Voltammetric sensing

Shiu and Chan [837] described a voltammetric sensing method for Cu(I/II) in which Alizarin Red S, a specific ligand for Cu, was incorporated into a P(Py) film on an electrode. After equilibration with a Cu(I/II) solution for a specific time period, the CP/ligand film was emersed and characterized by cyclic and square wave voltammetry. A linear calibration curve could be obtained for both oxidation states of Cu, with sensitivity greater but linear range less for Cu(II). 

Although glucose sensors have been the most widely studied sensors based on immobilized receptors, similar sensors based on immobilized antibodies have also been suggested [839]. 

Emge and Bauerle [840] studied several new uracil-substituted poly(bithiophenes), whose synthesis was described in Chapter 14 above, as voltammetric-mode sensors (more correctly molecular recognition modules ) for the complementary DNA/RNA bases the sensitivity of this technique however appeared to be poor. 

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Raj CR, Okajima T, Ohsaka T (2003) Gold nanoparticle arrays for the voltammetric sensing of dopamine. J Electroanal Chem 543 127-133... 

A. J. Tunon-Blanco P., Computational approach to the rational design of molecularly imprinted polymers for voltammetric sensing of homovanillic acid, Anal. Chem. 2005, 77, 6741-6746... 

Gholivand, M.B., Karimian, N., and Malekzadeh, G. (2012) Computational design and synthesis of a high selective molecularly imprinted polymer for voltammetric sensing of propazine in food samples. Talanta, 89, 513-520. 

In the case of screen-printed sets of electrodes intended for amperometric applications there is variety of proposed/available commercially arrangements, including different shapes and materials/modified materials of working and/or counter electrodes. Reference elements intended for amperometric/voltammetric sensing are most often silver or silver/silver chloride, used without any additional outer coating layer. The main concern is in reproducibility of obtained sets intended for disposable applications, not in stability of potential of reference element in time (which is hardly ever studied in amperometric applications). 

Emge and Bauerle [630] synthesized novel uracil-substituted poly(bithiophenes), through electrochemical, potential-sweep polymerization of tlie monomer which was synthesized as shown in Fig. 14-13. These were used for voltammetric sensing of DNA/RNA bases, described in Chapter 17. 

Heitzmann M, Basaez L, Brovelli E, Bucher C, Limosin D, Pereira E, Rivas BE, Royal G, Saint-Aman E, Moutet JC (2005) Voltammetric sensing of trace metals at a poly(pyrrole-malonic acid) film modified carbon electrode. Electroanalysis 17 1970-1976... 

A.2.5 IL Redox Chemistry Enabled Reliable Voltammetric Sensing... 

Wang J, Thongngamdee S, Lu D (2006) Sensitive voltammetric sensing of the 2,3-dimethyl-2,3-dinitrobutane (DMNB) explosive taggant. Electroanalysis 18 971-975... 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

In a wider sense the subject of voltammetric titration would include the polarographic mode as a type of amperometric titration however, we have already treated this in Section 3.3.1.3, because we prefer to use the term voltammetric titration in the strict sense, i.e., for faradaic non-polarographic titration techniques, a survey of which is given in Table 3.3. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

It follows that the time scale of cyclic voltammetry is a function of the potential scan rate v, in the sense that the higher the scan rate the higher is the competition of the voltammetric intervention with respect to the rate of the chemical complication. 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

The silicon-rubber-based graphite electrode has been employed as an appropriate voltammetric sensor to measure the rates of dissolution of several oxidizable pharmaceutical compounds [169]. The recirculating dissolution measuring arrangement with this sensing electrode and an Ag/AgCl reference are... 

The cyclic voltammetric experiment can give a great deal of information about the redox activity of a compound and the stability and accessibility of its reduced or oxidised forms. For a fully chemically reversible process, ipa must equal rpc, i.e. all of the material oxidised at the electrode surface on the forward scan must be re-reduced on the reverse scan (or vice versa). If this condition does not hold true, then the process may be partially reversible (rpc < ipa) or irreversible (rpc = 0). Observation of processes that are not fully reversible implies decomposition or chemical reaction of the reduced or oxidised species and the ratio of ipa to /p(. will show a strong dependence on scan rate since the reverse current is related to the lifetime of the redox-generated material. Note that processes that are chemically reversible (in the sense that the reduced and oxidised species are both stable) may not be electrochemically reversible (a term that relates to the relative rates of forward and back electron transfer). Electrochemically reversible processes are characterised by a separation between the forward and reverse potential peaks of exactly 59 mV. 

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

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