Colorimetric detection of fluoride ions in aqueous medium using thiourea derivatives: a transition metal ion assisted approach

This work explores the position of the hydroxyl moiety and its participation in intramolecular H-bonding towards dictating the fluoride selective colorimetric response in functionalized thiourea derivatives. The study reveals the pivotal aspect of the hydroxyl moiety in C2 towards attaining selectivity for fluoride over acetate and dihydrogenphosphate ion. Furthermore, a methodology employing stabilization of deprotonated thiourea through metal ion (Ni 2+ and Cu 2+ ) coordination is proposed for the colorimetric sensing of fluoride in water medium. The mechanism of interaction is thoroughly studied by UV-Vis, 1 H NMR, ESR spectroscopy, electrochemical techniques and further validated by DFT calculations. This study reveals the formation of an in situ Ni 2+ complex that shows greater stability in aqueous medium. The methodology is applied in the detection of fluoride in groundwater samples. A transition metal ion based methodology is demonstrated employing cooperative interaction between metal ions (Ni 2+ /Cu 2+ ) and fluoride with functionalized thiourea derivatives with a particular goal to detect fluoride under an aqueous environment.