the styrylfunctionalised T8-F (-81.49 ppm; JSi-F = 2.5 Hz, Fig. S9, ESI, † 55 ). Weak signals associated with small quantities of by-products and impurities are also observed at the base of the signals arising from the T8-F cage, which probably arise fromoligomeric species associated with the 19 F NMR signal observed at -25.43 ppm. The FTIR spectrum of the product (Fig. S10,ESI †) is also consistent with the proposed structure. In particular, a sharp Si-O-Si stretching band at 1072 cm -1 , characteristic of a well-defined cage silsesquioxane is observed, which is similar to that previously reported for the styryl-functionalised T8-F compound. 55 The loss of the CQC stretching mode at 1628 cm -1 associated with the vinyl group initially present on the styryl moiety is consistent with the hydrothiolation of the double bond. In addition, a weak peak is observed at 692 cm -1 (see inset to Fig. S10,ESI †) which is tentatively assigned to the C-S stretching mode. 59,60 The weak peak observed at around 3680 cm -1 is attributed to small quantities of isolated silanol species, which were presumably formed by hydrolysis of the ethoxy groups during acquisition of the spectra and associated exposure to atmospheric moisture. The flat spectral response observed from 3100 to 3600 cm -1 indicates that there are no H-bonded water, alcohol, or H-bonded silanol species in the precursor. In addition, a weak signal arising from the minor hydrolysis product can be observed in the 29 Si NMR spectrum of the precursor, upfield to the main T 0 peak at -45.69 ppm, when the sample is exposed to atmospheric water vapour. The intensity of the weak signal (not seen in Fig. 4) increases with increasing exposure to atmospheric moisture, confirming that it is not an intrinsic feature of the precursor compound and is an artefact arising from hydrolysis. Finally, a possible attribution of the minor signals observed in the 29 Si and 19 F NMR spectra (at -81.58 and -25.43 ppm, respectively) to the small quantity of hydrolysed species observed by FTIR may be discounted, in view of the fact that these hydroxyls are located more than 10 atoms distant from the silsesquioxane cage.

Formation of gels incorporating intact T8-F silsesquioxane cages

As indicated above, there now exists a rapidly growing body of work on the incorporation of functionalised Tn cages into a variety of materials, with many such systems and corresponding applications summarised in a recent review. 33 However, materials incorporating intact T8-F cages have not been unambiguously demonstrated previously. Hence a key goal of this work was to highlight an example of the successful conversion of the reactive precursor produced via the styryl/thiol-ene reaction platform into a nanohybrid material with retention of the T8-F moieties. Accordingly, we next investigated the fluoride-catalysed conversion of the precursor into a gel via sol-gel processing, as illustrated in Fig. 1.

The TEM micrograph of the material obtained, Fig. S11 (ESI †), is consistent with a dense, relatively uniform structure, with no porosity being evident. This was confirmed by N2 physisorption, with the low quantities of gas adsorbed precluding any meaningful analysis of porosity or surface area.

Retention of the fluoride species within the T8 cages embedded in the hybrid network was confirmed by 19 F SP-MAS NMR spectroscopy, which revealed a single broad signal centred at -21.9 ppm (Fig. 5, top). This is shifted downfield compared to the chemical shift of the styryl-functionalised T8-F (-25.17 ppm 55 ) and the triethoxysilylated precursor (-25.16 ppm, Fig. 3) in the corresponding solution NMR spectra. The width of the signal compared to the solid state NMR signal of the styryl T8-F,55 together with the asymmetry on the downfield side, also suggests that the T8-F cages experience a range of different environments within the gel matrix, consistent with a disordered gel structure. A broad signal is also evident at -132.0 ppm (Fig. S12,ESI †), which is attributed to residual TBAF not removed during washing of the gel, following earlier reports of a similar feature at -129 ppm in the 19 F NMR spectrum of a POSS system incorporating TBAF. 61 Fig. 5 19 F SP-MAS (top) and 13 C CP-MAS (bottom) solid state NMR spectra of the T8-F gel.

The 13 C CP-MAS NMR spectrum of the gel (Fig. 5, bottom) exhibits the expected signals arising from the phenylene moiety linked to the T8-F cage at 140.5, 134.8 and 127.3 ppm. 55 Aliphatic CH2 groups from the ethyl and propyl branches (the latter arising from the thiopropyl moiety), together with the TBA + cation, are also observed as a cluster of signals between 35.4 and 13.0 ppm. In addition, the spectrum indicates that hydrolysis is not complete, as demonstrated by the presence of signals arising from residual ethoxy groups at 58.6 ppm and in the aliphatic signal cluster between 35.4 and 13.0 ppm.

Silicon species associated with both the gel matrix and the intact T8-F cage are clearly evident in the 29 Si MAS spectra of the gel (Fig. 6). In particular, the spectrum obtained by cross polarisation using the 1 H nucleus (Fig. 6, middle) exhibits signals associated with T 2 (R-Si(OX)O, X = H, Et) and T 3 (R-SiO1.5) species at -60.0 and -66.4 ppm, which arise from hydrolysis and condensation of the thiopropyltriethoxysilylated moiety. An additional signal associated with this network is also observed as a weak shoulder downfield to the T 2 signal, which is attributed to the corresponding T 1 species. The observation of T 2 and T 1 species is consistent with the 13 C NMR spectrum, which indicated that hydrolysis was not complete. The additional narrow signal observed at -79.3 ppm is unambiguously attributed to the T 3 Si species, functionalised with aromatic moieties, that comprise the fully intact T8-F silsesquioxane cages. It is interesting that the chemical shift observed for these latter species is shifted downfield compared to the values obtained for styryl-functionalised T8-F (-80.72 and -81.2355), as previously observed for the corresponding T8 and T10 gel systems. 57 This deshielding is also similar to that observed in the associated 19 F solid-state NMR spectrum. The 1 H cross polarised 29 Si NMR spectrum of the corresponding T8 gel 57 is also shown for comparison in Fig. 6 (top). It should be noted that the T8 gel was prepared under acid-catalysed conditions, whereas the T8-F product was prepared using nucleophilic catalysis. As expected, the relative abundance of aliphatic T 3 species in the case of the T8-F gel is higher than that in the T8 gel (where T 2 and T 1 species predominate), due to the higher extent of condensation expected for nucleophilic catalysis compared to acid-catalysed conditions. 62 The T8 cage resonance for the T8 gel (-76.3 ppm) is also downfield with respect to that of the T8-F cage (-79.3 ppm), as is typically observed for these systems in liquid NMR spectroscopy. 55 Fig. 6 29 Si CP [ 1 H] of the T8 gel (top 57 ); compared with 29 Si CP [ 1 H] (middle) and 29 Si CP [ 19 F] (bottom) MAS solid state NMR spectra of the T8-F gel.

In order to unambiguously confirm the retention of the fluoride ion within the T8 cage of the T 8 -F gel, an additional solid state 29 Si cross polarisation NMR experiment was performed using the fluoride signal observed at -21.9 ppm in the 19 F NMR spectrum for polarisation transfer (Fig. 6, bottom). In this latter 29 Si CP-MAS NMR experiment, a single signal at -79.3 ppm can be seen. This signal corresponds to the T 3 silicon species of the T8-F cage and appears at the same chemical shift as in the corresponding 29 Si CP-MAS NMR spectrum with 1 H polarisation transfer (Fig. 6, middle). In addition, as expected, the T 2 and T 3 signals associated with the hydrolysed triethoxysilyl group are no longer observed when using 19 F as the polarisation transfer nucleus, since the corresponding silicon atoms do not interact with the fluoride anion. These observations further confirm the retention of the fluoride in the T8-F cages and show that the cage was not degraded during the synthesis of the gel.

In our earlier study, 55 we demonstrated the enhanced stability of the styryl-functionalised T8-F POSS under conditions that had been previously shown to release the fluoride anion from the POSS cage in other T8-F compounds. This stability might also contribute to the retention of the fluoride ion within the intact POSS structure during the thiol-ene click reaction and subsequent sol-gel processing.

Thermal decomposition of POSS compounds has been the subject of several studies. [63][64][65][66] For example, octaphenylfunctionalised T8 POSS was found to 95% of its initial mass up to 471 °C under an air atmosphere, with substantial decomposition subsequently observed from 500 to 700 ° C by Blanco et al. 63 A similar result was reported by Fan and Yang, 64 who indicated that the onset of thermal decomposition was 465 °C in the octaphenyl T8 POSS system. T8 silsesquioxane cages functionalised with aliphatic groups such as aminopropyl chains also exhibit relatively high decomposition temperatures, with the onset of decomposition of the POSS cage being observed at around 425 °C. 65 In this latter case, a mass loss at lower temperatures (<200 °C) was attributed to the desorption of water bound to the hydrophilic aminopropyl groups. A model to predict the onset of thermal decomposition in organo-functionalised T8 POSS compounds has also been reported. 66 However, as far as we are aware, there have been no reports of the thermal-decomposition characteristics of T8-F POSS compounds.

The thermogravimetric analyses of T8-F POSS, together with the T8-F and T8 gels, are compared in Fig. 7. As above, we note that the two gel samples were prepared using different sol-gel catalysts, although key differences in their respective TGA traces can be largely attributed to the presence of the encapsulated fluoride and TBA + counter-ion in the case of the T8-F gel. The ~3% mass loss below 200 °C in the case of the T8 gel is attributed to the loss of ''structural'' water, 57 with a much smaller evolution being observed in the case of the T8-F gel (<1%). No such mass loss is evident for T8-F POSS below 200 °C, consistent with the lack of water associated with the compound. The feature at 275 °C in the case of the T8-F gel (~6% mass loss) and T8-F POSS (~11%) is not observed in the T8 gel system and is attributed to the partial decomposition of the TBA + counter-ions, which account for 7 and 16% of the mass of the gel and T8-F POSS, respectively. Similar TGA profiles have been previously reported for a range of TBA + compounds. [67][68][69] The feature at ~350 °C in the dTGA traces of the gels is much less prominent in the case of the T8-F POSS, suggesting that it arises mainly from the loss of hydroxyl species in the gels, together with a minor contribution from decomposition of organics. This attribution is also consistent with the relative intensities of the corresponding T 1 and T 2 NMR signals for the T8 and T8-F gels (Fig. 6), with the larger hydroxyl content in the former sample giving rise to an increased mass loss in this region (Fig. 7).

The significant mass losses beginning at around 550 °C, and the associated maxima in the dTGA traces at ~600-630 °C for both gels and T8-F POSS, contain contributions resulting mainly from the decomposition of organic species and the T8 cages, both of which occur in multiple steps. The latter feature is similar to that typically observed for simple organo-functionalised T8 POSS compounds. 63,64 The weak shoulder observed at around 600 °C in the dTGA plot of the T8-F gel (which is not observed in the case of the T8 gel) is tentatively attributed to loss of fluoride species following decomposition of the T8 cages. Finally, the residual masses of the two gel samples at 800 °C (~40.4%) are comparable, while that of the T8-F precursor (34.7%) is close to the value of 32.0% expected following decomposition and associated formation of SiO2.

FTIR spectra of the triethoxysilyl T8-F precursor and gel,together with the corresponding T8 gel, 57 are compared in Fig. 8. The spectra were normalised against the intensity of the phenyl ring stretching band at 1604 cm -1 , since the phenyl ring is retained within the structure and not transformed during sol-gel processing. The broadening of the Si-O-Si stretching band at 1064 cm -1 in the gel is consistent with the formation of a disordered Si-O-Si network from the hydrolysiscondensation of the triethoxysilyl moiety, with the gel network being less organised than the Si-O-Si species within the cages. The decrease in the intensity of the CH3 stretching mode at 2972 cm -1 is also consistent with the hydrolysis of the ethoxy groups. It should be noted that a component of the 2972 cm -1 peak is also associated with the TBA + counter-ions retained within the structure, which contributes to the spectroscopic profile in the C-H stretching region. In addition, the peak at 3680 cm -1 attributed to isolated silanol species in the T8-F precursor is not evident in the spectrum of the corresponding gel, indicating that the majority of silanol species in the gel were condensed or are H-bonded. Fig. 8 FTIR spectra of the triethoxysilylated T8-F precursor (top) and of the material obtained via sol-gel processing of the latter (middle), compared to that of the corresponding T8 gel (bottom). All spectra were normalised to the intensity of the (C=C) mode.

Selected regions of the FTIR spectra of the T8-F and T8 gels are compared in greater detail in Fig. 9. The spectrum of the T8 gel exhibits a band at 1704 cm -1 (Fig. 9, bottom), which was previously attributed by us to ''structural'' water strongly interacting with the silsesquioxane gel network. 57 Briefly, earlier studies of the vibrational spectrum of water in a variety of systems demonstrated that increasing H bond strength in water led to an increase in the wavenumber of the bending mode within the range <1600 to >1700 cm -1 . 70 Similar conclusions were reported in subsequent computational and experimental studies, 71 while a peak in the spectrum of zeolite NaA observed at 1678 cm -1 was attributed to ''connective'', ice like water within the zeolite cage. 72 It was noted that the properties of this ''connective'' water were significantly different to those of bulk water. 72 A similar peak is also observed in the spectrum of the T8-F gel (Fig. 9, top) although it is significantly weaker, consistent with the presence of less water in the latter sample. Similar conclusions can be drawn from the corresponding broad (OH) profiles at 3400 cm -1 , where the profile intensity for the T8-F gel is significantly weaker than that of the T8 material. These results are consistent with the TGA data that indicate only small quantities of water are present in the T8-F gel. The low water content of the T8-F gel, despite the presence of the hydrophilic TBA + counter ion in the material, is intriguing. The structure of the styryl-functionalised T8-F was reported in our earlier paper, 55 which revealed that the TBA + cation is surrounded by phenyl groups in the compound. Given the Si-F interaction observed in this system, 55 the negative charge is presumably partially delocalised across the eight phenyl rings, with each ring bearing a partial negative charge which is balanced by the proximate counter ion. A similar short-range structure would also be anticipated in the gel. This suggests that the low water uptake of the gel might be influenced by shielding of the hydrophilic TBA + cation and by the nearby hydrophobic phenyl groups.

Finally, to enhance the range of potential applications for such materials, the possibility of intentionally degrading the T8-F cage to generate size-controlled porosity within the gel matrix was also considered. Cage silsesquioxanes are known to be cleaved under basic conditions, and hence preliminary experiments involving exposure of the gel to pyridine at ambient temperature were undertaken. However, 19 F SP NMR studies demonstrated that the attempt to cleave the cage was unsuccessful, with a signal still observed at -21.3 ppm and no new signals appearing. Additional experiments were undertaken with refluxing triethylamine, but no cleavage was observed under these more robust conditions either. These results suggest that the T8-F cage is at least partially stabilised in the hybrid material, due to (a) the intrinsic stability of the styryl-functionalised T8-F system; and/or (b) its incorporation within the dense gel matrix, which limits penetration of the base into the network.

Conclusions

In summary, despite the typically low reactivity of T8-F compounds toward traditional organic synthesis reactions, the styryl-functionalised T8-F has been successfully modified via a thiol-ene click reaction with mercaptopropyltriethoxysilane. In particular, we demonstrate for the first time the successful post-functionalisation of a T8-F compound without degradation of the POSS cage and with retention of the fluoride anion within the cage. A particular feature of this approach is that the styryl/thiol-ene platform enables a much wider variety of reactive functional groups to be grafted onto the T8-F cage, including electron-donating groups that cannot usually be covalently bound to T8-F. We note that the previous lack of such reactive T8-F compounds has limited progress in exploring potential applications for this intriguing family of compounds over the past few decades.

To provide an example of potential approaches for incorporating the reactive T8-F moieties within materials, we next explored the transformation of the triethoxysilylated T8-F compound into a gel via sol-gel processing. Solid-state multinuclear NMR studies ( 29 Si, 19 F, 13 C) demonstrated that the POSS cage structure remains intact, with fluoride retained within the cage, after sol-gel processing and formation of the nanohybrid material. This latter feature was shown by cross polarisation 29 Si NMR sequences using 19 F as polarisation transfer agent, which demonstrated unambiguously that the fluoride remains encapsulated within the POSS cage after formation of the sol-gel silsesquioxane network. The resulting material thus contains nanometre-sized charged domains, uniformly incorporated within a nanohybrid network. This is the first unambiguous example of reactive T8-F precursors that can be used to covalently incorporate T8-F POSS species within a functional material. Interestingly, TGA and IR analysis demonstrated that the material contained only small quantities of water, despite the presence of the hydrophilic TBA + cation within the matrix. This suggests a possible application for such materials as hydrophobic coatings, which remains to be explored. The availability of precursors for producing materials incorporating intact T8-F moieties also opens the way to the preparation of new materials with properties such as electrical conductivity, optical response, etc., that can be modulated by the presence of encapsulated fluoride anions within their structure.

Fig. 1

 1 Fig. 1 Synthesis of triethoxysilylated T8-F cage via thiol-ene click reaction and production of hybrid silica gel. For clarity, the functional group is shown on only one vertex of the cages.

Fig. 3

 3 Fig. 3 Solution 19 F NMR spectrum of the triethoxysilylated functionalised T8-F cage. For clarity, the functional group is shown on only one vertex of the cages.

Fig. 4

 4 Fig. 4 Solution 29 Si NMR spectrum of the triethoxysilylated functionalised T8-F cage. For clarity, the functional group is shown on only one vertex of the cages.

Fig. 7

 7 Fig. 7 TGA (top) and dTGA (bottom) of T8 (blue) and T8-F gels (orange); and T8-F POSS precursor (black).

Fig. 9

 9 Fig. 9 Expanded view of the FTIR spectra shown in Fig. 8, highlighting the 1500 to 1900 (left) and 2500 to 4000 cm -1 regions (right).

Acknowledgements

The authors gratefully acknowledge Frank Godiard (Plateforme Microscopie Electronique et Analytique, Université de Montpellier) for TEM measurements, as well as Cédric Totée and Emmanuel Fernandez (Plateforme d'Analyses et de Caractérisation Balard -PAC Balard Montpellier) for NMR experiments. Funding from the French Ministère de l'Enseignement Supérieur et de la Rechercheto support the PhD scholarship of Mathilde Laird and the Japan Student Services Organization for a travel scholarship awarded to Mathilde Laird are gratefully acknowledged.

Conflicts of interest

There are no conflicts to declare.