Exclusive Detection of Ni²⁺, Cu²⁺, Zn²⁺, and CO₃^2- Ions Using a Novel Schiff Base

This study reports the synthesis, characterization, and multifunctional sensing capabilities of a novel quinoline-based Schiff base ligand (L), designed for selective and sensitive detection of Ni²⁺, Cu²⁺, Zn²⁺ ions, and CO₃²⁻ anions. L exhibits distinct colorimetric responses visible to the naked eye-pale yellow to amber red for Ni²⁺, caramel brown for Cu²⁺, and canary yellow for Zn²⁺-enabling efficient and straightforward detection. Fluorescence studies reveal a selective green fluorescence "turn-on" response for Zn²⁺, complemented by fluorescence quenching in the presence of CO₃²⁻, demonstrating the ligand's reusability and robustness. Moreover, the fluorescent quenching of L-Zn by CO₃²⁻ enabled the detection of CO₃²⁻ using the Stern-Volmer plot, with a limit of detection (LOD), the limit of quantification (LOQ), and rate constant (K) for CO₃²⁻ calculated to be 3.594 µM, 10.891 µM, and 0.64 mol^-1L, respectively. Furthermore, a Job's plot experiment was conducted to determine the precise stoichiometric ratio of complex formation, revealing a 1:2 (ML₂) adduct between M and L, which was further confirmed by ESI-MS spectra and DFT calculation. Comprehensive analyses, including FTIR, ESI-MS, ¹H and ¹³C NMR, UV-Visible, fluorescence spectroscopy, and DFT calculations, elucidate the electronic properties, binding, and sensing mechanism of L. NBO, MEP mapping and HOMO-LUMO analysis further highlight nucleophilic and electrophilic reactive sites, confirming the L's suitability for selective coordination with metal ions. Compared to existing sensing systems, L offers competitive or superior detection limits, high selectivity, and visual responsiveness, making it a potential candidate for practical applications in metal ion sensing.