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UC Berkeley - Tiny laser sensor heightens bomb detection sensitivity

DCN Corp® - UC Berkeley engineers are developing plasmon laser sensors that could soon compete with bomb-sniffing dogs. Credit - iStockphotoInnovative technology developed at UC Berkeley could soon provide bomb-sniffing dogs some serious competition...

A teams of researchers led by Xiang Zhang, UC Berkeley Professor of Mechanical Engineering, have found a way to increase the sensitivity of light-based plasmon sensor to detect incredibly minute concentrations of explosives. The researchers noted that the sensor could potentially be used to sniff out a hard-to-detect explosive popular with terrorists.  The research was published in the advanced online publication of the journal Nature Nanotechnology.

The engineers put the sensor to test with various explosives, including 2,4-dinitrotoluene (DNT), ammonium nitrate (NH4NO3) and nitrobenzene (C6H5NO2), and came to find-out 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.  For example, one part per billion would be similar to a blade of grass on a football field.  It was noted that the results collected were more sensitive then those published to date for other optical sensors.

As Zhang stated - "optical explosive sensors are very sensitive and compact" - and - "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."  It is claimed that the new sensor could have many advantages over current bomb-screening methods.

As stated by the co-lead author Ren-Min Ma, an Assistant Professor of Physics at Peking University - "bomb-sniffing dogs are expensive to train, and they can become tired" - and - "the other things we see at airports is the use of swabs to check for explosive residue, but those have relatively low-sensitivity and require 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."

The researchers claim that the sensor could also be developed into an alarm for unexploded land mines that are otherwise difficult to detect.  According to the United Nations (UN), landmines kill 15,000-20,000 people/year.  Most of the victims are children, women and the elderly.

Unstable and hungry for electrons

DCN Corp - The plasmon laser sensor consists of a thin slab of semiconductor separated from the metal surface by a dielectric gap layer. Surface defects on the semiconductor interact with molecules of the explosive DNT. Credit - Ren-Min Ma and Sadao Ota, Peking University/The nano-scale plasmon sensor employed in the laboratory experiments is much smaller than other explosive detectors on the market.  Essentially it consists of a layer of Cadmium Sulfide (CdS), a semiconductor, that is laid on top of a sheet of Silver (Ag) with a layer of Magnesium Fluoride (MgF2) in the middle.

In designing the device, the researchers took advantage of the chemical make-up of many explosives, in particular 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 are also characteristically electron deficient.  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.

Potential use to sense a hard-to-detect explosive

As stated by another co-lead author Sadao Ota, a former PhD in Zhang's laboratory and now an Assistant Professor of Chemistry at the University of Tokyo - "we think that higher electron deficiency of explosives leads to a stronger interaction with the semiconductor sensor."

DCN Corp - The plasmon laser sensor is shown in this scanning electron microscope image. Credit - Ren-Min Ma and Sadao Ota, Peking University.Because of this phenomenon, the researchers are hopeful that their plasmon laser sensor could detect pentaerythritol tetranitrate (PETN) an explosive compound considered a favorite of terrorists.  Small amounts of it pack a powerful blast, and because it is plastic, it escapes X-ray machines when not connected to detonators.  For example, it is the explosive found in Richard Reid's shoe bomb in 2001 and Umar Farouk Abdulmtallab's underwear bomb in 2009.

Recently USA Attorney General Eric Holder Jr. was quoted in news reports as having "extreme, extreme concern" about Yemeni bomb makers joining forces with Syrian militants to develop these hard-to-detect explosives, which can be hidden in cell phones and mobiles devices.

Ma continues to state - "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 then with DNT."

Latest generation of plasmon sensors

The sensor represents the latest milestone in surface plasmon sensor technology, which is now used in the medical field 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, a limitation in fundamental physics that forces a trade-off between how long and in how small a space the 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 laboratory that compensated for this light leakage by using reflectors to bounce the surface plasmons back and forth inside the sensor - similar to the way sound waves are reflected across the room in a whispering gallery - and using the optical gain from the semiconductor to amplify the light energy.

Zhang stated that 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. He continues to state - "the difference in intensity is similar to going from a light bulb for a table lamp to a laser pointer" - and - "we create a sharper signal, which makes it easier to detect even smaller changes for tiny traces of explosives in the air."

In conclusion, the researchers note that the sensor could have applications beyond chemical and explosive detection, such as use in biomolecular research. Original article available here

The above research positively highlights the potential of nano-sensor plasmon sensors. Especially in the potential for earlier diagnosis of fatal diseases. As stated previously, DCN Corp strongly believes it can compete. Going forward, if you and/or your colleagues are interested in making DCN Corp's alternative process reality - please ensure to contact the company as soon as practicably possible.