Selective Detection of Divalent Cations (Cu 2+ , Zn 2+ , Pb 2+ ) and Anions (SO 4 2- , S 2- , CO 3 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, ¹ H, and ¹³ 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 ²⁺ , Zn ²⁺ , and Pb ²⁺ and anions such as CO ₃ ² ⁻, S ² ⁻, and SO ₄ ² ⁻. This color change results from forming complexes like M ₃ L ₂ 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 ²⁺ , S ² /CO ₃ ² ⁻ ion with L are fluorescent, enabling the detection of Cu ²⁺ and SO ₄ ² ⁻ 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 ²⁺ ions and restored when Cu ²⁺ is complexed with EDTA. A logic gate has also been constructed for understanding the TURN-OFF-TURN-ON mechanism involving Cu ²⁺ 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.
(© 2024. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)