Three researchers from Lehigh University have found a better, more natural way to produce quantum dots, the valuable semiconductor nanoparticles that support medical imaging applications. The method begins with engineered bacterial cells in a simple, aqueous solution and ends with functional semiconducting nanoparticles, all without resorting to high temperatures and toxic chemicals.

Industrial processes take many hours to grow the nanocrystals, which then need to undergo additional processing and purifying steps. The environmentally friendly biosynthesis process, however, takes minutes to make the full range of quantum dot sizes (about 2 to 3 nanometers).

No post-processing steps are needed to harvest the final, water-soluble product. Because bacterial cells are much larger than the nanocrystals, researchers simply use a centrifuge to pull the cells away from the quantum dots in the solution.

Cell-based growth needs only basic equipment in a typical biochemistry wet lab setting. Using a process called directed evolution, the Lehigh team altered the bacteria so they would selectively produce quantum dots.

Housed in a beaker containing water, cadmium and sulfur precursors, and minimum levels of carbon and nitrogen, the cells forgo most normal biological functions. The cells build quantum dots by sequestering metal ions from their environment, generating a reactive sulfur source and controlling the resultant structure to form a crystal.

The tiny semiconductor nanoparticles have light-emitting properties that are valuable for cellular and in vivo imaging.