Fission yeast makes chips
Since the late 1980s scientists have known that some yeast species can produce a semiconductor material used in advanced lasers and microchips. When cultured on cadmium salts Schizosaccharomyces pombe (S. pombe) produces cadmium sulphide in the form of peptide-coated crystals. Researchers have now developed an easy way to extract these cell deposits with a high degree of purity.
Tiny crystals of cadmium sulphide, about 1-2 nanometres in diameter, have highly specific electronic properties. When grown in a culture of cadmium salts, the yeast S. pombe consistently makes stable cadmium sulphide crystals of just the right size – 1.8nm. But scientists have struggled to extract the material from the cells. “The problem is that the yeast also secretes some cadmium sulphide into the culture medium,” explains Paul Williams of The Biotechnology Centre at Huddersfield University. “So far the only way to get the cadmium sulphide from inside the cells is to physically break them up. That releases the material, but it mixes with the cell debris. Separating them again, though, is hard work.”
Dr Williams has discovered that suddenly freezing the yeast cells and then letting them thaw out makes them release the internal cadmium sulphide. “Cadmium sulphide crystallites leak out during freeze-thaw treatment,” he says. “We first wash the cells free of all the extracellular secreted crystallites then freeze them for up to a month.
“We also found that the leeched 1.8 nanometre crystallites can be further purified from any other contaminating cadmium sulphide crystallites,” Dr Williams continues. “The freeze-thaw crystallites are surrounded by a peptide coat whereas the extracellular secreted crystallites are not. If you leave the extractions to age over a couple of weeks the less stable secretions clump together and thus lose their quantum characteristics, leaving just the 1.8 nanometre crystallites intact.”
Dr Williams is now refining his production process to ensure the large scale consistency of different yeast culture batches of other quantum semiconductors. He expects the first applications of biologically produced cadmium sulphide quantum semiconductor in the development of tunable lasers and as luminescent tags for biological molecules.
“Microbiology could provide sufficient material quickly and cheaply, hastening the application of high performance optoelectronics in computing and research,” he enthuses.