An Unbranched, Hybrid Conductive-Redox Polymer for Interfacing Intact Chloroplasts and Electrode Surfaces during Photobioelectrocatalysis

Abstract

Photobioelectrocatalysis (PBEC) adopts the sophistication of photosynthetic units, their abundant availability, easy processing and sustainability, to convert solar energy into electricity. Specifically, the use of chloroplasts as photobioelectrocatalysts is desirable due to their metabolic independence and photoprotection mechanisms. However, the protein-based outer membranes of chloroplasts impede efficient charge transfer onto electrode surfaces during PBEC. Typical bio-inspired redox polymers used to ameliorate this charge transfer are branched, and contain redox pendants peripherally attached to a backbone. Pendants extract, and propagate electrons via collision-based electron tunneling. Conversely, we investigate the ability of an unbranched hybrid conductive-redox polymer, polydihydroxy aniline (PDHA), which has its redox moiety embedded in a conductive backbone. The redox moiety of PDHA is a quinone derivative, with redox potentials higher than the quinone-based redox active sites of chloroplasts. The biohybrid chloroplast-PDHA electrode encapsulates chloroplasts in a conductive matrix of PDHA, improving charge transfer. Holistically, PDHA facilitates the immobilization of chloroplasts and mediate electron transfer between chloroplasts and electrode surfaces. Our results demonstrate that a 120 % photocurrent increment is obtained upon combining chloroplasts with PDHA. Sequentially layering chloroplasts and PDHA on electrode surfaces evinces a 260 % increment in photocurrent responses. We report the highest photocurrent recorded with chloroplasts during PBEC (-48±3 µA cm-2) by pairing chloroplast-PDHA electrodes with diffusible redox mediator, 2,6-dichlorobenzoquinone. The study shows that redox polymer designs for artificially mediating electron transfer from chloroplasts can extend to unbranched conductive polymers in PBEC systems. Figure 1