Semiconductor nanoparticles give bacteria a boost

Bacteria covered in semiconductor nanoparticles make more efficient use of light and could be used to create acetic acid, a key ingredient in fuel and plastics.

This is the conclusion of scientists in the US who have developed a process that outperforms natural photosynthesis and could be utilised in a range of industrial processes.

The researchers presented their work at the 254th National Meeting & Exposition of the American Chemical Society (ACS) on August 22, 2017.

Photosynthesis is the process in which plants and other organisms convert light into chemical energy but chlorophyll - the green pigment that plants use to harvest sunlight - is relatively inefficient. To help humans capture more of the sun's energy than natural photosynthesis, scientists have now covered bacteria in tiny, highly efficient solar panels to produce useful compounds.

"Rather than rely on inefficient chlorophyll to harvest sunlight, I've taught bacteria how to grow and cover their bodies with tiny semiconductor nanocrystals," said Kelsey K. Sakimoto, Ph.D., who carried out the research in the lab of Peidong Yang, Ph.D. "These nanocrystals are much more efficient than chlorophyll and can be grown at a fraction of the cost of manufactured solar panels."

The pursuit of alternatives to fossil fuels as sources of energy and feedstocks for chemical production has led to the creation of artificial photosynthetic systems that generate renewable energy and simple organic chemicals. Such systems, however, are not efficient enough for the commercial production of fuels and feedstocks.

Research in Yang's lab at the University of California, Berkeley focuses on harnessing inorganic semiconductors that capture sunlight and pairing them with organisms such as bacteria that can then use the energy to produce useful chemicals from carbon dioxide and water.

"The thrust of research in my lab is to essentially 'supercharge' non-photosynthetic bacteria by providing them energy in the form of electrons from inorganic semiconductors, like cadmium sulphide, that are efficient light absorbers," Yang said in a statement. "We are now looking for more benign light absorbers than cadmium sulphide to provide bacteria with energy from light."

Sakimoto worked with a naturally occurring, non-photosynthetic bacterium, Moorella thermoacetica, which, as part of its normal respiration, produces acetic acid from carbon dioxide (CO₂). Acetic acid can be modified to help produce a number of fuels, polymers, pharmaceuticals and commodity chemicals through complementary, genetically engineered bacteria.

When Sakimoto introduced cadmium and the amino acid cysteine, which contains a sulphur atom, to the bacteria, they synthesised cadmium sulphide (CdS) nanoparticles, which function as solar panels on their surfaces. The hybrid organism, M. thermoacetica-CdS, produces acetic acid from CO₂, water and light.

"Once covered with these tiny solar panels, the bacteria can synthesise food, fuels and plastics, all using solar energy," Sakimoto said. "These bacteria outperform natural photosynthesis."

The bacteria operate at an efficiency of more than 80 per cent, and the process is self-replicating and self-regenerating, making this a zero-waste technology.

"Synthetic biology and the ability to expand the product scope of CO₂ reduction will be crucial to poising this technology as a replacement, or one of many replacements, for the petrochemical industry," Sakimoto said.