A new plasmon laser sensor can identify extremely minute concentrations of explosives, as well as explosives that are typically difficult to detect. “Optical explosive sensors are very sensitive and compact,” said Dr. Xiang Zhang of the University of California, Berkeley, whose team developed the system. “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 sensor consists of a 50-nm cadmium sulfide semiconductor separated from the metal surface by an 8-nm dielectric gap layer. Images courtesy of UC Berkeley. The nanoscale sensor consists of a layer of cadmium sulfide that acts as a semiconductor and is laid on top of a sheet of silver, with a layer of magnesium fluoride in the middle. In designing the sensor, the researchers took advantage of the chemical makeup of many explosives, particularly nitro compounds such as TNT and dinitrotoluene (DNT). These nitro groups are unstable, according to the researchers, and also electron-deficient. This increases the interaction of the molecules with natural surface defects on the semiconductor. The new plasmon 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 researcher Dr. Sadao Ota, an assistant professor of chemistry at the University of Tokyo who is a former doctoral candidate in Zhang's lab. In the study, the researchers tested the sensor on 2,4-dinitrotoluene — also called DNT, which is the chemical compound CH3C6H3(NO2)2 — ammonium nitrate and nitrobenzene. They 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. The cadmium sulfide semiconductor on top of the silver metal surface, as shown under a microscope. The researchers said they are also hopeful that the laser sensor will be able to detect pentaerythritol tetranitrate (PETN, an explosive compound of plastics and other materials that the researchers said has been used by terrorists). It is quite powerful, they added, and because it is plastic, it is not detectable by x-ray machines when not connected to metal detonators. “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,” said researcher Dr. Ren-Min Ma, an assistant professor of physics at Peking University who also is a former postdoctoral candidate in Zhang's lab. The researchers said the new sensor could have applications beyond chemical and explosives detection, such as in biomolecular research. The work was funded by the U.S. Air Force Office of Scientific Research Multi-University Research Initiative program. The research was published in Nature Nanotechnology (doi: 10.1038/nnano.2014.135). For more information, visit www.berkeley.edu.