A colorimetric and fluorescent sensor for the detection of both fluoride ions and trifluoroacetic acid based on acylhydrazone derivatives

An acylhydrazone-based colorimetric and fluorescent sensor (PAH-8) for the detection of fluoride ions (F−) and trifluoroacetic acid (TFA)/triethylamine (TEA) has been studied. PAH-8 solution and organogel are highly selective and sensitive to F− among various tested anions (F−, Cl−, Br−, AcO−, and H2PO4−) in DMSO. Upon addition of F−, the maximum absorption wavelength of PAH-8 in DMSO solution shows a big red shift from 377 nm to 464 nm with a marked color change from colorless to yellow, and the fluorescence emission also displays a red shift from 438 nm to 532 nm with its fluorescence emission switching from blue to yellow. Both Job's plot and the Benesi–Hildebrand plot confirm a 1 : 1 stoichiometric relationship between PAH-8 and F−. The detection limit of PAH-8 for the analysis of F− can reach 8.31 × 10−7 M. Very interestingly, an expeditious ‘naked eye’ detection of F− can be realized by the reversible gel–sol transition, along with a color change from slight yellow to bright red and fluorescence quenching. In addition, the cast film of PAH-8 (the solution was developed from the DMSO gel upon the addition of TBAF) can detect water in air by color switching or fluorescence on/off. In both solution and solid states, PAH-8 also exhibits obvious acid–base stimulated fluorescence conversion. The solution of PAH-8 can convert the blue emission into intensive sky blue emission after adding trifluoroacetic acid (TFA), and the solid film exhibits no fluorescence after fuming with TFA vapors, while fuming with triethylamine (TEA) vapors resulted in intensive green emission.