Benzimidazolium ionic liquid tagged phenazine salophen as a bifunctional ‘off–on’ PET based fluorescent sensor for aqueous phase detection of trinitrotoluene and picric acid

Abstract

Due to their high toxicity and structural similarity to other nitroaromatics, there is a demand for the development of new molecules as sensors for rapid, selective and sensitive detection of 2,4,6-trinitrotoluene (TNT) and picric acid (PA). Herein, a novel benzimidazolium ionic liquid tagged phenazine salophen (PS-IL) has been synthesized and reported as a fluorescent probe for discriminative detection of TNT and PA in the aqueous phase. PS-IL exhibited a strong fluorescence at 562 nm, whose intensity was enhanced during the addition of TNT and was quenched upon addition of PA. The binding mechanism between the probe and the analytes was established using spectral investigations and theoretical studies. Addition of TNT to the PS-IL probe results in the formation of hydrogen bonding between the –NO₂ groups of TNT and the benzimidazolium (C2-H) moiety of PS-IL and this facilitates the restriction of photoinduced electron transfer (PET), which in turn results in fluorescence enhancement. Meanwhile, addition of PA results in the formation of hydrogen bonding between the phenazine (N atoms) moiety of PS-IL and the –OH group of PA, which results in fluorescence quenching due to PET. The proposed sensor showed a detection limit of 0.95 nM for TNT and 7.75 nM for PA. Also, the sensor was effectively applied for detection of TNT and PA using an electrospun nanofiber of PS-IL which showed prominent response.