Let us spray: low-cost explosive sensor

A low-power, realtime explosive trace sensor has emerged that is both highly selective and highly sensitive

     In Christmas Day 2009, a 23-year-old Nigerian student named Umar Farouk Abdulmutallab boarded Detroit-bound Northwest Airlines flight 253 in Nigeria with more than 80 grams of the plastic explosive PETN sewn into his underwear — enough to blow a hole in the fuselage. PETN, or pentaerythritol tetranitrate, is the same explosive used by the shoe bomber Richard Reid. The intent of the Christmas Bomber was to cause the aircraft to crash, but the attempt failed due to incompetence. Attempts by airport security to discover the explosive also failed.

     The incident illustrates not just how obsolete airport metal detectors have become in preventing explosive devices from finding their way on board airplanes, but also the evolving threat explosives present.

     Explosives may appear in virtually any form, from solid to liquid to gas. They can also be made to resemble anything from an iPhone to a baby bottle.

     One of the problems for law enforcement and airport security personnel is, explosive detection technologies do not adapt quite so fast. No single explosive detection technology can reliably and rapidly screen all people, cargo and vehicles, although new technologies continue to emerge that may help close the threat gap.

     Researchers at the University of California-San Diego have one solution.

     Chemists there have devised a simple new spray-on film that could one day allow airport security, police and hazardous material teams to quickly and confidently screen vehicles, passengers, luggage and cargo for traces of nitrogen-based explosive residue.

     The basic idea is, airport screeners would apply an extremely thin spray of fluorescent polymer film on a suspect surface to reveal the presence of dangerous chemicals, such as nitroglycerin. Contaminated fingerprints leave dark shadows on the film, which glow blue under ultraviolet light.

     Incriminating traces are revealed as soon as the solution dries, typically within 30 seconds.

Spray it again, Sam

     "No special instruments or training are needed to interpret the results because the polymers fluoresce brightly when exposed to explosive residue," says William Trogler, a UCSD professor of chemistry and biochemistry.

     Trogler says only a minute amount of film is necessary to provoke a chemical reaction. A single one-thousandth of a gram layer of the polymer is enough to detect as little as a few trillionths of a gram of residue on a palm-sized surface. Any surface, including fingers, that has come in contact with nitrogen-based explosives will have 1,000 times that quantity or more stuck to it.

     "It's a simple visual test for explosives that doesn't take a scientist to understand how to use or interpret it," Trogler says.

     One of the films can distinguish between classes of explosive chemicals, a property that could provide evidence to help solve a crime, or prevent one, Trogler says.

     Other explosive residue-sensing technologies exist, such as handheld electronic "sniffers" that sample the air; but Trogler says his technology is different because the UCSD films adhere directly to potentially contaminated surfaces.

     "This makes the films more sensitive than previous methods, which rely on capturing molecules that escape into the air," he says.

     Trogler explains the technology could also possibly be used to scan tickets or boarding passes to see if a person had recently handled explosives.

     Trogler's technology starred in an episode of the television series "CSI: Miami," where it was used to connect fingerprints left on a video camera to a bomb used in a bank heist. In reality, the security systems company RedXDefense has licensed the technology, and has developed a portable kit called XPAC.

Miniature explosive sensors

     Another new way to detect explosives has appeared at Oak Ridge National Lab. Researchers there have come up with what they believe to be a more reliable miniature explosive sensor technology. This one provides a way to mass produce miniature explosive sensors less likely to alarm falsely.

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