Using a silicon chip and parts from an inexpensive CD player, chemists at the University of California, San Diego have developed a portable nerve-gas sensor capable of detecting "G-type" nerve agents, such as sarin, soman and GF.
The achievement should eventually permit the development of a large number of small and inexpensive sensors that could be deployed by soldiers across a battlefield or by police after a terrorist explosion to rapidly detect the presence of certain nerve agents and to track the movements of the deadly plumes.
"With multiple sensors that have a radio transmitter attached to them, you can tell how big the cloud is and where it is moving and relay that information to a base station," says Michael J. Sailor, a professor of chemistry and biochemistry at UCSD. He will provide details of his group’s achievements today at the 220th national meeting of the American Chemical Society in Washington, DC.
The innovative silicon sensor was constructed by a team that included William C. Trogler, a professor of chemistry and biochemistry, and postdoctoral associates Sonia Letant and Honglae Sohn. It works by selectively detecting compounds with a phosphorus-fluorine chemical bond, such as sarin, at very low concentrations.
To accomplish this, the scientists used a catalyst that Trogler and his co-workers had developed for the Army to detoxify materials containing nerve agents and other deadly chemicals with phosphorus-fluorine bonds. This catalyst breaks the phosphorus-fluorine bond in "G"-type nerve agents, resulting in the production of hydrogen fluoride, which is used commercially to etch and frost glass.
The sensor detects the presence of hydrogen fluoride through a silicon interferometer—a stamp-sized silicon wafer, similar to a computer chip, with an optical coating containing the catalyst. The rainbow-colored optical coating, which is akin to the sheen left by a thin film of oil on water, changes color when molecules of hydrogen fluoride hit its surface. "These silicon interferometers can detect very, very small changes in color," says Sailor.
The key to their sensitive detection is the use of a small laser, similar to that found in CD players, which measures the small changes in intensity of light reflecting from the optical coating on the surface of the silicon chip. "It turns out that if you take a laser that’s at the right frequency that matches the properties of that layer, you can measure very small amounts of chemicals as they enter the coating," says Sailor.
While the diode laser that the UCSD scientists built for their sensor is a bit more sophisticated than those in inexpensive CD players, it can be reproduced cheaply. In fact, the researchers’ first sensors were constructed from five inexpensive CD players they purchased at Fry’s, a local electronics discounter. "Our program manager at the Defense Advanced Projects Research Agency, which sponsored our research, raised an eyebrow when I told him that story," says Sailor. "But for 24 bucks, we got an interferometer that was sensitive enough to detect chemicals in the parts per billion range."
The low-cost feature of the UCSD design should make it possible to deploy handfuls of sensors in a terrorist nerve-gas attack, like the 1995 Tokyo subway bombing, in which sarin was used. Because the laser is capable of recording the accumulation of hydrogen fluoride molecules on the silicon chip’s surface, the sensor can also be used as a dosimeter. "You can tell how much nerve gas an area has been exposed to," says Sailor.
He says the main advantage of the sensor is that it is more specific to the detection of G-type nerve agents than the surface acoustic-wave devices, which are currently used to detect nerve gas, but which tend to produce an excess of false alarms.
"The advantage of this new development is that we’ll be able to reduce the false-alarm rate," adds Sailor, whose team published the technical details of their development in a recent issue of the Journal of the American Chemical Society. "The disadvantage is that we’re specific to only one type of nerve agent."
Although the UCSD researchers have not tested their sensor on nerve gas, they have demonstrated that it can detect a compound called diisopropylfluorophosphonate, or DFP, which is structurally related to sarin and soman, at a level of 800 parts per million. They plan to test the UCSD sensor on live nerve gas at an Army research laboratory later this year.
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