Selective Detection of Divalent Cations (Cu, Zn, Pb) and Anions (SO, S, CO) Using a pH-Sensitive Multi-functional Schiff Base in Neutral Medium.

A new Schiff base-based multi-cation/anion probe (L) has been synthesized and characterized using HR-MS, FT-IR, H, and C NMR techniques. The Schiff base motif provides specific binding sites that detect cations and anions by generating distinct optical output signals upon interaction. A noticeable color change of the probe solution was observed from pale yellow to various shades of yellow upon adding cations such as Cu, Zn, and Pb and anions such as CO⁻, S⁻, and SO⁻. This color change results from forming complexes like ML with metal ions. Whereas origin of color in presence of anion were attributed due to the deprotonation of acidic proton in the ligand. Moreover, the complexes formed by Zn, S/CO⁻ ion with L are fluorescent, enabling the detection of Cu and SO⁻ using the Stern-Volmer plot, with a limit of detection (LODs) of 8.48 µM and 10.47 µM, respectively. Additionally, increasing the pH of the probe solution above 8 reveals a significant enhancement of fluorescence intensity due to the deprotonation of phenolic -OH and amide -NH in the presence of hydroxide ions. This emission in the basic medium is quenched by Cu ions and restored when Cu is complexed with EDTA. A logic gate has also been constructed for understanding the TURN-OFF-TURN-ON mechanism involving Cu ions and EDTA. Overall, the versatile performance of a single probe L opens up new possibilities as a multifunctional sensor, making it highly suitable for practical applications.