Schematic description of a model system for the photo-reduction of CO₂ to CO using visible light based on a metal oxide nanoparticle functionalised with the enzyme carbon monoxide dehydrogenase (CODH) and sensitised to visible light using a ruthenium bipyridyl photosensitiser. Using the light intensity and illuminated facial area of the reaction vessel an average quantum yield of approximately 0.07% is estimated. Reproduced from ref. 31 with permission from The Royal Society of Chemistry.