Configurations of Microelectrodes

FIGURE 4-24 Cyclic voltanimograms for the oxidation of ferrocene at a 6 pm platinum microdisk at different scan rates. (Reproduced with permission from reference 84.) 

---

FIGURE 4-26 Common configurations of microelectrodes, a, disk b, ring c, cylinder d, hemisphere e, line (bond) / = length, w = width, r = radius. 

A typical configuration of a SECM system is shown in Fig. 36.6. In this case the solution contains oxidized (Ox) species (denoted mediators) that are reduced on the active part of the microelectrode yielding the reduced (Red) species. The figure also shows a possible reaction of the Red species with the electrode, with the reaction rate If is very large, the approach of the tip to the surface will result in an increase in the reduction reaction (current) on the tip because the regeneration of Ox on the tip will be more efficient in a smaller gap. On tfie otfier fiand, if k is close to zero, the only effect of the tip approach to the surface wifi be the depletion of the Ox species in the gap upon reduction, whose diffusion from the bulk of the solution is now hindered by the probe. These two mechanisms, which result in the positive and negative feedback operation modes, can be used to map the reaction rate k, on the surface. 

Techniques have been devised for constructing microelectrodes of various geometries, such as hemispheres, disks, cylinders, rings, and bands. In addition, because the electrodes themselves are smaller than their associated diffusion layers, arrays of closely spaced microelectrodes have also been of considerable practical and theoretical interest. In this section, construction of the more frequently employed microelectrode configurations is described. First, however, it should be pointed out that several commercial sources of microelectrodes now exist, and these may represent an economically viable alternative to the do-it-yourself approach for those who anticipate requiring only a few electrodes for... 

The best configuration of ensembles of microelectrodes is a collection of micro discs of equal size, arranged on a uniform grid, at equal distances from each other. Sometimes this is not feasible, and ensembles containing electrodes of nonuniform size and/or distance have been considered. Ensembles of strip microelectrodes have also been constructed, and the variation of current with time for such configurations has been evaluated. 

This prevents water penetration and only the exterior of the electrode is exposed to electrolytes. When used with 30 nm sized carbon black powder, the conductive area of the electrode in contact with the electrolyte consists of less than 1 % of the geometric cross-sectional area of the electrode. This has been compared with an ensemble of microelectrodes. These CCEs showed an improvement of up to three orders of magnitude in terms of Faradaic signal-to-noise compared with glassy carbon electrodes [218]. In addition, the sol gel method of preparation affords a variety of structural configurations and it has been shown that CCEs can be produced by thick film and ink jet technology, which allows for their mass production [219]. 

FIG. 1 Schematic cross sections of selected microelectrode configurations, (a) Nomenclature for parts of microelectrode, (b) Na+-sensitive microelectrode (22), (c) recessed-tip Na+-sensitive microelectrode (27), (d) liquid ion-exchanger micropipette electrode (38), (e) coated wire electrode (16), (f) flow-through ISE (e.g., NOVA 6, Boehringer ISE 2020), (g) micro-capillary glass electrode of tubular shape (e.g., Radelkis OP-266), (h) planar sensor fabricated by microelectronic technology (93), (i) ISFET sensor (94). 

Fig. 7.15 Configuration of a poly aniline-based microelectrode device [217], (Reproduced with the permission of the American Chemical Society)...

Theoretical and experimental comparison of microelectrode sensing configurations for impedimetric cell monitoring... 

Kuhn et al. (2008) tested coplanar SC-SOFCs with different curvilinear microelectrode configurations of arbitrarily complex geometry. It was shown that the performance of the cells depends only on electrode and interelectrode dimensions, not on the electrode s shape. 

---