The storage of solar energy in the form of chemical fuels is an approach to expanding our use of the sun as a renewable and clean energy resource. Such systems require three main components: a visible light-absorbing photosensitizer, a catalyst for fuel production, and a source of electrons, although the photosensitizer and catalyst may be integrated into a single photocatalyst. In this project, we are developing living bio-nano systems for light-driven hydrogen production that use a nanocrystalline photocatalyst and metabolic electrons from a microbial electron donor.
Our systems employ nanometer-scale crystalline CdSe as photocatalysts. The CdSe is capped with ligands to yield a stable colloid in water. As a sustainable source of electrons, we have developed an approach to harvesting respiratory electrons from Shewanella oneidensis MR-1 (MR-1) to support hydrogen production by CdSe. MR-1 is capable of shuttling electrons to extracellular substrates via extracellular electron transfer (EET). We have shown that MR-1 metabolism sustains CdSe photocatalysis via EET to CdSe.
Here, we report our mechanistic studies of this living bio-nano system for photochemical hydrogen production. By knocking out genes for both MR-1 hydrogenases, we show that EET yielding hydrogen is directed from MR-1 to CdSe. We have also employed knockouts of genes in the EET pathways to determine that specific multiheme cytochromes c on the surface of MR-1 are critical for performing EET from MR-1 to CdSe. We also have determined that indirect EET pathways involving excreted flavins do not play a role in EET to CdSe under our conditions. Building on these results, we are developing strategies for enhancing system activity by overexpressing MR-1 cytochromes c and tuning properties of the CdSe surface that contacts the MR-1 surface cytochromes c.