This thesis reports the development of acid-operating microbial fuel cells (MFCs) for the investigation of elevated electrical conductivity and resulting enhanced bioelectricity generation. This project describes the use of extremophile microorganisms as the biological material in MFCs, for the investigation of low internal resistance biological fuel cells. In particular, this thesis focuses on BPV (biological photovoltaic) cells, a type of MFC that utilises autotrophic biological material, which relies on oxygenic photosynthesis and hence simply requires water as the electron donor (unlike traditional MFCs, which are dependent of an organic substrate feed). Novel reactor designs based on acidophilic and metallotolerant microorganisms, studied using electrochemical techniques, are reported for the first time. The novel strategy consists in the adoption of very low pH and elevated heavy metal concentration levels for biological fuel cell operation, which is possible due to the choice of suitable extremophile microorganisms that are able to thrive under such severe physicochemical conditions. In order to support the analysis of the subject MFCs, a series of electrochemical and fluorescence techniques were employed. Chapter 3 reports the study of standard BPV cells, focusing on classic cell configuration and choice of biological material. BPV cells based on the standard prokaryotic and eukaryotic strains Synechococcus elongatus and Chlorella vulgaris, respectively, were built and electrochemically characterised by means of polarisation curves and continuous power output monitorisation. Subsequently, a study on the potential conditioning of BPV cells was conducted using Pulse Amplitude Modulation (PAM) Fluorimetry; it is the first documented observation of short-term electrolytic potential conditioning effects on photosynthetic efficiency and associated parameters. The work in chapters 4 and 5 explores the extent to which acidophiles may be used as the biological material in MFCs. A search to find a set of naturally-occurring, metallotolerant acidophiles is undertaken throughout the Rio Tinto ecosystem, selected for its unique extreme physicochemical nature and reported extremophile presence. Chapter 4 informs about the physicochemical characterisation of the chosen sampling points, describing the evolution of pH, electrical conductivity, heavy metal concentration, ferric/ferrous ion balance and dissolved oxygen throughout a natural year, in order to identify the sites with the hardest physicochemical conditions. Finally, chapter 5 investigates the presence of living microorganisms in the sampled sites, enabling the identification of the best location for the purpose of this study. A tailored sediment cell was built and tested in situ (for the first time in an extremophilic environment), and compared to the electrical performance of a novel BPV cell based on commercially-available photosynthetic acidophile Dunaliella acidophila.

Doctor of Philosophy / Department of Chemistry / Jun Li / Solar energy as a sustainable resource is a promising alternative to fossil fuels to solve the tremendous global energy crisis. Development of three generation of solar cells has promoted the best sunlight to electricity conversion efficiency above 40%. However, the most efficient solar cells rely on expensive nonsustainable raw materials in device fabrication. There is a trend to develop cost-effective biophotovoltaics that combines natural photosynthetic systems into artificial energy conversion devices such as dye sensitized solar cells (DSSCs). In this research, a model system employs natural extract light-harvesting complex II (LHCII) as a light-absorbing sensitizer to interface with semiconductive TiO₂ and plasmonic nanoparticles in DSSCs. The goal of this research is to understand the fundamental photon capture, energy transfer and charge separation processes of photosynthetic pigment-protein complexes along with improving biophotovoltaic performance based on this model system through tailoring engineering of TiO₂ nanostructures, attaching of the complexes, and incorporating plasmonic enhancement. The first study reports a novel approach to linking the spectroscopic properties of nanostructured LHCII with the photovoltaic performance of LHCII-sensitized solar cells (LSSCs). The aggregation allowed reorganization between individual trimers which dramatically increased the photocurrent, correlating well with the formation of charge-transfer (CT) states observed by absorption and fluorescence spectroscopy. The assembled solar cells demonstrated remarkable stability in both aqueous buffer and acetonitrile electrolytes over 30 days after LHCII being electrostatically immobilized on amine-functionalized TiO₂ surface. The motivation of the second study is to get insights into the plasmonic effects on the nature of energy/charge transfer processes at the interface of photosynthetic protein complexes and artificial photovoltaic materials. Three types of core-shell (metal@TiO₂) plasmonic nanoparticles (PNPs) were conjugated with LHCII trimers to form hybrid systems and incorporated into a DSSC platform built on a unique open three-dimensional (3D) photoanode consisting of TiO₂ nanotrees. Enhanced photon harvesting capability, more efficient energy transfer and charge separation at the LHCII/TiO₂ interface were confirmed in the LHCII-PNP hybrids, as revealed by spectroscopic and photovoltaic measurements, demonstrating that interfacing photosynthesis systems with specific artificial materials is a promising approach for high-performance biosolar cells. Furthermore, the final study reveals the mechanism of hot electron injection by employing a mesoporous core-shell (Au@TiO₂) network as a bridge material on a micro-gap electrode to conduct electricity under illumination and comparing the photoconductance to the photovolatic properties of the same material as photoanodes in DSSCs. Based on the correlation of the enhancements in photoconductance and photovoltaics, the contribution of hot electrons was deconvoluted from the plasmonic near-field effects.

Photosynthetic biofilms have much higher cell density than suspended cultures and when grown in a stacked waveguide configuration, can have orders of magnitude higher areal productivity. Evanescent and plasmonic growth of biofilm cultures have been demonstrated, solving issues with light penetration impeding growth, but thus far the technology has been limited to biofuel production applications. In this thesis, plasmonically excited cyanobacterial biofilms are used to produce electrical power in a photosynthetic-plasmonic-voltaic device. This approach uses red lasers to deliver light to cells via an optical waveguide through the generation of surface plasmons at the interface between a metal and dielectric, in this case a glass-gold-air interface. This gold film serves a dual purpose as a current collector for electrons generated at the cell surface. Experiments presented here demonstrate positive power output light response under both direct light and plasmonic excitation and produced equivalent power output of 6 uW/m2 under similar light power intensities.

Photosynthetic biofilms have much higher cell density than suspended cultures and when grown in a stacked waveguide configuration, can have orders of magnitude higher areal productivity. Evanescent and plasmonic growth of biofilm cultures have been demonstrated, solving issues with light penetration impeding growth, but thus far the technology has been limited to biofuel production applications. In this thesis, plasmonically excited cyanobacterial biofilms are used to produce electrical power in a photosynthetic-plasmonic-voltaic device. This approach uses red lasers to deliver light to cells via an optical waveguide through the generation of surface plasmons at the interface between a metal and dielectric, in this case a glass-gold-air interface. This gold film serves a dual purpose as a current collector for electrons generated at the cell surface. Experiments presented here demonstrate positive power output light response under both direct light and plasmonic excitation and produced equivalent power output of 6 uW/m2 under similar light power intensities.

This two-part thesis focuses on biointerfaces of two different biological systems. It specifically examines the interplay of structure and functionality in these biointerfaces. Part one studies photo-bio-electrochemically active bacteria and the strong dependence of their electrical current generation on electrode structure and pigment organisation. Part two uncovers surprising design principles of photonic structures on flower petals and presents research tools to study disordered optical systems. Biophotovoltaics (BPV) is a newly described biophysical effect in which a biofilm of photosynthetic microorganisms associated with an anode produces electrical current that can be harvested and passed through an external circuit. In this thesis-part, an experimental set-up is presented to quantitatively measure photo-electric activity of cyanobacteria in BPVs. Using this set-up, a systematic study of anode morphologies reveals that large electrode surface areas enhance photocurrents by two orders of magnitude, identifying structuration as key design criterion for bioelectrochemical interfaces. Electrodes with micrometer-sized pores allow enhanced direct contact area with bacteria, but with tested cyanobacteria this did not result in a photocurrent increase, disproving recent speculations in the literature. Furthermore, a theoretic-mathematical framework is presented to estimate light-energy utilisation in biofilms. It is detailed how pigment concentration and distribution affects the light-level dependent saturation of electron harvesting biofilms. This study brings the theory together with experiments, such as genetic modification and photo-current measurements. Part two of this thesis approaches the interaction of light and biointerface structuration from a different angle. In a significant extension of the candidate’s MPhil project, it was discovered that the disorder in natural photonic structures can be an advantage rather than a limitation in biology. With biological image analysis, optics simulations and nano-manufacturing a new photonic effect is uncovered which is iridescent but surprisingly constant in chroma. In collaboration with plant scientists, it is shown that many flowers have co-evolved disordered surface structuration that generates this bee visible colouration.