An Ultrasensitive Explosives Detector

Nanowire arrays are a thousand times more sensitive than dog noses at sniffing out explosives.

A nanowire sensor made by researchers at Tel Aviv University can detect extremely small traces of commonly used explosives in liquid or air in a few seconds. The device is a thousand times more sensitive than the current gold standard in explosives detection: the sniffer dog.

Bomb detector: Chemically treated silicon nanowires at the center of this glass chip change conductance when exposed to minute traces of explosives in air or liquids.

The sensor could be cheaply produced and incorporated into a handheld instrument for detecting buried landmines or concealed explosives at security checkpoints, according to Fernando Patolsky, a chemistry professor who led the work, which was published in the journal Angewandte Chemie last week. The researchers are developing a portable instrument based on the technology. Their first prototype is about the size of a brick. “We could put the sensors everywhere in an airport, in every corner of a shopping mall,” Patolsky says.

Trained dogs have historically been used to sniff out bombs and landmines because they can smell explosives at concentrations of just a few parts per billion. But it takes tens of thousands of dollars to train and maintain a sniffer dog, so handheld detectors promise a cheaper and more portable solution.

The nanowire array isn’t 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 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.

The device consists of a chip containing an array of silicon nanowires coated with an organic amine compound that binds to the explosives, changing the wires’ conductance. Patolsky says the trick is to grow nanowires at desired spots and along a defined direction on the chip. Once the array is grown, the researchers coat the nanowires and deposit the electrodes. In laboratory tests, the chip was exposed to liquid solutions containing explosives, as well as to TNT vapors mixed with air. The researchers are working on packaging the chip with microfluidics pumps and electronics to make a low-power, portable detector.

Aimee Rose, a researcher at ICx Technologies, says that using nanowire arrays for sensing shows great promise because the method is so sensitive, allowing potential developers to “put many sensors in a small footprint,” but the method will have to prove its mettle with real-world vapor samples.

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.”

Patolsky and colleagues are now making larger nanowire arrays coated with different molecules for detecting other kinds of explosives.

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