Voltammetric electrodes construction

Construction and Mass-Transport Properties of Voltammetric Electrodes... 

Pendley, B. D., Abruna, H. D. Construction of submicrometer voltammetric electrodes. Anal. Chem. 1990, 62, 782-784. 

Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting current is measured. Amperometry is most often used in the construction of chemical sensors that, as with potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and water. 

From the foregoing treatment of voltammetry it is clear that for its application sophisticated apparatus is required, especially the electronics moreover, the construction of the electrodes and their mutual positions within the measuring vessel, of adapted size and without or with stirring, requires great care and experience. For this reason it is often advisable to purchase commercial apparatus, which has reached a high level of sophistication and reliability. Here the most desirable property is smooth recording of the voltammetric curve, which necessitates the kind of automation inherently required in the voltammetric method this is different from advanced automation, which is treated in Part C. 

FIGURE 5.25. Avidin-biotin construction of a monolayer glucose oxidase electrode with an attached ferrocenium cosubstrate and cyclic voltammetric response in a phosphate buffer (pH 8) at 25°C and a scan rate of 0.04 V/s. a attached ferrocene alone, h In the presence of 0.5 M glucose, c Variation of the inverse of the plateau current with the inverse of substrate concentration. Adapted from Figure 1 in reference 24, with permission from the American Chemical Society. 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

Hale et al. reported the use of an enzyme-modified carbon paste for the determination of acetylcholine [21], The sensor was constructed from a carbon paste electrode containing acetylcholineesterase and choline oxidase, and the electron transfer mediator tetrathiafulvalene. The electrode was used for the cyclic voltammetric determination of acetylcholine in 0.1 M phosphate buffer at +200 mV versus saturated calomel electrode. Tetrathiafulvalene efficiently re-oxidized the reduced flavin adenine dinucleotide centers of choline oxidase. The calibration graph was linear up to 400 pM acetylcholine, and the detection limit was 0.5 pM. 

Many designs of calomel electrodes exist, and several types are commercially available. Some of these are constructed for voltammetric work, whereas others are used in connection with glass electrodes. The latter type generally has a high resistance, which must be considered when the choice of voltmeter or potentiostat is made. 

Faradaic processes of electrode reactions, which are principle mechanism of obtaining analytical signal in amperometric sensors, significantly depend on working electrode material and state of its surface. The common working electrode materials include noble and seminoble metals, solid oxides of various elements and different kinds of carbon materials including carbon nanostructures. They are employed in conventional voltammetric measurements with various modes of electrode polarization, as amperometric chemical sensors, as well as for construction of amperometric biosensors. 

For high impedance electrodes, e.g., liquid membrane ion-selective tips (39), another method is essential. Using techniques borrowed from electrophysiology, it is possible to construct dual tips from theta glass (Chapter 3). One barrel of the double tip can be used for the liquid membrane, and the other may be filled with a low melting point metal such as gallium to make a voltammetric sensor (41). 

Small LL interfaces have been used by Girault and co-workers (33-38) and by Senda et al. (39, 40). We have used a small hole formed in a thin glass wall (41-43). Figure 16 shows the voltammetric response of lauryl sulfate anion transport between water and nitrobenzene. Recent analytical applications of these microinterfaces have resulted in construction of gel-solidified probes. The advantage of such a modification is ease of handling (44-47). The immobilization can be extended further to studies of frozen interfaces, or even to solid electrolytes. Significantly, ITIES theory also applies to interfaces that are encountered in ion-doped, conductive, polymer-coated electrodes. 

Voltammetric sensors based on chemically modified electrodes (conducting polymers, phthalocyanine complexes) with improved cross-selectivity were developed for the discrimination of bitter solutions [50], The performance and capability were tested by using model solutions of bitterness such as magnesium chloride, quinine, and four phenolic compounds responsible for bitterness in olive oils. The sensors gave electrochemical responses when exposed to the solutions. A multichannel taste sensor was constructed using the sensors with the best stabilities and cross-selectivities and PCA of the signals allowed distinct discrimination of the solutions. 

For example, a voltammetric sensor for the herbicide 2,4-D was constructed [33] where the electroactive compound 2,5-dihydroxyphenylacetic acid was used as a probe instead of the labeled analyte. MIP particles (0.5 mg) were coated as a thin layer onto a screen-printed carbon electrode, and incubated with the sample to which the probe was added. In the presence of the analyte, some of the probe was competed out of the imprinted sites, whereas the remaining probe was directly quantified by differential pulse-voltammetric measurements (Fig. 5). Section X resumes the steps for the preparation of this sensor. 

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