Anal. Chem. 2001, 73, 6088-6092
Laminated Microelectrodes: A Simple Approach to the Construction of Inexpensive Microelectrodes with a Variety of Geometries Peter J. Welford,† John Freeman,† Shelley J. Wilkins,† Jay D. Wadhawan,† Clive E. W. Hahn,‡ and Richard G. Compton*,†
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom, and Nuffield Department of Anaesthetics, Oxford University, Radcliffe Infirmary, Oxford OX2 6HE, United Kingdom
Designs for reusable microelectrodes, which are easy to assemble and so do not require special technical skills or services for their construction, are presented. Three types of microelectrodes are fabricated by lamination of metal foil, wire, and wire grids, producing microband, microdisk, and a linear array of microdisk geometries. The electrodes themselves do not need to be polished prior to their use but are observed to be slightly recessed from the surrounding insulating surface. Good agreement is observed between experimental steady-state voltammetric results in nonaqueous solutions and the relevant analytical theory for the voltammetric current. Microelectrodes are attractive tools for undertaking a variety of electrochemical experiments and, as such, have found wide application in electroanalysis and in the study of ultrafast kinetic processes,1,2 as mass transport to the electrode is fast even in quiescent solution, by virtue of the current density at the electrode scaling with the reciprocal of the electrode dimension.1 Furthermore, a variety of geometries of microelectrodes exist, such as disk, band, and ring shapes and arrays of these.3 We present here an inexpensive general method for the construction of microelectrodes of three geometries (band, disk, disk array), based upon thermal lamination of metal foil, wire, or mesh, and show that this approach is well suited for a variety of solvents and electrochemical systems. Previously, laminationbased methods have been used in the construction of macroelectrodes.4 A variety of methods have been used in the construction of microdisk electrodes.3 One of the most successful is that in which a wire is sealed in glass and the surface subsequently polished (or etched away) until the electrode is revealed.5 Since glass is insoluble in most solvents used for electrochemical * To whom all correspondence should be addressed: (e-mail) compton@ ermine.ox.ac.uk; (tel) +44 (0) 1865 275 413; (fax) +44 (0) 1865 275 410. † Physical and Theoretical Chemistry Laboratory. ‡ Nuffield Department of Anaesthetics. (1) (a) Aoki, K. Electroanalysis 1993, 5, 627. (b) Aoki, K., Tokuda, K, J. Electroanal. Chem. 1987, 237, 163. (2) Montenegro, M. I. Res. Chem. Kinet. 1994, 2, 1. (3) Wang, J. Microelectrodes; VCH: New York, 1990. (4) (a) Neudeck, A.; Kress, L. J. Electroanal. Chem. 1997, 437, 141. (b) Neudeck, A.; Petr, A.; Dunsch, L. J. Phys. Chem. 1999, 103, 912. (5) (a) Koppenol, M.; Cooper, J. B.; Bond, A. M. Am. Lab. 1994, 26, 25. (b) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, A.; Lewis, A. Anal. Chem. 1997, 69, 1627.
6088 Analytical Chemistry, Vol. 73, No. 24, December 15, 2001
purposes, electrodes thus formed are attractive.6 However, their major limitation is that if a careful polishing technique is not employed,7 the electrodes have a tendency to become recessed below the surface of the insulating glass, leading to voltammetric responses slightly different from those anticipated for a perfectly coplanar electrode. Furthermore, if the wire is less than 2 µm in diameter, a perfect microdisk microelectrode is unlikely to be formed.5b Recently, the manufacture and use of arrays of microdisk electrodes has been explored, using a variety of methods.8 Microdisk arrays, which typically consist of diffusionally independent microelectrodes wired in parallel, are better than a single microdisk electrode for sensing purposes, as the observed current is typically thousands of times larger in the former than in the latter.8a However, careful design is essential in ensuring that the diffusion spheres at each microelectrode do not interfere with others. Current methodologies involve sealing thousands of carbon fibers randomly in epoxy resin,8a deposition of electrode material into the pores of a nanoporous material,8b,c using ultrasonic ablation of nonconductive polymer films on electrode surfaces,8d and the formation of “microhole” arrays by laser ablation of an insulating monolayer.8e-g The last two methods necessarily create recessed microelectrodes, and all the methods form random arrays of electrodes. Varco Shea and Bard9a and Wightman and co-workers9b constructed microband electrodes by sealing metal foils inside a (6) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 267. (7) Cardwell, T. J.; Mocak, J.; Santos, J. H.; Bond, A. M. Analyst 1996, 121, 357. (8) (a) Fletcher, S.; Horne, M. D. Electrochem. Commun. 1999, 1, 502. (b) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920. (c) Hulteen, J. C.; Menon, V. P.; Martin, C. R. J. Chem. Soc., Faraday Trans. 1996, 92, 4029. (d) Madigan, N. A.; Hagan, C. R. S.; Coury, L. A. J. Electrochem. Soc. 1994, 141, L23. (e) Seddon, B. J.; Shao, Y.; Fost, J.; Girault, H. H., Electrochim. Acta 1994, 39, 783. (f) Osborne, M. C.; Shao, Y.; Pereira, C. M.; Girault, H. H. J. Electroanal. Chem. 1994, 364, 155. (g) Wilke, S.; Osborne, M. D.; Girault, H. H. J. Electroanal. Chem. 1997, 436, 53. (9) (a) Varco Shea, T.; Bard, A. J. Anal. Chem. 1987, 59, 2101. (b) Kovach, P. M.; Caudill, W. L.; Peters, D. G.; Wightman, R. M. J. Electroanal. Chem. 1985, 185, 285. (c) Compton, R. G.; Fisher, A. C.; Wellington, R. G.; Dobson, P. J.; Leigh, P. A. J. Phys. Chem. 1993, 97, 10410. (d) Paeschke, M.; Dietrich, F.; Uhlig, A.; Hintsche, R., Electroanalysis 1996, 8, 891. (e) Wehmeyer, K. R.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1913. (f) Williams, D. E.; Ellis, K.; Colville, A.; Dennison, S. J.; Laguillo, G.; Larsen, J. J. Electroanal. Chem. 1997, 432, 159. (g) Nagale, M. P.; Fritsch, I. Anal. Chem. 1998, 70, 2902. (h) Nagale, M. P.; Fritsch, I. Anal. Chem. 1998, 70, 2908. 10.1021/ac010434s CCC: $20.00
© 2001 American Chemical Society Published on Web 11/17/2001
sandwich of two glass insulating plates. Other previous approaches to the manufacture of microband electrodes include using photolithographic9c,d and sputtering9e methodologies or by screenprinting the electrode.9f Recently, Nagale and Fritsch developed a method for constructing assemblies of “nanoband” electrodes, with dimensions of 2 mm × 37.0 nm.9g,h A discussion on the construction of a variety of microelectrodes may be found in ref 10. A major limitation in most methods of construction is that the electrode is unlikely to be exactly coplanar with its surrounding insulating support and may be either elevated above the plane of the support or recessed below it, necessarily affecting the voltammetric response.11 The more successful construction methodologies have these errors minimized, while simultaneously maintaining the electrode shape. However, the preparation of these electrodes is not trivial. In contrast, the lamination method developed here is shown to be a more inexpensive alternative. Although the electrodes are observed to be marginally recessed with the surrounding lamination, the voltammetric responses are in close agreement with those predicted by theory.1 EXPERIMENTAL SECTION The polyester/polyethylene (in a 4:1 mass ratio) laminating foil used (obtained from Murodigital, Weston Super Mare, Somerset U.K.) was found to be resistant to solvents such as water, acetonitrile, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). Electrodes were prepared according to the general scheme in Figure 1. Microband electrodes were constructed by thermal lamination of a metal foil of known dimensions, and microdisk electrodes were prepared by laminating a metal wire of known diameter. In both cases, electrical connection to the foil or wire had been previously established either by “spot-welding” a thin metal wire (
