Enhancing the functionality of photovoltaic and photonic biointerfaces through structuration

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.