Naphthyridine derived colorimetric and fluorescent turn off sensors for Ni²⁺ in aqueous media

Highly selective and sensitive 2,7-naphthyridine based colorimetric and fluorescence "Turn Off" chemosensors (L1-L4) for detection of Ni²⁺ in aqueous media are reported. The receptors (L1-L4) showed a distinct color change from yellow to red by addition of Ni²⁺ with spectral changes in bands at 535-550 nm. The changes are reversible and pH independent. The detection limits for Ni²⁺ by (L1-L4) are in the range of 0.2-0.5 µM by UV-Visible data and 0.040-0.47 µM by fluorescence data, which is lower than the permissible value of Ni²⁺ (1.2 µM) in drinking water defined by EPA. The binding stoichiometries of L1-L4 for Ni²⁺ were found to be 2:1 through Job's plot and ESI-MS analysis. Moreover the receptors can be used to quantify Ni²⁺ in real water samples. Formation of test strips by the dip-stick method increases the practical applicability of the Ni²⁺ test for "in-the-field" measurements. DFT calculations and AIM analyses supported the experimentally determined 2:1 stoichiometries of complexation. TD-DFT calculations were performed which showed slightly decreased FMO energy gaps due to ligand-metal charge transfer (LMCT).