A Living Semiconductor that Could Sniff Out Poison Gas in a Bio-Terrorist Attack

Silicon Bugs - They’re Half Bacterium, Half Microchip
by Anil Ananthaswamy

Source: New Scientist Magazine

August 12, 2000

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’ 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.

"The micro-organisms were protecting themselves against our heroic measures to kill them," 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 "living cell" out of semiconducting material.

"This is where the imagination runs wild," says physicist John O’Hanlon, project leader and director of the Center for Microcontamination Control at the University of Arizona in Tucson. O’Hanlon 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--which 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’s 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, "capable of growing in areas with very low nutrients, fixing nitrogen and surviving in clean water".

Meanwhile, Baier’s 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. "It looks like diamond dust. It’s absolutely beautiful," 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’s still a lot of development needed.

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