Colorimetric sensor based on hydrazine moiety for rapid and selective detection of fluoride ion via intramolecular charge transfer

A colorimetric anion sensor (E)-3-((2-((4-(4-Methoxyphenyl)thiophen-2- yl)methyl)hydrazono)methyl)-1 H-indole ATH was synthesised and confirmed by different spectroscopic methods, such as 1 H NMR, 13 C NMR, ESI-Mass and FT-IR spectroscopy. The ATH sensor revealed the spectral change and detection in the presence of fluoride ion over the other anions such as OH-, Cl-, AcO-, I-, HSO4 (-), NO3 (-), Br-, S2-, CN- & NO2 (-). Interestingly, sensor ATH exhibited highly selective fluoride ion detection with significant changes in the colour from light yellow to dark yellow in ACN and ACN: H2O (8:2, v/v) medium. The colorimetric sensor effectively responds to fluoride ion stimulation through deprotonation and (ICT) intramolecular charge transfer sensing mechanism. The fluoride sensing mechanism was confirmed by 1 H NMR titration and was supported by density functional theory (DFT) and electrochemical studies. In this study, the B-H plot calculates the stoichiometry and binding constant (K-a) of the host-guest complex and strong interaction between the probe ATH with fluoride ions forming a 1:2 binding ratio. Furthermore, the test strips based on the probe ATH were used to detect F- ions qualitatively and quantitatively in 100% aqueous solution.

Automated summary

A new colorimetric anion sensor ATH was synthesized and confirmed, which showed high selectivity for detecting fluoride ions and responded to fluoride ion stimulation through deprotonation and intramolecular charge transfer sensing mechanism. The sensing mechanism of fluoride ions was confirmed and the binding constant and binding ratio between fluoride ions and probe ATH were determined.