Electrochemical Properties of Carbon Nanotube (CNT) Film

This paper describes electrochemical properties, such as electrode reactivity, electrode dimensions, and interfacial capacitance, of multiwalled carbo...
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Anal. Chem. 2006, 78, 2651-2657

Electrochemical Properties of Carbon Nanotube (CNT) Film Electrodes Prepared by Controllable Adsorption of CNTs onto an Alkanethiol Monolayer Self-Assembled on Gold Electrodes Lei Su, Feng Gao,† and Lanqun Mao*

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

This paper describes electrochemical properties, such as electrode reactivity, electrode dimensions, and interfacial capacitance, of multiwalled carbon nanotube (MWNT) film electrodes prepared by controllable adsorption of the MWNTs onto the self-assembled monolayer (SAM) of n-octadecyl mercaptan (C18H37SH) deposited onto Au electrodes. The adsorption of the MWNTs onto the SAMmodified Au electrode substantially restores heterogeneous electron transfer between bare Au electrode and redox species in solution phase that is almost totally blocked by the SAM of C18H37SH, and as a result, the prepared MWNT/SAM-modified electrode possesses good electrode reactivity without a remarkable barrier to heterogeneous electron transfer. In addition, the surface coverage of the MWNTs is readily controlled by adjusting the immersion time for the adsorption of the MWNTs onto the SAM of C18H37SH, which essentially endows the prepared MWNT/SAM-modified electrodes with tunable electrode dimensions ranging from a nanoelectrode array to a macro-sized conventional electrode. On the other hand, the MWNT/SAM-modified electrode is found to possess a largely reduced interfacial capacitance, as compared with the MWNT film electrodes prepared with existing methods by directly confining the MWNTs onto electrode surface. This demonstration offers a new approach to fabrication of stable MWNT film electrodes with excellent electrochemical properties that are believed to be very attractive for electrochemical studies and electroanalytical applications. Carbon nanotubes (CNTs) represent a new kind of carbonbased materials that possess structural and electronic features that are different from other kinds of carbon-based materials frequently used in electrochemistry, such as graphite, fullerene, and diamond.1 Other groups and we have demonstrated that the unique properties of the CNTs essentially endow them with distinct electrochemical properties that possess great potential for electrochemical and bioelectrochemical applications.1d-i For example, * Corresponding author. Phone: +86-10-62646525. Fax: +86-10-62559373. E-mail: [email protected]. † Present address: College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China. 10.1021/ac051997x CCC: $33.50 Published on Web 03/17/2006

© 2006 American Chemical Society

the special structure of the CNTs possibly enables them to bear more electrochemically favorable elements (e.g., edge plane-like carbons), as compared with, for example, glassy carbon and graphite, which eventually makes them very attractive for lowpotential electrochemical determinations.2 In addition, the CNTs bear highly π-conjunctive and hydrophobic sidewalls consisting of sp2 carbons and open ends containing oxygen-containing moieties, which actually enables them to act as a support for organic and inorganic electrocatalysts to form electrochemically functional nanodevices, such as sensors and fuel cells.3 Furthermore, the unique structural and electronic properties, such as a large length-to-diameter ratio and a good conductivity of the CNTs, make it possible to form a three-dimensional conducting matrix that can be used for immobilization of enzymes and proteins and, more importantly, for direct electron transfer of those biomacromolecules4 with enhanced faradiac responses.5 The striking electrochemical properties of the CNTs described above have substantially motivated intensive interest in their transition from electrochemically attractive nanostructures to practically useful electrochemical nanodevices,1d-h of which much (1) For reviews, see: (a) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (b) Sun, Y.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (c) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (d) Gong, K.; Yan, Y.; Zhang, M.; Xiong, S.; Mao, L. Anal. Sci. 2005, 21, 1. (e) Dai, L.; Soundarrajan, P.; Kim, T. Pure Appl. Chem. 2002, 74, 1753. (f) Wang, J. Electroanalysis 2005, 17, 7. (g) Gooding, J. J. Electrochim. Acta 2005, 50, 3049. (h) Zhao, Q.; Gan, Z.; Zhuang, Q. Electroanalysis 2002, 14, 1609. (i) McCreery, R. L. In Interfacial ElectrochemistrysTheory, Experiment, and Applications; Wieckowski , A., Ed.; Marcel Dekker, Inc.: New York, 1999. (2) (a) Zhang, M.; Liu, K.; Gong, K.; Su, L.; Chen, Y.; Mao, L. Anal. Chem. 2005, 77, 6234. (b) Gong, K.; Dong, Y.; Xiong, S.; Chen, Y.; Mao, L. Biosens. Bioelectron. 2004, 20, 253. (c) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (d) Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075. (e) Lawrence, N. S.; Deo, R. P.; Wang, J. Talanta 2004, 63, 443. (f) Musameh, M.; Wang, J.; Merkoci, A.; Lin, Y. Electrochem. Commun. 2002, 4, 743. (g) Luo, H.; Shi, Z.; Li, N.; Gu, Z.; Zhuang, Q. Anal. Chem. 2001, 73, 915. (h) Moore, R. R.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 2677. (i) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Chem. Commun. 2004, 1804. (j) Moore, R. R.; Banks, C. E.; Compton, R. G. Analyst 2004, 129, 755. (3) (a) Gong, K.; Zhu, X.; Zhao, R.; Xiong, S.; Mao, L.; Chen, C. Anal. Chem. 2005, 77, 8158. (b) Yan, Y.; Zhang, M.; Gong, K.; Su, L.; Guo, Z.; Mao, L. Chem. Mater. 2005, 17, 3457. (c) Zhang, M.; Gorski, W. J. Am. Chem. Soc. 2005, 127, 2058. (d) Zhang, M.; Gorski, W. Anal. Chem. 2005, 77, 3960. (e) Wildgoose, G. G.; Leventis, H. C.; Streeter, I.; Lawrence, N. S.; Wilkins, S. J.; Jiang, L.; Jones, T. G. J.; Compton, R. G. ChemPhysChem 2004, 5, 669. (f) Ye, J.; Wen, Y.; Zhang, W.; Cui, H.; Xu, G.; Sheu, F. Electroanalysis 2005, 17, 89.

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attention has been paid to depositing the CNTs onto a conducting solid substrate, that is, fabrication of stable CNT film electrodes.2-3,6-9 To this end, several methods have so far been developed for this key point, mainly by directly confining the CNTs onto the electrode surface. This includes dip-coating a CNT dispersion in organic solvents (typically, N,N-dimethylformamide),2f-j,6 layer-by-layer assembling CNT multilayer film,7 and aligning the CNTs onto the electrode surface.8 This study describes a new method for fabrication of stable MWNT film electrodes and investigates the electrochemical properties of the as-prepared electrodes. The method demonstrated here is based on the controllable adsorption of the MWNTs onto the hydrophobic monolayer of C18H37SH self-assembled onto a Au electrode through the hydrophobic interaction between the MWNTs and the monolayer (Scheme 1). This idea is essentially motivated by our earlier works on the interactions between the CNTs and polynuclear aromatic compounds and surfactants.3b,5,7d Basically, the CNTs consist of seamlessly rolled-up graphene sheets of carbon with π-conjugative and highly hydrophobic sidewalls and can interact with, for example, surfactants and some kinds of aromatic compounds through hydrophobic or π-π electronic interaction(s).10 The choice of C18H37SH in this study as a bifunctional linker to prepare MWNT film electrodes is due to the facts that such a compound can form a stable monolayer on a Au substrate through the formation of a Au-S bond and that the formed hydrophobic monolayer can interact with the (4) (a) 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. (b) Davis, J. J.; Coleman, K. S.; Azamian, B. R.; Bagshaw, C. B.; Green, M. L. H. Chem.sEur. J. 2003, 9, 3732. (c) Wang, J.; Li, M.; Shi, Z.; Li, N.; Gu Z. Anal. Chem. 2002, 74, 1993. (d) Katz, E.; Willner, I. ChemPhysChem 2004, 5, 1084. (e) Zhang, Y.; Shen, Y.; Li, J.; Niu, L.; Dong, S.; Ivaska, A. Langmuir 2005, 21, 4797. (f) Davis, J. J.; Coles, R. J.; Hill, H. A. O. J. Electroanal. Chem. 1997, 440, 279. (g) Yu, X.; Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F.; Rusling, J. F. Electrochem. Commun. 2003, 5, 408. (h) Zhao, G.; Zhang, L.; Wei, X.; Yang, Z. Electrochem. Commun. 2003, 5, 825. (i) Wang, L.; Wang, J.; Zhou, F. Electroanalysis 2004, 16, 627. (j) Cai, C.; Chen, J. Anal. Biochem. 2004, 332, 75. (5) Yan, Y.; Zheng, W.; Zhang, M.; Wang, L.; Su, L.; Mao, L. Langmuir 2005, 21, 6560. (6) (a) Wang, J.; Li, M.; Shi, Z.; Li, N.; Gu, Z. Electroanalysis 2002, 14, 225. (b) Tang, H.; Chen, J.; Nie, L.; Yao, S.; Kuang, Y. Electrochim. Acta, in press. (c) Chen, J.; Bao, J.; Cai, C.; Lu, T. Anal. Chim. Acta 2004, 516, 29. (7) (a) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59. (b) Zhang M.; Yan Y.; Gong K.; Mao L.; Guo Z.; Chen Y. Langmuir 2004, 20, 8781. (c) Zhang M.; Gong K.; Zhang H.; Mao L. Biosens. Bioelectron. 2005, 20, 1270. (d) Zhang, M.; Su, L.; Mao, L. Carbon 2006, 44, 276. (e) Xu, Z.; Gao, N.; Dong, S. Talanta 2006, 68, 753. (8) (a) Koehne, J.; Li, J.; Cassell, A. M.; Chen, H.; Ye, Q.; Ng, H. T.; Han, J.; Meyyappan, M. J. Mater. Chem. 2004, 14, 676. (b) Ye, J.; Wen, Y.; Zhang, W.; Gan, M. L.; Xu, G.; Sheu, F. Electroanalysis 2003, 15, 1693. (c) Lin, Y.; Lu, F.; Tu, Y.; Ren, Z. Nano Lett. 2004, 4, 191. (d) Liu, Z.; Shen, Z.; Zhu, T.; Hou, S.; Ying, L. Langmuir 2000, 16, 3569. (e) Chen, Z.; Yang, Y.; Wu, Z.; Luo, G.; Xie, L.; Liu Z.; Ma S.; Guo W. J. Phys. Chem. B 2005, 109, 5473. (f) Diao, P.; Liu, Z.; Wu, B.; Nan, X.; Zhang, J.; Wei, Z. ChemPhysChem 2002, 3, 898. (g) Diao, P.; Liu, Z. J. Phys. Chem. B 2005, 109, 20906. (h) Gao, M.; Dai, L.; Wallace, G. G. Electroanalysis 2003, 15, 1089. (i) Dai, L.; Patil, A.; Gong, X.; Guo, Z.; Liu, L.; Liu, Y.; Zhu, D. ChemPhysChem. 2003, 4, 1150. (9) Gong, K.; Zhang, M.; Yan, Y.; Su, L.; Mao, L.; Xiong, S.; Chen, Y. Anal. Chem. 2004, 76, 6500. (10) (a) Richard, C.; Balavoine, F.; Schultz, P.; Ebbesen, T. W.; Mioskowski, C. Science 2003, 300, 775. (b) Zhang, J.; Lee, J.-K.; Wu, Y.; Murray, R. W. Nano Lett. 2003, 3, 403. (c) Hedderman, T. G.; Keogh, S. M.; Chambers, G.; Byrne, H. J. J. Phys. Chem. B 2004, 108, 18860. (d) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379. (e) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (f) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838.

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Scheme 1. Schematic Illustration of (left panel) the Protocol for Fabrication of the MWNT Film Electrodes through Controllable Adsorption of the MWNTs onto a Self-Assembled Monolayer of C18H37SH Deposited onto a Au Electrode and (right panel) the Equivalent Circuit of the Prepared MWNT/SAM-Modified Electrodesa

a R is the electrochemical resistance of the soluble redox r species, containing the charge-transfer resistance and Warburg impedance.

MWNTs through hydrophobic interaction (Scheme 1).11 Similar to the cases with gold nanoparticles (Au-NPs), as first reported by Schiffrin and co-workers in 1996,12 the blocked heterogeneous electron transfer between a Au electrode and a redox species in solution phase by the SAM of C18H37SH is largely restored by the adsorption of the MWNTs, resulting in a good electrode reactivity’s being obtained at the MWNT/SAM-modified electrodes. Moreover, the MWNT/SAM-modified electrodes are also found to possess tunable electrode dimensions and a small interfacial capacitance. To the best of our knowledge, the study undertaken here has not been reported so far and could offer a new route to fabrication of stable CNT film electrodes with excellent electrochemical properties that are envisaged to be particularly useful for electrochemical studies and electroanalytical applications. EXPERIMENTAL Materials and Reagents. Multiwalled carbon nanotubes (MWNTs, with an average diameter of ∼10-30 nm) were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). The MWNTs were purified by refluxing the as-received MWNTs in 2.6 M nitric acid for 5 h, followed by centrifugation, resuspension, filtration, and drying under vacuum at 100 °C. n-Octadecyl mercaptan (C18H37SH) and hexaamine ruthenium(III) chloride (Ru(NH3)63+) were purchased from Sigma. N,N-dimethylformamide (DMF), potassium ferricyanide, potassium ferrocyanide, and other chemicals of at least analytical reagent were obtained from Beijing Chemical Corporation (Beijing, China). Aqueous solutions were prepared with doubly distilled water. Electrode Preparation and Modification. Au electrodes (0.18-cm diameter, Bioanalytical Systems Inc.) were polished with alumina powder (0.3 and 0.05 µm) and sonicated in acetone and doubly distilled water (each for ∼3-5 min). The electrodes were immersed into aqua regia (3:1 concentrated HCl/HNO3) for 5 min (11) Burghard, M.; Duesberg, G.; Philipp, G.; Muster, J.; Roth, S. Adv. Mater. 1998, 10, 584. (12) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137.

and then taken out of the solution and thoroughly rinsed with doubly distilled water. After that, the electrodes were subjected to electrochemical pretreatment by consecutive potential cycling in 0.50 M H2SO4 within a potential range between -0.20 and +1.60 V at 0.5 V s-1 until a cyclic voltammogram characteristic of a clean Au electrode was obtained. Au electrodes modified with SAM of C18H37SH were prepared by immersing the electrodes into ethanol solution of C18H37SH (40 mM) for at least 24 h at room temperature. The electrodes (denoted as SAM-modified Au electrodes, hereafter) were then thoroughly rinsed with ethanol and dried with pure N2. For adsorption of the MWNTs onto the SAM-modified electrodes, the SAM-modified electrodes were immersed into MWNT dispersion in DMF (1 mg/mL). The immersion time was adjusted to achieve electrodes with different surface coverages of the MWNTs. The electrodes (denoted as MWNT/SAM-modified electrodes) were thoroughly rinsed with DMF and doubly distilled water to remove the MWNTs unstably adsorbed onto the electrode surface and then dried with pure N2 before use. Apparatus and Measurements. Electrochemical measurements were performed with a computer-controlled electrochemical analyzer (CHI 660A, CH Instruments) with a three-electrode configuration. The SAM- and MWNT/SAM-modified Au electrodes were used as working electrode, a Ag/AgCl (KCl-saturated) electrode as reference electrode, and a platinum coil as counter electrode. The controllable growth of the MWNTs onto the SAM-modified Au electrode was characterized by scanning electron microscopy (SEM) (Hitachi S4300-F microscope, Hitachi Inc., Tokyo, Japan). For such a purpose, a 50-nm-thick Au film with a 2-nm-thick Cr underlayer was alternately deposited onto the glass slides with a high-vacuum evaporator (Hitachi, HUS-5GB, Japan) to mimic the Au electrodes used for the electrochemical measurements. Before that, the glass slides were pretreated by thoroughly rinsing with acetone, ethanol, and water. The SAM of C18H37SH and MWNTs were then alternately deposited onto the Au-coated glass slides using the same procedures as those for the Au electrodes. RESULTS AND DISCUSSION Controllable Adsorption of the MWNTs onto the SAMModified Au Electrode. Figure 1 displays typical SEM images of the MWNTs deposited on the SAM-modified Au substrate. The surface coverage of the MWNTs on the SAM-modified Au substrate was clearly increased with an increase in the immersion time; because the immersion time was shorter than 2 min, only a few of single and small bundles of the MWNTs were sparsely adsorbed onto the SAM-modified Au substrate (A), and the coverage of the MWNTs was largely increased as the immersion time was prolonged to 20 min (C). This observation essentially reveals that the surface coverage of the MWNTs on the SAMmodified Au substrate increases with the immersion time and is, thus, controllable by properly adjusting the immersion time. To confirm the essential role of the SAM of C18H37SH in the controllable adsorption of the MWNTs onto the electrodes, we have performed control experiments by simply immersing bare Au substrate (without SAM modification) into the same MWNT dispersion and found that no MWNTs were adsorbed onto such a substrate, even when the immersion time was increased to 40 min as shown in Figure 1D. This demonstration indicates that

Figure 1. Typical SEM images of the MWNTs deposited onto the SAM-modified Au electrode by immersing the SAM-modified Au electrodes into a MWNT dispersion in DMF (1 mg/mL) for 1 (A), 5 (B), and 20 min (C). (D) SEM image of Au electrode (without SAM modification) after being immersed in the same MWNT dispersion for 40 min and rinsed with DMF and distilled water. Scale bar, 5 µm.

Figure 2. CVs at the MWNT/SAM-modified Au electrodes in 0.10 M phosphate buffer (pH 7.0). Scan rate, 100 mV/s. The MWNT/SAMmodified Au electrodes were prepared by immersing the SAMmodified electrode into a MWNT dispersion for different times of (from inner to outer) 0, 0.3, 1.5, 7.5, 17.5, 32.5, 47.5, 77.5, 117.5, and 207.5 min. Inset, plots of charging (O) and faradic (9) currents obtained at the MWNT/SAM-modified Au electrodes against the immersion time. The charging currents were calculated by averaging the anodic and cathodic currents in the voltammograms at +0.40 V.

the adsorption of the MWNTs onto the electrode surface is essentially based on the hydrophobic interaction between the MWNTs and the SAM of C18H37SH confined onto Au electrode. The controllable adsorption of the MWNTs onto the SAMmodified Au electrode was also monitored with cyclic voltammetry using the MWNT/SAM-modified Au electrodes prepared by immersing the SAM-modified electrodes into the MWNT dispersion for different times, as depicted in Figure 2. As reported previously, the oxidation procedure in an acidic solution employed for purifying the MWNTs could lead to partial oxidation of the MWNTs into oxygen-containing moieties,2g,3a-b,13 of which some are electroactive and exhibit a pair of redox waves at -50 mV at the MWNT/SAM-modified electrode. As can be seen from Figure 2, the peak currents of such a redox wave increase with increasing immersion time, which is indicative of the adsorption of the Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

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MWNTs onto the SAM-modified electrode to fabricate the MWNT/SAM-modified electrode using a tunable surface coverage of the MWNTs. This feature could also be evident from the increase in the charging current in cyclic voltammograms (CVs) at +0.40 V as a function of the immersion time (Figure 2, inset). The prepared MWNT/SAM-modified electrodes were found to be very stable, which was illustrated by the almost unchanged redox peak currents at -50 mV for the oxygen-containing moieties produced onto the MWNTs after continuously cycling the electrodes for 100 cycles or after the electrodes’ being stored in distilled water for 1 week (data not shown). The electrodes were also found to be stable for the redox process of Fe(CN)63-/4- in solution phase: the redox peak currents were essentially unchanged after continuously cycling the electrodes for 50 cycles or after the electrodes’ being stored in distilled water for 1 week (data not shown). These demonstrations substantially suggest that the hydrophobic interaction between the MWNTs and the SAM of C18H37SH essentially makes it possible to stably adsorb the MWNTs onto the SAM-modified Au electrode in a controllable manner and, thus, to fabricate the MWNT film electrodes with a controllable surface coverage of the MWNTs. This property is very similar to the attachment of metallic nanoparticles, for example, Au and Pt nanoparticles, onto electrode surface modified with, for example, sol-gel and SAMs, in which the surface coverage of the metallic nanoparticles could also be controllable.14 In addition to their potential uses in other research fields, such as MWNT-based nanodevices,1c the MWNT film electrodes prepared in this study through controllable adsorption of the MWNTs onto the insulating SAM are believed to be electrochemically useful. It is believed that they have advantages over those prepared by directly depositing the CNTs onto the electrode surface by, for example, dip-coating and layer-by-layer,6-9 with respect to tunable electrode dimensions and small interfacial capacitance with a good electrode reactivity, and also with respect to ease in electrode preparation, as will be demonstrated below. Restored Electrode Reactivity of the MWNT/SAM-Modified Electrodes. To understand the electron-transfer properties of the MWNT/SAM-modified Au electrodes, we first study the electrochemical behavior of the SAM-modified electrode (i.e., without adsorption of the MWNTs onto the electrode). It is known that an alkanethiol with a long hydrocarbon chain, such as the C18H37SH used in this study, can form a compact insulating SAM on a Au electrode, resulting in a large change in the interfacial structure between the electrode and the electrolyte.15 This change eventually leads to a large difference in the electrochemical properties between the SAM-modified electrodes and the bare electrode. Figure 3 compares voltammetric responses of both kinds of redox couples (i.e., Fe(CN)63- and Ru(NH3)63+) at bare (A and C) and SAM-modified (B and D) Au electrodes. At a bare (13) (a) Kuznetsova, A.; Popova, I.; Yates, J. T., Jr.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. J. Am. Chem. Soc. 2001, 123, 10699. (b) Zhang, J.; Zou, H.; Qing, Q.; Yang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. J. Phys. Chem. B 2003, 107, 3712. (14) (a) Musick, M. D.; Pen ˜a, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (b) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss, B. D.; Natan, M. J. Chem. Mater. 2000, 12, 314. (c) Cheng, W.; Dong, S.; Wang, E. Anal. Chem. 2002, 74, 3599. (15) (a) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (b) Porter, M. D.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.

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Figure 3. CVs at bare (A and C) and SAM-modified (B and D) Au electrodes in 0.10 M phosphate buffer containing 1.0 mM Fe(CN)63(A and B) or 1.0 mM Ru(NH3)63+ (C and D). Dotted lines represent CVs obtained in the phosphate buffer containing no redox couples. Scan rate, 50 mV/s.

Au electrode (A and C), a pair of well-defined redox peaks were recorded for both redox couples with a peak-to-peak separation (∆Ep) of ∼60 mV (at 50 mV/s) and a near unity of anodic-tocathodic peak current ratio (ipa/ipc), which is characteristic of the reversible process of the redox species in solution phase.16 In contrast, only tailed voltammetric responses were recorded for both kinds of redox couples at the SAM-modified electrodes (B and D), indicating that electron transfer between Au electrode and the redox species was largely blocked by the SAM of the C18H37SH. This observation further indicates that the SAM of the C18H37SH confined on the Au electrode is compact and essentially pinhole-free. Figure 4 (A and C) depicts typical CVs of both redox couples, (i.e., Fe(CN)63-/4- and Ru(NH3)63+) at the MWNT/SAM-modified electrode prepared by immersing the SAM-modified electrode in a MWNT dispersion for a long time (207.5 min). By comparing the voltammetric responses of both kinds of redox couples at the SAM-modified (Figure 3B and D) and the MWNT/SAM-modified (Figure 4A and C) Au electrodes, we found that the prepared MWNT/SAM-modified electrodes possess a good electrode reactivity without a remarkable barrier to the heterogeneous electron-transfer kinetics. For example, well-defined redox peaks were obtained for the redox couples, of which the small ∆Ep (∼60 mV, at 50 mV/s) and near unity peak current ratio (ipa/ipc ) 0.98) obtained for the Ru(NH3)63+ redox couple at the MWNT/SAMmodified electrode reflect that the reversibility achieved at such electrode was essentially the same as that at the bare Au electrode. This demonstration reveals that the heterogeneous electrontransfer process blocked by the SAMs of the C18H37SH (Figure 3B and D) was largely restored upon the adsorption of the MWNTs onto the SAM-modified electrode, which was further (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001.

Figure 4. CVs at the MWNT/SAM-modified Au electrode in 0.10 M phosphate buffer (pH 7.0) in the absence (dotted lines) and presence (solid lines) of 1.0 mM Fe(CN)63-/4- (A and B) or 1.0 mM Ru(NH3)63+ (C and D). Scan rate, 50 mV/s. The MWNT/SAM-modified Au electrode was prepared by immersing the SAM-modified Au electrode into a MWNT dispersion in DMF (1 mg/mL) for 207.5 (A and C) and 3 min (B and D).

confirmed by electrochemical impedance spectroscopy (EIS), as shown in Figure 5. The charge-transfer resistance (Rct) of the Ru(NH3)63+ redox couple at the SAM-modified electrode (without the adsorption of MWNTs) is very large (i.e., >105 Ω) (A), indicating the insulating feature of the SAM of the C18H37SH used in this study. The Rct was largely decreased, even to the same level as that of a bare Au electrode (B, dotted line), upon the adsorption of the MWNTs (B), which is again indicative of the restoration of electron transfer with the adsorption of the MWNTs onto the SAM-modified electrode. The observed restoration in the heterogeneous electron transfer at the MWNT/SAM-modified electrode may not be due to the penetration of the MWNTs through the SAM. We reasoned this because the formation of the SAM of alkanethiols is known to undergo a crystallization step in which the hydrophobic interactions among the long alkyl chain drive the alkyl chain to form a compact and rigid structure17 that has been reported to be able to keep its integrity unless undergoing extreme conditions, such as an extreme electric field or high temperature.17a On the other hand, the SAM of the C18H37SH with a long alkyl chain prepared in this study is essentially rigid and pinhole-free. The latter property could be evident from the tailed, rather than steadystate, voltammetric responses of the soluble redox couples, as shown in Figure 3B and D, characteristic of the heterogeneous electron transfer through the intervenient SAM via a tunneling process, rather than a pinhole effect.15 In 1996, Schiffrin et al. first observed that the blocked heterogeneous electron transfer between a Au electrode and a (17) (a) Ulman, A. Chem. Rev. 1996, 96, 1533, and references therein. (b) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (c) Akram, M.; Stuart, M. C.; Wong, D. K. Y. Anal. Chim. Acta 2004, 504, 243. (d) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whiteside, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

Figure 5. Nyquist plots obtained with the SAM-modified (A) and the MWNT/SAM-modified (B) Au electrodes in 0.10 M phosphate buffer containing 1.0 mM Ru(NH3)63+. Dotted line in panel B represents Nyquist plot obtained with a bare Au electrode under the same conditions. EIS conditions: potential, -0.19 V; alternative voltage, 5 mV; frequency range, ∼100 kHz to 0.1 Hz.

soluble redox species by the SAM of 1,9-nonanedithiol was largely restored with the attachment of Au-NPs onto the SAM.12 They further studied the electron transfer of such film electrodes and concluded that the electron transfer occurs via activated electron hopping.18 Recently, the same group have also studied the electron transfer at the layer-by-layer self-assembled Pt-NPs with 1,8diisocyanooctane as the cross-linker and concluded that the electron-transfer process should contain at least two processes in series, that is, tunneling from the metal electrode to nanoparticles and from the nanoparticles to the redox species in solution phase.19 Probably similar to the metallic nanoparticles demonstrated recently,12,18-20 the MWNTs adsorbed onto the SAM of C18H37SH may also be able to relay electron transfer between a bare Au electrode and the redox species in solution phase through the (18) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (19) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2003, 107, 4844. (20) (a) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C. D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (b) Hicks, J. F.; Zamborini, F. P.; Osisek, A. J.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048. (c) Chen, S. J. Phys. Chem. B 2000, 104, 663. (d) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1999, 15, 866. (e) Wessels, J. M.; Nothofer, H. G.; Ford, W. E.; Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A. J. Am Chem, Soc. 2004, 126, 3349. (f) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D. J. Am. Chem. Soc. 2004, 126, 14287. (g) Lioubashevski, O.; Chegel, V. I.; Patolsky, F.; Katz, E.; Willner, I. J. Am Chem. Soc. 2004, 126, 7133. (h) Sagara, T.; Kato, N.; Nakashima, N. J. Phys. Chem. B 2002, 106, 1205. (i) Lu, M.; Li, H.; Li, H. J. Colloid. Interface Sci. 2002, 248, 376.

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insulating SAM. In such a case, the electron transfer at the MWNT/SAM-modified electrodes could be understood to proceed through two steps, the first between the bare Au and the MWNTs and the second between the MWNTs and the redox species in solution phase, as schematically shown in Scheme 1. The electron transfer in the first step was believed to proceed essentially quickly, and as a result, the flux of electron transferred can well satisfy Nernstian equilibrium, leading to good electrode reactivity of the MWNT/SAM-modified electrode, as shown in Figures 4 and 5. The restored electron transfer of the MWNT/SAM-modified electrode is very critical to such an electrode. This property, combined with the tunable dimensions and the small interfacial capacitance as demonstrated below, essentially makes the MWNT/ SAM-modified electrodes very useful for electrochemical studies and electroanalytical applications. Tunable Dimensions of the MWNT/SAM-Modified Electrodes. In addition to their good electrode reactivity described above, we interestingly found that the shape of the voltammograms obtained with the MWNT/SAM-modified electrodes varies with the time employed for immersing the SAM-modified electrodes into the MWNT dispersion, as shown in Figure 4. When the immersion time was shorter than 5 min, the voltammograms obtained for both redox couples (i.e., Fe(CN)63-/4- and Ru(NH3)63+) almost reach a steady state (B and D), but a longer immersion time (typically longer than 10 min) essentially leads to peak-shaped voltammograms for the same redox couples (A and C) under the experimental conditions employed. The former feature suggests a radial diffusion kinetics at the prepared MWNT/SAM-modified electrode; that is, the electrode resembles a nanoelectrode array, whereas the latter suggests a planar diffusion kinetics, that is, the electrode resembles an electrode of conventional dimensions. This observation essentially reveals that by simply controlling the immersion time for the adsorption of the MWNTs onto the SAM-modified electrodes, one may fabricate stable MWNT film electrodes with tunable dimensions from a nanoelectrode array to a conventional electrode. The preparation of these dimension-tunable electrodes was very reproducible, and both the peak-shaped and the state-steady currents for the Fe(CN)63-/4- redox couple obtained with both kinds of electrodes (i.e., conventional electrode and nanoelectrode array) bore no large variation from electrode to electrode of the same dimension, provided the formation of the SAM of the C18H37SH was kept identical in each experiment. Moreover, when compared with the sol-gel method demonstrated in our earlier work for preparing dimension-tunable CNT film electrodes9 and other methods for preparing a CNT nanoelectrode array,8 the method demonstrated in this study is simpler and may be more reliable for fabrication of dimension-tunable MWNT film electrodes with excellent electrochemical properties, for example, good electrode reactivity and small interfacial capacitance. Small Interfacial Capacitance of the MWNT/SAM-Modified Electrode. Figure 6 depicts CVs obtained at bare (1, dotted line), SAM-modified (2, dashed line), and MWNT/SAM-modified (3, solid line) Au electrodes in 0.10 M N2-saturated phosphate containing no redox couples. As shown, the formation of insulating SAMs onto Au electrode substantially reduces the interfacial capacitance of the electrode (2, dashed line), as compared with that at a bare Au electrode (1, dotted line). This could be readily 2656 Analytical Chemistry, Vol. 78, No. 8, April 15, 2006

Figure 6. CVs obtained at bare (1, dotted line), SAM-modified (2, dashed line), and MWNT/SAM-modified (3, solid line) Au electrodes in 0.10 M N2-saturated phosphate buffer. Scan rate, 100 mV/s. The MWNT/SAM-modified electrode was prepared by immersing the SAMmodified electrode into a MWNT dispersion for 207.5 min.

understood in terms of the low dielectric constant of the hydrocarbon chain of the alkanethiol, as predicted by the Helmholtz theory, in which the double layer interface was considered an ideal capacitor.17,21 Unlike the electron-transfer process that was mostly restored upon the adsorption of the MWNTs onto the SAM-modified electrodes (Figures 4 and 5), we interestingly found that the capacitance of the electrode was not largely restored by the adsorption of the MWNTs onto the SAM-modified electrodes. The capacitance of the MWNT/SAM-modified electrodes (3, solid line) is slightly larger than that of the SAM-modified electrode (2, dashed line), but it remains smaller than that of a bare Au electrode (1, dotted line), as displayed in Figure 6. The small interfacial capacitance of the MWNT/SAM-modified electrode is very remarkable, especially for the CNT film electrodes. This is because the CNT film electrodes prepared using existing methods, such as the dip-coating method that has been frequently used in most electrochemical studies,2f-j,6 layer-by-layer,7 and aligning the CNTs onto the electrodes,8 generally have a large interfacial capacitance, as displayed in Table 1. Although the capacitance of the CNT film electrodes prepared with existing methods varies with the amount of the CNTs confined onto the electrode surface, and thus, the CNT film electrodes with a small interfacial capacitance could be prepared, in principle, by decreasing the amounts of the CNTs confined onto the electrode substrate, the preparation of such CNT thin film electrodes is practically difficult with, for example, the dip-coating method. This is because, as mentioned above, due to the strong van der Waals interaction between the tubes, the nanotubes applied onto the electrodes readily tend to aggregate during the drying process for evaporating the solvent, which eventually leads to poor surface coverage, even leaving parts of the substrate electrode exposed for the electrode reaction, which essentially creates difficulties in differentiating the electrochemical process on the CNTs from that at the substrate electrode and further in understanding the mechanistic aspects of electrochemistry of the CNTs. In most (21) (a) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (b) Miller, C.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 5225. (c) Sur, U. K.; Subramanian, R.; Lakshminarayanan, V. J. Colloid. Interface Sci. 2003, 266, 175.

Table 1. Capacitance of CNT Film Electrodes Prepared by Various Methodsa CNTs

electrode preparation

capacitance (µF/cm2)

ref

MWNT SWNTe SWNTe SWNTe SWNTe MWNT MWNT MWNT MWNT MWNT

adsorption onto SAM dip-coating dip-coating dip-coating dip-coating dip-coating layer-by-layer aligned aligned aligned

29.1b 159b 710b 444b 500c 2140b 430b,d 2000c 2083c 640

this work 2g 2f 6a 4i 2f 7b 6b 6c 8b

a Capacitances were calculated by summing the charge current in the positive and negative scan directions and dividing the sum by twice scan rate. The charge currents were measured at +0.4 Vb and -0.2 Vc. d The capacitance of {PDDA/MWNT} was measured. e SWNT, single5 walled carbon nanotubes

cases, a large amount of the CNTs was essentially needed for achieving a full surface coverage of the CNTs, which inevitably generates a large interfacial capacitance of the as-prepared CNT film electrodes, as listed in Table 1. The small capacitance of the MWNT/SAM-modified electrode could be possibly elucidated in terms of the pinhole-based microelectrode array theory.22 In this regard, the total capacitance (CT) of the electrode could be considered a parallel combination of the capacitance of the MWNT-SAM/solution (CMWNT-SAM) and that of the SAM/solution (without the MWNT adsorption) (CSAM),

CT ) θCMWNT-SAM + (1 - θ)CSAM

(1)

where θ is the fraction of the surface coverage of the MWNTs on the SAM-modified electrode (or of the MWNT-SAM on the Au electrode), which depends on the time employed for the adsorption of MWNTs onto the SAM-modified electrodes, as described above (Figure 2). From eq 1, one may easily deduce that in the case of a low coverage of the MWNTs on the insulating SAM of (22) (a) Sˇ tulı´k, K.; Amatore, C.; Holub, K.; Marecˇek, V.; Kutner, W. Pure Appl. Chem. 2000, 72, 1483. (b) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (c) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041. (d) Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1991, 113, 5464. (e) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (f) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663.

the C18H37SH, the CT is mainly contributed from the CSAM. For an extreme case of θ ) 0, the CT tends to be the CSAM, which is consistent with the results obtained above (Figure 6). On the other hand, since the MWNTs adsorbed on the SAM of the C18H37SH can be simply considered partially functionalized with such an insulating monolayer with a low dielectric, thus, CSAM < CMWNT-SAM < CMWNT. CMWNT represents the capacitance of the MWNTs without adsorption or functionalization of the SAM of the C18H37SH. Consequently, the CT of the MWNT/SAM-modified electrode increases with an increase in the θ of the MWNTs, as could be evident from Figure 2, and is much smaller than those of the CNT film electrodes typically displayed in Table 1. The reduced capacitance of the MWNT film electrode prepared in this study is similar to those with metallic nanoparticles, for example, Au and Pt nanoparticles, assembled onto electrodes, as demonstrated previously.12,18-20 CONCLUSIONS We have described the electrochemical properties of stable MWNT film electrodes prepared by controllable adsorption of MWNTs onto the SAM of C18H37SH deposited onto a Au electrode. The electrodes were found to have excellent electrochemical properties, such as tunable electrode dimensions from a nanoelectrode array to a conventional electrode and very small interfacial capacitance with good electrode activity. This demonstration offers a facile approach to fabrication of stable MWNT film electrodes with excellent electrochemical properties that are believed to be particularly useful for electroanalytical applications. Moreover, the protocol demonstrated here may be further developed for preparation of electrochemically favorable conducting films with other kinds of conducting nanomaterials and could, thus, pave a new way to nanostructure-based electrochemistry and electroanalytical chemistry. ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants Nos. 20375043, 20435030, and 20575071), Chinese Academy of Sciences (Grant No. KJCX2-SW-H06), and State Key Laboratory for Electroanalytical Chemistry at Changchun Institute of Applied Chemistry. Received for review November 10, 2005. Accepted February 17, 2006. AC051997X

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