PHOTOELECTROCHEMICAL RESPONSES OF PHOTOSYSTEM-II PARTICLES IMMOBILIZED ON DYE-DERIVATIZED TIO2 FILMS

Oxygen-evolving photosystem II (PS II) particles prepared from spinach chloroplasts were deposited on the surfaces of different types of dye-derivatized polycrystalline TiO2 electrodes. A photoelectrochemical cell was set up with the PS II-covered TiO2 as the anode, a platinum cathode as the counterelectrode and a saturated calomel electrode (SCE) as reference electrode, with 50 mM NaCl as electrolyte. The cell was connected to a potentiostat maintained at 0.2 V vs. SCE. On irradiation of the cell with white light, in the presence of a PS II electron acceptor (benzoquinone derivatives), a steady photocurrent was observed. This photocurrent production was completely inhibited by 3-(3′,4′-dichlorophenyl)-1,1′-dimethylurea (DCMU), a photosynthetic oxygen evolution inhibitor; there was no decrease in the photocurrent response on addition of DCMU when no PS II was deposited on the TiO2 or when the TiO2 was covered with heat-inactivated PS II. Thus the source of electrons for the DCMU-sensitive photocurrent is water photolysis. The maximum monochromatic incident photon to current conversion efficiency observed was 12%. Photocurrents up to 35 μA cm-2 of electrode surface and 10 mA mg-1 of chlorophyll were obtained on illumination with white light of 35 mW cm-2 intensity. Approximately 20% of the photosynthetic electron transport rate of the PS II particles (measured as oxygen evolution) was manifested as photocurrent. © 1990.