Researchers at the Lawrence Berkeley National Laboratory and the University of California at Berkeley are claiming a major advance in artificial biosynthesis after developing a system which can capture carbon dioxide and, using sunlight, convert it into biodegradable polymers, pharmaceutical ingredients or liquid fuels. The system combines the use of semiconductor nanowires and bacteria in a hybrid network, as the team describes in the journal Nano Letters.
The system consists of an ‘artificial forest’ of nanowires made out of silicon and titanium oxides, which lead researcher Peidong Yang says are analoguous to the chloroplasts in green plants — the chlorophyll-containing cells where photosynthesis takes place. When sunlight hits these structures, electrons are freed from the titanium and silicon atoms, which absorb different light wavelengths, and are passed on to Sporomusa ovata bacteria which are nestled within the nanowires “like Easter eggs buried in tall grass,” as co-author Michelle Yang describes it; these bacteria then reduce the carbon dioxide, transforming the usually very stable carbon atom into a more reactive form. Meanwhile, the positively-charged ‘holes’ left by the electrons force water molecules in the air to split apart, generating reactive oxygen that reacts with the reduced CO2 to form acetate, a useful building-block for other organic molecules.
The acetate is then fed to genetically-engineered E coli bacteria which stynthesises it into a variety of product molecules. In the team’s experiment, the two bacterial populations were kept separate but they could be combined, the researchers say.
Sporomusa ovata was chosen because it readily accepts electrons from its environment. It is usually oxygen-sensitive, but the protective effect of the surrounding nanowires have a protective effect. “We were able to uniformly populate our nanowire array with S. ovata using buffered brackish water with trace vitamins as the only organic component,” said Chang.
Separating the light capture and solar conversion and the catalytic activity boosts the efficiency of the process, which the team claims was around 0.38 per cent: about the same as a leaf. They claim yields of about 26 per cent for the liquid fuel butanol; 25 per cent for the antimalarial drug precursor amorphadiene; and 52 per cent for the biodegradable plastic PHB. “We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three per cent,” Yang says. “Once we can reach a conversion efficiency of 10 per cent in a cost effective manner, the technology should be commercially viable.”
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