Unbranched Hybrid Conducting Redox Polymers for Intact Chloroplast-Based Photobioelectrocatalysis

Photobioelectrocatalysis (PBEC) adopts the sophistication and sustainability of photosynthetic units to convert solar energy into electrical energy. However, the electrically insulating outer membranes of photosynthetic units hinder efficient extracellular electron transfer from photosynthetic redox centers to an electrode in photobioelectrocatalytic systems. Among the artificial redox-mediating approaches used to enhance electrochemical communication at this biohybrid interface, conducting redox polymers (CRPs) are characterized by high intrinsic electric conductivities for efficient charge transfer. A majority of these CRPs constitute peripheral redox pendants attached to a conducting backbone by a linker. The consequently branched CRPs necessitate maintaining synergistic interactions between the pendant, linker, and backbone for optimal mediator performance. Herein, an unbranched, metal-free CRP, polydihydroxy aniline (PDHA), which has its redox moiety embedded in the polymer mainchain, is used as an exogenous redox mediator and an immobilization matrix at the biohybrid interface. As a proof of concept, the relatively complex membrane system of spinach chloroplasts is used as the photobioelectrocatalyst of choice. A "mixed" deposition of chloroplasts and PDHA generated a 2.4-fold photocurrent density increment. An alternative "layered" PDHA-chloroplast deposition, which was used to control panchromatic light absorbance by the intensely colored PDHA competing with the photoactivity of chloroplasts, generated a 4.2-fold photocurrent density increment. The highest photocurrent density recorded with intact chloroplasts was achieved by the "layered" deposition when used in conjunction with the diffusible redox mediator 2,6-dichlorobenzoquinone (-48 ± 3 μA cm^-2). Our study effectively expands the scope of germane CRPs in PBEC, emphasizing the significance of the rational selection of CRPs for electrically insulating photobioelectrocatalysts and of the holistic modulation of the CRP-mediated biohybrids for optimal performance.