Sigmoid voltammetry

This classification scheme may be extended to a fifth case [3]. In Case 5, the experimental time scale is very long (very slow scan rate) so that diffusion to the entire substrate is convergent, giving sigmoidal voltammetry. This behaviour is most readily observed experimentally when the supporting substrate itself is of microscale and the electroactive particles are of sub-micron size. 

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

Cyclic voltammetry at spherical electrodes. As discussed in Chapter 1, Section 4.2.3, diffusion laws at a spherical electrode must take into account the curvature r0 of the electrode. The mathematical treatment of diffusion at a spherical electrode becomes somewhat more complicated6 with respect to the preceding one for planar diffusion and we will not dwell on it. On the basis of what we will see in Chapter 11, Section 2, it is important to consider that, under radial diffusion, the cyclic voltammogram loses its peak-shaped profile to assume a sigmoidal profile, see Figure 6. 

Figure 5.2 Small-amplitude voltammetric techniques (a) various small-amplitude waveforms are imposed on a dc ramp (normally only one waveform is used in a given experiment) (b) the sigmoidal dc response is typical of dc polarography and hydrodynamic voltammetry. The greatest amplitudes for the small-amplitude current (Aiac) are achieved on the rising part of the dc current, where the small-amplitude voltage signal causes the greatest change in the surface concentrations (c) small-amplitude current response versus applied dc potential.

The conclusions drawn from analysis of the chronoamperometric response of sphere and disk electrodes apply equally to other electrochemical techniques, such as cyclic voltammetry. The characteristic time, tc, of a cyclic voltammetry experiment can be conveniently expressed by the reciprocal of the scan rate RT/nFv. When rc (Dtc),/ , the voltammogram will appear as predicted for a macroplanar electrode (Chap. 3), and when rc (Dtc) A, the voltammogram will take on a sigmoidal shape given by ... 

It is possible to use cyclic voltammetry in the presence of ferrocene carboxylic acid to confirm the presence of micro-electrodes due to the typical sigmoidal-shaped profile produced [2] (Fig. 24.3). Twenty different sensors comprising micro-electrode arrays formed by this technique were analysed for reproducibility. This analysis can be performed by holding the sensors at a potential of +100 mV for 60s and recording the... 

Among the double pulse techniques, DDPV is very attractive for the characterization of multi-electron transfer processes. Besides the reduction of undesirable effects, this technique gives well-resolved peak-shaped signals which are much more advantageous for the elucidation of these processes than the sigmoidal voltammograms obtained in Normal Pulse Voltammetry and discussed in Sect. 3.3. 

DDPV curves is shown for spherical and disc electrodes and different AEf values. As can be seen, independently of the electrode size, a peak-shaped response is obtained with the same peak potential and width (see the superimposed A/pDPV/ A/[, 5 pyk — E curves in the inserted Figures) since these responses are independent of the electrode geometry (see Eqs. (4.173) and (4.176)). This is a notable advantage over Cyclic Voltammetry where sigmoidal curves are obtained when small electrodes are employed which makes data analysis more difficult and less... 

It is of interest at this point to compare the study of Multipulse Chronoamperometry and Staircase Voltammetry with those corresponding to Single Pulse Chronoamperometry and Normal Pulse Voltammetry (NPV) developed in Chaps. 2 and 3 in order to understand how the same perturbation (i.e., a staircase potential) leads to a sigmoidal or a peak-shaped current-potential response as the equilibrium between two consecutive potential pulses is restored, or not. This different behavior is due to the fact that in SCV the current corresponding to a given potential pulse depends on the previous potential pulses, i.e., its history. In contrast, in NPV, since the equilibrium is restored, for a reversible process the current-potential curve is similar to a stationary one, because in this last technique the current corresponding to any potential pulse is independent of its history [8]. 

A steady state is independent of the details of the experiment used in attaining it. Thus, under conditions where a steady state is attained, e.g., under convective conditions in an - electrochemical cell, the application of a constant current leads to a constant potential and similarly the application of a constant potential leads to the same constant current. Voltammetric steady states are most commonly reached using linear potential sweeps (or ramps) in a single or cyclic direction at a UME or RDE. A sigmoidally shaped current (l)-potential (E) voltammogram (i.e., a steady-state voltammogram) is recorded in the method known as steady-state voltammetry as shown in the Figure. Characteristics of the... 

Fig. S Voltammetric shapes commonly encountered (a) asymmetric peak-shaped response (e.g. cyclic voltammetry) and (b) sigmoidal-shaped response (e.g. steady-state...

A reversible one-electron transfer process (19) is initially examined. For all forms of hydrodynamic electrode, material reaches the electrode via diffusion and convection. In the cases of the RDE and ChE under steady-state conditions, solutions to the mass transport equations are combined with the Nernst equation to obtain the reversible response shown in Fig. 26. A sigmoidal-shaped voltammogram is obtained, in contrast to the peak-shaped voltammetric response obtained in cyclic voltammetry. 

Figure 4. Sampled current voltammetry a) Potential step experiments for E equal to fg + 100 mV (lowest curve), Eq + 50 mV, Eq, Eq — 50 mV, and q — 100 mV. b) The current recorded at 50 ms is plotted versus the step potential E as the sigmoid curve. The points shown in (a) are included in (b).

If the direction of the potential ramp is reversed at the end of the sweep back to the starting potential, the technique is called cyclic voltammetry. In this case, one scan is due to the reduction of a species (cathodic current) and the other is due to oxidation of that species (anodic current). The two current plots combined make a voltammogram (Figure 5.8). Anodic and cathodic currents are always opposite in sign and sigmoidal in shape. Cyclic voltammetry allows variations in scan rate to study kinetics and can also characterise the oxidation and reduction behaviour of various electroactive compounds. 

Studies made with this instrumentation on other voltammetrlc techniques such as anodic stripping voltammetry allow one to conclude that the optimization of initial d.c. linear sweep or stripping data leads to optimum performance In the semi-integral, semi-differential and derivative approaches and that, under Instrumental equivalent conditions where d.c. experiments have been optimized with respect to electronic noise and background correction, detection limits are not markedly different within the sub-set of related approaches. Obviously, the resolution and ease of use of a method providing a peak-type readout (semi-differential) are superior to those with sigmoidally shaped read- outs (semi-integral). 

Cyclic voltammetry is a method frequently used to measure 7s,i ni. Mediated bioelectrocatalysis yields cyclic voltammograms (CVs) of different shapes as illustrated in Fig. 2, depending on the measuring conditions [11]. Curve (a) is the wave for a reversible electrode reaction of an Mox/Mred redox couple. Bioelectrocatalysis mediated with the Mqx/ Mred redox couple produces a sigmoidal catalytic wave as curve (c) under the conditions [Mred] - M and [S] Ks. When [Mred] is increased to higher concentrations, an anodic peak of the diffusion current of Mred rnay be overlapped on the catalytic current as depicted by curve (d) the current, however, becomes steady state after appropriate periods... 

For spherical microelectrodes in the limit of low scan rates T 1), the diffusive mass transport is able to keep the surface gradient constant with time and the steady state is attained. Under these conditions, a sigmoidal response is obtained in cyclic voltammetry with the current reaching a... 

As discussed for the case of (hemi)spherical microelectrodes in Chapter 4, the response in cyclic voltammetry at microdiscs varies from a transient, peaked shape to a steady-state, sigmoidal one as the electrode radius and/or the scan rate are decreased, that is, as the dimensionless scan rate, a = Y r lv/TZTD, is decreased. The following empirical expression describes the value of the peak current of the forward peak for electrochemically reversible processes [11] ... 

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