A Calcium(II) Coordination Polymer Gel as a Highly Selective Fluorescent Chemosensor in Water for Cu(NO₃)₂

The development of metal-organic gels as chemosensors for detecting metal ions has gained significant attention due to their critical roles in biological and environmental systems. Among these, the detection and quantification of Cu²⁺ ions are particularly important, given the widespread industrial applications and the toxicity of its salts at elevated concentrations. Coumarin-based fluorescent chemosensors have emerged as promising candidates due to their low toxicity and ease of functionalization. In this study, we report the synthesis and characterization of N-(7-hydroxy-4-methyl-8-coumarinyl)-leucine (H₂muleu) and its metal complexes. Remarkably, the Ca²⁺ complex forms a coordination polymeric (CP) gel, with gelation being highly specific to the choice of metal salt, ligand, and solvent. The resulting metal-organogel exhibits intense blue fluorescence at 455 nm, with significantly enhanced emission compared to the free H₂muleu ligand. Both H₂muleu and its Ca²⁺ complex demonstrate high selectivity and sensitivity as fluorescent chemosensors for Cu²⁺ ions, especially as nitrate ions in aqueous media, with low detection limits. Notably, the fluorescence is completely quenched upon interaction with Cu²⁺ ions, enabling naked-eye detection under UV light. This work introduces a rare water-soluble, highly selective, and sensitive chemosensor system for Cu²⁺ ions, with potential applications in biological and environmental monitoring.