Towards improved performance of mediated glucose oxidising enzyme electrodes for biosensing and biopower applications
Bennett, Richard W.
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Energy produced through enzymatic catalysis at electrodes, namely oxidation of glucose and reduction of oxygen may be used to power devices. The goal of powering implantable or semiimplantable medical devices through enzymes has driven forward the field of enzymatic bioelectrochemistry. Significant challenges must be overcome prior to their inclusion as a power source for medical devices such as low power output, poor operational stabilities and biocompatibility issues. Similar challenges hinder the development of continuous use enzymatic biosensors with poor operational stabilities and biocompatibility preventing long term deployment. This thesis aims to build on work carried out previously in the area of enzyme electrode assembly for fuel cell and biosensor development to improve the understanding and performance of glucose oxidising enzyme electrodes for enhanced operation. Novel enzyme electrode immobilisation strategies are investigated and compared with previously published enzyme electrode assemblies. Enzyme electrodes were prepared using the electropolymerisation of L-Dopa to poly(L-Dopa) as an immobilisation strategy. Improved optimisation methodologies are reported and used to assemble glucose oxidising electrodes with higher current output than those previously described.Enzyme electrodes consisting of co-immobilised redox polymer with FADGDH, MWCNTs and PEGDGE achieved current densities of 1.22 ± 0.10 mA cm-2 in PBS containing 5 mM glucose were achieved, 52 % higher than those using one factor at a time optimisation approaches. Enzyme electrodes consisting of co-immobilised redox polymer with FADGDH, MWCNTs and PEGDGE were tested in test solutions containing individual plasma components to investigate the reduced performance of enzyme electrodes in human physiological solutions. Electrodes tested in the presence of uric acid generated the lowest current responses and the lowest operational stabilities of all individual plasma components. Current densities of 0.9 ± 0.10 mA cm-2 were observed at 5 mM glucose concentrations in the presence of uric acid compared to 1.5 ± 0.10 mA cm-2 in PBS with no uric acid present. 46 % of the initial current remained after 12 hours continuous operation in the presence of uric acid compared to 72 % for PBS with no uric acid present. In comparison, glucose oxidase based electrodes maintained identical operational stabilities of 70 % with and without uric acid present while cellobiose dehydrogenase electrodes maintained 86 % stability in PBS, with 33 % stability in the presence of uric acid. Enzyme electrodes were prepared with Nafion coatings to minimise the effect of plasma on the enzymatic film. Current densities of 8.0 ± 2.0 mA cm-2 were recorded in 100 mM for glucose oxidase enzyme electrodes prepared with a 0.5 % w/v Nafion coating with an operational stability of 84 % for 12-hour continuous operation when tested in artificial plasma. FADGDH based electrodes in the same test conditions produced current densities of 4.5 ± 0.70 mA cm-2 and an operational stability of 58 %. Further work is required to fully understand changes to enzyme electrode performance in physiological solutions and strategies to minimise the impact of these solutions for operation in-vivo to allow for higher power outputs and longer operational lifetimes for prototype devices.
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