UC Berkeley

The subject of this dissertation focuses on designing biohybrid systems for efficient solar-fuel conversion. As the unlimited energy source for the ecosphere, solar energy could be captured via various devices to meet the rising demand on Earth’s energetic hungry. Among all efforts, solar-driven conversion of carbon dioxide to value-added multi-carbon products is an ambitious objective, which recycles the waste CO2 back into useful chemicals as the energy fuel, polymer, and pharmaceutical precursor. However, pure inorganic CO2 (photo)electrocatalysts suffer from low selectivity, high overpotential, and poor mass transfer. Therefore, we proposed integrating inorganic semiconducting nanomaterials that harvesting solar energy with biological organisms to microbial fix CO2 into target product. By taking advantage of solar energy harvesters, the metabolic pathway in the living organism is essentially powered by solar energy. This work herein explores the novel nanomaterial-biological photosynhtetic biohybrid system for efficient artificial photosynthesis, as well as cytoprotective strategy for further developments. Additionally, elucidation of charge transfer mechanisms at abiotic-biotic interface remains challenge, and this work delves into model system to probe the possible charge transfer process.

Chapter 1 introduces a brief history of new emerging biohybrid system. The diversity of possible biohybrid architectures has been thoroughly discussed, depicting the microorganisms coupled with photovoltaic devices, interfaced with photoelectrodes, and membrane-bound photocatalysts. The synergistic biohybrid design is capable to address the grand challenges for the efficient generation of solar fuels and chemicals. In parallel, the living anaerobe with less oxidative stress tolerance has been carefully treated with various cytoprotective strategies. To get insight of modern nano-biotechnology, the current progress on energy transfer mechanism and the challenges associated with this technology has been examined.

Chapter 2 presents one type of biohybrid system with gold nanoclusters as the photosensitizers. Gold nanocluster with discrete energy levels is regard as the light absorber, and translocation of these gold nanoclusters into nonphotosynthetic bacteria, Moorella thermoacetica, enabled photosynthesis of acetic acid from CO2. Due to the small-size, gold nanocluster enables strong direct interface between organism, which could circumvent the sluggish kinetics of electron transfer for existing biohybrid systems. Besides, biocompatible gold nanoclusters also serve as reactive oxygen species inhibitors to maintain high bacterium viability, realizing CO2 fixation continuously over several days.

Chapter 3 introduces a uniformly cyprotective strategy for anaerobes using metal-organic framework (MOF) monolayer. Biohybrid system has recently being taken as a promising approach to sustainable energy. Among these chemical-producing microbes are anaerobic bacteria, inherently susceptible to O2 and reactive oxygen species that are inevitably generated on anodes. Moorella thermoacetica as the strict anaerobic bacteria was dynamically wrapped by single MOF layer. The high definition of the MOF-bacteria interface involves direct bonding between phosphate units on the cell surface and zirconium clusters on MOF monolayer, enabling cell elongation and separation. The catalytic activitly of MOF enclosure toward decomposition of reactive oxygen species reduces the death of bacteria by fivefold.

Chapter 4 discusses the possible electron transfer process based on the work in chapter 2. The excited states behavior of gold nanoclusters attracts more and more attention due to their vast applications in artificial photosynthesis. The influence of charge-state on the cluster was studied by transient absorption spectroscopy, and interesting correlation between the core-gold and shell-gold relaxation was observed. The long-lived excited states components have been identified as ligand-to-metal transfer of gold nanoclusters with two different sizes. Their photocatalytic activity was evidenced from the electron transfer to methyl viologen. The elucidation of excited states and transfer process in model system could facilitate the understanding of complex biohybrid system.