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Scientists Create Artificial Photosynthesis System

A team of researchers from the University of California, Berkeley, the Lawrence Berkeley National Laboratory, and the Kavli Energy NanoSciences Institute, has created a hybrid system of semiconducting nanowires and bacteria that mimics the natural photosynthetic process.

The artificial photosynthesis system has four general components: (i) harvesting solar energy; (ii) generating reducing equivalents; (iii) reducing carbon dioxide to biosynthetic intermediates, and (iv) producing value-added chemicals.

The artificial photosynthesis system has four general components: (i) harvesting solar energy; (ii) generating reducing equivalents; (iii) reducing carbon dioxide to biosynthetic intermediates, and (iv) producing value-added chemicals.

“We believe our system is a revolutionary leap forward in the field of artificial photosynthesis. Our system has the potential to fundamentally change the chemical and oil industry in that we can produce chemicals and fuels in a totally renewable way, rather than extracting them from deep below the ground,” said Dr Peidong Yang of the University of California, Berkeley, who co-led the study published in the journal Nano Letters.

In natural photosynthesis, carbon dioxide is first reduced to common biochemical building blocks using solar energy, which are subsequently used for the synthesis of the complex mixture of molecular products that form biomass.

The artificial photosynthetic scheme developed by Dr Yang and his colleagues functions via a similar two-step process by developing a biocompatible light-capturing nanowire array that enables a direct interface with microbial systems.

“In natural photosynthesis, leaves harvest solar energy and carbon dioxide is reduced and combined with water for the synthesis of molecular products that form biomass. In our system, nanowires harvest solar energy and deliver electrons to bacteria, where carbon dioxide is reduced and combined with water for the synthesis of a variety of targeted, value-added chemical products,” explained co-author Dr Christopher Chang of the Lawrence Berkeley National Laboratory and the University of California, Berkeley.

By combining biocompatible light-capturing nanowire arrays with select bacterial populations, the new system offers a win/win situation for the environment: solar-powered green chemistry using sequestered carbon dioxide.

“Our system represents an emerging alliance between the fields of materials sciences and biology, where opportunities to make new functional devices can mix and match components of each discipline,” said co-author Dr Michelle Chang, also of the Lawrence Berkeley National Laboratory and the University of California, Berkeley.

The system starts with an ‘artificial forest’ of nanowire heterostructures, consisting of silicon and titanium oxide nanowires. Once the forest is established, it is populated with populations of Sporomusa ovata (an anaerobic bacterium that readily accepts electrons directly from the surrounding environment and uses them to reduce carbon dioxide) that produce enzymes known to selectively catalyze the reduction of carbon dioxide.

Once the carbon dioxide has been reduced by S. ovata to acetate or some other intermediate, genetically engineered Escherichia coli bacteria are used to synthesize targeted chemical products.

A key to the success of the system is the separation of the demanding requirements for light-capture efficiency and catalytic activity that is made possible by the nanowire/bacteria hybrid technology.

With this approach, the scientists achieved a solar energy conversion efficiency of up to 0.38-percent for about 200 hours under simulated sunlight, which is about the same as that of a leaf.

The yields of target chemical molecules produced from the acetate were also encouraging – as high as 26-percent for butanol, a fuel comparable to gasoline, 25-percent for amorphadiene, a precursor to the antimaleria drug artemisinin, and 52-percent for the renewable and biodegradable plastic PHB.

The team is currently working on a second-generation system which has a solar-to-chemical conversion efficiency of three-percent.

“Once we can reach a conversion efficiency of 10-percent in a cost effective manner, the technology should be commercially viable,” Dr Yang said.

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Chong Liu et al. Nanowire-Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals. Nano Lett., published online April 07, 2015; doi: 10.1021/acs.nanolett.5b01254