Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes

Light-driven enzymatic catalysis is enabled by the productive coupling of a protein to a photosensitizer. Photosensitizers used in such hybrid systems are typically costly, toxic, and/or fragile, with limited chemical versatility. Carbon dots (CDs) are low-cost, nanosized light-harvesters that are attractive photosensitizers for biological systems as they are water-soluble, photostable, nontoxic, and their surface chemistry can be easily modified. We demonstrate here that CDs act as excellent light-absorbers in two semibiological photosynthetic systems utilizing either a fumarate reductase (FccA) for the solar-driven hydrogenation of fumarate to succinate or a hydrogenase (H₂ase) for reduction of protons to H₂. The tunable surface chemistry of the CDs was exploited to synthesize positively charged ammonium-terminated CDs (CD-NHMe₂⁺), which were capable of transferring photoexcited electrons directly to the negatively charged enzymes with high efficiency and stability. Enzyme-based turnover numbers of 6000 mol succinate (mol FccA)^-1 and 43,000 mol H₂ (mol H₂ase)^-1 were reached after 24 h. Negatively charged carboxylate-terminated CDs (CD-CO₂^-) displayed little or no activity, and the electrostatic interactions at the CD-enzyme interface were determined to be essential to the high photocatalytic activity observed with CD-NHMe₂⁺. The modular surface chemistry of CDs together with their photostability and aqueous solubility make CDs versatile photosensitizers for redox enzymes with great scope for their utilization in photobiocatalysis.