Films Reveal Explosives

New spray-on films that glow blue under ultraviolet light and can distinguish between different types of volatile chemicals are being used to quickly and easily detect explosives.

A recent episode of “CSI: Miami” featured the technology, developed by University of California, San Diego (UCSD) chemists, which linked fingerprints left on a video camera to a bomb used in a bank heist, revealing the motive for the robbery. In real life, security systems company RedXDefense of Rockville, Md., has developed a portable kit based on the technology that security officers could use with minimal training.

Contaminated fingerprints leave dark shadows on the films, which glow blue under ultraviolet light, and one of the films can distinguish between different classes of explosive chemicals.

Top: A handprint contaminated by dynamite darkens the glow of an explosive-detecting gel. Bottom: After a minute or two, the handprint glows green if the explosive is a nitrate ester such as nitroglycerin. (Photos: Jason Sanchez/UCSD)

Detection relies on fluorescent polymers developed at UCSD by chemistry and biochemistry professor William Trogler and graduate student Jason Sanchez. “It’s a very intuitive detection method that doesn’t require a scientist to run,” Trogler said.

The polymers emit blue light when excited by ultraviolet (UV) radiation. Nitrogen-based explosive chemicals such as TNT quench that glow by soaking up electrons.

Because the polymers fluoresce brightly, no special instruments are needed to read the results. Only a very thin film sprayed on a suspect surface is needed to reveal the presence of a dangerous chemical. A single layer of the polymer, about one thousandth of a gram, is enough to detect minute amounts of some explosives, as little as a few trillionths of a gram (picograms) on a surface a half-foot in diameter. Handling explosives can leave 1000 times that quantity or more stuck to fingers or vehicles.

The films also adhere directly to potentially contaminated surfaces, making them more sensitive than previous methods, which rely on capturing molecules that escape into the air.

Detection can be fast, revealing incriminating fingerprints as soon as the solvents dry, within 30 seconds. Exposure to UV light for a minute or two alters one of the films so that traces of nitrate esters, a class of chemicals that includes the highly explosive PTEN, begin to glow green. Traces of other classes of explosives, such as nitroaromatics like TNT, stay dark.

Sanchez and Trogler describe the synthesis and properties of their polymers in an upcoming issue of the Journal of Materials Chemistry.

Trogler’s group is currently developing similar systems to detect explosives based on peroxides.

The Air Force Office of Scientific Research and RedXDefense funded the research; Sanchez was supported by the National Science Foundation.

Trogler serves on the strategic advisory board of RedXDefense, which has licensed the technology from UCSD.

For more information, visit: www.ucsd.edu