Microelectrode structure

With increasing miniaturisation, the area of electrodes in contact with the solution becomes smaller and its behaviour becomes more capacitive. Microelectrode structures typically have a resonant frequency [58]. Usually, this occurs at hundreds of MHz. While this is above the frequency of most generators it can accentuate harmonics when square wave drive is used. Also, by the addition of... 

Ramos A, Morgan H, Green NG, Castellanos A (1998) AC electrokinetics a review of forces in microelectrode structures. J Phys D 31 2338-2353... 

Fedkiw Jr PS. Preparing in situ electrocatalytic films in solid polymer electrolyte membranes, composite microelectrode structures produced thereby and chloralkali process utilizing the same. United States patent US 4959132. 1990 Sep 25. 

Other similar cyclic structures may present quite unexpected behaviour. Let us give the example of 46, where X is Cl or Br. Such structures are very easily reduced44 (polished platinum microelectrodes are preferred owing to the reaction of mercury with C—X linkages), and the presence of an anion radical of some stability can be demonstrated in the... 

To deposit Au structures, a Au probe is approached to the surface until a positive feedback is observed. This is due to the regeneration of Cl species on the substrate while Au is deposited from AUCI4 according to the reverse reaction, leading to an increase in the local concentration of Cl. The microelectrode is then left at this position above the substrate for a certain time, after which it is withdrawn from the surface. The potential of the substrate, the electrolyte, and the pH were found to be the most significant parameters determining in determining the rate of Au electrodeposition and its structure (Amman and Mandler, 2001). 

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

Interest in microelectrodes, in vivo analysis, and carbon-reinforced structural materials has stimulated research on the electrochemical behavior of carbon fibers. Such fibers have diameters ranging from a few micrometers to about 60 pm, with the majority in the range of 5-15 pm. Although carbon fibers have a wide variety of structures and properties and are often less well characterized than GC or graphite, they have been used successfully in several important electroanalytical experiments. 

The magnitude of the ohmic drop at a microelectrode can be evaluated quite readily for case 1 from a knowledge of the specific solution resistance (obtained from conductivity measurements such as in Table 12.1) and the expressions for the voltammetric current for the specific microelectrode employed. Case 2 is also straightforward if the free concentration of ions exceeds that of the electroactive species. However, the situation is somewhat more complicated for the third class. In this case, and in case 2 for fully associated electrolyte, migration as well as diffusion can affect the observed voltammetric signals. In all three cases, the situation may be further complicated by a change in structure of the double layer. However, this is ignored for now, and is considered in the section on very small electrodes. 

Theoretically, electrical patterning is one the simplest method to structure materials since they can be patterned directly on the surface of an electrode. Creating conducting microelectrodes is, nowadays, fast and simple using micro and nanotechnology tools. Deposition and etching, or deposition followed by lift-off, are the conventional methods [35], Other solutions based on electrodeposition of metals... 

The actual retina contact structure incorporates 12 or 24 independent electrodes, respectively. The electrodes were arranged concentrically to minimize the electrical stray field during stimulation. We established the microfabrication process for double metallization layers needed to obtain concentric microelectrodes. In a temper step, the electrodes were formed into a convex shape according to the curvature of the eye. The generation of convex shapes was possible since the stimulator was designed in concentric rings interconnected by s-shaped bridges (Fig. 26). 

Future work will focus on real three-dimensional electrodes that may slowly penetrate the superficial layer of the retina. We hope to improve the spatial selectivity of a stimulator structure and to lower the energy consumption during stimulation, when the microelectrode is in close proximity to the somata of the ganglion cells. A possible design of this structure is shown in Fig. 27. It demonstrates the design potentials that microfabrication of polymer based microstructure offer. 

Conducting carbon polymer ink, which filled a UV-ablated microchannel, was used to construct the integrated microelectrode on a plastic chip. Both chronoamperometry and CV were employed to detect a model compound (fer-rocenecarboxylic acid) down to 3 iM, corresponding to 0.4 fmol within a volume 120 pL [758], In another report, a carbon-paste electrode was constructed by filling a laser-ablated (PET or PC) channel with C ink. The whole structure was then cured at 70°C for 2 h [189]. 

The strong and specific biotin-streptavidin binding was used to assemble biomolecule-functionalized nanoparticles in multilayered structures.67 Application of an electrical field allowed the assembly of multilayer structures by using extremely low concentrations of nanoparticles with minimal nonspecific binding. A microelectrode array was used to facilitate the rapid parallel electrophoretic transport and binding of biotin- and streptavidin-functionalized fluorescent nanoparticles to specific sites. By controlling the current, voltage, and activation time at each nanoparticle adsorption step, the directed assembly of more than 50 layers of nanoparticles was accomplished within an hour. 

Figure 5.20 Cyclic voltammogram of a spontaneously adsorbed [Ru(bpy)2Qbpy]2+ monolayer, obtained at a scan rate of 1 V s-1 the surface coverage is 1.04 x 10 10 mol cm 2. The supporting electrolyte is 0.1 M TBABF4 in acetonitrile, with the radius of the platinum microelectrode being 25 pm. The cathodic currents are shown as up, while the anodic currents are shown as down. The complex is in the 2+ form between approximately +1 to —1 V. The inset shows the structure of the surface active complex. Reprinted with permission from R. J. Forster and T. E. Keyes, /. Phys. Chem., B, 102,10004 (1998). Copyright (1998) American Chemical Society...

---