From Microelectrodes to Scanning Electrochemical Microscopy

Salvatore Daniele and Guy Denuaulf University of Venice, Dipartimento Scienze Molecolari e Nanosistemi, Italy University of Southampton, Chemistry, UK 

Developments in Electrochemistry Science Inspired by Martin Fleischmann, First Edition. Edited by Derek Fletcher, Zhong-Qun Xian and David E. WilUams. 

In this chapter, two areas are considered where the unique properties of microelectrodes have had a significant impact (i) the use of microelectrodes and arrays of microelectrodes in electroanalytical studies (in foodstuffs, in concentrated industrial solutions, analysis with minimal sample preparation), especially in combination with pulsed amperometric techniques and (ii) in scanning electrochemical microscopy (SECM note that the acronym is used for both the instrument and the technique). 

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Despite the predominant use of fluorescence and/or patch clamp techniques in single cell measurements, there has been a steady increase in the demand for new electroanalytical tools applicable to single cell studies [4]. Traditionally, such methods have been confined to the development and production of hand crafted sensors including the aforementioned glass capillaries [1-3] for patch clamping, as well as conical microelectrodes for scanning electrochemical microscopy (SECM) [5, 6] and carbon fiber microelectrodes to measure for example, the release of neurotransmitter from single neurons [7, 8]. 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Scanning electrochemical microscopy seeks to overcome the lack of sensitivity and selectivity of the probe tip in STM and AFM to the substrate identity and chemical composition. It does this by using both tip and substrate as independent working electrodes in an electrochemical cell, which therefore also includes auxiliary and reference electrodes. The tip is a metal microelectrode with only the tip active (usually a metal wire in a glass sheath). At large distances from the substrate, in an electrolyte solution containing an electroactive species the mass-transport-limited current is therefore... 

Within a regular scanning electrochemical microscopy (SECM) system, the probe microelectrode, called the tip electrode, can be precisely positioned several micrometers away from a substrate under the control of a three-dimensional motorized positioner in the solution containing redox-active species. By scanning the SECM tip within the plane paralleling a substrate surface and simultaneously monitoring tip current (/ [ ), which is sensitive to the presence of conducting and... 

The properties and applications of microelectrodes, as well as the broad field of electroanalysis, have been the subject of a number of reviews. Unwin reviewed the use of dynamic electrochemical methods to probe interfacial processes for a wide variety of techniques and applications including various flow-channel methods and scanning electrochemical microscopy (SEM), including issues relating to mass transport (1). Williams and Macpherson reviewed hydrodynamic modulation methods and their mass transport issues (2). Eklund et al. reviewed cyclic voltammetry, hydrodynamic voltammetry, and sono-voltammetry for assessment of electrode reaction kinetics and mechanisms with discussion of mass transport modelling issues (3). Here, we focus on applications ranging from measnrements in small volumes to electroanalysis in electrolyte free media that exploit the uniqne properties of microelectrodes. 

FIGURE 3.12 Steady-state current-distance curve for a conical tip over conductive (A) and insulating (B) substrates corresponding to different values of the parameter k=hg/ag. (A) k=3 (curve 1), 2 (curve 2), 1 (curve 3), 0.5 (curve 4), and 0.1 (curve 5). The upper curve was computed for a disk-shaped tip from Equation 3.1. (B) From top to bottom, k=3, 2, 0.5, and 0.1. The lower curve was computed for a disk-shaped tip from Equation 3.2. (Reprinted from/. Electroanal. Chem., 328, Mirkin, M.V., Ean, E.-R.R, and Bard, A.J., Scanning electrochemical microscopy Part 13. Evaluation of the tip shapes of nanometer size microelectrodes, 47-62, Copyright 1992, with permission from Elsevier.)... 

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