A novel crown ether-acylhydrazone turn-on fluorescent chemosensor for Al3+ ion.

Abstract

A novel fluorescent chemosensor based on crown ether-acylhydrazone (L) was synthesized and characterized. The sensor L exhibited high selectivity and sensitive recognition towards Al3+ through a significant "turn-on" fluorescence response at 444 nm in methanol solution. While, other competitive ions had no such significant effects on the fluorescence emission. The associated spectra behavior might be attributed to the formation of 2:1 L-Al3+ complex and the proposed binding mode of L-Al3+ was demonstrated by the fluorescence titration profiles, Job's plot, ESI-MS spectrometry, 1H NMR titration and the IR analysis. Besides the high reproducibility of the optical signals, very low detection limit of 0.24 μM was calculated. The association constant (Ks) for L-Al3+ complex was about 5.56 × 109 ± 8.19 × 107 M-2. The results of various experiments showed the sensor L followed a turn-on mechanism via of the suppression of CN isomerization and excited state intramolecular proton transfer (ESIPT) process and the activation of chelation-enhanced fluorescence (CHEF). Theoretical calculations using DFT/B3LYP method was also used to get insight into the sensing mechanism and electronic structure of L. The sensor L could be conceived as an effective and practical fluorescent chemosensor for determination of Al3+.