Photosystem II providing direct electron transport with electrodeposited poly(mercapto-p-benzoquinone) was immobilised onto modified gold electrodes. The properties of the biosensor were characterized using linear sweep voltammetry, atomic force microscopy and photo-induced chronoamperometry. I0=9nM and LOD=0.7nM for DCMU showed improved sensitivity compared to all previously studied PSII biosensors.