Ortho-Vanillin-based fluorescent sensor with ESIPT characteristics for selective detection of Al(III) in aqueous solutions and its application for plant and live-cell imaging

A new acylhydrazone fluorescent sensor (VN62) with excited intramolecular proton transfer (ESIPT) charac-teristics has been designed and synthesized through condensation of o-vanillin with 6-hydroxy-2-naphthoic hydrazide for selective detection of Al3+ in aqueous medium. The structure of the VN62 sensor was characterized by various techniques such as H-1 and C-13 NMR and mass spectrometry, and also confirmed by single crystal X-ray diffraction. The VN62 sensor showed a remarkable fluorescence enhancement response to Al3+ over a broad pH range in a nearly 100 % aqueous solution without interference by most of tested metal ions and anions. Through fluorescence titration and a Job's plot experiment, the binding stoichiometry of VN62 with Al3+ was found to be 1:1. The limit of detection of VN62 toward Al3+ was determined as 39.6 nM and the binding constant was 2.66 x 104 M-1. Comprehensive analyses by HR-MS, H-1 NMR and density functional theory calculations illustrated that the VN62 sensor coordinated with Al3+, forming a stable trigonal bipyramid structure complex, which blocked the ESIPT and photoinduced electron transfer processes and simultaneously promoted an intramolecular charge transfer process, thereby leading to strong cyan fluorescence. Moreover, VN62-loaded test strips were success-fully applied for rapid and visual detection of Al3+. Furthermore, the VN62 sensor was used to visualize Al3+ in plants and live cells. The results indicated that the VN62 sensor has great potential for detecting Al3+ in biological systems.