Organic photovoltaic cells based on photoactive bacteriorhodopsin proteins

Recent advances in materials engineering have enabled photovoltaic (PV) cells to be fabricated from solid state semiconductors, photosensitive organic dyes, and photoactive proteins. One type of organic PV cell is based on the natural light-harvesting protein bacteriorhodopsin (bR) found in the plasma membrane of a salt marsh archaebacteria. When exposed to sunlight, each bR molecule acts as a simple proton pump which transports hydrogen ions from the cytoplasmic to the extracellular side through a transmembrane ion channel. Two types of bR-PV cells comprised of photosensitive dry and aqueous (wet) bR thin films are described in this paper. The self-assembled monolayer of oriented purple membrane (PM) patches from the bR protein is created on a bio-functionalized gold (Au) surface using a biotin molecular recognition technique. The dry bR monolayer is covered with an optically transparent Indium Tin Oxide (ITO) electrode to complete the dry bR-PV device. In contrast, the aqueous bR-PV cell is created by immobilizing the bR monolayer on an Au-coated porous substrate and then inserting the assembly between two micro-reservoirs filled with KCl solutions. Platinum wire probes are then inserted in the opposing liquid reserviors near the porous bR monolayer. The dry bR-PV cell generated a photo-electric response of 9.73 mV/cm(2), while the aqueous bR-PV produced 41.7 mV/cm(2) and 33.3 mu A/cm(2). Although the generated voltages appear small, it may be sufficient to power various microelectromechanical systems (MEMS) and microfluidic devices.