Cantilevers detect trace amounts of explosives.

Plastic explosives are difficult to detect because a bomb maker can mold them into concealable or inconspicuous objects. Consider shoe bombs. Existing technologies for sensing explosives are bulky and expensive. Now, however, researchers have fabricated a cheap sensor that can detect the barest whiff of these materials in the air and do so in a matter of seconds.

At the heart of the device is a V-shaped silicon cantilever, 180 micrometers long by 25 micrometers wide, Researchers already use such microcantilevers for detecting minute quantities of biological molecules such as DNA and proteins (SN: 10/13/01, p. 237). In the new scheme, the researchers adapted the technology to detect two chemicals typically found in plastic explosives: pentaerythritol tetranitrate (PETN) and hexahydro-l,3,5-triazine (RDX).

A team led by Thomas Thundat at Oak Ridge (Tenn.) National Laboratory first coated the upper surface of a cantilever with a layer of gold. The researchers then added a one-atom-thick layer of an acid that normally binds to both PETN and RDX. When a stream of air containing trace amounts of the explosives passed over the cantilever, molecules of PETN and RDX attached to the cantilever's coated surface, causing the cantilever to bend "like a diving board," says Oak Ridge team member Lal Pinnaduwage.

"But [the bending] isn't because of the added weight," he explains. When the molecules bind to the acid, they cause the cantilever's coated surface to stretch relative to its uncoated surface. This makes the structure curve. In the Oak Ridge work, a laser pointing at the tip of the cantilever detected the degree of bending. The more explosives present, the greater is the curvature.

Compared with other technologies for detecting plastic explosives, "ours is a thousand times more sensitive," says Pinnaduwage. In recent experiments, the Oak Ridge sensor could detect just 14 parts per trillion of PETN in 20 seconds and about 30 parts per trillion of RDX in 25 seconds. Specifically, the device revealed these compounds when an airflow generator delivered just a few femtograms--10[sup.-15] grams--of the material, as described in the Aug. 18 Applied Physics Letters.

Mechanical engineer Arun Majumdar of the University of California, Berkeley says, "This is a fantastic piece of work and very relevant." The sensor, with its high speed and sensitivity, would be a welcome addition at airport-security checkpoints, border crossings, and ports, he notes. Because each silicon cantilever costs about a dollar and the entire device is the size of a shoe box, deploying the technology could be relatively easy.

"This is really great work," adds Scott Manalis, an applied physicist at the Massachusetts Institute of Technology. He says the next important step will be to demonstrate how well the cantilevers can distinguish between the plastic explosives and other chemicals in the air.

Researchers can already fabricate arrays of thousands of cantilevers. With different coatings on each cantilever, array-containing devices might simultaneously detect not just explosives but many different chemicals and biological agents.