New laser sensor for higher bomb detection sensitivity

Sniffer dogs may soon face serious competition from a tiny new light-based sensor that can detect incredibly minute concentrations of explosives.
A team led by Xiang Zhang, at the University of California, Berkeley, noted that the sensor could potentially be used to sniff out a hard-to-detect explosives popular among terrorists.
They put the sensor to the test with various explosives - 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene - and found that the device successfully detected the airborne chemicals at concentrations of 0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively.
One part per billion would be akin to a blade of grass on a football field.

The researchers noted that this is much more sensitive than the published results to date for other optical sensors.
"Optical explosive sensors are very sensitive and compact," said Zhang.

"The ability to magnify such a small trace of an explosive to create a detectable signal is a major development in plasmon sensor technology, which is one of the most powerful tools we have today," said Zhang.
The new sensor could have many advantages over current bomb-screening methods.
"Bomb-sniffing dogs are expensive to train and they can become tired," said study co-lead author Ren-Min Ma, an assistant professor of physics at Peking University.

"The other thing we see at airports is the use of swabs to check for explosive residue, but those have relatively low-sensitivity and require physical contact. Our technology could lead to a bomb-detecting chip for a handheld device that can detect the tiny-trace vapour in the air of the explosive's small molecules," said Ren-Min.
The sensor could also be developed into an alarm for unexploded land mines that are otherwise difficult to detect, researchers said.
The nanoscale plasmon sensor used in the lab experiments is much smaller than other explosive detectors on the market.
It consists of a layer of cadmium sulfide, a semiconductor, laid on top of a sheet of silver with a layer of magnesium fluoride in the middle.

The researchers took advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and its more well-known relative, TNT.
Not only do the unstable nitro groups make the chemicals more explosive, they are also characteristically electron deficient, the researchers said.
This quality increases the interaction of the molecules with natural surface defects on the semiconductor. The device works by detecting the increased intensity in the light signal that occurs as a result of this interaction.
The findings are published in the journal Nature Nanotechnology.