Easier Detection of Explosives

A new substance could be used in portable sensors to reveal traces of explosives.

Researchers at MIT have created a molecule that glows 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.

Lighting explosives: Vials containing a new compound glow only in the presence of two explosives, RDX (in the vial marked with an R) and PETN (marked with a P), and not in the presence of TNT (second vial from the right). The vial on the far left is a control. All four vials are illuminated with invisible ultraviolet light. Credit: Trisha Andrew, MIT.

In experiments recently reported in the Journal of the American Chemical Society, the researchers demonstrated that the substance fluoresces in response to RDX and PETN, explosives used by the military in warheads. RDX is also a component of plastic explosives. Both explosives are among those that cause the most concern for security officials guarding against terrorists. The explosives can be detected now, but they typically require bulky and expensive instruments such as ion-mobility spectrometers used in airports today. The new substance produces an easy-to-read signal that could allow for a small, simple device.

The reaction of the new compound, says Timothy Swager, a professor of chemistry at MIT, 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.

The new compound could be even more sensitive than other fluorescence-based compounds because it uses a different mechanism. With the others, a fluorescent material that is already glowing becomes dimmer in the presence of explosives. The new compound is dark at a particular wavelength until the explosive is present, producing a signal that's 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.

Previously, Swager and his coworkers developed a highly sensitive device based on a novel polymer for sensing TNT and other explosives; it's now being used in Iraq. (See "Stopping Roadside Bombs.") The technology can register the presence of an explosive by detecting the TNT vapors even if the explosive device is inside a sealed container. To identify liquid explosives, the Transportation Security Administration is now testing another fluorescent detector in airports using the same device platform.

The new compound was inspired by a known enzyme that produces a green-glowing substance in the presence of the explosives. But that enzyme reacts with oxygen and isn't stable enough to be used in sensors. So the researchers synthesized a similar molecule based on zinc that is much more stable.

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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 not when exposed to other materials often associated with explosives. This suggests that the reactions are selective, and therefore less likely to register false positives. Swager says that he believes the selectivity will prove good enough to be used in environments such as battlefields and airports.

So far, the MIT researchers have only detected the explosives in solution. The next step is to optimize the molecules for use in a device that can detect dry particles of explosives. Then the researchers can establish the device's sensitivity. If those tests prove successful, the first device could be similar to handheld devices now being used in Iraq for sensing explosives. It could have two potential advantages over the current device, says Aimee Rose, a researcher at ICx-Nomadics, a company based in Stillwater, OK that makes the device used in Iraq based on Swager's earlier research. First, while the ICx-Nomadics device can detect RDX, the new compound shows the promise of being much more sensitive--even 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 know how to deal with the threat they face.