Bomb Detecting Sensors

At a binational armaments and security research center here in eastern France, Dr. Spitzer and his colleagues are working on a sensor to detect vapors of TNT and other explosives in very faint amounts, as might emanate from a bomb being smuggled through airport security. Using microscopic slivers of silicon covered with forests of even smaller tubes of titanium oxide, they aim to create a device that could supplement, perhaps even supplant, the best mobile bomb detector in the business: the sniffer dog.

But emulating the nose and brain of a trained dog is a formidable task. A bomb-sniffing device must be extremely sensitive, able to develop a signal from a relative handful of molecules. And it must be highly selective, able to distinguish an explosive from the “noise” of other compounds.

While researchers like Dr. Spitzer are making progress — and there are some vapor detectors on the market — when it comes to sensitivity and selectivity, dogs still reign supreme.

“Dogs are awesome,” said Aimee Rose, a product sales director at the sensor manufacturer Flir Systems, which markets a line of explosives detectors called Fido. “They have by far the most developed ability to detect concealed threats,” she said.

But dogs get distracted, cannot work around the clock and require expensive training and handling, Dr. Rose said, so there is a need for instruments.

Flir’s state-of-the-art detectors, including hand-held models that weigh just a few pounds, are used in the military and elsewhere and are generally considered on par with dogs for detecting TNT at extremely low concentrations, a few parts per quadrillion. Even so, Dr. Rose said, “We see our technology as complementary” to dogs.

The devices use a fluorescent polymer technology developed by Timothy M. Swager, a chemist at M.I.T. under whom Dr. Rose studied. Thin films of the polymers emit visible light when exposed to ultraviolet rays, but molecules of TNT stop the fluorescence. A single TNT molecule can quench many thousands of fluorescence reactions, greatly increasing sensitivity.

Dr. Spitzer’s approach is far different, and also far from being commercialized. The slivers of silicon, called microcantilevers, are fixed at one end and made to vibrate, like a diving board. As molecules of explosives are captured by the cantilever, the added mass alters the rate of vibration, which can be measured by a laser or other means.

Microcantilevers are already used in many sensing applications, but for explosives the cantilevers alone are not sensitive enough. Growing hollow cylinders of titanium oxide, called nanotubes, increases the surface area, allowing more molecules to be captured.

Sensitivity is critical because many explosives compounds, including powerful ones like RDX and PETN, are not very volatile — at normal temperatures, very few molecules vaporize. As a result, bomb detection — which became a priority in aviation after a hidden device destroyed a Pan Am jetliner over Lockerbie, Scotland, in 1988, killing 270 people — has focused more on finding explosive particles on surfaces than on detecting molecules in the air.

David Atkinson, chief scientist for explosives detection research at the Pacific Northwest National Laboratory in Richland, Wash., said, “We’ve had a particle-based detection paradigm for the past two decades.” Even today, when a laptop or other object is deemed suspicious after being X-rayed, an agent wipes the surface and puts the wipe in a spectrometer that ionizes any explosive compounds present, allowing them to be quickly identified.

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