Fluorescence response of a thiazolidine carboxylic acid derivative for the selective and nanomolar detection of Zn(II) ions: quantum chemical calculations and application in real samples

A thiazolidine carboxylic acid derivative (L) was conveniently synthesized and characterized by spectral techniques and single crystal X-ray crystallography. The complexation of L with zinc (L-Zn2+) was studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and further substantiated by H-1 NMR and ESI-MS analysis. The L showed excellent specificity and sensitivity towards a zinc-induced fluorescence response by forming a 1 : 1 complex (Job's plot) in a 95% (v/v) water-methanol mixture. The presence of zinc ions causes similar to 40 fold fluorescence enhancement at 481 nm (quantum yield, Phi = 0.19) and can be detected with the naked eye under a UV-lamp. The L can detect Zn2+ in nanomolar levels (13.90 nM) with good tolerance in the presence of other interfering metal ions. The reversibility of L-Zn2+ complexation was checked by EDTA titration. The maximum fluorescence enhancement by Zn2+ binding of L was observed in the pH range of 7.0-9.0. The dissociation constants of L and the stability constant of the L-Zn2+ complex in 0.15 M NaClO4 were determined by pH metrically. Theoretical calculations were done using density functional theory (DFT) to support the above findings. L was successfully applied for the determination of Zn2+ in water samples. The compound (L) detects the presence of, Klebsiella pneumoniae and E. coli in water samples.