Recessed electrodes

One major advantage of finite element methods over finite difference methods is the way they naturally incorporate non-uniform meshes. They can therefore be applied to problems with a complex geometry (Stevens et al, 1997), for example elevated and recessed electrodes (Ferrigno et ah, 1997), and, in principle, simulation of rough electrodes. On the downside, finite element methods are more complex to program, especially when simulating chemical steps, and result in a linear system of equations which is not neatly banded. 

When a submicrometer-sized UME is used as an SECM tip, it is essential to distinguish between planar and recessed geometries. In the latter case, the metal core of an UME is recessed into the solution-filled microcavity inside the glass insulator. Such electrodes are usually unsuitable for kinetic measurements (21). On the other hand, a nm-thick solution layer trapped between such a nm-sized recessed tip and the substrate can be used for single-molecule detection (22). Fan et al. (22) simulated a special case of the recessed electrode with the radius of the circular hole in the insulator (ra) equal to the disk radius. They also derived an approximate expression for the iT, ., at a recessed tip with ra = a ... 

The advantages of the recessed electrode can be reahzed in a simpler design that lends itself to mass production through automation. This results in low per-unit cost so that these electrodes can be considered... 

FIGURE 4.4 Examples of different skin electrodes (a) metal plate electrodes, (b) suction electrode for EGG, (c) metal cup EEG electrode, (d) recessed electrode, (e) disposable electrode with electrolyte-impregnated sponge (shown in cross section), (f) disposable hydrogel electrode (shown in cross section), (g) thin-film electrode for use with neonates (shown in cross section), and (h) carbon-filled elastomer dry electrode. 

If a small (submicrometer) tip is to be used in quantitative measurements, it is important to show that the metal surface is not recessed into the insulator since the mass transport coefficient will be altered. This is possible via a study of the approach curve a recessed tip does not produce as large a positive feedback [26]. Recessed tips have also been produced intentionally for single molecule experiments [79, 80]. Fan and coworkers [80] simulated a special case of the recessed electrode with the radius of the circular hole in the insulator equal to the disk radius. An analytical approximation for recessed tips with the hole in the insulator sheath different from the disk radius is also available [25]. The feedback effect is also reduced for tips, which are conical or spherical caps, but the shape of the approach curve provides some information on the tip geometry, for example, the solid angle [26]. Accurate numerical simulations of such tips by the finite difference method are more costly than for microdisks, but have been... 

Figure 6.13.1 Different shapes of ultramicroelectrodes (UMEs). (A) A perfect microdisk electrode. (B) A conical disk-type planar UME due to sealing of the tnicrowire at an angle relative to the insulating shroud. (C) An irregular cylindrical protruding from the insulator. (D) A recessed microdisk electrode. (E) A recessed electrode in which the cavity in the insulator is large than the...

Fig. 10 Combination of AEM with SECM. a Schematic illustration of the AFM—SECM setup. AEM—SECM in b contact mode, c Uft mode, and d recessed electrode mode. Reproduced from [30] with permission of the Royal Society of Chemistry...

A wide variety of electrodes have been reported for special purposes. It is hard to categorize them. Some of these miscellaneous types are cutting electrodes, depth electrodes, and recessed electrodes. 

Many electrodes qualify as recessed electrodes. A metal recording electrode linked by a salt bridge to a potential source is a recessed electrode. Recessed skin-surface electrodes are generally cup-shaped. The gap between the skin surface and the metallic cup electrode is filled with electrode paste or some other electrolyte to maintain good electrical contact between the electrode and the surface which it contacts. The advantage of this type of... 

Some recessed electrodes have suction bulbs attached to facilitate application to a body surface. This type is used in EKG recording from the chest surface. 

Frequently recessed electrodes are circular in cross-section and are equipped with a rubber gasket ring which forms a tight seal between the electrode assembly and the surface which the electrode contacts. 

Since suction electrodes are basically recessed electrodes, problems with electrical contact can develop. One must use a good wetting electrolyte to insure maintenance of a high-integrity electrical connection. 

Figure 10.6. Recessed-electrode cell for electrolyte impedance measurements.

In other electrode configurations, the electrodes, either a microdisc or an array of electrodes, can be recessed, that is, the electrode is not planar to the insulating material and consequently the diffusional profiles will quantitatively change [10] recessed electrodes are typically produced unintentionally when photolithography is used. In the case of an array of recessed microdiscs. 

Metters, J.R, Tan, F, Kadara, R.O. and Banks, C.E. (2012) Electroanalytical properties of screen printed shallow recessed electrodes. Anal. Methods, 4, 3140. 

Rubinstein JT, Spelman FA, Soma M (1987) Current-density profiles of surface mounted and recessed electrodes for neural prostheses. IEEE Trans Biomed Eng 34 (11) 864-875... 

Recessed electrodes have a more uniform current distribution both at the electrodeelectrolyte interface and at the carrier-tissue junction [9,11,14], For patient s safety, clinical use of recessed electrodes in cochlear implants is recommended [14]. 

A uniform current density electrode was designed in [11] by combining the shapes of a hemispherical electrode and a recessed electrode. This electrode had... 

In the recessed electrode, the active surface is located at the bottom of a hole. Simple geometry of the electrode is a disk as is shown in Fig. 15.11. These electrodes can be fabricated by etching the metal wires. The steady-state current for such geometry is expressed by equation T2,is. ... 

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