Liquid Sunlight: The Evolution of Photosynthetic Biohybrids

This is an excerpt of an article in Nano Letters by Peidong Yang, S.K. and Angela Chan Distinguished Professor of Energy and Professor of Chemistry. 

Liquid sunlight can be considered as a new form of chemical energy converted and stored in chemical bonds from solar energy. Natural photosynthesis in green plants represents one of the most elegant and powerful examples of such a process. As the only energy input into the ecosphere, solar energy positions itself as one of the most promising solutions to address the crisis on the environment and climate change. Efficient capture and storage of solar energy can provide unlimited renewable power sources and drive the capture and conversion of greenhouse gases such as CO₂ into valuable chemicals. Such an artificial photosynthetic process presents one of the most important solutions, if not the only one, toward net-zero carbon emission or even negative emission society in the near future.

Despite intense chemistry, biology, materials, and engineering efforts to develop such CO₂ fixation processes, comprehensive solutions remain elusive. Traditionally this CO₂ fixation problem has been approached with either a purely synthetic material approach or biological approach. Biology achieves CO₂ fixation to energy-dense multicarbon compounds with impressive selectivity yet suffers from relatively poor photosynthetic light absorption. The synthetic material method typically uses light-absorbing semiconductors coupled with synthetic catalysts for solar-driven CO₂ reduction; however, the progress of this approach is hampered by the lack of a suitable synthetic catalyst with sufficient activity and selectivity, especially for multicarbon compounds.

In this regard, our recent work focuses on a novel cyborgian design for solar fuel production: the introduction of photosynthetic biohybrid systems (PBSs) for photochemical biosynthesis using CO₂ where the optoelectronic properties of inorganic material systems are integrated with the self-regenerative properties of biological systems. Biological microorganisms engage a collection of enzymes and reductive pathways to produce long-chain hydrocarbons from simple building blocks, including CO₂, N₂, and H₂O. Enzymes and proteins within the metabolic pathways of the living cells benefit from an embedded building code in genetic information and can be repaired and replicated as necessary. For these reasons, the integration of inorganic light-harvesters and whole-cell biocatalysts would take advantage of the best functions of each component. In particular, semiconducting nanomaterials are highly configurable with tunable broadband light absorption and surface charge, pair well with microorganisms and enable significant sunlight capture, hence solar-to-chemical conversion. It has been demonstrated that by using an autotropic homoacetogen Sporomusa ovata or M. thermoacetica as a CO₂-reducing electrocatalyst, it is possible to couple these microorganisms with silicon nanowires, CdS quantum dots, and gold nanoclusters for solar-powered CO₂ reduction. Under solar irradiation, the integrated biohybrids could effectively capture sunlight and reduce CO₂ into a common chemical intermediate, acetic acid, without additional external energy input, and the energy conversion efficiency could be as high as 3.6%. This solar-to-chemical energy conversion efficiency can be further optimized by tuning the medium pH, semiconductor bandgap, microorganism selection, the semiconductor and bacteria loading density, as well as the charge transfer interface. In addition, several other microorganisms including R. eutropha, S. cerevisiae, A. vinelandii, and C. necator have also been introduced into this biohybrid family.

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