Ultramicro electrode

Note that couples which show electrochemically reversible behavior at macro-electrodes may display quasi-reversibility or irreversibility at very small electrodes (ultramicro-electrodes, nano-electrodes)... 

SECM has also been used to deposit a gold microstructure on n-Si(lll) and indinm oxide. In these experiments, Au is dissolved anodically from the ultramicro electrode under constant potential according to the reaction... 

Ultramicro electrodes are electrodes of any shape with dimensions of the order of a few micrometers and possessing a different electrochemical behaviour compared with its millimetre shaped counterparts. The reason is a different mechanism for the transport of electroactive species towards the electrode (Fig. 1.10). For ultramicro electrodes, the main fraction of electroactive species is transport through a spherical diffusion, where it is a... 

Schematic representation of transport of electroactive species towards the surface of a micro (a) and an ultramicro (b) electrode. 

Despite much smaller current being recorded, the current density at ultramicro electrodes is much higher. 

Converting the small currents measured at ultramicro electrodes results in higher current densities compared with the values obtained at micro electrodes. This is also caused by the spherical diffusion path. This effect improves on the sensitivity and the detection limit for analytical purposes, and the small size of the electrodes in combination with application of high scan rates allow measurement to be performed in vivo and with a minimal (negligible) disturbance of the conditions of the system to be analysed. 

The current obtained at ultramicro disc electrodes is described in... 

The main disadvantage of ultramicro electrodes is the need to use equipment that is able to measure very small direct currents (nA) with high precision and low noise (pA-fA). This equipment is fairly expensive, and special precautions should be taken when it is used in an industrial envi-... 

Different methods are described to construct ultramicro electrodes48-62 such as insulation in glass or epoxy resin, the construction of ultramicro electrode arrays (an array of individual ultramicro electrodes to increase the current signal without losing the benefits of ultramicro electrode behaviour) by template synthesis, metal depositions in pores of membranes and by using the connecting wires in microchips. The latter has the advantage that each ultramicro electrode is individually addressable. 

Scheme of the construction of ultramicro electrodes according to a procedure described in the text. 

Before ultramicro electrodes became important in voltammetry, a supporting electrolyte always needed to be used and was of more importance than... 

For analytical purposes, the method can be used to obtain time-independent signals when a constant thickness of the diffusion layer is obtained by convection or using ultramicro electrode configurations. 

Preliminary experiments were performed to study the (electroactive) behaviour of styrene containing TRAP at a platinum ultramicro disc electrode, which was obtained as described in Chapter 1, pages 21-24. However, in the range where copper activity is observed (see later) no additional reactions of the solvent or the electrolyte were observed. Outside this potential window, oxidation of styrene and reduction of the electrolyte were observed. As these potential ranges are not interesting for the purpose of detection of Cu(II) and Cu(I), they are not further described in this chapter. 

Using ultramicroelectrodes, it is possible to study reactions under the conditions of synthesis, including electrosynthesis. An example is the electrohydrodimerisation of acrylonitrile to adiponitrile (Scheme 6.11, top) mentioned in the introduction in industry this is typically carried out with an emulsion of acrylonitrile in an aqueous phosphate buffer as electrolyte. At substrate concentrations in the mM level, the reduction of acrylonitrile takes another route leading to saturation of the C—C double bond (Scheme 6.11, bottom). This precludes studies of the dimerisation using substrate concentrations at the mM level and thereby working electrodes of conventional sizes. The transition between the two mechanisms could be studied conveniently using an ultramicro electrode as the working electrode... 

Cylindrical cells are easier to construct and thermostat than flat cells and dark-field illumination can be obtained by the ultramicro-scopic method of illuminating the sample perpendicular to the direction of observation (see page 52 and Figure 7.6). The volume of dispersion required is usually less for cylindrical cells than for flat cells and, owing to the relatively small cross-section, it is more often possible to use platinum black rather than reversible electrodes with cylindrical cells. However, unless the capillary wall is extremely thin, an optical correction must be made with cylindrical cells to allow for the focusing action of the tube, and optical distortion may prevent measurements from being made at the far stationary level. Cylindrical cells are unsatisfactory if any sedimentation takes place during the... 

Approach curve (SECM) — A current-distance curve recorded as an ultramicro electrode (tip) approaches... 

Nanoelectrodes in SECM — For the improvement of the lateral resolution in - scanning electrochemical microscopy (SECM) the size of the - ultramicro electrode... 

We should perhaps conclude this section by noting that the above considerations are valid for large electrodes, operating under semi-infinite linear diffusion conditions. If ultramicro electrodes are... 

When an ultramicro electrode is used as an eleclroanalytical tool, the diffusion-limited current density is not affected by the rate of... 

We assumed here a solution of medium conductivity, and yet arrived at an ohmic potential drop of less than 1 mV at the very large current density of 0.8 A/cm. Thus, using an ultramicro electrode extends the range of measurable current densities because (a) the limiting current density is inversely proportional to the radius and (b) the resistivity is proportional to the radius. More concisely, we could say that both the diffusion-limited current density and the conductivity are inversely proportional to the radius. 

Another simple calculation that will emphasize the advantage of using ultramicro electrodes refers to poorly conducting solutions. The specific resistivity of deionized water, for example, is about... 

Microelectrodes have many advantages over regular-sized electrodes, but they also have two major disadvantages. First, since the electrode is very small, the total current flowing in the circuit is minute and may be difficult to measure accurately. The ultramicro electrode just discussed has a total surface area of about 2x10 cm. For a current... 

Fig. I9L An ensemble of ultramicro electrodes. Note that the distance between the electrodes is large compared to their diameter.

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