Redox state, chronoamperometry

IR spectra of the RC in this cell were recorded on a Bruker IFS 25 FTIR spectrophotometer equipped with a MCT detector selected for high sensitivity. The optical setup was modified to provide a second measuring beam in the 350-1100 nm range at the IR sample focus for the control of the RC redox state. All spectra were recorded at 8 C. Electrochemical measurements (chronoamperometry, coulometry), as well as optical and IR spectrosopy were all controlled from software developed in our laboratory. 

Chronoamperometry is a useful technique in those cases where cyclic voltammetry does not succeed in identifying the electrode mechanisms underlying certain redox changes. It is important to state that chronoamperometric measurements can be performed using the same equipment of cyclic voltammetry. 

Transient technique — A technique whose response is time dependent and whose time dependence is of primary interest, e.g., -> chronoamperometry, -> cyclic voltammetry (where current is the transient), -> chronopotentiometry and -> coulostatic techniques (where voltage is the transient). A transient technique contrasts with steady-state techniques where the response is time independent [i]. Some good examples are cyclic voltammetry [i, ii] (fast scan cyclic voltammetry), the indirect-laser-induced-temperature-jump (ILIT) method [iii], coulostatics [i]. The faster the transient technique, the more susceptible it is to distortion by -> adsorption of the redox moiety. 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

One line in bioelectrochemistry is rooted in electrochemical techniques, spectroscopy, and other physical chemical techniques. Linear and cyclic voltammetry are central.Other electrochemical techniques include impedance and electroreflectance spectroscopy," ultramicro-electrodes, and chronoamperometry. To this come spectroscopic techniques such as infiared, surface enhanced Raman and resonance Raman,second harmonic generation, surface Plasmon, and X-ray photoelectron spectroscopy. A second line has been to combine state-of-the-art physical electrochemistry with corresponding state-of-the-art microbiology and chemical S5mthesis. The former relates to the use of a wide range of designed mutant proteins, " the latter to chemical synthesis or de novo designed synthetic redox metalloproteins. " " ... 

The majority of measurements for electroanalysis with microelectrodes are recorded under steady-state conditions by using either chronoamperometry (CA), linear sweep voltammetry (LSV) or cyclic voltammetry (CV) [1,2, 9,10]. Moreover, to solve problems related to the selectivity between species with similar redox potentials, pulsed techniques such as differential pulse voltammetry (DPV) [1, 7, 43 5] and square-wave voltammetry (SWV) [1, 45-49] have been employed. The use of the latter technique also minimizes the influence of oxygen in aerated natural samples [47]. In order to enhance sensitivity in these measurements, fast-scan voltammetry (FSV) [50] or the accumulation of analytes onto an electrode surface has also been performed, in conjunction with stripping analysis (SA) [51]. 

Let us now consider potential step chronoamperometry in some detail. We consider a surface-deposited polymer film of uniform thickness L to which a small amplitude potential step is applied. This ensures that only a small change in the polymer oxidation state is effected. Structural changes will be minimal. Initially at time t = 0 before applying the step the redox center concentration in the layer is uniform and has the value c, = where denotes the total redox center concentration in the layer. After applying the small amplitude step, the redox centre concentration at jc = 0 (which defines the support electrode/film interface) is given by Cf (see Fig. 1.46). Let c x, t) denote the concentration of redox centers in the film as a function of distance x and time t. The boundary value problem can be stated as follows... 

Much more accurate measurements of diffusion coefficients can be obtained with LSV or CV using UMEs. These measurements are much less dependent on the electrochemical reversibility of the redox couple. Measurement of the diffusion-limited current from a voltammogram recorded at a microelectrode is demonstrated in Figure 19.3c. The concept is identical to that already discussed for chronoamperometry at UMEs at slow scan rates (i.e., long times) the current becomes steady state as long as the potential is well past Ey2-The dependence of D on the steady-state current is given by equation (19.3) for a hemispherical UME and equation (19.4) for a disk UME. The time considerations for CV are the same as those discussed above for chronoamperometry, except that the time is estimated from the scan rate and the difference between the final potential and Ey . 

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