Microband electrodes

OPTIMISATION OF MICROBAND ELECTRODE SIZES AND LOCATIONS WITHIN A RECTANGULAR MICROFLUIDIC CHANNEL FOR ELECTROCHEMICAL MONITORING OF HYDRODYNAMIC FLOW PROFILES... 

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.)... 

C.X. Cai, H.X. Ju, and H.Y. Chen, Cobalt hexacyanoferrate-modified microband gold electrode and its electrocatalytic activity for oxidation of NADH. J. Electroanal. Chem. 397,185-190 (1995). 

L. Authier, C. Grossiord, P. Brossier, and B. Limoges, Gold nanoparticle-based quantitative electrochemical detection of amplified human cytomegalovirus DNA using disposable microband electrodes. Anal. Chem. 73, 4450-4456 (2001). 

The oxidation of sulfite and thiosulfate becomes facile in the presence of iodide and novel disposable microband sensor electrodes have been developed by Williams and coworkers [187] to allow fast amperometric determination. A similar approach was proposed for the determination of sulfite in wine [188]. In this method, a coulometric titration is carried out in which S(IV) is indirectly oxidized to S(VI). Speciation of SO2 and sulfite was achieved down to micromolar levels. Sulfide and hydrogen sulfide can be determined elec-trochemically in the presence of an iodide mediator [189]. This process may be further enhanced at elevated temperatures. 

Actually, there are reports on transforming the negative effect of separation electric field into new detection approaches. A potentiostat-less detection scheme for amperometric detection in CE based on the use of microband array electrodes situated in the CE electric field has been proposed [53] as well as the use of an indirect amperometric detection with a carbon fibre in-channel configuration [54]. In this case, the potential difference induced by the CE separation electric field produces a change in the reduction potential of oxygen, which can be used to determine nonelectroactive analytes. 

SPMBE = screen-printed microband electrode, ASV = anodic stripping voltammetry, HCMV = human cytomegalovirus, PGE = pencil-graphite electrode, DPV = differential pulse voltammetry, SPEs = screen-printed electrodes, PSA = potentiometric stripping analysis, M-GECE = magnetic graphite-epoxy composite electrode. 

He, Y. and H.K. Lee. 1997. Application of a 32-microband electrode array detection system for liquid chromatography analysis. Anal. Chem. 69 4634-4640. 

In PDMS chips, metal electrodes cannot easily be made on PDMS because of its pliable nature. So the Au/Cr microband electrodes have first been formed on a glass plate, which was then aligned and sealed with a PDMS channel plate [748],... 

SECM employs an UME probe (tip) to induce chemical changes and collect electrochemical information while approaching or scanning the surface of interest (substrate). The substrate may also be biased and serve as the second working electrode. The nature of the tip and the way it interacts with the substrate determine what information can be obtained in an SECM experiment. Many different types of UMEs have been fabricated, for example, microband electrodes, cylindrical electrodes, microrings, disk-shaped, and hemispherical electrodes [10, 11]. For reasons discussed below, the disk geometry is preferred... 

It was soon realised that at least unequal intervals, crowded closely around the UMDE edge, might help with accuracy, and Heinze was the first to use these in 1986 [300], as well as Bard and coworkers [71] in the same year. Taylor followed in 1990 [545]. Real Crank-Nicolson was used in 1996 [138], in a brute force manner, meaning that the linear system was simply solved by LU decomposition, ignoring the sparse nature of the system. More on this below. The ultimate unequal intervals technique is adaptive FEM, and this too has been tried, beginning with Nann [407] and Nann and Heinze [408,409], and followed more recently by a series of papers by Harriman et al. [287,288,289, 290,291,292,293], some of which studies concern microband electrodes and recessed UMDEs. One might think that FEM would make possible the use of very few sample points in the simulation space however, as an example, Harriman et al. [292] used up to about 2000 nodes in their work. This is similar to the number of points one needs to use with conformal mapping and multi-point approximations in finite difference methods, for similar accuracy. 

The ways to simulate our chosen example, the UMDE, are described here. The integral equation approach, taken by Coen and coworkers over a number of years [167,176,177,178, 179, 180, 219] for microband electrodes, can be used on the UMDE as well [179], The reader is referred to these papers for the method. Also, although the adaptive FEM approach might be thought to be about the most efficient, and has been developed by a few workers (see above, references to Nann and Heinze, and Harriman et al), it does not seem the most popular it is not trivial to program, and as Harriman et al. found, it appears that a rather large number of nodes were required. The reason is probably that this is a kind of discretisation in the original cylindrical (A, Z)... 

The derivation leading to equation (10.14) assumes that mass transport to the electrode by diffusion occurs only normal to the surface. This is not true at the edge of the electrode, where axial diffusional rates can be quite high. The effect becomes increasingly evident when the electrode behaviour is dominated by the edges. For a channel microband electrode this is evident with short length electrodes and low solution velocities [23-25]. For an UME (i.e., an electrode where at least one dimension is comparable to 5D) implanted in the wall of a flow channel, when the... 

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... 

Fig. 10.6. Schematic diagrams of a microband electrode prepared by screen-printing gold onto an alumina substrate, over-printing with an insulator and then snapping to expose a fresh line electrode (Reference [33]). Substrate (1) 500 p.m thick gold (2) 10 im thick insulator (3) 20 p.m thick, (a) Cross section showing the different layers (b) cross-section of the exposed surface at the snap line (c) scheme of oscillation.

The periodic asymmetry of the signal, (a small and a large peak which regularly alternate with the period of the motion—Fig. 10.8(a) is due to the geometry of the fabricated microband electrodes, as shown in Fig. 10.6. On one side of the electrode there is only a 20-40 pm thick layer of insulator while on the other this layer is 500 pm thick (the ceramic substrate). It has already been stated in Section 10.3.2 and shown in Fig. 10.1 that for flow impinging parallel to a wall in which an electrode is embedded, SH over the electrode will differ depending on the distance of the electrode... 

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. 

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