Can science end the threat of terrorism? Michael Knapp and Meaghan Germain at the University of Massachusetts, Amherst, US, explain chemistry's key role.

In this modern age, marred by terrorism and deadly explosives, identifying and then neutralising hidden threats is crucial to restoring the personal safety of both civilian and military populations. Without such confidence, travel and economic development would suffer. Consider the civil disruption that results in war-torn countries when a trip to visit loved ones or purchase food at the market carries with it the danger of a hidden landmine or an improvised explosive device (IED).

Explosive sensors can detect the tri- and dinitrotoluene given off by landmines © Mattphoto/Dreamstime.com

In the distant past, landmines and related devices had metal casings and could be identified with metal detectors. Modern explosives, however, rely increasingly on plastic components. Asymmetric warfare has increased our awareness of the danger of IEDs manufactured by terrorists in clandestine labs. Consequently, intelligence and military agencies need to identify threats by detecting the chemical components used in explosives manufacturing.

Chemists play a key role in this search for new explosives sensors. Chemical explosives encompass a variety of compounds, including nitro-organics, nitramines and peroxides, making universal explosives detection challenging. Each class of chemical explosives presents its own hurdles to detection, such as nitramine and peroxide's low vapour pressures, but clever chemistry and engineering can overcome such challenges. Because explosives combine strong oxidants with fuel, detection is possible through a combination of chemical reactivity and molecular recognition. For example, trinitrotoluene (TNT) and its degradation product, dinitrotoluene (DNT), are unique chemical signatures of many landmines, making them a common target for explosives detection.

Detection requires that a chemical response, such as the binding of TNT, leads to a sensor output, such as a change in a spectroscopic signal or a unique molecular decomposition pattern. Each detection approach must balance various factors, such as the analyte's physical properties, the potential for false positives or negatives and the instrument's portability. A single sensor is seldom universally applicable.

Some of the most exciting advances have been made in optical sensors. They show great promise for minimising the cost and operator training needed for threat detection while maximising portability and sensitivity.

Optical detection techniques have focused on either fluorescence quenching, which relies on molecular recognition and the electron-accepting nature of explosives, or colorimetric assays, which rely on a chemical reaction of explosives to produce a new colour. Each of these approaches can be implemented as hand-held kits or sensors that are both cheap and easy to use.

Because current fluorescence-quenching sensors rely on molecular recognition, they are typically specific for only one class of explosives (nitroaromatics but not nitramines, for example). Developing fluorescence sensors responsive to a broad range of explosives would represent a significant advance for this field, as it would enable a richer identification of threats. Further utility could come from discriminating between related compounds, such as different nitroaromatics (for example DNT versus TNT) through a sensor array, which could reduce the instances of false positives and negatives.

Fluorescence turn-on sensing, in which explosives chemically react to create a new fluorophore, is one approach that is growing in importance. It combines the advantage of sensing by chemical specificity with an increased signal intensity to indicate the presence of explosives.

Chemists have much to contribute to maintaining our confidence in personal security. Through continued improvements in explosives sensors, we may one day eliminate the threat posed to civilians by minefields or the threat to our military posed by IEDs.

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