Plasmon detector finds minute traces of chemical explosives

A team of researchers led by Xiang Zhang, UC Berkeley professor of mechanical engineering, has found a way to increase the sensitivity of a light-based plasmon sensor to detect minute concentrations of explosives.

Their findings were published yesterday, July 20 in Nature Nanotechnology.

They put the sensor to the test with various explosives – 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene – and found that the device detected the airborne chemicals at concentrations of 0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively.

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, who is also director of the Materials Science Division at the Lawrence Berkeley National Laboratory and director of the National Science Foundation Nanoscale Science and Engineering Center at UC Berkeley. ‘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.’

The new sensor could have many advantages over current bomb-screening methods, such as deploying sniffer dogs or taking swabs.

‘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 study co-lead author Ren-Min Ma, an assistant professor of physics at Peking University

The sensor could also be developed into an alarm for unexploded land mines that are otherwise difficult to detect, the researchers said in a statement. 

The nanoscale plasmon sensor used in the lab experiments is claimed to be much smaller than other explosive detectors on the market and consists of a layer of cadmium sulphide, a semiconductor, laid on top of a sheet of silver with a layer of magnesium fluoride in the middle.

We create a sharper signal which makes it easier to detect even smaller changes for tiny traces of explosives in the air

In designing the device, the researchers are said to have taken advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and 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.

Because of this, the researchers are hopeful that their plasmon laser sensor could detect pentaerythritol tetranitrate (PETN).

‘PETN has more nitro functional groups and is more electron deficient than the DNT we detected in our experiments, so the sensitivity of our device should be even higher than with DNT,’ said Ma.

The ability to increase the sensitivity of optical sensors had traditionally been restricted by the diffraction limit, a limitation in fundamental physics that forces a trade-off between how long and how small light can be trapped. By coupling electromagnetic waves with surface plasmons, the oscillating electrons found at the surface of metals, researchers were able to squeeze light into nanosized spaces, but sustaining the confined energy was challenging because light tends to dissipate at a metal’s surface.

The new device builds upon earlier work in plasmon lasers by Zhang’s lab that compensated for this light leakage by using reflectors to bounce the surface plasmons back and forth inside the sensor and using the optical gain from the semiconductor to amplify the light energy.

Zhang said the amplified sensor creates a much stronger signal than the passive plasmon sensors currently available, which work by detecting shifts in the wavelength of light.

‘The difference in intensity is similar to going from a light bulb for a table lamp to a laser pointer,’ he said. ‘We create a sharper signal which makes it easier to detect even smaller changes for tiny traces of explosives in the air.’