Electrochemical wiring of cyanobacteria to anodes using polymers towards biohybrid devices for solar-chemical production Rachel M. Egan 1 , Jonas Honacker 2 , Kaizhong Xing 1 , Lin Su 3 , Evan Wroe 1 , Linying Shang 1 , Joshua M. Lawrence 1,4 , Laura T. Wey 5 , Bartosz Witek 6 , Nicolas Plumeré 2 , Erwin Reisner 1 , Jenny Z. Zhang 1 1 Department of Chemistry, University of Cambridge, the United Kingdom, 2 Straubing Campus for Biotechnology and Sustainability, Technical University of Munich, Germany, 3 School of Biological and Behavioural Sciences, Queen Mary University of London, the United Kingdom, 4 Department of Biochemistry, University of Cambridge, the United Kingdom, 5 Department of Life Technologies, University of Turku, Finland, 6 School of Natural and Environmental Sciences, Newcastle University, the United Kingdom Bio-photoelectrochemical devices using living microorganisms as catalysts could be used as sustainable technologies for solar-chemical conversion. As a material, living microorganisms are abundant, capable of self-repair and reproduction, and possess excellent catalytic capabilities [1] . The integration of photosynthetic microorganisms which perform solar-driven water oxidation into such devices is hindered by poor electron exchange efficiencies with the electrode. Polymeric mediators may be introduced at the cell-electrode interface to improve charge transfer by providing a stable and direct route for electrons to reach the electrode. At present, the polymer properties that are key for efficient wiring of cyanobacteria to electrodes are poorly understood. Bridging this knowledge gap will be essential for guiding rational design of next-generation polymers specifically tailored to achieve maximal output [2] . In this work, we systematically tested two common polymers — the conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and an osmium-based redox polymer — in terms of their ability to act as wiring tools for the model cyanobacterium Synechocystis sp. PCC 6803 on three-dimensional electrode structures. By using tailored analytical photoelectrochemistry methods, we were able to identify the conditions under which each polymer served only as an immobilisation matrix, enhancing the photocurrent by means of increasing the cell loading rather than by mediation. The contribution of various parameters including polymer type (redox vs conductive), deposition method (dropcasting vs electropolymerisation), immobilisation geometry (layered vs mixed), light management properties, polymer morphology and polymer loading towards optimal photocurrent outputs were deconvoluted. Under the conditions tested here, the osmium-modified electrodes produced higher photocurrents vs the PEDOT-modified electrodes (62-fold vs 27-fold enhancement respectively) relative to unmodified electrodes at a low light intensity. The success of the osmium polymer system was partially attributed to the electrostatic interaction between the polymer chains and the extracellular polymeric substances produced by the cells, enabling it to adopt a configuration conducive to efficient wiring. Longevity tests revealed that the photocurrent mediated by the osmium polymer exhibited superior stability over 24 hours compared to that mediated by a state-of-the-art lipid-soluble diffusional species. The practical application of these high-performing cyanobacteria anodes was demonstrated through a proof-of-concept bio‑photoelectrochemical device that relies solely on microbial catalysts to drive solar-to-chemical energy conversion. References 1. Lawrence, J. M.; Egan, R. M.; Hoefer, T.; Scarampi, A.; Shang, L.; Howe, C. J.; Zhang, J. Z. Rewiring Photosynthetic Electron Transport Chains for Solar Energy Conversion. Nature Reviews Bioengineering 2023 , 1 (12), 887–905. https://doi. org/10.1038/s44222-023-00093-x 2. Wroe, E. I.; Egan, R. M.; Willyam, S. J.; Shang, L.; Zhang, J. Z. Harvesting Photocurrents from Cyanobacteria and Algae. Current Opinion in Electrochemistry 2024 , 46 , 101535. https://doi.org/10.1016/j.coelec.2024.101535
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