Colorimetric receptors for naked eye detection of inorganic fluoride ion in aqueous media using ICT mechanism

A new series of receptors were designed and synthesized based on benzohydrazide for the colorimetric detection of fluoride ion. The receptors L1 and L2 are highly selective towards fluoride ion over other anions. These receptors are able to detect inorganic fluoride such as NaF in aqueous solutions. The presence of two carbonyl groups in the receptor molecule makes the -NH proton highly acidic and hence these receptors are capable of competing with water molecules to bind fluoride ion. The receptors L1 and L2 showed a significant colour change from colourless to yellow in aqueous solutions of NaF with a Δλmax of 149 nm and 147 nm respectively. The mechanism involved in the colour change was deprotonation, formation of imidic acid intermediate followed by stabilization of complex through Intramolecular Charge Transfer (ICT). This was further confirmed by 1H NMR titrations where the formation of imidic acid was observed. The receptor L1 proved itself to be potentially useful for real-life applications by detecting fluoride ion quantitatively in sea water and commercially available mouth wash. © 2012 The Royal Society of Chemistry.