Dipeptide-functionalized mesoporous silica spheres

Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 Dipeptide-functionalized mesoporous silica spheres Alain Walcarius a,∗ , Stéphanie Sayen a , Christine Gérardin b , Faouzia Hamdoune b,1 , Ludwig Rodehüser b a Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, Unité Mixte de Recherche UMR 7564, CNRS-Université H. Poincaré Nancy I, 405 rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France b Laboratoire de Chimie Physique Organique et Colloı̈dale (CNRS UMR 7565), Faculté des Sciences, Université H. Poincaré Nancy I, BP 239, F-54506 Vandoeuvre-lès-Nancy, France Received 29 April 2003; accepted 12 November 2003 Abstract The preparation of surfactant-templated mesoporous silica spheres functionalized with carnosine moieties (dipeptide ␤-alanyl-l-histidine) is described. The procedure involves the hydrolysis and co-condensation of a carnosine-containing organotriethoxysilane and tetraethoxysilane (TEOS) in water/ethanol solution in the presence of ammonia acting as catalyst and cetyltrimethylammonium bromide (CTAB) as templating agent. The resulting organic–inorganic hybrid displays spherical shape as evidenced by electron microscopy and narrow particle size distribution. It exhibits high specific surface area (775 m2 g−1 ), as determined from the N2 adsorption isotherm, and the XRD pattern is characteristic of a mesostructure with wormholelike framework. Functionalization was confirmed by IR spectroscopy and solid-state 29 Si and 13 C MAS NMR. The regular mesoporous structure can be advantageously exploited in processes where diffusion is the rate-determining step, as exemplified for the electrochemical detection of trace copper(II) after preconcentration at a carbon paste electrode modified with the material. © 2003 Published by Elsevier B.V. Keywords: Ordered mesoporous silicas; Organic–inorganic hybrid composites; Carnosine; Copper; Electrochemical detection 1. Introduction Silica-based organic–inorganic hybrid materials that are mixing the inorganic and organic components at the nanometric scale have garnered great interest in recent years for applications in various fields including heterogeneous catalysis, separation sciences, environmental remediation, electrochemistry, chemical and biological sensors, and optical devices [1–17]. Two classes have been distinguished: class I corresponds to materials with weak bonding between organic and inorganic phases, whereas class II corresponds to materials where both phases are chemically grafted [1]. The latter can be prepared either by postsynthesis grafting of an organic component on an as-synthesized inorganic substrate ∗ Corresponding author. Tel.: +33-383-91-6343; fax: +33-383-27-5444. E-mail address: [email protected] (A. Walcarius). 1 Present address: Faculté des Sciences et Techniques de Tanger, Tanger, Morocco. 0927-7757/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2003.11.010 [2,12,13], or by sol–gel chemistry from a mixture of silicon alkoxides and organo-functional Si-alkoxides, according to a “hydrolysis co-condensation” sequence [1,3,11]. They are very attractive since the nonhydrolysable character of the Si–C bond prevents leaching of organic groups out of materials when used in solution. The discovery of the micelle-templated mesoporous silica [18] in the early 1990s has greatly expanded the possibilities in the development of advanced materials with unprecedented performances [8,9,12,14–17]. They can be functionalized via covalent bonding of organic molecules, either by postsynthesis grafting [19] or by the co-condensation route [20,21], leading to a wide range of high surface area mesoporous organic–inorganic hybrids with tailor-made composition, displaying a regular pore system of monodispersed size. The second method (co-condensation) usually leads to a uniform distribution of organic functions throughout the channels whereas postsynthesis grafting results often in larger amounts of organic groups at the outermost surface of particles and near the channel openings [16,22,23]. 146 A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 However, one-pot syntheses of mesoporous silicas with organically-modified surfaces remained often limited to the production of materials containing rather simple organic groups [14,23], while the introduction of more sophisticated components was usually achieved by postsynthesis procedures (primary or secondary modification of either pure silica materials or prefunctionalized structures [12,16,23]). An illustrative example is the case of a guanidine moiety grafted onto micelle-templated silicas, which were then applied for catalytic purposes [24–27]. In this work, we have evaluated the possibility to obtain micelle-templated dipeptide-functionalized mesoporous silica spheres by one-pot synthesis. The selected dipeptide is Carnosine (I), a naturally occurring compound (␤-alanyl-l-histidine), which is present in many human tissues in relatively high concentrations (1–20 mM) [28,29]. The synthesis starts with the preparation of a carnosinecontaining organotriethoxysilane [30] which is then hydrolysed and co-condensed with tetraethoxysilane (TEOS) in hydroalcoholic medium. The protocol involves the use of ammonia as catalyst, to obtain particles of spherical shape [31], and cetyltrimethylammonium bromide (CTAB) as templating agent to induce mesostructuration, as previously applied to the production of pure silica MCM-41 and MCM-48 materials [32–34] or organically-modified ordered mesoporous solids containing amine or thiol groups [35,36]. The mesoporous hybrid denoted MTS-Car will be characterized by various physico-chemical techniques and used as an electrode modifier to evaluate its performance with respect to Cu(II) binding. 2. Experimental 2.1. Chemicals and reagents Analytical grade chemicals from the following suppliers were used as purchased for the preparation of the silica-based material: tetraethoxysilane (>98%, Merck), cetyltrimethylammonium bromide (98%, Fluka), ammonia (28% aqueous, Prolabo), ethanol (95–96%, Merck), HCl (36%, Prolabo). The carnosine-containing organosilane precursor (II) was prepared as previously described [30], via amide coupling between the amine group of 3-aminopropyltriethoxysilane (APTES, >99%, Aldrich) and the carboxyl group of l-carnosine (>99%, Fluka), which requires beforehand protection of its amine groups by tert-butyloxycarbonyl (Boc) substituents. All solvents used for the synthesis of both the precursor and the silica-carnosine were reagent grade and used without further purification except for THF which was dehydrated by distillation from sodium wire. Copper(II) solutions were prepared from analytical grade Cu(NO3 )2 (Prolabo) and high-purity water (18 M cm−1 ) obtained from a millipore milli-Q water purification system. Adjustment of pH was made by using an acetate buffer (Riedel de Haën). 2.2. Synthesis of the mesoporous silica-carnosine hybrid material One-pot synthesis of carnosine-functionalized silica was performed in the presence of a surfactant templating agent, according to our previously published procedure, which was designed to yield organically-modified mesoporous silica spheres with MCM-41 architecture [35]. In this case, however, the precursor (II) was used as the organosilane reactant allowed to condense with TEOS. Typical molar composition of the starting sol was 1:0.40:12:65:175 (precursors:CTAB:ammonia:ethanol:water), with precursors made of (II)/TEOS ratios of 5% or 10%. CTAB (1.4 g) was dissolved in 30 ml deionized water and 25 ml ethanol (95–96%) to which 7.7 ml of 28% aqueous ammonia were added. The precursor mixture was prepared by dissolving appropriate ratios of (II) and TEOS in 5 ml ethanol, which was then added to the “surfactant + catalyst” solution under vigorous stirring. Condensation occurred within 2 min. The resulting white precipitate was stirred for 2 h at room temperature. The product was then isolated by vacuum filtration on a Büchner funnel and washed alternatively with water and ethanol. The resulting powder was dried under vacuum (<10−2 bar) for 24 h. The surfactant was removed from the hybrid material via acid/solvent extraction by suspending the solid particles in 1 M HCl in ethanol (0.5 g solid in 50 ml solution), which was then allowed to reflux for 18 h. After filtration and washing with ethanol, the solid was dried according to the aforementioned conditions. This step led also to the deprotection of N functions of carnosine via the acid-catalyzed hydrolysis of Boc groups, yielding the final hybrid mesoporous copolymer MTS-Car (III): A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 For sake of comparison, an amorphous version of this hybrid material was prepared by performing the copolymerization of precursor (II) and TEOS in the absence of the structure-directing agent, according to a procedure published previously [30]. 2.3. Instrumentation and electrochemical procedure Various techniques were applied to characterize the carnosine-functionalized mesoporous silica. The specific surface area was measured on the basis of nitrogen adsorption/desorption isotherms at the temperature of liquid nitrogen, by using a Coulter SA 3100 apparatus. Powder X-ray diffraction data were collected at room temperature using a classical powder diffractometer (X’PERT PRO, Philips), equipped with a Cu anode (quartz monochromator, Kα1 radiation, λ = 0.154056 nm). Scanning electron microscopy (SEM) was performed with a Philips XL30 apparatus, while transmission electron microscopy (TEM) pictures have been obtained from a Philips CM20 microscope operating at 200 keV. Particle size distribution was measured using a light scattering analyzer (model LA920, Horiba), based on the Mie scattering theory. The 29 Si and 13 C MAS solid-state NMR spectra were recorded on a Bruker 300 apparatus. The IR spectra were recorded on a Perkin-Elmer 1600 FT-IR spectrometer in either ATR or transmission mode (KBr pellets). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, plasma 2000, Perkin-Elmer) was applied to the quantitative analysis of Cu(II) in solution for evaluating the extent of copper adsorption from batch suspensions of the organic–inorganic hybrid. The electrochemical measurements were performed at room temperature with a ␮-Autolab potentiostat and GPES electrochemical analysis system (Eco Chemie), equipped with an undivided three-electrode cell. The working electrode was a homemade carnosine-silica modified carbon paste, an Ag/AgCl electrode (Metrohm, no. 6.0733.100) was used as reference and a platinum plate served as counter-electrode. The carnosine-silica modified carbon paste was prepared as previously described [37], by carefully mixing 60% high purity graphite (Ultra F, 200 mesh, from Alfa-Johnson Matthey), 30% paraffin wax (Fluka), 147 and 10% of the hybrid material. The electrochemical experiments were performed according to the “preconcentration— voltammetric detection” sequence, which was carried out in two successive steps. First, the electrode was immersed in a copper-containing preconcentration medium, where the accumulation of Cu(II) was achieved by chemical binding to carnosine ligands, at open circuit, under constant stirring. The electrode was then removed from the analyte cell, rinsed with distilled water, and transferred to a fresh electrolyte solution (0.1 M NaNO3 and 0.01 M HNO3 ). Immediately after immersion in this separate voltammetric cell, a cathodic potential (−0.4 V) was applied to the electrode to reduce all the previously accumulated species, and voltammetric monitoring was performed by anodic stripping voltammetry in the differential pulse mode (modulation amplitude, 50 mV; modulation time, 5 ms; potential scan rate, 50 mV s−1 ). 3. Results and discussions The observation of the micelle-templated silica-carnosine sample, MTS-Car (III), by SEM reveals that particles are spherical, submicrometric, and monodisperse, yet under the form of larger aggregates when deposited on solid support as required to get the SEM images (Fig. 1), in agreement with other organically-modified mesoporous silicas prepared according to the same procedure [35]. Ultrasonic treatment of particles in aqueous suspension leads to the separation of most aggregates into isolated individual particles and smaller aggregates. Particle size analysis reveals a bimodal distribution centred around 400–500 nm for isolated spheres and 2–4 ␮m for closely associated particles, when expressed in volume, whereas the data presentation with respect to the number of particles confirms a narrow size distribution, centered at mean diameters in the 400–500 nm range. The same results were obtained whether the precursors (II) to TEOS ratio in the starting sol was 5% or 10%. The XRD pattern of the 5% MTS-Car sample (Fig. 2) features a dominant broad correlation peak corresponding to an average lattice spacing of 35 Å (d1 0 0 reflection), which is characteristic of wormholelike mesostructures [38–40], indicating a significant degree of order in this material. This is 148 A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 However, higher order reflections can be noticed on Fig. 2 (in the 2θ region ranging between 4 and 5.5), suggesting the existence of some long-range order. Enlargement of this range enables even to distinguish two contributions at 2θ equal to 4.5 and 5.2, which might be attributed to d1 1 0 and d2 0 0 reflections (with corresponding lattice spacings of 19.6 and 16.8 Å, respectively), providing evidence for some hexagonal arrangement as in MCM-41 structures. This is supported by the observation of regular arrays of channels on some portions of MTS-Car particles, as revealed by high resolution TEM (Fig. 3B). Increasing the organic group content in the material (from 5% precursor (II) in the starting sol to 10%) leads to disappearance of the d1 1 0 and d2 0 0 reflections with a concomittant decrease in the intensity of the d1 0 0 peak. This may be partially attributed to lower local order, or due to contrast matching between the silicate framework and Fig. 1. SEM micrograph of the carnosine-functionalized silica sample MTS-Car. Intensity (a.u.) confirmed by the TEM picture of a single sphere (Fig. 3A) where the framework porosity can be readily observed over large continuous areas. The pores are regular in diameter as they originate from the space initially occupied by the uniform supramolecular assemblies of surfactant. The structure of MTS-Car is less ordered than those characteristic of aminopropyl- or mercaptopropyl-functionalized silicas produced according to the same synthetic procedure, which display typical MCM-41 long-range packing [35]. This can be rationalized in terms of different interactions between the templating micelle and the organosilane precursors (as expected from major differences between the bulky carnosine-containing precursor (II) and smaller organosilanes such as aminopropyl- or mercaptopropyltrialkoxysilanes); the voluminous carnosine-based organic moiety may perturb the self-assembly of surfactant aggregates and somewhat hinder the mesophase architectures. 2 4 6 8 10 2θ Fig. 2. Powder X-ray diffraction pattern for cetyltrimethylammoniumassembled MTS-Car derivative (5% carnosine precursor in the starting sol, relative to TEOS). Inset: enlargement of the 2θ range 3.2–6.2. Fig. 3. TEM micrographs of the surfactant-extracted MTS-Car derivative (5% carnosine precursor in the starting sol, relative to TEOS): (A) single sphere and (B) enlargement of a portion of the particle. 149 A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 250 (a) 3 -1 Volume adsorbed (cm g ) 300 200 150 (b) 100 50 0 0,0 0,2 0,4 0,6 0,8 1,0 P/P0 Fig. 4. Nitrogen adsorption isotherms ((䊏), (䊉) adsorption branch; (䊐), (䊊) desorption data) obtained for MTS-Car derivatives, respectively prepared from (a) 5% and (b) 10% carnosine precursor in the starting sol, relative to TEOS. increasing amounts of organic components that are located inside the mesopores [22]. Fig. 4 presents typical N2 adsorption–desorption isotherms for MTS-Car samples, respectively prepared from 5 and 10% precursor (II) in the starting sol. The first isotherm is clearly of type IV, characteristic of mesoporous materials [38], giving a specific surface area of 775 m2 g−1 , a total pore volume of 0.419 cm3 g−1 , and a mean pore diameter of 21 Å. This constitutes a significant improvement in comparison to the corresponding amorphous material for which the specific surface area was 62 m2 g−1 , with a total pore volume of 0.279 cm3 g−1 , and pore diameter of about 160 Å. As expected, the pore volume of MTS-Car decreased upon rising the carnosine content in the material, due to larger space occupancy by the organic groups in the mesopore channels. The efficiency of functionalization was confirmed by IR spectroscopy and solid-state 29 Si and 13 C MAS NMR. These data also point out that carnosine groups were not damaged during the synthesis of the organic–inorganic hybrid. In particular the 13 C NMR resonance lines at 172 and 56 ppm, corresponding to the C=O and C–H groups of carnosine indicate the presence of the organic substituent in the mesoporous structure. When preparing amorphous silica gels grafted with carnosine moieties, the Boc protecting groups were removed from the polymeric material by treatment with gaseous HCl [30]. In the case of MTS-Car, however, this additional step was not required because the acidic treatment applied to extract the surfactant from the mesoporous structure ensured at the same time the deprotection operation. The efficiency of this operation has been checked by the disappearance of its characteristic absorption (νCO = 1680 cm−1 ) on the IR spectra of MTS-Car and also by the disappearance of two signals in solid-state 13 C NMR, at 30 ppm (CH3 of Boc) and 160 ppm (C=O of Boc). The analysis by IR spectrometry proves also that the surfactant CTAB used during the preparation of the mesoporous silica is removed quan- Fig. 5. IR Spectrum of MTS-Car (a) and pure surfactant CTAB (b) showing that the surfactant used for the synthesis has been removed successfully from the mesoporous silica. The arrow indicates one of the characteristic bands of the carnosine substituent. titatively by the extraction process while the characteristic bands of carnosine, e.g. for the amide C=O stretching around 1640 cm−1 , are still present (Fig. 5). Solid-state 29 Si MAS NMR provides information about the silicon environment and about the degree of functionalization by the organic groups. A typical spectrum recorded for the extracted 10% MTS-Car sample is depicted in Fig. 6. Distinct resonances can be observed for the siloxane [Qn = Si–(OSi)n –(OH)4−n , n = 2–4; Q4 at −111 ppm, Q3 at −102 ppm, and a small contribution of Q2 at −93 ppm] and organosiloxane [T m = RSi–(OSi)m –(OH)3−m , m = 1–3; T3 at −66 ppm, T2 at −57 ppm] species. These data indicate on one hand that the organic groups have been readily incorporated into the mesoporous structure and on the other that the silica network is characterized by a relatively low degree of cross-linking (Q4 /Q3 ∼ 0.5). The latter fact can be partially explained by the use of a cationic quaternary ammonium template, which favors the presence of unlinked SiO-groups that are needed for charge compensation [41]. No attempt was made here to increase the degree of cross-linking by any postsynthesis treatment. The relative integrated intensities of the siloxane (Qn ) and organosiloxane (Tm ) NMR signals (Tm /Qn ) allow the quantitative assessment of the Q 3 Q Q 3 T 4 2 2 T -40 -60 -80 -100 -120 -140 ppm Fig. 6. Deconvoluted 29 Si MAS NMR spectrum of the surfactant-free mesoporous MTS-Car (10% carnosine precursor in the starting sol, relative to TEOS) in the shift range from −40 to −140 ppm. A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 incorporation degree of the organic moiety [42]. Firstly, one observes a good agreement among the Tm /Qn ratios expected on the basis of the composition of the starting sol and those measured by NMR, which indicates a high degree of carnosine-based precursor (II) incorporation in the final material (>90%). Secondly, these ratios enable to estimate the carnosine loading in the materials, as 0.4 and 0.9 mmol g−1 , respectively, for the 5% MTS-Car and 10% MTS-Car samples. The carnosine-type peptidoamine has a natural tendency to form stable complexes with copper(II), for which both the stoechiometry and quantitativity are strongly dependent on pH, the analyte and ligand concentrations, as well as the carnosine-to-copper ratio [43–47]. It has been shown recently that carnosine grafted on amorphous silica gels is also able to trap copper(II) species from aqueous media [30,48]. It is not the purpose of this work to provide a detailed study of the binding properties of MTS-Car materials, but we would like to point out their binding ability toward copper(II) species and to discuss the interest of the ordered mesoporous structure on this process, if any, in comparison to the corresponding amorphous material. Typical uptake experiments have been performed in batch conditions, from MTS-Car suspensions (1 g l−1 ) containing selected Cu(II) concentrations (in the 20–100 ␮M range), in acetate buffer (0.01 M, pH 5.6) where monomeric complexes are expected to be formed [37,46]. Under these conditions, after 2 h of reaction, the 5% MTS-Car sample had accumulated Cu(II) up to 0.045 mmol g−1 , which corresponds to about 10% of the carnosine binding sites in the material, whereas only 6% filling was achieved when the corresponding amorphous adsorbent was used under the same experimental conditions. This illustrates the importance of the mesopore structure for improving the accessibility to the active centers in silica-based organic–inorganic hybrids, as otherwise demonstrated for thiol-functionalized mesoporous silica samples [49–51]. An even more interesting point concerns the speed at which the analytes are reaching the active sites in the porous hybrid material, which constitutes a key parameter in the design of efficient adsorbents for remediation purposes [51–54] or in order to ensure high sensitivity for electrochemical devices comprising such organically-modified silicates [55–57]. We have recently demonstrated that diffusion processes in ordered mesoporous silica samples grafted with either aminopropyl or mercaptopropyl groups are faster than mass transfer rates in the corresponding amorphous solids grafted with the same organic functions, pointing out the advantage of a regular internal structure to speed up the diffusion-controlled reactions [51,54]. The electrochemical detection of electroactive analytes subsequent to chemical accumulation at modified electrodes constitutes such a diffusion-controlled process, which is especially limiting when using porous electrode modifiers [57]. With this respect, Fig. 7 compares the voltammetric signals recorded for the preconcentration analysis of 1 × 10−5 M Cu(II) at 1 µA (a) Current 150 (b) -0,4 -0,2 0,0 0,2 Potential (V) Fig. 7. Typical electrochemical responses of carbon paste electrodes modified with carnosine-silica derivatives: (a) ordered MTS-Car material and (b) amorphous solid. Preconcentration experiments were performed during 10 min in an aqueous solution containing 1 × 10−5 M Cu(II) at pH 6. Differential pulse stripping voltammograms were recorded in 0.1 M NaNO3 + 0.01 M HNO3 after 1 min electrolysis at −0.4 V. carbon paste electrodes modified either with the ordered mesoporous 5% MTS-Car (curve a) or with a corresponding amorphous solid (curve b). It is clear that the electrode modified with the ordered MTS-Car sample gave rise to a larger voltammetric signal (by a factor of about 2.5 in peak area) in comparison to that obtained with the amorphous material, in spite of its less open structure (21 Å pore size relative to about 160 Å for the amorphous one). This higher preconcentration efficiency and better detection sensitivity underline the beneficial effect of the ordered structure in improving access of guest species to the binding sites of adsorbents designed from a mesostructure with well-defined pore channels, relative to disordered materials [37]. 4. Conclusions Micelle-templated dipeptide-functionalized mesoporous silica spheres can be obtained by one-pot synthesis, via co-condensation of a carnosine-containing organotriethoxysilane and tetraethoxysilane in water/ethanol medium containing ammonia as catalyst and cetyltrimethylammonium as templating agent. The resulting material displays a wormholelike framework mesostructure with high specific surface area, resulting in improved complexation behaviour due to enhanced mass transport kinetics with respect to the corresponding amorphous solids. Acknowledgements We are grateful to P. Tekely for recording the solid-state NMR spectra. We also thank the “Region Lorraine” for financial support through the “Interfaces” project. A. Walcarius et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 234 (2004) 145–151 References [1] C. Sanchez, F. Ribot, New J. Chem. 18 (1994) 1007. [2] E.F. Vansant, P. van der Voort, K.C. Vrancken, Characterisation and Chemical Modification of the Silica Surface, Elsevier, The Netherlands, 1995. [3] J. Wen, G.L. Wilkes, Chem. Mater. 8 (1996) 1667. [4] O. Lev, Z. Wu, S. Bharathi, V. Glezer, A. Modestov, J. Gun, L. Rabinovich, S. Sampath, Chem. Mater. 9 (1997) 2354. [5] E. Bescher, J.D. Mackenzie, Mater. Sci. Eng. C 6 (1998) 145. [6] Z. Ahmad, J.E. Mark, Mater. Sci. Eng. C 6 (1998) 183. [7] A. Walcarius, Electroanalysis 10 (1998) 1217; A. Walcarius, Electroanalysis 13 (2001) 701. [8] T. Maschmeyer, Curr. Opin. Solid State Mater. Sci. 3 (1998) 71. [9] K. Moller, T. Bein, Chem. Mater. 10 (1998) 2950. [10] D. Avnir, L.C. Klein, D. Levy, U. Schubert, A.B. Wojcik, Chem. Org. Silicon Compounds 2 (1998) 2317. [11] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35. [12] D. Brunel, Microporous Mesoporous Mater. 27 (1999) 329. [13] N.R.E.N. Impens, P. van der Voort, E.F. Vansant, Microporous Mesoporous Mater. 28 (1999) 217. [14] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403. [15] Special issue of Chem. Mater. 13 (2001). [16] A.P. Wight, M.E. Davis, Chem. Rev. 102 (2002) 3589. [17] D. Brunel, A.C. Blanc, A. Galarneau, F. Fajula, Catal. Today 73 (2002) 139. [18] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [19] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [20] S.L. Burkett, S.D. Sims, S. Mann, Chem. Commun. (1996) 1367. [21] D.J. Macquarrie, Chem. Commun. (1996) 1961. [22] M.H. Lim, A. Stein, Chem. Mater. 11 (1999) 3285. [23] A. Sayari, S. Hamoudi, in: [15], p. 3151. [24] Y.V. Subba Rao, D. De Vos, P.A. Jacobs, Angew. Chem., Int. Ed. Engl. 36 (1997) 2661. [25] A. Derrien, G. Renard, D. Brunel, Stud. Surf. Sci. Catal. 117 (1998) 445. [26] A.C. Blanc, S. Valle, G. Renard, D. Brunel, D.J. Macquarrie, C.R. Quinn, Green Chem. 2 (2000) 283. [27] D.J. Macquarrie, K.A. Utting, D. Brunel, G. Renard, A. Blanc, Stud. Surf. Sci. Catal. 142B (2002) 1473. [28] K.G. Crush, Comp. Biochem. Physiol. 34 (1970) 3. [29] R. Cohen, Y. Yamamoto, K.C. Cundy, B.N. Ames, Proc. Natl. Acad. Sci. U.S.A. 85 (1988) 3175. 151 [30] F. Hamdoune, C. El Moujahid, L. Rodehüser, C. Gerardin, B. Henri, M.-J. Stébé, J. Amos, M. Marraha, A. Asskali, C. Selve, New J. Chem. 24 (2000) 1037. [31] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [32] M. Grün, I. Lauer, K.K. Unger, Adv. Mater. 9 (1997) 254. [33] M. Grün, K.K. Unger, A. Matsumoto, K. Tsutsumi, Microporous Mesoporous Mater. 27 (1999) 207. [34] D. Kumar, K. Schumacher, C. du Fresne von Hohenesche, M. Grün, K.K. Unger, Colloids Surf. A 187–188 (2001) 109. [35] M. Etienne, B. Lebeau, A. Walcarius, New J. Chem. 26 (2002) 384. [36] M. Etienne, S. Sayen, B. Lebeau, A. Walcarius, Stud. Surf. Sci. Catal. 141 (2002) 615. [37] S. Sayen, C. Gerardin, L. Rodehüser, A. Walcarius, Electroanalysis 15 (2003) 422. [38] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865. [39] E. Prouzet, T.J. Pinnavaia, Angew. Chem., Int. Ed. Engl. 36 (1997) 516. [40] L. Mercier, T.J. Pinnavaia, Chem. Mater. 12 (2000) 188. [41] C.-Y. Chen, H.-X. Li, M.E. Davis, Microporous Mesoporous Mater. 2 (1993) 17. [42] D. Margolese, J.A. Melero, S.C. Christiansen, B.F. Chmelka, G.D. Stucky, Chem. Mater. 12 (2000) 2448. [43] R.F. Pasternack, K. Kustin, J. Am. Chem. Soc. 90 (1968) 2295. [44] R.J. Sundberg, R.B. Martin, Chem. Rev. 74 (1974) 471. [45] I. Sovago, E. Farkas, A. Gergely, J. Chem. Soc., Dalton Trans. (1982) 2159. [46] P.G. Daniele, E. Prenesti, V. Zelano, G. Ostacoli, Spectrochim. Acta A 49 (1993) 1299. [47] A. Torreggiani, M. Tamba, G. Fini, Biopolymers 57 (2000) 149. [48] F. Hamdoune, C. El Moujahid, M.-J. Stébé, C. Gerardin, P. Tekely, C. Selve, L. Rodehüser, Prog. Colloid Polym. Sci. 118 (2001) 107. [49] M.H. Lim, C.F. Blanford, A. Stein, Chem. Mater. 10 (1998) 467. [50] J. Brown, R. Richer, L. Mercier, Microporous Mesoporous Mater. 37 (2000) 41. [51] A. Walcarius, M. Etienne, B. Lebeau, Chem. Mater. 15 (2003) 2161. [52] S.V. Mattigod, X. Feng, G.E. Fryxell, J. Liu, M. Gong, Sep. Sci. Technol. 34 (1999) 2329. [53] A. Bibby, L. Mercier, Chem. Mater. 14 (2002) 1591. [54] A. Walcarius, M. Etienne, J. Bessière, Chem. Mater. 14 (2002) 2757. [55] A. Walcarius, N. Lüthi, J.-L. Blin, B.-L. Su, L. Lamberts, Electrochim. Acta 44 (1999) 4601. [56] M. Etienne, J. Bessière, A. Walcarius, Sensors Actuators B 76 (2001) 531. [57] S. Sayen, M. Etienne, J. Bessière, A. Walcarius, Electroanalysis 14 (2002) 1521.