Benzoyl thiourea-derived chemosensor for colorimetric and fluorescent detection of Cu ²⁺ in water and soils

Abstract 4-bromo-N-(phenethylcarbanmothioyl)benzamide ( LBr ) was successfully synthesized and characterized using FTIR, DSC, 1 H and 13 C NMR spectroscopy, and elemental analysis, confirming its proposed structure. Its sensing ability toward Cu 2+ ions in aqueous solution and soil extracts was investigated, addressing the challenges posed by complex environment matrices to conventional sensors. FTIR showed distinct shifts in the C=O and C=S stretching bands upon Cu 2+ coordination, while Job’s plot confirmed a 2:1 ligand-to-metal stoichiometry. The detection limits were 5.39 μM (UV‒vis) and 6.70 μM (fluorescence), which are below the WHO guideline of 2.0 mgL −1 for Cu 2+ in drinking water. The binding constants were 2.29 × 10 4 M −1 (UV–vis) and 3.18 × 10 4 M −1 (fluorescence). Coordination occurs via the C=S and C=O groups, blocking the ligand’s intrinsic intramolecular charge transfer (ICT)-based emission. The resulting LBr-Cu 2+ complex undergoes ligand-to-metal charge transfer (LMCT), providing a non-radiative relaxation pathway that quenches fluorescence. Density functional theory (DFT) calculations indicated electron density redistribution and a reduced HOMO–LUMO gap, facilitating electronic transitions and enhancing sensing performance. The fluorescence response was stable for 30 min and reversible over six consecutive cycles. Practical application was demonstrated through rapid on‒site Cu 2+ detection using test strips and in soil extracts, highlighting LBr as a versatile chemosensor for environmental monitoring.