SOD-based biosensors, for

The sensitivity of the SOD-based biosensors for 02" determination was found to be dependent on the operation potential and the surface coverage of each kind of SOD. The analytical properties of three kinds of 02 biosensors under optimized conditions are summarized in Table 6.6. 

The protocol demonstrated for the development of Cu, Zn-SOD-based third-generation 02 - biosensors is also sutiable for other kinds of SODs, such as Fe-SOD and Mn-SOD in the SOD family [138], In those cases, MPA was used as promoter for the SODs instead of cysteine because cysteine was unable to promote electron transfer of Fe-SOD and Mn-SOD. Figure 6.10 compares the CVs obtained at the Cu, Zn-SOD- (a), Fe-SOD- (b), and Mn-SOD- (c) based electrodes in the absence and presence of 02". As shown, the presence of 02 - in solution obviously increases both anodic and cathodic peak currents of the SODs confined on the electrodes, suggesting the good bifunctional catalytic activity for the reduction and oxidation of 02 - at the SODs, which is similar to the results obtained with the Cu,Zn-SOD/cysteine-modified Au electrode. It should be mentioned that the same response was observed neither at the MPA-modified Au electrode nor at the bare Au electrode under the same conditions. Such a bidirectional electromediation of the SOD-based biosensors is essentially based on the inherent specificity... 

UA, AA, and DOPAC, with the concentrations approximating their extracellular fluid levels were investigated at +300 and -lOOmV at the SOD-based biosensors at +300mV, the interferences from AA and UA were considerable, for instance 15% and 23% current responses were obtained for 500 pM AA relative to 13 nM 02 with Fe-SOD/MPA-modified and Mn-SOD/MPA-modified Au electrodes, respectively. In addition, 10% current response was obtained for 50 pM UA relative to 13 nM 02 at both electrodes. Fortunately, such interferences were well suppressed when the electrodes were polarized at -lOOmV. Besides, the interferences of H202, 5-HIAA, HVA, and DOPAC were negligible at both +300 and -lOOmV at both electrodes [138],... 

The interference from 02 was also investigated with the SOD-based biosensors, in which 02 was generated from K02 rather than from the xanthine-XOD system since the enzymatic system requires 02 for the 02, generation. The removal of 02 from the phosphate buffer by bubbling N2 into the solution was found to produce no observable change in the current response of the SOD-based biosensors toward K02, suggesting that 02 does not interfere with 02, determination under the present experimental conditions. 

In addition to a family of SODs, several other kinds of enzymes and proteins, including tyrosinase [87], galactose oxidase [87], hemin, and cytochrome c (Cyt. c), have been employed to construct enzyme-based biosensors for the O2 determination. Here, we will use Cyt. c as an example to illustrate the analytical mechanism of such a kind of 02 biosensors. For constructing a Cyt. c-based biosensor, Cyt. c is normally inunobilized on the electrode surface and acts as an electron transfer mediator between the electrode and 02. The O2 radical reduces the immobilized Cyt. c (Fe(lll)) to Cyt. c (Fe(II)) and the Cyt. c (Fe(ll)) is reoxidized on the electrode at a potential of... 

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],... 

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