A decaheme cytochrome as a molecular electron conduit in dye-sensitized photoanodes

In nature, charge recombination in light-harvesting reaction centers is minimized by efficient charge separation. Here, we aimed to mimic this by coupling dye-sensitized TiO2 nanocrystals to a decaheme protein, MtrC from Shewanella oneidensis MR-1, where the ten hemes of MtrC form a ~7 nm long molecular wire between the TiO2 and the underlying electrode. The system is assembled by forming a densely-packed MtrC film on an ultra-flat gold electrode, followed by the adsorption of approximately 7 nm TiO2 nanocrystals that are modified with a phosphonated bipyridine Ru(II) dye (RuP). The step-by-step construction of the MtrC/TiO2 system is monitored with (photo)electrochemistry, quartz-crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). Photocurrents are dependent on the redox state of the MtrC, confirming that electrons are transferred from the TiO2 nanocrystals to the surface via the MtrC conduit. In other words, TiO2/MtrC molecular wires function as hybrid photodiodes in which MtrC traps the conduction-band electrons from TiO2 before transferring them to the electrode. To the best of our knowledge, this report is the first demonstration of a photobioelectrochemical system that uses a redox protein to mimic efficient charge separation found in biological photosystems.