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Sept. 16, 2003 – Few people remember Richard Reid by name, but many can recall his media-made moniker: the shoe bomber. An Osama bin Laden sympathizer, Reid attempted to blow up an American Airlines transatlantic flight in late 2001 by igniting plastic explosives hidden in his shoes.
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He failed, thanks to alert crewmembers and passengers who overpowered him. But his attempt showed that even in times of heightened airport security, some explosives elude detection. In this case, it was pentaerythritol tetranitrate, or PETN, which at room temperature produces a very diluted vapor.
Oak Ridge National Laboratory scientists are devising a MEMS-based detector that accurately registers the presence of the explosive at a sparse 30 parts per trillion. They reported their findings in the Aug. 18 issue of Applied Physics Letters.
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“The detection limits are much better” than the technique used in airports today, said Lal Pinnaduwage, a senior scientist at Oak Ridge and the lead investigator. “When we make a sensor, hopefully we can keep those sensitivities.”
Detectors in airports expose a piece of luggage or whatever is being sampled to a stream of air, which dislodges chemicals into the air as vapors. The vapor is put in a concentrator in an attempt to bring it up to detectable levels.
Pinnaduwage is collaborating with Oak Ridge’s Thomas Thundat to create small sensors for inexpensive, portable explosives detectors. Thundat pioneered the microcantilevers that are the heart of the detection system. Micro Sensor Technologies Inc., a subsidiary of SENSE Holdings Inc., acquired some patent licenses in 2001 for explosives detection technology developed at Oak Ridge.
Smiths Detection based in the United Kingdom, Scintrex Trace Corp. in Ottawa, Ontario, and Electromax International in Houston already provide detectors that they claim can spot PETN. They compete with dozens of companies selling explosives screening equipment in a burgeoning market. Homeland Security Research Corp. forecasted in 2002 that the explosives screening market could reach $3.5 billion by 2006 and $9.9 billion by 2010.
The Oak Ridge team is experimenting with various detection methods based on the microcantilevers that they say greatly outperform today’s detectors. The method reported in August relies on a silicon cantilever coated on one side with gold and a monolayer that attracts PETN molecules. The cantilevers, which resemble a diving board, quiver at room temperature. When PETN vapor interacts with the monolayer, the cantilever bends. They use a laser system to detect the bending.
Commercially available microcantilevers are only about 200 microns long, 25 microns wide and a micron thick. At those dimensions, the cantilevers can be assembled into a microarray. A microarray would allow researchers to measure variables such as temperature and vapors that chemically resemble PETN.
“The cantilever is sensitive to temperature,” Pinnaduwage said. “We’ll have to have at least two cantilevers: one as a reference that is just gold-coated to respond to temperature.”
Other cantilevers could be coated with monolayers that detect common vapors that often trip up detectors — alcohol or perfume, for instance. Using software that can compare the various cantilevers’ vibrations and bending, they could eliminate the false positives.
“This is why the idea of arrays is so important,” Pinnaduwage said.
But for a detector to really work in the field, it must be able to test samples quickly and repeatedly. Humidity, an environmental factor that can play havoc with some types of sensors, is beneficial with cantilevers because it helps them release molecules once they are captured on the surface.
The monolayer attracts moisture in the air as well as the explosives’ molecules, Pinnaduwage said. Water molecules will adhere to the cantilever’s surface and the explosives then attach onto the water molecule. Turn off the vapor stream and the water molecule detaches, freeing the explosive’s molecule.
Their experiments showed that when a vapor stream containing minute amounts of the explosive was on, the cantilever would bend within 10 seconds. When the stream was turned off, the cantilever resumed its normal shape within a minute. They put it through many cycles to show it worked repeatedly.