Redox probes

Oxidation-reduction potential Because of the interest in bacterial corrosion under anaerobic conditions, the oxidation-reduction situation in the soil was suggested as an indication of expected corrosion rates. The work of Starkey and Wight , McVey , and others led to the development and testing of the so-called redox probe. The probe with platinum electrodes and copper sulphate reference cells has been described as difficult to clean. Hence, results are difficult to reproduce. At the present time this procedure does not seem adapted to use in field tests. Of more importance is the fact that the data obtained by the redox method simply indicate anaerobic situations in the soil. Such data would be effective in predicting anaerobic corrosion by sulphate-reducing bacteria, but would fail to give any information regarding other types of corrosion. 

Apart from the instruments described in previous paragraphs, there are others that, while not directly connected with cathodic protection as such, are extremely useful tools to a corrosion engineer. They include pH meters. Redox probes, protective-coating test instruments and buried-metal-location instruments. 

Our approach to studies of CT in DNA relies on two key features. First is the use of well-characterized DNA assemblies, which include redox probes that are strongly coupled to the DNA w-stack. The importance of well-characterized DNA assemblies, including the redox participants and DNA bases, cannot be overstated. Differences in structural and energetic properties of DNA assemblies, particularly when unaccounted for, may be responsible for drastically different conclusions regarding DNA CT. Furthermore, in order to characterize the DNA w-stack as a medium for CT, it is necessary to employ redox probes that are directly coupled to the w-stack. 

Redox participants are chosen to facilitate spectroscopic, biochemical and electrochemical probing of DNA CT. These include metallointercalators, organic intercalators, and modified bases that possess useful, well-described, and varied redox, photophysical and photochemical properties (Table 1). Our probes are readily incorporated into DNA assemblies where CT distances ranging from 3.4 to 200 A and driving forces spanning over two volts can be modulated with certainty. Most importantly, all redox probes which afford fast and/or efficient CT through DNA are well-coupled to the 7r-stack. 

The first experiments characterizing DNA-mediated CT over a precisely defined distance between covalently appended redox probes were reported in 1993 [95]. Remarkably, the luminescence of a photoexcited Ru(II) intercala-tor was quenched by a Rh(III) intercalator fixed to the other end of a 15-mer DNA duplex over 40 A away (Fig. 4). Furthermore, non-intercalating, tethered Ru(II) and Rh(III) complexes did not undergo this quenching reaction. In this way the importance of intercalative stacking for efficient CT was demonstrated. 

In many investigations of CT, pendant redox probes interact with both bases of abase pair. However, studies of base-base charge transfer can differentiate between discrete intra- and interstrand reactions (Fig. 7). These investigations further attest to the critical role of base stacking in DNA-mediated CT. In B-DNA duplexes, stacking interactions are largely restricted to... 

These observations were significant to our choice of reactants for probing CT at DNA-modified surfaces. In particular, an upright orientation of the DNA relative to the surface is required to probe DNA-mediated reactions otherwise a more direct reaction between an intercalating probe and the electrode might be possible. Consequently, reactants were selected such that a negative potential could be apphed, thereby initiating reduction of an intercalated redox probe distantly bound within DNA helix. Importantly, the... 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

As the immunocomplex structure is generally electroinactive, its coverage on the electrode surface will decrease the double layer capacitance and retard the interfacial electron transfer kinetics of a redox probe present in the electrolyte solution. In this case, Ra can be expressed as the sum of the electron transfer resistance of the bare electrode CRbare) and that of the electrode immobilized with an immunocomplex (R immun) ... 

There are several ways to present the Faradaic impedance data obtained at an electrode immobilized with an immunocomplex in the presence of a redox probe. For example, ZIm is plotted vs ZRe as a function of decreasing frequency to obtain a... 

FIGURE 5.10 An equivalent circuit representing the interfacial features of an electrochemical immuno-sensor in the presence of a redox probe. 

M. Kanungo and M.M. Collinson, Diffusion of redox probes in hydrated sol-gel-derived glasses. Effect of gel structure. Anal. Chem. 75, 6555-6559 (2003). 

Figure 1.26 Scheme of immuno-biosensor developed by Liu and Gooding, exploiting the size of proteins and the space that a protein takes up to block ion access to the redox probe. (Reproduced by permission of The Royal Society of Chemistry from [142].)... 

Figure 3.13 (a) Values of charge-transfer resistance of different systems based on carbon, using the redox probe Fe(CN)6 . (b) Nyquist plot of different carbon nanotube composites in the presence of the redox couple, (c) Table with the electron-transfer rate constants calculated from cyclic voltammet data by using Nicholson method. Adapted with permission from Ref [103]. Copyright, 2008, Elsevier. 

Fig. 4.1 Scheme of the three-phase electrode with a droplet configuration, consisting of a paraffin-impregnated graphite electrode, modified with a macroscopic droplet of an organic solvent that contains a neutral redox probe... 

Fig. 4.2 Scheme of a part of the three-phase electrode consisting of pyrolytic graphite electrode modified with an uneven thin film of an organic solvent covering partly the electrode surface and containing a neutral redox probe... 

Figure 4.3 shows a representative voltarmnogram recorded at a three-phase electrode with a droplet configuration consisting of DMFC as a redox probe and nitrobenzene as the organic solvent. The oxidation of DMFC to decamethyUerrocenium cation... 

Equation (4.2) predicts a linear dependence of Efvs. A with a slope 1, and a linear dependence between Ef vs. log(o ) with a slope 2.303. These two dependencies can serve as diagnostic criteria to identify the electrochemical mechanism (4.1). Figure 4.4a shows the effect of different anions on the position of the net peak recorded at the three-phase electrode with a droplet configuration, where DMFC is the redox probe and nitrobenzene is the organic solvent. Figure 4.4b shows the linear variation of the net peak potential with A, with a slope close to 1. Recalling that the net peak potential of a reversible reaction is equivalent to the formal potential of the electrochemical reaction (Sect. 2.1.1), the results in Fig. 4.4 confirm the validity and applicability of Eq. (4.2). 

The three-phase electrode with a thin-film configuration (Fig. 4.2) has been mainly used in combination with nitrobenzene as an organic solvent and lutetium bis(tetra-i-butylphthalocyaninato) complexes as a redoxprobe (LBPC) [21,23]. Figure 4.5 depicts a typical voltammogram recorded with this redox probe in contact with 0.1 mol/L aqueous solution of KNO3. LBPC can be both oxidized and re-... 

As the electrochemical reaction is confined to the boundaries of the thin film, the voltammetric response exhibits a quasireversible maximum. The position of the quasireversible maximum on the log frequency axis depends on the kinetics of the overall reaction at the thin-film electrode, i.e., reflecting the coupled electron-ion transfer (4.3). Analyzing the evolution of the quasireversible maximum measured with different redox probes and various transferring ions, it has been demonstrated... 

Fig. 4.6 Thin-film electrode consisting of a pyrolytic graphite electrode covered with a film of an organic solvent containing a neutral hydrophobic redox probe and a suitable electrolyte...

As the transferring ion is present in a large exeess in both liquid phases compared to the redox probe, its concentrations at the O-W are virtually constant and equal to their concentrations in the bulk of the liquid phases. Thus, (4.10) is transformed into the following form ... 

The aforementioned methodology has been applied to measure the kinetics of a series of monovalent ions by using the oxidation of LBPC [26-29], As the redox probe LBPC is oxidized to the stable hydrophobic cation LBPC+, and the electrode reaction is accompanied by either anion ingress from the aqueous phase (4.12) or cation expulsion from the organic phase (4.13), which depends on the type of ions and their relative affinity for both liquid phases. 

Passivating behavior of self-assembled octadecanethiol on pc-Au electrodes has been tested by CV using Ru(NH3)6Cl3, K4Fe(CN)6, and benzoquinone [106] as the redox probes. Inhibiting properties toward these reactants were found to be different. 

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