An optical sensor under development at University of California, Berkeley (USA) could soon give bomb-sniffing dogs some serious competition.
UC researchers led by SPIE Fellow Xiang Zhang have engineered a way to dramatically increase the sensitivity of a light-based plasmon sensor to detect minute concentrations of explosives.
The engineers tested the sensor with various explosives – 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene – and found it 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.
“Optical explosive sensors are very sensitive and compact,” said Zhang, who is also director of the Materials Science Division at Lawrence Berkeley National Lab 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.”
The new sensor could have many advantages over bomb-screening dogs, which are expensive to train and can become tired, said study co-lead author Ren-Min Ma, an assistant professor of physics at Peking University who did this work when he was a postdoctoral researcher in Zhang’s lab. Swabs used at airports to check for explosive residue have relatively low-sensitivity and require physical contact, he added.
“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,” Ma said.
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 UN, land mines kill more than 15,000 people every year.
SENSOR IS HUNGRY FOR ELECTRONS
The nanoscale plasmon sensor used in the lab experiments is much smaller than other explosive detectors on the market. It consists of a cadmium sulfide semiconductor 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. Not only do the unstable nitro groups make the chemicals more explosive, they also are 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.
“We think that higher electron deficiency of explosives leads to a stronger interaction with the semiconductor sensor,” said co-lead author Sadao Ota, a former PhD student in Zhang’s lab and now assistant professor of chemistry at University of Tokyo.
Because of this, the researchers are hopeful that their plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, considered a favorite of terrorists. Small amounts of it pack a powerful punch, and because it is plastic, it escapes detection by x-ray machines when not connected to detonators.
“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,” Ma said.
NEW GENERATION OF SENSOR TECHNOLOGY
The sensor represents the latest milestone in surface-plasmon sensor technology, which is also used to detect biomarkers in the early stages of disease.
The ability to increase the sensitivity of optical sensors traditionally had been restricted by the diffraction limit. By coupling electromagnetic waves with surface plasmons, researchers squeezed light into nano-sized 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. (Volker Sorger, Zhang, Ma, and other researchers reported on such a semiconductor plasmon laser in an invited paper at SPIE Optics + Photonics 2010. See dx.doi.org/10.1117/12.859136.)
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.
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