Single-Step Synthesis of Solid-State Sensors for Detecting Explosives

Photo: Lindsey Elder

In the past few decades, the sensitive and accurate detection of explosives and their precursors has become a pressing issue of global concern, owing to its implications for civilian and homeland security, and in combating the deepening terrorism crisis and the environmental pollution arising from the widespread use of such compounds in industries.

Nitroaromatic compounds, such as picric acid (PA), 2,4,6-trinitrotoluene, and 2,4-dintirotoluene, are the most commonly used chemicals for the manufacture of explosives. These chemicals, even in trace amounts, are also highly toxic to humans, animals, and the environment. As such, their fast and accurate detection can be effective in preventing explosions and environmental contamination.

Most existing techniques to detect nitroaromatic compounds cannot be used in practical situations. For example, costly laboratory equipment, like gas chromatography or X-ray imaging, cannot be deployed on the field. Hence, lately, portable and cost-effective fluorescence based chemical detectors have gained popularity.

Fluorescence-based detectors operate on a phenomenon called aggregation-induced emission (AIE), which indicates that the constituent polymers emit strong fluorescence at high concentrations, under aggregated condition. This fluorescence shows quenching (reduction in intensity) behavior in contact with nitroaromatic compounds. Chemicals that are often 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 the need of the hour.

Fortunately, a group of researchers from Pusan National University, Republic of Korea, led by Prof. Il Kim, were successful in developing a single-step synthesis procedure for an AIE active polymer—TPE conjugated-hyperbranched polyglycidol (HPG). Speaking of the advantages of this method in their study published in Dyes and Pigments, Prof. 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.”

Solid-state sensors embedded with TPE-HPG fluorescent NPs display highly sensitive quenching towards PA and can be used for its accurate in situ detection. Limit of detection of picric acid (PA) is 40 ppb.
Aggregation-induced emission-active sensors show quenching of fluorescence emission on contact with nitroaromatic compounds. Credit: Pusan National University

The novel single step protocol produces a TPE-HPG polymer solution that is added to water for formation of bright blue fluorescent TPE-HPG aggregated nanoparticles. Through empirical analysis, the researchers observed that the strong blue fluorescence of these nanoparticles is quenched by almost 95% on addition of 90 µM concentration of PA. The minimum concentration of PA that can be detected using TPE-HPG nanoparticles is 40 parts per billion.

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

Highlighting the applications of their study, Prof. Kim remarked, “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 chemosensing, bioimaging, and optoelectronics.”

Overall, by enabling fast, accurate, and in situ detection of explosives, the findings of this study could offer a significant technical support to security agencies worldwide.

Aggregation-induced emission-active hyperbranched polymers conjugated with tetraphenylethylene for nitroaromatic explosive detection. Dyes and Pigments, 29 June 2021.

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