Dithieno[3,2-b:2′,3′-d]pyrrole-based hyperbranched conjugated copolymers as efficient fluorescent probes for the trace detection of picric acid

Abstract

The rapid, selective and trace detection of nitroaromatic compounds is a critical global concern for public security and pollution control due to their potent explosive properties, toxic effects, and significant environmental impact. In this study, we developed a simple and efficient fluorescent sensor using hyperbranched conjugated copolymers (HPCs) containing dithieno[3,2-b:2′,3′-d]pyrrole (DTP) units for the first time to detect picric acid (PA) in all three states: solution, solid, and vapor. Two novel DTP-based HPCs, poly(TPA-alt-DTP) and poly(TPB-alt-DTP), were synthesized through a one-step A₂ + B₃ direct arylation polycondensation under aerobic conditions of DTP and tribromoaryl monomers such as triphenylamine (TPA) and triphenylbenzene (TPB). These hyperbranched polymers exhibited strong fluorescence and effective PA detection, achieving a very low detection limit of less than 1 μM. Furthermore, they demonstrated excellent selectivity for PA, reaching a fluorescence quenching efficiency of up to 90 % compared to other nitroaromatic interferences. The resulting polymers exhibited superior fluorescent quenching performance due to the hyperbranched structure in the backbone, which provided multi-dimensional transport pathways for excitons to migrate from the electron-rich polymer to the electron-deficient PA through a combination of static quenching and photo-induced electron transfer (PET) mechanisms. In comparison to previous reports, polymer-based fluorescent probes performed exceptionally well in solid and vapor states, sensing PA concentrations as low as 10⁻¹⁵ M using paper tests. The paper sensors proved effective in determining PA in solid, solution, and vapor forms, thereby enhancing their potential for on-site applications.