Nanobionic Architectures of Photosystem I on π-System Modified Graphene Electrodes

Artificial systems exploiting the features of natural photosynthesis are increasingly becoming a focus of current research. Particularly the two photosystems (PS) of the oxygenic photosynthesis have attracted the attention of researchers to build up new solar energy-converting systems.^[1,2] Besides the light-to-current conversion, PSI may also be used for light-driven redox and/or enzymatic reactions to be applied in photobiocatalysis.

To date a couple of approaches for coupling PSI to gold surfaces via covalent^[3] and non-covalent^[4] protocols have been described with the aim to obtain a short distance of the reaction center to the electrode for direct electron transfer (DET).^[4,5] Nevertheless, DET from PSI to a transducer results rather often in minor photocurrent densities, mainly due to long electron tunnelling distances between the reaction center and the electrode. A main reason is related to the lack of controlled orientation of PSI on the electrode surface.

In this contention we have been focusing on the unidirectional assembly of PSI on highly conductive graphene electrodes using different π-systems as interface modifiers for a proper and directed assembly of PSI. The different π-systems serve as an artificial scaffold harbouring functional groups which interplay with PSI for site-directed assembly. Particularly important is the hydrophilic-hydrophobic balance. Based on the strong interaction between conjugated aromatic compounds and the graphene material via π-π-stacking, we have designed a simple but smart platform to fabricate light-driven photoelectrochemical devices.^[6] Due to the possibility of surface property adaptation and the excellent conductivity of graphene, the modified biohybrid electrodes exhibit a well-defined photoelectrochemical response. Different groups of π-systems have been studied, but in particular the PSI–graphene electrode applying pyrene butyric acid NHS ester displays a very high photocurrent output of 23 µA cm² already at the open circuit potential. This can be further increased by an overpotential and the use of an electron acceptor (methyl viologen) under air saturation up to 135 µA cm².^[6] Comparing the graphene–PSI biohybrid systems based on different π-system-modifiers reveals that the pyrene derivatives result in higher current outputs compared to the anthracene derivatives and that the covalent fixation during immobilization appears more efficient compared to simple adsorption. Interestingly, the pyrene-based PSI electrodes also display a nearly unidirectional photocurrent generation, establishing the feasibility of conjoining these nanomaterials as potential constructs in next-generation photovoltaic devices. A systematic investigation on this topic will be presented.

[1] A. Badura et al, Energy Environ. Sci., 2011, 4, 3263–3274.

[2] F. Wang, et al, Adv. Mater., 2013, 25, 349–377.

[3] P. N. Ciesielski, et al, ACS Nano, 2008, 2, 2465–72.

[4] H. A. Kincaid, et al, Langmuir, 2006, 22, 8114–20.

[5] K. R. Stieger, S. C. Feifel, et al, PCCP, 2014, 16, 15667–15674.

[6] S. C. Feifel K. R. Stieger, et al, J. Mater. Chem. A, 2015, DOI: 10.1039/c5ta00656b.