A highly sensitive fluorescent sensor for reversible visual detection of fluoride ion and trace water in food products

A simple diphenylimidazole-based fluorescent sensor (DIP) had been designed for reversible fluorescence sensing of F^- and trace water. The sensor showed superior selectivity for F^- compared to other anions with fluorescence "turn on" mode. After adding F^- to CH₃CN solution of DIP, the emission color of DIP changed markedly from weak blue to strong cyan, which could be facilely discerned by the naked eye. The various technical analysis, including spectroscopies, ¹H NMR titration and DFT calculation, indicated that the deprotonation of imidazole -NH and -OH occurred in the presence of F^-. The detection limit of DIP toward F^- could reach as low as 72.3 nM. Moreover, F^--induced deprotonation of DIP-F^- system exhibited high sensitivity for water with the detection limit of 0.0015 vol%. The reversible switching characteristic of DIP prompted it suitable for the construction of molecular logic gate. In addition, DIP and DIP-F^- were successfully applied to the determination of F^- content in toothpaste and water content in table salt, sugar and tea, respectively. Moreover, DIP and DIP-F^--coated filter paper strips could be used as fluorescent display materials for inkless writing with repeatability.