Sol−Gel-Derived Ceramic−Carbon Nanotube Nanocomposite

Oct 5, 2004 - A high content of the MWNT (i.e., higher than. 1.5 mg/mL in the sol) leads to the formation of the. CCNNE characteristic of an electrode...
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Anal. Chem. 2004, 76, 6500-6505

Sol-Gel-Derived Ceramic-Carbon Nanotube Nanocomposite Electrodes: Tunable Electrode Dimension and Potential Electrochemical Applications Kuanping Gong,† Meining Zhang,† Yiming Yan,† Lei Su, Lanqun Mao,* Shaoxiang Xiong, and Yi Chen

Center for Molecular Science, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, China

Nanocomposite electrodes made of sol-gel-derived ceramic-carbon nanotube are fabricated by doping mutliwalled carbon nanotubes (MWNTs) into a silicate gel matrix. The electrochemical behavior and potential electrochemical applications of the ceramic-carbon nanotube nanocomposite electrodes (CCNNEs) are also studied. The as-prepared CCNNEs exhibit a tunable dimension ranging from conventional electrode to nanoelectrode ensemble (NEE), depending on the amount of the MWNT dispersed in the silica sol and finally doped within the gel matrix. A high content of the MWNT (i.e., higher than 1.5 mg/mL in the sol) leads to the formation of the CCNNE characteristic of an electrode of conventional dimension, while a low content (typically lower than 0.10 mg/mL) essentially yields the CCNNE like a nanoelectrode ensemble. The NEE is demonstrated to possess good electrocatalytic activity toward the oxidation of ascorbic acid (AA), and the CCNNE of conventional dimension is found to possess remarkable electrocatalytic activity toward the oxidation of glutathione (both reduced and oxidized forms, GSH and GSSG). These properties of the CCNNEs essentially offer a new electrochemical approach for the detection of AA, GSH, and GSSG. The possible essence of the tailor-made dimensions of the CCNNEs is also presented and discussed. Carbon-based materials, e.g., glassy carbon, graphite, and diamond, have been extensively used in electrochemistry due to their wide potential window, low background response, and good conductivity.1,2 On the other hand, sol-gel electrochemistry that stemmed from the intersection between sol-gel chemistry and electrochemistry has drawn extensive interest over past decades because of its remarkable facilitation of electrochemical studies and applications.3-5 Several reviews concerning this subject have * Corresponding author. E-mail: [email protected]. Fax: +86-10-6255-9373. † Also in Graduate School of the CAS. (1) Terashima, C.; Rao, T. N.; Sarada, B. V.; Kubota, Y.; Fujishima, A. Anal. Chem. 2003, 75, 1564. (2) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958. (3) Wang, B.; Li, B.; Wang, Z.; Xu, G.; Wang, Q.; Dong, S. Anal. Chem. 1999, 71, 1935. (4) Salimi, A.; Pourbeyram, S. Talanta 2003, 60, 205. (5) Pandey, P. C.; Upadhyay, S.; Pathak, H. C. Electroanalysis 1999, 11, 59.

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been appeared in the literature.6-8 Increasing interest has been focused on sol-gel-derived ceramic-carbon electrodes (CCEs), which are generally prepared by doping carbon or graphite powder into silicate gel matrix.9 This is because the CCEs can have the benefit of advantages both from carbon-based electrodes and from sol-gel electrochemistry, for instance, ease in fabrication at room temperature, capability for encapsulation of electrocatalyst and biocatalysts (e.g., proteins and enzymes), wide potential window, tunable polarity, and high stability. As a result, the CCEs have been demonstrated to be very useful for electrochemical investigations, e.g., electroanalysis, biosensors, electrocatalysis, and energy conversion and storage.9-12 The carbon nanotube (CNT) represents a new kind of carbonbased material and is superior to other carbon materials mainly in, for example, special structural feature and unique electronic and mechanical properties.13 Over the last several years, extensive interest has been shown in the preparation, physical and chemical properties, and possible applications of the CNTs.14 Besides their striking applications in other fields,15 recent efforts in CNT electrochemistry have revealed that the special structural and electronic features of the CNTs make them useful for electrochemical investigations,16,17 e.g., electrocatalysis,18 direct electrochemistry of proteins,19 and construction of electrochemical sensors and biosensors.20-22 These striking properties of the CNTs coupled with the advantages of sol-gel electrochemistry essentially suggest that the sol-gel-derived ceramic-carbon nano(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)

Walcarius, A. Electroanalysis 2001, 13, 701. Wang, J. Anal. Chim. Acta 1999, 399, 21. Collinson, M. M.; Howells, A. R. Anal. Chem. 2000, 72, 702A. Lev, O.; Rabinovich, L. Electroanalysis 2001, 13, 265. Cordero-Rando, M. M.; Hidalgo-Hidalgo de Cisneros, J. L.; Blanco, E.; Naranjo-Rodrı´guez, I. Anal. Chem. 2002, 74, 2423. Wang, J.; Pamidi, P. V. A.; Rogers, K. R. Anal. Chem. 1998, 70, 1171. Tsionsky, M.; Gun, G.; Glezer, V.; Lev, O. Anal. Chem. 1994, 66, 1747. Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799; Haddon, R. C. Acc. Chem. Res. 2002, 35, 997-997. Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. Science 2002, 297, 787. Dai, L.; Soundarrajan, P.; Kim, T. Pure Appl. Chem. 2002, 74, 1753. Zhao, Q.; Gan, Z.; Zhuang, Q. Electroanalysis 2002, 14, 1609. Luo, H.; Shi, Z.; Li, N.; Gu, Z.; Zhuang, Q. Anal. Chem. 2001, 73, 915. Gooding, J. J.; Wibowo, R.; Liu, J.; Yang, W.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006. Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075. Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413. Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. 10.1021/ac0492867 CCC: $27.50

© 2004 American Chemical Society Published on Web 10/05/2004

tube nanocomposite electrodes are more useful for electrochemical investigations than the CCEs. However, so far little attempt has been paid to the preparation, electrochemical characterization, and potential electrochemical applications of the CCNNEs.23 On the other hand, it is known that the CNTs could not dissolve in most solvents. The poor solubility essentially makes it difficult to construct CNT-based electronic devices. Although some creative efforts on the construction of the CNT-based electrodes have almost witnessed the possibility to accomplish the above purpose,20-22 a simple method for fabrication of stable CNT-based electrodes with tunable properties is still desired for electrochemical studies and for reliable and durable electrochemical applications. This work demonstrates a new kind of ceramic-carbon nanotube nanocomposite electrodes prepared by doping multiwalled carbon nanotubes into silicate gel. The intersection of CNT science with sol-gel chemistry not only provides a facile protocol for preparation of CNT-based electrodes (i.e., CCNNEs) that are relatively useful for electrochemical studies but also essentially allows the prepared CCNNEs to efficiently integrate the advantages both from the CNTs and from sol-gel electrochemistry. More notably, the present intersection provides the CCNNEs another advantage, i.e., tunable electrode dimension from conventional to microscopic dimensions. Such an intrinsic feature of the CCNNEs, which is very similar to that of other graphite composite electrodes reported in the literature,24-26 has not been observed in any CNT/sol-gel composite prepared previously23,27,28 and is superior to the CCEs.8 While the CCNNEs of conventional dimensions are useful, e.g., for electrochemical determinations and energy conversion and storage, the CCNNEs of microscopic dimensions are believed to be very attractive for electrochemical studies. This is because they can benefit from the following advantages: (1) of electrodes of microscopic dimensions, e.g., enhanced mass transportation, improved signal-to-noise ratio, and high temporal resolution, (2) associated with the CNTs, e.g., good electrocatalytic activity and high electronic conductivity, and (3) of sol-gel electrochemistry, e.g., ease in fabrication at room temperature, encapsulation of electrocatalyst and proteins or enzymes for electrocatalysis and biosensing, tunable polarity, and high stability. Consequently, the CCNNEs (both conventional and microscopic) would be very distinct for various electrochemical studies and pave a new way to CNT-based electronic (bio)devices. EXPERIMENTAL SECTION Chemicals and Materials. Multiwalled carbon nanotubes (MWNTs) were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). The MWNTs were purified as reported previously.29,30 Briefly, 50 mg of the as-received MWNTs was (23) Gavalas, V. G.; Andrews, R.; Bhattacharyya, D.; Bachas, L. G. Nano Lett. 2001, 12, 719. (24) Ramı´rez-Garcı´a, S.; Alegret, S.; Ce´spedes, F.; Forster, R. J. Anal. Chem. 2004, 76, 503. (25) Sleszynski N.; Osteryoung J. Anal. Chem. 1984, 56, 130. (26) O’Hare, D.; Macpherson, J. V.; Willows, A. Electrochem. Commun. 2002, 4, 245. (27) Dong, W.; Sakamoto, J.; Dunn, B. J. Sol-Gel Sci. Technol. 2003, 26, 641. (28) Wang, X. F.; Wang, D. Z.; Liang, L. J. Inorg. Mater. 2003, 18, 331. (29) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. Carbon 2002, 40, 985. (30) Zhao, X.; Inoue, S.; Jinno, M.; Suzuki, T.; Ando, Y. Chem. Phys. Lett. 2003, 373, 266.

heated at 673 K for 30 min to remove amorphous carbon particles. The residuals were dispersed in a mixture consisting of 100 mL of concentrated HF, 100 mL of concentrated HNO3, and 50 mg of sodium dodecyl sulfate. The suspension was then sonicated for 5 h followed by filtering and rinsing with 400 mL of NaOH solution (0.01 M) and 800 mL of absolute methanol. The product was finally vacuum-dried at 373 K for 16 h. These procedures were performed to remove amorphous carbon particles, metal catalysts, and other impurities. Reduced and oxidized glutathions (GSH, GSSG), ascorbic acid (AA), and methyltriethoxysilane (MTEOS) were purchased from Sigma. Other chemicals were of analytical grade or higher and used without further purification. Aqueous solutions were prepared with doubly distilled water. CCNNE Preparation. Glassy carbon (GC, 3-mm diameter) electrodes obtained from Bioananlytical Systerm Inc. (BAS, West Lafayette, IN) were used as substrate to prepare the CCNNEs. The GC electrodes were first polished with emery paper (No. 2000) and 0.3- and 0.05-µm alumina slurry on a woolen cloth, then cleaned under bath sonication for 10 min, and finally thoroughly rinsed with distilled water. A typical silica sol was prepared by mixing MTEOS precursor (0.5 mL) with absolute ethanol (1.2 mL), distilled water (0.6 mL), and hydrochloric acid (10.0 µL, 12.1 M). The mixture was vigorously stirred for 30 min. Then, the various amounts of MWNTs were added to the sol and the as-prepared dispersions were sonicated to give homogeneous suspensions containing different contents of the MWNT, i.e., 2.0, 0.5, and 0.08 mg/mL. An aliquot of 0.5 µL of the suspension was coated on the GC electrodes with a microsyringe, and the electrodes were air-dried for 2 h to evaporate alcohol and water. Apparatus and Measurements. Cyclic voltammetry was performed in 0.10 M phosphate-buffered solution (PBS, pH 7.0) with BAS 100 B/W electrochemical analyzer (BAS) and CHI 660A instrument (CH Instruments). The CCNNEs were used as the working electrode and a Pt coil as the counter electrode. The potential was biased versus an Ag/AgCl electrode (saturated with KCl). Amperometric flow injection analysis was performed with LB-1 micro LC pump (Xingda, Beijing, China), an HPLC valve with 5-µL injection loop (BAS), and a radial flow cell (BAS) integrating with the CCNNE on GC substrate (6-mm diameter) as working electrode, Ag/AgCl (3 M NaCl) electrode as reference electrode, and stainless steel as counter electrode. The thickness of gasket used was 25 µm. The working electrode was polarized at +0.60 and +0.90 V for the oxidation of GSH and GSSG, respectively. The 0.10 M PBS was used as a carrier solution. All experiments were performed at room temperature. Tunneling electronic microscopy (TEM) used for characterization of the MWNTs and the MWNT/sol-gel nanocomposite was performed with an HITACHI 9000 Instrument (Hitachi Co. Ltd., Tokyo, Japan). RESULTS AND DISCUSSION Electrochemical Behavior of Sol-Gel-Derived CCNNEs. Figure 1 depicts typical cyclic voltammograms (CVs) for the Fe(CN)63-/Fe(CN)64- couple at the sol-gel-derived ceramic-carbon nanotube nanocomposite electrodes. A large difference in the voltammetric responses was clearly recorded; typically, the CCNNEs prepared with the MWNT with a content of silica sol higher than 1.5 mg/mL (e.g., 2.0 mg/mL in this case) exhibit a Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

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Figure 2. CVs for 1.0 mM K3Fe(CN)6 in 0.10 M PBS at the CCNNEs prepared by dispersing 2.0 (A) and 0.08 mg/mL (B) MWNTs into the silica sol. Potential scan rates (A) (from inner to outer) 10, 20, 50, 100, 200, 300, 400, and 500 mV s-1 and (B) 10 (solid line), 50 (dotted line), and 200 mV s-1 (dashed line).

Figure 1. Typical CVs for 1.0 mM K3Fe(CN)6 at the CCNNEs in 0.10 M PBS. The content of the MWNT in the silica sol was 2.0 (A), 0.5 (B), and 0.08 (C) mg/mL. Scan rate, 10 mV s-1.

pair of well-defined and peak-shaped waves (Figure 1A), indicative of a semi-infinite linear diffusion-controlled redox process of the Fe(CN)63-/Fe(CN)64- couple at the CCNNEs. The small peak separation (∆Ep is 60 mV at 10 mV s-1) and the near unity of the ratio of the cathodic-to-anodic peak current (ipc/ipa is 0.99) suggest a reversible redox process of the solution-phase Fe(CN)63-/Fe(CN)64- couple facilitated at the CCNNE since the GC substrate electrode exhibits a quasi-reversible process for such a redox couple (not shown). In addition, both anodic and cathodic peak currents clearly increase with increasing potential scan rate as shown in Figure 2A and are linear with the square root of the scan rate in a range from 10 to 500 mV s-1 (not shown), again suggesting the semi-infinite linear diffusion-controlled feature of the redox process of the Fe(CN)63-/Fe(CN)64- couple at the CCNNE. These observations reveal that the CCNNEs prepared by encapsulating a large amount of MWNTs in the gel matrix are characteristic of an electrode of conventional dimensions. Interestingly, when the content of the MWNT initially dispersed in the silica sol and doped within the gel matrix was 6502 Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

decreased, typically lower than 0.1 mg/mL (e.g., 0.08 mg/mL in this case) in the sol, the CV obtained for the Fe(CN)63-/Fe(CN)64redox couple changed dramatically as shown in Figure 1C; a sigmoidal voltammetric response was clearly recorded with a large steady-state current (µA level), characteristic of steady-state and radial diffusion to spherical ultramicroelectrode ensembles or arrays in the characteristic time of the voltammetric experiment. Such a sigmoidal voltammetric response with a very slight hysteresis between the forward and backward sweeps is consistent with the almost identical electron-transfer rate of the forward and backward redox process of the Fe(CN)63-/Fe(CN)64- couple enhanced at the nanotubes and with a steady-state diffusion-layer thickness greater than the electrode dimension31 even though both parameters were difficult to be quantified here. This observation essentially reveals that the CCNNE prepared with a low content of the MWNT has features like a nanoelectrode ensemble (NEE). Further supportive results were given with the CVs obtained with the prepared CCNNE for the Fe(CN)63-/Fe(CN)64- redox couple at various potential scan rates as shown in Figure 2B. Unlike that obtained, the CCNNE prepared with a high content of MWNT, i.e., of conventional dimension (Figure 2A), the steady-state current recorded at the electrode used in Figure 1C did not change with a potential scan rate up to 0.20 V s-1, again confirming that the CCNNEs prepared by doping a low content of MWNTs within the gel matrix behave as an NEE. Earlier reports have addressed the fact that the ohmic potential drop could distort the voltammetric results, e.g., a decrease in (31) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications. 2nd ed.; John Wiley & Sons Inc.: New York, 2001.

the peak current, increase in the peak width and a peak potential displacement, depending on the peak current.26,32 A large ohmic potential drop, which depends on the loading of the MWNT in the silicate matrix, may also result in the transition of the electrode dimension from an ensemble of nanoelectrodes to a conventional electrode. To test such a possibility, we measured the uncompensated resistance of the CCNNEs by electrochemical impendence spectroscopy (EIS), which was performed at +0.40 V in 0.10 M PBS containing 1.0 mM Fe(CN)63-. The cell resistance (real component in the EIS), which was mainly composed of the uncompensated resistance of the CCNNEs since the solution resistance was negligible in this case, was determined to be 420 and 50 Ω for the ceramic NEE and the conventional CCNNE, respectively. Such an ohmic potential drop was not found to seriously distort the voltammetric response and thus could not constitute a consequence for the transition of electrode dimension observed above because the cyclic voltammetric responses obtained with the NEE before and after the compensation of ohmic potential drop were almost identical (not shown). Similarly, by adjusting the content of the MWNT in the silica sol and finally in the gel matrix, we can efficiently tune the dimension of the CCNNEs to be of submicroscopic dimensions as was evident from Figure 1B, in which the CV for the Fe(CN)63-/Fe(CN)64- couple recorded with such an electrode was typically displayed. The electrode used was prepared with the MWNTs dispersed in the sol with a content (i.e., 0.5 mg/mL) between those for the conventional CCNNEs (A) and the ceramic NEE (C). As shown, the CV of the as-prepared CCNNE exhibits a peak-shaped sigmoid CV response, demonstrating that mass transport process is dominated by a radial (slow scan rates) or semi-infinite linear (fast scan rates) diffusion process. Although the sol-gel strategy demonstrated here suffers from a slight limitation; currently it is not possible to control the alignment and placement of the MWNTs, and thereby, the distribution, number, and size of individual MWNT nanoelectrodes as well as the electrode area and interelectrode spacing of the as-prepared CCNNEs were difficult to be accurately quantified; the present intersection of CNT science and sol-gel chemistry to construct the CCNNEs with tailor-made dimensions is yet remarkable. The advantages of this method could be illustrated by comparison of the present CCNNEs with the CCEs and other CNT-based electrodes reported in the literature with respect to electrode property and method simplicity. For example, the present CCNNEs are probably more useful for many electrochemical studies than the CCEs by virtue of their good electrochemical catalytic activities associated with the CNTs and tunable electrode dimensions. Additionally, while the earlier methods, e.g., by mixing the CNTs with carbon paste or Teflon,20,21 are very effective for the preparation of CNT-based electrodes, which are quite comparable to our CCNNEs of conventional dimensions, those methods may not be readily used for the preparation of the CNT-based NEEs. It has been reported that the CNT-based NEE could also be constructed by a creative method based on growth of well-aligned CNTs on nickel nanoparticles,33,34 and this method (32) Navarro-Laboulais, J.; Vilaplana, J.; Lo´pez, J.; Garcı´a-Jaren ˜o, J. J.; Benito, D. Vicente, F. J. Electroanal. Chem. 2000, 484, 33. (33) Tu, Y.; Lin, Y.; Ren, Z. F. Nano Lett. 2003, 1, 107. (34) Li. J.; Ng, H. T.; Cassell, A.; Fan, W.; Chen, H.; Ye, Q.; Koehne, J.; Meyyappan, M. Nano Lett. 2003, 3, 597.

Figure 3. Typical TEM images of the MWNTs used (A) and the MWNT/sol-gel nanocomposite prepared by dispersing 0.5 mg/mL MWNTs into the silica sol (B).

may also be used to prepare CNT-based electrodes of conventional dimension. Nevertheless, compared with such a method, the present sol-gel technique is more straightforward and can afford enormous flexibility and intrinsic properties to the as-prepared CCNNEs, e.g., encapsulation of electrocatalysts or biomacromolecules for electrocatalysis and biosensing and tunable polarity. The demonstrated advantage of the CCNNEs with a tailormade dimension is closely associated with the unique structure and properties of the MWNTs and sol-gel matrix. Figure 3 displays typical TEM images of the MWNTs (A) and the as-prepared MWNT/sol-gel nanocomposite (B). As shown, the MWNTs possess a structure different from the carbon materials used for the preparation of the CCEs, e.g., carbon or graphite powder. For instance, the purified MWNTs used here are more than 5 µm in length and ∼30 nm in diameter (Figure 3A). The special structural feature of the MWNTs enables them to act as nanowires that, on one hand, could be tailored into several individual nanoelectrodes and, on the other hand, could maintain the conductivity among each individual nanoelectrode. The formation of silicate gel undergoes multiprocesses of hydrolysis, condensation, polycondensation, and drying.35,36 The siloxane polymers formed with a MTEOS precursor in the (35) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (36) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990.

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Figure 4. Typical CVs obtained at the ceramic NEE (solid line) and carbon fiber microelectrode (dotted line) for the oxidation of AA (2.50 mM) in 0.10 M PBS. Scan rate, 10 mV s-1.

condensation and polycondensation reactions are hydrophobic and can thus interact with the hydrophobic sidewall of the nanotubes through a hydrophobic interaction.37 Such a noncovalent adsorption and the growth of the silicate particles on the nanotubes were considered to effectively separate the aggregated nanotubes (Figure 3B). Moreover, such a process, which leads to the growth of sol particles on the separated nanotubes, eventually separates a single nanotube or small MWNT bundles into several independent parts (Figure 3B), depending on the ratio of the amount of MWNT to silicate particles, i.e., the content of MWNT in the initial sol. The parts of the MWNTs shielded by the silicate particles are not electrochemically accessible and thereby cannot be used for the electrode reaction, while the exposed parts are readily accessible for solution species and actually serve as individual nanoelectrodes for the electrode reaction. Such a unique property of the MWNT/sol-gel nanocomposite essentially endows the novel property of tailor-made dimensions to the prepared CCNNEs. Electrocatalysis of the Ceramic NEE toward AA oxidation. Similar to the electrodes of microscopic dimensions,38,39 the asprepared sol-gel-derived ceramic NEE is believed to be capable of various electrochemical experiments that could not be readily performed with the electrodes of conventional dimensions. Besides, the ceramic NEE is anticipated to be particularly attractive for investigating the electrochemical process of redox species with poor electrochemical properties since the MWNTs have been documented to be capable of accelerating the electron-transfer rate of some compounds.18 Figure 4, using AA as an example, compares the electrochemical process of AA at the ceramic NEE (solid line) with a commercial carbon fiber microelectrode (CFME, 10-µm diameter) (dotted line). The larger nearly steady-state current response (∼1000 times in relative to that at the CFME) and the obvious negative shift in the potential obtained at the NEE for AA oxidation are indicative of the NEE nature and the excellent electrocatalytic activity of the prepared sol-gel-derived ceramic NEE. It is known that the overpotential in the voltammetric experiments is associated with the current density, where a low current density generally leads to a low overpotential. The recorded lower (37) Richard, C.; Balavoine, F.; Schultz, P.; Ebbesen, T. W.; Mioskowski, C. Science 2003, 300, 775. (38) Martin, C. R. Science 1994, 266, 1961. (39) Cheng, W.; Dong, S.; Wang, E. Anal. Chem. 2002, 74, 3599.

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overpotential for the AA oxidation at the NEE, relative to that at the CFME, could be due to the probable low current density of the NEE. Such a possibility should be first clarified before the catalytic activity is to be ascribed to the NEE. However, as mentioned above, the determination of active surface area of the NEE is difficult, and thus, the comparison of the current density at the NEE and the CFME is not possible here. We consequently performed control experiments for AA oxidation with the bare GC electrode (3 mm in diameter) and the conventional CCNNE in 0.10 M PBS (data not shown). Voltammetric results show that the AA oxidation almost commences at the same potential (+0.20 V) at the CFME and the bare GC electrode, both of which electrodes have a large difference in the current density. Moreover, the AA oxidation was found to commence also at the same potential (-0.15 V) at the NEE and the conventional CCNNE, again precluding the effect of the current density on the lower overpotential for AA oxidation at the NEE in the present case. The observed lower overpotential for AA oxidation, relative to that at the CFME, may be due to the catalytic activity of the MWNTs in the NEE. It is reported that the slow process of AA oxidation at a conventional GC and gold electrodes is due to the fouling effect from the product of AA oxidation.40,41 Such an attribution may also be responsible for the slow electron transfer of AA at the CFME (dotted line, Figure 4). While more evidence is still needed to understand the catalytic activity of the NEE toward AA oxidation, the observed catalytic activity of the NEE may be due to the catalytic property of the oxygen-containing groups at the MWNTs or the strong ability against electrode fouling of the MWNTs. Electrocatalysis of the Conventional CCNNE toward Glutathione. The CCNNEs of conventional dimensions are believed to possess electrochemical properties as reported in the CCEs, e.g., encapsulation of enzymes or proteins for biosensor development.9 Demonstrated here is a typical example of the conventional CCNNE for electrocatalysis toward glutathione (both reduced and oxidized forms). The GSH/GSSG redox couple is an important indicator to assess exposure of cells to oxidative stress.42 The determinations of GSH and GSSG remain very challenging to electrochemical methods because of their poor electrochemical behavior, even though some strategies, such as those based on dual electrode, diamond electrode, or enzymemodified electrode, have been previously proposed.1,43,44 Figure 5 compares the typical CVs for the oxidation of GSH (A, B) and GSSG (C, D) at the conventional CCNNE (A, C) with those at the bare GC electrode (B, D). At the bare GC electrode, the oxidations of GSH and GSSG, commencing from +0.80 and +1.20 V, respectively, were ill-defined, suggesting a slow electron transfer of GSH and GSSG at the GC electrode. The oxidation processes, commencing from +0.30 V for GSH and +0.85 V for GSSG, were largely facilitated at the CCNNE. Such an essential negative shift of the potential for the oxidation of both species suggests that the CCNNE possesses an efficient electrocatalytic (40) Raj, C. R.; Ohsaka, T. J. Electroanal. Chem. 2001, 496, 44. (41) Raj, C. R.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2003, 543, 127. (42) Schulz, J. B.; Lindenau, J.; Seyfried, J.; Dichgans, J. Eur. J. Biochem. 2000, 267, 4904. (43) Mao, L.; Yamamoto, K. Electroanalysis 2000, 12, 577. (44) Hiraku, Y.; Murata, M.; Kawanishi, S. Biochim. Biophys. Acta 2002, 1570, 47.

Figure 5. Typical CVs at the conventional CCNNE (A, C) and bare GC electrode (B, D) in 0.10 M PBS in the absence (dotted lines) and presence (solid lines) of 0.10 mM GSH (A, C) or 0.15 mM GSSH (B, D). Scan rate, 50 mV s-1.

activity toward the oxidation of GSH and GSSG, which could be further exploited for the detection of both species. Figure 6 displays typical amperometric responses for GSH (A) and GSSG (B) of the conventional CCNNE in a continuous-flow system. Well-defined peaks with a low noise were recorded with a proportion to the concentration of GSH and GSSG. Reproducible results were achieved for both species (60 µM each) with relative standard deviation less than 3.0% (n ) 20). The good reproducibility and high stability of the CCNNEs suggest their potential applications for the detection of GSH and GSSG, probably combined with a preseparation, such as HPLC or capillary electrophoresis. CONCLUSIONS We have demonstrated that the sol-gel-derived ceramiccarbon nanotube nanocomposite electrodes prepared by doping the MWNTs into a silicate gel matrix are a new class of nanocomposite electrodes that are very useful for electrochemical studies. Besides the properties already demonstrated with the sol-gel-derived ceramic-carbon electrodes, e.g., ease in fabrication at room temperature, high stability, and capability for encapsulation of electrocatalysts and biomacromolecules, the

Figure 6. Flow injection amperometric responses of the CCNNE of conventional dimensions toward GSH (A) and GSSG (B). The concentrations of GSH and GSSG injected are cited in the figure. Mobile phase, PBS. Flow rate, 100 mL min-1. The electrodes were polarized at +0.60 and +0.90 V for GSH and GSSG, respectively.

present intersection of CNT science and sol-gel electrochemistry essentially provides the CCNNEs with several additional advantages, e.g., tunable dimensions and excellent electrocatalytic activities. These attractive properties substantially make the CCNNEs very useful for electrochemical investigations and practical applications, e.g., electrocatalysis, electrochemical measurements, and development of CNT-based electronic biodevices. ACKNOWLEDGMENT We gratefully acknowledge the financial support from Chinese Academy of Sciences (Grant KJCX2-SW-H06) and National Natural Science Foundation of China (Grants 20375043 and 20175033). The authors thank Professor Zhi-Xin Guo for valuable discussions. Received for review May 14, 2004. Accepted August 19, 2004. AC0492867

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