Editorial Feature

Innovation in Bomb Detection Using Nanosensors

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Cynophobics may soon be able to avoid the anxiety of being sniffed by the canine police at airports thanks to the development of a nanosensor by a team of researchers at the University of California, Berkeley. The new technology, which uses light based plasmon sensors, can identify the smallest and most difficult to detect explosives and so could be employed at airport security in the near future.

Detecting a Needle in a Haystack

The new sensor was vigorously tested by engineers using a variety of explosives including 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene. The ultra-sensitive sensor managed to detect all of the airborne chemicals in minute concentrations (0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively).

These findings were published in the advanced online publication of the journal of Nature Nanotechnology and compared to other optical sensors published to date, the new nanosensor developed at UC Berkeley is far more sensitive.

The tiny sensor consists of a semiconductor (cadmium sulfide) on top of a dielectric gap-layer (magnesium fluoride) which sits on a silver metallic substrate. The chemicals being detected interact with the semiconductor itself (see figure 1), via defects which naturally occur on its surface.

The ability to magnify such a small trace of an explosive to create a detectable signal is a major development in plasmonsensor technology, which is one of the most powerful tools [against terrorism] we have today.

Xiang Zhang, Professor of Mechanical Engineering - UC Berkeley

The sensor is also versatile in that it can detect unexploded landmines which, according to the United Nations, kill between 15,000 and 20,000 people worldwide every year.

Intelligent Design

Developers exploited the chemical makeup of the most common explosive materials, particularly the nitro compounds such as DNT and TNT. Such compounds are electron deficient and the instability of the nitro group makes them more explosive. The electron deficiency causes greater molecular interaction between the chemicals and surface defects, such as those found on the semiconductor. The increased intensity in the light signal caused by these interactions is then detected by the semiconductor.

Possibly the most significant potential use for this sensor is the detection of pentaerythritol tetranitrate (PETN). This has previously eluded airport security as it is plastic. It can therefore escape detection by x-rays and, due to its explosive potential, can be used in very small amounts. PETN is often associated with terrorism and was used in the shoe bomb scare of 2001 and more recently the underwear bomb in 2009, both of which were fortunately thwarted. Ren-Min Ma discusses the potential of these devices exclusively with AZoSensors below:

Plasmon laser sensors can detect the tiny-trace vapor in the air of the explosive’s molecules at a very low cost. Even on the cross-section of a human hair, we can make thousands of them. A sensing matrix can be formed by a vast number of plasmon laser sensors at airports and other public places for social security and defense.

Ren-Min Ma, Co-lead Author and Assistant Professor of Physics - Peking University

Ma also states that PETN contains more unstable nitro compounds than DNT and so should be even easier to detect. This particular class of explosives has been highlighted as an "extreme, extreme concern" by U.S. Attorney General Eric Holder Jr. as it could be incorporated into small devices such as cell phones and mobile devices.

Evolving Technology

Surface plasmon sensors are a growing technology and they are already employed in the medical industry as dectectors for biomarkers that indicate the early stages of various diseases.

It was an advancement in the way light is trapped that has enabled the vast increase in the sensitivity of optical sensors to the stage where they can be adopted for detecting explosives. The diffraction limit, which dictates how long and how small a space light can fit, has restricted conventional optical sensors from having increased sensitivity. This new technology couples surface plasmons and electromagnetic waves which allows light to be squeezed into nanosized spaces.

The researchers behind the technology state that the already accomplished medical applications and now chemical applications are just the beginning of this unique sensor, and that they could be adapted for use in a number of industries, including biomolecular research.

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