Highly selective turn-on fluorogenic chemosensor for Zn2+ based on chelation enhanced fluorescence

Abstract

A new fluorescent zinc sensor was constructed based on cyclic and noncyclic Schiff's bases obtained by the condensation of 4‑(diethylamino) salicylaldehyde and 2‑aminobenzenethio under two different catalytic conditions such as sulphuric acid yields 2‑(benzo[d]thiazol‑2‑yl)‑5‑(diethylamino) phenol (L1) and acetic acid gives to (E)‑5‑(diethylamino)‑2‑(((2‑mercaptophenyl)imino)methyl) phenol (L2). Both L1 and L2 alone exhibits a week fluorescence in CH3OH/H2O (1:1, v/v) due to excited state intra-molecular change transfer (ESIPT) process but upon interaction with Zn2+ shows strong fluorescence due to chelation-enhanced fluorescence (CHEF) but with selected metal ions, there is no such a fluorescence change was observed. The Job's plot and B-H plot studies were revealed the formation of 2:1 and 1:1 stoichiometry with an estimated association constant (Ka) of 4.9 × 104 M−1 and 2.1 × 104 M−1 with L1 and L2 respectively. The detection limit of both sensors L1 and L2 were found to be 6.7 × 10−8 M and 3.6 × 10−7 M respectively. The fluorescence reversibility study was done by adding Zn2+ and EDTA sequentially to L1 and L2. Both sensors were successfully used for the determination of Zn2+ in the different water samples and pharmaceutical multivitamins tablet. The sensing mechanism was studied by 1H NMR, ESI-mass analysis as well as theoretical calculations using the Gaussian 09 program.