A.c. voltammetry

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

Active electrochemical techniques are not confined to pulse and linear sweep waveforms, which are considered large ampHtude methods. A-C voltammetry, considered a small ampHtude method because an alternating voltage <10 mV is appHed to actively couple through the double-layer capacitance, can also be used (15). An excellent source of additional information concerning active electroanalytical techniques can be found in References 16—18. Reference 18, although directed toward clinical chemistry and medicine, also contains an excellent review of electroanalytical techniques (see also... 

Trojanek, A. and De Jong, H. G., Fast scan A. C. voltammetry for better resolution of chromatographically overlapping peaks, Anal. Chim. Acta, 141,115,1982. 

Burke LD, O Sullivan JF (1992) A study of electrocatalytic behavior of gold in acid using a.c. voltammetry. Electrochim Acta 37 2087-2094. 

When a reaction is reversible, i ct—>0 and Zf = Zw = oco 1/2( 1-i). The observed phase angle is jt/4, and the impedance is the least possible for that value of co (o depends not only on the transport but also on the reciprocal of the concentration of electroactive species see (11.19)). If Rct > 0, Zf increases from its minimum value and there will be a lower phase angle. It is this phase-angle variation according to the rate of the electrode reaction that is used in the technique of a.c. voltammetry. 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

The use of pulse techniques for electroanalytical determinations has been much publicized, and is applicable to both solid electrodes and the HMDE/SMDE. The development in recent years of square wave voltammetry (SWV)39 widens the possibilities beause of its rapidity (Section 10.9) it is especially useful because the time necessary to do an experiment is only 2 s, which means that a SMDE drop in the dropping mode can also be used for micromolar determinations. Progress obtained with pulse techniques40,41 has meant that applications of a.c. voltammetry have been few, but there is no theoretical reason for this. The very low detection limits achieved in stripping voltammetry result not only from the pre-concentration step but also from the use of pulse waveforms in the determination step. 

Yunus K. and Fisher A. C., Voltammetry under microfluidic control, a flow cell approach, Electroanalysis, 15(22), 1782-1786, 2003. 

At the d.m.e., with the aid of cyclic a.c. voltammetry, two successive one-electron redox reactions of iron-sulfur clusters were found. Conversely, at the h.m.d.e. a steady state is reached (Figure 3) which is similar to that of apoferredoxin. This observation is explained in terms of a RSH/RSHg redox reaction [cf. Eq. (5)] of two forms of apoferredoxin. 

Figure 3. Cyclic a.c. voltammetry (in-phase, / = 500Hz) of ferrodoxin (1.9 x 10" M, pH 9.2) after adsorption at h.m.d.e. (-1.1 V). Al, A2, A3 first, second, third anodic scans Cl, C2, C3 first, second, third cathodic scans. The peak at -0.65 V of scan Cl disappears. AS and CS are steady states of the anodic and cathodic scans for apoferredoxin (iron removed).

Figure 12. Hysteresis of peak-fading process for ss-DNA by a.c. voltammetry in the potential range of peak 1 and 2 at the same h.m.d.e. 12 cathodic sweeps (upper trace) 12 anodic sweeps (lower trace).

The effect of solution pH on the magnitude of a.c. voltammetry peaks for cytochrome c at the 4,4 -bipyridine/gold electrode was studied in 0.02 M phosphate, 1.0 M NaC104 electrolyte. The a.c. peak current exhibited a maximum at pH 7 and decreased on either side of this value over the range pH 5 to pH 10. On the basic side the decrease is due to the cytochrome c State III to State IV conformational transition and the decrease on the acidic side was attributed to protonation of the adsorbed 4,4 -bipyridine molecules. A pK for the alkaline transition was given as 9.1 and it was noted to be close to the previously reported value of 9.3. However, this value is different from the values of 7.8 to 8.0 previously reported for perchlorate-containing solutions and the reason for this difference is not clear. 

Various electrochemical methods have been applied for the analysis of nucleic acids, including DPP and DPV, cyclic voltammetry, a.c. voltammetry, and constant current chronopotentiometry, which are particularly useful. Guanine (G) and adenine (A) residues in DNA and RNA molecules are oxidized at carbon electrodes however, their voltammetric peaks are poorly developed, providing insufficient sensitivity in... 

Various electrochemical methods have been appfied for the analysis of NAs, including DPP [5, 11] and DPV[13, 269, 270], linear sweep and CV [13, 271] square wave [138] and a.c. voltammetry [272-274], and recently constant current chronopotentiometry [249, 255-257, 275, 276] and elimination voltammetry [139, 277-279]. DPP was applied for the analysis of DNA in 1966 [280], and in a short time, it replaced OP and d.c. po-larography used in the early NA studies [4, 5]. The main advantage of DPP is its better sensitivity and resolution of peaks. Calf thymus ssDNA produced a well-developed DPP peak III (Fig. 6d) at concentrations of about 10 to 20 igml while dsDNA was inactive at the same concentration. At higher concentrations (hundreds of pgml ), dsDNA produced peak II at potentials by about 70 mV more positive than peak III (Fig. 6c). For years, DPP was the most sensitive instrumental method of determination of traces of ssDNA in dsDNA samples [5]. 

There are in fact two slightly different types of non-steady state technique. In the first an instantaneous perturbation of the electrode potential, or current, is applied, and the system is monitored as it relaxes towards its new steady state chronoamperometry and chronopotentiometry are typical examples of such techniques. In the second class of experiment a periodically varying perturbation of current or potential is applied to the system, and its response is measured as a function of the frequency of the perturbation cyclic and a.c. voltammetry are examples of this type of approach. In both cases the rate of mass transport varies with the time (or frequency), and by obtaining data over a wide range of these variables and by using curve fitting procedures, kinetic parameters are obtainable. Pulse techniques will be discussed later in this chapter, whilst sweep methods are described in Chapter 6 and a.c. methods in Chapter 8. 

While the current is measured at a constant voltage in amperometric detectors, in voltammet-ric detectors it is measured while the voltage is changing rapidly. Either linear scan, staircase, square-wave, pulse or a.c. voltammetry can be used for the measurement. When compared to stationary methods, nonstationary methods with superimposed voltage pulses often result in higher selectivity and sensitivity and fewer problems from the adsorption of impurities and electrode reaction products on the electrode surface [53]-[58j. 

Electron microscopy and spectroscopic techniques are employed to identify the structural characteristics of the modified surface. Cyclic voltammetry is largely used to define the electrochemical behaviours of the sensor and to elucidate mechanistic aspects of the electrode reaction. A.C. voltammetry has also been applied . As both spectroscopic and electrochemical results are often required to obtain valuable information, the use of spectroelectrochemical methods may be very useful2 -28 ... 

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