Microelectrodes insulating

Loeb GE, Bak Ml, Salcman M et al. (1977) Parylene as a chronictilly stable, reproducible microelectrode insulator, IEEE Trans. Biomed. Eng., 24 (2) 121-128... 

Recently, several molecule-based microelectrochemical devices have been developed by the Wrighton group.(14.15.21-22) A microelectrode array coated with poly(I) results in a microelectrochemical transistor with the unique characteristic that shows "turn on" in two gate potential, Vq, regimes, one associated with the polythiophene switching from an insulator to a conductor upon oxidation and one associated with the v2+ + conventional redox centers. 

For the sake of completeness, glass microelectrodes [48, 59, 184] will first be mentioned. Two types of these electrodes are used, spear-shaped microelectrodes [59] and recessed-tip microelectrodes [165] (see fig. 4.5). In the former case, the microelectrode is drawn from a capillary of an ion-exchanger glass and is insulated on the outside, except for the tip, by inserting the microelectrode into a micropipette made of an inactive glass. In the latter case, the outer micropipette extends over the microelectrode tip. The two capillaries are sealed together and the ISM is in contact with the liquid between the two capillaries. 

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,... 

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-... 

Double- and triple-band electrodes have been constructed in a similar fashion by placing thin insulating films (e.g., Mylar) between the metal foils. These have been used in generator-collector experiments, in which one electrode (the collector) is used to monitor a product formed at the other electrode (the generator). This can be considered the microelectrode equivalent of the rotated ring-... 

There is, of course, a limit on the amount of solution resistance that can be overcome even with microelectrodes. Attempts have been made to perform electrochemistry in the gas phase [79] or in supercritical C02 180] with microelectrodes. In each case, however, the conduction path was shown to be not through the bulk phase, but rather across the insulating surface between the microelectrode and the counter electrode. This mechanism enables electrochemical detection in highly unusual media for voltammetry and illustrates that only very small conduction pathways are required to obtain well-defined electrochemical behavior. 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Fig. 15.1. Schematic of sonochemical microelectrode formation (a) formation of the insulating layer on the electrode surface, (b) sonochemical ablation leading to formation of microelectrode pores, (c) electropolymerisation of aniline and AChE at the pores to form enzyme microelectrodes.

A microelectrode array consists of a series of microelectrodes separated by an insulating material [36]. The microelectrodes can be regularly or randomly distributed (in the latter case the term ensemble is also used). Arrays containing hundreds or even thousands of microelectrodes wired in parallel have been... 

As illustrated in Figure 3.2, when the microelectrode is distant from the surface by several electrode diameters, a steady-state current, ij., is observed at the tip. The magnitude of the current is the same as that observed for a microdisk in a conventional experiment. When the tip is near a surface, the tip current, ij, differs from ij.oc, and depends on both the distance between the surface and tip, and the chemical nature of the surface. If the interfacial assembly efficiently blocks electron transfer, i.e. it is an electronic insulator, the mediator will not be regenerated, thus causing to be less than unity. If the Red species becomes re-oxidized at... 

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. 

Contact electrodes insulated from the sample prevent a dc current between the current feed electrodes and therefore also facilitate reasonable dc microelectrode measurements. However, in this case, impedance measurements can be impeded by capacitive currents across the insulation layer between the large contacts and the sample. 

Fig. 12.16. Schematic effect of substrate type on microelectrode tip response (a) insulating—current reduction (b) conducting—current enhancement by...

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