Electrochemically Deposited Sol−Gel-Derived Silicate Films as a

Sol−Gel-Derived Silicate Films as a Viable Alternative in Thin-Film Design ... In this work, the electrodeposition of TMOS on conducting electro...
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Anal. Chem. 2003, 75, 5399-5405

Electrochemically Deposited Sol-Gel-Derived Silicate Films as a Viable Alternative in Thin-Film Design P. N. Deepa, Mandakini Kanungo, Greg Claycomb, Peter M. A. Sherwood, and Maryanne M. Collinson*

Department of Chemistry, 111 Willard Hall, Kansas State University, Manhattan, Kansas 66506-3701

Sol-gel-derived silicate films were electrochemically deposited on conducting surfaces from a sol consisting of tetramethoxysilane (TMOS). In this method, a sufficiently negative potential is applied to the electrode surface to reduce oxygen to hydroxyl ions, which serves as the catalyst for the hydrolysis and condensation of TMOS. The electrodeposition process was followed by the electrochemical quartz crystal microbalance and cyclic voltammetry. The electrodeposited films were characterized for their surface morphology, porosity, and film thickness using atomic force microscopy, electrochemical probe techniques, surface area and pore size analysis, and profilometry. The electrodeposited films were found to have a completely different surface structure and to be significantly rougher relative to spin-coated films. This is likely due in part to the separation of the gelation and evaporation stages of film formation. The electrodeposited films were found to be permeable to simple redox molecules, such as ruthenium(III) hexaammine and ferrocene methanol. Film thickness can be easily varied from 15 µm by varying the electrode potential from -600 mV to more than -1000 mV, respectively. The electrodeposition process was further applied for the electroencapsulation of redox molecules and organic dyes within the silicate network. Cyclic voltammograms for the gel-entrapped ferrocene methanol (FcCH2OH) and ruthenium(II) tris(bipyridine) (Ru(bpy)32+) exhibited the characteristic redox behavior of the molecules. The electroencapsulation of organic dyes in their “native” form proved to be more difficult because these species typically contain reducible functionalities that change the structure of the dye. The sol-gel approach has been used to develop new materials for catalysis, chemical sensors, separations, optical gain media, photochromic and nonlinear applications, and solid-state electrochemical devices.1-7 A typical procedure involves mixing an * Corresponding author. Phone: 785-532-1468. Fax 785-532-6666. E-mail: [email protected]. (1) Avnir, D. Acc. Chem. Res. 1995, 28, 328. (2) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67, 22A. (3) Collinson, M. M.; Howells, A. R. Anal. Chem. 2000, 72, 702A-709A. (4) Dunn, B.; Farrington, G. C.; Katz, B. Solid State Ionics 1994, 70/71, 3-10. (5) Lin, J.; Brown, C. W. Trends Anal. Chem. 1997, 14, 200-211. 10.1021/ac026459o CCC: $25.00 Published on Web 09/12/2003

© 2003 American Chemical Society

alkoxysilane (i.e., TMOS) with water, alcohol, and a catalyst, such as HCl or NH3. The silane hydrolyzes and condenses, as depicted in the simplified reaction mechanism shown below.8

Si(OCH3)4 + H2O f ≡Si-OH + CH3OH ≡Si-OH + HO-Si≡ f ≡Si-O-Si≡ + H2O Much of the interest in sol-gel chemistry stems from the fact that these materials can be easily prepared and modified and their physical and chemical properties tailored via judicious choice of the sol-gel processing conditions.1-8 Two of the most common configurations of sol-gel-derived materials are monoliths and thin films. Monoliths are prepared by pouring the silica sol into a container (i.e., polystyrene cuvette) and allowing it to gel, age, and dry slowly. Thin films of silica have been prepared by dipcoating or spin-coating the silica sol on the surface of a suitable substrate.8,9 At room temperature, an aerosol deposition technique has also been used to prepare thin films for sensing application.10 The ability to form thin films with fast response times is particularly vital to the development of advanced applications, because it decreases the response time of the sensor device. Although sol-gel monoliths can be quite porous, thin films are significantly less porous.8-9,11-12 This is because gelation and solvent evaporation occur simultaneously in the preparation of films, whereas in monoliths, evaporation, aging, and drying can proceed for weeks. The porosity of the film is highly dependent on the manner in which it is formed, the structure of the sol, and the rate of evaporation and drying.8-9,11-12 Spin-coated films prepared from acid-catalyzed sols can be quite dense and nonporous.8-9,13-14 Four years ago, a preliminary note appeared that reported on the fabrication of sol-gel-derived hydrophobic silica films using (6) Levy, D. Chem. Mater. 1997, 9, 2666-2670. (7) Blum, J.; Avnir, D.; Schumann, H. CHEMTECH 1999, 29, 32-38. (8) Brinker, J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1989. (9) Brinker, C. J.; Hurd, A. J.; Schunk, P. R.; Frye, G. C.; Ashley, C. S. J. NonCryst. Solids 1992, 147-148, 424-436. (10) Jordan, J. D.; Dunbar, R. A.; Hook, D. J.; Zhuang, H.; Gardella, J. A.; Colon, L. A.; Bright, F. V. Chem Mater. 1998, 10, 1041-1051. (11) Frye, G. C.; Ricco, A. J.; Martin, S. J.; Brinker, C. J. Mater. Res. Soc. Symp. Proc. 1988, 12, 348-354. (12) Brinker, C. J.; Frye, G. C.; Hurd, A. J.; Ashley, C. S. Thin Solid Films 1991, 201, 97-108. (13) Collinson, M. M.; Rausch, C. G.; Voigt, A. Langmuir 1997, 13, 7245-7251. (14) Collinson, M. M.; Wang, H.; Makote, R.; Khramov, A. J. Electroanal. Chem. 2002, 519, 65-71.

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an electrodeposition procedure.15 In this method, an electrode (gold or indium tin oxide) was placed in a sol prepared from methyltrimethoxysilane (MTMOS).15 A negative potential was applied to increase the pH at the electrode surface, which caused the immediate condensation of the silane on the electrode surface. The one unique aspect of this procedure is that gelation occurs separately from evaporation and drying, potentially leading to films with greater porosity. In this work, the electrodeposition of TMOS on conducting electrode surfaces to form hydrophilic films for chemical sensing related applications has been studied. In particular, we have sought to investigate the deposition procedure using TMOS as the silicon precursor, thoroughly characterize the physical properties of the resultant films, and demonstrate the pros and cons associated with the entrapment of species in the film during electrodeposition. The deposition procedure has been followed using an electrochemical quartz crystal microbalance and cyclic voltammetry. Furthermore, the films have been characterized using AFM, electrochemical probe techniques, surface area and pore size analysis, and profilometry. We have also shown that it is possible to trap reagents into the film by simply adding them at relatively high concentrations to the sol. The results show that electrodeposition is a viable method for the formation of films for many possible applications. EXPERIMENTAL SECTION Reagents and Materials. Tetramethoxysilane (TMOS, 99%) was purchased from Fluka. Ferrocene methanol (FcCH2OH) (97%) and ruthenium(II) tris(bipyridine) (Ru(bpy)32+) (98%) were purchased from Aldrich. Ruthenium(III) hexaammine (Ru(NH3)63+) (99%) was obtained from Strem. Potassium ferricyanide (Fe(CN)63-), potassium chloride (KCl), potassium nitrate (KNO3), and indicator dye solution were obtained from Fisher Scientific. Indicator dyes (i.e., the sulfothalein, azo, and nitro dyes) were obtained from either Aldrich, Fisher, or Allied Chemical. The working electrode was typically a 5-mm-diameter glassy carbon electrode fabricated by sealing a glassy carbon rod in a glass tube with epoxy. Other electrodes used were either a 2-mm-diameter gold electrode (CH instruments), a 2-mm-diameter Pt electrode (BAS), an antimonydoped tin oxide electrode (12 × 12 mm, Delta Technologies), or an ITO electrode (12 × 12 mm, Delta Technologies). For the electrodeposition experiments, the auxiliary electrode was a glassy carbon electrode and the reference electrode was a Ag/AgCl wire electrode. It is not a good idea to use a fritted reference electrode because the TMOS will eventually clog the pores. In all other experiments, the auxiliary electrode was a coiled Pt wire, and a Ag/AgCl (0.1M KCl) was used as a reference. The electrodes were polished with 0.05-µm alumina particles on a napless polishing cloth and cleaned by sonication in water for 10 min prior to use. Instrumentation. Water was purified to type I by a Labconco four-cartridge water purification system. A one-chamber, threeelectrode cell was used with a Bioanalytical System (BAS) CV50 potentiostat to carry out the electrochemical work. The film thickness was measured with a Tencor Alpha step 500 surface profilometer. The AFM images were acquired in the tapping mode with a Nanoscope IIIa multimode SPM microscope (Digital Instruments, Inc., Santa Barbara, CA) using a microfabricated Nanopore silicon nitride tip at a scan rate of 0.5-1 Hz. Electro(15) Shacham, R.; Avnir, D.; Mandler, D. Adv. Mater. 1999, 11, 384-388.

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chemical quartz crystal microbalance (EQCM) experiments were performed using a computerized time-resolved EQCM assembly obtained from CH Instruments Inc. In this setup, the QCM is integrated with the potentiostat and is controlled by an external computer. The quartz crystal (International Crystal Manufacturing) had a fundamental frequency of 10 MHz. The gold disk (0.2 cm2) on top of the crystal in contact with the electrolyte solution served as the working electrode. The reference and counter electrodes were a Ag/AgCl wire and a platinum wire, respectively. A Quantachrome Autosorb 1 MP surface area and pore size analyzer was used to acquire the N2 adsorption-desorption isotherms. The sample was outgassed at 120 °C for 24 h prior to analysis. An equilibration time of 3 min was used. XPS spectra were collected using a SPECS Sage100 spectrometer operating in the fixed analyzer transmission mode using achromatic Mg KR (1253.6 eV) radiation at 240 W (12 kV and 20 mA). The base pressure of the chamber was ∼2 × 10-8 Torr, and the energy scales were calibrated using copper16,17 and the separation between photoelectron peaks generated by Mg and Al KR X-rays. Survey spectra were collected with a pass energy of 30 eV; a pass energy of 15 eV was used for both core and valence band spectra. Spectra were calibrated by taking the C1s peak due to residual hydrocarbon as being at 284.6 eV. Data were collected on the film prepared from a sol containing 200 mM FcCH2OH. The sample was examined at two different takeoff angles, 90° and the more surface-sensitive angle 30°, with respect to the sample surface. Procedures. The electrochemical deposition of the silica on conducting substrates was carried out predominately using chronoamperometry. The electrochemical cell typically contained 2 mL of 0.2 M KCl, 2 mL of ethanol, and 0.5 mL of TMOS. The solution was sonicated for 5 min. In some cases, the sol was prepared without ethanol; in this case, the sol was sonicated for 15-20 min until one phase was observed. Potentials in the range of -700 to -1100 mV were applied to the working electrode for a period of 30 min. The sol-gel electrodeposited electrode was taken out of the cell, rinsed, and dried overnight at a relative humidity of 35%. The electroencapsulation of reagents within the sol-gel matrix was achieved by adding the molecule of interest to the starting sol and applying a negative potential to the working electrode as described earlier. A base-catalyzed sol was prepared by mixing 0.3 mL of TMOS, 0.5 mL of methanol, and 0.2 mL of water. After 40 min, 50 µL of 0.01 M NH3 was added. The sol was spin-cast at ∼7000 rpm on a glassy carbon electrode just prior to its gelation. An acid-catalyzed sol was prepared by mixing 0.75 mL of TMOS, 1.2 mL of methanol, 0.65 mL of water, and 0.15 mL of 0.1 M HCl. After 6 days, the sol was spin-coated at ∼7000 rpm on a glassy carbon electrode. RESULTS AND DISCUSSION Electrodeposition. In the electrodeposition process, the application of negative potential to the working electrode results in an increase in the pH at the electrode surface via the oxidation of oxygen, water, or both (see below). These ions catalyze the hydrolysis and condensation of the sol, and hence, a base-catalyzed sol-gel process occurs at the electrode surface. Electrodeposition (16) Seah, M. P.; Gilmore, I. S.; Beamson, G. Surf. Interface Anal. 1998, 26, 642. (17) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.

of organically modified silicate films via a base-catalyzed condensation of MTMOS in an acid-catalyzed sol containing KNO3 has been previously described in a preliminary note by Mandler and co-workers.15 The reactions involved in film formation were believed to be an increase in pH via the reduction in H+ concentration, nitrate (from the supporting electrolyte), or both at the ITO surface.15 In this work, we have taken a closer look at the deposition process, the physical characteristics of the films thus produced, and their ability to entrap reagents. TMOS was used as the starting precursor because it produces hydrophilic films that are potentially useful in many applications.1-7 Electrodeposition was carried out via application of a negative potential to an electrode placed in a sol prepared by mixing TMOS with water in the presence (or absence) of ethanol as a mutual solvent. A colored film is immediately observed after the electrode is removed from the sol. In contrast, no film is observed if the electrode is just left in the sol for 30 min without application of sufficiently negative potentials. Typically, films were prepared from sols that contained 0.7 M TMOS. However, concentrations as low as 1 mM can still be used to form thin films (∼75 to 100 nm) on glassy carbon electrodes. In an attempt to quantify the pH at the electrode surface during electrodeposition, a few drops of a commercial indicator dye solution was added to the sol. Upon application of -900 mV, the surface of the glassy carbon electrode turned blue-green, indicative of a pH of 8.5 at the electrode surface (compared to 5-5.5 in bulk). If the bulk pH of a silica sol was suddenly increased to pH 8.5 via the addition of OH- ions, the sol would immediately gel. Therefore, this surface pH is high enough to cause the sol to gel at the electrode surface. Hydroxide can be generated and H+ depleted at electrode surfaces via a number of electrochemical process, all of which have very large overpotentials, and all are dependent on the electrode material. Therefore, it is difficult to precisely state which reaction is occurring conclusively. In this case, it will likely be several reactions that involve both the production of OH- and maybe even the reduction of H+ at the electrode surface through oxygen reductions (see below). As judged from the literature, the most pertinent reactions are18-21

2H2O + 2e- f 2OH- + H2 O2 + 2H2O + 4e- f 4OHO2 + 2H2O + 2e- f H2O2 + 2OH-

Figure 1 shows the cyclic voltammograms acquired at a glassy carbon electrode in the silica sol containing TMOS, water, ethanol, and potassium chloride. The KCl was needed as a supporting electrolyte. As can be seen, a reduction wave commencing at potentials more negative than -600 mV was observed. This peak (18) Bowyer, W. J.; Xie, J.; Engstrom. R. C. Anal. Chem. 1996, 68, 2005-2009. (19) Ratcliff, B. B.; Klancke, J. W.; Koppang, M. D.; Engstrom, R. C. Anal. Chem. 1996, 68, 2010-2014. (20) Bard, A. J.; Faulkner, LR. Electrochemical Methods. Fundamentals and Applications; John Wiley and Sons: New York: 1980; p 175. (21) The pH of the sol used to electrodeposit the film is ∼5 to 5.5. Since the concentration of H+ is not high, the redox reaction between H+ and oxygen to form water will likely not be as important as the reactions shown.

Figure 1. Cyclic voltammograms acquired at a glassy carbon electrode in the TMOS sol before (a) and after deoxygenation (b). Scan rate: 100 mV/s.

Figure 2. Current-time (chronoamperometric) and ∆F-time (QCM) plots obtained at a gold electrode in a TMOS sol following a potential step from 0.0 to -800 mV.

is due to the reduction of oxygen to hydroxyl ions at the electrode surface. To verify this, a CV was recorded in the sol after it has been deoxygenated with water-saturated nitrogen. As can be seen, no reduction wave was observed. In the absence of oxygen, no film could be obtained at the working electrode at potentials less negative than -1000 mV, stressing the fact that electrodeposition at the given potentials likely results from the production of hydroxyl ions from the reduction of oxygen.18-20 The electrodeposition process can be followed in a number of different ways. Perhaps the simplest method is to use the electrochemical quartz crystal microbalance. The technique has been used for studying the growth or dissolution of thin films on electrode surface.22-24 In this experiment, a gold-coated quartz crystal resonating at a fundamental frequency of 10 MHz is used as the working electrode. As mass deposits on the crystal, the frequency will drop.24 Figure 2 shows the current-time and frequency (∆F)-time plots recorded at the gold electrode following a potential step from 0 V to -800 mV. As can be seen, ∆F drops as soon as the potential is stepped to -800 mV. The OHgenerated at the electrode surfaces causes the immediate condensation of TMOS. ∆F continues to steadily drop until the crystal stops oscillating. At this point, the silicate film has become very (22) Schumacher, R.; Muller, A.; Stockel, W. J. Electroanal. Chem. 1987, 219, 311. (23) Hinsberg, W.; Wilson, C.; Kanazawa, K. K. J. Electrochem. Soc. 1986, 133, 1448. (24) Buttry, D. A. In Electroanalytical Chemistry, Bard, A. J., Ed.; Marcel Dekker: New York, 1991; p 1-85.

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Figure 3. Repetitive cyclic voltammetric curves obtained at a glassy carbon electrode in a TMOS sol containing 52 mM Ru(NH3)63+. Scan rate: 10 mV/s.

thick (µm), and when the electrode is removed from the cell and dried, a distinctive white film flakes off the surface. As noted below, on gold (and platinum, as well), it is very difficult to form a thin adherent film on the surface. Similar results are observed when the electrode potential is linearly scanned as in a cyclic voltammetric experiment from 0 to -1.4 V and back again. ∆F starts to significantly drop at potential more negative than -800 mV on the second cycle and continues to drop until the crystal stops oscillating after three cycles. Another method that can be used to follow the deposition process involves examination of the Faradaic current of a redox probe added to the sol at relatively high concentrations. In this experiment, Ru(NH3)63+, which is reversibly reduced at -0.17 V, is used as the redox probe. Figure 3 shows the cyclic voltammograms (CVs) recorded at a glassy carbon electrode in a TMOSbased sol containing 52 mM Ru(NH3)63+. As the potential is scanned negative, Ru(NH3)63+ is reduced at the electrode surface, as signified by an increase in current. As the film forms on the surface of the glassy carbon electrode, it blocks the electron transfer of Ru(NH3)63+, and hence, the current continually decreases with each successive scan. The magnitude at which the current for the reduction of Ru(NH3)63+ decreases depends on the number of potential cycles, the scan rate, and the switching potential. In this particular example, the film begins to form during the first cycle and becomes significantly compact by the sixth cycle. Silicate films can be deposited on any conducting surface. We have specifically deposited films on gold, platinum, tin oxide, ITO, and glassy carbon. The potential needed to electrochemically deposit a film on the electrode surface depends somewhat on the electrode material. For glassy carbon, films can be formed using potentials more negative than about -700 mV, whereas for tin oxide, potentials of about -1000 mV are needed. At relatively low electrode potentials (-700 to -900 mV), thin colored films that range in thickness from ∼75 nm (dark brown), 100 nm (light blue), and 180 nm (yellow/violet) can easily be obtained on glassy carbon. The greater the negative potential, the thicker the films. Application of large negative potentials (typically more negative than -1.1 V) results in white films that can be >15 µm in 5402 Analytical Chemistry, Vol. 75, No. 20, October 15, 2003

thickness. For platinum and gold, only thick white films that usually flake off the electrode surface can be obtained, regardless of the electrode potential. Both tin oxide and glassy carbon have surface oxides that can bond to the silica films, thus promoting its adhesion to the surface. Pemberton and co-workers have noted that the formation of strongly adherent silicate films on gold and silver surfaces is best achieved by first modifying the surface with a binding agent, such as (3-mercaptopropyl) trimethoxysilane.25 It is important to note that hydronium ions also act as a catalyst for the hydrolysis and condensation of TMOS. H+ can be generated in solution via applications of potentials positive enough to oxidize water at the electrode surface. We attempted to carry out the electrodeposition of silicate films by application of potentials near and at the solvent oxidation limits. No films were obtained, regardless of the magnitude of the electrode potential (1.6-2.2 V), the electrodeposition time (10-60 min), and the electrode used (i.e., platinum, tin oxide, and glassy carbon). Under acid-catalyzed conditions, the rate of condensation is much smaller relative to base-catalyzed systems, and this undoubtedly plays an important role in film formation.8 Film Characterization. The electrodeposited silicate films have been characterized using AFM, electrochemical probe techniques, and surface area and pore size analysis. AFM is a powerful tool that can give information about the surface roughness and morphology of the films. Figure 4 shows AFM images of two silica films (brownish-violet and blue) on glassy carbon substrates. As can be seen, the surfaces of the films are very rough and irregular. In fact, it looks like the film consists of aggregates of particles that are linked together, forming a gel. For comparison, AFM images were acquired for films spin-coated on a glassy carbon substrate prepared from an acid-catalyzed sol and from a basecatalyzed sol (Figure 5). Upon examination of these images, it can be seen that the film prepared from the acid-catalyzed sol is more compact (less rough) than the film prepared from the basecatalyzed sol. This is expected, because spin-cast films prepared from acid-catalyzed sols, which are predominantly composed of entangled linear chains, will yield low-pore-volume, dense films.8,11-12 Films prepared from base-catalyzed sols, in contrast, will be more particulate as a result of the particulate nature of the sol.8,11-12 This is observed in Figure 5. Both spin-cast films, however, are more compact and less rough than the electrodeposited films.8,11-12 The major differences between the structure of the electrodeposited and spin-coated films can likely be attributed to differences in the formation process. When spin-casting a sol, gelation and evaporation occur simultaneously and very rapidly - less than a second.8-9,11-12 In case of the spin-coated film the overlap of the deposition and evaporation stages established a competition between evaporation (which compacts the structure) and continuing condensation reactions (which makes the structure stronger).8-9,11-12 The only process occurring during electrodeposition is however gelation; only after the film is formed is it then removed and dried. Electrochemical probe techniques can also be used to indirectly evaluate whether the films are compact with few direct channels to the underlying electrode surface or more porous with (25) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1995, 7, 130-136.

Figure 4. AFM images of a silicate film (A, blue/brown film; B, blue/ yellow film) electrodeposited on a glassy carbon electrode.

channels/pores.14,26

many In this method, the film coated electrode is placed in a solution of a redox probe and a cyclic voltammetric curve obtained. Transport of redox molecule can occur either via diffusion or by partitioning across the film. In previous work, spincoated films prepared from an acid-catalyzed sol were relatively compact with very little direct channels to the underlying electrode surface.14,26 In this work, electrodeposited silicate films of varying thickness were characterized by recording CVs of the film in 1 mM FcCH2OH, 1 mM Ru(NH3)63+, and 1 mM Fe(CN)63-. These molecules were chosen because they have relatively fast electrontransfer kinetics and also have different charges, which enables any probe dependent effects to be evaluated. Figure 6 shows the voltammograms obtained at a dark brown film acquired via application of -800 mV in a solution of FcCH2OH, Ru(NH3)63+, and Fe(CN)63-. Well-defined peaks due to the oxidation of ferrocene to ferrocinium (Figure 6A) and the reduction of Ru(NH3)63+ (Figure 6B) can be seen signifying that the redox probe has direct access to the underlying electrode surface. For comparison, the peak current for the bare glassy carbon electrode in the same solutions are 38 µA, 31 µA, and 29 µA for FcCH2OH, Ru(NH3)63+, and Fe(CN)63-, respectively. Ru(26) Khramov, A. N.; Munos, J.; Collinson, M. M. Langmuir 2001, 17, 81128117.

Figure 5. AFM images of a silicate film spin-cast (7000 rpm) on a glassy carbon electrode from an acid-catalyzed sol and a basecatalyzed sol.

(NH3)63+, being a positively charged probe, can ion-exchange to some extent into the negatively charged films (pI of silica ∼227), thus its current appears higher in magnitude relative to the other two probes. Fe(CN)63-, on the other hand, is negatively charged and cannot easily diffuse/partition into the film thus little Faradaic current can be observed for this redox probe. The magnitude of the Faradaic current observed for these redox probes does depend on the thickness of the film. The thicker the films the smaller the resultant Faradaic current with “white” films giving no response. As the film sits in solution (>3 h), the Faradaic current for the redox probes increases slightly. As noted in prior work, this is due to the development of pinholes in the films as it sits in solution.14 In most cases, however, the film does not fall off the electrode surface as evident from the voltammetry and via examination of the film until 24-48 h. Perhaps the best way to directly assess porosity is via the acquisition of nitrogen adsorption-desorption isotherms from which surface area and pore diameter can be obtained. While this is relatively simple to do with monoliths, it is very difficult to do (27) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.

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Figure 7. N2 adsorption-desorption isotherm of electrodeposited films prepared on tin oxide electrodes.

Figure 6. Cyclic voltammograms acquired at a electrodeposited film on a glassy carbon electrode in a solution containing 1 mM ferrocene methanol, 1 mM ruthenium(III) hexaammine, and 1 mM potassium ferricyanide. Supporting electrolyte: 0.1 M KCl. Scan rate: 50 mV/s.

with thin films as there is very little material on the surface (micrograms or less). Brinker and co-workers addressed this problem by using a surface acoustic wave device.11 They have shown that films prepared from acid and base-catalyzed sols are much less porous than corresponding monoliths.12 In fact, the acidcatalyzed films they made were nonporous.11,12 In this work, we do not have access to such instrumentation. As a result, we were unable to directly determine the porosity of spin-cast films. In the case of the electrodeposited films, however, thick white films that easily flake off the electrode surface can be easily prepared in a short period of time. Figure 7 shows the N2 adsorption-desorption isotherm for a sample collected from ∼20 electrodeposited films grown on tin oxide electrodes. The isotherm is characteristic of a mesoporous solid.28-30 The observed hysteresis loop can be classified as Type H3, which has been reported “for materials comprised of aggregates of platelike particles forming slitlike pores.”28-30 The BET surface area is ∼50-100 m2/g and the material exhibits little microporosity. The BJH (28) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619. (29) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: New York 1999; Chapter 7. (30) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169-3183.

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surface pore size distribution obtained from the desorption branch is shown as an inset in Figure 7. Electroencapsulation. Much of the interest in sol-gel-derived materials stems from the fact that these materials can be used to entrap receptors of various sizes and types and they are porous enough to allow external analyte species to diffuse through the pores and channels to react with the entrapped receptor.1-3 In this work, we sought to evaluate if reagents can be entrapped into the network during electrodeposition and the pros and cons associated with this procedure. Theoretically, it should be possible to entrap reagents into the matrix by adding the dopant to the sol prior to the application of a negative potential. Questions that need to be considered, however, include (1) does the dopant contain reducible functionalities, (2) are these reducible functionalities irreversibly reduced at the potentials used to form the silicate films, and (3) does the dopant or its reduced product significantly adsorb onto the electrode surface from the relatively high concentrations that need to be used. In this work, several different dopants were investigated (redox molecules and pHsensitive organic dyes). The entrapment of redox probes into the film can easily be assessed via acquisition of a cyclic voltammogram, whereas for organic dyes, UV-vis absorption spectra can be obtained. Figure 8 shows a representative cyclic voltammetric response for ferrocene methanol and Ru(bpy)32+ entrapped in a bluishbrown colored film obtained on a glassy carbon electrode. To obtain a decent signal, relatively high concentrations of the dopant must be added to the sol (>100 mM). As can be seen, the molecules exhibit their characteristic redox behavior even when trapped in the silicate network. For gel-encapsulated Ru(bpy)32+, the Faradaic peaks are generally not as well defined as observed for ferrocene methanol, in part as a result of its larger size and, hence, slower diffusion. The electrochemical behavior of the electroencapsulated redox probes is similar to that obtained when the probe is trapped in a spin-cast film.13 The Faradaic current is relatively small, and it decreases with time, particularly when the electrode potential is cycled. The electroactive fraction of the entrapped reagent is likely that portion of the total concentration that is able to diffuse through small channels or voids in the film.13 The leaching of small molecules from sol-gel-derived films is very common.13,31-32 To further ensure the doped species was entrapped throughout the film and not just located at the electrode surface (due to adsorption), XPS was used to examine the outermost layer of the

from a slightly acidic sol. More experiments are being conducted to further understand the differences between these films. This will be the subject of future work. The entrapment of organic dyes in their functional form proved to be more difficult, because many dyes contain quinone groups that can irreversibly be reduced at the potentials needed to electrodeposit the films on either ITO or tin oxide. A few dyes from several classes of indicator dyes were evaluated by cyclic voltammetry. The dyes used were (1) sulfothalein dyes (bromocresol purple, bromophenol blue, cresol red), (2) Azo dyes (methyl red, methyl orange, alizarin red), (3) triphenylmethane dyes (crystal violet, malachite green), (4) nitro dyes (dinitrophenol), and (5) metallochromic indicators (methylene blue). Most of these dyes were irreversibly reduced at potentials ranging from -0.6 to -1.0 V on glassy carbon. In the case of methylene blue, strong adsorption on the electrode surface occurred, which prevented its use. For methyl red, thin films were successfully made on optically transparent tin oxide. However, the absorption spectrum was different from that for the dye in solution, and the spectrum did not change as expected when the pH of the solution was changed.

Figure 8. Cyclic voltammetric response for gel-encapsulated ferrocene methanol and gel-encapsulated ruthenium(II) tris(bipyridine). Supporting electrolyte: 0.1 M KCl. Scan rate: 100 mV/s for FcCH2OH and 50 mV/s for Ru(bpy)32+.

film (top ∼100Å). XPS data (binding energies in parentheses) collected at a normal takeoff angle of 90° revealed the presence of carbon (284.6 eV), oxygen (532.4 eV), silicon (102.8 eV), iron (710.1 eV), and a very small amount of potassium (from the supporting electrolyte) in a film prepared from a sol containing 200 mM FcCH2OH. By collecting data at the more surfacesensitive angle of 30°, it would be expected that the relative amount of iron should increase if the doped species is simply adsorbed to the surface.33 However, it was observed that there was no change in the relative atomic percentages of oxygen, silicon, iron, and potassium, indicating a fairly uniform film and that the doped species is not solely adsorbed on the electrode surface or the outermost surface of the film. There was the expected increase in the amount of carbon consistent with the expected overlayer of hydrocarbon, which is seen on nearly all solids that have been exposed to the atmosphere. The binding energy of the iron 2p (3/2) was consistent with iron in the 3+ state,34 in contrast to that obtained with a spin-coated film prepared (31) Aharonson, N.; Altstein, M.; Avidan, G.; Avnir, D.; Bronshtein, A.; Lewis, A.; Liberman, K.; Ottolenghi, M.; Polevaya, Y.; Rottman, C.; Samuel, J.; Shalom, S.; Strinkovski, A.; Turniansky, A. Mater. Res. Soc. Symp. 1994, 346, 519-529. (32) Zusman, R.; Rottman, C.; Ottolenghi, M.; Avnir, D. J. Non-Cryst. Solids 1990, 122, 107. (33) Ansell, R. O.; Dickinson, T.; Povey A. F.; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom. 1977, 11, 301. (34) Umana, M.; Rolison, D. R.; Nowak, R.; Daum, P.; Murray, R. W. Surf. Sci. 1980, 101, 295-309.

CONCLUSIONS Silicate films can be electrodeposited on conducting surfaces from TMOS via application of a potential significantly negative to reduce oxygen to hydroxide ions. This method has several advantages relative to the formation of films using more traditional methods, such as spin-casting or dip-coating. The thickness of the films can easily be varied without having to significantly dilute the sol; gelation and drying proceed independently of each other, enabling films with potentially greater porosity to be produced; and films can be prepared with very low amounts of TMOS, thereby minimizing the hazards associated with its use and disposal. A potentially promising application of this method could lie in the entrapment of biomolecules. Conventional silica sols often contain high amounts of TMOS, which produces methanol during hydrolysis that can potentially denature biomolecules. Films electrodeposited on conducting surfaces can be prepared from sols that contain very low amounts of TMOS, which in turn will produce comparably low amounts of methanol. Furthermore, acid is not needed to hydrolyze the sol; therefore, it may be possible to just dope in the protein/enzyme without first buffering the sol. The only concern that needs be contended with would be the slightly basic pH at the electrode surface and the need to use relatively high concentrations of a dopant that contains preferably nonreducible functionalities. ACKNOWLEDGMENT We gratefully acknowledge support of this work by the National Science Foundation (NSF) and the Office of Naval Research through DURIP. We also thank Helene Maire for the preparation of silicate films for thickness and AFM measurements and Professor Ed Bowden for helpful discussions.

Received for review December 21, 2002. Accepted August 6, 2003. AC026459O Analytical Chemistry, Vol. 75, No. 20, October 15, 2003

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