UCLA

Interfacing light-harvesting semiconductors with microbial biochemistry is a viableapproach of producing chemicals with high efficiency from air, water, and sunlight. Yet it remains unclear whether all of the absorbed photons in the semiconductors will be transferred through the materials-biology interface for solar-to-chemical production and whether the presence of inorganic light-absorbers will beneficially affect the microbial metabolism. Here we report a microbe-semiconductor hybrid for CO2 and N2 fixation with internal quantum efficiencies approaching the biochemical limits (Chapter 2). Integrating CO2/N2-fixing bacterium Xanthobacter autotrophicus with CdTe quantum dots results in a photocatalytic system with internal quantum yields of 47.2 � 7.3% and 7.1 � 1.1% for the fixation of CO2 and N2, iii respectively, in comparison to the theoretical upper limits of 46.1% and 6.9% imposed by the stoichiometry in biochemical pathways. Photophysical studies suggest a microbe-semiconductor interface endowed with fast charge-transfer kinetics (Chapter 3). The establishment of charge transfer between the microbes and the quantum dots is discovered to be a time-dependent process from the stationary emission spectra and the flow cytometry experiments, which is calculated to be at least one magnitude faster than the other processes of electron quenching from the results of lifetime measurements. Then, we conducted the Stern-Volmer study and found the charge transfer at the microbe-material interfaces to be a hybrid of static quenching in addition to a dynamic quenching behavior. In Chapter 4, we applied proteomic and metabolomic analyses to study microbial metabolism in the hybrid condition and discovered a material-induced regulation of microbial metabolism possibly favoring higher quantum efficiencies compared to the biological counterparts alone. The proteins of charge transfer and nitrogen fixation are observed to be significantly upregulated, together with the detection of all enzymes necessary for the fixation of carbon dioxide. Interestingly, we found a frugal ATP policy in the microbes which would favor the synthesis of small N-containing metabolites instead of large biomolecules, contributing to the observed high IQYs. The interdisciplinary study unveils the cyborg effects of semiconducting nanomaterials for altered microbial metabolisms and heralds additional tunability for custom-designed materialsbiology interface in solar-to-chemical production and beyond.