New technology to detect common but difficult to detect explosives

Published 11 June 2007

MIT researchers synthesized a molecule based on zinc to allow the detection not only of RDX, but of RDX vapors, which are about 1,000 times more difficult to detect than TNT vapors

Researchers at MIT have created a molecule which emits a distinctive light-blue color in the presence of two common but difficult to detect explosives. The compound could be incorporated into small, easy-to-use devices for detecting traces of hidden explosives at airports and on the battlefield.

In experiments recently reported in the Journal of the American Chemical Society (see reference below) the researchers demonstrated that the substance fluoresces in response to RDX and PETN, explosives used by the military. RDX is also a component of plastic explosives. Both explosives are among those which make security officials guarding against terrorists most anxious. The explosives can be detected now, but to do so requires bulky and expensive instruments such as ion-mobility spectrometers which is currently in use in airports. The new substance produces an easy-to-read signal which could allow for a small, simple device.

Timothy Swager, a professor of chemistry at MIT and a coauthor of the JACS article, says that the reaction of the new compound is “sort of like seeing a very small light in a completely dark room.” The substance fluoresces brightly at a specific wavelength of light. Although the sensitivity limits of the new compound have not yet been determined, other fluorescence-based detectors have proved extremely sensitive in the past.

Technology Review’s Kevin Bullis reports that the new compound could be even more sensitive than other fluorescence-based compounds because it uses a different mechanism. With other compounds, a fluorescent material which is already glowing becomes dimmer in the presence of explosives. The new compound is dark at a particular wavelength until the explosive is present, when it produces a signal which is easier to read. “The fact that we can make a new fluorescence on a dark background suggests that we can expect very high sensitivity,” Swager says.

Swager and his fellow researchers had already developed a sensitive device — already in use in Iraq — based on a novel polymer for sensing TNT and other explosives. The technology registers the presence of an explosive by detecting the TNT vapors even if the explosive device is inside a sealed container. The Transportation Security Administration (TSA) is now testing another fluorescent detector in airports, using the same device platform, to identify liquid explosives.

For the technically inclined: The researchers were inspired to discover the new compound by a known enzyme which produces a green-glowing substance in the presence of the explosives. That enzyme, however, reacts with oxygen and is not stable enough to be used in sensors. The researchers synthesized a similar molecule based on zinc which is much more stable. They demonstrated that the new compound fluoresces brightly at a wavelength of 480 nanometers in the presence of RDX or PETN in a solution, but — and this is important — not when exposed to other materials often associated with explosives. This offers the possibility that the reactions are selective, and thus less likely to register false positives. Swager believes the selectivity will prove good enough to be used in environments such as battlefields and airports.

The MIT researchers have detected the explosives only in solution, so the next step is to optimize the molecules for use in a device that can detect dry particles of explosives. The next step is to establish the device’s sensitivity. If these two steps are successful, then the technology would have two potential advantages over the current device, says Aimee Rose, a researcher at Stillwater, Oklahoma-based ICx-Nomadics, a company making the device used in Iraq, which is based on Swager’s earlier research. First, the ICx-Nomadics device can detect RDX, but the new compound promises to be more sensitive — to the point of being able to detect RDX vapors, which are about 1,000 times more difficult to detect than TNT vapors. Second, the new compound can distinguish between TNT and RDX, which could help first responders identify and deal with the threat they face.

-read more in Trisha L. Andrew and Timothy M. Swager, “A Fluorescence Turn-On Mechanism to Detect High Explosives RDX and PETN,” JACS Communications (27 May 2007): 7254-55 (sub. req.)