Electron transfer of SOD

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

To fulfill both the requirement of CFME for the practical applications and the necessity of Au substrate to assemble so-called promoters to construct the third-generation biosensor, Tian el al. have combined the electrochemical deposition of Au nanoparticles (Au-NPs) onto carbon fiber microelectrodes with the self-assembly of a monolayer on these Au-NPs to facilitate the direct electron transfer of SOD at the carbon fiber microelectrode. The strategy enabled a third-generation amperometric 02 biosensor to be readily fabricated on the carbon fiber microelectrode. This CFME-based biosensor is envisaged to have great potential for (he detection of 02" in biological systems [158],... 

Another application of Zinc oxide nanostructure is immobilization of uricace onto ZnO nanorod and fabrication a sensitive biosensor for uric acid detection [167], The biosensor successfully used for micromolar detection of uric acid in the presence serious interferences, glucose, ascorbic acid, and 1-cysteine. The apparent KM value for the uric acid biosensor is 0.238 mM, showing high affinity of the biosensor. Direct electron transfer of SOD at a physical vapor deposited zinc oxide nanoparticles surface was investigated [168], In comparison to SOD immobilized onto ZnO nanodisks [169], the electron transfer rate constant is small and a quasi- reversible electrochemical behavior observed. A novel... 

As can be seen the ZnO nanodisks are typically 50-80 nm in thickness and several micrometers in dimensions. Many nanodisks are rather regular hexagons, and the contrast on a hole sheet is homogenous. For investigating the direct electron transfer of SOD onto ZnO nanodisks the cyclic voltammograms of ZnO nanodisks and ZnO nanodisks-SOD in phosphate buffer solution free of SOD recorded As shown a well defined redox couple for immobilized enzyme observed (Fig. 10B). Furthermore, the CVs remained essentially unchanged on consecutive potential scanning up to 1000 cycles at a sweep rate of 20 mV s 1,... 

Figure 10-37 shows the voltammetric response of native and reeonstituted SODs at the Cys/Au electrode. The E2Zn2SOD does not shows any redox response in the potential window of -0.5 to 0.5 V vs. Ag/AgCl. On the other hand, as can be seen from the Fig. 10-37 the CU2E2SOD shows a reversible redox peak, which is very similar to that of the native SOD. This implies that the observed direct electron transfer of SOD at the Cys-modifed electrode is associated with the redox reaction of the copper moiety (not of the zinc moiety). It further demonstrates that... 

As described above, the self-assembly of Cys promotes the electron transfer between SOD and the Au electrode. This Cys-promoted rapid and direct electron transfer of SOD and its relevance to the redox reaction of the copper complex moiety in SOD formed a strong basis for the development of a SOD-based third-generation biosensor for 02 because the copper complex moiety has been well documented as the active site for the catalytic dismutation of O2 [128]. Figure 10-39 shows CVs at the bare Au, Cys/Au and SOD/Cys/ Au electrodes in PBS (O2 saturated) containing 0.002 unit of XOD and 50 pM xanthine, i.e. in the presence of O2. Both cathodic and anodic peak currents corresponding to the redox reaction of the SOD confined on the electrode are significantly increased, compared with those in the absence of O2 (Fig. 10-36a) [133—135]. Such a redox response was not... 

Such a bidirectional electromediation (electrocatalysis) by SOD/Cys/Au is essentially based on the inherent specificity of SOD for the dismutation of O2 i.e. SOD catalyzes both the reduction of 02 to H2O2 and the oxidation to O2 via a redox cycle of its Cu complex moiety as well as the direct electron transfer of SOD we obtained at Cys/Au. Thus, this coupling between the electrode and enzyme... 

Further, the influence of scan rate on the performance of the SODl-CNT-PPy-Pt electrode in 0.1 M PBS containing 100 pM of NO solution was also investigated (Figure 3.40). It was observed that the anodic peak currents negatively increased hneady with increasing the scan rate of 50 to 300 mV s and also that the characteristic CV remains unchanged. This indicates the favorable orientation of SODl at the CNT-PPy-Pt electrode leading to a facilitated electron transfer of SOD 1, and the linear variation of anodic peak currents (IP) with scan rate (y) indicate that the electrochemical process is diffusion controlled. 

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