New Laser Technology can Sniff Out Explosives

Soon bomb-sniffing dogs may be out of a job, because UC Berkeley scientists are currently developing new laser technology that can sniff out minute concentrations of explosives, a new study says.

These super-sensitive light-based Plasmon sensors can even pick up on hard-to-detect explosives popularly used by terrorists.

"Optical explosive sensors are very sensitive and compact," Xiang Zhang, UC Berkeley professor of mechanical engineering, said in a statement. "The ability to magnify such a small trace of an explosive to create a detectable signal is a major development in plasmonsensor technology, which is one of the most powerful tools we have today."

The Plasmon sensor can detect airborne molecules of 2,4-dinitrotoluene (DNT) and ammonium nitrate at concentrations below one part per billion - that's equivalent to finding a blade of grass on a football field.

According to the study, the new sensor could have many advantages over current bomb-screening methods. Though bomb-sniffing dogs may be cute, they are expensive to train and get tired.

Also, swabs, commonly used at airports to detect explosive residue, have relatively low-sensitivity and involve physical contact.

"Our technology could lead to a bomb-detecting chip for a handheld device that can detect the tiny-trace vapor in the air of the explosive's small molecules," explained study co-lead author Ren-Min Ma.

The sensor also could be developed into an alarm for unexploded land mines that otherwise are difficult to detect, the researchers said. According to the United Nations, landmines kill 15,000 to 20,000 people every year, most of the victims women and children.

The findings are published in the advanced online publication of the journal Nature Nanotechnology.

The engineers put the sensor to the test with various explosives - 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.

Not only is the nanoscale plasmon sensor among the most sensitive explosive detection methods ever produced, it's also one of the smallest. It's made of a layer of cadmium sulfide, a semiconductor, which is laid on top of a sheet of silver with a layer of magnesium fluoride in the middle.

In designing the device, the researchers took advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and its more well-known relative, TNT.

The nitro groups make the chemicals more explosive, and being electron deficient, they interact with natural surface defects on the semiconductor sensor.

The device works by detecting the increased intensity in the light signal as a result of this interaction.

Researchers are hopeful that their Plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, an explosive compound considered a favorite among terrorists.