Microbial Photosynthesis & Soft Polymers: the yellow brick road to sustainability

In bio-electrochemical systems, living and metabolically active microorganisms can be used for the sustainable production of energy. Conductive polymer layers on the surface of several bacterial species have been used to intercept the electron flow produced by microbial metabolism, funnel it outside the cells, and eventually transfer it toward the electronic circuit of a biohybrid device. Purple photosynthetic bacteria are anoxygenic microorganisms with very versatile metabolisms that use sunlight1 to oxidize a broad variety of organic compounds in addition to heterotrophic and photoautotrophic alternative metabolisms. Biocompatibility of several monomers, such as gallic acid, L-DOPA, EDOT and dopamine were tested by in vivo addition in the growth media of the photosynthetic purple non sulphur Rhodobacter (R.) sphaeroides. Furthermore, the ability of these monomers to self-assemble and polymerize was considered. Among the tested monomers, polydopamine (PDA), produced by self-assembly of dopamine, is a very versatile and bioinspired polymer which has found widespread applications2 due its ability to adhere and cover surfaces of different chemical composition. The oxidative conditions employed for the formation of this dark insoluble polymer are mild and biocompatible and have inspired scientists to develop novel nanomaterials for optoelectronics. We have used PDA conductive coatings as biotic-abiotic interfaces in biohybrid photoelectrochemical devices through the encapsulation of entire bacterial cells or single components – e.g. photosynthetic reaction center (RC) - of R. sphaeroides3, ensuring electronic communication of the biological component with the electrodes’ surfaces in photoelectrochemical cells. Post-functionalization of PDA also enables fine-tuning of properties.