Development of a light-driven biofuel cell

Inspired by Nature’s photosynthetic system, this Thesis describes the development of a light-driven enzyme biofuel cell. In this type of electrochemical cell, light can be used to initiate biologically-catalysed reactions, such as the reduction of nitrate to nitrite. This biofuel cell was created by combining synthetic chromophores in the form of iridium and ruthenium terpyridine complexes with Saccharomyces cerevisiae cytochrome c, a biological electron transfer protein. The resulting bioconjugates were successfully shown to become reduced upon light exposure in the presence of a sacrificial donor. When attached to electrode surfaces, devices were produced capable of reducing either oxygen to water or nitrate to nitrite. A number of strategies for introducing functional groups onto terpyridine complexes were explored that allow for the covalent attachment of cytochrome c, and the synthesis of both homoleptic and heteroleptic complexes is described. A “chemistry on the complex” approach was used for the direct conversion of alcohol to azide on the complexes, which was then reacted with an alkyne-maleimide to introduce a maleimide group for bioconjugation. The iridium complexes prepared in this work had not been previously described, so further structural, photophysical, and electrochemical studies were conducted. It was found that the introduction of an amine group resulting in unusual behaviour due to an intraligand charge transfer process (ILCT). A number of pure biologically active bioconjugates were synthesised using two approaches. Cytochrome c, possessing a single cysteine, was reacted with a maleimide-appended iridium or ruthenium complex, or in the second approach, an alkyne-tagged cytochrome c was first synthesised and this was then reacted with azide functionalised complexes to produce the desired bioconjugate. Room temperature luminescence and lifetime studies of the bioconjugates showed quenching of luminescence not seen for the free complexes, and the reduction of the heme group upon light exposure was ascribed to electron transfer from the complex to cytochrome c. Complexes and bioconjugates were attached to fluorine tin oxide electrodes to produce a light-driven biofuel cell capable of reducing nitrate to nitrite. Such a biofuel cell may find application in the generation of renewable fuel, in the treatment of waste, in biocatalysis, or in bioelectronics