N, N coordinating schiff base ligand acting as a fluorescence sensor for zinc(II) and colorimetric sensor for copper(II), and zinc(II) in mixed aqueous media

•N, N coordinating ligands derived from fluorene and substituted imidazole moiety.•Detection of Zn2+ through fluorogenic and Cu2+ and Zn2+ by chromogenic method.•Determination of 2:1 binding stoichiometry of the ligand:metal in complexes.•DFT and TDDFT investigation.

Two simple and low cost new schiff base compounds of fluorene and derivative of imidazole based chemo sensor were synthesized and characterized by 1H NMR, IR and mass spectroscopic technique. These sensors could simultaneously detect two biologically important metal ions through fluorogenic (Zn2+) and chromogenic (Cu2+ and Zn2+) methods in mixed aqueous solution. However, the sensor functioned as a “turn-on” fluorescence receptor only to Zn2+ ion in presence of other metal ions. The detection limit of HL for Zn2+ (10 μM) was below the guidelines of the WHO (76 μM). The 2:1 binding stoichiometry of the ligand metal ratio in complexes were established by combined UV–Vis, fluorescence and ESI-MS spectroscopy. In addition, the electronic structure and photo physical properties of the ligands and complexes were calculated by DFT and time-dependent DFT (TDDFT) method. The sensor also showed color changes from colorless to yellow upon selective binding with Zn2+ and Cu2+ metal ions, which were visible to the naked eye. The detection limit of the sensor for Cu2+ (10 μM) was found to be below the maximum permissible copper concentration in drinking water as per the guidelines of the World Health Organization (30 μM) and therefore it is capable of being a practical system for the monitoring of Cu2+ concentrations in aqueous samples.

Graphical abstractTwo N, N coordinating ligands derived from fluorene and substituted imidazole moiety. These sensors could simultaneously detect two biologically important metal ions through fluorogenic (Zn2+) and chromogenic (Cu2+ and Zn2+) methods in mixed aqueous solution. The 2:1 binding stoichiometry of the ligand metal ratio in complexes were established by combined UV–Vis, fluorescence and ESI-MS spectroscopy. A full density functional theory (DFT) and time-dependent density functional theory (TDDFT) has been investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide