Microelectrodes materials

Metal particles also behave as microelectrodes, and electron exchange between the microelectrode material and redox species present in the solution occurs. Upon conjugation of the metal particle with a reduced species, the electrode is charged and in the presence of an oxidant the electrode material is discharged. The ET exchange rate between a metal particle and a redox relay is given by... 

Keywords Cochlear implants (CFs) Microelectrode material Neural stimulation and sensing Titanium Nitride (TiN)... 

The requirements for microelectrode materials used in CTs and neural implants is increasing nowadays for achieving high performance and stability in their dedicated applications. The microelectrode material selection is an key factor for the success of such implants. Here in this section we talk about the microelectrode material requirements with the charge transfer techniques between the electrode material and the electrolyte. 

Lead materials lead-antimony-silver, lead with platinum alloy microelectrodes, lead/magnetite, lead dioxide/titanium, lead dioxide/ graphite. 

Thus the time during which the transport process attains the steady state depends strongly on the radius of the sphere r0. The steady state is connected with the dimensions of the surface to which diffusion transport takes place and does, in fact, not depend much on its shape. Diffusion to a semispherical surface located on an impermeable planar surface occurs in the same way as to a spherical surface in infinite space. The properties of diffusion to a disk-shaped surface located in an impermeable plane are not very different. The material flux is inversely proportional to the radius of the surface and the time during which stationary concentration distribution is attained decreases with the square of the disk radius. This is especially important for application of microelectrodes (see page 292). 

Various pH microelectrodes and numerous fabrication techniques have been developed. Some selected examples of microelectrodes based on the formats of pH sensitive materials are discussed in this section. 

Some earlier developments and applications of various implantable pH sensors or measurement systems have been reported [128, 129, 130, 131]. However, reliable pH sensors for long-term implantations are still not available, and widespread clinical usage of implantable pH sensors has not been reached. Similar to other implantable sensors, the development of implantable pH microelectrodes, either fully implanted in the body or needle type sensors applied through the skin (percutaneous), has faced serious obstacles including sensor stability deterioration, corrosion, and adverse body reactions [48, 132, 133], Among them, encapsulation to prevent corrosion represents a major challenge for the implantable sensor devices [51]. Failure of encapsulation can cause corrosion damage on internal components, substrate materials, and electrical contacts [48], The dissolution of very thin pH sensitive layers will also limit the stability and lifetime of implantable micro pH sensors. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Electronically active chips (e.g.. Nanogen s NanoChip Electronic Microarray) are true microchips in which microelectrodes (pads) become elements of the array (Figure 2.13). The microelectrodes are covered with materials that allow immobilization of probes. Each electrode is individually addressable so that specific probes can be attached to different electrodes. Hybridization is accelerated by electromotive force (emf) on the target. Enhanced stringency is also achieved by modulation of the emf (Heller et al., 2000). 

A novel application of ionic liquids in biochemistry involved duplex DNA as the anion and polyether-decorated transition metal complexes. When the undiluted liquid DNA-or molten salt-is interrogated electrochemically by a microelectrode, the molten salts exhibit cyclic voltammograms due to the physical diffusion (D-PHYS) of the polyether-transition metal complex. These DNA molten salts constitute a new class of materials whose properties can be controlled by nucleic acid sequence and that can be interrogated in undiluted form on microelectrode arrays (Leone et al., 2001). 

Arrays. One can compensate for the tiny currents produced by microelectrodes by working with many of them placed together within a board of an insulating material (connected at the back so that all the currents add) (see Fig. 7.34). Then, if r is the radius of each electrode (assumed to be disklike in shape) and ti the number per unit area, rrtrp is the total active area. If L is the distance between the spots," (VnL)2 is the total area. Hence,... 

Interest in microelectrodes, in vivo analysis, and carbon-reinforced structural materials has stimulated research on the electrochemical behavior of carbon fibers. Such fibers have diameters ranging from a few micrometers to about 60 pm, with the majority in the range of 5-15 pm. Although carbon fibers have a wide variety of structures and properties and are often less well characterized than GC or graphite, they have been used successfully in several important electroanalytical experiments. 

For the purposes of considering diffusion at microelectrodes, it is convenient to introduce two categories of electrodes those to which diffusion occurs in a linear fashion and those to which diffusion occurs in a nonlinear fashion. The former category consists of cylindrical and spherical electrodes. As shown schematically in Figure 12.2A, the lines of flux (i.e., the pathway followed by material diffusing to the electrode) are straight, and the current density is the same at all points on the electrode. Thus, the diffusion problem is one-dimensional (i.e., distance from the electrode surface) and involves solution of the appropriate form of Fick s second law, Equation 12.7 or 12.8, either by Laplace transform methods or by digital simulation (Chap. 20). 

Phenomenologically speaking, the reason that a steady-state response is obtained with a hemispherical microelectrode is that the lines of flux converge on the electrode surface from all directions. Thus, the volume of the solution providing electroactive material to the electrode is very large in comparison to the electrode surface area. In fact, this can be seen in Figure 12.1 where the... 

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