Solvent-free mechanochemical synthesis of a novel benzothiazole-azine based ESIPT-coupled orange AIEgen for the selective recognition of Cu2+ ions in solution and solid phase

Abstract Herein, a novel benzothiazole derived unsymmetrical azine with aggregation-induced orange emission has been designed and synthesized for the selective detection of Cu2+ both in the solution and solid phase. The probe, benzothiazol-diphenyl-azine (BTDPA) was synthesized following a short two-steps synthetic route, in a green and sustainable way by adopting mechanochemistry. BTDPA exhibited aggregation-induced emission (AIE) property based on solvent polarity dependent luminophores aggregation in aqueous-organic media and displayed ESIPT phenomenon arising from the benzothiazole unit. In the presence of Cu2+ ions, BTDPA showed an immediate turn-off response of orange fluorescence and a color change from colorless to yellow in 10 % DMF-H2O medium. The saturation point was observed at 0.5 equiv of Cu2+ and the stoichiometry of BTDPA-Cu was established as 2:1 from Job’s plot, time-resolved fluorescence measurements, and DFT studies. BTDPA forms a strong complex with Cu2+ having binding constant (Ka) 3.37 × 105 M−1 and is selective in the presence of many other cations and anions. The limit of detection (LOD) was found to be 5 × 10-9 M (0.3 ppb), which is much lower than the standard set by US-EPA (15–25 μM) in drinking water. The practical utility of the probe in real sample analysis was validated by spiking Cu2+ in water samples collected from various water bodies. The detection of Cu2+ on the solid phase was demonstrated by coating BTDPA on a silica-coated TLC plate and drop-casting variable concentrations of Cu2+ on it for a visual turn-off response. The smart design, cost-effective, green synthesis, high selectivity and sensitivity, low detection limit are few merits of the new AIE probe for the detection and quantitation of Cu2+.