Anal. Chem. 2003, 75, 4250-4256
Electrogenerated Chemiluminescence from Tris(2,2′-bipyridyl)ruthenium(II) Immobilized in Titania-Perfluorosulfonated Ionomer Composite Films Han Nim Choi, Sung-Hee Cho, and Won-Yong Lee*
Department of Chemistry, Yonsei University, Seoul 120-749, Korea
Electrochemical behavior and electrogenerated chemiluminescence (ECL) of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+) immobilized in sol-gel-derived titania (TiO2)Nafion composite films coated on a glassy carbon electrode have been investigated. The electroactivity of Ru(bpy)32+ ion exchanged into the composite films and its ECL behavior were strongly dependent upon the amount of Nafion incorporated into the TiO2-Nafion composite films. The ECL sensor of Ru(bpy)32+ immobilized in a TiO2-Nafion composite with 50% Nafion content showed the maximum ECL intensities for both tripropylamine (TPA) and sodium oxalate in 0.05 M phosphate buffer solution at pH 7. Detection limits were 0.1 µM for TPA and 1.0 µM for oxalate (S/N ) 3) with a linear range of 3 orders of magnitude in concentration. The present ECL sensor showed improved ECL sensitivity and long-term stability, as compared to the ECL sensors based on pure Nafion films. The present Ru(bpy)32+ ECL sensor based on TiO2-Nafion (50%) composite films was applied as an HPLC detector for the determination of erythromycin in human urine samples. The present Ru(bpy)32+ ECL sensor was stable in the mobile phase containing a high content of organic solvent (30%, v/v), in contrast to a pure Nafion-based Ru(bpy)32+ ECL sensor. The detection limit for erythromycin was 1.0 µM, with a linear range of 3 orders of magnitude in concentration. Electrogenerated chemiluminescence (ECL) is light emission from the generation of emitting excited states via an electrontransfer reaction of electrogenerated reactant.1 In recent years, ruthenium (II) complex-based ECL has gained importance as a sensitive and selective detection method for the analysis of a wide range of compounds2-5 such as oxalate,6 alkylamines,7 amino acids,8 NADH,9,10 and organic acids.11 In particular, the ECL * To whom correspondence should be addressed. Phone: 82-2-2123-2649. Fax: 82-2-364-7050. E-mail address:
[email protected]. (1) Faulkner, L. R.; Bard, A. J. J. Electroanal. Chem. 1977, 1. (2) Lee, W.-Y. Mikrochim. Acta 1997, 127, 19. (3) Lee, W.-Y.; Nieman, T. A. Anal. Chem. 1995, 67, 1789. (4) Knight, A. W.; Greenway, G. M. Analyst 1996, 121, 101R. (5) Gerardi, R. D.; Barnett, N. W.; Lewis, S. W. Anal. Chim. Acta 1999, 378, 1. (6) Rubinstein, I.; Bard, A. J. Anal. Chem. 1983, 55, 1580. (7) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865.
4250 Analytical Chemistry, Vol. 75, No. 16, August 15, 2003
observed with a tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+) system is among the most intense and the best characterized.12 The oxidation-reduction reaction mechanism for Ru(bpy)32+ ECL has been postulated by Rubinstein and Bard.13 In this case, the oxidized product of analyte (e.g., oxalate or tripropylamine) is a reductant.
Ru(bpy)32+ f Ru(bpy)33+ + eRu(bpy)33+ + reductant f [Ru(bpy)32+]* + product [Ru(bpy)32+]* f Ru(bpy)32+ + light (610 nm) The reaction scheme for Ru(bpy)32+ ECL reveals that the starting material Ru(bpy)32+ is regenerated in situ when it is immobilized on an electrode surface. Therefore, extensive efforts have been directed toward the immobilization of Ru(bpy)32+ on electrode surfaces to fabricate a regenerable ECL sensor. The immobilization of Ru(bpy)32+ on the electrode surface can overcome several flaws in the solution-phase Ru(bpy)32+ ECL. The Ru(bpy)32+-immobilized ECL sensor is cost-effective because the ECL reagent is reused. When the Ru(bpy)32+-immobilized ECL sensor is used as a HPLC detector, there is no need for an extra pump to continually deliver the Ru(bpy)32+ to a reaction/observation zone in front of photomultiplier tube. To date, a number of different approaches have been tried to immobilize Ru(bpy)32+ or its derivatives on a variety of different electrode surfaces. For example, the derivatives of Ru(bpy)32+ have been immobilized on an electrode surface as Langmiur-Blodgett films or self-assembled monolayers.14 However, their analytical application is very limited, because they are easily desorbed in (8) (a) Brune, S. N.; Bobbitt, D. R. Talanta 1991, 38, 419. (b) Brune, S. N.; Bobbitt, D. R. Anal. Chem. 1992, 64, 166. (c) Uchikura, K.; Kirisawa, M. Chem. Lett. 1991, 1373. (9) Downey, T. M.; Nieman, T. A. Anal. Chem. 1992, 64, 261. (10) Martin, A. F.; Nieman, T. A. Anal. Chim. Acta 1993, 281, 475. (11) (a) Chen, X.; Sato, M. Anal. Sci. 1995, 11, 749. (b) Zorzi, M.; Pastore, P.; Magno, F. Anal. Chem. 2000, 72, 4934. (12) Tokel, N. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862. (13) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. (14) (a) Zang, X.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (b) Miller, S. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781. (c) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195. (d) Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 384, 57. (e) Wang, H.; Xu, G.; Dong, S. Talanta 2001, 126, 1095. 10.1021/ac0206014 CCC: $25.00
© 2003 American Chemical Society Published on Web 07/15/2003
organic solvent and also are unstable in positively biased potential. Different approaches have been tried based on metallopolymers, such as a electropolymerized Ru(vinyl bpy)32+-modified electrode15 and a [Ru(bpy)2PVP10]2+-coated electrode,16 in which bpy is 2,2′bipyridyl and PVP is poly(4-vinylpyridine). They have positive ECL properties, and the analytical usefulness in the case of the [Ru(bpy)2PVP10]2+-coated electrode is quite selective to oxalate determination. Another powerful approach is the immobilization of Ru(bpy)32+ in perfluorosulfonated ionomer Nafion films.3,9,17 Such an ECL sensor was used not only for the determination of oxalate, amines, NADH, and antibiotics9 but also for the enzymatic biosensors to determine glucose or ethanol.10 However, the stability of this sensor is problematic because the immobilized Ru(bpy)32+ migrates into the electroinactive hydrophobic region with time.3,9 Recently, sol-gel chemistry has offered a new and interesting possibility in the fields of chemical sensors and biosensors.18 In particular, this class of sol-gel-derived silica films possesses physical rigidity, chemical inertness, negligible swelling, tunable porosity, thermal stability, and optical transparency. These characteristics have led to intensive research in this area based mainly on optical and electrochemical sensors.19 For the ECL sensor, Ru(bpy)32+-modified chitosan-silica composite films have been reported, and the presence of silica gel in the ECL sensor improved the long-term stability of the sensor, as compared to that of the Nafion-based ECL sensor.20 Ru(bpy)32+-derivatized polystyrene polymer-silica composite films have been reported, but their analytical application has not been studied in detail.21 Khramov and Collinson have studied the ECL sensor consisting of Ru(bpy)32+ immobilized in Nafion-silica composite films.22 The ECL sensor exhibited improved the ECL sensitivities for tripropylamime and oxalate, as compared to a pure Nafion-based ECL sensor. Similarly, Dong and co-workers have reported the immobilization of Ru(bpy)32+ in PSS-silica composite films, in which PSS is poly(sodium 4-styrene sulfonate), and the ECL sensor showed very good storage stability during a period of 6 months.23 Although all reported ECL sensors based on the sol-gel-derived silicate films exhibited improved ECL characteristics when compared to those based on pure Nafion films, new materials and immobilization methods are still needed in order to improve both the sensitivity and the long-term stability of ECL-based sensors. Therefore, we report here on an alternative matrix of composite films consisting of sol-gel-derived titania (TiO2) and Nafion for the immobilization of Ru(bpy)32+ on an electrode surface. TiO2 is (15) Abruna, H. D.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 2641. (16) Forster, R. J.; Hogan, C. F. Anal. Chem. 2000, 72, 5576. (17) (a) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 6642. (b) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817. (18) (a) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (b) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A. (c) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Jun, J. Anal. Chem. 1995, 67, 22A. (d) Lin, J.; Brown, C. W. Trends Anal. Chem. 1997, 16, 200. (19) (a) Sampath, S.; Lev, O. Anal. Chem. 1996, 68, 2015. (b) Wang, J.; Pamidi, P. V. A. Anal. Chem. 1997, 69, 4490. (c) Wang, B.; Li, B.; Dong, Q.; Dong, S. Anal. Chem. 1998, 70, 3170. (d) Lee, W.-Y.; Lee, K. S.; Kim, T.-H.; Shin, M.-C.; Park, J.-K. Electroanalysis 2000, 78. (20) Zhao, C.-Z.; Egashira, N.; Kurauchi, Y.; Ohga, K. Anal. Sci. 1998, 14, 439. (21) Sykora, M.; Meyer, T. J. Chem. Mater. 1999, 11, 1186. (22) Khramov, A. N.; Collinson, M. M. Anal. Chem. 2000, 72, 2943. (23) (a) Wang, H.; Xu, G.; Dong, S. Analyst 2001, 126, 1095. (b) Wang, H.; Xu, G.; Dong, S. Electroanalysis 2002, 14, 853.
a wide-band-gap semiconductor (band gap ) 3 eV), and the TiO2 films can be easily prepared via the sol-gel process.24 Since the TiO2 films are very stable and optically transparent, such films have been widely utilized in recent years for the immobilization of ruthenium(II) complexes in photoelectrochemical cells and for the immobilization of proteins in bioanalytical applications.25 In this present work, we describe the preparation of titania-Nafion composite films from the sol-gel process using titanium tetraisopropoxide as a precursor. The electrochemical behavior and ECL characteristics of the films have been studied as a function of the amount of Nafion incorporated into the TiO2 sol. The ECL characteristics obtained with the present ECL sensor based on Ru(bpy)32+ immobilized in a TiO2-Nafion composite-modified glassy carbon electrode will be evaluated in terms of sensitivity, detection limits, and long-term stability relative to a pure Nafionbased ECL sensor for the determination of tripropylamine or oxalate. Finally, the present ECL sensor will be applied to HPLCRu(bpy)32+ ECL detection system in a simple manner for the determination of erythromycin in human urine samples. EXPERIMENTAL SECTION Reagents. Titanium(IV) isopropoxide(Ti(OR)4 (R ) CH(CH3)2, 99.99%), Nafion (perfluorinated ion-exchange resin, 5% (w/v) solution in a solution of 90% aliphatic alcohol/10% water mixture), tripropylamine (TPA, 99%), sodium oxalate (99.5%), and tris(2,2′bipyridyl)dichlororuthenium(II) (Ru(bpy)32+, 98%) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Water for all solutions was purified using a Milli-Q water purification system (Millipore, Bedford, MA). Apparatus. Cyclic voltammetric experiments were performed with an EG&G 273A potentiostat (Oak Ridge, TN). All experiments were carried out with a conventional three-electrode system in a 15-mL electrochemical cell. The working electrode was glassy carbon (0.07 cm2) coated with the composite films, and a platinum wire was the counter electrode. All of the potentials quoted here were relative to an Ag/AgCl (3 M NaCl) reference electrode. The photon-counting system used was a Hamamatsu Photonics HC 135-02 photon-counting module (Hamamatsue City, Japan) in conjunction with a computer for recording the output. The electrochemical cell was also used in the ECL experiments. The ECL cell was placed directly in front of the photomultiplier tube (PMT) window. The entire ECL cell was enclosed in a light-tight box. All experiments were conducted in ambient conditions at ∼25 °C. In the HPLC-Ru(bpy)32+ ECL detection system, a flow cell was assembled from a conventional LC-EC dual platinum electrode (Bioanalytical Systems, West Lafayette, IN) and placed against a transparent Plexiglass window for the detection of ECL emission, as described in the previous paper.26 In addition to a dual platinum electrode, a stainless steel counter electrode at the cell outlet and silver wire quasi-reference electrode were used. The entire flow cell was placed directly in front of the photo multiplier tube (PMT) window. A Younglin M930 HPLC pump (24) Lakshmi, B. B.; Patrissi, C. J.; Martin, C. R. Chem. Mater. 1997, 9, 2544. (25) (a) Topoglidis, E.; Cass, A. E. G.; Gilardi, G.; Sadeghi, S.; Beaumont, N.; Durrant, J. R. Anal. Chem. 1998, 70, 5111. (b) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731. (c) Andersson, A.-M.; Isovitsch, R.; Miranda, D.; Wadlwa, S.; Schmehl, R. H. Chem. Commun. 2000, 505. (d) Chen, X.; Cheng, G.; Dong. S. Analyst 2001, 126, 1728. (26) Skotty, D. R.; Lee, W.-Y.; Nieman, T. A. Anal. Chem. 1996, 68, 1530.
Analytical Chemistry, Vol. 75, No. 16, August 15, 2003
4251
(Young-Lin Instrument, Seoul, Korea) was used to deliver a buffered mobile phase. Injection was made using a Rheodyne model 9725 injector equipped with a 20-µL injection loop. An XTerra RP18 column (5 µm, 150 mm × 4.6 mm i.d.) from Waters (Milford, MA) was used for reversed-phase separation of erythromycin. The potential of the ECL detection system was held at 1.3 V vs the Ag quasi-reference electrode using a Won-A Tech potentiostat (Seoul, Korea). The photon multiplier tube used was the same Hamamatsu Photonics H5784 optical sensor module. The output of the PMT module was fed into an Autochro Data Module (Young-Lin Instrument). BET surface area, pore diameter, and BJH cumulative pore volume were obtained using a particle size analyzer (UPA-150, Microtrac, U.S.A.) at the Korea Basic Science Institute (KBSI). Preparation of the ECL Sensor. The TiO2 sol was prepared by the hydrolysis and condensation by mixing 1 mL of 0.15 M Ti(OC4H9)4 dissolved in propan-2-ol and 5 mL of deionized water in a manner similar to the previous report.27 Around 7 µL of 0.1 M HCl solution was added to the above solution as a catalyst. The solution was vigorously stirred for 1 h at room temperature until transparent. The volume of Nafion added to the TiO2 sol was varied from 25 to 80% (v/v, %), assuming complete conversion of Ti(OC4H9)4 to TiO2 sol. A 2-µL aliquot of the composite was hand-cast on the surface of a glassy carbon electrode. The film was uniform and consistently transparent. The thin films were dried for 1 min at room temperature. Prior to the casting, the electrodes were polished with 0.05-µm alumina on a polishing cloth, rinsed thoroughly with doubly distilled water, and then allowed to dry at room temperature. The composite electrode was then placed in an electrochemical cell containing 0.5 mM Ru(bpy)32+ solution in 0.05 M phosphate buffer at pH 7. The incorporation of Ru(bpy)32+ into the composite film was electrochemically monitored by running consecutive cyclic potential scans, from an initial potential of +800 mV to a high of +1350 mV and back to +800 mV at a scan rate of 100 mV/s. The electrode containing immobilized Ru(bpy)32+ was then placed in 0.05 M phosphate buffer solution at pH 7 and was ready for use. The surface coverage, Γ, was determined for a TiO2-Nafion (50%) composite-modified electrode by graphical integration of background-corrected cyclic voltammogram (1 mV/s). The surface coverage was ∼6 × 10-8 mol/cm2. Applications of ECL Sensor to HPLC Detection. In the HPLC-ECL experiments, an aliquot of a human urine sample was diluted 4-fold with 0.05 M phosphate buffer solution at pH 7, and the mixture was spiked with a solution containing accurately measured erythromycin. The urine sample was filtered using a syringe filter of 0.45-µm pore size before injecting it into the HPLC column. The column was operated at ambient temperature (25 ( 2 °C). The mobile phase consisting of 30% acetonitirile in 0.05 M phosphate buffer (pH 7.0) was filtered using a vacuum filter system equipped with a 0.45-µm nylon membrane filter (Millipore, Bedford, MA) and sonicated for 30 min. The flow rate of the mobile phase was maintained at 1.0 mL/min. RESULT AND DISCUSSION Voltammetric Behavior. The electrochemical behavior of Ru(bpy)32+ immobilized in TiO2-Nafion composite-modified elec(27) Yusuf, M. M.; Imai, H.; Hirashima, H. J. Non-Cryst. Solids 2001, 285, 90.
4252 Analytical Chemistry, Vol. 75, No. 16, August 15, 2003
Figure 1. Cyclic voltammograms of 0.5 mM Ru(bpy)32+ in 0.05 M phosphate buffer solution (pH 7) at a bare glassy carbon electrode (‚‚‚) and at a TiO2-Nafion (50%) composite-modified electrode (s) with a scan rate of 100 mV/s.
trode has been studied using cyclic voltammetry. Since Nafion is known to contain a hydrophobic domain or phase composed of fluorocarbon skeleton,28 a hydrophobic cation such as Ru(bpy)32+ can be easily incorporated into the composite films composed of cation-exchangeable Nafion and sol-gel-derived titania via both an ion-exchange process and hydrophobic interactions. The Ru(bpy)32+ was incorporated into the composite films by simply placing them in 0.5 mM Ru(bpy)32+ solution prepared in 0.05 M phosphate buffer at pH 7 for a certain period of time or by running consecutive cyclic potential scans from an initial potential of +800 mV to a high of +1350 mV and back to +800 mV at a scan rate of 100 mV/s. Figure 1 shows typical cyclic voltammograms obtained at a bare glassy carbon electrode and at a TiO2-Nafion (50%) composite-modified electrode in 0.5 mM Ru(bpy)32+ solution. The voltammogram for the composite-modified electrode shown in Figure 1 was recorded after the electrode had reached a steadystate response in the Ru(bpy)32+ solution. The voltammogram of the Ru(bpy)32+ obtained at the composite-modified glassy carbon electrode is similar in shape to that obtained at a bare glassy carbon electrode. The oxidation current obtained at the compositemodified electrode was almost 7-fold greater than that obtained at the bare electrode. This indicates that the TiO2-Nafion (50%) composite is an effective medium for the preconcentration of Ru(bpy)32+. The oxidation potential was shifted positively by ∼13.8 mV at the modified electrode, which indicates that the Ru2+ species has been stabilized by Nafion in the composite film relative to the Ru3+ species. This can be attributed to a strong hydrophobic interaction between Ru(bpy)32+ and the fluorocarbon phase of Nafion. A similar positive potential shift in formal potential has been previously reported by Heineman group for Re(DMPE)3+ (where DMPE ) 1,1-bis(dimethylphosphino)ethane) in Nafion films.29 The Nafion in the TiO2-Nafion composite electrode causes the peak separation to increase to ∼110 mV, as compared to 60 mV, at the bare electrode. For all the modified electrodes with various amounts of Nafion in the composite films, the anodic and (28) Lee, P. C.; Meisel, D. J. Am. Chem. Soc. 1980, 102, 5477. (29) Deng, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4045.
cathodic peak currents vary linearly with the square root of the scan rate over the range 10-200 mV/s. At those experimental time scales, the thickness of the diffusion layer is less than the thickness of the composite films (∼10 µm), and therefore, the semi-infinitive diffusion process prevails. After a steady-state cyclic voltammogram was obtained, the composite-modified electrode was removed from the Ru(bpy)32+ solution, rinsed with water, and placed in a pure phosphate buffer solution at pH 7. The cyclic voltammogram of the Ru(bpy)32+-immobilized composite-modified electrode in the buffer solution was comparable with that obtained in the Ru(bpy)32+ solution. The anodic peak current obtained with the Ru(bpy)32+-immobilized composite-modified electrode in the buffer solution was ∼5% lower than that of the steady-state current obtained in the Ru(bpy)32+ solution. The uptake rate of Ru(bpy)32+ into composite films is relatively fast; however, the time to obtain a steady-state cyclic voltammogram is highly dependent upon the amount of the Nafion incorporated into the composite films and also the Ru(bpy)32+ concentration. As the amount of the Nafion in the composite films increases, the time to reach a steady-state cyclic voltammogram also increases (e.g., 25% ) 9 min, 50% ) 11 min, 75% ) 15 min, and pure Nafion ) 33 min) in 0.5 mM Ru(bpy)32+ solution. For the TiO2-Nafion (50%) composite-modified electrode, the time to reach a steady-state current was 3-fold shorter than that obtained with a corresponding pure Nafion-modified electrode under the identical experimental conditions. In addition, the uptake rate is faster at higher Ru(bpy)32+ concentrations. Similar increased uptake rates have been previously reported by other groups for the incorporation of 2-propranol,30 Re(DMPE)3+,31 and Ru(bpy)32+ into the silica-Nafion composite films.22 Harmer and co-workers have reported that ion-exchange sites in Nafion-silica composite films are more accessible to 2-propanol than those in pure Nafion films, therefore offering the potential for a significantly enhanced catalytic activity.30 Heineman, Seliskar, and co-workers have observed that the dispersing the Nafion in sol-gel-derived silica matrix improves the slow diffusion of Re(DMPE)3+ in pure Nafion films.31 Khramov and Collinson have also noted an ∼2-3 times faster uptake rate of Ru(bpy)32+ into the silica-Nafion composite films than into pure Nafion films.22 The increased uptake rate of Ru(bpy)32+ into the TiO2-Nafion composite films relative to the pure Nafion films in the present study is possibly due to the greater accessibility of the ionexchange sites, SO3-, in the composites, therefore leading to a faster diffusion of Ru(bpy)32+ in the composite films. The apparent diffusion coefficient, Dapp, for Ru(bpy)32+ in the TiO2-Nafion (50%) composite films was calculated from the slope (from linear regression analysis) of the anodic peak current vs square root of the scan rate plot by the Randles-Sevick equation. The calculated Dapp is ∼4 × 10-9 cm2/s, which is slightly larger than the previously reported diffusion coefficient for the Ru(bpy)32+ in the pure Nafion films (5 × 10-10 cm2/s).17b In addition, the pore sizes in both titania-Nafion (50%) composite films and pure Nafion films have been examined. When compared to the pure Nafion films, the titania-Nafion (50%) composite films are more porous, and the pore size is greater (Table 1). Therefore, the ion-exchange (30) Harmer, M. A.; Farneth, W. E.; Sun, Q. J. Am. Chem. Soc. 1996, 118, 7708. (31) (a) Hu, Z.; Seliskar, C. J.; Hemineman, W. R Anal. Chem. 1998, 70, 5230. (b) Hu, Z.; Slaterbeck A. F.; Seliskar, C. J.; Ridgway, T. H.; Hemineman, W. R Langmuir 1999, 15, 767.
Table 1. Surface Area, Pore Volume, and Pore Size of Titania-Nafion Composite and Pure Nafion Films
surface area, m2g-1 pore diameter, nm pore vol., cm3g-1 a
titania-Nafion compositea
Nafion
353 3.14 0.28
58 2.82 0.02
Titania-Nafion (50%) composite film.
Figure 2. Dependence of anodic peak current (b) and ECL intensity (O) on the amount of Nafion incorporated into the composite films obtained at a scan rate of 100 mV/s. The ECL intensity was measured for 0.5 mM TPA solution prepared in 0.05 M phosphate buffer at pH 7. The points shown are the mean of at least five determinations ((SD).
sites appear to be easily accessible to Ru(bpy)32+ through the interconnected porous channels in the composite films. Further AFM study for the surface characterization is underway. Since the amount of Nafion incorporated into sol-gel derived TiO2 sol significantly affects the electrochemical behavior of the composite films, the effect of the composition of TiO2-Nafion composite on oxidation current was investigated by changing the Nafion content in the composite films. The anodic peak current obtained from cyclic voltammograms similar to those shown in Figure 1 is plotted versus the amount of Nafion incorporated into sol-gel-derived TiO2 sol. As shown in Figure 2 (b), the anodic peak current steadily increased as the amount of Nafion in the TiO2 sol increased up to 50%. However, as the amount of Nafion in the TiO2 sol further increased, the current gradually decreased. Our results are somewhat different from the previous results of Re(DMPE)3+ and Ru(bpy)32+ incorporated into relatively thin (
