Ultramicroelectrodes , scanning

Figure 17.14 Steady-state voltammetry of a liquid and polymer (PVDF-HFP) gel electrolyte at a Pt ultramicroelectrode. Scan speed lOmV/s. Reprinted by permission from Mac Millan Publishers Ltd Nature Materials, 2003, 2, 402.

Fig.6. Cyclic voltammetry of 2,6-anthraquinone disulfonic acid at mercury ultramicroelectrodes. Scan rate = 10240 mV/s. The top voltam-mogram was obtained with a platinum electrode overcoated with mercury. The bottom voltammogram was obtained using a platinum electrode that was etched and silanized before coating with mercury as shown in Fig.5.

Figure 2 High scan rate cyclic voltametry of 3, 5,7-trimethylcatechin (C° = 0.98 mM) in acetonitrile-0,1M NBU4BF4 on a lO-pm-diameter glassy carbon ultramicroelectrode (scan rate v = 11500 Vs ). SCE, saturated calomel electrode reference.

Selzer Y and Manler D 2000 Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes steady-state and transient behavior Anal. Chem. 72 2383... 

Explain clearly why and how a change of the scan rate affects the shape of the cyclic voltammetric response of an ultramicroelectrode. 

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... 

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

FIGURE 2.28. Comparison of high-scan-rate ultramicroelectrode cyclic voltammetry (A), redoc catalysis (A), and laser flash photolysis (x) for the determination of the rate constant of deprotonation of methylacridan cation radical by bases of increasing pKa. Adapted from Figure 6 in reference 20, with permission from the American Chemical Society. 

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

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-...

The kinetics of AgGl dissolution in aqueous solutions without supporting electrolyte have been studied utilizing well-defined and high mass transport properties of the scanning electrochemical microscope [376]. An ultramicroelectrode probe positioned close to the AgGl surface was used to induce and monitor dissolution of the salt via reduction of Ag+ from the initially saturated solution. 

In this method, a wide range of scan rate (v) is possible, i.e. from 1 mV s-1 to 1000 Vs-1 with a conventional apparatus, and up to 1000000 V s 1 or even more with a combination of a sophisticated apparatus and an ultramicroelectrode. 

Fig. 5.23 Influence of the scan rate on the cyclic voltammograms of ferrocene at an ultramicroelectrode. Electrode radius 5 pm supporting electrolyte 0.6 M Et4NCI04 in AN.

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

Fig. 8.19 Cyclic voltammogram for the oxidation of 9,10-diphenylanthracene (1 mM) at a platinum ultramicroelectrode in 0.5 M Bu4NCI04-AN. Scan rate 1000 Vs"1 [62].

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

Figure 15 Diagram of scanning electrochemical microscope for ECL-based detection. ECL generated at an ultramicroelectrode via annihilation of R and R+ and detected with a photomultiplier. (From Ref. 85. With permission from the American Chemical Society.)...

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