Acylthiourea derivatives for the colorimetric detection of Cu(II), Hg(II) and Fe(III) in aqueous solution

Abstract Acylthiourea compounds, functionalized with carbonyl (C=O), thioyl (C=S), and nitrogen-containing groups, offer multiple coordination sites for binding metal ions in environmental media. These interactions promote stable ligand-metal complexes, making them effective colorimetric sensors for metal ion detection. In this study, six acylthiourea derivatives were synthesized and evaluated as naked-eye colorimetric sensors in aqueous media. Five derivatives namely 4-(3-benzoylthioureido)benzoic acid), N -((4-methoxyphenyl)carbamothioyl)benzamide), N -((4 acetamidophenyl)carbamothioyl)benzamide), N -((3-methylpyridin-2-yl)carbamothioyl) benzamide), and N -((2-nitrophenyl)carbamothioyl)benzamide) exhibited visual response towards Cu(II) and Hg(II) with absorption bands at 420–445 nm and a noticeable response toward Fe(III), producing orange solutions with absorption below 490 nm. One derivative, N -(pyridin-2-ylcarbamothioyl)benzamide) demonstrated limited colorimetric sensing responses toward Fe(III) and Cu(II). Meanwhile, N -((2-nitrophenyl)carbamothioyl)benzamide, exhibited broad multi-ion recognition behaviour toward Mo(VI), Cr(III), and several divalent metal ions. Detection efficiency was assessed through naked-eye observations and UV–vis absorption spectra. Significant optical changes were observed in the UV region, confirming complex formation, while broad peaks in the visible range limited precise wavelength assignment. Although sensitivity limitations were noted, the results highlight the potential of acylthiourea-based single molecule colorimetric sensors for rapid, low-cost environmental monitoring, with scope for further optimization through improved color intensity and signal enhancement.