Microelectrode Configurations

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... 

Fig. 13. (a) Sketch of the microelectrode configuration used to investigate the distribution of grain boundary properties, (b) Typical impedance spectrum calculated for a model sample (inset) consisting of 24 cubic grains and two microelectrodes on adjacent grains. An equivalent circuit consisting of two serial RC-elements (inset) can be used to fit the spectrum. 

Conventional impedance spectroscopy does not allow a separation of grain boundary and bulk contributions in samples with highly conductive grain boundaries (cf. Sec. 3.2). The two microelectrode configurations sketched in Fig. 17a, however, can be applied i) to easily check whether highly conductive grain boundaries exist and ii) to quantitatively determine the bulk conductivity obuik and the grain boundary conduc-... 

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). 

Early two-dimensional simulations focused on the evaluation of the current distribution at microdisc electrodes [107, 108] and simulations of a variety of electrode geometries [109-111] including the influence of recessed microelectrode configurations [112]. Work has been also extended to cases involving coupled homogeneous kinetics, adsorption [113], and time-dependent redox polymer electrochemistry [114]. 

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. 

Figure 9.7 illustrates the microelectrode configurations with narrow coplanar or point WE and RE, where the CE has an effective infinite width and the potential... 

Fig. 15.5 Microelectrode configurations (a) disk, (b) cylinder, (c) hemisphere, (d) band, (e) ring, (f) sphere cap, (g) cone, (b) nanopore, and (i) recessed microelectrode...

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. 

Figure 1 Schematic diagrams illustrating the patch-clamp technique. (A) Overall setup for isolating single ionic channels in an intact patch of cell membrane. P = patch pipet R = reference microelectrode I = intracellular microelectrode Vp = applied patch potential Em = membrane potential Vm = Em — Vp = potential across the patch A = patch-clamp amplifier. (From Ref. 90.) (B) Five different recording configurations, and procedures used to establish them, (i) Cell attached or intact patch (ii) open cell attached patch (iii) whole cell recording (iv) excised outside-out patch (v) excised inside-out patch. Key i = inside of the cell o = outside of the cell. (Adapted from Ref. 283.)...

Fig. 25. (a) Sketch of a possible set-up to perform microcontact measurements using one microelectrode and an extended counter-electrode. The microelectrode is contacted under the microscope by a sharp needle, (b) Set-up for two microelectrodes, (c) Evaporated Au microelectrodes on SrTiC>3 and the needlelike tungsten tip to contact the electrodes. Such a configuration is used to perform spatially resolved bulk conductivity measurements (Sec. 6.2). (d) Ag-coated YBajCujOs+s-microelectrodes on a SrTiC>3 polycrystal contacted by two tungsten tips. The corresponding local grain boundary measurements are discussed in Sec. 6.3. 

This can be understood from the electrode configuration shown in Fig. 27b extended electrodes on two sides of a square sample are used as current feed electrodes and very thin, highly conductive lines connect these contacts and circular microelectrodes on top of the sample. In the case of a single crystal, the resistance between two circular microelectrodes Rme is given by (ubuik me)-1, whereas the resistance between the two contact electrodes reads (owk j with being the sample height. The... 

Fig. 27. (a) Electrode configuration frequently used to perform microelectrode measurements. The end of the connection line represents the microelectrode. However, if the sample surface and the contact electrodes for current feed are not separated by an insulator, such a set-up often measures the overall properties between the contact electrodes rather than the local properties, (b) Sketch of a model sample with extended contact electrodes, very thin highly conductive connection lines, and circular microelectrodes. 

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