Selective Detection of Divalent Cations (Cu²⁺, Zn²⁺, Pb²⁺) and Anions (SO₄^2-, S^2-, CO₃^2-) 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, ¹H, and ¹³C 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 Cu²⁺, Zn²⁺, and Pb²⁺ and anions such as CO₃²⁻, S²⁻, and SO₄²⁻. This color change results from forming complexes like M₃L₂ 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 Zn²⁺, S²/CO₃²⁻ ion with L are fluorescent, enabling the detection of Cu²⁺ and SO₄²⁻ 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 Cu²⁺ ions and restored when Cu²⁺ is complexed with EDTA. A logic gate has also been constructed for understanding the TURN-OFF-TURN-ON mechanism involving Cu²⁺ 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.