Green synthesis of thiadiazole Schiff bases and specific recognition of Cr2O72−, SiO32− and S2−

Abstract

Sulfide (S²⁻), silicate (SiO₃²⁻) and dichromate (Cr₂O₇²⁻) ions are prevalent contaminants in industrial effluents, and their rapid and accurate detection is of critical importance for environmental monitoring. In this study, we utilized an environmentally friendly, highly efficient, and biodegradable deep eutectic solvent (DES) to catalyze the synthesis of 2-(((5-phenyl-1,3,4-thiadiazol-2-yl)imino)methyl)phenol (Probe M). Probe M exhibits excellent selectivity for S²⁻, SiO₃²⁻ and Cr₂O₇²⁻, and the corresponding color changes can be readily detected by the naked eye. Upon addition of S²⁻ and SiO₃²⁻ to the probe solution, the solution turns a distinct orange-red, while the addition of Cr₂O₇²⁻ results in a pronounced darkening of the solution. The detection of S²⁻ and SiO₃²⁻ by probe M under alkaline condition is stable, and the detection of Cr₂O₇²⁻ under acidic condition is stable. Fluorescence titration experiments revealed that probe M forms a 1:2 stoichiometric complex with S²⁻, SiO₃²⁻ and Cr₂O₇²⁻, with binding constants of 3.84 × 10⁴ M⁻¹, 1.86 × 10⁴ M⁻¹ and 3.19 × 10⁴ M⁻¹, respectively. The detection limits were determined to be 1.76 × 10⁻⁴ M for S²⁻, 9.15 × 10⁻⁵ M for SiO₃²⁻, and 2.13 × 10⁻⁴ M for Cr₂O₇²⁻. The dipstick experiment demonstrates the probe M can be used as a solid-state sensor to identify S²⁻, SiO₃²⁻ and Cr₂O₇²⁻.