Ultrasensitive fluorescence detection of nitro-explosives by dihydro-oxoisobenzofuranyl-phthalazinone obtained from the Cd(ii)-catalyzed cyclization of azinodimethylidyne-benzoic acid

Abstract

Chemical explosive detection through a feasible, straightforward, and efficient technique is essential for human safety. A Schiff base 2,2′-(azinodimethylidyne)-bis-benzoic acid (2) was synthesized, characterized, and cyclized to form 2-(1,3-dihydro-1-oxoisobenzofuran-3-yl)phthalazin-1(2H)-one (1) upon treatment with various transition and lanthanide metal ions, especially with Cd(II). Further, the cyclized product 1 could act as a nanomolar sensitive fluorescent ‘turn-off’ probe for 2-nitrophenol (o-NP, 86%) and picric acid (PA, 81%), wherein, the selectivity of 1 was higher for PA over o-NP. The fluorescence quenching of 1 in the presence of PA and o-NP may be attributed to the energy-transfer mechanism. The binding constants for o-NP and PA were in the order of 10⁴ M⁻¹, while the Stern–Volmer constants (K_sv) were estimated to be in the order 10⁵. The limits of detection (LoDs) for 1 toward o-NP and PA were determined to be 4.52 ppb and 6.81 ppb, respectively, while the limits of quantification (LoQs) were determined to be 15.06 ppb and 22.69 ppb, respectively in DMF/H₂O. Probe 1 showed excellent selectivity toward o-NP and PA over a wide range of analytes, including cations, nitroaromatics, phenols, and aromatic amines. The present findings demonstrated that Cd(II) induced the rapid production of 1, which could serve as an ultrasensitive probe for o-NP and PA and was highly selective for PA with no interference of ions, nitro-explosives, phenols, or aromatic amines. The nitro-explosives o-NP and PA could also be determined in real Ganga river water samples using 1, which strongly suggest its possibility for practical applications.