Light-Powered Nano-Microbial ‘Factories’ Could Help Reduce Carbon

Using light-activated quantum dots to activate specific enzymes within microbial cells, researchers at the University of Colorado Boulder (CU Boulder) created nanobio-hybrid organisms (nanorgs) that can consume harmful gases such as carbon dioxide (CO₂) and convert them into biodegradable plastic, gasoline, ammonia, and biodiesel. According to the researchers, the technique could improve CO₂ capture and one day potentially replace carbon-intensive manufacturing for plastics and fuels.

A research team led by professor Prashant Nagpal began exploring the potential of quantum dots to offset carbon production in 2013. The goal was to see if quantum dots could act as a sort of “spark plug,” triggering enzymes within microbial cells that have the capability to convert airborne CO₂ and nitrogen, but are not able to make the conversion naturally. An interface between living cells and nanomaterials required chemical coupling through affinity binding and self-assembly; energetic coupling between optoelectronic states of artificial materials and the cellular process; and interfaces designed for biocompatibility.

A gram of biodegradable plastic created by nanobio-hybrid microbes developed by CU Boulder engineers. Courtesy of the Nagpal Lab/University of Colorado Boulder.

When the researchers diffused their tailored quantum dots into the cells of a common microbial species found in soil, they found that exposure to even small amounts of indirect sunlight activated the microbes’ appetite for CO₂, causing the microbes to carry out biochemical conversions without any other source of energy.

The team demonstrated that seven different core-shell quantum dots, with excitations ranging from the UV to the near-infrared (NIR), could couple with targeted enzyme sites in bacteria. When illuminated by light, these quantum dots could drive the renewable production of different biofuels and chemicals using CO₂, water, and nitrogen as substrates. The quantum dots used their zinc-rich shell facets for affinity attachment to the proteins. The nanobio-hybrid organisms catalyzed light-induced air-water-CO₂ reduction to biofuels such as isopropanol and 2,3-butanediol; chemicals such as formic acid and ammonia; and degradable bioplastics.

The microbes, which lie dormant in water, release the converted product to the surface, where it can be skimmed off and harvested for manufacturing. Different combinations of quantum dots and wavelengths produce different products: green wavelengths cause the bacteria to consume nitrogen to produce ammonia, while red wavelengths encourage the microbes to consume CO₂ to produce plastic.

The researchers said that their process shows promise as a technique that can be scaled. The study found that even when the microbial “factories” were activated consistently for hours at a time, they showed few signs of exhaustion or depletion, indicating that the cells can regenerate and limit the need for rotation.

University of Colorado Boulder assistant professor Prashant Nagpal. Courtesy of Casey A. Cass.

The ideal futuristic scenario, Nagpal said, would be to have single-family homes and businesses pipe their CO₂ emissions directly to a nearby holding pond, where microbes would convert them to a bioplastic. Owners of the homes and businesses would be able to sell the resulting product for a small profit while essentially offsetting their own carbon footprint.

“If we could convert even a small fraction of local ditch ponds, it would have a sizable impact on the carbon output of towns,” Nagpal said.

The team’s focus now, he said, will be on optimizing the conversion process and bringing on new undergraduate students. Nagpal is looking to convert the project into an undergraduate lab experiment in the fall semester, funded by a CU Boulder Engineering Excellence Fund grant. He credits his current students with sticking with the project over the course of many years. “It has been a long journey and their work has been invaluable,” he said. “I think these results show that it was worth it.”

The research was published in the Journal of the American Chemical Society (https://doi.org/10.1021/jacs.9b02549).