Quantum dot interactions with and toxicity to Shewanella oneidensis MR-1

Combining abiotic photosensitisers such as quantum dots (QDs) with non-photosynthetic bacteria presents an intriguing concept into the design of artificial photosynthetic organisms and solar-driven fuel production. Shewanella oneidensis MR-1 (MR-1) is a versatile bacterium concerning respiration, metabolism and biocatalysis, and is a promising organism for artificial photosynthesis as the bacterium's synthetic and catalytic ability provides a potential system for bacterial biohydrogen production. MR-1's hydrogenases are present in the periplasmatic space. It follows that for photoenergised electrons to reach these enzymes, QDs will need to be able to enter the periplasm, or electrons need to enter the periplasm via the Mtr pathway that is responsible for MR-1's extracellular electron transfer ability. As a step towards this goal, various QDs were tested for their photo-reducing potential, nanotoxicology and further for their interaction with MR-1. CdTe/CdS/TGA, CdTe/CdS/Cysteamine, a commercial, negatively charged CdTe and CuInS₂/ZnS/PMAL QDs were examined. The photoreduction potential of the QDs was confirmed by measuring their ability to photoreduce methyl viologen with different sacrificial electron donors. The commercial CdTe and CuInS₂/ZnS/PMAL QDs showed no toxicity towards MR-1 as evaluated by a colony-forming units method and a fluorescence viability assay. Only the commercial negatively charged CdTe QDs showed good interaction with MR-1. With transmission electron microscopy, QDs were observed both in the cytoplasm and periplasm. These results inform on the possibilities and bottlenecks when developing bionanotechnological systems for the photosynthetic production of biohydrogen by MR-1.