Surface modification of electrodes for enzymatic fuel cell application
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The immobilisation chemistry of enzymes and redox complexes capable of shuttling electrons between enzymes and electrode surface can have an impact on the magnitude and stability of current response, with implications for application as biosensor and enzymatic biofuel cell development. Enzyme electrode prepared using co-immobilisation of redox mediators, multiwalled carbon nanotubes and polymer support using a chemical crosslinker provide 3-dimensional biofilms for an electrocatalytic response for substrate, such as sugar, important in biosensor and biofuel cell applications. The objective of this thesis was to investigate the interactions of redox complexes, enzyme and nanostructure on electrode surface for application to sensor and with view to developing a semi-or fully implantable, membrane-less enzymatic biofuel cell anode for energy generation. Furthermore, the optimisation of the electrochemical response of enzyme electrode was evaluated using a design of experiment approaches, in seeking to improve the current density under physiological conditions. Chapter 2 reports a simple immobilisation strategy using electrochemically-induced grafting of osmium-based redox mediators onto carbon electrodes surface. The redox-active layer is produced by electrochemical oxidation of alkyl-amine functional group, distal to a ligand of the redox complex, to form reactive radicals that couple to carbon surfaces, with coupling characterized by XPS and voltammetry. The electrode surface modified by electrografting of an osmium complex was therefore examined for its ability to mediate electron transfer from glucose oxidase, in solution, as a result of glucose oxidation. The simple and efficient methodology for modifying carbon surfaces to obtain redox active monolayers offers many potential applications to biosensor and biofuel cell device development. Chapter 3 reports on thicker bioelectrocatalytic films produced by the addition of nanostructured supports, and by cross-linking alkyl-amine functional groups, distal to a ligand of the redox complex, to the redox enzyme and functionalised polymers, with concomitant adsorption/grafting to the electrode surface. Co-immobilisation of enzyme, redox complex and polymer support using a chemical crosslinker, therefore, provides a 3-dimensional biofilm for catalytic electro-oxidation of glucose. The chapter three focus on an investigation of the effect of the selection of polymers, possessing carboxylic acid functional groups, as chemical supports for immobilisation of mediators and enzymes at electrode surfaces. The electrochemical response was calculated for the enzyme electrode composed of glucose oxidase, multiwalled carbon nanotubes and a range of redox mediators and polymer supports, in seeking to improve the current density and stability of glucose-oxidising enzyme electrode under physiological conditions. Overall, a maximum current density of 3.4 mA cm-2 at 0.2 V vs. Ag/AgCl, in pH 7.4 phosphate buffers at 37 °C, is achieved for oxidation of glucose, showing promise for application to glucose determination in blood and as an anode in a biofuel cell for power generation. Chapter 4 reports on the design of experiment method to provide a more statistically relevant approach to optimise the amount of different components used to construct enzyme electrodes. The enzyme electrodes perofrmance was evaluated and optimised under physiological conditions using design of experiment model. Based on the optimised amount of components, enzyme electrodes display improved current densities of 1.2 ± 0.1 mA cm-2 and 5.2 ± 0.2 mA cm-2 at 0.2 V vs. Ag/AgCl in buffer containing 5 mM and 100 mM glucose, respectively. Design of experiment model was experimentally validated. The observed current density of enzyme electrodes in physiological conditions was consistent with the predicted values of the model. Therefore, design of experiment approach can be applied effectively and efficiently to improve the performance of enzyme electrodes for application to glucose oxidising enzymatic biofuel cell device. Chapter 5, focus on the genipin based immobilisation procedure to fabricate enzymes and redox complexes on the electrode surfaces. The performance of enzyme electrode prepared by co-immobilisation of redox complexes and enzymes to chitosan matrix via genipin (a natural crosslinker) was evaluated for glucose oxidation. The genipin based crosslinking offer many biomedical applications for its ability to crosslink amine based biomaterials and exhibits lower cytotoxicity. The genipin crosslinked enzyme electrodes displayed current density of 0.74 ± 0.08 mA cm-2 ¬at 0.45 V (vs. Ag/AgCl) in 50 mM PBS (pH 7.4, 37°C, 150 rpm) containing 100 mM glucose showing potential for incorporation a biocompatible technology in bioelectrochemical devices for in vivo applications. Finally, Chapter 6 summarises the main research findings and provides some opinion on the future research direction to continue this work.