A Burst of Light: A New Fluorescent Sensor to Detect Nitrate Explosives

Scientists discover a sensing system that amplifies fluorescent signals and traces their chemical signatures in hard-to-detect nitro-based explosives

A team from Sookmyung Women’s University has found that fluorescence indicator displacement assays, which use optical emission-based molecular sensors to detect explosives, made of perylenediimide derivatives can identify nitro-based explosives with negligible fluorescent activity by amplifying it and tracing their chemical signatures. Published in the Journal of the American Chemical Society, this research will help in military operations, homeland security, and environmental and personal health risk assessment to save lives and property.

Much of contemporary history is wrought with instances of the use of explosives. While the first impression about them might be negative, they are actually regularly used for productive purposes, like in the mining and construction industries. Nevertheless, their improper use or misuse can lead to devastating consequences. One sobering example is the recent explosion of tons of nitrate at the Port of Beirut, Lebanon, killing at least 200 and injuring thousands.

In Lebanon, the authorities happened to be aware of the presence of the explosives. In most cases, however, explosives are hidden and harder to detect, necessitating effective explosive detection technologies for the functioning of military operations, homeland security, and environmental and personal health risk assessment.

A common detection technology involves molecular sensors with “optical emissions,” which show a visual indicator of the presence of explosives. Of these sensors, the most common are of the fluorescence “turn-off” type, which means that they reduce their light emission when in the presence of explosives. Unfortunately, these sensors show only a simple monotonic decrease in the emission intensity.

A global cross-institutional study led by Associate Professor of Chemistry Jung Su Park from Sookmyung Women’s University in South Korea, sought to establish alternatives. Turn-on’ fluorescence systems, unlike their turn-off counterparts, increase light emission when in contact with explosives, have higher sensitivity, better selectivity, easier readout, and more accurate measurements of targeted explosive concentrations, are thus the need of the hour,” reports Prof Park.

To address this need, the team developed a “fluorescence amplification sensor,” which emits light when in contact with nitro-based aromatic explosives and modifies its identifiable wavelength to generate specific signals according to the type of explosive. This sensor is “ratiometric,” which means that it emits two signals, one of which increases and the other decreases, as the input increases. Their findings, published in Journal of the American Chemical Society, tell us the full story.

To find out whether weak fluorescence signals could be boosted to detectable levels after being exposed to nitroaromatic compounds, the team synthesized a set of synthetic “supramolecular” ensembles containing structures with a definite number of molecules, tested them for their fluorescent responsiveness by introducing a series of di- and tri-nitroaromatic compounds, and examined the change in their fluorescence spectral characteristics.

Depending on the ensemble and explosive, they observed either a “turn-on” or a “turn-off” fluorescent response. Using these fluorophores, they then formulated fluorescence indicator “displacement assays” and found a ratiometric fluorescence enhancement, meaning that depending on the type of explosive, there were identifiable wavelength changes.

These novel ensembles thus provide easy visual identification for the detection of explosives while ensuring excellent chemical selectivity, high sensitivity, and low false alarms. “This is crucial,” suggests Prof Park, “because current molecular and supramolecular-based explosive detection fluorescence-based chemical sensors lack sensitivity, selectivity, and are particularly vulnerable to false positives.”

This is the first study to present a set of small-molecule probes that produce readily distinguishable responses for different nitroaromatic explosives via a fluorescence turn-on mechanism. The authors believe these findings can be taken a step further to provide new ensembles capable of acting as sensors for both broad classes of molecular targets and specific individual explosives. Ultimately, this invention takes us one step closer to a safer future.


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