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

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 (L-1) and acetic acid gives to (E)-5-(diethylamino)-2-(((2-mercaptophenyl)imino)methyl) phenol (L-2). Both L-1 and L-2 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 (K-a) of 4.9 x 10(4) M-1 and 2.1 x 10(4) M-1 with L-1 and L-2 respectively. The detection limit of both sensors L-1 and L-2 were found to be 6.7 x 10(-8) M and 3.6 x 10(-7) M respectively. The fluorescence reversibility study was done by adding Zn2+ and EDTA sequentially to L-1 and L-2. 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 H-1 NMR, ESI-mass analysis as well as theoretical calculations using the Gaussian 09 program.