Copper ion detection in aqueous media using a carboxamide-based colorimetric sensor and test kit: Synthesis, crystal structures, antimicrobial activity, and computational studies

In the present work, a colorimetric chemosensor, N-(2-ethoxyphenyl)pyrazine-2-carboxamide (L^pz-2-OEt) compound, was synthesized via condensation of pyrazine-2-carboxylic acid and 2-ethoxyaniline, and characterized by elemental analysis, various spectroscopic methods (IR, UV–Vis, ¹H NMR, and ¹³C{¹H}NMR), DFT calculation, and single-crystal X-ray diffraction techniques. UV–Vis studies revealed that L^pz-2-OEt exhibited high selectivity and sensitivity toward Cu²⁺ ions in acetonitrile, with a distinct color change from colorless to yellow, even in the presence of competing metal ions. Job's plot analysis indicated a 2:1 ligand-to-metal stoichiometry. The detection limit (LOD) and binding constant (K_a) for Cu²⁺ were determined to be 1.10 × 10⁻⁵ M and 4.14 × 10⁹ M⁻², respectively. The sensing performance was further validated using a test kit, confirming its practical applicability. Notably, the sensor demonstrated a fast response, reversibility, and reusability. The structure of the new mononuclear Cu²⁺ complex [Cu (L^pz-2-OEt)₂(η²-NO₃)₂(H₂O)].CH₃CN (1) was determined by X-ray crystallography, and its composition was confirmed by elemental analysis and IR spectroscopy. This complex adopts a distorted pentagonal bipyramidal (6 + 1) coordination geometry. DFT calculations were used to optimize ground-state geometries and calculate global reactivity descriptors and nonlinear optical (NLO) properties. Intermolecular interactions in the crystal lattice were examined via Hirshfeld surface analysis. The synthesized compounds were also evaluated for in vitro antibacterial and antifungal activities. The Cu²⁺ complex exhibited stronger antibacterial activity than the free ligand, while both compounds lacked antifungal activity. This study provides a comprehensive insight into the design of efficient and reusable chemosensors for selective Cu²⁺ detection.