Photocurrent generation by direct electron transfer using photosynthetic reactioncentres

Abstract

Photosynthetic reaction centres (RCs) convert light into separated charges with nearlyperfect quantum efficiency, and have been used to generate photocurrent. Previous workhas shown that electron tunnelling rates between redox centres in proteins dependexponentially on the tunnelling distance. In this work the RC from Rhodobactersphaeroides was genetically modified with the aim of achieving the shortest tunnellingdistances yet demonstrated between the RC’s electron-accepting P site and underlyinggraphite and gold electrodes, and between the electron donor Q site and graphiteelectrodes. Opposite charges are carried to counter electrodes using mobile mediators, as indye-sensitised solar cells. Native RCs are bound to graphite surfaces throughN-(1-pyrene)iodoacetamide. Although the linker’s length is only 4 Å, the electrontransfer pathway between the Q electron donor site on the RC and the electrodesurface is still too large for current to be significant. A mutant version with theelectron acceptor P side close to the graphite surface produced currents of 15 nA cm−2 upon illumination. Direct binding of RCs to a gold surface is shown, resulting in currents of 5 nA cm−2. In both cases the current was unaffected by mediator concentration but increased withillumination, suggesting that direct electron transfer was achieved. The engineering of anRC to achieve direct electron transfer will help with long term efforts to demonstrateRC-based photovoltaic devices.