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Controlling Diffusion in Sol-Gel Derived Monoliths Mandakini Kanungo and Maryanne M. Collinson* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-3701 Received October 7, 2004. In Final Form: December 13, 2004 Redox probes were trapped within a silica monolith prepared in part with organoalkoxysilanes containing a quaternary ammonium functional group. The diffusion coefficients of the entrapped molecules were measured as the gels were slowly dried using chronoamperometry and cyclic voltammetry with ultramicroelectrodes. Gel-entrapped cobalt(II) tris(bipyridine) (Co(bpy)32+) diffuses at rates similar to that measured in the sols by incorporating a small amount of the positively charged functional group in the matrix. In comparison, the diffusion coefficient of gel-entrapped ferricyanide (Fe(CN)63-) drops an order of magnitude relative to its value in the sol soon after gelation. These results demonstrate the ease at which diffusion in hydrated gels can be easily controlled by simply changing the charge on the walls of the silica host.
Introduction The sol-gel process1 provides a versatile means to prepare inorganic and organic-inorganic host structures for catalysis,2,3 chemical analysis,4 ion-exchange applications,5-7 solid-state electrochemical devices,8,9 and photonics.10,11 In this process, a sol is first made through the hydrolysis and condensation of metal alkoxides (i.e., tetramethoxysilane) and then typically doped with a reagent or receptor.12,13 After a certain period of time, the sol gels, thus trapping the reagent in a hydrated matrix. During drying, solvent evaporates, the gel shrinks, the surface-area-to-volume ratio in the pores increases, and interactions between the entrapped reagent and the walls of the host structure become increasingly important.14 Of utmost importance to many applications is the rotational and translational freedom at which the gel-entrapped receptor can react with an analyte species in solution. Equally important is the rate at which a diffusing analyte species can move through the porous matrix to react with the entrapped receptor. Understanding how molecules diffuse in these materials and developing procedures to alter the rate at which reagents diffuse in a solid host is a necessary first step to improve the performance of solgel-based devices. Several methods have been used to measure the diffusion coefficients of various guests entrapped in sol-gel derived materials. Fluorescence correlation spectroscopy,15-17 fluorescence microscopy,18 Raman spectroscopy,19,20 and time-dependent methods21 have been used to study the diffusion of dyes and other molecules in thin * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 785-532-1468. Fax: 785-532-6666. (1) Brinker, J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1989. (2) Blum, J.; Avnir, D.; Schumann, H. CHEMTECH 1999, 32-38. (3) Schubert, U. New J. Chem. 1994, 18, 1049-1058. (4) Collinson, M. M. Crit. Rev. Anal. Chem. 1999, 29 (4), 289-311. (5) Hsueh, C.; Collinson, M. M. J. Electroanal. Chem. 1997, 420 (1-2), 243-249. (6) Wei, H.; Collinson, M. M. Anal. Chim. Acta 1999, 397 (1-3), 113121. (7) Petit-Dominguez, M. D.; Shen, H.; Heineman, W. R.; Seliskar, C. J. Anal. Chem. 1997, 69, 703-710. (8) Dunn, B.; Farrington, G. C.; Katz, B. Solid State Ionics 1994, 70/71, 3-10. (9) Rolison, D. R.; Dunn, B. J. Mater. Chem. 2001, 11, 963-980. (10) Knobbe, E. T.; Dunn, B.; Fuqua, P. D.; Nishida, F. Appl. Opt. 1990, 28 (18), 2729-2733. (11) Levy, D. Chem. Mater. 1997, 9, 2666-2670. (12) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67 (1), 22A-30A. (13) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (14) Dunn, B.; Zink, J. I. Chem. Mater. 1997, 9, 2280-2291.
silicate films and thoroughly dried monoliths. Electrochemical methods have also been used to evaluate the diffusion of redox-active reagents in hydrated monoliths.22-27 Our work, in particular, has utilized gel-immobilized ultramicroelectrodes to measure the diffusion coefficients of redox molecules of varying size and charge trapped within hydrated gels prepared under different conditions.25-27 This work has attested to the importance of intermolecular interactions over constrained pore environments on diffusion in partially dried gels. In this study, a simple method to influence or “control” the rates at which cationic and anionic probes diffuse in these solids is described. This method involves the manipulation of electrostatic forces between the gel-entrapped receptor and the walls of the matrix. Experimental Section Reagents. Tetramethoxysilane (TMOS, 99%) and ferrocene methanol (FcCH2OH) were purchased from Aldrich. Trimethoxysilylpropyl-modified poly(ethylene imine), 50% in 2-propanol, (polyamine-Si), and N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, 50% in methanol (QAPS), were obtained from Gelest Chemicals. Hydrochloric acid (HCl), potassium ferricyanide (Fe(CN)63-/4-), potassium chloride, methanol, and 2-propanol were purchased from Fisher Scientific. All reagents were used as received without further purification. Cobalt(II) tris(bipyridine) (Co(bpy)32+/3+) and ferrocenylmethyl trimethylammonium hexafluorophosphate (FcN+PF6-) were synthesized as described previously.26 Water was purified to type I (18 MΩ) using a Labconco four-cartridge system. (15) McCain, K. S.; Harris, J. M. Anal. Chem. 2003, 75, 3616-3624. (16) McCain, K. S.; Schluesche, P.; Harris, J. M. Anal. Chem. 2004, 76 (4), 930-938. (17) Mahurin, S. M.; Dai, S.; Barnes, M. D. J. Phys. Chem. B 2003, 107 (48), 13336-13340. (18) McCain, K. S.; Hanley, D. C.; Harris, J. M. Anal. Chem. 2003, 75, 4351-4359. (19) Nikiel, L.; Hopkins, B.; Zerda, T. W. J. Phys. Chem. 1990, 94, 7458-7464. (20) Watson, J.; Zerda, T. W. Appl. Spectrosc. 1991, 45, 1360-1365. (21) Koone, N.; Shao, Y.; Zerda, T. W. J. Phys. Chem. 1995, 99, 1697616981. (22) Audebert, P.; Griesmar, P.; Hapiot, P.; Sanchez, C. J. Mater. Chem. 1992, 2, 1293-1300. (23) Audebert, P.; Griesmar, P.; Sanchez, C. J. Mater. Chem. 1991, 1, 699-700. (24) Audebert, P.; Sallard, S.; Sadki, S. J. Phys. Chem. B 2003, 107, 1321-1325. (25) Collinson, M. M.; Zambrano, P. J.; Wang, H.; Taussig, J. S. Langmuir 1999, 15 (3), 662-668. (26) Howells, A. R.; Zambrano, P. J.; Collinson, M. M. Anal. Chem. 2000, 72 (21), 5265-5271. (27) Kanungo, M.; Collinson, M. M. Anal. Chem. 2003, 75 (23), 65556559.
10.1021/la047518r CCC: $30.25 © 2005 American Chemical Society Published on Web 01/06/2005
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Procedures. The QAPS/TMOS sols were prepared by mixing 7.5 g of the TMOS with either 0.75, 1.1, or 1.8 g of QAPS. A 7.35-mL portion of the resultant mixture was mixed with 3.8 mL of methanol, 8.2 mL of water, and 1.2 mL of 0.01 M HCl. The redox species and the supporting electrolyte (KCl) were added to the sol to give a final concentration of 5 mM and 0.15 M, respectively. For the case of Fe(CN)63- (0.75 g of QAPS), 0.6 mL of 0.01 M HCl was added and the water amount was increased to 8.8 mL. The polyamine-Si/TMOS sols were prepared by mixing 7.5 g of the TMOS with either 0.80 g of polyamine-Si (for Co(bpy)32+) or 0.25 g of polyamine-Si (for Fe(CN)63-). A 7.35-mL portion of the resultant mixture was added to 3.8 mL of 2-propanol, and the pH of the sol was adjusted to around 6-7 by adding an appropriate amount of dilute HCl. The pH of the sol was very important for the gelation of the sol. The 5 mM redox species and 0.15 M KCl were added to the sol as described above. TMOS-only sols were prepared as described.26 All the doped sols were vigorously stirred for 30-45 min and then poured into polystyrene vials. The microelectrode assembly, which consisted of a Pt microelectrode (r ) 13.3 µm) and a Ag/ AgCl wire (serves as reference and counter electrode), was inserted into the sol and the resultant electrochemical cell secured in a darkened Faraday cage. The electrochemical data were collected at regular intervals as the gels were slowly dried at 60-70% relative humidity.26 The sols gelled within 12 h. The relative change in mass of the gel during the slow drying period was ∼15% over a 2-week period for all the gels. “Ambigels”28 formed from 3-week-old gels had approximately the same average pore diameter (3.8 nm) calculated from the desorption branch of a N2 adsorption-desorption isotherm using the Barrett, Joyner, and Halenda (BJH) method.
Letters Chart 1
ficiently hydrated that electrochemistry can be done in the matrix. By utilizing ultramicroelectrodes, the apparent diffusion coefficient (Dapp) of entrapped redox-active molecules can be easily measured as the gel dries by coupling slow scan cyclic voltammetry with chronoamperometry.26 Figure 1 shows the steady-state cyclic voltammograms (CVs) of gel-encapsulated Fe(CN)63- in a matrix prepared
Results and Discussion Diffusion in porous solids can be significantly more complex than that measured in solution because of potential interactions between the entrapped guest and the walls of the porous network and because of confinement effects. The surfaces of the pore walls are complex and contain different functional groups including siloxane (SiO-Si), silanol (Si-OH), and siloxide (SiO-) groups.14,29 In addition, the walls will be negatively charged under most conditions, as the isoelectric point (pI) of silica is around 2.30 A few studies have demonstrated the importance of intermolecular interactions on dopant mobility in both hydrated and fully dried gels, particularly with regard to charged molecules or ones that are able to hydrogen bond with the matrix.19,29,31-33 One simple method for altering the diffusion rates of charged molecules would thus be to manipulate the charge on the porous matrix and change the extent of electrostatic interactions. In this study, positively charged functional groups were incorporated into the negatively charged silica matrix by cohydrolyzing and condensing tetramethoxysilane (TMOS) with either QAPS or polyamine-Si. The structures of the two precursors are shown in Chart 1. Prior to gelation, an ultramicroelectrode (r ) 13.3 µm), a reference/counter electrode, the redox molecule (either Fe(CN)63-, Co(bpy)32+, FcCH2OH, or FcN+), and potassium chloride are added to the sol. After a certain period of time, the sol turns into a gel, and then, the gel is slowly dried under a controlled humidity environment. Throughout the course of the experiment (typically 2 days to 3 weeks), the gels are suf(28) Harreld, J. H.; Dong, W.; Dunn, B. Mater. Res. Bull. 1998, 33 (4), 561-567. (29) Shen, C.; Kostic, N. M. J. Am. Chem. Soc. 1997, 119, 13041312. (30) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (31) Sieminska, L.; Zerda, T. W. J. Phys. Chem. 1996, 100, 4591-4597. (32) Badjic, J. D.; Kostic, N. M. J. Phys. Chem. B 2000, 104, 1108111087. (33) Badjic, J. D.; Kostic, N. M. J. Mater. Chem. 2001, 11, 408-418.
Figure 1. Cyclic voltammograms (2 mV/s) of gel-encapsulated Fe(CN)63- (A and B) and Co(bpy)32+ (C and D). Gels were prepared from a QAPS/TMOS sol (B and D) or solely from TMOS (A and C). The potentials are with respect to a Ag/AgCl wire reference electrode.
solely from TMOS (Figure 1A) and from QAPS (0.75 g)/ TMOS (Figure 1B). In the TMOS-only gel, the steadystate limiting current increases slightly as the gel dries, whereas, in the modified gel, it drops quickly. For an ultramicroelectrode, the steady-state current depends on the product DappC, where Dapp is the apparent diffusion coefficient and C is the concentration.34 As the gel dries, C slowly increases and Dapp will either drop or remain the same. Since C does not significantly increase during the first few days, most of the changes in the voltammetry can be attributed to Dapp (at least early on). For Fe(CN)63in a TMOS-only gel, it appears that Dapp stays the same, while, in the QAPS-modified gel, it obviously drops. Just the opposite is observed for gel-entrapped Co(bpy)32+ (Figure 1C (TMOS) and D (QAPS(1.8 g)/TMOS)). In the TMOS-only gel, the limiting current for Co(bpy)32+ quickly decreases (indicative of Dapp significantly decreasing), whereas, in the QAPS-modified gel, it stays relatively constant (Dapp likely does not vary too much) after the first hour. Nearly identical results were obtained with gels prepared from the cationic polymer, polyamine-Si (i.e., the CVs of Co(bpy)32+ and Fe(CN)63- showed the same behavior as they did in the QAPS gels). (34) Wightman, R. M.; Wipf, D. O. Voltammetry at Ultramicroelectrodes; Marcel Dekker: New York, 1989; Vol. 15, pp 267-353.
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Figure 2. Normalized diffusion coefficients (D/D0) as a function of drying time for gel-encapsulated Fe(CN)63- (top) or Co(bpy)32+ (bottom). Gels doped with Fe(CN)63- were prepared from ([) TMOS only, (O) 0.75 g of QAPS/TMOS, or (1) polymer-Si/TMOS. Gels doped with Co(bpy)32+ were prepared from (b) TMOS only, (1) 1.1 g of QAPS/TMOS, (9) 1.8 g of QAPS/TMOS, or (O) polymer-Si/TMOS. D0 is the diffusion coefficient of the redox probe obtained within the first hour after the sol was made. The error bars represent the standard deviations in Dapp for three to six gels. Lines have been added to guide the eye.
The apparent diffusion coefficient can be calculated without knowledge of C from the normalized chronoamperometric response according to the following equation, where id(t) is the chronoamperometric current during the potential step, iss is the steady-state current, and r is the electrode radius (13.3 µm).35
id(t)/iss ) 0.7854 + 0.4431(Dt/r2)-1/2 + 0.2146 exp(-0.3911(Dt/r2)-1/2) (1) The chronoamperometric response was obtained by stepping the potential ∼ (250 mV beyond the redox formal potential. This current is normalized by the steady-state limiting current obtained from the steady-state CVs acquired at 2 mV/s and then fitted to the above equation to obtain Dapp independent of C. Only data that yielded regression coefficients greater than 0.96 were used. Figure 2 shows how the diffusion coefficient of the entrapped reagent changes as the gel dries in a controlled humidity environment. The mean and standard deviations (error bars) from individual measurements in three to six different gels prepared at different times are shown. For Fe(CN)63- trapped in a gel prepared from TMOS, Dapp does not change and is nearly identical to that measured in solution. As previously described, this is consistent with the dopant residing in the center of solvent filled pores.26 (35) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1982, 140, 237-245.
Significant changes are observed, however, when a relatively small amount of the positively charged precursor is incorporated in the gel. For gels prepared with a higher amount of QAPS, the steady-state current drops too quickly to accurately measure Dapp. For gels containing a smaller amount (i.e., 0.75 g of QAPS), it was possible to calculate Dapp. As can be seen in Figure 2, Dapp drops over an order of magnitude in just a few days. Similar results were observed when a small amount of the cationic polymer (polyamine-Si) was incorporated in the matrix. Under these conditions, gel-entrapped Fe(CN)63- diffuses at a slower rate due to electrostatic interactions with the positively charged groups on the surface. As the sol turns into a gel and the porous network starts to form, there will be a greater interaction between the dopant and the modified silica walls. In addition, as solvent evaporates, there may also be a greater attraction between charged sites due to reduced screening. Hence, Dapp drops as the gel is slowly dried. Just the opposite is observed for gel-entrapped Co(bpy)32+ (Figure 2). In gels prepared solely from TMOS, Dapp drops an order of magnitude in a very short period of time (similar to that for Fe(CN)63- in the QAPS gel). As previously described, the decreased diffusion rate is due to electrostatic interactions between the positively charged dopant and the negatively charged surface.23 Co(bpy)32+ can, however, be made to diffuse at rates similar to what it does in solution by changing the charge on the porous network. Figure 2 shows relative changes in Dapp as a function of drying time for Co(bpy)32+ trapped in gels containing different amounts of QAPS or the cationic polymer (polyamine-Si). As the amount of QAPS copolymerized with TMOS increases, the rate of change of Dapp becomes less. Eventually, Dapp stays approximately constant and at a value similar to that measured for Co(bpy)32+ in the sol. The diffusion coefficients of two other dopants (FcN+ and FcCH2OH) trapped in the materials prepared from QAPS have also been measured during gel formation and slow drying. For both compounds, the drop in Dapp is not as large during the early stages of drying (
