Directed Nanoscale Self-Assembly of Natural Photosystems on Nitrogen-Doped Carbon Nanotubes for Solar-Energy Harvesting

Natural photosystems (PSs) have received much attention as a biological solar energy harvester because of their high quantum efficiency for energy transfer. However, the PSs hybridized with solid electrodes exhibit low light-harvesting efficiencies because of poor interface properties and random orientations of PSs, all of which interfere with efficient charge extraction and transfer. Herein, we report the linker-free, oriented self-assembly of natural PSs with nitrogen-doped carbon nanotubes (NCNTs) via electrostatic interaction. Protonated nitrogendoped sites on the NCNTs facilitate spontaneous immobilization of the negatively charged stroma side of PSs, which provides a favorable orientation for electron transfer without electrically insulating polymer linkers. The resulting PS/NCNT hybrids exhibit a photocurrent density of 1.25 +/- 0.08 mu A cm(-2), which is much higher than that of PS/CNT hybrids stabilized with polyethylenimine (0.60 +/- 0.01 mu A cm(-2)) and sodium dodecyl sulfate (0.14 +/- 0.01 mu A cm(-2)), respectively. This work emphasizes the importance of the linker-free assembly of PSs into well-oriented hybrid structures to construct an efficient lightharvesting electrode.