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Single-Walled Carbon Nanotube Network Ultramicroelectrodes Ioana Dumitrescu,† Patrick R. Unwin,† Neil R. Wilson,‡ and Julie V. Macpherson*,† Departments of Chemistry and Physics, University of Warwick, Coventry, CV4 7AL, U.K. Ultramicroelectrodes (UMEs) fabricated from networks of chemical vapor deposited single-walled carbon nanotubes (SWNTs) on insulating silicon oxide surfaces are shown to offer superior qualities over solid UMEs of the same size and dimensions. Disk shaped UMEs, comprising two-dimensional “metallic” networks of SWNTs, have been fabricated lithographically, with a surface coverage of 0.4 cm s-1,42 k°Hg ) 0.35 cm s-1).43 These initial results thus indicate that defect sites alone are unlikely to explain the electrochemical activity of SWNTs. To provide further insight on the electron transfer kinetics at SWNTs, we are in the process of carrying out studies to greatly enhance mass transport, for example through the use of hydrodynamicUME flow systems.44 Interestingly, increasing the concentration of Ru(NH3)63+ from 1 mM (black line) to 10 mM (blue line) causes a broadening and a displacement of the wave toward more negative potentials, as shown in Figure 5. E1/2 shifts by 40 mV and |E1/4 - E3/4| increases from 59 mV (1 mM) to 67 mV (5 mM) and 86 mV (10 mM). The low solubility of FcTMA+ in water prohibited measurements above 1 mM for this mediator. The shift of E1/2 to more negative potentials as the concentration of Ru(NH3)63+ increases can be attributed to either sample resistance (R) or kinetic effects.45 Although R plays a significant role in the observed shift in E1/2, typical values measured for R from wet gate experiments30,31 (upper limit ca. 100 kΩ) do not account fully for the observed shifts and we cannot rule out a possible contribution to the wave shape from electron transfer kinetics.19,46 Influence of Supporting Electrolyte Concentration. Aside from the ability to deliver high mass transfer rates, another advantage of employing UMEs is compatibility of the electrode with resistive media, given the small currents which flow.23 Decreasing the supporting electrolyte concentration compared to that of charged redox species also results in an increased (41) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118–1121. (42) Cline, K. K.; McDermott, M. T.; McCreery, R. L. J. Phys. Chem. 1994, 98, 5314–5319. (43) Gennett, T.; Weaver, M. J. Anal. Chem. 1984, 56, 1444–1448. (44) Macpherson, J. V.; Marcar, S.; Unwin, P. R. Anal. Chem. 1994, 66, 2175– 2179. (45) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical properties of carbon nanotubes; Imperial London Press: London, 1998. (46) Heller, I.; Kong, J.; Williams, K. A.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2006, 128, 7353–7359. (47) (a) Amatore, C.; Fosset, B.; Bartelt, J.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1988, 256, 255–268. (b) Cooper, J. B.; Bond, A. M.; Oldham, K. B. J. Electroanal. Chem. 1992, 331, 877–895. (c) Amatore, C.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1987, 225, 49–63. (d) Oldham, K. B. J. Electroanal. Chem. 1992, 337, 91–126.
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Figure 8. Chronoamperometric characteristics for (a,c) the oxidation of 1 mM FcTMA+ and (b,d) the reduction 1 mM Ru(NH3)63+ at a 100 µm diameter SWNT disk UME and 25 µm diameter glass sealed Pt disk UME, respectively. Solid line shows the behavior of a simple diffusioncontrolled process predicted by the Shoup and Szabo equation.50
contribution of migration to mass transport,47 and it is of interest to examine the electrochemical response of this new electrode material under the influence of varying supporting electrolyte concentrations. Solutions consisted of either 1 mM FcTMA+ or 1 mM Ru(NH3)63+, respectively, in NaCl supporting electrolyte, at concentrations 0, 0.001 M, 0.01 M, 0.1 to 1 M. Figure 6a,b shows the voltammetric response for the oxidation of FcTMA+ and the reduction of Ru(NH3)63+ with different concentrations of NaCl, recorded at a potential sweep rate of 4 mV s-1, at the same SWNT network UME. The potential of the Ag/AgCl RE in solutions of varying chloride concentration was measured against a saturated calomel electrode (SCE) to allow comparison of the voltammograms on the same potential scale. Inspection of Figure 6 reveals two main effects associated with a decrease of background electrolyte concentration from 1 to 0 M. First, iss decreases by 25% for the oxidation of FcTMA+ and increases by 19% for the reduction of Ru(NH3)63+. Second, E1/2 shifts by 30 mV to more positive potentials for the oxidation of FcTMA+ and by 110 mV to less negative potentials for the reduction of Ru(NH3)63+. The change in iss with background electrolyte concentration for both FcTMA+ oxidation and Ru(NH3)63+ reduction is consistent with previous findings which considered the influence of migration on the CV response of UMEs; with decreasing electrolyte concentration the limiting current decreases for oxidation of a cationic species, while it increases for reduction of a cationic species.47 The change in E1/2 with supporting electrolyte concentration is most likely an influence of the formation of ion pairs between the charged redox species and the supporting electrolyte 3604
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anion.48 The highly charged cations, product FcTMA2+ and reactant Ru(NH3)63+ respectively, form stronger ion pairs with the chloride anion48 than their corresponding other half of the redox couple, and are selectively stabilized. This results in the observed shifts of E1/2 with increasing chloride concentration: the reduction of Ru(NH3)63+ shifts to more negative potentials, while the oxidation of FcTMA+ shifts to less positive potentials. The predictable behavior of CVs taken at different supporting electrolyte concentrations indicates that SWNT network electrodes behave as conventional UMEs in low ionic strength solutions. Time Dependent Characteristics. The time response of the SWNT network UME was investigated using chronoamperometry. While the time scale for double layer charging (nonfaradaic) at a UME is relatively short,49 this current may dominate the electrochemical response for measurements at short times. The current–time response of a typical SWNT network UME (b) is given in Figure 7. Also shown are the responses for 25 µm (9), 50 µm (2) and 100 µm diameter Pt disk UMEs (0), all sealed in glass, and a 100 µm diameter Pt disk UME lithographically defined (O). For all measurements in Figure 7, the potential of the UMEs was stepped from 0.0 to 0.2 V vs Ag/AgCl, in a solution containing 0.1 M NaCl. The charging current, i, decreases exponentially with time, t, as shown in Figure 7, at a rate dictated by RC, the cell time constant.24 However, the exponential decay of the charging current can be clearly seen to be significantly faster for the 100 µm SWNT network UME, even when compared with a 25 µm Pt (48) Beriet, C.; Pletcher, D. J. Electroanal. Chem. 1994, 375, 213–218. (49) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268–1288.
UME. This is because capacitance scales with electrode area and, even though the geometric area of the 100 µm diameter SWNT UME is 16 times that of the 25 µm diameter Pt UME, 1 mM for the reduction of Ru(NH3)63+, the CVs became distorted, most likely due to contributions from both SWNT network resistance and electron transfer kinetics. Due to the low surface area of the network electrode and the low capacitance of pristine SWNTs, the SWNT network electrodes are shown to have very fast response times, with the charging current decaying significantly faster at a 100 µm diameter SWNT network UME than for a conventional 100 µm Pt UME, sealed in glass. This has critical implications for the use of such electrodes in the measurement of fast kinetic processes especially when employed in conjunction with perturbation techniques such as rapid pulse voltammetry. ACKNOWLEDGMENT J.V.M. thanks the Royal Society for the award of a University Research Fellowship. We thank the EPSRC for funding (EP/ C518268/1), and I.D. thanks the University of Warwick for a Postgraduate Fellowship Award and the Overseas Research Students Awards Scheme. Received for review December 11, 2007. Accepted February 26, 2008. AC702518G
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