Anal. Chem. 2000, 72, 5265-5271
Diffusion of Redox Probes in Hydrated Sol-Gel-Derived Glasses Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson*
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-3701
Cyclic voltammetry and chronoamperometry have been used to characterize the translational mobility of eight different redox probes trapped in hydrated silica gel monoliths and evaluate the extent of surface interactions. The redox probes, selected for their size and charge, were introduced into a silica sol prepared by the acid-catalyzed hydrolysis of tetramethoxysilane along with an ultramicroelectrode (r ) 13 µm) and a Ag/AgCl reference/ counter electrode. Relative changes in the magnitude of the Faradaic current, the half-wave potential, E1/2, and the apparent diffusion coefficient (D) were evaluated for each redox probe as the sol gelled, aged, and dried. Significant variations were observed in the diffusion rates between molecules of similar size and structure but varying ionic charge. Large shifts in the redox potential were also observed, with the direction of shift dependent on the entrapped reagent. These results demonstrate the importance of internal surface interactions versus surface confinement in wet and partially dried sol-gel glasses. The sol-gel process provides a versatile way to prepare porous silica glasses through the hydrolysis and condensation of silicon alkoxide precursors.1 In a typical procedure, tetramethoxysilane (or tetraethoxysilane) is mixed with water in a mutual solvent such as methanol followed by the addition of a suitable catalyst.1-5 As the sol becomes interconnected, a macroscopically rigid, hydrated gel is formed. Specific reagents such as proteins, organic dyes, and redox species can be trapped into this optically transparent, stable host matrix by simply adding them to the sol prior to its gelation. These materials have been used in numerous applications including solid-state electrochemical devices, chemical sensors, catalysts, and nonlinear and optic applications.2-7 Of utmost importance to these and many other applications is the extent to which the entrapped reagents maintain their chemical and physical properties when immobilized in this solid host. The silica gel matrix, while stable, is not a completely inert support. The surface of the pore walls contain several kinds of * To whom correspondence should be addressed: (phone) 785-532-1468; (fax) 785-532-6666; (e-mail)
[email protected]. (1) Brinker, J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1989. (2) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (3) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I; Gun, J. Anal. Chem. 1995, 67, 22A-30A. (4) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A-1127A. (5) Collinson, M. M. Crit. Rev. Anal. Chem. 1999, 29, 289-311 (6) Dunn, B.; Farrington, G. C.; Katz, B. Solid State Ionics 1994, 70/71, 3-10. (7) Dunn, B.; Zink, J. I. Chem. Mater. 1997, 9, 2280-2291. 10.1021/ac000910z CCC: $19.00 Published on Web 10/04/2000
© 2000 American Chemical Society
functional groups including siloxane (SiOSi), silanol (SiOH), siloxide (SiO-), and possibly unreacted silicon alkoxide groups (SiOCH3).7,8 In addition, the walls will be negatively charged (pI of silica ∼2)9 under most conditions. The degree of surface interactions between an entrapped dopant and the walls of the silica host and the extent of surface confinement can strongly affect the rotational and translational mobility of the entrapped guest and impact the overall performance of the sol-gel-based devices. The size, charge, and functionality of the entrapped species as well as the average pore size, pore connectivity, tortuosity, and interfacial polarity of the pore walls are important variables that need to be considered.7 During the past few years, there have been a number of studies aimed at elucidating and understanding surface confinement and surface interaction in porous sol-gel-derived glasses. Fluorescence spectroscopy, Raman spectroscopy, and time-dependent methods have been used to examine specific intramolecular interactions and monitor the reorientational movements and translational mobility of small molecules entrapped in sol-gelderived matrixes.7,10-16 These investigations have largely focused on thoroughly dried gels (e.g., xerogels). Since many applications involve the use of hydrated, or partially dried gels, there exists a strong need to study surface confinement and surface interactions in these materials as well. Electrochemical methodologies provide a promising approach for the characterization of inorganic and organic materials.17,18 Cyclic voltammetry and chronoamperometry, in particular, have been used to probe charge transfer and diffusion in polymeric structures such as agarose gels,19 poly(ether) films and melts,20-22 (8) Shen, C.; Kostic, N. M. J. Am. Chem. Soc. 1997, 119, 1304-1312. (9) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (10) Yi, J.; Jonas, J. J. Phys. Chem. 1996, 100, 16789-16793. (11) Wallen, S. L.; Nikiel, L.; Yi, J.; Jonas, J. J. Phys. Chem. 1995, 99, 1542115427. (12) Watson, J.; Zerda, T. W. Appl. Spectrosc. 1991, 45, 136-165. (13) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17-22. (14) Sieminska, L.; Zerda, T. W. J. Phys. Chem. 1996, 100, 4591-97. (15) Koone, N. D.; Zerda, T. W. J. Non-Crystalline Solids 1995, 183, 243-51. (16) Koone, N. D.; Guo, J. D.; Zerda, T. W. J. Non-Crystalline Solids 1997, 211, 150-7. (17) Murray, R. W. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992. (18) Inzelt, G. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 89-241. (19) Ciszkowska, M.; Guillaume, M. D. J. Phys. Chem. A 103, 5, 607-613. (20) Watanabe, M.; Longmire, M. L.; Murray, R. W. J. Phys. Chem. 1990, 94, 2614-2619. (21) Haas, O.; Velazquez, C. S.; Porat, Z.; Murray, R. W. J. Phys. Chem. 99, 1527915284.
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polyelectrolytes such as Nafion,23 and to a limited extent solgel-derived solids.24-29 In rigid solids, the use of ultramicroelectrodes affords many advantages relative to commonly used large macroscopic electrodes for the measurement of diffusion.29 Their small size minimizes cracking of the gel upon drying, their reduced double-layer capacitance enables high sweep rates to be employed allowing the gel-electrode interface to be probed, and they provide a means for the calculation of diffusion coefficients without prior knowledge of solution concentration.30-32 In this work, ultramicroelectrodes have been incorporated into a silica gel monolith and used to measure the diffusion coefficient of gel-entrapped redox probes during the sol-gel transformation. This study is a continuation of previous work in our laboratory that showed vastly different behavior in the rate of diffusion of gel-entrapped potassium ferricyanide relative to gel-entrapped ferrocenemethanol.29 In the current investigation, eight redox species of varying size, structure, and charge were selected for study to more thoroughly examine diffusion in these partially dried, hydrated gels. By directly comparing diffusion rates and shifts in the half-wave potentials of structurally and chemically different redox species, surface confinement and interfacial interactions between the immobilized reagent and the walls of the silica gels can be elucidated. EXPERIMENTAL SECTION Reagents. Tetramethyl orthosilicate (TMOS, 99%), ferrocenemethanol (FcCH2OH), ferrocenedimethanol (Fc(CH2OH)2), and ammonium hexafluorophosphate were purchased from Aldrich. Hydrochloric acid, potassium ferricyanide, potassium chloride, and methanol were purchased from Fisher Scientific. Dicyanobis(1,10phenanthroline)iron(II), (Fe(CN)2(phen)2), was obtained from Sigma Chemical Co. Ferrocenylmonocarboxylic acid (FcCOOH) and ferrocenylmethyltrimethylammonium iodide (FcN+I-) were purchased from Strem. All reagents were used as received. Potassium octacyanomolybdate(IV) was provided by Professor Richard M. Crooks at Texas A&M University. Cobalt(II) tris(bipyridine) was synthesized according to literature procedures.33 Ferrocenylmethyltrimethylammonium hexafluorophosphate (Fc(22) Longmire, M. L.; Watanabe, M.; Zhang, H.; Wooster, T. T.; Murray, R. W. Anal. Chem. 1990, 62, 747-752. (23) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 685-689. (24) (a) Zhang, Y. Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1991. (b) Oliver, B. N.; Coury, L. A.; Egekeze, J. O.; Sosnoff, C. S.; Keller, C.;. Umana, M. X In Biosensor Technology. Fundamentals and Applications; Buck, R. P., Hatfield, W. E., Umana, M., Bowden, E. F., Eds.; Marcel Dekker: New York, 1990. (25) (a) Audebert, P.; Griesmar, P.; Sanchez, C. J. Mater. Chem. 1991, 1, 699700. (b) Audebert, P.; Griesmar, P.; Hapiot, P.; Sanchez, C. J. Mater. Chem. 1992, 2, 1293-1300. (26) Cattey, H.; Audebert, P.; Sanchez, C.; Hapiot, P. J. Phys. Chem. B 1998, 102, 1193-1202. (27) Cox, J. A.; Wolkiewicz, A. M.; Kulesza, P. J. J. Solid State Electrochem. 1998, 2, 247-252. (28) Holmstrom, S. D.; Karwowska, B.; Cox, J. A.; Kulesza, P. J. J. Electroanal. Chem. 1998, 456, 239-243. (29) Collinson, M. M.; Zambrano, P. J.; Wang, H.; Taussig, J. S. Langmuir 1999, 15, 662-668. (30) Wightman, R. M.; Wipf, D. O. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol 15. (31) Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A.; Lefrou, C.; Rollin, Y. J. Electroanal. Chem. 1990, 288, 45-63. (32) Denuault, G.; Mirkin, M. V.; Bard, A. J. J. Electroanal. Chem. 1991, 308, 27-38. (33) Burstall, F. H.; Nyholm, R. S. J. Chem. Soc. 1952, 3570-3579.
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N+PF6-) was precipitated from FcN+I- and NH4PF6 in aqueous solution.24 Water was purified to type I using a Labconco Water Pro PS four-cartridge system. Procedures. The preparation of platinum microdisk electrodes has been described previously.29,30 The Ag/AgCl reference electrodes were prepared by electrochemically depositing AgCl on a clean silver wire followed by aging for a least 1 day. The glassencased microelectrode and the Ag/AgCl reference electrode were secured in a nylon cap with epoxy. The microelectrode was gently polished with 1500-grit sandpaper and thoroughly rinsed with water and ethanol prior to use. The silica sol was prepared by combining TMOS with water, methanol, and 0.01 M hydrochloric acid in a mole ratio of 1:10: 3:2 × 10-4 Si/H2O/MeOH/HCl. The redox species and the supporting electrolyte (KCl) were added to the sol. Unless otherwise noted, the concentration of the redox species and KCl in the sol was initially 5 mM and 0.15 M, respectively. The doped sol was stirred for at least 20 min and then poured into a 5-dram polystyrene vial in 10-mL aliquots. The microelectrode assembly was inserted into the sol, and the level of the sol in the vial was adjusted so that each electrode was immersed ∼4 mm into the sol. A Parafilm plug was inserted into a 3-mm-diameter hole previously drilled in the nylon cap, and the vial was secured inside a Faraday cage. Gelation typically occurred within 12 h. After 3 days of aging, the Parafilm plug was removed from the cap and the gels were slowly dried under a high humidity environment (60-70% relative humidity) typically for 3-5 weeks. In the case of FcCOOH, a slight modification was made to the procedure described due to the insolubility of this complex in pure water or methanol. The sol was prepared using a mole ratio of 1:4:3:2 × 10-4 Si/H2O/MeOH/HCl. KCl was added so that its final concentration in the sol was 0.15 M. The redox species was dissolved in phosphate buffer (0.3 M, pH 7). The buffer solution was added to the sol after the electrode was inserted. A gel formed within a few minutes after mixing due to the high pH. The Si/ H2O ratio in the final sol was 1:14. Instrumentation. Cyclic voltammetry (CV) and chronoamperometry (CA) were performed with a BAS 100-W potentiostat equipped with a model PA-1 preamplifier. The electrochemical data were collected at regular intervals as samples aged and dried. N2 adsorption-desorption isotherms were acquired 2-3 months after preparation using a Quantachrome Autosorb 1MP analyzer with an equilibration time of 3 min. Prior to analysis, the hydrated gels were dried overnight in a vacuum oven followed by drying under vacuum at 40 °C for 12-24 h to thoroughly remove moisture. The average pore diameter of the resultant thoroughly dried gel (e.g., xerogel), which was about one-eighth the size of the original hydrated gel, was determined to be ∼40-50 Å from the desorption branch using the BJH model. Because the gels had to be thoroughly dried to make this measurement, these values underestimate the pore size of the hydrated gels used herein but can be used as a comparison to other work with similar solids. Thermogravimetric data were also collected using a Shimadzu TGA-50H analyzer unit using a scan from about 25 to 400 °C at 10 °C/min. RESULTS The solid-state electrochemical cell used in this work consists of a 13.3-µm Pt ultramicroelectrode and a Ag/AgCl reference
Figure 1. Relative change in the mass of the gel during aging and drying.
electrode imbedded in the doped silica sol/gel housed in a polystyrene container.29 This assembly was securely mounted in a Faraday cage to ensure that the gel-electrode remains intact for an extended length of time. The gels were aged for 3 days and then dried for 3-5 weeks under a high-humidity environment during which time the electrochemical data were obtained for the encapsulated redox probes. A plot of the average change in the mass of three gels during aging and drying is shown in Figure 1 for gel-encapsulated potassium ferricyanide. Similar results were obtained for the gels containing the other redox probes. As can be seen from the change in mass, drying begins once the Parafilm plug is removed from the sample vial. After 60-90 days, the aged and dried gels had a solvent (water and methanol) content of 5565% as determined from thermogravimetric analysis. During drying, the gel structure contracted and pulled away from the sides of the polystyrene vial. The volume of the gel changed by a factor of 2 during this time period. No visible cracks were observed in the gels over this time frame as long as the gels were dried slowly and a small electrode assembly was used.29 The voltammetry of eight redox species immobilized into the silica gel glasses were examined as the sol gelled, aged, and dried. These reagents, selected for their size and charge, can be broken up into three main groups: (a) negatively charged redox probes (e.g., Fe(CN)63-/4-, Mo(CN)84-/3-, and [FcCOO-]0/+), (b) neutral or positively charged redox probes (e.g., FcCH2OH0/+1, Fc(CH2OH)20/+, and [FcN+]0/+), and (c) large redox probes (e.g., Fe(CN)2(phen)20/+ and Co(bpy)32+/3+). Representative cyclic voltammograms for one species from each of the three groups are shown in Figures 2-4. In all cases, the voltammetry is consistent with that obtained at ultramicroelectrodes in solution.30 The sigmoidal-shaped voltammetry at slow sweep rates (10 V/s) the CVs obtained are characteristic of planar diffusion (peak current (ip) proportional to (scan rate)1/2). Distortions in the characteristic sigmoidal- or peak-shaped waveforms indicate a corruption of the gel-electrode interface as described in prior work.29 When this occurred (typically 3-5 weeks after gelation), the experiment was stopped.
Figure 2. Cyclic voltammograms of gel-encapsulated FcN+ (initially 5 mM) acquired 1 (solid line) and 23 (dashed line) days after gelation.
Figure 3. Cyclic voltammograms of gel-encapsulated Co(bpy)32+ (initially 5 mM) acquired 1 (solid line) and 4 (dashed line) days after gelation.
As observed in the CVs depicted in Figures 2-4, the faradaic current is highly dependent on the type of redox probe doped into the gel and changes as the gel dries. At high sweep rates, the peak current, ip, is proportional to the product D1/2C, where C is the concentration of the encapsulated redox species and D is the apparent diffusion coefficient (physical diffusion plus electron exchange).18 At low sweep rates, the limiting steady-state current, iss, is proportional to the product DC.30 As the gel dries, solvent evaporates and the concentration of the redox probe trapped within increases. Depending of the molecule entrapped Analytical Chemistry, Vol. 72, No. 21, November 1, 2000
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Figure 4. Cyclic voltammograms of gel-encapsulated FcCOO(initially 3 mM) acquired 1 (dotted line), 2 (solid line), and 26 (dashed line) days after gelation.
and the extent to which it is influenced by the matrix, D will increase, decrease, or remain constant. For gel-encapsulated FcN+, the current decreases as the gel dries. This drop is most obvious at the low scan rates. Similar results were also observed for the other small positively charged or neutral redox probes such as FcCH2OH29 and Fc(CH2OH)2. For gel-encapsulated FcCOO-, the current drops the first day but then increases thereafter. Similar results were also observed for Fe(CN)63- (see ref 29 for example) and Mo(CN)84- with the exception that the current steadily increased from day 1 until completion of the experiment. The magnitude of the current for gel-encapsulated Co(bpy)32+ drops very quickly during the solgel state and during the first few days after gelation. To obtain true steady-state behavior, a scan rate of
