A highly selective pyrazole-hydrazide-based colorimetric and "turn on" fluorometric sensor for fluoride with portable...

(E)-N'-((5-(4-Chlorophenoxy)-3-methyl-1-phenyl-1H-pyrazol-4-yl)methylene)-4-nitrobenzohydrazide (PNH), a pyrazole-hydrazide-based chemosensor, was designed for the selective detection of fluoride ions using dual-mode colorimetric and fluorometric responses. When PNH interacts with fluoride ions, it shows a clear colour shift from colourless to brilliant yellow and a turn-on fluorescence emission at 510 nm, but it is insensitive to other common anions. Its high sensitivity was confirmed by UV-visible and fluorescence titration tests, which showed a 12 nM detection limit and satisfactory linearity. The sensing study was further supported by ¹H NMR titrations. Additionally, DFT investigations were used to theoretically validate the suggested binding mechanism of the F-ion interaction with the receptor. Crucially, the development of polymer-based films and paper strip devices, which allow for quick, portable, and visible detection of fluoride ions without the need for complex equipment, demonstrates the sensor's practical application.