Solid-State Sensors Detect Explosive Nitroaromatic Compounds

by Mat Dirjish

Researchers at Pusan National University in South Korea are developing solid-state sensors for reliably detecting explosives. The scientists’ aggregation-induced emission-active sensors show quenching of fluorescence emission on contact with nitroaromatic compounds. An explanation is necessary here.

The majority of terrorist-based and other attacks involve explosives and nitroaromatic compounds, such as picric acid (PA), 2,4,6-trinitrotoluene, and 2,4-dintirotoluene, are the most common chemicals used for explosives. Also, in trace amounts, these chemicals are highly toxic to humans, animals, and the environment. Obviously, fast and accurate detection of these elements will prevent explosions and/or environmental contamination.

Current detection methods for these chemicals are impractical as they involve bulky laboratory equipment, like gas chromatography or X-ray imaging, that are undeployable in the field. As a workaround, portable and fluorescence based chemical detectors are a popular remedy.

Fluorescence-based detectors operate via aggregation-induced emission (AIE), which indicates that the constituent polymers emit strong fluorescence at high concentrations, under aggregated condition. This fluorescence shows quenching behavior (reduction in intensity) when in contact with nitroaromatic compounds.

Chemicals used to formulate AIE polymers are tetraphenylethylene (TPE), thriphenylamine (TPA), and distyrylanthracene derivatives. Conventional methods available for the formulation of AIE polymers are highly complicated, involving multi-step purification processes, proving detrimental in the application of AIE polymer-based probes. Therefore, simplification of their chemical synthesis is necessary.

Led by Professor Il Kim, a group of researchers from Pusan National University, successfully developed a single-step synthesis procedure for an AIE active polymer—TPE conjugated-hyperbranched polyglycidol (HPG). Professor Kim says, “We synthesized AIE active nanoparticles using commercially available HPG, that too in a simple single-step process. This new protocol will eliminate earlier difficulties faced in the synthesis of AIE active nanoparticles and promote their use in the detection of nitroaromatic compounds.”

The researchers’ single step protocol produces a TPE-HPG polymer solution that they add to water to form bright-blue fluorescent TPE-HPG aggregated nanoparticles. The strong blue fluorescence of these nanoparticles quenches by almost 95% on addition of 90 µm concentration of PA. The minimum concentration of PA detected using TPE-HPG nanoparticles is 40 parts per billion.

As solid-state sensors are more convenient for field applications than nanoparticle solutions, the team immobilized the TPE-HPG nanoparticles on strips of paper by following a simple dip and dry process. They dip strips of paper in nanoparticle solution for 10 minutes then dry them under a vacuum at 50ᵒC for 24 hours. The prepared paper-based sensors emit bright blue fluorescence under UV light, which progressively quenches on addition of higher concentrations of PA.

Professor Kim concludes, “We demonstrated a facile synthesis protocol for a fluorescent nanoparticle that can be deployed as a solid-state sensor for detecting explosives. The full potential of this technology is as yet untapped; it can be further customized for applications in chemo sensing, bioimaging, and optoelectronics.”

Need to know more? If so, reacquire the original paper, titled “Aggregationinduced emission-active hyperbranched polymers conjugated with tetraphenylethylene for nitroaromatic explosive detection.” Also, you can visit the Pusan National University lab.

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