Development of nanoporous gold based bioelectrodes

Dealloyed nanoprous gold (NPG) is a porous material that possesses three dimensional frameworks of bicontinuous pores and ligaments, fabricated by electro/- chemical dissolution of the less noble component from an Au alloy. In this thesis, bioelectrodes were prepared by immobilising enzymes onto NPG and characterised in detail. For redox enzymes, osmium polymers were used to host the enzyme and as electron transfer mediators. Enzymatic biofuel cells (EBFCs) were assembled with redox enzyme modified bioelectrodes and characterised for a number of applications. A glucose/O2 EBFC device has been developed that can harvest electricity in nonaqueous solvents, which may inspire new applications of EBFCs in bioelectrosynthesis. The EBFC was comprised of a NPG/[Os(4,4′-dimethyl-2,2′- bipyridine)2(polyvinyl -imidazole)10Cl]+/2+ (Os(dmbpy)2PVI)/glucose oxidase (GOx) bioanode and a NPG/[Os(2,2′-bipyridine)2(polyvinylimidazole)10Cl]+/2+ (Os(bpy)2PVI)/bilirubin oxidase (BOx) biocathode. The power output of the cell decreased with increasing solvent hydrophobicity in the alcohols examined and the response of each electrode was restored when the electrodes were placed in phosphate buffer solution after operation in organic solutions. To further expand the range of EBFC, a proof-of-concept “self-powered pulse generator” based on a supercapacitor/EBFC hybrid device has been developed. The device was prepared by immobilising redox enzymes with electrodeposited poly(3,4- ethylenedioxythiophene) (PEDOT) and Os(bpy)2PVI on NPG. Once charged by the internal EBFC, the device can be discharged as a supercapacitor at a current density of 2 mA cm-2 providing a maximum power density of 608.8 μW cm-2, an increase of a factor of 468 when compared to the power output from the EBFC itself. To address the constrained oxygen supply that occurs at the biocathode, an oxygen-independent and membrane-less glucose biobattery was prepared by replacing the BOx based biocathode, with a solid-state NPG/MnO2 cathode. The potential of the discharged MnO2 could be recovered, enabling the development of a proof-of-concept biobattery/supercapacitor hybrid device. The resulting device exhibited a stable performance for 50 cycles of self-recovery and galvanostatic discharge as a supercapacitor at 0.1 mA cm-2. Wearable EBFCs are emerging as potential power sources for wearable microelectronic devices. A key requirement of such cells is the need for flexible electrodes. Mechanically stable and flexible NPG electrodes were prepared using an electrochemical dealloying method consisting of a pre-anodization process and a subsequent electrochemical cleaning step. A flexible lactate/O2 EBFC consisting of a lactate oxidase based bioanode using electrodeposited Os(bpy)2PVI, and a BOx biocathode was placed between two commercially available contact lenses to avoid direct contact with the eye. When tested in air-equilibrated artificial tear solutions (3 mM lactate), a maximum power density of 1.7±0.1 μW cm-2 and an open-circuit voltage of 380±28 mV was obtained, values slightly lower than in phosphate buffer solution (2.4±0.2 μW cm-2 and 455±21 mV, respectively). The decrease was mainly attributed to interference from ascorbate. After 5.5 h of operation, the EBFC retained 20% of its initial power output. Finally, the utilization of NPG in fluidic biocatalysis was investigated. An electrochemically triggered sol-gel process was used to generate a thin silica layer for the immobilisation of lipase onto dealloyed NPG. The catalytic response of the entrapped lipase was examined using the hydrolysis of 4-nitrophenyl butyrate (4- NPB) as a model reaction. A deposition time of 180 s and a lipase concentration of 3 mg/mL was used to prepare the optimised electrode. The operational stability of the silica immobilised enzyme was enhanced on NPG in comparison to that on planar gold, which may arise from confinement of the enzyme in the porous structure. The modified electrodes were incorporated into a 3D printed flow cell with conversion efficiencies of up to 100% after 8 cycles.