Anal. Chem. 2004, 76, 503-512
Articles
Carbon Composite Microelectrodes: Charge Percolation and Electroanalytical Performance Sonia Ramı´rez-Garcı´a,†,⊥ Salvador Alegret,† Francisco Ce´spedes,‡ and Robert J. Forster*,§
Grup de Sensors i Biosensors, Departament de Quı´mica, Universitat Auto` noma de Barcelona, 08193 Bellaterra, Barcelona, Spain, Escola Universita` ria Polite` cnica de Medi Ambient de Mollet del Valle` s, 08100 Mollet del Valle` s, Barcelona, Spain, School of Chemical Sciences, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland
Microelectrodes based on two different epoxy-graphite composites (Araldite-M/HY5162 and Araldite-PY302-2/ HY943) that are compatible with organic solvents have been developed and characterized. The variation in the bulk conductivity with graphite particle loading is described by percolation theory and indicates that the particles interact strongly with one another. The percolation threshold is 52% v/v loading of graphite, and this composite exhibits a bulk conductivity of 15 S m-1. Microdisk electrodes of 25-µm diameter were produced by first etching a microcavity at the tip of a platinum microelectrode, which was then packed with a composite containing 60% v/v graphite so as to optimize both electrical conductivity and the electrode stability in acetonitrile and methanol solutions. Solution phase voltammetry of ferrocene is nearly ideal, and the responses are dominated by radial diffusion (slow scan rates) and semi-infinite linear diffusion (fast scan rates). The microelectrodes display high signal-to-noise ratios, good sensitivity, and low detection limits. The response times given by the product of the resistance, R, and capacitance, C, are 7.5 × 10-4 and 1.4 × 10-1 s for the Araldite M and PY302-2 composites, respectively. Although these response times are significantly slower than those associated with microelectrodes based on carbon fibers or metal wires, they are sufficient for time-resolved electroanalytical applications. The long response times arise from the large composite resistances, 3.1 × 1011 and 8.3 × 1011 Ω cm-2 for Araldite M and PY302-2, respectively. Volta†
Universitat Auto`noma de Barcelona. Escola Universita`ria Polite`cnica de Medi Ambient de Mollet del Valle`s. § Dublin City University. ⊥ Current address: School of Chemical Sciences, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland. ‡
10.1021/ac034536p CCC: $27.50 Published on Web 11/26/2003
© 2004 American Chemical Society
mmetry of ferrocene in the absence of deliberately added supporting electrolyte is also reported. Significantly, indistinguishable slopes and intercepts for a calibration curve of peak current vs ferrocene concentration where 2 < [ferrocene] < 50 µM are obtained in the presence and absence of supporting electrolyte.
Microelectrodes have proven to be extremely useful tools for clinic analysis in vivo1 and for trace analysis,2 as well as for the determination of kinetic parameters.3 It is possible to find a wide range of shapes and materials used for the construction of microelectrodes, and even strategies for their modification in order to produce sensitive and selective analytical sensors.2 This drive toward miniaturized sensors arises because of the desire to decrease the ohmic potential drop, to rapidly establish a steadystate current signal, and to exploit enhanced mass transport at the electrode boundary so as to improve the signal-to-noise ratio. Carbon-based microelectrodes play an especially important role in electroanalysis,4 particularly for investigations of biological systems and for the development of chemically selective sensors based on redox proteins,5 enzymes,6,7 antibodies,8 and immobilized DNA strands.9 Beyond traditional forms, such as pyrolized carbon and carbon fibers, conducting composite materials have been applied in the construction of electroanalytical devices since (1) Wightman, R. M. Anal. Chem. 1981, 53, 1125A. (2) Forster, R. J.; Diamond, D. Anal. Commun 1996, 33. (3) Forster, R. J. Chem. Soc. Rev. 1994, 289. (4) Chen, S.; Kucernak, A. Electrochem. Commun. 2002, 4, 80. (5) Bianco, P.; Lattuca, C. Anal. Chim. Acta 1997, 353, 53. (6) Lvovich, V.; Scheeline, A. Anal. Chem. 1997, 69, 454. (7) Liu, T. Z.; Wang, Y.; Kounaves, S. P.; Brush, E. J. Anal. Chem. 1995, 67, 1679. (8) Turyan, I.; Matsue, T.; Mandler, D. Anal. Chem. 2000, 72, 3431. (9) Caruana, D. J.; Heller, A. J. Am. Chem. Soc. 1999, 121, 769.
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Adams’ seminal work.10 However, despite the great variety of electrode shapes and sizes that can be created using disperse rigid conducting composites, only a few examples of microelectrodes based on these materials can be found in the literature.11-14 Moreover, the smallest composite “microelectrode” reported thus far has a diameter on the order of 200 µm.12 In addition, despite the great variety of available materials for the preparation of rigid conducting composites, only epoxy-graphite11-13 and silicagraphite14 composites have been used for electrode construction. In this contribution, the construction and properties of diskshaped microelectrodes of 12.5-µm radius based on rigid conducting composite is described. The two different epoxy resins investigated are compatible with organic solvents.15,16 The stability of these composites in organic solvents opens up applications in electrosynthesis,17 biosensors based on stabilized enzymes,18-23 batteries,24 and nanotechnology.25 In particular, chemical recognition elements, such as enzymes, antibodies, and molecular hosts can be incorporated into the composite prior to hardening, making the composites useful for microsensor development.26 Moreover, unlike conventional electrochemical sensors that are predominantly based on thin films, functionalized composites can be mechanically polished to produce a pristine sensing surface. The ability to simultaneously record topographic images and maps of the conductivity of disperse conducting composites using conducting AFM demonstrated by MacPherson and co-workers has been pivotal in developing applications of this kind.27 One attractive feature of microelectrodes is their immunity to ohmic effects because of the low currents observed. However, the conductivity of composite materials is highly sensitive to the carbon loading, which can contribute significantly to the overall cell resistance in electroanalytical experiments.28 To address this issue, we have systematically varied the carbon loading and measured the bulk and dry state conductivity of the resulting composites. These data are described by the percolation theory and have allowed the optimum carbon loading that simultaneously maximizes both the composite conductivity and chemical stability to be identified. The shortest time scale at which the microelectrodes provides meaningful analytical data, as well as their interfacial capacitance and cell resistance, has been determined using chronoamperom(10) Adams, R. N. Anal. Chem. 1958, 30, 1576. (11) Lipka, S. M.; Cahen, G. L.; Stoner, G. E.; Scribner, J. R.; Gileadi, E. J. Electrochem. Soc. 1988, 135, 368. (12) Falat, L.; Cheng, H.-Y. Anal. Chem. 1982, 5, 2108. (13) Liberti, A.; Morgia, C.; Mascini, M. Anal. Chim. Acta 1985, 173, 157. (14) Tsionsky, M.; Gun, G.; Glezer, V.; Lev, O. Anal. Chem. 1994, 66, 1747. (15) Ramı´rez-Garcı´a, S.; Ce´spedes, F.; Alegret, S. Electroanalysis 2001, 13, 529. (16) Ramı´rez-Garcı´a, S.; Alegret, S.; Ce´spedes, F.; Forster, R. Analyst 2002, 127, 1512. (17) Little, R. D.; Moeller, K. D. Interface Winter 2002; p 36. (18) O Ä ’Fa´ga´in, C. Enzyme Microb. Technol. 2003, 33, 137. (19) Klibanov, A. M. Nature 2001, 409, 241. (20) Carrea, G.; Riva, S. Angew. Chem. 2000, 39, 2227. (21) Zaks, A.; Klibanov, A. M. Science 1984, 224,1249. (22) Zaks, A.; Klibanov, A. M. J. Biol. Chem. 1988, 263, 8017. (23) Lee, M. Y.; Dordick, J. S. Curr. Opin. Biotechnol. 2002, 13, 376. (24) Electrochemistry in Nonaqueous Solutions; Izutsu, K., Ed.; John Wiley and Sons: New York, 2002. (25) Aiken, J. D.; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1. (26) Moreno-Baron, L.; Merkoc¸i, A.; Alegret, S. Electrochim. Acta 2003, 48, 2599. (27) O’Hare, D.; MacPherson, J. V.; Willows, A. Electrochem. Commun. 2002, 4, 245. (28) Navarro-Laboulais, J.; Trijueque, J.; Garcı´a-Jaren ˜o, J. J.; Benito, D.; Vicente, F. J. Electroanal. Chem. 1998, 443, 41.
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etry and compared with conventional carbon fiber- and platinumbased microelectrodes. Composite-based microelectrodes offer enhanced current at the electrode/insulator boundary as well as the possibility of individual conducting particles acting as microelectrodes. To characterize the temporal evolution of the diffusion field and to assess the contribution to the overall current from these processes, cyclic voltammetry has been performed using ferrocene as a solution-phase electrochemical probe across a wide range of scan rates. The voltammetry is close to ideal, with both steady-state and semi-infinite linear diffusion-controlled responses being observed at slow and fast scan rates, respectively. Calibration curves for ferrocene are linear down to concentrations of at least 2.5 µM, and the limit of detection is approximately an order of magnitude lower, as compared to composite-based macroelectrodes. EXPERIMENTAL SECTION Reagents and Materials. All experiments were carried out in HPLC grade acetonitrile and methanol obtained from Labscan Ltd. Tetrabutylammonium perchlorate (TBAP) (99.99%) and ferrocene (99.99%) were obtained from Aldrich. Instrumentation. Cyclic voltammograms and potential step chronoamperometry were performed using a potentiostat from CH Instruments, model 660, which has a rise time of
