Chronopotentiometric response

FIGURE 6-14 DNA hybridization biosensors detection of DNA sequences from the E. coli pathogen. Chronopotentiometric response of the redox indicator upon increasing the target concentration in 1.0 pg/ml steps (a-c), in connection with a 2 min hybridization time. (Reproduced with permission from reference 46.)... 

R.O. Kadara and I.E. Tothill, Resolving the copper interference effect on the stripping chronopotentiometric response of lead (II) obtained at bismuth film screen-printed electrode, Talanta, 66 (2005) 1089-1093. 

H.-W. Rosier, F. Maletzki and E. Staube, Ion transport across electrodialysis membranes in the overlimiting current range Chronopotentiometric studies, J. Membr. Sci., 1992, 72, 171-178 R Sistat and G. Pourcelly, Chronopotentiometric response of an ion-exchange membrane in the underlimiting current-range. Transport phenomena within the diffusion layers, J. Membr. Sci., 1997, 123, 121. 

Clearly, constant current methods are not as useful as chronocoulometric ones for determination of adsorbed reactant. However, once Fq is determined, the chronopotentiometric response can yield information about the order in which dissolved and adsorbed species are reduced. 

Pons BS (1981A) Approximation of chronopotentiometric responses by orthogonal collocation. Electrochim Acta 25 601. 

W. S. Li, S. Q. Cai, J. L. Luo, Chronopotentiometric responses and capacitance behaviors of passive films formed on iron in borate buffer solution, J. Electrochem. Soc., 2004,151,4, pp. B220-B226. 

Fig. 6 Chronopotentiometry (a) typical excitation signal (b) potential response (c) concentration profiles of educt for a chronopotentiometric experiment (three profiles at various times, increasing time shown by arrow).

Fig. 18. A chronopotentiometric experiment, (a) Current step (b) potential vs. time response.

The reader familiar with controlled-potential methodology will have no trouble understanding a controlled-current apparatus. Figure 6.18 illustrates classical approaches to two- and three-electrode constant-current chronopotentiometric experiments (see Chap. 4). The simplicity of these circuits was for many years an attractive feature of chronopotentiometry. Improvements in potentiostats have been largely responsible for a decline in the popularity of chronopot in recent years. Nevertheless, constant-current experiments are even more important with respect to coulometric titrations and stripping potentiometry (Chap. 24). 

As is well known, the steady-state behavior of (spherical and disc) microelectrodes enables the generation of a unique current-potential relationship since the response is independent of the time or frequency variables [43]. This feature allows us to obtain identical I-E responses, independently of the electrochemical technique, when a voltammogram is generated by applying a linear sweep or a sequence of discrete potential steps, or a periodic potential. From the above, it can also be expected that the same behavior will be obtained under chronopotentiometric conditions when any current time function I(t) is applied, i.e., the steady-state I(t) —E curve (with E being the measured potential) will be identical to the voltammogram obtained under controlled potential-time conditions [44, 45]. 

The non-reversible behavior is plotted in Fig. 7.46, which corresponds to the corrected (Jcv/v) — E curves (dashed lines) and the (QDSCVC/AE) — E ones (symbols) of the system 4-PhenylazoPhenol. From these curves, it can be seen that although the DSCVC curves are perfectly superimposable, the CV ones clearly show smaller peak heights in both scans. This systematic decrease of the CV signals, which cannot be theoretically predicted, is 5-10 %, and it has been reported when the response of electro-active monolayers in CV has been compared with other voltammetric and chronopotentiometric electrochemical techniques [71, 72], Due to the quasi-reversible nature of the charge transfer reduction of the 4-PhenylazoPhenol, no simple equations for the peak parameters are available. So, a numerical comparison between theoretical and experimental curves for different sets of parameters should be made in order to obtain the kinetic and thermodynamic parameters of the system. 

It is always interesting to plot the effect of the chronoamperometric transient at a certain prefixed potential (Figure 12.3) and to check the stability of the modified layer with a chronopotentiometric transient (Figure 12.4) by the open circuit potential response. The break of the (3-oxide growth on the platinum is observed after 250 s (Figure 12.4) when the current density becomes stable (Figure 12.3). 

FIGURE 4.12 Pulsed chronopotentiometric (a) and differential (b) response of a calcium pulstrode in artificial ten times diluted blood serum [59]. Cathodic current pulses of lOpA (125 pA cm ) and Isec duration are followed by a stripping potential of +30mV appUed for ISOsec. Potentials were sampled at 0.5 and l.Osec of each pulse and are the averages over preceding 100msec. The Nernstian response slope of 29 mV decade is shown for comparison. 

Figure 4 Experimental derivative chronopotentiograms of the reduction of mercurous cysteine thiolate in 0.1 mol I phosphate buffer, pH 7.4 (A) before and (B) after deaeration. The presence of dissolved oxygen, which is simultaneously being reduced at the working electrode, enhances the response. The dissolved oxygen is reduced in two steps (peaks on either side of a). The peak on the left is the reduction to hydrogen peroxide and the peak on the right is the reduction of hydrogen peroxide to water. (Reproduced with permission from Honeychurch MJ and Ridd MJ (1995) The effect of a non-adsorbing electroactive species on the transition time in derivative adsorptive chronopotentiometric stripping analysis. Electroanalysis 1047-1054 Wiley-VCH.)...

In PNA the entire sugar-phosphate is replaced by (Af-(2-amino-ethyl) glycine units. In contrast to DNA and RNA (with negatively charged backbones), PNA has an electrically neutral backbone. Electrochemical responses of PNA were similar to DNA and RNA (i.e. A, C and G were reduced on mercury electrodes G producing an anodic CV peak due to oxidation of the G reduction product and A and G were oxidized on carbon electrodes) [247]. Peak potentials of ssPNA at the HMDE were shifted to negative values as compared with ssDNA. Differences in backbones of PNA and DNA were manifested by the different adsorption behavior of these two compounds, as detected by a.c. impedance [108] on mercury (Fig. 3), and by chronopotentiometric measurements on carbon electrodes [129]. 

FIG. 5—A schematic diagram showing (a) the input applied current steps, and (b) the resultant potential response from a single current cycle for the cyclic current reversal chronopotentiometric experiment... 

Voltammetric and chronopotentiometric detection modes are mostly used [20]. Together with them, electrochemical impedance spectroscopy (EIS) becomes to be popular at DNA-based biosensors [13]. According to electrochemi-cally active species which responses are evaluated at the detection of damage to DNA, the experimental techniques can be classified as follows [1] ... 

---