6.2.2. Whole-Cell Based Semi-Artificial Photosynthesis

Semi-artificial photosynthesis has attracted great attention for harvesting solar energy for electricity or chemical generation [5]. Instead of isolated photosynthetic proteins (e.g., PSII), photosynthetic microorganisms such as algae [221], cyanobacteria [222–225] and purple bacteria [226], or extracted organelles such as thylakoids [227] have been studied for biophotoelectrochemical systems. The development of 3D structured electrodes increased the loading capacity which enhances

photocurrents [228]. However, low EET rates still limit the efficiency of these photoelectrodes. It has been shown that redox polymers can improve electron transfer kinetics between microorganisms and electrode [221,226]. Furthermore improvement in EET kinetics and new electrode designs for accommodating microorganisms are required to boost the performance of photoelectrochemical devices that are based on photosynthetic microorganisms [111].

An emerging direction in the semi-artificial photosynthesis is to interface synthetic light-harvesting materials with non-photosynthetic microorganisms for value-added chemicals [229,230]. As a model microorganism, *S. oneidensis* has been widely studied for integration with artificial light-harvesting materials for solar-driven microbial synthesis. The transmembrane cytochrome MtrCAB, helped by OmcA located on the outside of the membrane, are known to transfer electrons between the interior of the cell and extracellular materials (Figure 6) [231,232]. In the presence of a sacrificial electron donor, MtrC and OmcA can be photoreduced by water-soluble photosensitisers including eosin Y, fluorescein, proflavine, flavin, and adenine dinucleotide, riboflavin and flavin mononucleotide [233]. In an in vitro approach, it was showed that dye-sensitised TiO<sup>2</sup> nanoparticles can photoreduce MtrC or OmcA, either in solution or one an electrode surface [233–235] and it might be possible to extend this process to reductive photosynthesis in *S. oneidensis* [236]. To provide a proof-of-concept, we recently reconstituted MtrCAB into proteoliposomes encapsulating a redox dye, Reactive Red 120 (RR120), which can be reductively bleached. Using light-harvesting nanoparticles including dye-sensitised TiO<sup>2</sup> nanoparticles, amorphous carbon dots and nitrogen doped graphitic carbon dots, we observed RR120 reductive decomposition inside the lumen of the MtrCAB proteoliposomes, confirming that electrons were transferred from the nanoparticles via transmembrane MtrCAB complex into the liposome lumen [237]. These results show that the rational design of light-harvesting nanoparticles and protein hybrids can lead to the development of semi-artificial photosynthetic systems for solar fuels synthesis.
