Electrically Connecting Bacteria to Nanoparticles for Biotechnological Applications

Combining abiotic photosensitisers such as semiconductor fluorescence emitting nanoparticles – quantum dots (QDs), with non-photosynthetic bacteria ‘in vivo’ 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 very promising organism for artificial photosynthesis. The bacteria’s synthetic and catalytic abilities, together with their longevity, provide a promising system for bacterial biohydrogen production. MR-1’s hydrogenases are present in the periplasmatic space, and it follows QDs or their electrons will need to enter the periplasm via the Mtr pathway that is responsible for the extracellular electrontransfer ability of MR-1. Firstly, various QDs were tested for their nanotoxicology and further for interaction with MR-1 by fluorescence and electron microscopy. CdTe/CdS/TGA, CdTe/CdS/Cysteamine, commercial negatively charged CdTe and CuIn2S/ZnS/PMAL QD were examined, and it was found that the latter two showed no toxicity for MR-1 as evaluated by a colony-forming units method and a fluorescence viability assay. Only commercial negatively charged CdTe QDs showed good interaction with MR-1. Detailed investigation of the above interaction by transmission electron microscopy showed QDs were placed both inside the cell and close to the membrane. Subsequently, the photoreduction power of QDs was evaluated by the methyl viologen assays with different sacrificial electron donors. It was indeed found that QDs have reduction potential sufficiently low to perform MV photoreduction. As assessed by gas chromatography, CdTe/CdS/TGA and negatively charged CdTe QDs supported hydrogen evolution in Shewanella putrefaciens CN-32. The above results establish a proof of concept for photosynthetic production of biohydrogen by CN-32. Further research should be invested in the use of biocompatible sacrificial electron donors and the development of appropriate bacteria mutants that would help to understand the assisted by QDs hydrogen evolution in this bacterium.