A research team from the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley has developed a hybrid system of bacteria and semiconducting nanowires, which imitate the natural process of photosynthesis, by which plants use solar energy for carbohydrate synthesis from water and carbon dioxide.
Acetate, which is currently the most common basis for biosynthesis, is produced from the combination of water and carbon dioxide in this novel artificial photosynthetic system.
This system is capable of capturing carbon dioxide emissions before they are let out into the atmosphere, and then, by using the energy in sunlight, this carbon dioxide is transformed into useful chemical products such as pharmaceutical drugs, biodegradable plastics and also liquid fuels.
“We believe our system is a revolutionary leap forward in the field of artificial photosynthesis,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and one of the leaders of this study. “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.”
Yang is one of the three corresponding authors of a paper about this research, which was published in the journal Nano Letters titled “Nanowire-bacteria hybrids for unassisted solar carbon dioxide fixation to value-added chemicals.” Yang also holds appointments with the Kavli Energy NanoSciences Institute (Kavli-ENSI) at Berkeley and the UC Berkeley.
Chemists Christopher Chang and Michelle Chang are the other corresponding authors and leaders of this research. Both hold joint appointments with Berkeley Lab and UC Berkeley. Further, Chris Chang is a Howard Hughes Medical Institute (HHMI) investigator.
The atmosphere becomes warmer when more carbon dioxide is released. Mainly due to the burning of fossil fuels, the level of atmospheric carbon dioxide is at an all-time high for the last three million years. Nevertheless, fossil fuels, particularly coal, will be an important energy source to meet human needs in the near future. Technologies for carbon sequestering, before the gas is vented into the atmosphere, are being researched. However, all of them require storage of the captured carbon, which again has its own ecological challenges.
Berkeley researchers have solved this challenge using the artificial photosynthetic technique by using the captured carbon dioxide.
“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,” says Chris Chang, an expert in catalysts for carbon-neutral energy conversions. “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.”
The combination of light-capturing biocompatible nanowire arrays and specific bacterial populations causes this system to be advantageous on two counts – it is solar-powered green chemistry and uses 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,” says Michelle Chang, an expert in biosynthesis. “For example, the morphology of the nanowire array protects the bacteria like Easter eggs buried in tall grass so that these usually-oxygen sensitive organisms can survive in environmental carbon-dioxide sources such as flue gases.”
Earlier, Yang and his team developed a nanowire heterostructure artificial forest comprising titatnium oxide nanowires. This was the starting point.
“Our artificial forest is similar to the chloroplasts in green plants,” Yang says. “When sunlight is absorbed, photo-excited electron−hole pairs are generated in the silicon and titanium oxide nanowires, which absorb different regions of the solar spectrum. The photo-generated electrons in the silicon will be passed onto bacteria for the CO2 reduction while the photo-generated holes in the titanium oxide split water molecules to make oxygen.”
Once the nanowire array forest was created, it was filled with microbial populations, which secrete enzymes that catalyze carbon dioxide reduction selectively. The Berkeley team made use of Sporomusa ovate for this research, which is an anaerobic bacterium capable of directly receiving surrounding electrons and using them to reduce carbon dioxide.
“S. ovata is a great carbon dioxide catalyst as it makes acetate, a versatile chemical intermediate that can be used to manufacture a diverse array of useful chemicals,” says Michelle Chang. “We were able to uniformly populate our nanowire array with S. ovata using buffered brackish water with trace vitamins as the only organic component.”
Once carbon dioxide has been reduced by S. ovata to acetate, or some other biosynthetic intermediate, genetically engineered E.coli are used for the synthesis of targeted chemical products. In order to enhance the yields of specific chemical products, the E.coli and S.ovata were kept separate. These two activities- synthesis and catalysis- can be integrated into a one-step process in future.
The success of their artificial photosynthesis system relies upon being able to separate the challenging needs for catalytic activity and light-capture efficiency, rendered possible by the nanowire/bacteria hybrid technology. Under simulated sunlight, similar to that of a leaf, the Berkeley team attained a solar energy conversion efficiency of up to 0.38% for 200 hours.
The target chemical molecule yields obtained from acetate were also positive: as high as 25% for amorphadiene, a precursor to the antimaleria drug artemisinin, 26% for butanol, a fuel similar to gasoline, and 52% for the biodegradable and eco-friendly plastic PHB. As the technology is refined further, better performances are expected.
“We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three-percent,” Yang says. “Once we can reach a conversion efficiency of 10-percent in a cost effective manner, the technology should be commercially viable.”
Along with the corresponding authors, other co-authors of the Nano Letters paper describing this research were Chong Liu, Joseph Gallagher, Kelsey Sakimoto and Eva Nichols.
This research funding was undertaken by the DOE Office of Science.