Photobioelectrocatalysis of Intact Chloroplasts for Solar Energy Conversion

Recently, interest in photosynthetic energy conversion has substantially increased. Chloroplasts, the photosynthetic organelle inside higher plants and algae, are the ultimate source of carbon-based fuels. However, they have been less studied in a photobioelectrochemical cell, because their electrochemical communication at an electrode surface is challenging due to their complex membrane system. Although redox polymers are widely used for mediating bioelectrocatalysis, they have never been explored for wiring chloroplasts to electrodes. Herein, a naphthoquinone-functionalized linear poly(ethylenimine) (NQ-LPEI) redox polymer is used as an electron transfer (ET) mediator as well as the immobilization matrix for chloroplasts. They are immobilized on Toray carbon paper electrodes (TPs), and the photoexcited ET from water oxidation is evaluated, showing that intact chloroplasts can undergo direct electron transfer (DET) and mediated electron transfer (MET). Photocurrent generation by DET of chloroplasts results in an oxidative current of 1.5 ± 0.2 μA cm-2. On NQ-LPEI modified electrodes, the oxidative photocurrent increased to 4.7 ± 0.7 μA cm-2 and further improved to 29 ± 6 μA cm-2 in the presence of an additional diffusive mediator, 2,6-dichlorobenzoquinone (DCBQ). The oxidative current produced in the presence of light confirms the ability to oxidize water (H2O) at a chloroplast-modified electrode surface. (Chemical Equation Presented)