Characterization of Platinum Nanoparticle-Embedded Carbon Film

Apr 2, 2003 - The film electrode was prepared by RF cosputtering carbon and Pt, which requires only a one-step formation process. This method can cont...
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Anal. Chem. 2003, 75, 2080-2085

Characterization of Platinum Nanoparticle-Embedded Carbon Film Electrode and Its Detection of Hydrogen Peroxide Tianyan You,† Osamu Niwa,*,† Masato Tomita,†,§ and Shigeru Hirono‡

NTT Microsystem Integration Laboratories, 3-1 Morinosato, Wakamiya, Atsugi, Kanagawa 243-0198, Japan, and NTT Afty Corporation, 4-16-30 Shimorenjyaku, Mitaka, Tokyo, 181-0013 Japan

A method for the highly sensitive determination of acetylcholine (ACh) and choline (Ch) that employs a graphitelike carbon film electrode containing 6.5% platinum (Pt) nanoparticles was developed for use as a detector in microbore liquid chromatography (LC) with a postcolumn enzyme reactor. The film electrode was prepared by RF cosputtering carbon and Pt, which requires only a onestep formation process. This method can control the Pt content of the film at a relatively low deposition temperature (below 200 °C). The average size of the Pt nanoparticles was 2.5 nm. The film electrode showed excellent electrocatalytic activity, high sensitivity, and negligible baseline drift when detecting hydrogen peroxide. The electrode was modified with glucose oxidase and responded rapidly to glucose with a much more stable baseline current than at a Pt bulk electrode based sensor. Therefore, it is appropriate to employ the electrode to detect trace amounts of biomolecules, such as neurotransmitters and hormones combined with various oxidase enzymes. We used the electrode as a detector for microbore LC and observed a low detection limit of 2.5 and 2.3 fmol (10-µL injection) for ACh and Ch, respectively, which is ∼1 order of magnitude lower than that of a Pt bulk electrode. Acetylcholine (ACh) is a well-known neurotransmitter. Trace amounts are widely distributed throughout the human body. ACh plays a crucial role in transferring important biological information both in the central nervous system and in vertebra and nerveskeletal junctions.1 Choline (Ch) is a precursor and a metabolite of ACh. Many sensors have been developed to detect ACh and Ch on the basis of the determination of the hydrogen peroxide generated by ACh esterase (AChE) and Ch oxidase (ChOx) enzymatic reactions.2-16 Therefore, it is very important to improve the sensitivity of hydrogen peroxide detection with good stability * Corresponding author. Tel: +81-46-2403517. Fax: +81-46-2404728. E-mail: [email protected]. † NTT Microsystem Integration Laboratories. ‡ NTT Afty Corporation. § Present address: Corning Technology Center, Osuka-cho, Ogasa-gun, Shizuoka 437-1397, Japan. (1) Hasselmo, M. E.; Bower, J. M. Trends Neurosci. 1993, 16, 218-222. (2) Cui, J.; Kulagina, N. V.; Michael, A. C. J. Neurosci. Methods 2001, 104, 183-189. (3) Zhang, S.; Zhao, H.; John, R. Biosens. Bioelectron. 2001, 16, 1119-1126. (4) Chen, J.; Lin, X.; Chen, Z.; Wu, S.; Wang, S. Anal. Lett. 2001, 34, 491-501.

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in electroanalysis. Platinum (Pt) electrodes are very widely used for the electrochemical oxidation of hydrogen peroxide.17-22 However, the baseline current of the electrode gradually decreases with constant potential measurement because the electrode surface is gradually deactivated, mainly as a result of surface oxide formation. It is also necessary to leave the electrode for a long time (usually overnight) after applying the potential when it is used to determine trace amounts of biochemicals. Various groups have been trying to improve the performance of Pt-based electrodes, including that of a Pt-black electrode,23 a Pt-black microarray electrode,24 and mesoporous Pt microelectrodes,25 to achieve a low detection limit and a stable response. The Pt-black electrode shows better long-term stability than bare Pt because it has a much greater surface area than the Pt bulk electrode, and this helps maintain a fast apparent electron transfer (5) Niwa, O.; Horiuchi, T.; Kurita, R.; Torimitsu, K. Anal. Chem. 1998, 70, 1126-1132. (6) Kehr, J.; Dechent, P.; Kato, T.; O ¨ gren, S. O. J. Neurosci. Methods 1998, 83, 143-150. (7) Osborne, P. G.; Yamamoto, K. J. Chromatogr., B 1998, 707, 3-8. (8) Yang, L.; Janle, E.; Huang, T.; Gitzen, J.; Kissinger, P. T.; Vreeke, M.; Heller, A. Anal. Chem. 1995, 67, 1326-1331. (9) Huang, T.; Yang, L.; Gitzen, J.; Kissinger, P. T.; Vreeke, M.; Heller, A. J. Chromatogr., B 1995, 670, 323-327. (10) Tsai, T.-R.; Cham, T.-M.; Chen, K.-C.; Chen, C.-F.; Tsai, T.-H. J. Chromatogr., B 1996, 678. 151-155. (11) Kato, T.; Liu, J. K.; Yamamoto, K.; Osborne, P. G.; Niwa, O. J. Chromatogr., B 1996, 682, 162-166. (12) Fossati, T.; Colombo, M.; Castiglioni, C.; Abbiati, G. J. Chromatogr., B 1994, 656, 59-64. (13) Nevera, E. N.; Suzuki, M.; Tamiya, E.; Takeuchi, T.; Karube, I. Electroanalysis 1993, 5, 17-22. (14) Ruiz, B. L.; Dempsey, E.; Hua, C.; Smyth, M. R.; Wang, J. Anal. Chim. Acta 1993, 273, 425-430. (15) Isaksson, K.; Kissinger, P. T. J. Chromatogr. 1987, 419, 165-175. (16) Potter, P. E.; Meek, J. L.; Neff, N. H. J. Neurochem. 1983, 41, 188-194. (17) Domı´nguez Sa´nchez, P.; Tunˇon Blanco, P.; Ferna´ndez Alvarez, J. M.; Smyth, M. R.; O’Kennedy, R. Electroanalysis 1990, 2, 303-308. (18) Ferapontova, E. E.; Grigorenko, V. G.; Egorov, A. M.; Bo¨rchers, T.; Ruzgas, T.; Gorton, L. Biosens. Bioelectron. 2001, 16, 147-157. (19) Lundback, H.; Johansson, G.; Holast, O. Anal. Chim. Acta 1983, 155, 4756. (20) Hendji, N.; Bataillard, P.; Jaffrezic-Renauly, N. Sens. Actuators B 1993, 1516, 127-134. (21) Westbroek, P.; Huate, B. Van; Temmerman, E. Fresenius’ J. Anal. Chem. 1996, 354, 405-409. (22) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochim. Acta 1998, 43, 579588, 2015-2024; 1999, 44, 2455-2462, 4573-4582; 2000, 45, 3573-3579. (23) Ikariyama, Y.; Yamauchi, S.; Yukiashi, T.; Ushioda, H. Anal. Lett. 1987, 20, 1407-1416. (24) Niwa, O.; Horiuchi, T.; Morita, M.; Huang, T.; Kissinger, P. T. Anal. Chim. Acta 1996, 318, 167-173. 10.1021/ac026337w CCC: $25.00

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rate. However, this greater surface area results in a high background current, which makes it difficult to improve the detection limit. Metal particle electrodes formed on carbon electrodes by vacuum deposition or electroplating have been used as electrochemical detectors. Since each particle works as a microelectrode, a lower detection limit can be obtained because of the high current densities at each particle. However, the surface of the metaldeposited electrode is not flat, and this often increases the noise level as a result of the turbulence of the flow when the electrodes are used for flow analysis, for example, as detectors for FIA and LC. In addition, an electrode whose surface metal particles are held in place solely by physical adsorption can easily lose its sensitivity if the particles become detached from the surface. Highly dispersed nanoscale Pt particles have been attracting growing interest because of their unique physical and chemical properties as well as with regard to their applications.26,27 Several methods have been used for preparing metal clusters.28-35 Most of these methods can produce nanoparticles on a supported surface or colloidal dispersions. Very few papers have reported the preparation of uniform Pt cluster dispersed carbon film.36-40 McCreery et al. reported nanoscale Pt(0) clusters in glassy carbon (Pt-GC) that they obtained by incorporating Pt into carbon precursors, poly(phenylene-1,3-diacetylene) or poly(1,2,3,5-tetrafluorophenylene-1,3-diacetylene), and then pyrolizing them at 600 °C. Although this process can be used to prepare Pt nanoparticles in glassy carbon, the synthesis of carbon precursors and the incorporation of Pt require sophisticated polymer synthesis skills and many steps to obtain the final products. In addition, hightemperature pyrolysis limits the use of substrate materials. We proposed a simple RF cosputtering method for preparing a homogeneous film containing Pt nanoparticles embedded in graphite-like carbon film (Pt-NEGCF).41 This method needs only one step, and the film can be formed on a Si wafer substrate by sputtering carbon and Pt together at low temperature (200 °C). In the work reported here, we used Pt-NEGCF electrodes (containing 1.3, 2.9, and 6.5% Pt atoms) to detect hydrogen (25) Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.; Denuault, G. Anal. Chem. 2002, 74, 1322-1326. (26) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622-624. (27) Tong, Y.; Rice, A.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 1123-1129. (28) Kao, W.-H.; Kuwana, T. J. Am. Chem. Soc. 1984, 106, 473-476. (29) Itaya, K.; Takahashi, H.; Uchida, I. J. Electroanal. Chem. 1986, 208, 373. (30) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401-7402. (31) Zoval, J. V.; Gorer, J. L.; Penner, R. M. J. Phys. Chem. B 1998, 102, 11661175. (32) Awada, M.; Strojek, J. W.; Swain, G. M. J. Electrochem. Soc. 1995, 42, L42L45. (33) Enea, O.; Riedo, B.; Dietler, G. Nano Lett. 2002, 2, 241-244. (34) Liu, S.; Tang, Z.; Wang, E.; Dong, S. Electrochem. Commun. 2000, 2, 800804. (35) Wang, J.; Swain, G. M.; Tachibana, T.; Kobashi, K. Electrochem. Soc. Proc. 1999, 99-32, 428. (36) Callstrom, M. R.; Neenan, T. X.; McCreery, R. L.; Alsmeyer, D. C. J. Am. Chem. Soc. 1990, 112, 4954-4956. (37) Pocard, N. L.; Alsmeyer, D. C.; McCreery, R. L.; Neenan, T. X.; Callstrom, M. R. J. Mater. Chem. 1992, 2, 771-784. (38) Pocard, N. L.; Alsmeyer, D. C.; McCreery, R. L.; Neenan, T. X.; Callstrom, M. R. J. Am. Chem. Soc. 1992, 114, 769-771. (39) Howard, H. D.; Pocard, N. L.; Alsmeyer, D. C.; Schueller, O. J. A.; Spontak, R. J.; Huston, M. E.; Huang, W.; McCreery, R. L.; Neenan, T. X.; Callstrom, M. R. Chem. Mater. 1993, 5, 1727-1738. (40) Schueller, O. J. A.; Pocard, N. L.; Huston, M. E.; Spontak, R. J.; Neenan, T. X.; Callstrom, M. R. Chem. Mater. 1993, 5, 11-13.

peroxide, which is the product of enzymatic reactions with various biomolecules, such as glucose, ACh, and Ch. We also compared the properties of the film electrode with those of a Pt bulk electrode when they were employed in LC to detect ACh and Ch coupled with microbore liquid chromatography (LC) and a postcolumn enzyme reactor. EXPERIMENTAL SECTION Reagents. Glucose, glucose oxidase (GOx), bovine serum albumin (BSA), acetylcholine (ACh), and choline (Ch) were obtained from Sigma (St. Louis, MO). Hydrogen peroxide (30%), 0.5 M H2SO4, and 0.1 M phosphate buffer solution (PBS, pH 7) were purchased from Kanto Chemical Co. Ltd. (Tokyo, Japan). Glutaraldehyde was purchased from Wako (Tokyo, Japan). Water used in the experiments was purified by Milli-Q (Millipore, Bedford, MA). All reagents were used as received. Film Electrode Preparation. We prepared the Pt-NEGCF on a Si wafer by the RF sputtering method (ANELVA FP-21, Japan). During sputtering, the background pressure of the chamber was kept at 1.0 × 10-7 Torr. The Ar gas pressure was 1.0 × 10-2 Torr. The temperature of the Si substrate was controlled at 200 °C, and the RF power was 200 W. The Pt content could be widely changed without difficulty by changing the Pt and carbon target area. For example, when the target surface area ratio of Pt/carbon was 0.7%, we obtained an atomic concentration of 6.5% Pt in the film. After deposition, the film thickness was 40 nm. The wafer was then cut into a rectangle, and a plastic tape (with a 3-mm-diameter hole) was fixed to it to form a Pt-NEGCF disk electrode. When measuring glucose, the Pt-NEGCF and Pt bulk electrodes (d ) 3 mm) were modified with a 4-µL drop of aqueous solution containing 1% BSA, 5% GOx, and 0.2% glutaraldehyde. Apparatus. X-ray photoelectron spectra were collected with an X-ray photoelectron spectroscope (XPS) (XPS5700, Physical Electronics, Inc., USA) using monochromatic Al KR radiation (1486.6 eV). A compositional quantitative estimation was carried out with a simple quantification procedure,42 where the atomic fractional contents were calculated from the XPS signal intensities and the relative sensitivity factors. We observed the film using a transmission electron microscope (TEM) (H-9000UHR, Hitachi Ltd., Japan) by peeling the film from the silicon substrate. We used an acceleration voltage of 200 kV for the TEM observation. We measured the Pt-NEGCF conductivity with the conventional four-terminal method at room temperature. We undertook atomic force microscopy (AFM) in air using a Nano Scope III (Digital Instruments, USA). An NSC12/15 tip was used for AFM observation in the tapping mode. The setpoint voltage was 1.4 V. Hydrogen Peroxide, Glucose, ACh and Ch Measurement. Cyclic voltammetry measurements were carried out with an ALS/ CHI 802 Electrochemical Analyzer (CH Instruments, Inc. USA) using a three-electrode cell consisting of a Pt-NEGCF (or a Pt bulk) working electrode (d ) 3 mm), a Ag/AgCl (3 M KCl) reference electrode, and a Pt auxiliary electrode. The flow injection analysis (FIA) experiments were performed with an HPLC class(41) You, T.; Niwa, O.; Horiuchi, T.; Tomita, M.; Iwasaki, Y.; Ueno, Y.; Hirono, S. Chem. Mater. 2002, 14, 4796-4799. (42) Seah, M. P. in Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1990; Vol. 1, Chapter 5.

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Figure 2. High-resolution XPS spectrum of Pt(4f) line of 6.5% PtNEGCF.

Figure 1. Transmission electron micrograph (TEM) image of front view of 6.5% Pt-NEGCF.

VP system (Shimazu Co., Japan) and a thin-layer radial flow cell (BAS, West Lafayette, IN). We conducted LC measurements with a PM-80 pump, an LC4C amperometric detector, a CC-5 flow cell, and a column compartment (BAS, West Lafayette, IN). The mobile phase was pH 8.0 Na2HPO4 buffer (50 mM) containing 0.5 mM EDTA, and the flow rate was 120 µL/min. A microbore ACh and Ch kit, consisting of a UniJet ACh analytical column (530 × 1 mm i.d.) and a UniJet ACh/Ch immobilized enzyme column (BAS, West Lafayette, IN), was used for ACh and Ch LC determination. RESULTS AND DISCUSSION Film Characterization. Figure 1 shows a transmission electron micrograph (TEM) of the front view of the Pt-NEGCF. The dark spots and light features correspond to Pt nanoparticles and the carbon matrix, respectively. We can see that the carbon matrix in the Pt-NEGCF contains small crystalline structures. We obtained lattice images of the carbon with a 3.71-Å spacing, indicating that the carbon matrix is not amorphous, but has a disordered graphite-like structure. The Pt particles in the PtNEGCF were ∼2.5 nm in size and highly dispersed in the graphitelike carbon film. The lattice spacing of the Pt-NEGCF measured by the electron diffraction of Pt (111) was 2.34 Å. The TEM micrograph of the film cross section shows that there are Pt nanoparticles throughout the film. Concerning the electron beam diffraction of the cross-sectional view, we obtained images of the carbon lattice, which had a 3.64-Å spacing and Pt (111) with a 2.30-Å spacing. From the data obtained with both the front and cross-sectional views, we can conclude that the film consists of Pt nanoparticles and a graphite-like carbon matrix and that the Pt nanoparticles are highly dispersed in the graphite-like carbon matrix throughout the film. The conductivity of the Pt-NEGCF is 124.7 S/cm. This high conductivity is because the carbon matrix structure is graphitelike and the conductivity of RF carbon film without Pt nanoparticles is 24 S/cm. Since the carbon matrix exhibits a large overpotential to H2O2, the Pt-NEGCF electrode functioned as a Pt nanoarray electrode. 2082 Analytical Chemistry, Vol. 75, No. 9, May 1, 2003

We investigated the Pt-NEGCF by XPS analysis. The XPS spectrum of Pt-NEGCF reveals that there was 6.5 at. % of Pt in the film. We used C(1s) as an internal reference (284.6 eV). Figure 2 shows the high-resolution XPS spectrum of the Pt(4f) line of the 6.5% Pt-NEGCF. The binding energies of the Pt(4f7/2) and Pt(4f5/2) electrons in the high-resolution spectrum of the Pt(4f) line were 71.9 and 75.2 eV, respectively, indicating that the Pt particles in the film were in the Pt(0) state. A slightly higher binding energy (∼0.9 eV) than that for bulk polycrystalline Pt was also observed,36-40 which may be due to the size effect of the small particles.43-45 This means we can expect the Pt-NEGCF surface to be more stable than a bulk electrode surface in terms of resistance to surface oxidation during electrochemical processes. The structure of 6.5% Pt-NEGCF is very similar to the structures of Pt-NEGCF films with less Pt content (1.3, 2.9%) that we reported before.41 Pt particles with the same average size (∼2.5 nm) can be obtained for metallic Pt with different atomic concentrations in the films (1.3, 2.9, and 6.5%). The lattice spacing values of the Pt (111) and the carbon matrix obtained from the electron diffraction patterns of these films were 2.30 ( 0.04 Å, and 3.65 ( 0.06 Å, respectively. The conductivity of RF carbon (RF-C), 1.3, 2.9, and 6.5% Pt-NEGCF is 24, 92.4, 103, and 124.7 S/cm. The conductivity increases as the Pt content of the films increases. These results suggest that the RF method can provide a simple way of preparing Pt-NEGCFs with the same structure and same average Pt particle size, but with a wide range of Pt concentrations. We studied the 6.5% Pt-NEGCF surface by AFM analysis. Within the selected area (3 × 3 µm), the root-mean-square (RMS) roughness is 0.71 nm, and the average roughness (Ra) is 0.56 nm. The film surface is so smooth that it is difficult to distinguish the carbon and Pt atoms on the surface. The RF method has several advantages over previous methods in terms of preparing carbon film containing Pt nanoparticles. First, this method is a simple method that requires only one step to form a uniform Pt-NEGCF film, whereas the previously reported uniform Pt-GC film fabrication process requires several steps, including the synthesis of carbon precursors, the incorporation of Pt into the carbon precursors, and themolysis.36-40 Processes for forming Pt particles electrochemically on GC or (43) Takasu, Y.; Unwin, R.; Tesche, B.; Bradshaw, A. M.; Grunze, M. Surf. Sci. 1978, 77, 219-232. (44) Kim, K. S.; Winograd, N. Chem. Phys. Lett. 1975, 30, 91-95. (45) Eberhardt, W.; Fayet, P.; Cox, D.; Fu, Z.; Kaldor, A.; Sherwood, R.; Sondericker, D. Phys. Scr. 1990, 41, 892-895.

Figure 3. Cyclic voltammograms of RF-C (a), 1.3% (b), 2.9% (c), 6.5% (d) Pt-NEGCF and Pt bulk (inset curve) electrodes in 0.5 M H2SO4 (Ar bubbled) and 6.5% Pt-NEGCF in O2-saturated 0.5 M H2SO4 solution (e). Scan rate, 50 mV/s.

diamond electrodes have also been reported.32-34 However, these approaches still need at least two steps, namely carbon or diamond film formation and Pt plating. Second, our process provides a flat and uniform film, which is advantageous for reducing the noise level during amperometric measurement and microfabrication. In contrast, the Pt-plated carbon electrodes have a rougher surface than Pt-NECGF film, and the adhesion of the particles is usually weak, as compared with that of carbon film with uniformly dispersed Pt nanoparticles.36-41 The sputter-deposited Pt-NECGF also realizes a homogeneous Pt particle distribution on the electrode surface. In contrast, the electrochemical method tends to deposit more Pt particles at the edge of the electrode as a result of the higher current density during plating. This is very serious when the electrode is small. Third, the sputtering process requires a much lower temperature (e200 °C) than the other processes, such as diamond film formation by CVD (700 °C)33 and the pyrolysis of the precursor (600 °C).36-40 The low-temperature process allows use of a variety of substrates, such as polymeric materials. In addition, a low-temperature process, such as the liftoff process, might be employed for the sputtered film. Finally, the Pt atomic concentration can be controlled by changing the ratio of the Pt/carbon surface area without changing the sputtering parameters, such as power and pressure. Electrochemical Behavior of 6.5% Pt-NEGCF. We studied the electrochemical properties of the Pt-NEGCF electrodes (1.3, 2.9, 6.5%) and used them for the electrochemical determination of hydrogen peroxide and biomolecules (glucose, acetylcholine, and choline). Figure 3 shows cyclic voltammograms of RF-C (a), 1.3% (b), 2.9% (c), 6.5% (d) Pt-NEGCF and Pt bulk (inset curve) electrodes in 0.5 M H2SO4. Before the experiments, the H2SO4 solution was always bubbled with Ar gas. Curve (e) is the CV obtained at 6.5% Pt-NEGCF in O2-saturated 0.5 M H2SO4 solution. No obvious reduction or oxidation peaks were observed in the potential range at the RF-C electrode (a). The CV curves (b, c, d) at the PtNEGCF electrodes are different from that of the Pt bulk electrode (inset figure). As for the Pt-NEGCF electrodes, the Pt oxidation current starts increasing gradually and reaches its peak potential at 1.0 V. The peak potential shifts to a higher value than that at

Figure 4. Cyclic voltammograms of 1 mM H2O2 and buffer at 6.5% Pt-NEGCF (a, c) and Pt bulk electrodes (b, d), respectively. Buffer solution, 0.1 M PBS, pH 7; scan rate, 50 mV/s. Lines a and b, 1 mM H2O2 solution; lines c and d, buffer.

the Pt bulk electrode. This could be due to the higher binding energy of Pt (4f), as mentioned in the XPS discussion. The anodic oxidation current increased as the Pt content of the films increased (b f d). The magnitude of the oxidation current at 6.5% PtNEGCF was ∼6% that obtained at the Pt bulk electrode. This also suggests that the observed anodic peak is due to platinum oxide formation, not oxygen evolution, which almost overlaps the Pt oxidation (compare curves d and e). Concerning the platinum oxide reduction, we observed a reduction peak at each Pt-NEGCF electrode, starting from 0.4 V. As the Pt atom concentration increases at the Pt-NEGCF electrode, the reduction current can be expected to increase. This peak is assumed to be the reduction peak of platinum oxide. However, it is also difficult to separate the platinum oxide and oxygen reduction. In the O2-saturated 0.5 M H2SO4 solution (e), the 6.5% Pt-NEGCF film electrode exhibited a dramatically electrocatalytic current increase for O2 reduction, as compared with that obtained in O2-free solution (d), which was prepared by bubbling Ar gas for more than 2 h. Therefore, we can confirm that the main origin of the reduction peak at the 6.5% Pt-NEGCF electrode (d) is platinum oxide reduction. However, we can see that the charge passed for oxide reduction was much larger than the charge passed to form the oxide on the anodic scan at the Pt-NEGCF electrodes. Concerning the Pt bulk electrode, we obtained ideal CV behavior under the same conditions (inset figure). The effect of a small amount of oxygen on the Pt-NEGCF electrode was much larger than that on the Pt bulk electrode. We assumed the reason to be as follows. Although we degassed the sulfuric acid solution with Ar gas, as mentioned, it is possible that a very small amount of oxygen remained in the solution. The Pt-NEGCF electrode functioned as a Pt nanoarray electrode. Therefore, the current was not proportional to the total Pt area, because the current density increases as the size of each electrode decreases. In contrast, the magnitude of the platinum oxide reduction is proportional to the Pt surface area. As a result, the contribution of the small amount of oxygen that remains after the degassing process to the total reduction current could be larger than that at the Pt bulk electrode. We evaluated the electrocatalytic ability of 6.5% Pt-NEGCF with respect to the electro-oxidation of H2O2. Figure 4 shows cyclic voltammograms (CVs) of H2O2 oxidation at a 6.5% Pt-NEGCF electrode and a conventional Pt bulk electrode. The H2O2 oxidation peak potential at the Pt-NEGCF electrode is at 0.5 V, which is Analytical Chemistry, Vol. 75, No. 9, May 1, 2003

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∼170 mV lower than that at the Pt bulk electrode (0.67 V) under the same experimental conditions. This suggests a faster electrontransfer rate at the Pt-NEGCF than at the Pt bulk electrode. A higher electrocatalytic current can be obtained at the 6.5% PtNEGCF electrode (a) than at the Pt bulk electrode (b). Moreover, the background current at the film electrode (c) is lower than the value obtained at the Pt bulk electrode (d). This can improve the sensitivity of the H2O2 detection by improving the signal-tonoise ratio. Hall et al.22 published a series of articles on the mechanism of electrochemical oxidation of H2O2 at platinum electrodes. They confirmed that the platinum oxide film plays an important role in the kinetics of the reaction. They also confirmed that the oxidation reaction is an ECC process, namely an adsorptioncontrolled mechanism, which depends on the potential, temperature, phosphate buffer, pH value, and chloride concentration. The mechanism should be applicable to the oxidation of H2O2 at the Pt-NEGCF electrodes. In addition to the above factors reported by Hall et al., we observed that the thickness of the platinum oxide film greatly affects the reaction. We pretreated the Pt bulk electrode with O2 plasma for a certain time and found that a thick platinum oxide film can be formed on the Pt electrode surface. The electrocatalytic oxidation current of H2O2 at the pretreated Pt electrode decreased to