Arrays of microelectrodes

Ion-selective electrodes and amperometric ceUs have had a long history of success in a wide variety of appHcations (8,9). A microelectronics-inspired revolution is also occurring in these devices, brought about by the advent of photoHthographicaHy defined arrays of microelectrodes on planar substrates... 

Preparation of Microelectrode Arrays. The microelectrode arrays used in the work were arrays of microelectrodes each 80 pm long, 2.3pm wide and 0.1 pm thick and 3paced 1.7 pm apart. Fabrication and encapsulation of the microelectrode arrays has been described previously.<14.15.21-22) Prior to use, arrays of microelectrodes were cleaned by an rf 02 plasma etch to remove residual photoresist, followed by cycling the potential of each electrode between -1.5 V... 

Figure 6. Generation/collection experiments for different fixed collector potentials in an interdigitated array of microelectrodes coated with poly(I) in CH3CN/O.I 14 [n-Bu4N]PF6. The potential of the collector electrodes is held at 0.0 V, -0.50 V, -0.59 V, -0.62 V, or -0.80 V vs. Ag+/Ag while the potential of the generator electrodes is swept between 0.0 V and -0.9 V vs. Ag+/Ag at 20 mV/s.

Figure 7. Cyclic voltammetry of an interdigitated array of microelectrodes coated with poly(I) in CH3CN/0.I H [n-Bu NlPFg (a) The potential of all eight electrodes is scanned together at 10 mV/s. (b) The potential of electrodes 2,4,6, and 8 is scanned at 10 mV/s while the potential of electrodes 1,3,5, and 7 is held at 0 V vs. Ag+/Ag.

Figure 8. Generation/colleotion experiments as a function of temperature at an interdigitated array of microelectrodes coated with poly(I) at 50 mV/s in CH3CN/O.I H [n-B NIPFg.

Alternatively, an assembly of microelectrodes can alleviate some of the problems associated with the individual microelectrodes. Such a random array of microelectrodes (RAM) comprises about 1000 carbon fibres (each of diameter 5-7 pm) which are embedded randomly within an inert adhesive such as an epoxy resin. (The ends of the fibres need to be widely spaced.) The net result is to generate an electrode system with a superior response time and a current which is IfKK) times that of a single microelectrode. By increasing the current in this way, the sensitivity of measurement is further increased. 

Random array of microelectrodes (RAM) A microelectrode system comprising about 1000 carbon fibres embedded randomly within an inert adhesive such as an epoxy resin. 

Ciosek, P., Maminska, R., Dybko, A., and Wroblewski, W. (2007). Potentiometric electronic tongue based on integrated array of microelectrodes. Sens. Actuators B 127(1), 8-14. 

Great activity has also been evidenced in microlithographically fabricated arrays of microelectrodes, which are typically formed in one plane on an insulating substrate [7,8,13,34-45] for experiments involving either an array of electrodes held at a common potential [37,40,42,43], or an array of noninteracting electrodes held at two or more different applied potentials [42,44], or an array of interdigitated electrodes held at two different potentials [13,34,36,38,39,45-47]. Arrays have significantly better analytical detection limits than continuous electrodes of the same overall dimensions, due to enhanced mass transport fluxes that arise from an increase in the spatial dimensionality of mass transport due to the alternation of electrode zones with pas-... 

The main purpose of this contribution, however, is to review recent advances in solid state ionics achieved by means of microelectrodes, i.e. electrodes whose size is in the micrometer range (typically 1-250 pm). In liquid electrolytes (ultra)-microelectrodes are rather common and applied for several reasons they exhibit a very fast response in voltametric studies, facilitate the investigation of fast charge transfer reactions and strongly reduce the importance of ohmic drops in the electrolyte, thus allowing e.g. measurements in low-conductive electrolytes [33, 34], Microelectrodes are also employed to localize reactions on electrodes and to scan electrochemical properties of electrode surfaces (scanning electrochemical microscope [35, 36]) further developments refer to arrays of microelectrodes, e.g. for (partly spatially resolved) electroanalysis [37-39], applications in bioelectrochemistry and medicine [40, 41] or spatially resolved pH measurements [42], Reviews on these and other applications of microelectrodes are, for example, given in Ref. [33, 34, 43-47],... 

Arrays of microelectrodes are being developed which may have individually addressable elements. For example, Kakerow et al. (1994) have produced arrays with as many as 400 individually accessible working microelectrodes in a chip with centimetre dimensions. These advances coupled with the... 

Figures 1IC-E show SEM images of test patterns of silver that were fabricated using pCP with hexadecanethiol, followed by selective chemical etching [102], The SAMs protect the underlying substrates from dissolving by blocking the dilSisional access of etchants. The ability to generate arrays of microstructures of coinage metals with controlled shapes and dimensions is directly useful in fabricating sensors and arrays of microelectrodes.

Metal nanoparticles housed in zeolites and aluminosilicates can be regarded as arrays of microelectrodes placed in a solid electrolyte having shape and size selectivity. Remarkably, the chemical and electrochemical reactivity of metal nanoparticles differ from those displayed by bulk metals and are modulated by the high ionic strength environment and shape and size restrictions imposed by the host framework. In the other extreme end of the existing possibilities, polymeric structures can be part of the porous materials from electropolymerization procedures as is the case of polyanilines incorporated to microporous materials. The electrochemistry of these types of materials, which will be termed, sensu lato, hybrid materials, will be discussed in Chapter 8. 

At porous electrodes, diffusion will be conditioned by the electrode geometry and pore-size distribution, so that under several conditions, semi-infinite diffusion holds however, under several other conditions, the porous electrode can be treated as an array of microelectrodes (Rolison, 1994). 

Davies and Compton simulated a regular array of microelectrodes, varying the diffusion domain approach, as illustrated in Figure 6.20. 

The modeling of coupled electrochemical microcells, progress in capacitive biocompatible microelectrodes, and the creation of precisely tailored arrays of microelectrodes are particularly relevant to the coupling of microelectronics and nerves. 

Recently it was proposed that the apparently slow heterogeneous electron-transfer rates for such proteins as cytochrome c, cytochrome b5, plasto-cyanin, and ferredoxin are an artifact of the experimental approach (25). Instead of assuming that protein molecules react at a planar and essentially homogeneous surface, it is assumed instead that movement of the protein occurs predominantly by radial diffusion to very small, specific sites. These sites are presumed to facilitate very rapid electron transfer at the reversible potential while the rest of the surface remains inactive. Thus, the modified electrode surface behaves like an array of microelectrodes. If this theory is used to treat previous data, much higher electron-transfer rate constants are obtained. Although this theory deserves more detailed scrutiny, it may serve... 

Computer-brain interfaces can work two ways. Cochlear implants have been developed to detect sound with an external microphone and relay the electrical signal to electrode arrays that directly stimulate inner ear nerve fibers. A visual prosthesis promises to similarly help the blind by applying electrical signals from a camera to an array of microelectrodes implanted into the visual cortex of the brain. Electrical signals from the brain can be used to control prosthetic limbs, computer software, or robots. Electrodes implanted into the pleasure centers of the brains of rats have been used to train rats to respond to investigators commands. 

Plate electrode with (a) array of holes and (b) U-shaped hole, (c) array of microelectrodes and (d) U-shaped microelectrode fabricated by reverse EDM [10],... 

If we approximate each of the inert blocking particles as being discshaped and of the same size, the modelling of a PBE is only very slightly different from modelling a random array of microelectrodes. In the latter case we considered an array of electroactive discs on an inert surface whereas for a PBE we consider an array of inert discs on an electroactive surface. The simple solution then is to use exactly the same simulation model as for the random array of microdiscs except that the surface boundary conditions... 

A recent experimental system consisted of an electroactive layer deposited across an array of microelectrodes, each of which was addressable. A finite-... 

FIGURE 6.12. Discrete model (not drawn to scale) of the electrode array solution system, where the reduced form B and the oxidized form A species diffuse into adjacent boxes that may partition between the polymer and solution phases. In this case the array of microelectrodes are not used. [Reprinted with permission from I. Fritsch-Faules and L. R. Faulkner, J. Electroanal. Ghent. 331,997 (1992).]... 

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