MIT researchers have created a new detector so sensitive it can pick up a single molecule of an explosive such as TNT.

To create the sensors, chemical engineers led by Michael Strano coated carbon nanotubes — hollow, one-atom-thick cylinders made of pure carbon — with protein fragments normally found in bee venom. This is the first time those proteins have been shown to react to explosives, specifically a class known as nitro-aromatic compounds that includes TNT.

If developed into commercial devices, such sensors would be far more sensitive than existing explosives detectors — commonly used at airports, for example — which use spectrometry to analyze charged particles as they move through the air.

"Ion mobility spectrometers are widely deployed because they are inexpensive and very reliable. However, this next generation of nanosensors can improve upon this by having the ultimate detection limit, [detecting] single molecules of explosives at room temperature and atmospheric pressure," says Strano, the Charles (1951) and Hilda Roddey Career Development Associate Professor of Chemical Engineering.

A former graduate student in Strano's lab, Daniel Heller (now a Damon Runyon Fellow at MIT's David H. Koch Institute for Integrative Cancer Research), is lead author of a paper describing the technology in the Proceedings of the National Academy of Sciences. The paper appears online this week.

Strano has filed for a patent on the technology, which makes use of protein fragments called bombolitins. "Scientists have studied these peptides, but as far as we know, they've never been shown to have an affinity for and recognize explosive molecules in any way," he says.

In recent years, Strano's lab has developed carbon-nanotube sensors for a variety of molecules, including nitric oxide, hydrogen peroxide and toxic agents such as the nerve gas sarin. Such sensors take advantage of carbon nanotubes' natural fluorescence, by coupling them to a molecule that binds to a specific target. When the target is bound, the tubes' fluorescence brightens or dims.

The new explosives sensor works in a slightly different way. When the target binds to the bee-venom proteins coating the nanotubes, it shifts the fluorescent light's wavelength, instead of changing its intensity. The researchers built a new type of microscope to read the signal, which can't be seen with the naked eye. This type of sensor, the first of its kind, is easier to work with because it is not influenced by ambient light.