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Electrochemical and Spectroscopic Characterization of Surface Sol-Gel Processes Xiaohong Chen and George S. Wilson* Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045 Received May 29, 2003. In Final Form: June 25, 2004 (3-Mercaptopropyl)trimethoxysilane (MTS) forms a unique film on a platinum substrate by self-assembly and sol-gel cross-linking. The gelating and drying states of the self-assembled MTS sol-gel films were probed by use of electrochemical and spectroscopic methods. The thiol moiety was the only active group within the sol-gel network. Gold nanoparticles were employed to detect the availability of the thiol group and their interaction further indicated the physicochemical states of the sol-gel inner structure. It was found that the thiol groups in the open porous MTS aerogel matrix were accessible to the gold nanoparticles while thiol groups in the compact MTS xerogel network were not accessible to the gold nanoparticles. The characteristics of the sol-gel matrix change with time because of its own irreversible gelating and drying process. The present work provides direct evidence of gold nanoparticle binding with thiol groups within the sol-gel structures and explains the different permeability of “aerogel” and “xerogel” films of MTS on the basis of electrochemical and spectroscopic results. Two endogenous species, hydrogen peroxide and ascorbic acid, were used to test the permeability of the self-assembled sol-gel film in different states. The MTS xerogel film on the platinum electrode was extremely selective against ascorbic acid while maintaining high sensitivity to hydrogen peroxide in contrast to the relatively high permeability of ascorbic acid in the MTS aerogel film. This study showed the potential of the MTS sol-gel film as a nanoporous material in biosensor development.
Introduction Materials that are permselective to a diversity of molecules on the basis of size, charge, shape, or chemical affinity are needed to prepare more sensitive, selective, and efficient sensors. Sol-gel chemistry has been the subject of many studies because it offers a low-temperature methodology for the production of ceramic glass materials.1 The sol-gel process has been extensively studied,2 and sol-gel derived materials have shown a variety of applications in catalysis, sensors, coatings, optics, and specialty polymers.3 One of the distinct advantages that sol-gel-derived materials provides over other commonly used organic polymers is the inherent flexibility associated with material preparation and processing. During the past few years there have been a number of studies aimed at tailoring porosity and elucidating the permselectivity and ion exchange properties by functional group4 or molecular imprinting.5 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.6 Basically, the terminology “sol-gel” is used to describe a broad class of processes in which a solid phase is formed through gelation of sol. A sol is the dispersion of colloidal particles in a solution, and a gel results from cross-linking * Corresponding author. E-mail:
[email protected]. Phone: (785) 864-5152. Fax: (785) 864-5156. (1) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (2) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33-72. (3) Collinson, M. M. Crit. Rev. Anal. Chem. 1999, 29 (4), 289-311. (4) Hsueh, C.; Collinson, M. M. J. Electroanal. Chem. 1997, 420, 243-249. (5) (a) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1701. (b) Makote, R.; Collinson, M. M. Chem. Mater. 1998, 10, 2440-2445. (c) Wei, Y.; Jin, D.; Ding, T.; Shih, W.; Lin, X.; Cheng, S. Z. D.; Fu, Q. Adv. Mater. 1998, 3, 313-316 and references therein. (d) Khramov, A. N.; Collinson, M. M. Chem. Commun. 2001, 8, 767-768. (6) Dunn, B.; Zink, J. I. Chem. Mater. 1997, 9, 2280-2291.
of the sol to form an interconnected, rigid network with pores of submicrometer dimensions. When the liquid within the pores is removed from the interconnected solid gel network as a vapor, the network does not collapse and a low-density aerogel is produced. If the liquid evaporation process continues and the pore liquid is removed accompanied with pore shrinkage, then the monolith is termed a xerogel. So the aerogel and xerogel represent the different stages of the gelation procedure. Sol-gel materials have become more appealing in electrochemistry since Murray and co-workers reported on redox modified siloxane-based cross-linked films on silicon, platinum, and other metal electrodes.7 For the convenience of material fabrication on electrode substrates, self-assembly of a silica gel network has been achieved from a thiolated silicon alkoxide precursor, (3mercaptopropyl)trimethoxysilane (MTS).8 MTS is comprised of two reactive functional groups. The thiol tail is able to form a strong covalent bond with a variety of metals, including platinum, through cleavage of the S-H bond and formation of a Pt-S bond.9 The methoxy headgroup can undergo hydrolysis and condensation reactions, which are the basis for sol-gel synthesis.10 The combination of self-assembled monolayers of organosulfur compounds and the silica gel network provides a convenient methodology (7) (a) Lenhard, J. R.; Murray, R. W. J. Electroanal. Chem. 1977, 78, 195-201. (b) Lenhard, J. R.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 7870-7875. (c) Kuo, K.-N.; Moses, P. R.; Lenhard, J. R.; Green, D. C.; Murray, R. W. Anal. Chem. 1979, 51, 745-748. (d) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. (8) Wang, J.; Pamidi, P. V. A.; Zanette, D. R. J. Am. Chem. Soc. 1998, 120, 5852-5853. (9) (a) Bravo, B. G.; Mebrahtu, T.; Soriaga, M. P.; Zapien, D. C.; Hubbard, A. T.; Stickney, J. L. Langmuir 1987, 3, 595-597. (b) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885-4893. (c) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753-756. (10) Brinker, C. J.; Scherer, G. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990.
10.1021/la034940j CCC: $27.50 © 2004 American Chemical Society Published on Web 08/21/2004
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by which compact membranes with controlled thickness and structure can be prepared. MTS has been used to provide surface protection, promote adhesion of thin metal films to oxide surfaces,11 and tether materials such as zeolites to metal surfaces.12 The electrochemical preparation of two-dimensional monolayers of MTS, possessing different defect sizes, was reported recently in connection with molecular recognition.13 Many methods have been used to study the state of the bulk sol-gel material (e.g., NMR, IR, and Raman spectroscopies). Electrochemical techniques have also proven useful in probing transitions during sol-gel processing. Free redox probes or reactive redox monomers have been introduced into the starting sol-gel solution and have monitored the electrochemical response during the propagation of polymerization.14 Audebert and co-workers15 employed chronoamperometry and cyclic voltammetry to study the diffusion coefficient of three different redox probes during the sol-gel process. Dunn et al.16 introduced alternating current impedance spectroscopy for studies of the evolution of the sol-gel process and gel aging. In addition, Tenan et al. applied an electrochemical quartz crystal microbalance to the study of silica gelation from an aqueous silica sol prepared by the hydrolysis of the metasilicate ion.17 Our study concentrates on characterization of the MTS sol-gel process on platinum substrates by use of gold nanoparticles as markers. Electrochemical methods are employed to probe the interaction between the thiol moiety and gold nanoparticles within the MTSderived sol-gel matrix. The binding of thiol groups and gold nanoparticles is also explored by spectroscopic techniques such as UV-vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray spectroscopy (EDXS). The results are consistent with the electrochemical data and give first-hand information about the interaction of gold nanoparticles and the thiol group within the sol-gel structure. The different features of aerogel and xerogel states of MTS films also define their respective permselectivity. Experimental Section Reagents. MTS, N-propyl trimethoxysilane (PTS), hydrogen peroxide, and potassium ferrocyanide were purchased from Fluka (NY). L-Ascorbic acid was obtained from Aldrich. All the sols and solutions were prepared immediately before testing because they are subject to oxidative decomposition in solution. Potassium ferrocyanide solution was 1.0 mM with 100 mM KCl as the electrolyte. Gold colloid (5 nm, approximately 0.01% as HAuCl4) was purchased from Sigma (St. Louis, MO). Teflon-coated platinum (Pt) wire (composition: 90% Pt/10% Ir and 0.17-mm diameter) was purchased from Medwire Corp. (Mount Vernon, NY) and used for all electrochemical tests. Phosphate buffer (0.05 M) was prepared from the corresponding phosphate salts. Phosphate-buffered saline (PBS, pH 7.4) was prepared from phosphate salts (0.1 M) and sodium chloride (0.15 M). (11) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85-88. (12) Yan, Y.; Bein, T. J. Phys. Chem. 1992, 96, 9387-9393. (13) Che, G.; Cabrera, C. R. J. Electroanal. Chem. 1996, 417, 155161. (14) (a) Zhang, Y. Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1991. (b) Howells, A. R.; Zambrano, P. J.; Collinson, M. M. Anal. Chem. 2000, 72, 5265-5271. (15) (a) Audebert, P.; Griesmar, P.; Sanchez, C. J. Mater. Chem. 1991, 1, 699-700. (b) Audebert, P.; Griesmar, P.; Hapiot, P.; Sanchez, C. J. Mater. Chem. 1992, 2, 1293-1300. (16) (a) Durakpasa, H.; Breiter, M. W.; Dunn, B. Electrochim. Acta 1993, 38, 371-377. (b) Durakpasa, H.; Breiter, M. W.; Dunn, B. J. Sol.-Gel Sci. Technol. 1994, 2, 251-3. (17) Tenan, M. A.; Soares, D. M.; Bertran, C. A. Langmuir 2000, 16, 9970-9976.
Langmuir, Vol. 20, No. 20, 2004 8763 Apparatus. Amperometry was performed using a model 814 electrochemical detector (CH Instruments, TX) connected to a Dell (L500r) computer. Pt sheet and Ag/AgCl (3 M NaCl) were used as the counter and reference electrodes, respectively. FTIR spectra were collected on a BioRad FTS40 operating in the reflectance mode with attenuated total reflectance spectroscopy. A KRS-5 crystal and an incident angle of 58° were utilized. The EDXS was carried out on an EDAX phenix detector with LEO ISSO FESEM system under an accelerating voltage of 20 keV. Glass microscope slides (Richard-Allan Scientific) cut into 8 × 25 mm pieces were used as substrates for the UV-vis absorbance measurement of the MTS film with or without the immobilization of gold nanoparticles. The UV-vis absorption spectra were obtained using a Cary 50 Bio UV-visible spectrophotometer. Preparation of the MTS Sol. Typically, 38.3 µL of MTS and 0.4 µL of 1 M HCl solution were added to 43.2 µL of Nanopure water. The molar ratio of MTS/H2O/HCl is 1:12:0.005. The mixture was stirred vigorously for 15 min, and a homogeneous transparent solution was obtained. The resulting MTS sol was used for the modification of platinum or glass slides. Fabrication and Characterization of the MTS Film with/ without Gold Nanoparticles. The Pt wires used for the electrochemical measurements were of cylindrical geometry (with a diameter of 0.17 mm and length of 1 mm) and were made using the same method as described in a previous publication.18 The clean Pt electrode was soaked in the MTS sol for 3 h to let the film grow on the surface. There were then two ways to treat these electrodes. Some electrodes were immersed immediately in the gold colloid (5-nm gold nanoparticles) for 6 h, and the resulting MTS films on the Pt electrodes were in the aerogel state (defined as aMTS). Other electrodes were dried in a bottle containing CaCl2 as the desiccant (the relative humidity was less than 20%) for 3 days first to allow the MTS xerogel formation (defined as xMTS), followed by immersion in gold colloid (5-nm gold nanoparticles) for 6 h. After the electrodes were rinsed with water, cyclic voltammetry was utilized to test the heterogeneous electron-transfer characteristics of potassium ferrocyanide on these MTS/gold-coated Pt electrodes. These Pt electrodes were also used for the EDXS. Chronoamperometry was performed to investigate the distribution of gold nanoparticles in the MTS film. The same threeelectrode system as mentioned above was used in 1 mM potassium ferrocyanide solution. The applied potential was stepped from -0.05 to +0.5 V and back with a pulse width of 500 s. The aMTS- and xMTS-coated Pt electrodes were also tested for the permselectivity of L-ascorbic acid and hydrogen peroxide, using the apparatus described above. The solution was pH 7.4 PBS. The potential applied to the working electrode was 0.6 V (vs Ag/AgCl), and the analyte concentrations were 0.1 mM for both L-ascorbic acid and hydrogen peroxide. Glass slides and thin Pt sheets were employed for the UV-vis absorption and FTIR spectroscopy tests, respectively. Both glass and Pt sheets were coated with MTS films by soaking in the MTS sol for 3 h. The aMTS- and xMTS-coated substrates were immersed in 5-nm gold colloid for 6 h, respectively. After drying in air, glass slides and Pt sheets with MTS, aMTS/gold, and xMTS/gold films were rinsed with water and dried in air prior to their respective spectroscopic tests.
Results and Discussion Probing Aerogel and Xerogel States of the SolGel-Derived MTS Films on Platinum by Electrochemical Methods. The thiolated silicon alkoxide, MTS, was utilized in this research to design three-dimensional silica gel interfacial structures via the direct coupling of sol-gel and self-assembly technologies. The orientation of the silane monomer should be dominated by the affinity of the MTS thiol moiety for the Pt, although the trimethoxysilyl groups also have the ability to react with the possible surface oxides of Pt to form siloxy bonds. Like (18) Chen, X.; Matsumoto, N.; Hu, Y.; Wilson, G. S. Anal. Chem. 2002, 74, 368-372.
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Figure 1. Cyclic voltammograms of Pt electrodes modified with silane films: (a) aMTS film, (b) xMTS film, (c) aMTS/gold, and (d) xMTS/gold in 1 mM potassium ferrocyanide solution with 100 mM KCl. Scan rate: 50 mV/s.
Figure 2. Chronoamperometric response of Pt electrodes modified with aMTS/gold in 1 mM potassium ferrocyanide solution with 100 mM KCl. Potential range, -0.05 to +0.5 V; pulse width, 500 s.
other alkanethiols, MTS is able to self-assemble on the Pt surface through a Pt-S bond.19 However, MTS films on electrodes have not been well characterized although they are of obvious interest as chemically modified surfaces.8 A silica gel is usually prepared by the formation of an interconnected three-dimensional network through the simultaneous hydrolysis and polycondensation of a precursor. When applied to the surface sol-gel processes, a series of materials reflecting pore size changes at different stages will lead to variable permeability. MTS molecules on an electrode result in a cross-linked three-dimensional structure. Besides the first layer of the thiol moiety selfassembled onto the Pt surface, there should be a large number of thiol groups within the sol-gel matrix. Because thiol is the active functional group in the sol-gel structure, it provides the possibility of probing the silica network on the basis of the availability of thiol for further reaction. Colloidal gold was utilized to detect the thiol groups in this study because of the high affinity of gold nanoparticles for thiol groups.20-25 Evaluation of redox processes at filmcoated electrode surfaces by cyclic voltammetry can provide information about the ability of various species to penetrate the film, thereby reflecting the film morphology. Potassium ferrocyanide was used as an electrochemical indicator to detect the interaction of colloidal gold with the thiol groups. As is known, the bonds between the metal electrode and the thiol group reduce the electrode area and slow electron-transfer rates for electroactive probes such as the ferricyanide ion.26 Therefore, it is expected that the self-assembled MTS sol-gel film may greatly diminish the heterogeneous electron transfer rate of the ferrocyanide marker. Figure 1 displays the voltammetric
response of platinum electrodes modified with aMTS, xMTS, and their respective complexes with gold nanoparticles. It was observed that a platinum electrode coated with an aMTS film showed an irreversible electrochemical reaction in 1 mM potassium ferrocyanide solution (Figure 1a). The characteristic peak of the ferrocyanide ion was completely blocked for a xMTS film-coated Pt electrode (Figure 1b). This obviously suggests a more compact structure and smaller pores of the MTS film after drying. When gold nanoparticles self-assemble into the aMTS film, the electroactivity of the ferrocyanide ion is “recovered” (Figure 1c) with an apparent heterogeneous rate constant of 0.003 cm/s based on simulation of cyclic voltammetric curves. Potential step chronoamperometric experiments were carried out to investigate the distribution of gold nanoparticles in the MTS film, with the results shown in Figure 2. Data after 3 s were considered for the calculation of the slope because of the delay in charging the electrode, and good linearity was obtained for a Cottrell plot up to about 25 s. At longer times, convection usually results in currents larger than those predicted by the Cottrell equation.27 For the calculation of gold nanoparticle distribution in MTS films, it is assumed that the diffusion coefficient of the ferrocyanide ion does not change at the MTS film/solution interface. The diffusion coefficient of ferrocyanide is 6.5 × 10-6 cm2/s, and the bulk concentration of ferrocyanide is 1 mM. By analyzing the chronoamperometric results (Figure 2), the effective area of the gold nanoparticles (A in the Cottrell equation) was 8.2 × 1011 nm2, equivalent to about 1010 gold nanoparticles of 5-nm diameter. Moreover, it is assumed that formation of a sulfur-gold bond does not significantly affect the surface area of the gold nanoparticles. When considering that only the thiol functional groups of the aMTS film on the Pt electrode (geometric area 5.34 × 1011 nm2) could bind with gold nanoparticles, each layer could at most be occupied by 5 × 109 gold nanoparticles if all thiol groups are available. So finally the chronoamperometry data suggest that the gold nanoparticles correspond to around two equivalent monolayers of 5-nm gold nanoparticles distributed in the aMTS film on the Pt electrode. Thus, approximately two layers of gold nanoparticles entrapped in the aMTS films are accessible to the electrode although more layers of nanoparticles have been assembled.
(19) Brito, R.; Rodriguez, V. A.; Figueroa, J.; Cabrera, C. R. J. Electroanal. Chem. 2002, 520, 47-52. (20) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425-5429. (21) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499-1501. (22) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844-850. (23) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-61. (24) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148-1153. (25) Tseng, J.-Y.; Lin, M.-H.; Chau, L.-K. Colloids Surf., A 2001, 182, 239-245. (26) Hsueh, C.; Lee, M.; Freund, M. S.; Ferguson, G. S. Angew. Chem., Int. Ed. 2000, 39, 1227-1230.
(27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001.
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Figure 3. Cyclic voltammogram of Pt electrodes modified with PTS/gold film in 1 mM potassium ferrocyanide solution with 100 mM KCl. Scan rate: 50 mV/s.
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Figure 4. Cyclic voltammograms of Pt electrodes modified with different films: (a) aMTS/gold and (b) xMTS/gold after drying in air at day 26, (c) day 28, (d) day 32, and (e) day 44. The solution was 1 mM potassium ferrocyanide solution with 100 mM KCl. Scan rate: 50 mV/s.
“Recovery” of electroactivity of the ferrocyanide ion was not observed for the xMTS film under the same procedure (Figure 1d). This suggests that the gold nanoparticle “link” between the MTS film and the ferrocyanide ion did not exist for the xerogel state. It was reasonably concluded here that the aMTS film had an open structure and the thiol groups were free and available for the colloidal gold binding while the thiol moieties in the compact xMTS film were embedded and blocked within the siloxane network. It has been previously suggested that gold nanoparticles are distributed predominantly inside the network rather than as aggregates on the surface.28 The “recovery” of the redox peaks of the ferrocyanide ion on the aMTS/goldcoated Pt electrode is likely due to the entrapment of gold nanoparticles into the sol-gel pores rather than binding with the thiol groups. To clarify this point, the following control experiment was done. First, a Pt electrode was immersed in 50 mM MTS solution in methanol for 12 h to allow the formation of a self-assembled MTS monolayer on the Pt electrode. After rinsing with methanol and water, the electrode was immersed in a PTS sol (prepared by the same method and ratio as for MTS sol) for 3 h to allow the PTS film to form on the electrode. After that, the electrode was immersed in the gold colloid solution (5 nm) for another 6 h to let gold nanoparticles “communicate” with the pores in the PTS film. Finally, cyclic voltammetric results were obtained that showed an irreversible electron transfer reaction in potassium ferrocyanide solution (Figure 3). Thus, the gold nanoparticles were not successfully loaded into the PTS film. Because the only difference between MTS and PTS is the mercapto- group, it can be concluded that the entrapment of gold nanoparticles in the MTS film was triggered by the selfassembly of gold nanoparticles with thiol groups rather than simple entrapment in the pores, although the pore size is clearly important. Did gold nanoparticles alter the drying process of the MTS aerogel after the self-assembly within the sol-gel matrix? To address this question, it was necessary to further characterize the changing states of the sol-gel matrix in the presence of gold nanoparticles. A clean Pt electrode was soaked in the MTS sol for 3 h to let the film grow on the surface. At this time, an MTS film on a Pt electrode was in the aerogel state. It was immersed immediately in the 5-nm gold colloid solution for 6 h for the self-assembly of gold nanoparticles. The heterogeneous
electron transfer rate of the ferrocyanide marker was not expected to change at the resulting Pt electrode with aMTS/gold if gold nanoparticles perturbed the drying process of the MTS aerogel. However, Figure 4 demonstrates the changing voltammetric response with time of a Pt electrode modified with aMTS/gold. A Pt electrode with aMTS/gold exhibited a reversible electrochemical reaction in ferrocyanide solution (Figure 4a), and the voltammetry was governed by linear diffusion because the relatively high area of the fresh gold nanoparticles in the aMTS film resulted in overlapping diffusion layers.29 By contrast, the cyclic voltammogram of the same electrode showed obviously slower kinetics for the ferrocyanide ion with mixed linear/radial diffusion at the sixth day after aging and drying in air (Figure 4b). The state of the solgel matrix on the Pt electrode should be a mixture of aerogel/xerogel, and the loaded gold nanoparticles seemed partly shielded by the siloxane structure. It has been amply demonstrated that redox centers on chemically modified electrodes exhibit electrochemical reactivity even when they are entangled in electrochemically insulating chains.7d We propose that the kinetic picture is also appropriate to describe the electrochemical communication of the assembled gold nanoparticles with the substrate Pt electrode. Changes in the redox state would be accompanied by counterion migration toward and could explain the asymmetry of the two peaks of Figure 4b. What was more interesting was that voltammetry of nearly pure radial diffusion (a sigmoidal-shaped curve) was observed at the same electrode at the 28th day (Figure 4c), and this is a typical electrochemical characteristic of an array of ultramicroelectrodes. This observation can be explained by the diminished surface area of gold nanoparticles due to siloxane shielding resulting in a high ratio (over 10) for the distance between electrode centers versus their average diameters.29 As the drying process continued, the electron transfer of the ferrocyanide ion became slower leading to greatly diminished faradic current (Figure 4d). At the 44th day, further barrier properties were observed and the electrode became stable after that (Figure 4e). All these changes indicated the reorientation of gold nanoparticles inside the siloxane matrix with a concomitant drying process. These observations suggest the following conclusions: (1) gold nanoparticles were in the pores rather than the outer surface; (2) the drying process resulted in pore shrinkage, and the gold nanoparticles are entrapped
(28) Bharathi, S.; Nogami, M.; Ikeda, S. Langmuir 2001, 17, 1-4.
(29) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329-1340.
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Figure 5. UV-vis absorbance spectra of the silane films: (a) aerogel and (b) xerogel MTS film-functionalized colloidal gold on glass slides. The absorbance spectrum of the xMTS film is labeled as c. Figure 6. EDXS spectra of silane films: (a) aMTS/gold and (b) xMTS/gold.
in the siloxane network and are, thus, inaccessible for electron transfer. Characterization of the Interaction between Thiol Groups and Gold Nanoparticles by Spectroscopic Techniques. The molecular binding of gold nanoparticles with thiol groups could be monitored spectrophotometrically by use of UV-vis absorption spectroscopy because of the high-extinction surface plasmon resonance of gold nanoparticles.23 As is known, the UV-vis spectrum of a ∼6-nm gold colloid multilayer with MTS as a linker molecule has an absorption band at about 530-575 nm.25 The spectrum of the aMTS/gold network in this study did show a characteristic band at 545 nm (Figure 5a) while the aMTS film showed the same UV spectrum as xMTS without any obvious absorbance in this region (Figure 5b). However, the xMTS network did not give any characteristic absorption signal even after the same time of immersion in gold colloid (Figure 5c). These contrasts suggested that gold nanoparticles self-assembled in the aerogel MTS film but not in the xerogel MTS film. The presence of gold nanoparticles in the aMTS film was also examined with EDXS measurements, and the results are displayed in Figure 6. The dispersive X-ray penetrated through the MTS film to the platinum substrate, giving rise to a Pt signal. Therefore, the EDXS spectra are characteristic of the entire film and not just the outer layer of MTS. The EDXS spectrum of aMTS/ gold indicated the presence of gold at 9.7 keV (Figure 6a), the characteristic energy of the gold LR line. In contrast, there was no evidence of gold in the xMTS matrix (Figure 6b). The reason behind this observation was that gold nanoparticles were not actually self-assembled into the xMTS film. When the MTS multilayer film formed, one would expect to see evidence of S-H vibration or S-S bond formation.30 From the previous results and discussion, it was clear that gold nanoparticles could bind with the available -SH groups within the MTS sol-gel matrix. Therefore, the S-H evidence is expected to disappear for the MTS solgel matrix with gold nanoparticles. Infrared measurements were performed to confirm the chemical reaction of gold nanoparticles with thiol groups in the sol-gel structure. Figure 7 shows the FTIR spectra of sol-gels under various conditions. The strong signal at 1000-1200 cm-1 in each spectrum was characteristic of a Si-O-Si asymmetric vibration stretching associated with the siloxane network.31 It indicated that the sol-gel network
had been predominantly “polymerized”. The intense band at 2800-3000 cm-1 is typical of a C-Hx stretching mode connecting the alkyl chain between the thiol group and silicon. A weak band at about 2560 cm-1 could be observed for both the MTS film (Figure 7a) and the xMTS/gold (Figure 7b) and is the characteristic stretching mode for the thiol group.32 The xMTS/gold appears to keep the same chemical structure as MTS, and gold nanoparticles could not bind to the thiol group within the xerogel matrix. Alternatively, the thiol group in the xerogel sol-gel network was not available for the binding of the gold nanoparticles. The absence of the characteristic peak of thiol for the aMTS/gold (Figure 7c) suggests that the thiol group reacts with the gold nanoparticles by self-assembly to form a S-Au bond. Sol-Gel Process of MTS at Platinum in the Absence/Presence of Gold Nanoparticles. The processing steps involved in making sol-gel-derived silica monoliths are considered to explain the above observations. The typical sol-gel process can be divided into four steps: (A) hydrolysis (to form a sol); (B) gelation; (C) aging; and (D) drying. Hydrolysis results in the formation of silanol groups (Si-OH). The silanol groups react further to form siloxane polymers in the condensation reaction. Polycondensation continues along with localized solution and reprecipitation of the gel network, which increases the thickness of the interparticle necks and decreases the porosity. During aging and drying, the remaining H2O and alcohol expelled from the hydrolysis reaction are removed from the interconnected pore network. When the substrate is immersed in the prehydrolyzed MTS sol, selfassembly and concomitant gelation occurs. As gelation proceeds, the cross-linking of the structure becomes more dominant. When the substrate is removed from the sol, an aerogel film forms on the surface. The aerogel consists of amorphous primary particles of variable size (5-10 nm or smaller) with an interstitial liquid phase. At this stage, the pores remain filled with the liquid phase. This makes the entry of gold nanoparticles possible. With continuous aging and drying, the liquid phase in the interior of the pores is removed and considerable weight loss and shrinkage results. At this stage pore collapse occurs, decreasing the pore size (the average pore radius of the acid-catalyzed alkoxide silica gels when dry is 2.3 nm).2 The combination of these effects explains why xMTS films
(30) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1993, 5, 241244. (31) Pai, P. G.; Chao, S. S.; Takigi, Y.; Lucovsky, G. J. Vac. Sci. Technol., A 1986, 4, 689-694.
(32) (a) Crews, P.; Rodriguez, J.; Jaspars, M. Organic Structure Analysis; Oxford University Press: New York, 1998; pp 317-348. (b) Kim, C. H.; Han, S. W.; Ha, T. H.; Kim, K. Langmuir 1999, 15, 83998404.
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Figure 7. FTIR spectra: (a) xMTS film; (b) xMTS/gold; and (c) aMTS/gold on platinum sheets.
do not take up gold nanoparticles. In another words, easier access in the more open structures is seen while the -SH groups located in the smaller pores of the less porous materials are not accessible to the reactant.33 As for the MTS aerogel loaded with gold nanoparticles, the entry of gold nanoparticles did not stop the aging and drying process of the volume containing gold nanoparticles. The cross-linking network forms around the gold nanoparticles and finally blocks the heterogeneous electron transfer of the ferrocyanide redox marker. This result is consistent with the observation in the previous electrochemical probe studies13-15 that the connectivity of the gel voids was retained throughout the gelation process with macroscopic gel formation. In contrast to the previous work by Ikeda et al.28 we have been able to observe and clarify the different states of MTS film and the continuous sol-gel process of MTS in the absence and presence of gold nanoparticles. Permselectivity of the MTS Film on Pt Electrodes. Materials with tailor-made pore sizes and shapes are particularly important in applications where molecular recognition is needed, such as shape-selective catalysis, molecular sieving, chemical sensing, and selective adsorption.34 Sol-gel matrixes contain interconnected “bottleneck-like” pores formed by a three-dimensional SiO2 network, so it is important to consider such films as possible candidates for a permselective membrane for sensor fabrication. From the different features of gold nanoparticles interaction with aMTS and xMTS, it is expected that the state of the sol-gel material is critical to its permselectivity against some electroactive species. An impressive observation was that the MTS xerogel and aerogel films on Pt electrodes showed very different
permselectivity to hydrogen peroxide and L-ascorbic acid. It is found that 0.1 mM L-ascorbate produces a high current response of 7.3 nA at aMTS film-coated Pt electrode although most of the interference is excluded compared with that of a bare Pt electrode. By contrast, the current response of the same concentration of ascorbate on a xMTS film-coated Pt electrode was too small to observe while 0.1 mM hydrogen peroxide still resulted in a relatively high current response of 35.7 nA. It was concluded that the xMTS film showed much better selectivity (with a selectivity index of 1488, defined as the ratio of response of 0.1 mM hydrogen peroxide to that of 0.1 mM ascorbate) compared to the aMTS film (with selectivity index of 12) on a Pt electrode. The preparation conditions for a satisfactory MTS film need to be carefully controlled because many factors affect the quality of the final films, including temperature, pH, particular alkoxide precursor, solvent, and ratio of reactants as well as the relative humidity of drying.35 It was observed in this research that satisfactory permselectivity of the MTS film could be achieved when the conditions were carefully optimized. Moreover, the xerogel MTS film was stable for at least 1 month. It is easy to understand because pore shrinkage of silica gel upon drying is an irreversible process.6 Acknowledgment. We thank Linda Olafsen, Katherine Greene, Russ Middaugh, Donald Maclean, and Bruce Cutler for use of the FTIR and EDXS instruments and helpful discussions. Support by National Institutes of Health Grants DK55297 and NS37608 is gratefully acknowledged. LA034940J
(33) Walcarius, A.; Etienne, M.; Bessiere, J. Chem. Mater. 2002, 14, 2757-2766. (34) (a) Davis, M. E. Nature 1993, 364, 391-393. (b) Davis, M. E. Nature 2000, 403, 286-292.
(35) Collinson, M. M.; Wang, H.; Makote, R.; Khramov, A. J. Electroanal. Chem. 2002, 519, 65-71.
