Ion-Exchange Properties and Electrochemical Characterization of

Dec 3, 2005 - Ion exchange and ion exchange voltammetry with functionalized mesoporous silica materials. V. Ganesan , A. Walcarius. Materials Science ...
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Langmuir 2006, 22, 469-477

469

Ion-Exchange Properties and Electrochemical Characterization of Quaternary Ammonium-Functionalized Silica Microspheres Obtained by the Surfactant Template Route Alain Walcarius*,† and Vellaichamy Ganesan‡ Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement Unite´ Mixte de Recherche UMR 7564, CNRSsUniVersite´ Henri Poincare´ Nancy I, 405 rue de VandoeuVre, F-54600 Villers-les-Nancy, France, and Department of Chemistry, Faculty of Sciences, Banaras Hindu UniVersity, Varanasi 221 005, India ReceiVed July 15, 2005. In Final Form: NoVember 2, 2005 Porous silica spheres functionalized with quaternary ammonium groups have been prepared by co-condensation of N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride (TMTMAC) and tetraethoxysilane (TEOS) in the presence of cetyltrimethylammonium as a template and ammonia as a catalyst. The physicochemical characteristics of the resulting ion exchangers have been analyzed by various techniques and discussed with respect to the amount of organofunctional groups in the materials. For comparison purposes, both an ordered MCM-41 type mesoporous silica and two silica gels of different pore size have been grafted with TMTMAC. The ion-exchange capabilities were first evaluated from batch experiments (determination of anion-exchange capacities) and then by ion-exchange voltammetry at carbon paste electrodes modified with these hybrid materials. Effective concentration of Fe(CN)63species in the anion exchangers was pointed out, while no significant accumulation of Ru(NH3)63+ was observed. The preconcentration efficiency was discussed on the basis of the organic group content in the materials as well as their structure and porosity. A second series of materials displaying zwitterionic surfaces was finally prepared and characterized with respect to their physicochemical properties and ion-exchange voltammetric behavior. They consisted of sulfonic acid-functionalized mesoporous silica samples resulting from the oxidation of thiol-functionalized silica spheres obtained by co-condensation of mercaptopropyl-trimethoxysilane (MPTMS) and TEOS, which were then grafted with TMTMAC at various functionalization levels. Possible interactions between the ammonium and sulfonate moieties in the confined medium were pointed out from X-ray photoelectron spectroscopy. The competitive accumulationrejection of Fe(CN)63- and Ru(NH3)63+ redox probes was finally studied by cyclic voltammetry.

1. Introduction Silica-based organic-inorganic hybrids become increasingly attractive as they combine in a single material both the mechanical stability of a rigid inorganic framework with the particular reactivity of the organofunctional groups.1-6 They can be obtained by postsynthesis grafting7,8 or, in one step, by the sol-gel process.1,2,4 They have found applications in various fields, including heterogeneous catalysis,9-12 separation sciences,12-17 * Corresponding author. E-mail: [email protected]. Fax: (+33) 3 83 27 54 44. † Universite ´ Henri Poincare´ Nancy I. ‡ Banaras Hindu University. (1) Mackenzie, J. D. Polym. Mater. Sci. Eng. 1993, 70, 380. (2) Sanchez, C.; Ribot, F New J. Chem. 1994, 18, 1007. (3) Chujo, Y. Curr. Opin. Solid State Mater. Sci. 1996, 1, 806. (4) Wen, J.; Wilkes, G. L Chem. Mater. 1996, 8, 1667. (5) Avnir, D.; Klein, L. C.; Levy, D.; Schubert, U.; Wojcik, A. B. In Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U. K., 1998; Vol. 2, Part 3, pp 2317-2362. (6) Polarz, S. In Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites, Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2003; Vol. 1, pp 165-205. (7) Vansant, E. F.; Van der Voort, P.; Vrancken, K. C. Characterisation and Chemical Modification of the Silica Surface; Elsevier: The Netherlands, 1995. (8) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (9) Morea, J.; Wong Chi Man, M. Mater. Res. Soc. Symp. Proc. 1998, 519, 41. (10) Brunel, D. Microporous Mesoporous Mater. 1999, 27, 329. (11) Brunel, D. Spec. Publ.sR. Soc. Chem. 2001, 266, 38. (12) Price, P. M.; Clark, J. H.; Macquarrie, D. J. J. Chem. Soc., Dalton Trans. 2000, 101. (13) Haas, K. H.; Amberg-Schwab, S.; Ballweg, T. AdV. Sci. Technol. 2003, 31B, 581. (14) Tavlarides, L. L.; Lee, J. S. Ion Exch. SolVent. Extr. 2001, 14, 169. (15) Guizard, C.; Bac, A.; Barboiu, M.; Hovnanian, N. Mol. Cryst. Liq. Cryst. Sci. Technol., A 2000, 354, 91.

sensors,15-19 electrochemistry,19-23 optics,5,24,25 or biology.5,19,26-28 Of special interest are the ordered mesoporous materials synthesized by using surfactants as templates,29-33 as their regular structure made of mesopore channels of uniform size and their very high specific surface area offer great accessibility34-38 and (16) Collinson, M. M. Trends Anal. Chem. 2002, 21, 30. (17) Collinson, M. M. Crit. ReV. Anal. Chem. 1999, 29, 289. (18) Bescher, E.; Mackenzie, J. D. Mater. Sci. Eng., C 1998, 6, 145. (19) Wang, J. Anal. Chim. Acta 1999, 399, 21. (20) Lev, O.; Wu, Z.; Bharathi, S.; Glezer, V.; Modestov, A.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 9, 2354. (21) Walcarius, A. Electroanalysis 1998, 10, 1217. (22) Walcarius, A. Electroanalysis 2001, 13, 701. (23) Walcarius, A. Chem. Mater. 2001, 13, 3351. (24) Sanchez, C.; Ribot, F.; Lebeau, B. J. Mater. Chem. 1999, 9, 35. (25) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J.-P. AdV. Mater. 2003, 15, 1969. (26) Avnir, D. Acc. Chem. Res. 1995, 28, 328. (27) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (28) Pierre, A. C. Biocatal. Biotransform. 2004, 22, 145. (29) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (30) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (31) Boury, B.; Corriu, R. J. P. In Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U. K., 2001; Vol. 3, pp 565-640. (32) Asefa, T.; Ozin, G. A.; Grondey, H.; Kruk, M.; Jaroniec, M. Stud. Surf. Sci. Catal. 2002, 141, 1. (33) Lebeau, B.; Patarin, J.; Sanchez, C. AdV. Technol. Mater. Mater. Process. J. 2004, 6, 298. (34) Mercier, L.; Pinnavaia, T. J. AdV. Mater. 1997, 9, 500. (35) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemmer, K. M. Science 1997, 276, 923. (36) Bezombes, J.-P.; Chuit, C.; Corriu, R. J. P.; Reye, C. J. Mater. Chem. 1999, 9, 1727. (37) Brown, J.; Richer, R.; Mercier, L. Microporous Mesoporous Mater. 2000, 37, 41. (38) Walcarius, A.; Etienne, M.; Delacote, C. Anal. Chim. Acta 2004, 508, 87.

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fast access39-42 to a high number of organofunctional groups with improved performance in comparison to the corresponding amorphous gels.34,38,41,43-45 In recent years, organic-inorganic hybrid materials displaying cation46-50 or anion51-59 exchange properties have been developed to try to overcome some drawbacks associated with the commonly used ion-exchange resins (limited surface areas, chemical and thermal instability in harsh conditions, swelling, hydrophobicity of the polymer backbone).53,57,60,61 Cation exchangers were mostly based on silica gels or mesoporous silicas functionalized with sulfonate46-50,62-64 or carboxylate65 moieties. Anion exchangers were made of quaternary ammonium salts immobilized by covalent binding to silica gel particles54-56,59,66 or membranes51,58 or within mesostructured particles53,57,67-69 or films,70-72 which are expected to ensure long-term stability in comparison to solgel doping with, i.e., long-chain quaternary ammonium salts.73 An alternative can be achieved with protonated N-bearing groups (i.e., aminopropyl, propyl-ethylenediamine, pyridine derivatives) that have been successfully bonded to various silica frameworks,38,52,54,68,74 but great care should be taken to use these (39) Bibby, A.; Mercier, L. Chem. Mater. 2002, 14, 1591. (40) Walcarius, A.; Etienne, M.; Bessie`re, J. Chem. Mater. 2002, 14, 2757. (41) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161. (42) Walcarius, A.; Delacote, C. Chem. Mater. 2003, 15, 4181. (43) Brown, J. S.; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 69. (44) Im, H.-J.; Barnes, C. E.; Dai, S.; Xue, Z. Microporous Mesoporous Mater. 2004, 70, 57. (45) Etienne, M.; Delacoˆte, C.; Walcarius, A. In Progress in Electrochemistry Research; Columbus, F., Ed.; Nova Science Publishers: Hauppauge, New York, 2005; in press. (46) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467. (47) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448. (48) Clark, J. H.; Elings, S.; Wilson, K. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3, 399. (49) Ogawa, M.; Kuroda, K.; Nakamura, T. Chem. Lett. 2002, 31, 632. (50) Hamoudi, S.; Kaliaguine, S. Microporous Mesoporous Mater. 2003, 59, 195. (51) Kogure, M.; Ohya, H.; Paterson, R.; Hosaka, M.; Kim, J.-J.; McFadzean, S. J. Membr. Sci. 1997, 126, 161. (52) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z.; Ferris, K. F.; Mattigod, S.; Gong, M.; Hallen, R. T. Chem. Mater. 1999, 11, 2148. (53) Ju, Y. H.; Webb, O. F.; Dai, S.; Lin, J. S.; Barnes, C. E. Ind. Eng. Chem. Res. 2000, 39, 550. (54) Markowitz, M. A.; Deng, G.; Gaber, B. P Langmuir 2000, 16, 6148. (55) Tien, P.; Chau, L.-K.; Shieh, Y.-Y.; Lin, W.-C.; Wei, G.-T. Chem. Mater. 2001, 13, 1124. (56) de Campos, E. A.; da Silva Alfaya, A. A.; Ferrari, R. T.; Costa, C. M. M. J. Colloid Interface Sci. 2001, 240, 97. (57) Lee, B.; Bao, L.-L.; Im, H. J.; Dai, S.; Hagaman, E. W.; Lin, J. S. Langmuir 2003, 19, 4246. (58) Touzi, H.; Sakly, N.; Kalfat, R.; Sfihi, H.; Jaffrezic-Renault, N.; Rammah, M. B.; Zarrouk, H. Sens. Actuators, B 2003, 96, 399. (59) Li, W.; Fries, D.; Alli, A.; Malik, A. Anal. Chem. 2004, 76, 218. (60) Biernat, J. F.; Konieczka, P.; Tarbet, B. J.; Bradshaw, J. S.; Izatt, R. M. Sep. Purif. Methods 1994, 23, 77. (61) Varshney, K. G. Diffus. Defect Data, Pt. B 2003, 90-91, 445. (62) Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. Chem. Commun. 1998, 317. (63) Wilson, K.; Lee, A. F.; Macquarrie, D. J.; Clark, J. H. Appl. Catal., A 2002, 228, 127. (64) Ganesan, V.; Walcarius, A. Langmuir 2004, 20, 3632. (65) Hsueh, C. C.; Collinson, M. M. J. Electroanal. Chem. 1997, 420, 243. (66) Xiao, H.; Cezar, N. J. Colloid Interface Sci. 2003, 267, 343. (67) Che, S.; Garcia-Bennett, A. E.; Tokoi, T.; Sakamoto, K.; Kunieda, H.; Terasaki, O.; Tatsumi, T. Nature Mater. 2003, 2, 801. (68) Garcia-Bennett, A. E.; Terasaki, O.; Che, S.; Tatsumi, T. Chem. Mater. 2004, 16, 813. (69) Chao, K.-J.; Cheng, M.-H.; Ho, Y.-F.; Liu, P.-H. Catal. Today 2004, 97, 49. (70) Wong, E. M.; Markowitz, M. A.; Qadri, S. B.; Golledge, S. L.; Castner, D. G.; Gaber, B. P Langmuir 2002, 18, 972. (71) Markowitz, M. A.; Wong, E. M.; Gaber, B. P Stud. Surf. Sci. Catal. 2002, 141, 213. (72) Wong, E. M.; Markowitz, M. A.; Qadri, S. B.; Golledge, S.; Castner, D. G.; Gaber, B. P J. Phys. Chem. B 2002, 106, 6652. (73) Levy, D.; Kuyavskaya, B. I.; Gogozin, I.; Zamir, I.; Ottolenghi, M.; Avnir, D.; Lev, O. Sep. Sci. Technol. 1992, 27, 589. (74) Kaneko, Y.; Iyi, N.; Kurashima, K.; Matsumoto, T.; Fujita, T.; Kitamura, K. Chem. Mater. 2004, 16, 3417.

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material in strongly acidic media because of their rather poor chemical stability in aqueous alkaline, neutral, and even slightly acidic solutions.75 Few organosilica ion exchangers have been involved in electrochemistry.38,49,64,65,76-81 The Collinson group has prepared sol-gel films functionalized with either ammonium or carboxylate groups exhibiting permselective properties at electrode surfaces.65,76 Extending this approach to sol-gel coatings modified with quaternary ammonium moieties led to possible preconcentration of anions while rejecting cations,77,78 consistent with related works with organoclay film modified electrodes.82 Similar accumulation of electroactive anions has been reported for electrodes modified with mesoporous silica functionalized with amine groups in their protonated form.38 Just the opposite was observed for electrodes modified with sulfonic acidfunctionalized solids for which cation preconcentration was promoted.49,64,79 Of related interest is the electrochemical monitoring of mass-transport rates in silica-based materials functionalized with ammonium or quaternary ammonium groups.80,81 In a previous paper, we have investigated the cation-exchange64 characteristics exhibited by mesoporous silicas functionalized with sulfonic acid moieties and pointed out the critical influence of the organofunctional group contents on their structure and applications in ion-exchange voltammetry. In the present work, we have examined the anion-exchange and charge-selective properties of various silica-based organic-inorganic hybrid materials containing quaternary ammonium groups covalently attached to the solid framework alone or together with sulfonic acid moieties. The ammonium-functionalized solids were prepared by co-condensation of tetraethoxysilane (TEOS) and N-((trimethoxysilyl)propyl)-N,N,N-trimethylammonium chloride (TMTMAC) precursors in the presence of a surfactant template and by using ammonia as the catalyst to get particles with a spherical shape.83 Although a positively charged surfactant is probably not the best choice to obtain highly ordered structures with positively charged groups in the mesopores,67,68 our goal was here to be as close as possible to our previous work on sulfonic acid-functionalized materials. Some other silica samples (either amorphous gels or MCM-41 type) were grafted with the same quaternary ammonium groups and used for comparison purposes. The physicochemical properties and the ion-exchange capacity of the materials were determined and discussed with respect to the density of functional groups. Cyclic voltammetry was also used to describe their preconcentration behavior once incorporated into carbon paste electrodes. In addition, bifunctional hybrid materials were prepared by grafting various quantities of TMTMAC on/within sulfonic acid-functionalized mesoporous silicas synthesized according to our previous report.64 These porous solids were also characterized by various physicochemical techniques, and special attention was directed to point out the eventual existence of ion pairs in the hybrid materials between the two functions of opposite charge. The effect of the simultaneous presence of the two different organofunctional groups was then studied by ion-exchange voltammetry by using Fe(CN)63- and Ru(NH3)63+ as redox probes. (75) Etienne, M.; Walcarius, A. Talanta 2003, 59, 1173. (76) Wei, H.; Collinson, M. M. Anal. Chim. Acta 1999, 397, 113. (77) Lin, W.-C.; Tien, P.; Chau, L.-K. Electrochim. Acta 2004, 49, 573. (78) Liu, A.; Zhou, H.; Honma, I. Electrochem. Commun. 2005, 7, 1. (79) Aylward, W. M.; Pickup, P. G. J. Solid State Electrochem. 2004, 8, 742. (80) Walcarius, A.; Delacote, C.; Sayen, S. Electrochim. Acta 2004, 49, 3775. (81) Kanungo, M.; Collinson, M. M. Langmuir 2005, 21, 827. (82) Tonle, I. K.; Ngameni, E.; Walcarius, A. Electrochim. Acta 2004, 49, 3435. (83) Etienne, M.; Lebeau, B.; Walcarius, A. New. J. Chem. 2002, 26, 384.

Quaternary Ammonium-Functionalized Silica Microspheres

2. Experimental Section Synthesis of Trimethylammonium Chloride (TMAC) Functionalized Silicas. The anion exchangers were prepared by cocondensation of TMTMAC (50% in methanol, ABCR, Germany) and TEOS (>98%, Merck) precursors in hydro-alcoholic medium by using cetyltrimethylammoniun bromide (CTAB, 98%, Fluka) as a template and ammonia (28% aqueous, Prolabo) as the catalyst. The procedure involves typically the dissolution of 2.4 g CTAB in a solution of 50 mL of deionized water, 45 mL of ethanol, and 13 mL of 28% ammonia. Suitable amounts of TMTMAC and TEOS (18.3 mmol, in various ratios) were then dissolved in 5 mL of ethanol and added to the surfactant solution. The resulting medium was stirred for 2 h. The white precipitate was then filtered off, washed with ethanol, and dried under vacuum ( 90% for amino- or mercaptopropyl groups83). After template removal, the N2 adsorption/desorption isotherms measured on the 5%-TMAC sample were clearly of type IV (Figure 1B), characteristic of mesoporous materials. Their BET analysis revealed a significant decrease of both the specific surface area and pore volume in comparison to the pure silica (not functionalized) prepared according to the same protocol (Table 1) with, however, an increase in pore size from 2.5 (MCM41) to 3.1 nm (5%-TMAC). This can be rationalized again in terms of TMAC-template repulsion, the TMAC groups occupying some place outside the template micelles. Increasing the TMTMAC/TEOS ratio above 5% in the starting sol resulted in a dramatic decrease of the ordering level (the 15%-TMAC and 20%-TMAC materials being totally amorphous) for the aforementioned reasons of unfavorable TMAC-template interaction. Nevertheless, all these hybrids were characterized by TMAC incorporation levels higher than 70%, in agreement with previously reported TMAC-functionalized mesoporous silica thin films.70 All TMAC moieties were counterbalanced by the presence of Cl- species, as confirmed by elemental N and Cl

analyses revealing a 1:1 N/Cl stoichiometry (Table 1). This1:1 N/Cl stoichiometry and the absence of measurable bromide species also account for effective removal of the template (i.e., no or very low amount of CTAB in the extracted materials). As expected from the space occupied by Pr-N(Me)3+-Cl- moieties in the functionalized materials, both the specific surface area and pore volume were found to decrease when increasing the organofunctional group content (Table 1). These groups were homogeneously distributed in the whole volume of the hybrid materials, as pointed out by the good agreement between the results of N content measurements performed by elemental analysis (i.e., bulk characterization) and X-ray photoelectron spectroscopy (XPS) via the N1s line at 402 eV (i.e., surface analysis) for samples 5%-TMAC and 10%-TMAC (Figure 2). For the nonordered 15%-TMAC and 20%-TMAC samples, however, some enrichment by TMAC groups was observed on the surface of the materials (N content by XPS > N content from elemental analysis, see Figure 2), indicating some restrictions in the incorporation of these positively charged groups in the bulk due to electrostatic repulsion induced by the CTAB template. Some TMAC-grafted silica samples (a mesoporous MCM-41 and two silica gels of different pore size) were also prepared for comparison purposes. In this case, the amount of organofunctional groups in the materials was only limited by the specific surface area of the pristine silica sample, so that it is not surprising to

Quaternary Ammonium-Functionalized Silica Microspheres

Figure 2. Variation of the nitrogen content in propyl-trimethylammonium-functionalized silica materials as a function of the relative molar ratio of TMTMAC (n%) in the starting sol (n ) {[TMTMAC]/ ([TEOS] + [TMTMAC])} × 100), as measured from elemental analysis (O, right axis) or by XPS (9, left axis).

observe a higher TMAC content in the MCM-41 sample than in the amorphous silica gels (Table 1). Ion-exchange capacities of TMAC-functionalized materials have been determined for two anions: the monovalent ClO4and the divalent SO42-, in large excess (> 100 times) over the amount of anion-exchanging sites (TMAC). The results have been gathered in Table 1, and they show dramatic influence of the anion charge and significant effects of both the structure of the materials and their functionalization level. Ion-exchange capacities (expressed in meq g-1 to allow easier comparison between mono- and divalent anions) for ClO4- were always larger than that for SO42- despite the usually higher affinity of divalent species over the monovalent ones for the anion exchangers.85 This is most probably due to unfavorable charge distribution in the material (one SO42- is expected to require two N(Me)3+ sites located closely enough to each other), but also possibly due to the smaller size of ClO4- in comparison to that of SO42- (ClO4- could penetrate the smallest pores while SO42cannot). Increasing the functionalization level in the n%-TMAC materials caused the ion-exchange capacity to decrease relative to the calculated maximum one (on the basis of the N(Me)3+ groups content): from 80% filling to 70% exchange when passing from 5%-TMAC to 20%-TMAC for ClO4-, and from 60% to 41% for SO42- (Table 1). This is explained by poorer structural order and lower porosity for materials containing higher TMAC group contents. Grafted samples behaved similarly (Table 1), i.e., better accessibility to the ion-exchange sites for TMACgrafted silica gels of larger porosity (84% for ClO4- in the 5.3 nm pore-sized G-60-TMAC sample in comparison to only 76% filling of the 3.4 nm pore-sized K-40-TMAC by the same ClO4anion) and higher ion-exchange capacity for the well-ordered materials (86% of the ion-exchange sites occupied by ClO4species in the MCM-41-TMAC sample displaying a pore size of only 2.2 nm). 3.1.2. Ion-Exchange Voltammetry. Ion-exchange voltammetry (IEV)86 is an elegant technique to characterize the ion-exchange properties of various materials, including both organic86 and inorganic87 films coated on solid electrode surfaces or powdered (85) Helfferich, F. Ion Exchange; Dover Publications: New York, 1995. (86) Ugo, P.; Moretto, L. M. Electroanalysis 1995, 7, 1105.

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Figure 3. Cyclic voltammograms recorded in solutions containing either (A) 5 × 10-5 M K3Fe(CN)6 (in 0.01 M KCl) or (B) 5 × 10-5 M Ru(NH3)6Cl3 (in 0.01 M NaNO3) using carbon paste electrodes containing (a) the unmodified MCM-41 silica and (b) the 5% TMAC mesoporous silica sample. Scan rate: 100 mV s-1. All the curves were recorded in multisweep conditions and did not vary upon the multiple potential scans, except those of part (A, b) for which the 1st, 5th, 10th, 15th, and 20th curves have been depicted.

ion exchangers dispersed in a conductive composite electrode matrix.64,88 It is based on the application of multisweep cyclic voltammetry (CV) in a solution containing a suitable redox probe, usually at a rather low concentration (90%) of the electrode surface area was not covered with the 5%-TMAC materials and thus was still available for the electrochemical reactions to occur. As preconcentration of Fe(CN)63- species in 5%-TMAC particles and their subsequent electrochemical detection at the modified carbon paste electrode are dependent on the ion exchanger properties (active sites content, accessibility to external reagents, speed of mass transfer processes in such confined medium), one can expect some variations in the electrode response as a function of the physicochemical characteristics of the materials used to modify the carbon paste. This is notably illustrated in Figure 4, depicting the maximum peak currents (stationary values) sampled in multisweep cyclic voltammetry when using carbon paste electrodes modified with the TMACfunctionalized materials described in Table 1. As shown in the n%-TMAC series, increasing the functionalization level (theo(89) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (90) Walcarius, A. In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003; Chapter 14, pp 721-783.

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Figure 4. Maximum peak currents sampled by cyclic voltammetry after 20 successive potential scans in 5 × 10-5 M K3Fe(CN)6 using carbon paste electrodes modified with the various TMAC silica samples. Other conditions as in Figure 3.

retical ion-exchange capacity) did not result in any increase in peak currents but rather in a significant decrease in the electrode sensitivity. When operating in conditions where the voltammetric data were always obtained under diffusion control, the determining factor is thus not the amount of ion exchanging sites in the electrode modifier, but should be discussed in terms of kinetic limitations. Indeed, the electrode sensitivity was found to be less when using the less-ordered and less-porous materials (i.e., 15%TMAC or 20%-TMAC samples) in comparison to the 5%-TMAC and 10%-TMAC particles, displaying a higher level of structural order, larger pore volume, and specific surface area, ensuring thereby faster mass-transfer rates and, consequently, more effective Fe(CN)63- preconcentration and detection. This tendency was also observed when using electrodes modified with the grafted materials (Figure 4), except that the electrode comprising the MCM-TMAC sample exhibited an unexpectedly low signal in comparison to those recorded from electrodes prepared with particles of the co-condensed n%-TMAC series. This can be rationalized by taking into account the fact that the grafting process applied to functionalize ordered mesoporous materials gives rise to organofunctional groups not uniformly distributed on the silica walls (i.e., more concentrated at the pore entrance), while the same hybrid materials prepared by the direct assembly pathway are characterized by a regular distribution of these groups in the whole volume of the mesostructure.90-93 So, even if n%-TMAC materials suffer from some lack of longrange structural order in comparison to the MCM-41-TMAC one displaying a regular hexagonal packing of mesopores, the uniform distribution of the N(Me)3+ groups in the mesoporous exchangers obtained by the co-condensation route ensures good accessibility and fast mass transfer for the ion-exchange preconcentration of Fe(CN)63- probes. Such effective preconcentration behavior and the high sensitivity of the associated voltammetric detection make these promising for applications in the field of electrochemical sensors. Another evidence sustaining these fast accumulation processes is the fact that the first CV curve recorded directly after immersion of the electrode in the diluted Fe(CN)63- solution was always characterized by (91) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285. (92) Mercier, L.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 188. (93) Corriu, R. J. P.; Lancelle-Beltran, E.; Mehdi, A.; Reye´, C.; Brande`s, S.; Guilard, R. J. Mater. Chem. 2002, 12, 1355.

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peak heights more than half of the steady-state maximum current values recorded (usually) after 20 successive potential scans. This points out again the interest of the template route to get mesoporous ion exchangers directed to electroanalytical applications, as they provide faster response time and higher sensitivity than their unordered amorphous analogues. For instance, ion-exchange voltammetry applied to sulfonated organically modified silica hydrogels has revealed steady-state currents (for Ru(NH3)63+) obtained only after 5 h of continuous potential cycling,79 while the same experiment performed with ordered materials of comparable composition gave rise to steadystate values after only ca. 15 min.64 One should finally mention that when operating under thin-layer-like conditions (i.e., at the very low scan rate of 0.1 mV s-1), all the ion-exchanged redox probes are likely to participate to the charge-transfer processes, and the integration of voltammetric peak currents were indeed directly proportional to the ion-exchange capacities reported in Table 1. 3.2. Bifunctionalized Silica Materials (Quaternary Ammonium and Sulfonate). In a second step, to complete the results presented above for anion exchange in TMAC-functionalized mesoporous materials and our previously published report on the cation-exchange properties of sulfonated mesoporous silicas,64 we provide hereafter an investigation of the ion-exchange voltammetric response of electrodes modified with mesoporous materials functionalized with both quaternary ammonium and sulfonate moieties. 3.2.1. Physicochemical Characterization: EVidence to Support Zwitterionic Species. All solids were prepared by co-condensation of MPTMS and TEOS at variable relative ratios (subsequently oxidized into sulfonate moieties64), which were then grafted with variable amounts of TMAC groups. Two sets of bifunctionalized materials have been obtained: the first one is based on a 1:1 sulfonate-to-ammonium ratio with variable organofunctional group contents (ranging from 0.1 to 2.0 mmol g-1), and the second series corresponds to solids containing 2.0 mmol g-1 of sulfonate groups with variable amounts of grafted TMAC groups (leading to sulfonate-to-ammonium ratios of 0.05, 0.20, 0.50, 0.75, and 1.00). The amounts of the respective of N(Me)3+ and SO3- groups have been determined by elemental N and S analyses. It was reported previously that increasing the amount of SO3groups in mesoporous silicas led to the continuous decrease of their porosity.64 As expected from the space occupied by the Pr-N(Me)3+ groups, it is not surprising that this decrease in porosity was even more important when the sulfonated mesoporous silicas were grafted with these propylammonium groups. For example, a mesoporous silica sample containing 2.0 mmol g-1 of sulfonate groups, initially characterized by a pore volume of 0.27 cm3 g-1, fell to 0.12 cm3 g-1 upon grafting 0.5 mmol g-1 of TMAC groups, and to less than 0.02 cm3 g-1 when containing 1.5 mmol g-1 TMAC. Similar variations were observed in both the specific surface area and pore volume for the hybrid materials based on a 1:1 sulfonate-to-ammonium ratio when increasing the functionalization level from 0.1 to 2.0 mmol g-1 of both N(Me)3+ and SO3- groups (Figure 5). All bifunctionalized samples have been analyzed by XPS and some typical results are depicted in Figure 6 for solids containing 2.0 mmol g-1 of sulfonate groups with increasing amounts of grafted TMAC groups (final materials with relative N(Me)3+/ SO3- ratios between 5 and 100%). A first observation can be made from the contribution of the S2p signal (shown at the left of part A in Figure 6), indicating the successful oxidation of thiol groups into sulfonic acid moieties (major contribution of the S2p line at 168 eV and minor at 163 eV, corresponding to -SO3H

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Figure 5. Variation of the specific surface area and the total pore volume (inset) of quaternary ammonium and sulfonic acidbifunctionalized silica materials, as a function of the organofunctional groups content (x mmol -NMe3 + x mmol -SO3H per gram of material).

and -SH groups, respectively63,64). Although the S content in the materials was the same for all samples (bulk composition), the S2p signal upon grafting N(Me)3+ groups seemed to decrease, especially at high N(Me)3+/SO3- ratios. This indicates (once again90-92) that the grafting process led to the presence of an excess of the organofunctional groups (here, TMAC) at the entrance of the mesopore channels, while the co-condensation approach resulted in a more uniform distribution of the groups (here, sulfonate) in the whole mesoporous volume. This observation is also corroborated by the variation in the N1s signal, which was anomalously high for materials characterized by N(Me)3+/SO3- ratios higher than 50% (Figure 6B). Another very attractive feature can be pointed out via the Cl2p/N1s signal ratio. This ratio was always equal to unity for all the monofunctionalized TMAC-based materials because all the N(Me)3+ groups were counterbalanced by Cl- species. This was also more or less true for the bifunctionalized solids containing large amounts of TMAC groups (i.e., mostly present at the external boundaries of the particles), as shown in Figure 6 (spectra b in part A and right portion of plot B), but this was no more the case of samples containing less N(Me)3+ groups for which the Cl- content was by far below that of N(Me)3+ (see spectra a in part A and left portion of plot B in Figure 6). This strongly suggests that a significant portion of these N(Me)3+ groups (>50%) are no longer counterbalanced by Cl- species, but should be in electrostatic interaction with the sulfonic acid (sulfonate) moieties located in close vicinity in the mesopore channels. This series of bifunctionalized materials thus displays zwitterionic properties via the existence of N(Me)3+, SO3- centers. The second series of hybrids based on a 1:1 sulfonate-toammonium ratio with variable organofunctional groups contents (ranging from 0.1 to 2.0 mmol g-1) behaved similarly. The N(Me)3+, SO3- zwitterions were especially visible by XPS on samples of low functionalization levels (i.e., Cl2p/N1s signal ratios of about 0.5 or less for materials containing less than 0.7 mmol g-1 of organofunctional groups). This ability to form N(Me)3+, SO3- zwitterions by rejecting the Cl- counterions from TMAC out of the materials is probably driven by the presence of the N(Me)3+ and SO3- groups located close to each other in the confined medium of the mesopore channels. 3.2.2. Electrochemical Response. All the above materials displaying zwitterionic surfaces were incorporated in carbon paste

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Figure 6. XPS data obtained from sulfonic acid (2 mmol g-1) functionalized silica samples grafted with various amounts of TMAC. (A) Typical XPS spectra showing the N1s, S2p, and Cl2p lines for samples containing respectively (a) 50% and (b) 100% TMAC relative to the amount of -SO3H groups. (B) Variation of the XPS response of the N1s, S2p, and Cl2p lines (molar %) as a function of the amount of grafted TMAC groups relative to that of -SO3H moieties.

Figure 7. Cyclic voltammograms recorded in 5 × 10-5 M K3Fe(CN)6 (in 0.01 M KCl) using carbon paste electrodes containing (a) a 5% TMAC silica sample, (b) a 5% SO3H mesoporous silica, and (c) the same solid as in (b) but grafted with an amount of TMAC groups equal to that in sample (a). All the curves were recorded in multisweep conditions and the depicted ones represent the 20ths.

electrodes and characterized by multisweep ion-exchange voltammetry by using Fe(CN)63- and Ru(NH3)63+ as the redox probes. Again, a stationary response was obtained rather rapidly, with steady-state currents attained within the first 20 potential scans. As expected, both of these probes were likely to interact with these bifunctionalized materials, but the electrode response was greatly affected by both the functionalization level and the N(Me)3+/SO3- ratios. Figure 7 displays illustrative results obtained when using the Fe(CN)63- redox probe. Consistent with the anion-exchange behavior of the 5%-TMAC sample and the cation-exchange (anion exclusion) properties of the 5%-SO3H-MCM-41 material, significant accumulation of Fe(CN)63- species was achieved in the first case (with correspondingly high peak currents, see curve a in Figure 7), while no preconcentration was observed in the second case (see curve b in Figure 7). When using the zwitterionic 5%-TMAC-5%-SO3H-MCM-41 sample, the voltammetric experiment gave rise to an intermediate situation (see curve c in Figure 7), indicating an effective preconcentration behavior, but less important than that observed in the absence of the sulfonate

Figure 8. Variation of the cathodic peak currents recorded in multisweep cyclic voltammetry (20th scan) in 5 × 10-5 M K3Fe(CN)6 (+ 0.01 M KCl) using carbon paste electrodes containing quaternary ammonium and sulfonic acid-bifunctionalized silica materials, as a function of the organofunctional groups content (x mmol -NMe3 + x mmol -SO3H per gram of material).

groups in the material. There are at least two reasons to justify such an intermediate behavior: (1) the existence of N(Me)3+, SO3- zwitterions somewhat restricts the (an)ion-exchange capacity of the material with respect to foreign anions, and (2) the accessibility and access rates are lower in the less porous material (0.25 cm3 g-1 for 5%-TMAC-5%-SO3H-MCM-41 against 0.60 cm3 g-1 for 5%-TMAC), thus lowering the intensity of the voltammetric peaks. At this stage, it is difficult to state what parameter is the most important one in controlling the ionexchange voltammetric response, but both of them are expected to play a role, as suggested by the variation of peak currents sampled with electrodes containing bifunctionalized materials with variable organic groups contents (Figure 8). As shown, the voltammeric peaks were small at low functionalization levels; where the quantity of zwitterionic species is expected to be high and the number of ion-exchange sites low, they passed by a maximum at about 0.5 mmol g-1 of organofunctional groups, and finally, their intensity decreased at higher functionalization

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tration effectiveness when increasing the amount of N(Me)3+ groups in the sulfonate-functionalized cation exchanger because of both the formation of N(Me)3+, SO3- zwitterions and concomitant decrease in the material porosity. Finally, one has to mention that both anions and cations can be incorporated at the same time at the same modified electrode, giving rise to voltammetric signals characteristics of both Fe(CN)63- and Ru(NH3)63+ species, but no attempt was made in this work to describe the competitive effects that would result from such mixed systems.

4. Conclusions

Figure 9. Cyclic voltammograms recorded in 5 × 10-5 M Ru(NH3)6Cl3 (in 0.01 M NaNO3) using carbon paste electrodes containing 20% SO3H mesoporous silica samples (2.0 mmol -SO3H per gram of material): (a) as-synthesized, (b) same as (a) but grafted with 0.5 mmol g-1 TMAC, and (c) same as (a) but grafted with 1.0 mmol g-1 TMAC groups. All the curves were recorded in multisweep conditions and the depicted ones represent the 5ths.

levels corresponding to the less porous solids. A bell-shaped curve was also observed when plotting peak currents recorded in cyclic voltammetry when using electrodes modified with ion exchangers containing 2.0 mmol g-1 of sulfonate groups (with variable N(Me)3+/SO3- ratios) as a function of the amount of grafted TMAC groups. One can thus conclude that the sensitivity of the electrode toward the anionic redox probe is enhanced when increasing the amount of N(Me)3+ groups, decreasing the amount of N(Me)3+, SO3- zwitterions, and maintaining the material porosity as high as possible (i.e., lowest resistance to mass transport). This trend was also observed for the preconcentration/ voltammetric analysis of the positively charged Ru(NH3)63+ redox probe, but in this case, the effect of the N(Me)3+/SO3- ratio was opposite to that observed for Fe(CN)63- as SO3- moieties contribute to accumulate the probe cations while the N(Me)3+ centers reject them. An illustration is given in Figure 9 comparing the relative sensitivity of three carbon paste electrodes modified with mesoporous silica samples containing 2.0 mmol g-1 of SO3- groups alone (curve a), 2.0 mmol g-1 of SO3- and 0.5 mmol g-1 of N(Me)3+ groups (curve b), and 2.0 mmol g-1 of SO3- and 1.0 mmol g-1 of N(Me)3+ groups (curve c). The voltammetric curves clearly show a decrease of the preconcen-

Propyltrimethylammonium-functionalized mesoporous silica samples prepared in a single step by coupling the co-condensation and surfactant template routes display effective anion-exchange properties in aqueous medium. When incorporated in carbon paste electrodes, they enable the sensitive detection of negatively charged redox probes (i.e., Fe(CN)63-), owing to their preconcentration behavior prior to the voltammetric analysis. Despite their rather poor structural order and specific surface areas in the 200-700 m2 g-1 range, these surfactant templated materials gave rise to superior performance in comparison to the ion exchangers obtained by the postsynthesis grafting route, which is mainly due to better accessibility and faster mass transport in the mesopore channels of the wormhole-like structure. The bifunctionalized ion exchangers based on the incorporation of both sulfonate and quaternary ammonium moieties in mesoporous silicas were characterized by zwitterionic properties, most probably due to favorable electrostatic interactions between N(Me)3+ and SO3- groups in the confined medium of the mesopore channels. These materials were likely to accumulate both anions and cations by ion exchange. Their behavior in ionexchange voltammetry has, however, revealed lower performance in terms of sensitivity in the electrochemical response in comparison to materials containing only one type of organofunctional group (N(Me)3+ or SO3- alone). Acknowledgment. This work has been supported by the French Ministry of Foreign Affairs under the form of a postdoctoral fellow for one of us (V.G.). We would like to thank gratefully J. Cortot and J. Lambert (LCPMEsNancy) for help in CE and XPS experiments. We also acknowledge J.-P. Emeraux for recording XRD measurements and J. Ghanbaja for TEM images. LA051916S