Microelectrodes diffusion, enhancement

Tallman et al. (1984) presented data to suggest that microelectrodes further enhance response by remixing between electroactive zones on the surface. As discussed earlier, in the case of continuous planar electrodes, the diffusion layer increasingly extends out in the cell volume as flow moves down the length of the electrode. In a microarray. significant areas are inactive, and diffusion layers dwindle in thickness over these relaxation zones before moving on to the next active area. The net effect is to maximize the concentration gradient over the entire macroscopic electrode area. This... 

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]. 

What happens when the dimensions are furthermore reduced Initially, an enhanced diffusive mass transport would be expected. That is true, until the critical dimension is comparable to the thickness of the electrical double layer or the molecular size (a few nanometers) [7,8]. In this case, diffusive mass transport occurs mainly across the electrical double layer where the characteristics (electrical field, ion solvent interaction, viscosity, density, etc.) are different from those of the bulk solution. An important change is that the assumption of electroneutrality and lack of electromigration mass transport is not appropriate, regardless of the electrolyte concentration [9]. Therefore, there are subtle differences between the microelectrodic and nanoelectrodic behaviour. 

Fig. 16.8. Schematic effect of substrate on microelectrode tip response in SECM (a) nonconducting substrate hindered diffusion and reduction in current (b) conducting substrate recycling (positive feedback) and current enhancement (c) conducting substrate recycling with current enhancement attenuated by slow electrode kinetics at substrate electrode.

Decreasing the size of the electrode so that the rate of radial diffusion of material to the electrode surface is enhanced as is the case for microelectrodes. 

We have considered so far only disc-shaped microelectrodes, for which spherical diffusion can be applied, to a good approximation. Other forms have been used, mainly because they might be easier to fabricate. Most noted among these is the linear or strip niicroelec-trode, which is macroscopic in length but microscopic in width. The diffusion field at such electrodes can be approximated. satisfactorily by diffusion to a cylinder. The enhancement of diffusion is less than that... 

In electroanalysis, electrodes of millimeter dimensions are termed millielec-trodes, while the more recently developed very small area electrodes of micron dimensions are termed microelectrodes there are differences in properties beyond simply the change of dimension. Thus in millielectrode-scale experiments the enhancement of the diffusion-limited current plateau has been observed by a number of other workers—for example, in the reduction of methylviologen in aqueous acetonitrile [32], in the oxidation of bis(cyclopentadienyl) molybdenum dichloride in acetonitrile [33], as well as in several other studies on the aqueous ferrocyanide/ferricyanide couple using wire or disc millielectrodes to study diffu-sional phenomena [34—36], Typical values of the diffusion layer thickness of approximately 5 pm are found under ultrasound [35] in contrast to the normal value of approximately 500 pm in silent conditions. 

Amperometric electrodes made on a microscale, on the order of 5 to 30 /rm diameter possess a number of advantages. The electrode is smaller than the diffusion layer thickness. This results in enhanced mass transport that is independent of flow, and an increased signal-to-noise ratio, and electrochemical measurements can be made in high-resistance media, such as nonaqueous solvents. An S-shaped sigmoid current-voltage curve is recorded in a quiet solution instead of a peak shaped curve because of the independence on the diffusion layer. The hmiting current, q, of such microelectrodes is given by... 

Given an appropriate system of potential control and current measurement (or vice versa), microelectrode experiments are essentially identical to those made using conventionally sized electrodes. All aspects of cell design, 02 removal etc., are therefore normal. Adequate cleaning and solvent/elec-trolyte purification is, of couse, essential and in fact may be rather more important than with conventional electrodes. The enhanced diffusion to a microelectrode applies also to the impurities, which can therefore build up rapidly near the electrode and, in view of the small electrode size, it can readily be blocked. Having seen how to construct microelectrodes and how to make measurements with them, we will now consider some applications. 

Microelectrodes exhibit faster response, reduced iR-drop as a consequence of reduced capacitance, and lower currents. However, owing to the hemispherical diffusion profile in front of the microelectrode the mass transfer is enhanced leading to a higher current density and an improved signal-to-noise ratio (see Chapter 2.5). They not only permit measurements in small volumes and very low concentrations but additionally in samples with low conductivity. Because of the reduced size of miniaturized sensors, smaller amounts of rare and expensive... 

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