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Tailoring Perfluorosulfonated Ionomer-Entrapped Sol-Gel-Derived Silica Nanocomposite for Spectroelectrochemical Sensing of Re(DMPE)3+ Zhongmin Hu, Andrew F. Slaterbeck, Carl J. Seliskar,* Thomas H. Ridgway, and William R. Heineman* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172 Received June 29, 1998. In Final Form: November 30, 1998 The influence of the initial molar ratio of water relative to tetraethyl orthosilicate (TEOS) precursor and the content of Nafion ionomer in sol-gel-derived silica composites on the voltammetric response of electrodes modified with these composites for [ReI(DMPE)3]+ was investigated. The slow diffusion of [ReI(DMPE)3]+ in Nafion can be significantly improved by dispersing Nafion in sol-gel-derived silica, and the diffusion of [ReI(DMPE)3]+ in such a composite increases with the increase in water/TEOS molar ratio and the decrease in Nafion content. With the mass ratio of Nafion relative to sol-gel-derived silica being 40:100 and the initial molar ratio of water relative to TEOS being 20:1, the electrodes modified with the derived Nafion-silica nanocomposite exhibited an apparent peak current increase rate, during preconcentration of [ReI(DMPE)3]+, that was approximately three times faster than the corresponding Nafion-modified electrode. Compared with bare indium-tin oxide (ITO) glass, the composite-coated ITO glass showed a 25-fold enhancement in voltammetric response to [ReI(DMPE)3]+. The suitability of the developed optically transparent Nafion-silica composite for spectroelectrochemical sensing of [ReI(DMPE)3]+ was demonstrated. The [ReI(DMPE)3]+ extracted into the coating (∼0.4 µm in thickness) was electrolyzed to [ReII(DMPE)3]2+. Under attenuated total reflection mode, the in-situ electrogenerated chromophore [ReII(DMPE)3]2+ was monitored by probing its interaction with the evanescent field of light of a selected wavelength. Thus, the elements required for a spectroelectrochemical sensor with three modes of selectivity were demonstrated: partitioning into the film on an electrode surface and an electrochemically modulated optical signal.
Introduction Spectroelectrochemical sensing, which incorporates three modes of selectivity, partitioning, electrochemical, and spectroscopic, in a single device, has been demonstrated.1,2 Coated with a thin film of chemically selective material, the spectroelectrochemical device is capable of selectively extracting an analyte from a sample based on one or more chemistry-dependent driving forces. By selectively electrolyzing the analyte which partitions into the film and by selectively monitoring the electrolyzed analyte spectroscopically, the spectroelectrochemical device is capable of responding to the analyte and discriminating against interferents. [ReI(DMPE)3]+ complex ion, where DMPE ) 1,2-bis(dimethylphosphino)ethane, is a nonradioactive analogue of the radioactive cation, [99mTcI(DMPE)3]+, which represents a step in the development of technetium myocardial perfusion imaging agents.3,4 [ReI(DMPE)3]+ is electroactive. It undergoes reversible one-electron electrochemical oxidation to [ReII(DMPE)3]2+ with a formal potential around 0.0 V versus Ag/AgCl,5-7 depending on the nature of the electrode and the electrolyte solution. (1) Shi, Y.; Slaterbeck, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679-3686. (2) Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4819-4827. (3) Gerson, M. C.; Deutsch, E. A.; Libson, K. F.; Adolph, R. J.; Ketring, A. R.; Vanderheyden, J.-L.; Williams, C. C.; Saenger, E. L. Eur. J. Nucl. Med. 1984, 9, 403-407. (4) Deutsch, E.; Libson, K.; Vanderheyden, J.-L.; Ketring, A. R.; Maxon, H. R. Nucl. Med. Biol. 1986, 13, 465-477. (5) Hu, Z.; Seliskar, C. J.; Heineman, W. R. Anal. Chem., in press. (6) Deng, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4045-4050. (7) Kirchhoff, J. R.; Allen, M. R.; Cheesman, B. V.; Okamoto, K.; Heineman, W. R.; Deutsch, E. Inorg. Chim. Acta 1997, 262, 195-202.
The oxidized form, [ReII(DMPE)3]2+, absorbs light in the visible region, whereas [ReI(DMPE)3]+ is colorless. With a suitable coating which allows the partitioning of [ReI(DMPE)3]+ and limits the access of some potential interferents to the electrode surface, [ReI(DMPE)3]+ can be selectively preconcentrated in the film. With an applied electrolysis potential, at which [ReI(DMPE)3]+ is oxidized to [ReII(DMPE)3]2+, and with light of a selected wavelength at which [ReII(DMPE)3]2+ absorbs, [ReI(DMPE)3]+ would be able to pass all of the three levels of interrogation. Other species in the sample, which may interfere with the response of a sensor with only one or two modes of selectivity, might not be able to “answer correctly” to all of these three interrogations in sequence and, hence, be discriminated against. Sol-gel processing involves hydrolysis and polycondensation of molecular precursors, mostly metal and semimetal alkoxides, under ambient conditions and leads to the formation of ceramic materials.8-10 The sol-gel process is amenable to the incorporation of organic moieties in inorganic matrixes, in both hybrid and composite forms. Organic-inorganic hybrid materials have been synthesized using functionalized organic components, such as organosilanes, where chemical bonding is established between organic and inorganic moieties.11-16 Sol-gel techniques have been exploited to encapsulate (8) Schubert, U. J. Chem. Soc., Dalton Trans. 1996, 3343-3348. (9) Brinker, C. J. J. Non-Cryst. Solids 1988, 100, 31-50. (10) Corriu, R. J. P.; Leclercq, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 5, 1420-1436. (11) Hsueh, C.-C.; Collinson, M. M. J. Electroanal. Chem. 1997, 420, 243-249. (12) Wen, J.; Wilks, G. L. Chem. Mater. 1996, 8, 1667-1681. (13) Panusa, A.; Flamini, A.; Poli, N. Chem. Mater. 1996, 8, 12021209.
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proteins17,18 and enzymes.19,20 A variety of ionomers have been entrapped in sol-gel-derived silica. For example, poly(acrylic acid) and sodium poly(styrene sulfonate) were entrapped in sol-gel-derived silica and phase separation, which occurs in parallel with the hydrolysis, condensation, and gelation of silicon alkoxide solution containing the ionomers, was investigated.21,22 Sodium poly(vinyl sulfonate) and poly(dimethyldiallylammonium chloride) (PDMDAAC) were doped in sol-gel-derived silica and characterized electrochemically with [Ru(bipy)3]2+ and [Fe(CN)6],4- respectively.23 Sol-gel-derived silica entrapped with PDMDAAC and several other ionomers were investigated as optical materials.24 The electroactivity of redox probes encapsulated in sol-gelderived silica has been investigated to provide specific information about dopant stability, entrapment, and activity.25 Nafion, a perfluorosulfonated ionomer, has been entrapped in sol-gel-derived silica for use as a new class of solid acid catalysts.26 The porous Nafion-entrapped solgel-derived silica (Nafion-silica) nanocomposites offer the potential for significantly enhanced catalytic activity, due to the increased accessibility of the Nafion resin-based acid sites. Conductivity and morphology of Nafion-silica composites were investigated.27,28 Inorganic moieties were dispersed in Nafion films, and the composites formed via sol-gel-reaction by exposing hydrated Nafion films to silicon and other metal alkoxide solutions were investigated.29-32 The benefit of incorporation of organic moieties in solgel-derived silica is that the incorporated organic moieties remain accessible to species which are in contact with the organic-inorganic hybrid or composite, due to the porosity of the matrix. Furthermore, the properties of the hybrid or composite can be tuned to meet specific needs by varying reaction conditions. Additionally, by choosing an appropriate organic moiety and maintaining optical transparency, the silica-based material would be suitable for spectroscopic and spectroelectrochemical applications. In the present paper we describe the incorporation of Nafion in sol-gel-derived silica and the tailoring of Nafion-silica composites by manipulating Nafion loading (14) Motakef, S.; Suratwala, T.; Roncone, R. L.; Boulton, J. M.; Teowee, G.; Neilson, G. F.; Uhlmann, D. R. J. Non-Cryst. Solids 1994, 178, 31-36. (15) Motakef, S.; Suratwala, T.; Roncone, R. L.; Boulton, J. M.; Teowee, G.; Uhlmann, D. R. J. Non-Cryst. Solids 1994, 178, 37-43. (16) Wei, Y.; Wang, W.; Yang, D.; Tang, L. Chem. Mater. 1994, 6, 1737-1741. (17) Ellerby, L. M.; Nishida, C. R.; Nishida, F.; Yamanaka, S. A.; Dunn, B.; Valentine, J. S.; Zink, J. I. Science 1992, 255, 1113-1115. (18) Wu, S.; Ellerby, L. M.; Cohan, J. S.; Dunn, B.; El-Sayed, M. A.; Valentine, J. S.; Zink, J. I. Chem. Mater. 1993, 5, 115-120. (19) Li, J.; Tan, S. N.; Ge, H. Anal. Chim. Acta 1996, 335, 137-145. (20) Braun, S.; Rappoport, S.; Zusman, R.; Avnir, D.; Ottolenghi, M. Mater. Lett. 1990, 10, 1-5. (21) Nakanishi, K.; Soga, N. J. Non-Cryst. Solids 1992, 139, 1-13. (22) Nakanishi, K.; Soga, N. J. Am. Ceram. Soc. 1991, 74, 25182530. (23) Petit-Dominguez, M. D.; Shen, H.; Heineman, W. R.; Seliskar, C. J. Anal. Chem. 1997, 69, 703-710. (24) Shi, Y.; Seliskar, C. J. Chem. Mater. 1997, 9, 821-829. (25) Collinson, M. M.; Rausch, C. G.; Voigt, A. Langmuir 1997, 13, 7245-7251. (26) Harmer, M. A.; Farneth, W. E.; Sun, Q. J. Am. Chem. Soc. 1996, 118, 7708-7715. (27) Zoppi, R. A.; Nunes, S. P. J. Electroanal. Chem. 1998, 445, 3945. (28) Zoppi, R. A.; Yoshida, V. P.; Nunes, S. P. Polymer 1997, 39, 1309-1315. (29) Mauritz, K. A.; Stefanithis, I. D.; Davis, S. V.; Scheetz, R. W.; Pope, R. K.; Wilkes, G. L.; Huang, H.-H. J. Appl. Polym. Sci. 1995, 55, 181-190. (30) Shao, P. L.; Mauritz, K. A.; Moore, R. B. Chem. Mater. 1995, 7, 192-200. (31) Apichatachutapan, W.; Moore R. B.; Mauritz, K. A. J. Appl. Polym. Sci. 1996, 62, 417-426.
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and the initial molar ratio of water relative to the precursor, tetraethyl orthosilicate, for improving the slow diffusion of [ReI(DMPE)3]+ in Nafion. In addition, we demonstrate the potential utility of the Nafion-silica composites for three-mode-selectivity spectroelectrochemical detection of [ReI(DMPE)3]+. Experimental Section Chemicals. [ReI(DMPE)3]CF3SO3 was prepared as previously described.4 Nafion perfluorosulfonated ionomer, 5 wt % solution in a mixture of lower aliphatic alcohols and 10% water, and tetraethyl orthosilicate (TEOS, 98%) were purchased from Aldrich and used as received. Phosphate buffer stock solution (Fisher Scientific), after adjustment to pH 7.4 with NaOH, was used to control the pH of [ReI(DMPE)3]+ solutions. Other chemicals used were of analytical or higher grade. Solutions were prepared with water purified from a Barnstead Nanopure water system (Barnstead Sybron Corp.). [ReI(DMPE)3]+ Solution. [ReI(DMPE)3]+ in solution is apt to be oxidized by dissolved oxygen,33 although solid [ReI(DMPE)3]CF3SO3 is stable under ambient conditions. Therefore, [ReI(DMPE)3]+ solutions were freshly prepared prior to measurements by dissolving [ReI(DMPE)3]CF3SO3 in 0.14 M NaCl (pH 7.4) supporting electrolyte solution which had been deoxygenated by argon purging. Perfluorosulfonated Ionomer-Loaded Silica Sol. Silica sols were prepared at room temperature from TEOS precursor. The initial composition of the reactant mixture was controlled in terms of molar ratio of water relative to TEOS, with HCl catalyst being ca. 1.5 × 10-3 M. After about 3 h of vigorous stirring with a magnetic stirrer, the resulting silica sols were, in most cases, homogeneous and transparent, depending on the initial H2O/TEOS molar ratio of the reactant mixture. An aliquot of 1.0 mL of the above sols was then mixed with 5 wt % Nafion perfluorosulfonated ionomer solution by sonicating for 5 min. The ionomer content in the sols was controlled as to the mass ratio of the ionomer to SiO2 calculated stoichiometrically from the amount of TEOS. Preparation of Electrodes. Spectroscopic grade graphite rods (4 mm diameter, Poco Graphite) were sealed with liquid Teflon (Teflon AF, 6%, DuPont), and the ends were then polished with Fibrmet polishing disks (Buehler) to yield a visually shiny surface with a defined sensing area, as previously described.34 Indium-tin oxide (ITO) deposited on tin float glass (Thin Film Devices) was cut into slides of 1.0 × 2.5 cm2 for electrochemical investigations and 2.5 × 7.6 cm2 for attenuated total reflection (ATR) spectroelectrochemical measurements. After thorough rinsing with absolute ethanol and deionized water, the graphite rods and ITO glass slides were spin-coated with Nafion-silica composite at 3500 rpm with a laboratory-made spin-coating system or a Photo-Resist Spinner (model 1-PM101DT-R485, Headway Research, Inc.). For electrical contact, a portion of each ITO glass slide was protected from coating with Scotch tape. The coated graphite and ITO glass electrodes were allowed to dry overnight under ambient conditions and were then conditioned in 0.14 M NaCl (pH 7.4) supporting electrolyte solution overnight before electrochemical or ATR spectroelectrochemical measurements were performed. Electrochemistry. Electrochemical measurements were carried out with a BAS 100B/W electrochemical analyzer (Bioanalytical Systems). The electrochemical cell consisted of a BAS Ag/AgCl reference electrode, a platinum wire auxiliary electrode, and a working electrode which was the modified spectroscopic graphite and bare or modified ITO glass. The solution volume was 15 mL. [ReI(DMPE)3]+ solutions were purged with argon and then blanketed with argon during measurement. UV-vis Spectrophotometry. UV-visible absorption spectra were taken by using a Hewlett-Packard 8453 diode array spectrophotometer. To obtain the absorbance spectra of [ReI(32) Deng, Q.; Wilkie, C. A.; Moore, R. B.; Mauritz, K. A. Polymer 1998, 39, 5961-5972. (33) Swaile, B. H.; Blubaugh, E. A.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1998, 70, 4326-4332. (34) Hu, Z. Seliskar, C. J.; Heineman, W. R. Anal. Chim. Acta 1998, 369, 93-101.
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Figure 1. Cyclic voltammograms at spectroscopic graphite electrodes modified with Nafion (A) and Nafion-silica composites (B, C, and D). For the composites, (B) ri ) 20:1, (C) ri ) 30:1, and (D) ri ) 50:1, with Nafion/SiO2 (wt/wt) being fixed at 40:100. Voltammetric parameters: 2 s prescan quiet time; 14 s cyclic voltammetric scan between -0.25 and 0.45 V at 100 mV/s; and 104 s postscan open-circuit quiescence. 60 potential cycles for each of the modified electrodes with each cycle being initiated at -0.25 V. Solution: 1 × 10-4 M [ReI(DMPE)3]+ in 0.14 M NaCl (pH 7.4). (DMPE)3]+ and [ReII(DMPE)3]2+, monolithic disks (∼0.5 mm thickness) cast on polystyrene dishes (35 mm diameter, Becton Dickinson) were used. For measuring the thickness of composite coating films on glass with the optical interference fringe method,24 a normal microscopic glass slide was used as the substrate. The spectra were taken against a blank of the corresponding polystyrene dish or glass slide. A refractive index of 1.46 (i.e., that for the silica host matrix) was assumed as an approximate value for the Nafion-silica.24 ATR Spectroelectrochemistry. ATR spectroelectrochemical measurements were performed with an instrumental setup similar to that described elsewhere.35 Briefly, a homemade potentiostat was used to supply the electrolysis potential. A tungsten light source was filtered (no. S143050, Esco Products), focused into an optical fiber and prism coupled to the Nafionsilica composite modified ITO glass-based optically transparent electrode (OTE). The incident light filtered from the tungsten light source has a bandwidth of 360-560 nm (10% cutoff relative to the maximum transmittance at around 450 nm). The prismdecoupled transmitted light was detected with a photodiode. The spectroelectrochemical cell consisted of a Ag/AgCl reference electrode, a platinum grid auxiliary electrode, and the OTE, with solution volume being ∼8 mL. Potential, current, and absorbance signals were simultaneously acquired with a PC (Gateway 2000, 200 MHz), which was also used to control the potentiostat.35
Results and Discussion Voltammetric Characterization of Nafion-Silica Composites. Nafion-silica composites on spectroscopic graphite were evaluated by cyclic voltammetry in terms of the increased rate in voltammetric response of the modified electrodes to [ReI(DMPE)3]+ during preconcentration. Spectroscopic graphite is attractive as a substrate material for the construction of polymer-modified electrodes because the slight porosity promotes anchoring of films.36 Spectroscopic graphite electrodes are available at low cost. However, they have a relatively large background current, compared to gold, platinum, and glassy carbon electrodes. Nafion ionomers have been shown to incorporate hydrophobic complex cations preferentially over simple or hydrophilic complex cations.37,38 It was also shown that [ReI(DMPE)3]+ strongly partitions into films of Nafion,6,33 owing to the combined effect of electrostatic (35) Slaterbeck, A. F.; Ridgway, T. H.; Seliskar, C. J.; Heineman, W. R. Submitted for publication in Anal. Chem. (36) Coury, L. A., Jr.; Heineman, W. R. J. Electroanal. Chem. 1988, 256, 327-341.
and hydrophobic interactions. However, the partitioning is rather sluggish,33 compared to the partitioning of hydrophilic cations. Dispersing Nafion in sol-gel-derived silica matrix has been shown to speed up the extraction of [ReI(DMPE)3]+ by Nafion.5 Ionomer Content and Initial Sol H2O/TEOS Molar Ratio. Figure 1 shows sets of representative cyclic voltammograms observed at spectroscopic graphite electrodes modified with Nafion (A) and Nafion-silica composites (B, C, and D). For each set of voltammograms, the base voltammogram, which exhibits the lowest voltammetric current, was recorded right after the modified electrode began its contact with 1 × 10-4 M [ReI(DMPE)3]+ solution. As can be seen from Figure 1, with all of the coatings, the anodic and cathodic peak currents increase with the increase in contact time. However, the peak current increase rate for different coatings varies. With Nafion-silica composite coatings the electrodes exhibited a significantly faster peak current increase rate than the electrode with Nafion coating. The peak current increase rate for the Nafion-silica composites is further dependent on the mass ratio of Nafion relative to SiO2 in the composites (designated as Nafion/SiO2) and the initial molar ratio of water to the precursor TEOS of the sols (designated as ri), as detailed in the following. The effect of the composition of Nafion-silica composite on voltammetric response was first investigated by varying the Nafion content in the composite. The anodic peak current taken from cyclic voltammograms similar to those shown in Figure 1 is plotted versus the contact time of a coating film with [ReI(DMPE)3]+ solution to illustrate the peak current increase kinetics (Figure 2). As shown in Figure 2A, the Nafion content in a composite affects both peak current increase rate and the maximum peak current, which corresponds to the maximum concentration of [ReI(DMPE)3]+ in the composite after the coating film reaches equilibrium with the 1 × 10-4 M [ReI(DMPE)3]+ solution. At Nafion/SiO2 (wt/wt) ) 20:100, the coated electrode reached 90% of its maximum anodic peak current in ca. 30 min (curve a in Figure 2A). It took ca. 75 min for the electrode modified with the Nafion/SiO2 (wt/wt) ) 80: 100 composite to reach 90% of its maximum anodic peak (37) Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 18981902. (38) Shi, M.; Anson, F. C. Anal. Chem. 1997, 69, 2653-2660.
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Figure 2. Anodic peak current as a function of contact time of electrode coatings with 1 × 10-4 M [ReI(DMPE)3]+ solution. (A) Effect of ionomer content. With ri ) 4:1, Nafion/SiO2 (wt/ wt): (a) 20:100, (b) 30:100, (c) 40:100, and (d) 80:100. (B) Effect of initial H2O/TEOS molar ratio. Except for the pure Nafion coating (e), the H2O/TEOS molar ratio in initial sol: (f) ri ) 10:1, (g) ri ) 20:1, (h) ri ) 30:1, (i) ri ) 40:1, and (j) ri ) 50:1, with Nafion/SiO2 (wt/wt) being at 40:100. Voltammetric parameters are the same as those for Figure 1. Each coating was evaluated with three electrodes, and results for the electrode with intermediate response are shown. Reproducibility among electrodes was typically 5-10%.
current (curve d in Figure 2A). On the other hand, the maximum anodic peak current attained at the electrode modified with the Nafion/SiO2 (wt/wt) ) 20:100 composite is ca. 85% of that of the electrode modified with the Nafion/ SiO2 (wt/wt) ) 80:100 composite. This was most likely due to the fact that a lower content of Nafion is equivalent to fewer SO3- sites present in a given electrode area. In general, the lower the Nafion content, the faster the apparent peak current increase rate is, with the tradeoff being a lower maximum attainable voltammetric response. It should, however, be noted that with a Nafion ionomer content lower than 20:100 Nafion/SiO2 (wt/wt), fracture of the composite coatings was observed under optical microscopy, especially with Nafion/SiO2 (wt/wt) < 10:100. To investigate the influence of the initial molar ratio of water to the precursor TEOS in the sols, ri, on voltammetric response of the Nafion-silica composite modified electrodes to [ReI(DMPE)3]+, the ionomer content was fixed at Nafion/SiO2 (wt/wt) ) 40:100. With this ionomer content, the maximum response is almost equal to that observed with the Nafion/SiO2 (wt/wt) ) 80:100 ionomer content, and the peak current increases relatively fast (compare c with d in Figure 2A). As shown in Figure 2B, ri markedly affects the peak current increase rate of the Nafion-silica composites for [ReI(DMPE)3]+. With ri )
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50:1 (curve j in Figure 2B, cf. Figure 1D), the modified electrode reached 90% of its maximum anodic peak current in ca. 20 min, which is only one-third of the time required for the electrode modified with the ri ) 10:1 composite to reach 90% of its maximum anodic peak current (curve f in Figure 2B). With the pure Nafion coating, the modified electrode had not reached an equilibrium with the [ReI(DMPE)3]+ solution in 2 h (curve e in Figure 2B, cf. Figure 1A), indicating the slowness of the partitioning process. On the other hand, increasing ri results in a decrease in maximum attainable response, as can be seen in Figure 2B. In addition, with ri greater than 30, the resulting sols are not transparent and thus inappropriate for spectroelectrochemical investigations. In making the Nafion-silica stock solutions, the asreceived Nafion solution was diluted by addition of the silica sol. When the Nafion/SiO2 mass ratio was lowered with ri being fixed or when ri was raised with the Nafion/ SiO2 mass ratio being kept constant, the Nafion ionomer concentration in the stock solution was lowered. Since the electrodes were spin-coated at a fixed rpm, a decrease in Nafion ionomer concentration in the stock solution would correspond to a decrease in ionomer loading and solution viscosity and, thus, a thinner coating. To take the decrease in coating thickness with the decrease in Nafion content and the increase in ri into consideration, control experiments were carried out by diluting the asreceived Nafion solution with water in the same dilution ratio as with sols. For instance, the dilution of 0.7 mL of the as-received Nafion solution with 1.0 mL of water is analogous to the dilution of 0.7 mL of the Nafion solution with 1.0 mL of the ri ) 30:1 sol for yielding the Nafion/ SiO2 (wt/wt) ) 40:100 composite. It was observed that the dilution of the as-received Nafion solution with water significantly affects the peak current increase rate. For example, with the above dilution ratio, the water-diluted Nafion coating reached 90% of its maximum response in ca. 50 min. This response is significantly faster than the coating from the as-received Nafion (∼2 h, curve e in Figure 2B, cf. Figure 1A) but is substantially slower than the Nafion-silica composite coating with the corresponding dilution ratio (curve h in Figure 2B, cf. Figure 1C), confirming that the presence of sol-gel-derived silica matrix does significantly contribute to the improvement in voltammetric response of the modified electrodes. In the voltammetrically monitored preconcentration, the voltammetric signal directly reflects both the diffusion coefficient and the concentration of [ReI(DMPE)3]+ in an electrode coating. The latter, on the other hand, depends on the diffusion of [ReI(DMPE)3]+ in the coating and the supply of [ReI(DMPE)3]+ to the coating/solution interface. Since the concentrations of [ReI(DMPE)3]+ and background electrolyte in solution phase were kept constant and the solutions were kept quiescent, and thus the rate of [ReI(DMPE)3]+ supply to the coating/solution interface was essentially unchanged at room temperature, a faster increase rate in voltammetric response is indicative of a faster diffusion of [ReI(DMPE)3]+ in the coating. It is likely that when Nafion is dispersed in sol-gelderived silica, at a lower content of Nafion, the ionexchange sites of Nafion in the composite are more accessible to [ReI(DMPE)3]+. Consistent with this influence is the fact that the peak current was observed to increase faster. This is in agreement with the observation that at higher Nafion loadings the surface area, pore volume and pore size of Nafion-loaded Si(OCH3)4-derived silica composite decrease dramatically, compared to lower Nafion loadings.26 Furthermore, the properties of sol-gel-derived silica depend on hydrolysis and condensation conditions.
Sol-Gel-Derived Silica Nanocomposites
Variations in processing parameters, such as the nature and concentration of catalyst, temperature, and water/ precursor molar ratio, affect the structures and morphology of sol-gel-derived silicate network. Of these parameters, the nature and concentration of catalyst are known to dramatically affect the porosity and pore size of the resulting silica.39,40 By variation of pH, Nafion-silica nanocomposites with pore sizes from 2 to 25 nm were prepared.26 With other parameters being constant, the water/precursor molar ratio significantly affects the porosity of the resulting silica.9,39 It is generally agreed that the formation of more hydrolyzed species is favored with increasing ri.9,41,42 During condensation, this would bring about polymers with more branched structures9,42 and thus a more porous silica, as confirmed by gas adsorption measurements.39 An increase in porosity of the sol-gel-derived silica matrix would increase the accessibility of ion-exchange sites of the entrapped Nafion. Consistent with this body of evidence, faster increase rate in voltammetric responses was observed with increasing ri. Regeneration. [ReI(DMPE)3]+ was strongly retained in the Nafion-silica composite. The regeneration of the Nafion-silica composite coating was investigated with supporting electrolyte solution and HCl solution. In 0.14 M NaCl supporting electrolyte solution (pH 7.4), only ca. 30% decrease in voltammetric response was observed after 2 h of soaking. Following 2 h of soaking in 0.1 M HCl, the voltammetric signal was observed to decrease 80% from that recorded before the soaking. After immersion in 1 M HCl overnight, no voltammetric response of [ReI(DMPE)3]+ was observable in supporting electrolyte solution. In 1 × 10-4 M [ReI(DMPE)3]+ the regenerated electrode exhibited a voltammetric response comparable to that recorded before the overnight immersion, indicating the feasibility of the regeneration approach. Nafion-Silica Composite on ITO Glass. The Nafionsilica composite optimized on the spectroscopic graphite substrate as described above was tested by cyclic voltammetry on ITO glass. As shown in Figure 3A, with ri ) 20:1 the Nafion/SiO2 (wt/wt) ) 40:100 composite on ITO glass reached 90% of its maximum attainable response in ca. 20 min. This is significantly faster than that observed at the spectroscopic graphite substrate with the corresponding composite, which took ca. 45 min to reach 90% of its maximum response (curve g in Figure 2B). The difference might be due to the difference in thickness of the coatings on spectroscopic graphite substrate and ITO glass substrate. By use of the interference fringe method and assuming the refractive index of 1.46 (dry silica film),24 the thickness of the composite coating on ITO glass was determined to be ∼0.4 µm. The thickness of composite coatings on spectroscopic graphite substrate was not directly determined. It is probable that the coatings on spectroscopic graphite are relatively thicker. The spectroscopic graphite electrode was polished to yield a shiny surface. The surface, however, is slightly porous, as can be observed by optical microscopy. This slight porosity would be conducive to the adhesion of the composite on the electrode surface, as is in the case of highly swollen polymer films.36 This porosity might result in an increase in effective thickness of the coatings, compared to the coatings on smooth surface (e.g., ITO glass), even though (39) Elferink, W. J.; Nair, B. N.; de Vos, R. M.; Keizer, K.; Verweij, H. J. Colloid Interface Sci. 1996, 180, 127-134. (40) Suh, D. J.; Park, T.-J. Chem. Mater. 1996, 8, 509-513. (41) Boonstra, A. H.; Bernards, T. N. M. J. Non-Cryst. Solids 1989, 108, 249-259. (42) Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1982, 48, 31-46.
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Figure 3. (A) Voltammetric response as a function of contact time of a Nafion-silica composite modified ITO glass electrode with [ReI(DMPE)3]+ solution. (B) Cyclic voltammograms at bare (dashed curve) and at composite-modified (solid curve) ITO glass electrodes. The inset in (B) is a magnification of the dashed voltammogram. Each potential scan was initiated at -0.35 V at the scan rate of 100 mV/s. Solution: 1 × 10-4 M [ReI(DMPE)3]+ in 0.14 M NaCl (pH 7.4).
the coatings were made with the same coating solution and the same spin speed. Shown in Figure 3B are the cyclic voltammograms observed at bare and Nafion-silica composite modified ITO glass electrodes for 1 × 10-4 M [ReI(DMPE)3]+. Modification of the ITO glass electrode with the Nafionsilica composite results in a 25-fold enhancement in peak current. At the bare ITO glass electrode, the voltammogram observed is well defined with a formal potential E°′, estimated from (Epa + Epc)/2, being -0.10 V at a scan rate of 100 mV/s (dashed voltammogram and the inset in Figure 3B). At the Nafion-silica composite modified ITO glass electrode (solid voltammogram in Figure 3B), the E°′ was calculated to be -0.04 V at the same scan rate. At the modified ITO glass electrode the voltammogram is asymmetric in shape, especially at a faster scan rate, as can be seen from Figure 3B. Compared to the cathodic wave, the anodic wave is flatter. By variation of scan rate over the range of 4-400 mV/s, it was observed that both anodic and cathodic peak currents are linearly proportional to square root of scan rate. Peak potentials are also shifting with increased scan rate, so that peak separation is larger at a faster scan rate. However, an increase in scan rate causes a greater shift in the anodic peak potential than that in the cathodic peak potential. Some of this shift is due to uncompensated iR drop in the ITO glass electrode. An Ohm’s law plot (Ep vs ip) reveals that the
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Hu et al.
Figure 4. Absorbance spectra of [ReI(DMPE)3]+ (a) and [ReII(DMPE)3]2+ (b) impregnated in a monolithic disk cast from asreceived 5 wt % Nafion solution. The disk was loaded with [ReI(DMPE)3]+ by soaking it in 1 × 10-4 M [ReI(DMPE)3]+ (0.14 M NaCl, pH 7.4) solution blanketed with argon for 5 h. After the spectrum (a) was taken, the [ReI(DMPE)3]+-loaded disk was left in air overnight before the spectrum (b) was taken.
apparent anodic resistance is about four times higher than the apparent cathodic resistance, suggesting that factors other than iR drop might contribute to the significant shift in anodic peak potential. The underlying mechanism causing the flatness of the anodic wave and the anodic peak shift being significantly greater than the corresponding cathodic peak shift with the increase in scan rate is not clear at present. This phenomenon was also observed in other systems (other composite coatings on ITO glass for other electroactive species) and is the subject of further investigations. ATR Spectroelectrochemistry of [ReI(DMPE)3]+. With a three-mode-selectivity ATR spectroelectrochemical configuration, detection is based on the optical attenuation resulting from the interaction of the evanescent field of light propagating in total internal reflection mode with chromophores on the coating side of the coating/substrate interface. Since the evanescent wave penetrates into the coating layers to a distance of about one wavelength of the propagating light, it mainly probes the chromophores in the coating, instead of the ones in the solution phase which is in contact with the coating. Therefore, for a species of interest in a sample to be detected, the species must first partition into the thin coating, then be reduced or oxidized at the applied electrolysis potential, and finally interact with the evanescent wave and exhibit an absorbance change, associated with electrolysis, at the chosen wavelength. The absorbance spectra for [ReI(DMPE)3]+ and [ReII(DMPE)3]2+ are distinct, as shown in Figure 4. When the disk film cast from the as-received 5 wt % Nafion solution was impregnated with [ReI(DMPE)3]+, the dominant absorption is in the near-UV region (spectrum a). The nonzero absorbance in the visible region was due to the oxidation of a small portion of [ReI(DMPE)3]+ to [ReII(DMPE)3]2+ by residual oxygen in the soaking solution. The [ReII(DMPE)3]2+-impregnated film shows absorption in the visible region, with a maximum absorbance at 528 nm (spectrum b). To probe the interaction of light with [ReII(DMPE)3]2+, incident light of this wavelength would be desirable. Shown in Figure 5 are the electrochemical and optical responses simultaneously observed under ATR spectroelectrochemical mode during extraction of [ReI(DMPE)3]+ into the coating. Similar to the previous electrochemical investigations, as soon as the composite coating contacted the [ReI(DMPE)3]+ solution, the potential scan was initiated. In the continuous 30 cyclic scans, [ReI(DMPE)3]+,
Figure 5. Cyclic voltammograms (A) and the associated absorbance changes (B) obtained with ATR spectroelectrochemical configuration. The potential was initiated at -0.35 V and continuously scanned for 30 cycles at a scan rate of 10 mV/s. Coating on ITO glass-based OTE: Nafion/SiO2 (wt/wt) ) 40:100 composite derived with ri ) 20:1. Solution: 1 × 10-4 M [ReI(DMPE)3]+ in 0.14 M NaCl (pH 7.4).
which partitioned into the coating and arrived at the coating/ITO interface, was oxidized to [ReII(DMPE)3]2+ and reduced back to [ReI(DMPE)3]+ periodically. As a result of the continuous extraction of [ReI(DMPE)3]+ by the coating, an increase in voltammetric response was observed. Eventually, the voltammetric response reached its maximum value, indicating that the coating had reached an equilibrium with the [ReI(DMPE)3]+ in the ATR spectroelectrochemical cell. The optical response follows the periodic change in [ReII(DMPE)3]2+ concentration at the coating/ITO interface. During the forward scan of the first potential cycle, before the [ReI(DMPE)3]+ oxidation potential was reached, the absorbance was essentially zero, since no [ReII(DMPE)3]2+ was generated. After the [ReI(DMPE)3]+ oxidation potential was reached, the absorbance increased with the progress of the potential scan. During the reverse potential scan, the absorbance continued increasing until the [ReII(DMPE)3]2+ reduction potential was reached. While the electrogenerated [ReII(DMPE)3]2+ was reduced back to [ReI(DMPE)3]+, a decrease in absorbance was recorded. As the potential starts a new cycle, the absorbance follows the general change pattern observed in the first potential cycle, with the exception that in the oxidation region the reverse potential scan brought about no observable absorbance increase after ca. 10 cycles, due to the fact that no appreciable amount of [ReI(DMPE)3]+ was being taken up after the coating had reached an equilibrium with the [ReI(DMPE)3]+ solution in the cell. It is interesting to note that the base absorbance, the absorbance in the far negative potential region in Figure 5B, increases slightly with the progress of the potential
Sol-Gel-Derived Silica Nanocomposites
Figure 6. Effect of potential scan rate on current and absorbance response signals: (A) cyclic potential excitation signal; (B) corresponding cyclic voltammogram; and (C) associated absorbance change. Scan rates from 4 to 625 mV/s in an increment of 1 (mV/s)1/2 in square root of scan rate. Initial potential ) -0.35 V, switching potential ) 0.5 V. Solution: 1 × 10-4 M [ReI(DMPE)3]+ in 0.14 M NaCl (pH 7.4).
scan and reaches a steady-state value as the coating reaches equilibrium with [ReI(DMPE)3]+ solution, suggesting that the electrolyzed [ReII(DMPE)3]2+ was not completely reduced back to its original form. The magnitude of the base absorbance is about 10% of the maximum absorbance in the oxidation region for each individual potential cycle. It may thus be estimated that, at this scan rate and in this potential range, about 10% of the electrolyzed [ReII(DMPE)3]2+ in the region probed by the evanescent wave was left unreduced in the reverse potential scan. On the other hand, this effect is apparently not cumulative, because the base absorbance value was observed not to increase further as the maximum absorbance in the oxidation region reached steady state. Figure 6 illustrates the effect of scan rate on the electrochemical and optical responses for a preloaded film. The cyclic excitation potential (A), the corresponding current response (B), and the associated absorbance change (C) at various potential scan rates are plotted against the square root of scan rate. The first potential scan was started after the composite coating had reached equilibrium with 1 × 10-4 M [ReI(DMPE)3]+ solution in the ATR spectroelectrochemical cell. Each subsequent potential scan was initiated ∼1 min after the previous one. During the 1 min pause, the data were saved and the scan rate was set to increase from its previous one in an increment of 1 (mV/s)1/2 in square root of scan rate. This 1 min pause also allows the composite coating to reequilibrate with the [ReI(DMPE)3]+ solution. As shown in Figure 6B, increasing scan rate results in an increase in electrochemical response, since peak current is proportional to the square root of scan rate. The optical response, contrary to the electrochemical response, decreases with the increase in scan rate (Figure 6C). This is attributed to the diffusion layer thickness being about or less than the penetration depth of the evanescent wave in the
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composite coating. As a result of the decrease in diffusion layer thickness with the increase in scan rate, which corresponds to a decrease in the amount of [ReII(DMPE)3]2+ generated electrolytically, a decrease in optical signal was observed. A plot of the maximum absorbance versus square root of scan rate gives a linear decrease (cf. Figure 6C), which is consistent with the technique of derivative cyclic voltabsorptometry.43 From Figure 6C it can also be seen that with increase in scan rate the base absorbance increases. This can be ascribed to the incomplete reversion (reduction) of the generated [ReII(DMPE)3]2+ (vide supra) at a faster scan rate. The capability of simultaneously monitoring the electrochemical and optical events of the ATR spectroelectrochemical cell is clearly seen from the above demonstration. It should be noted that for practical detection with a three-mode-selectivity spectroelectrochemical sensor, the use of potential excitation signals other than a triangular waveform, e.g., a steady-state potential, would also be feasible and might be advantageous. Conclusions Dispersing Nafion ionomer in a sol-gel-derived silica matrix significantly improves the slow extraction of Nafion for hydrophobic cations, as demonstrated with [ReI(DMPE)3]+. By manipulation of Nafion content and the initial molar ratio of water relative to the TEOS precursor, the Nafion-silica composite can be tuned to give increased accessibility of Nafion active sites to [ReI(DMPE)3]+. The approach would also be applicable to other hydrophobic cations. In addition, the properties of sol-gel-derived silica are not only dependent on precursor concentration but also affected by the nature and concentration of catalyst, reaction temperature, and the nature of the precursor. Hence, the accessibility of the active sites of ionomer entrapped in the matrix may also be tuned by the manipulation of these reaction conditions. Nafion-silica composite is optically transparent in the visible and near-ultraviolet region. It is thus suitable for optical applications, as demonstrated with ATR spectroelectrochemistry for [ReI(DMPE)3]+. With a three-modeselectivity spectroelectrochemical sensor, only the species which pass all of the three sequential interrogations (selective partitioning, electrochemical, and spectroscopic) are detected. The selectivity would thus be greatly improved, compared to an electrochemical or spectroscopic sensor. The effectiveness of the trimodal selectivity has been previously demonstrated in [Fe(CN)6]4-/[Ru(CN)6]4and [Fe(CN)6]3-/[Ru(bipy)3]2+ binary mixture systems2 and is the subject of further investigations involving [ReI(DMPE)3]+. Acknowledgment. Financial support provided by DOE (Grant DE-FG07-96ER62311) is gratefully acknowledged. LA980777U (43) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981, 53, 1390-1394.
