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Portable Plastic Explosives Detector

An explosives detector that uses luminescent polymers is being field-tested in Iraq.

In 2001, Richard Reid boarded an American Airlines flight with plastic explosives inside his shoes. Since then, Americans have had to remove their shoes during airport security checks and, at some airports, go through an air puffer that uses spectrometry to check passengers for traces of explosives. A new, easy to use explosives detector developed by RedXDefense of Rockville, MD, could provide a quick, simple visual diagnostic for the plastic explosives favored by terrorists like Reid. The device is portable and designed for use by nonscientists at security checkpoints and under harsh conditions. The detector is currently undergoing field tests in Iraq.

Explosive handprint: A trace of TNT left behind from a handprint is visible when the surface is sprayed with a fluorescent polymer and illuminated with ultraviolet (UV) light (top). This technology, which can detect a range of explosives, is being commercialized by RedXDefense as a portable kit (bottom) that includes batons for sampling a surface, canisters of the luminescent polymer, and a UV viewing box.

“Most explosives-detection methods go after sensing vapors,” says William Trogler, a chemistry professor at the University of California, San Diego, who developed the technology behind the device. This works well for detecting buried land mines and other devices that use volatile explosives, such as TNT, that form a gas that can be detected in the air. But the plastic explosives often used by terrorists are not very volatile, and technologies for their detection usually require dislodging the explosive from a surface, such as with a puff of air, before running a chemical analysis. And these systems are not portable.

Trogler developed a sprayable polymer that fluoresces blue-green under ultraviolet (UV) light, unless in the presence of explosive molecules, including PETN and TNT, that turn off this fluorescence. When the polymer is sprayed on a surface and examined under UV light, explosives appear as black spots. The polymer is described in a recent issue of the Journal of Materials Chemistry.

The product, called the XPak, consists of a plastic viewing box and a removable baton that resembles a lint roller. First, the soldier or police officer using the device rolls the paper-covered baton over the surface to be tested or has the subject being screened grip the roller with his hand. The baton is then placed inside the viewing box, where it is sprayed with the luminescent polymer. The user then looks through the viewfinder and employs a knob to rotate the baton under UV light. If there are no explosives present, the baton will look entirely blue. Wherever there are traces of explosives, even as small as a few picograms, there will be black spots.

Trogler says that the advantage of this system is that the results can be interpreted visually by nonscientists. “This is intuitive,” he says. “The human eye is very good, and you don’t have that overhead” of image-analysis software or spectrometry.

Timothy Swager, head of the chemistry department at MIT, says that the chemistry behind the XPak is not new. His research in explosives detection has been commercialized by another company, ICx Technologies, of Arlington, VA. The ICx detector is slightly more sensitive, Trogler readily admits, but it’s meant for different uses–in particular, the detection of buried land mines. But such sensitivity is not always necessary. “If you’re directly detecting solid explosives,” as in the case of the XPak, “you don’t have to detect such small amounts,” he says.

Harold Weinstock, a program manager at the U.S. Air Force Office of Scientific Research, confirms that the office funded Trogler’s research. Trogler says that the XPak is currently being field-tested in Iraq by the military. The company cannot disclose which branch.

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