DetectionNew TNT detector 1,000 more sensitive than sniffer dogs

To thwart possible terrorist attacks and to detect contamination on sites of former military installations, researchers have been concentrating their efforts in recent years on methods for the detection and analysis of explosives.

Fernando Patolsky and his team at the University of Tel Aviv have now developed a novel sensor chip that detects trinitrotoluene (TNT), as well as other explosive species, with high sensitivity and without a concentration step. As the Israeli researchers report in the journal Angewandte Chemie, their detector is superior to sniffer dogs and all other previous detection methods for this explosive.

The difficulty with the detection of explosives such as TNT is their extremely low volatility. Methods available for the analysis of air samples are expensive and time-consuming, and require large, bulky instruments, laborious sample preparation, and expert handling. “There is a need for an inexpensive, miniaturizable method that allows for quick, easy, and robust high-throughput analysis in the field,” says Patolsky.

The scientists built their sensor using the principle of a nanoscale field-effect transistor. In contrast to a current-controlled classical transistor, a field-effect transistor is switched by means of an electric field. At the core of the device are nanowires made of the semiconductor silicon. These were coated with a molecular layer made from special silicon compounds that contains amino groups (NH2). TNT molecules bind to these amino groups in the form of charge-transfer complexes. The binding process involves the transfer of electrons from the electron-rich amino groups to the electron-poor TNT. This change in the charge distribution on the surface of the nanowires modulates the electric field and leads to an abrupt change in the conductivity of the nanowires, which is easily measured.

To improve the signal-to-noise ratio and thus increase the sensitivity, the scientists equipped their chip with an array of about 200 individual sensors. “We are thus able to analyze liquid and gaseous samples without prior concentration or other sample preparation at previously unattainable sensitivities,” says Patolsky. “We were able to analyze concentrations down to 0.1 ppt (parts per trillion); that is, one molecule of TNT in 10 quadrillion other molecules.” The sensor can be quickly regenerated by washing and is selective for TNT; other related molecules do not react the same way.

“We are now creating a chip based on large arrays of nanosensors chemically modified with a large number of chemical receptors, with different binding capabilities, in order to detect a whole spectrum of explosive species in parallel,” says Patolsky.

Prachi Patel writes in Technology Review that the nanowire array is not the first device to achieve canine levels of explosive-sniffing sensitivity. A system developed by ICx Technologies, based in Arlington, Virginia, can detect vapors given off by explosives with a sensitivity matching that of a canine nose. Instead of nanowires, the ICx system uses polymers that glow or stop glowing in response to traces of explosive in a vapor in a few seconds. This device is being used in battlefields in Iraq and Afghanistan, and the U.S. Transportation Security Administration (TSA) recently started using it at airports, but most airports still rely on microwave oven-sized instruments that take minutes, rather than seconds, to detect explosives in swabs taken from luggage or passengers’ skin.

The new Tel Aviv University device is a thousand times more sensitive than any existing detector, including the ICx device. The researchers have used it to detect TNT and the plastic explosives RDX and PETN at concentrations lower than one part per trillion in a few seconds.

MIT chemistry professor Timothy Swager agrees, pointing out that currently the array only works convincingly when detecting explosives in a solution. It is less effective at picking out vapors of explosives from a person’s skin or belongings, he says, noting that the array works best when TNT vapor-containing air samples are blown directly at the nanowires.

Harvard University chemistry professor Charles Lieber says that the nanowire sensor approach is much more sensitive than the ICx polymer technology, which was developed in Swager’s lab, but it has not yet been proven the way the ICx technology has. Lieber, who focuses on biomedical applications of nanowire transistors, says the Israeli research shows that nanowire sensing could be applied for explosives detection and could be readily commercialized. “There are no limitations to the methodology from my perspective…it has potential to revolutionize explosives detection.”

—Read more in Yoni Engel et al., “Supersensitive Detection of Explosives by Silicon Nanowire Arrays,” Angewandte Chemie International Edition 49, no. 38 (10 September 2010): 6830-35