Salicylaldehyde-based anion sensors featuring turn-on fluorescence, colorimetry, and the anti-counterfeiting application

Abstract

Two simple salicylaldehyde-based sensors BDQ and BQ were developed, which had been applied in anion sensing and anti-counterfeiting. Their chemical structures were determined by the 1H, 13C NMR and mass spectra. In the anion sensing investigations, F− ions exhibited more competitive response over the others, thus the related detecting limits, anti-interference ability, stoichiometry and sensing mechanism had been carefully carried out. Impressively, once the F− ions were added, the color of the sensor solutions obviously transformed from colorless to yellow, directly recognizing by naked eyes. On the other hand, these sensors were almost non-emissive in solutions, the green fluorescence were successfully turned on after adding fluoride anions. Moreover, the F− ion detection limits achieved by these sensors were as low as 7.5 × 10−7 M, much lower than that of World Health Organization (WHO) permission level. Most importantly, the mechanism of colorimetric change and turn-on fluorescence was safely attributed to the excited-state proton transfer (ESPT) process coming from the sensor-F− complex constructed by the hydrogen bond, which had been deeply interpreted by the NMR and mass titration experiments as well as the theoretical calculations. Finally, the practical application of these sensors for fluoride ion detections in real environment were conducted as well. Meanwhile, considering the reversible fluorescence transformation from blue to green, the high-level and repeatable anti-counterfeiting patterns were developed. Based on all of the results, it's expected that this contribution could provide useful clues for design the high-performance chemo-sensors and anti-counterfeiting materials.