Electron transfer between GOx and electrode surfaces

Further improvements (and attention to the above errors) can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the redox center of the enzyme to the surface of the electrode. Glucose oxidase does not directly transfer electrons to conventional electrodes because a thick protein layer surrounds its flavin redox center. Such a thick protein shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate [29]  

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

FIGURE 3.2 Sequence of events that occur in second-generation (mediator-based) glucose biosensor mediated systems. 

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In addition to the previously described dehydrogenase-based CNT electrodes, electrochemical biosensors that employ other types of enzyme-modified CNTs have also been reported. Kowalewska and Kulesza applied CNTs with adsorbed redox mediator tetrathiafulvalene (TTF) for electrochemical detection of glucose." TTF-modified CNTs were found to facilitate electron transfer between GOx and the electrode surface for glucose detection. Jia et al. reported a similar strategy for the detection of lactate using MWCNTs modified with TTF and lactate oxidase. Since TTF does not cause skin irritation and the CNT/TTF platform also enables low-potential sensing of lactate, CNT/ TTF/lactate oxidase-based electrochemical biosensors conld be used to detect lactate in perspiration directly on human skin. This was accomplished by preparing temporary tattoos from CNT/ TTF/lactate oxidase-conductive carbon ink that was transferred onto a human subject s skin. ... 

Sharma et al. reported application of poly(2-fluoroaniline) films which was electrochemically deposited on ITO coated glass plates to produce glucose sensors. GOx was immobilized on the polymer films by physical adsorption methods. Sensors constructed by this method showed efficient electron transfer between the adsorbed GOx and the electrode surface and were found to be stable up to 32 days [128]. 

Bioelectronics is another apphcation area, in which rotaxanes, particularly redox-active rotaxanes, could make a significant impact Enzyme electrodes are altered in these apphcations by direct electron transfer between the electrode surface and the redox enzyme. Electronic communication between the surface and the redox enzyme centers is hindered, because a separation exists. This impediment can be circumvented by aligning the enzyme with the electrode and utilizing the redox relay units as go-betweens. The aforementioned concept has been exploited to associate an apoprotein, apo-gjucose oxidase (apo-GOx), onto relay-functionalized materials including flavin adenine dinucleotide (FAD) monolayers, nanoparticles, and carbon nanotubes [85-88]. Katz etal. used reversible redox-active rotaxane shuttles in the bioelectrocatalyzed oxidation of glucose [80]. 

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