Photosystem I (PSI)/Photosystem II (PSII)-based photo-bioelectrochemical cells revealing directional generation of photocurrents.

Layered assemblies of photosystem I, PSI, and/or photosystem II, PSII, on ITO electrodes are constructed using a layer-by-layer deposition process, where poly N,N'-dibenzyl-4,4'-bipyridinium (poly-benzyl viologen, PBV(2+) ) is used as an inter-protein "glue". While the layered assembly of PSI generates an anodic photocurrent only in the presence of a sacrificial electron donor system, such as dichlorophenol indophenol (DCPIP)/ascorbate, the PSII-modified electrode leads, upon irradiation, to the formation of an anodic photocurrent (while evolving oxygen), in the absence of any sacrificial component. The photocurrent is generated by transferring the electrons from the PSII units to the PBV(2+) redox polymer. The charge-separated species allow, then, the injection of the electrons to the electrode, with the concomitant evolution of O2 . A layered assembly, consisting of a PSI layer attached to a layer of PSII by the redox polymer PBV(2+) , leads to an anodic photocurrent that is 2-fold higher, as compared to the anodic photocurrent generated by a PSII-modified electrode. This observation is attributed to an enhanced charge separation in the two-photosystem assembly. By the further nano-engineering of the two photosystems on the electrode using two different redox polymers, vectorial electron transfer to the electrode is demonstrated, resulting in a ca. 6-fold enhancement in the photocurrent. The reversed bi-layer assembly, consisting of a PSII layer linked to a layer of PSI by the PBV(2+) redox polymer, yields, upon irradiation, an inefficient cathodic current. This observation is attributed to a mixture of photoinduced electron transfer reactions of opposing effects on the photocurrent directions in the two-photosystem assembly.