July 1, 2008 A tiny sensor that monitors electrical conductivity allows scientists to detect the presence of explosives. The sensor measures the conductivity of two different thin films, one made of a cobalt compound and another made of a copper compound. When reacting to most fumes, the two films respond in similar ways, but when exposed to hydrogen peroxide the films show a difference in electrical conductivity. When the sensor indicates this difference, that means that trace amounts of hydrogen peroxide are present, a common ingredient of explosives.
From terrorist bombings on the ground and in the air, TATP, a peroxide-based explosive has been used in many suicide bombings. There's no easy way to detect the chemical in the field. But now, that is about to change.
In 2005 terrorists blew up subway trains in London using homemade bombs made of a peroxide called TATP. The same explosive shoe bomber Richard Reid tried to use to blow up an airliner in 2001.
But terrorists may soon find their bombs harder to hide. Inside a machine, a team of physicists and chemists built an electronic nose able to sniff out the explosive chemical.
No bigger than a penny, the sensor chip can detect the tiniest traces of hydrogen peroxide vapor. Normally, thin films of copper and cobalt phthalocyanine conduct the same level of electrical current when exposed to gas. But add hydrogen peroxide vapor … and copper's current strength increases while cobalt's decreases.
"This material that is absorbed on the chip is a peroxide," Ivan Schuller, professor of physics at the University of California, San Diego in La Jolla, Calif., told Ivanhoe.
"About 50 or so molecules we, and others, have looked at it's the only one that will give this opposite behavior," William Trogler, professor of chemistry at UCSD said. Small and low-powered, it fits in a variety of packages for use by the military or homeland security.
"As a checkpoint monitor in airports that sort of thing to screen people as they're coming through. Have they been handling this type of explosive?" Todd Mlsna, Ph.D., president of Seacoast Science, Inc., told Ivanhoe.
The chip can distinguish between types of peroxide, so your whitening toothpaste won't raise alarms. And they're cheap -- once mass produced these sensors could be made for less than a dollar a chip. A small price to pay to save lives.
HOW DOES THE DETECTOR WORK? The sensor detects hydrogen peroxide, which is often used in homemade explosives. It can detect trace amounts indirectly by monitoring the electrical conductivity of a thin metal film. The film responds to certain chemicals (oxidizers) by showing an increase in electrical current, and to others (reducers) by reducing the amount of electrical current. Hydrogen peroxide is an oxidizing agent, but cobalt films will respond to it by decreasing current, and copper or nickel films will show increased current. It is this unusual reaction that made the sensor possible. The tiny sensor capitalizes on this property to detect very small amounts of hydrogen peroxide.
The Materials Research Society, the Institute of Electrical and Electronics Engineers, Inc., The American Physical Society, and the American Industrial Hygiene Association contributed to the information contained in the TV portion of this report, with support from The Camille and Henry Dreyfus Foundation, Inc.