An imidazole-based Schiff-base chemosensor for selective, sensitive and reversible optical detection of Ni²⁺ ions in aqueous media: analytical and biological application

A novel imidazole-based Schiff-base chemosensor (LH) has been synthesized and investigated for the selective optical detection of Ni²⁺ ions in a 1 : 1 methanol-water medium. The receptor showed a highly selective response toward Ni²⁺ over other tested metal ions, producing a distinct colorimetric change and significant fluorescence quenching. The sensor exhibited low detection limits of 2.1 × 10^-7 M and 1.98 × 10^-7 M for absorption and fluorescence measurements respectively, with a 1 : 1 L-Ni²⁺ binding stoichiometry. The association constants were 4.7 × 10⁶ M^-1 from absorption studies and 2.6 × 10⁶ M^-1 from fluorescence studies. The sensing process was effective over a broad pH range and was reversible using Na₂EDTA, allowing repeated sensing cycles. DFT/TDDFT calculations support formation of a high-spin tetrahedral [Ni(L)(H₂O)₂]⁺ complex, with Ni-N and Ni-O coordination reinforced by EDA and NBO analyses showing substantial electrostatic/orbital interactions and ligand-to-metal charge transfer. LH displayed antibacterial activity against E. coli and S. aureus, with MIC values of 6.25 µg mL^-1 and 25 µg mL^-1, respectively. It also demonstrated an INHIBIT-type molecular logic gate using Ni²⁺ and Na₂EDTA as chemical inputs. Molecular docking with VEGFR-2 gave a best predicted binding affinity of -9.4 kcal mol^-1, suggesting potential biomolecular recognition properties. Overall, LH represents a multifunctional system for selective Ni²⁺ sensing with analytical, biological, and molecular-recognition applications.