A team of researchers led by Xiang Zhang, professor of mechanical engineering at the University of California, Berkeley, has created a lasing plasmon explosives-sniffing nanosensor that is free of metal losses and operates at a scale far below the diffraction limit for visible light.1 The sensor consists of a layer of the semiconductor cadmium sulfide on top of a silver film with a layer of magnesium fluoride between.
The ability to increase the sensitivity of optical sensors has traditionally been restricted by the diffraction limit; however, by coupling electromagnetic waves with surface plasmons, they researchers were able to use nanosized spaces, but sustaining the confined energy was challenging, as light tends to dissipate at a metal's surface.
The device relies on a whispering-cavity resonator to serve as the plasmon laser cavity. Zhang says 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 says.
They put the sensor to the test with various explosives such as 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 (ppb), 0.4 ppb, and 7.2 ppb, respectively. The researchers note that this is much more sensitive than the published results to date for other optical sensors.
Sensing PETN
The researchers say the sensor could potentially be used to sniff out a hard-to-detect explosive popular among terrorists. Pentaerythritol tetranitrate, or PETN, is an explosive compound considered a favorite of terrorists. Small amounts of it pack a powerful punch, and because it is plastic, it escapes x-ray machines when not connected to detonators. It is the explosive that was found in Richard Reid's shoe bomb in 2001 and Umar Farouk Abdulmtallab's underwear bomb in 2009.
"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," says 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.
REFERENCE:
1. Ren-Min Ma et al., Nature Nanotechnology (2014); doi: 10.1038/nnano.2014.135
