Microelectrodes features

Two different microelectrode arrays are presented the first array consists of 100 parallel connected ultramicroelectrodes. Counter as well as reference electrodes are integrated on the same chip. Although still exhibiting microelectrode features, this array yields analytical currents very similar to those of macroelectrodes. The second array consists of 400 individually addressable microelectrodes. It enables redundant as well as multianalyte measurements. Here, we demonstrate the electrochemical imaging of two-dimensional distributions of ammonium chloride as well as urea. 

Microelectrodes are well-established instruments in medical science and diagnosis. Due to their small size, they are always used when bulky instruments fail. However, the current of a single microelectrode is very small. For that reason, we designed microelectrode arrays in silicon thin-film technology combining large overall currents with typical microelectrode features [1]. 

It is a typical feature of the diffusion processes at electrodes of small size, which are reached by converging diffusion fluxes, that a steady state can be attained even without convection (e.g., in gelled solutions). Such electrodes, which have dimensions comparable to typical values of 8, are called microelectrodes. 

An optically transparent, porous platinum film has been produced by photoelectrodeposition on an InP semiconductor substrate [15], Polyester sheet covered with a thin film of sputtered gold has also proved suitable as an OTE [71]. When overcoated with a layer of Ti02, these electrodes exhibited electrochemical behavior consistent with a microelectrode array, including cyclic voltammetric current plateaus instead of clearly defined peaks, although this feature was not recognized at the time [71]. 

Another important feature is the relative size of the diffusion layer with respect to the double layer. The diffusion-layer dimensions are proportional to the size of the electrode, and can approach the size of the double layer with electrodes of molecular dimensions [95]. This can also occur in other situations explored with microelectrodes. For example, in solutions of very dilute electrolyte, the diffuse double layer extends several nanometers into solution [62]. Alternatively, very fast cyclic voltammetry results in a very small diffusion layer, which may be of dimensions similar to the double layer [46]. In all these situ-... 

Data at smaller scan rates and higher temperatures allowed determination of the rate constant for cleavage of nitrite from the radical anion of this and other v/c-dinitro compounds. In addition to the use of microelectrodes to reduce iR drop, another interesting feature of this study was the observation of the effect... 

The current for a reversible EE mechanism can achieve a stationary feature when microelectrodes are used since in these conditions the function fG(t, qa) that appears in Eq. (3.150) transforms into fG,micro given in Table 2.3 of Sect. 2.6. For microelectrode geometries for which fo.micro is constant, the current-potential responses have a stationary character, which for microdiscs and microspheres can be written as [16] ... 

This unusual feature in RPV response is more apparent the longer the duration of the second pulse (see Fig. 4.14a) and it is promoted by large electrodes, so that the greatest peak is obtained at planar electrodes whereas it is not observed at microelectrodes (see Fig. 4.14d). 

As is well known, the steady-state behavior of (spherical and disc) microelectrodes enables the generation of a unique current-potential relationship since the response is independent of the time or frequency variables [43]. This feature allows us to obtain identical I-E responses, independently of the electrochemical technique, when a voltammogram is generated by applying a linear sweep or a sequence of discrete potential steps, or a periodic potential. From the above, it can also be expected that the same behavior will be obtained under chronopotentiometric conditions when any current time function I(t) is applied, i.e., the steady-state I(t) —E curve (with E being the measured potential) will be identical to the voltammogram obtained under controlled potential-time conditions [44, 45]. 

Attractive features of microelectrodes relative to conventionally sized electrodes include increased current density, reduced charging currents and reduced ohmic drop (see Section 2). The last of these permits experiments to... 

The size of the working electrode is also important. This will influence the electropolymerization process in that the conductivity decrease during deposition can be minimized also, depletion effects are more pronounced with large electrodes. With smaller (< 20 pm diameter) electrodes, electropolymerization can be carried out in low-conductivity media also, the rate of transport to and from the electroactive center is markedly enhanced. In some cases, this latter feature can be a problem in that the enhanced rate of transport away from the microelectrode causes increased difficulty in obtaining a polymer deposit. 

A detailed discussion of the foregoing electrokinetic model as well as simplified versions of Eqs. (5.8) and (5.9) pertaining to the case of mass transfer controlled as well as irreversible electrode reactions are given in a recent paper by McLendon et al.I94) Suffice it here to evoke some general features concerning the tuning of the potential of the microparticle by the two redox couples. In the case where both redox couples are reversible on the microelectrode in question (k - < , k °°) the particle potential will lie approximately in the middle between the Nernst potentials E and E . If E and E are sufficiently separated, the reaction current will be high and approach the diffusion controlled limit. If, on the other hand, one couple is reversible while the other is not, the particle potential will remain near to the Nernst potential of the reversible couple. 

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