Microelectrode array behavior

Microelectrode arrays can be classified according to the way the array is operating. 

If all of the microelectrodes in the array are polarized at the same potential, higher currents per unit area are possible (88). The signal-to-noise ratio of each individual electrode can be preserved and this so-called amplification effect is maintained as long as no overlap between individual diffusion layers occurs. Amplification remains effective at long times if the interelectrode spacings are at least 10 times larger than the radius of an individual microelectrode on the array. For a microelectrode of 5 pm radius, a minimal spacing of 50 pm would be necessary (88, 89). 

Identical or different potentials can be applied to each electrode in an individually addressable array allowing multiparametric analysis while avoiding diffusional cross-talk. The development of an individually addressable microelectrode array places sophisticated demands on microfabrication technology. The conventional way to individually address each electrode of an array is to connect an electrode line to a corresponding bonding pad 

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Preparation and Electrochemical Behavior of Microelectrodes Modified with Poly(I). A microelectrode array consisting of eight Pt microelectrodes is cleaned and platinized as described in the Experimental Section. Poly(I) is deposited on the platinized microelectrode array by scanning the potential of the electrode(s) from 0 V to 1.5 V vs. Ag+/Ag in a solution of 0.2 MX in CH3CN/O.I M [U-BU4N]PFg. The resulting array is illustrated in Scheme I. The... 

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]. 

A voltammetric experiment in a microelectrode array is highly dependent on the thickness of the individual diffusion layers, <5, compared with the size of the microelectrodes themselves, and with the interelectrode distance and the time experiment or the scan rate. In order to visualize the different behavior of the mass transport to a microelectrode array, simulated concentration profiles to spherical microelectrodes or particles calculated for different values of the parameter = fD Ja/r s can be seen in Fig. 5.17 [57] when the separation between centers of... 

H. P. Wu. Fabrication and characterization of a new class of microelectrode arrays exhibiting steady-state current behavior, Anal. Chem. 65, 1643-1646 (1993). 

The electrochemical behavior of redox molecules in polymer films and gels has been investigated [4,7,18,23,61-66], but such behavior has usually been studied by using a modified electrode coated with a polymer film or gel in the presence of an outer electrolyte solution. In a few examples, entirely solid-state voltammetry was also achieved, but by using a microelectrode array [65,66] composed of working, counter, and reference electrodes because of the slow ionic or molecular diffusion in the soHd matrices. The apparent diffusion coefficient (Dapp) of a redox substrate in the films or solids coated on an electrode was very small [4,7,18,23,62-66], usually of the order of less than 10 cm s Another example of solid state votammetry is a report on the electrochemistry of Prussian blue in silica sol-gel electrolytes [67], but only Pt gauze working and counter electrodes for a - 1 mm-thick silica solid were used. Moreover, it is well known that solid electrolytes have been used on various sensors, electro chromic devices, etc. [68,69]. However, in spite... 

Current research is also directed at decreasing the dimensions of the individual electrodes in the array in order to produce nanoelectrode arrays. In these nanoelectrode arrays, the critical dimension is decreased to the same order as the thickness of the electrical double layer or the molecular size of redox species, and the experimental behavior starts to deviate from extrapolations of behavior at larger electrodes. This point may be viewed as the separation point between nanoelectrodes and microelectrodes arrays (191). 

The so-called partially blocked electrodes containing active and passive sites can be also treated as microelectrode arrays, when the charge transfer is utterly impossible at the passive area. Simulations of their electrochemical behavior under linear potential sweep conditions showed [15] that a good agreement... 

Amatore et al. developed a theoretical framework to describe the electrochanical responses of ultramicroelectrode ensemble and NEEs by considering mass transport for assemblies of microdisk and microband electrodes. Lee et al. used finite element simulation to solve 3D diffusion equations and found that a collection of 10 pm diameter microdisk electrodes required a separation distance of more than 40R to exhibit a sigmoidal simulated CV response typical for radial diffusion. " CV response typical of reversible linear diffusion at macroelectrodes was observed when the separation distance was less than 6R . Assemblies of microelectrodes for which the separation distances were between 6R and 4QR exhibited peak-shaped simulated CVs indicative of a mixture of radial and linear diffusion behavior. Thus, 12/ seems to be too small a separation distance for the design of ideal microelectrode arrays. 

All these array geometries show peculiar electrochemical behavior which can be properly exploited for electroanalytical purposes. The goal of this chapter is to give an overview of the different types of micro- and nanoelectrodes arrays (ensembles, ordered, interdigitated, etc.) as well as a brief description of the main techniques used for their fabrication. Note that in most cases the fabrication of nanoelectrodes arrays requires more specialized fabrication techniques with respect to microelectrode arrays, and therefore they will be presented separately. Specific electrochemical properties of micro and nanoelectrodes arrays will be described in relation to the specific diffusion mechanisms observed in such electrochemical sensors. 

Fig. 20.13 Zone diagram of cyclic voltammetric behavior at microelectrode arrays, d is the center-to center distance of individual electrodes in the array (measured in units of a),...

In view of the above-mentioned points, the offset of category 2/onset of category 3 behavior (beginning overlap of diffusion zones) is a crucial criterion in the voltammetry of regular arrays of microparticles/microelectrodes. Usually, a linearly dependent expression on the microparticle disk radius for the size of the diffusion zones is given and used, for example, d> 20 Ri, [47]. Davies et al., in contrast, proposed a condition to ensure diffusion of category 2 behavior as follows [35] ... 

Microdisk array (MDA) electrodes of boron-doped diamond (BDD) were fabricated on structured silicon substrates. The BDD-MDA electrodes exhibited sigmoidal voltammetric curves, which show that they function as assemblies of single microelectrodes. The microelectrode behavior was also confirmed with biologically... 

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