THE innards of next-generation optical computers could be manufactured inside microbes, following the first-ever successful attempt to grow semiconductor nanocrystals inside bacteria.
Bacteria could eventually be used to make devices such as transistors or light-emitting diodes only a few nanometres across, say the researchers who achieved this feat.
Such tiny LEDs will be needed to generate light in the planned ultra-fast microchips that will use optical rather than electrical signals to process data. Controlling the shape, size and crystal structure of the LEDs is crucial because, at these scales, quantum-physical effects come into play. These effects can subtly alter the wavelength of the light emitted by nanocrystals, possibly even making it undetectable. So researchers have looked to biology for ways to control the physical properties of nanocrystals.
Advertisement
Brent Iverson and his colleagues at the University of Texas in Austin wondered if they could manufacture semiconductor crystals inside the bacterium Escherichia coli by encouraging it to ingest the necessary ingredients as separate ions, using its normal ion-transport systems. They placed a culture of E. coli in a solution of cadmium chloride and then added sodium sulphide. The bacteria did indeed take up cadmium and sulphide ions, which reacted inside the microbes to make nanocrystals of the semiconductor cadmium sulphide (Chemistry and Biology, vol 11, p 1553).
A typical bacterium produced around 10,000 nanocrystals, each of which was 2 to 5 nanometres across 鈥� 25,000 times finer than a human hair.
But the researchers have yet to discover what controls crystal size and why they do not grow any larger. The determining factor will have to be identified and controlled before crystals of exactly the same size can be produced and then used in optical chips.
However, Dennis Winge from the University of Utah in Salt Lake City is sceptical about the Texan team鈥檚 chances. He has carried out similar experiments growing crystals in plant and yeast cells and, even after identifying the substance responsible for limiting crystal size, he has found it very hard to control crystal formation.
鈥淭he bacteria took up cadmium and sulphide ions, and these reacted to make semiconductor nanocrystals鈥�
Iverson is still looking for the substance that limits crystal size in bacteria. E. coli are very easy to grow and manipulate in the lab so he hopes that, once it is found, he will be able to make quicker progress than would be possible in yeast or plant cells. His aim is to genetically engineer bacteria to grow crystals of a particular size. 鈥淓. coli are nice little reactors,鈥� he says. 鈥淭hey鈥檙e the bread and butter of the research world, so we should be able to cover ground quickly.鈥�
