Differential pulse voltammetry experiments

FIGURE 25-30 Voltammogram for a differential-pulse voltammetry experiment. Here. A/ - - i. i (see Fig-... 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

Rajca and co-workers have studied star-branched and dendritic high-spin polyradicals which are potential organic magnets. Representative data were obtained for the model tetra-anionic compound 55. Three redox waves were observed by cyclic voltammetry and differential pulse voltammetry for a four-electron process between the potentials of -2.00 and -1.20 V (vs. SCE). Electrochemical experiments with these materials have usually been performed at 200 K. The polyradicals, which are less stable for systems with more unpaired electrons, have been characterized by spectroscopic studies, ESR data, and SQUID magnetometiy. 

Cai et al. [28] have synthesised AuNPs-labelled ssDNA as a probe to be hybridised with their complementary strand on chitosan-modified glassy carbon electrode (GCE). In their experiments, SH-ssDNA was self-assembled onto an AuNP (16 nm in diameter) (details given previously in Ref. [54]). After hybridisation, the AuNPs tags were detected by differential pulse voltammetry (DPV). The detection limit was 1.0 x 10-9 mol L 1 of 32-base synthesised complementary oligonucleotide. 

Pang et al. [54] studied the electrochemical behavior of L-dopa at SWCNT-modified GCE. Before starting, the electrode was immersed for 120 s in the L-dopa solution. L-dopa showed an irreversible behavior at bare GCE with peak potential separation of 161 mV. On the contrary, a quasi reversible behavior with peak potential separation of 55 mV was obtained at the SWCNTs-modified electrode. Experiments performed by differential pulse voltammetry showed a... 

A mixed self-assembled monolayer was used for the aptamer immobilization on the gold electrode. The aptamer-modified electrodes were then incubated for 1 h at 37°C with thrombin (18 ag/mL). Electrochemical measurements were recorded in the thin-layer cell configured to contain a total volume of 20 [xL. Thrombin chromogenic substrate (/3-Ala-Gly-Arg-p-nitroaniline) was injected into the cell and differential pulse voltammetry (DPV) measurements between -0.2 and -1V with a pulse height of—0.05 V and pulse duration of 70 ms were carried out. The DPV measurements showed that /3-Ala-Gly-Arg-p-nitroaniline substrate and the p-nitroaniline product have different redox potentials. Moreover, the DPV experiments showed a current peak at -0.45 V in the presence of the thrombin substrate. After 5 min, the peak at —0.45 V decreased and a new peak was detected at -0.70 V, indicating the formation of p-nitroaniline. The same measurements carried out on a control electrode in order to test the specificity of the assay in this experiment bovine serum albumin (BSA) substituted thrombin and in this case only the peak at 0.45 V was measured. 

The most elementary biosensors are fruit pulps or slices which have been combined with amperometric electrodes. A well-known example is the ba-nanatrode (Wang and tin 1988). This sensor, most useful for demonstration experiments, contains a paste mix of banana pulp, nujol and carbon powder which has been pressed into a glass tube with an electric contact (Fig. 7.39). The mass contains the enzyme polyphenolase, which catalyses the oxidation of polyphenols, among them important biological messengers like dopamine. The sensor can be tested by means of simple compounds like catechol, which can be detected in beer. As a result of air oxidation, o-quinone is formed. The latter is an electrochemicaUy active compound which can be detected e.g. by differential-pulse voltammetry. 

Very recently a differential pulse voltammetry device has been developed through imprinting smart polymers. In this experiment, which was described by N. Karimian et al, a temperature sensitive amine-terminated poly (N-isopropylacrylamide) block, and (N,N -methylenebisacryl amide) cross-linker with o-phenylenediamine were electropolymerised on the surface of a gold electrode, using folic acid as the template. This led to a thermally switchable MIP sensor with selectivity towards the template [355]. 

Solution concentrations for cyclic and differential pulse voltammetry and macroscale electrolysis were about 5.0 x 10 i mM. Similar concentrations were used in spectroelectrochemical experiments. Products of macroscale electrolysis were separated by thin layer chromatography, using silica gel, and identified by using FTIR and NMR spectroscopy. 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

Cyclic voltammetric methods, or other related techniques such as differential pulse polarography and AC voltammetry,3 provided a convenient method for the estimation of equilibrium constants for disproportionation or its converse, comproportionation. In this respect, the experimentally measured quantity of interest in a cyclic voltammetric experiment is E>A, the potential mid-way between the cathodic and anodic peak potentials. For a one-electron process, E,A is related to the thermodynamic standard potential Ea by equation (4).13 In practice, ,/2 = E° is usually a good approximation. 

This technique is based on the derivative of the NPV curve introduced by Barker and Gardner [2]. In DDPV, two consecutive potentials E and E2 are applied during times 0 < fi < ti and 0 < t2 < z2. respectively, with the length of the second pulse being much shorter than the first (t /t2 = 50 100). The difference AE = E2 — E is kept constant during the experiment and the difference A/DDPV = h h is plotted versus E or versus an average potential E y2 = (E +E2)/2. When the two pulses are of similar duration, the technique is known as Differential Normal Double Pulse Voltammetry (DNDPV) (Scheme 4.3). 

The range of time scales for the differential pulse experiment is the same as for normal pulse voltammetry, hence a given system ordinarily shows the same degree of reversibility toward either approach. However, the degree of reversibility toward pulse methods may differ from that shown toward conventional polarography for reasons discussed in Section 7.3.2. 

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