UC Berkeley hybrid semiconductor nanowire-bacteria system for direct solar-powered production of chemicals from CO2 and water
Researchers at UC Berkeley have developed an artificial photosynthetic scheme for the direct solar-powered production of value-added chemicals from CO2 and water using a two-step process involving a biocompatible light-capturing nanowire array with a direct interface with microbial systems.
As a proof of principle, they demonstrated that, using only solar energy input, such a hybrid semiconductor nanowire–bacteria system can reduce CO2 at neutral pH to a wide array of chemical targets, such as fuels, polymers, and complex pharmaceutical precursors A paper on their work is published in the ACS journal Nano Letters.
Natural photosynthesis, which harvests 130 TW of solar energy to generate up to 115 billion metric tons of biomass annually from the reduction of CO2, provides motivation for the development of artificial systems that can capture the energy of the sun to convert CO2 and H2O to value-added chemicals of societal benefit. However, such an approach has not been fully realized owing to a host of unmet basic scientific challenges. For example, enzymes isolated from microorganisms and plants can selectively catalyze CO2 reduction with low energy barriers; however, they do not self-repair outside their native cellular context and are often intolerant to oxygen. Consequently, bio-derived CO2-reducing catalytic systems are not directly applicable to oxygen- containing CO2 sources such as flue gas.
Another challenge for artificial photosynthesis is the selective synthesis of complex organic molecules. Nature transforms CO2 into a variety of complex molecules using a limited number of biosynthetic intermediates as building blocks. However, in the case of artificial photosynthesis, the selection of such an intermediate is difficult: ideally, mass transport requires it to be water-soluble, and it should also be easily incorporated into many biosynthetic pathways.—Liu et al.
The basic concept of the UC Berkeley strategy is to interface biocatalysts in their native cellular environments directly with semiconductor light-absorbers for unassisted solar CO2 reduction. They envisioned a two-step strategy that mimics natural photosynthesis, in which light capture by a biocompatible nanowire array can interface and directly provide reducing equivalents to living organisms for the targeted synthesis of a variety of value-added chemical products from CO2 fixation.
This is, the researchers point out, a different approach that of conventional microbial electrosynthesis, in which microbes do not directly interact with light-absorbing devices.
For the proof of principle, they used silicon and titanium dioxide nanowire arrays as the light-capturing units and the anaerobic bacterium Sporomusa ovata as the bio-catalyst. S. ovata can reduce CO2 under mild conditions and produce acetate. Acetate is a biosynthetic precursor to a variety of value added chemicals—including functionalized aliphatics and aromatics, lipids, alkanes and complex natural products—via acetyl coenzyme A (acetyl-CoA).
The hybrid nanowire array/bacteria system produced acetic acid under aerobic conditions (21% O2) with low overpotential (η < 200 mV), high Faradaic efficiency (up to 90%), and long-term stability (up to 200 h).
The acetate (∼6 g/L) produced by the nanowire/S. ovata combination was then activated to acetyl-CoA by genetically engineered Escherichia coli and used as a building block for a variety of value-added chemicals, such as n-butanol, polyhydroxybutyrate (PHB) polymer, and three different isoprenoid natural products.
The yield of target molecules was as high as 26% for n-butanol, 25% for one of the isoprenoid compounds (amorphadiene), and up to 52% for PHB biopolymer. Taking into account the 0.38% efficiency from CO2 to acetic acid, the researchers calculated a solar energy-conversion efficiency of 0.20% from CO2 to PHB biopolymer.
Overall, the production of different organic products with vastly different synthetic pathways proves the versatility of the integrated approach starting from one common bio-chemical building block, analogous to natural photosynthesis.
The results reported here outline a solar-energy conversion process that combines the strengths of semiconductor nano-devices and bacterium-based biocatalysts. Key advantages of the nanowire-based device are the enhanced oxygen tolerance that allows exhaust gas to be directly fed into the system, thereby enabling use of strict anaerobes with aerobes, as well as the high measured CO2 fixation activity of the nanowire−bacteria hybrid. Moreover, this modular platform simplifies the overall system design by allowing for the production of a variety of molecular targets, without any setup change in the components for light capture and CO2 reduction into acetate, by varying only the downstream microorganisms.—Liu et al.
Chong Liu, Joseph J. Gallagher, Kelsey K. Sakimoto, Eva M. Nichols, Christopher J. Chang, Michelle C. Y. Chang, and Peidong Yang (2015) “Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals” Nano Letters doi: 10.1021/acs.nanolett.5b01254