Electrodes ultramicroelectrode

The influence of the nonlinearity of diffusion on the observed complex plane plots is shown in Fig. 13. Spherical mass transfer causes the formation of a depressed semicircle at low frequencies instead of the linear behavior observed for linear semi-infinite diffusion. For very small electrodes (ultramicroelectrodes) or low frequencies, the mass-transfer impedances become negligible and the dc current becomes stationary. On the Bode phase-angle graph, a maximum is observed at low frequencies. 

Aqueous diffusion coefficients are usually on the order of 5 x 10 cm /s. A second is typically a long time to an electrochemist, so 6 = 30 fim. The definition of far is then 30 ]lni. Short is less than a second, perhaps a few milliseconds. Microseconds are not uncommon. Small, referring to the diameter of the electrode, is about a millimeter for microelectrodes, or perhaps only a few micrometers for ultramicroelectrodes (13). 

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

Carbon-Fiber Electrodes The growing interest in ultramicroelectrodes (Section 4-5.4) has led to widespread use of carbon fibers in electroanalysis. Such materials are produced, mainly in connection with the preparation of high-strength composites, by high-temperature pyrolysis of polymer textiles or via... 

The measurement of the electrode impedance has also been ealled Faradaie impedanee method. Since measurements are possible by applying either an electrode potential modulated by an AC voltage of discrete frequeney (which is varied subsequently) or by applying a mix of frequencies (pink noise, white noise) followed by Fourier transform analysis, the former method is sometimes called AC impedance method. The optimization of this method for the use with ultramicroelectrodes has been described [91Barl]. (Data obtained with these methods are labelled IP.)... 

Electrode processes are often studied under steady-state conditions, for example at a rotating disk electrode or at a ultramicroelectrode. Polarog-raphy with dropping electrode where average currents during the droptime are often measured shows similar features as steady-state methods. The distribution of the concentrations of the oxidized and reduced forms at the surface of the electrode under steady-state conditions is shown in Fig. 5.12. For the current density we have (cf. Eq. (2.7.13))... 

The ohmic potential difference in an electrolytic cell consisting of a spherical test electrode, termed, for a small radius r0, ultramicroelectrode, in the centre and another very distant concentrical counter-electrode is given by the equation... 

Obviously, the ohmic potential difference does not depend on the distance of the counterelectrode (if, of course, this is sufficiently apart) being situated mainly in the neighbourhood of the ultramicroelectrode. At constant current density it is proportional to its radius. Thus, with decreasing the radius of the electrode the ohmic potential decreases which is one of the main advantages of the ultramicroelectrode, as it makes possible its use in media of rather low conductivity, as, for example, in low permittivity solvents and at very low temperatures. This property is not restricted to spherical electrodes but also other electrodes with a small characteristic dimension like microdisk electrodes behave in the same way. 

The kinetic investigation requires, as already stated in Section 5.1, page 252, a three-electrode system in order to programme the magnitude of the potential of the working electrode, which is of interest, or to record its changes caused by flow of controlled current (the ultramicroelectrode is an exception where a two-electrode system is sufficient). 

Fig. 5.19 Electrodes used in voltammetry. A—dropping mercury electrode (DME). R denotes the reservoir filled with mercury and connected by a plastic tube to the glass capillary at the tip of which the mercury drop is formed. B—ultramicroelectrode (UME). The actual electrode is the microdisk at the tip of a Wollaston wire (a material often used for UME) sealed in the glass tube...

The UMEs used in bioarrays can be divided into three types disk, ring, and strip electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied [97-100], Due to the edge effect, the profile of the mass diffusion to the ultramicroelectrode surface is three dimensional, and can significantly enhance the mass transportation in comparison to the conventional large electrode with one-dimensional mass transportation. The steady-state measurement at a planar UME can be expressed as... 

Diffusion of electroactive species to the surface of conventional disk (macro-) electrodes is mainly planar. When the electrode diameter is decreased the edge effects of hemi-spherical diffusion become significant. In 1964 Lingane derived the corrective term bearing in mind the edge effects for the Cotrell equation [129, 130], confirmed later on analytically and by numerical calculation [131,132], In the case of ultramicroelectrodes this term becomes dominant, which makes steady-state current proportional to the electrode radius [133-135], Since capacitive and other diffusion-unrelated currents are proportional to the square of electrode radius, the signal-to-noise ratio is increased as the electrode radius is decreased. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

Topics discussed above are some basic principles and techniques in voltammetry. Voltammetry in the frequency domain where i-E response is obtained at different frequencies from a single experiment known as AC voltammetry or impedance spectroscopy is well established. The use of ultramicroelectrodes in scanning electrochemical microscopy to scan surface redox sites is becoming useful in nanoresearch. There have been extensive efforts made to modify electrodes with enzymes for biosensor development. Wherever an analyte undergoes a redox reaction, voltammetry can be used as the primary sensing technique. Microsensor design and development has recently received... 

Numerical simulation of the experiments [7] became increasingly available during the 1980s, and ultramicroelectrodes [8] opened the way not only to ever-faster timescales but also to finer lateral resolution when characterizing electrode processes. Finally, combinations with spectroscopic and mass-sensitive devices opened new ways to augment information available from molecular electrochemical experiments. 

The current through the electrode is proportional to the flux of redox-active material to the surface, which, in turn is related to the concentrations c of various species near the interface. Thus, an equivalent description is based on the dependence of c on space x and t. Often a single space-coordinate suffices. More complex systems (e.g. ultramicroelectrodes) may require up to three space-coordinates. 

Electroanalytical application of hemispherical [35,36], cylindrical [37,38] and ring microelectrodes [39] has been described. A hemispherical iridium-based mercury ultramicroelectrode was formed by coulometric deposition at -0.2 V vs. SSCE in solution containing 8 x 10 M Hg(II) and 0.1M HCIO4 [35]. The radius of the iridium wire was 6.5 pm. The electrode was used for anodic stripping SWV determination of cadmium, lead and copper in unmodified drinking water, without any added electrolyte, deoxygenation, or forced convection. The effects of finite volume and sphericity of mercury drop elecPode in square-wave voltammetiy have been also studied [36]. 

SECM is a useful electrochemical technique for imaging the smface topographical structure at solid/liquid interfaces. " " Briefly, the electrochemical system consists of a 10 pm Pt-ultramicroelectrode (UME) with Ag/AgCl (3 M KCl) as the reference and Pt as the coimter electrode. The unmodified- and Pyc modified-Nafion membranes (side-1) are carefully moimted on a homemade plastic plate on the bottom of the SECM cell. 

Figure 4. Electrochemistry of dendrimer 35 (a) classical CV, in the presence of 2,3-dichloronaphthoquinone as internal standard, which gives rise to the wave at negative potential (solvent MeCN, electrolyte Bu N PF" Pt electrode, versus SCE, scan rate 100 mV s )- (b) Ultramicroelectrode CV, in the presence of 2,3-dichloronaphthoquinone [solvent MeCN/CHjClj (1 1 v/v) electrolyte Bu N PF j, Pt electrode, versus SCE, scan rate 50 mV s l-...

However, advantageous applications of micro- and ultramicroelectrodes are not limited to fundamental investigations. Such electrodes open up possibilities for work in very low concentrations of solute. Whatever can be done at a planar electrode can be done at a concentration about a thousand times lower by using an ultramicroelectrode without reaching the limiting diffusion current. This means that one could even obtain responses from solutes of 1 ppb (assuming a measured current density of 1 pA cm-2). 

Fig. 7.34. An ensemble of ultramicroelectrodes. (Reprinted from E. Gileadi, Electrode Kinetics for Chemists, Chemical Engineers, and Materials Scientists, VCH Publishers, 1993, p. 450. Copyright 1993 John Wiley. Reprinted by permission of John Wiley Sons, inc.)...

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