Microdisc array

Fig. 3 Schematic illustration of a microfabricated multi-element array A comprising 32 interdigitated microsensor electrodes, and B comprising 64 independently addressable microdisc voltametric electrodes. Each device shows the large area counter electrode (middle) and the reference electrode as a band around the counter electrode...

For shallow recessed microdisc electrode arrays, the hemispherical diffusion is larger than that for coplanar microdisc arrays. The minimum interelectrode distance necessary for hemispherical diffusion becomes smaller as recess depth increases [58],... 

Anion detection at microelectrodes has not been studied widely. Amongst the first was the work of de Beer et al. [ 111 ] who manufactured a nitrite sensor with a tip just a few microns in diameter, which could detect nitrite ions down to 1 pM. This proved to be suitable for profiling the concentrations of nitrite anion within biofilms less than 1-mm thick inside water treatment plants. Other workers have found that use of an interdigitated microelectrode array [ 112] allows measurement of iodide via monitoring of its redox peak down to sub-micromole levels, making it a suitable technique for analysing mineral water. Carbon nanotubes coated onto Pt microdiscs have been utilised to make a nitrite sensor [113,114] with detection levels of 0.1 pM. Sulphide has also been detected at nickel microdiscs (50 pm diameter) [115]. 

Figure 11.3 exhibits few examples of microelectrodes such as (a) microdisc electrode with diameter of 10 pm within a glass tube of 20 pm for amperometric measurements [5], (b) STM-Tip, which can measure with resolution in atomic dimension, (c) pH-sensitive microelectrode used in biological research, (d) microelectrode array for research [14], and (f) electrode array for glucose... 

A number of methods exist for fabricating microelectrode arrays [6] and a variety of array geometries are encountered with the most common being arrays of microdiscs and arrays of microbands. Microdiscs are most frequently arranged as a regularly distributed (i.e., a square or... 

In Chapter 9, we studied the problem of a single electroactive microdisc on an infinite supporting surface. Here we consider the situation where an array of such microdiscs are embedded in a surface in a regular distribution as illustrated in Figure 10.1. It is assumed that electroactivity only occurs at the microdisc electrodes, not on the supporting surface. 

Fig. 10.2. Hexagonal array of microdisc electrodes with centre-to-centre separation, d.

As with the isolated microdisc simulations in Chapter 9, we here consider the simulation of the cyclic voltammetry of a simple fully reversible one-electron reduction. For an array, since each unit cell is identical, the concentrations of the electroactive species will necessarily be the same on either side of the cell boundary and there can be no flux of electroactive material across the boundary. After using the diffusion domain approximation, this boundary is at a distance r = Vd, therefore... 

Apart from this change in boundary condition, the simulation space for a microdisc array is exactly the same as that used for the single microdisc, depicted in Figure 9.3. 

The simulation procedure for a microdisc array is almost identical to that developed using the ADI method for a single microdisc in Chapter 9 there are only two significant changes. First, as we have discussed, is the imposition of the diffusion domain boundary at r = rd (where before we had a bulk concentration boundary condition at r = Tmax)- In discrete form, this condition is represented as = 0 this is implemented by... 

It is useful at this point to examine the diffusional and voltammetric behaviour of a microdisc array as it varies with the surface coverage (or equivalently with the disc-to-disc separation) and disc size. It is observed that... 

Fig. 10.4. The four limiting cases of diffusional behaviour to an array of microdisc electrodes.

We now turn our attention to randomly distributed arrays of microdisc electrodes as illustrated in Figure 10.5 though they are not as commonly encountered as regularly distributed microdisc arrays, techniques do exist for their fabrication [22] and so here we consider the simulation of such arrays. Though the specific example of a randomly distributed microdisc array is of limited utility, the techniques for generating a random distribution of particles are applicable to a range of electrochemical problems. 

If we approximate each of the inert blocking particles as being discshaped and of the same size, the modelling of a PBE is only very slightly different from modelling a random array of microelectrodes. In the latter case we considered an array of electroactive discs on an inert surface whereas for a PBE we consider an array of inert discs on an electroactive surface. The simple solution then is to use exactly the same simulation model as for the random array of microdiscs except that the surface boundary conditions... 

Apart from this change in boundary condition, the simulation space for a microband array is exactly the same as for the single microband, depicted in Figure 9.8. As with the microdisc array, it is necessary to alter the spatial grid in the X-direction so that there is a high concentration of spatial points at this outer boundary to ensure accurate simulation. 

The unit cell and coordinates are illustrated in Figure 10.14(b). As with the array of microdiscs model, the unit cell is cylindrically symmetrical about an axis that passes through the centre of the pore, perpendicular to the electrode surface. The problem may thus be reduced from a three-dimensional one to a two-dimensional one. As with the microdisc electrode, this is a two-dimensional cylindrical polar coordinate system, and Pick s second law in this space is given by Eq. (9.6). The simulation space for the unit cell with its attendant boundary conditions is shown in Figure 10.15. 

T. J. Davies, S. Ward-Jones, C. E. Banks, J. del Campo, R. Mas, F. X. Mnnoz, and R. G. Compton. The cyclic and linear sweep voltammetry of regnlar arrays of microdisc electrodes Fitting of experimental data, J. Elec-troanal. Chem. 585, 51-62 (2005). 

The limits of independently addressable microbiosensors in an array have been explored on the basis of microdisc electrode arrays fabricated by thin-film technology. Cross-talk between the discrete enzyme-containing sensor elements was observed and the proximity limit was found to be about 100 (xm [261]. 

FIGURE 12.14 Schematic of various microfabricated electrode array devices manufectured by ABTECH Scientific, Inc. (A) Interdigitated microsensor electrodes (IMEs), (B) independently addressable microband electrodes (lAMEs), (C) independently addressable interdigitated microsensor electrodes (lAlMEs), (D) microdisc electrode array (MDEA), and (E) electrochemical cell-on-a-chip MDEA (ECC MDEA 5037). 

The diamond growth can also be patterned to produce microelectrode array structures [18,19]. Several possible microstructures are possible, such as microbands, microdiscs, and microcolumns. Micropyramids are another microstructure that can be produced, and an image of such an array is shown in Fig. 5. The SEM image reveals a monolithic diamond-tip array. The tips are ca. 2 pm in base diameter and are equally positioned over the surface with a spacing of ca. 5 pm. 

Lee HI, Beriet C, Ferrigno R, Girault HH (2001) Cyclic voltammetry at a regular microdisc electrode array. J Electroanal Chem 502 138-145... 

Davies TJ, Ward-Jones S, Banks CE, del Campo J, Mas R, Munoz FX, Compton RG (2(X)5) The cyclic and linear sweep voltammetry of regular arrays of microdisc electrodes fitting of experimental data. J Electroanal Chem 585 51-62... 

Davies TJ, Compton RG (2005) The cyclic tmd linetir sweep voltammetry of regular and random arrays of microdisc electrodes theory. J Electroanal Chem 585 63-82... 

In other electrode configurations, the electrodes, either a microdisc or an array of electrodes, can be recessed, that is, the electrode is not planar to the insulating material and consequently the diffusional profiles will quantitatively change [10] recessed electrodes are typically produced unintentionally when photolithography is used. In the case of an array of recessed microdiscs. 

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