Current microband

FIGURE 4-30 Cyclic voltammogram for ferrocene at a 3 pm width, 2 pm gap interdigitated microband (solid line). The dotted line represents the current of the collector electrode held at a potential of —0.1 V (Reproduced with permission from reference 95.)... 

In the case of microcyclinders and microbands, fG,micro is time dependent (Table 2.3) and only a pseudo-stationary response can be achieved. This is because all the microelectrode dimensions have to fall in the range of the microns to attain a true steady state. The expressions for the pseudo-stationary current-potential responses when the diffusion coefficients of species O and R fulfills Dq = Dr are ... 

Nevertheless, it is possible to obtain a constant relationship between the current at both microelectrodes for certain geometrical conditions. Thus, for microbands and microhemicylinders fulfilling rc = w/4, a constant ratio is obtained, but in this case it is necessary to use the same experimental timescale [10] ... 

The UMDE evinces strong edge effects or very uneven current densities along the radius of the disk. It shares this problem with all the other flat microelectrodes such as microbands or rings. The exception of course are the hemispherical or hemicylindrical microelectrodes, which have no effective edges and behave as half of a sphere or cylinder, and also deeply recessed disks or bands, which approach the shrouded types. 

The behaviour of a vibrating microband electrode [33] (Fig. 10.6) also illustrates these effects. The use of these devices in electroanalysis is described comprehensively in Section 10.4.2.2. Typical microband current (/)-potential (V) characteristics for the oxidation of 5.5 x 10-3 mol dm-3... 

A rather simple interpretation of the behaviour of vibrating electrodes can be obtained by considering the response to a square-wave motion, to which a sinusoid rather crudely approximates [33]. Here, it is considered that the concentration boundary layer is periodically renewed by the instantaneous rapid motion and that in the intervals between the square-wave steps the solution is at rest. This is a reasonable approximation for most practical purposes because the hydrodynamic boundary layer relaxation time is short, (Section 10.3.3). In this simple model, the waveform would instantaneously rise to a limit during the motion, decaying as a function of t m during the static phase. This decay rate will obviously be dependent on the size and geometry of the electrode wire, microwire, band or microband. If the delay time between steps were r then the mean current would vary as (l/r,)/o f 1/2df, i.e., as t, i/2 or as fm. 

If a stationary multiple microband electrode is used, then the collector current is rather sensitive to adventitious vibrations. If the electrode assembly is vibrated parallel to the inter-electrode gap, then although the collection efficiency is reduced the collector current is now insensitive to such random vibrations (of a non-modulatory nature). Repeatable, reliable titration using electrogenerated reagents has been demonstrated in this way [33]. 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

For many mechanisms, the steady-state Eia or N tt value is a function of just one or two dimensionless parameters. If simulations are used to generate the working curve (or surface) to a sufficiently high resolution, the experimental response may be interpolated for intermediate values without the need for further simulation. A free data analysis service has been set up (Alden and Compton, 1998) via the World-Wide-Web (htttp //physchem.ox.ac.uk 8000/wwwda/) based on this method. As new simulations are developed (e.g. for wall jet electrodes), the appropriate working surfaces are simulated and added to the system. It currently supports spherical, microdisc, rotating disc, channel and channel microband electrodes at which E, EC, EC2, ECE, EC2E, DISP 1, DISP 2 and EC processes may be analysed. 

Figure 4. (A) Current-time trace recorded upon introduction of [Fe(CN)6] , menadione and glucose to S. cerevisiae cells on an electrode microchip. (B) A silicon microchip for mediated amperometry (upper panel) and a microscope image of S. cerevisiae cells on a microband electrode (widtMength 25/1,000 pm). (Reprinted with permission from Ref. [8], 2009 Elsevier BV.) (Lower panel).

Figure 3. Effect of the enzymatic reaction with accompanying schemes on the collection current of a dual microband gold elec-...

Figure 4. Dependence of the collecting current on glucose concentration for a dual microband gold electrode of Wg p=82p.m at different DMHAE-ferrocene and GOD concentrations.

The current response of a microband electrode may be determined from ... 

The major drawback of the technology of coplanar interdigitated microband electrodes is related to the fabrication technique the electrode materials are restricted to those that can be screen-printed and the voltage drop in the thin layer electrodes can be important, leading to non uniform overvoltage and current distributions [19]. Finally, the specific electrode areas developed by the interdigitated design are smaller than those developed with simple plane electrodes. 

I is the current produced by the system consisting of m microband anodes/cathodes of the length b. is influenced by the width of bands, Wb, and the distance in gaps, Wg. It was shown [39] that the quasi-steady state may be achieved very quickly. The time t is proportional [40] to the quotient w /D, where w = Wb + Wg is the width of the unit system. 

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