Third-generation enzyme-based biosensors

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". ...

Enzymes were historically the first molecular recognition elements [2-6] included in biosensors and continue to be the basis for a significant number of pubUcations in this field. Amperometric biosensors based on glucose oxidase (GOx) are the most famous example of biosensors applied to medical diagnostic. The three modes of oxidation reactions that occur in redox enzyme-based biosensors, like GOx, are referred to as first, second, and third generation as follows [7] ... 

CNTs and other nano-sized carbon structures are promising materials for bioapplications, which was predicted even previous to their discovery. These nanoparticles have been applied in bioimaging and drag delivery, as implant materials and scaffolds for tissue growth, to modulate neuronal development and for lipid bilayer membranes. Considerable research has been done in the field of biosensors. Novel optical properties of CNTs have made them potential quantum dot sensors, as well as light emitters. Electrical conductance of CNTs has been exploited for field transistor based biosensors. CNTs and other nano-sized carbon structures are considered third generation amperometric biosensors, where direct electron transfer between the enzyme active center and the transducer takes place. Nanoparticle functionalization is required to achieve their full potential in many fields, including bio-applications. 

Also, third-generation biosensors for superoxide anion (O ) have been developed based on superoxide dismutase (SOD) immobilised by thin silica-PVA sol-gel film on a gold electrode surface [633]. The preparation of SOD electrode is easy and simple. The uniform porous structure of the silica-PVA sol-gel matrix results in a fast response rate of immobilised SOD and is very efficient for stabilising the enzyme activity. 

A different approach to realize biosensor architectures is the immobihzation of a redox enzyme on the electrode surface in such a manner that direct ET is possible between the active side of the enzyme and the transducer [22, 140]. Thus, free-diffusing redox mediators are not necessary for these types of biosensors [164-169]. Biosensor designs based on direct ET have been investigated thoroughly and comprehensively reviewed [22-26, 170-182]. Figure 1.5 schematically illustrates how such direct ET can be realized within third-generation biosensors. 

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