Array microelectrode

1LEPMI, UMR CNRS/INPG/UJF 5631, ENSEEG, Institut National Polytechnique de Grenoble, Domaine Universitaire, 38042 Saint Martin 

The behavior of microelectrodes (radius smaller than 50 pm) differs from conventionalsized electrodes (radius 1 mm or greater) in that nonlinear diffusion is the predominant mode of transport. This difference in mass transport from the bulk solution toward the electrode has several important implications that make microelectrodes very attractive in many areas of electroanalytical chemistry. These include reduced ohmic potential drop, a decreased time constant, a fast establishment of steady-state signals, and an increased signal-to-noise ratio. 

Since the beginning of the 1980s (1,2), the development of microfabrication techniques has allowed these electrodes to become widely used even though the benefits of the properties of small electrodes was recognized much earlier. Microelectrodes have thus been employed for those applications demanding electrochemistry in restricted volumes, in solutions of high resistance as well as in short-time regimes (1,2). For a more detailed discussion about microelectrodes, see Chapter 6. 

It was around the same time that the first reports on microelectrode arrays appeared. Both experimental and theoretical works (3-27) have demonstrated the advantages of such electrode assemblies, which result from the specific mass transport of electroactive materials or diffusion regimes taking place at their interface. These include the following  

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Fig. 6. Molecular transistor based on a microelectrode array. P is a polymer layer that can be switched conductive or nonconductive by the potential of the gate electrode (from ref.

Both microstructured layer electrodes and microelectrode arrays have... 

Recently, there has been a growth of interest in the development of in vitro methods for measuring toxic effects of chemicals on the central nervous system. One approach has been to conduct electrophysiological measurements on slices of the hippocampus and other brain tissues (Noraberg 2004, Kohling et al. 2005). An example of this approach is the extracellular recording of evoked potentials from neocortical slices of rodents and humans (Kohling et al. 2005). This method, which employs a three-dimensional microelectrode array, can demonstrate a loss of evoked potential after treatment of brain tissue with the neurotoxin trimethyltin. Apart from the potential of in vitro methods such as this as biomarkers, there is considerable interest in the use of them as alternative methods in the risk assessment of chemicals, a point that will be returned to in Section 16.8. 

Kohhng, R., Melani, R., and Koch, U. et al. (2005). Detection of electrophysiological indicators of neurotoxicity in human and rat brain slices by a three dimensional microelectrode array. ATLA 33, 579-589. 

Armstrong FA, Bond AM, Buchi FN, Hanmett A, Hill HAO, Lannon AM, Lettington OC, Zoski CG. 1993. Electrocatalytic reduction of hydrogen-peroxide at a stationary pyrol3ftic-graphite electrode surface in the presence of cytochrome-c peroxidase— A description based on a microelectrode array model for adsorbed enzyme molecules. Analyst 118 973-978. 

FIGURE 1.8 (a) A representative SEM image of the microelectrode array and (b) a schematic representation of the experimental setup. (Reprinted with permission from Elsevier Publishing [124].)... 

FIGURE 10.7 A thin film planar pH microelectrode array with 16 sputtered IrOx electrodes in a 4 X 4 arrangement. The needle type pH electrode as a control can be seen on the upper left comer of the array. (Reproduced from [19], with permission from the Electrochemical Society, Inc.)... 

W.E. Morf and N.F. de Rooij, Performance of amperometric sensors based on multiple microelectrode arrays. Sens. Actuators B. 44, 538-541 (1997). 

C. Belmont, M.L. Tereier, J. Buffle, G. Fiaccabrino, and M. Koudeldahep, Mercury-plated iridium-based microelectrode arrays for trace metals detection by voltammetry optimum conditions and reliability. Anal. Chim. Acta 329, 203-214 (1996). 

In the case of a single electrode, however, the decrease of its dimensions requires the measurement of very low currents. To overcome this problem it is convenient to use microelectrode arrays [136, 137], Despite the fact that in such arrays microelectrodes are electronically connected to each other, analytical properties of such assemblies are advantageous over those of a conventional macro-electrode [138, 139],... 

Bard AJ, Crayston JA, Kittlesen GP, Shea TV, Wrighton MS (1986) Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays consequences arising from closely spaced ultramicroelectrodes. Anal Chem 58 2321-2331... 

Johnstone AF, Gross GW, Weiss DG, Schroeder OH, Gramowski A, Shafer TJ (2010) Microelectrode arrays a physiologically based neurotoxicity testing platform for the 21st century. Neurotoxicology 31 331-350... 

Recently, several molecule-based microelectrochemical devices have been developed by the Wrighton group.(14.15.21-22) A microelectrode array coated with poly(I) results in a microelectrochemical transistor with the unique characteristic that shows "turn on" in two gate potential, Vq, regimes, one associated with the polythiophene switching from an insulator to a conductor upon oxidation and one associated with the v2+ + conventional redox centers. 

Preparation of Microelectrode Arrays. The microelectrode arrays used in the work were arrays of microelectrodes each 80 pm long, 2.3pm wide and 0.1 pm thick and 3paced 1.7 pm apart. Fabrication and encapsulation of the microelectrode arrays has been described previously.<14.15.21-22) Prior to use, arrays of microelectrodes were cleaned by an rf 02 plasma etch to remove residual photoresist, followed by cycling the potential of each electrode between -1.5 V... 

Scheme I. Arrangements of platinized microelectrode arrays used for experimentation in this work.

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... 

Figure 3. Cyclic voltammetry of adjacent electrodes of a poly(I)-coated microelectrode array driven individually and together at 200 mV/s in the region of the oxidative potential of polythiophene in CH3CN/O.I II [11-BU4N] PFg.

The conductivity ofthe film was calculated for 30 monolayers. The film was deposited onto a Ag microelectrode array with a 1-mm distance between fingers. The thickness ofthe monolayer was taken to be 2 x 10"7 cm. For an air humidity value of 60% the conductivity equals 1.3 x 10"6 (Q/cm)-1 The current through the film has an ionic character, and there is apparently layered solid electrolyte... 

Figure 7.2. Dependence of the film conductivity on the number of LB monolayers deposited onto an AI microelectrode array with a 0.1-mm distance between fingers.

Pei, J., Tercier-Waeber, M.-L. and Buffle, J. (2000). Simultaneous determination and speciation of zinc, cadmium, lead and copper in natural waters with minimum handling and artefacts by voltammetry on gel-integrated microelectrode arrays, Anal. Chem., 72, 161-171. 

The physical approach uses alternating current (ac-) dielectrophoresis to separate metallic and semiconducting SWCNTs in a single step without the need for chemical modifications [101]. The difference in dielectric constant between the two types of SWCNTs results in an opposite movement along an electric field gradient between two electrodes. This leads to the deposition of metallic nanotubes on the microelectrode array, while semiconducting CNTs remain in the solution and are flushed out of the system. Drawbacks of this separation technique are the formation of mixed bundles of CNTs due to insufficient dispersion and difficulties in up-scaling the process [102]. 

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