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A LIVING semiconductor that could sniff out poison gas in a bio-terrorist
attack has been accidentally discovered by researchers in the US and Northern
Ireland. The discovery followed the scientists鈥� failure to eliminate some
particularly persistant bacteria from computer chip production lines.

The researchers had tried everything to destroy the microbes, from
ultraviolet light to powerful oxidants. But the bacteria survived every
attempt.

鈥淭he micro-organisms were protecting themselves against our heroic measures
to kill them,鈥� says biophysicist Robert Baier, director of the Center for
Biosurfaces at the State University of New York in Buffalo.

When microchips are cleaned with ultra-pure water, the water can dissolve
some semiconducting materials, such as germanium oxide, which can then
crystallise around the bacteria. The bacteria survive extremely well inside
their crystal homes, impervious to the best human efforts to eradicate them. But
the problem has a silver lining. The microbes have created a 鈥渓iving cell鈥� out
of semiconducting material.

鈥淭his is where the imagination runs wild,鈥� says physicist John O鈥橦anlon,
project leader and director of the Center for Microcontamination Control at the
University of Arizona in Tucson. O鈥橦anlon and Baier believe the
semiconductor-encrusted bacteria can be used for building bio-transistors.

In a normal three-terminal transistor, the current flow between the source
and the drain is controlled by the voltage across the gate terminal. In the
bio-transistor, the gate would be replaced by the bacteria-semiconductor
crystal.

The idea is to tap into biological processes, such as respiration and
photosynthesis, which result in electron transfers, says Baier. The bacteria
could be induced to produce electrons when exposed to light or organic
vapours鈥攚hich would turn on the bio-transistor. Such an exquisitely
sensitive device could detect poison gas during bio-terrorist attacks, says
Baier.

Michael Larkin, a microbiologist at the Queen鈥檚 University of Belfast, has
been identifying the bacteria. He declines to name them, as the results of the
research have yet to be published. However, he says that the bacteria are
extremophiles, 鈥渃apable of growing in areas with very low nutrients, fixing
nitrogen and surviving in clean water鈥�.

Meanwhile, Baier鈥檚 group is making the crystals in the lab by trickling
bacteria-infested ultra-pure water over semiconductor wafers. Bio-crystals form
on the surface of the wafers and can be scraped off. 鈥淚t looks like diamond
dust. It鈥檚 absolutely beautiful,鈥� says Baier.

The next step is to get the crystals to behave like a transistor, says Baier.
Sandip Tiwari, director of the Cornell Nanofabrication Facility at Cornell
University in Ithaca, New York, says that while the device should work there鈥檚
still a lot of development needed.