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 mu M by UV-Visible data and 0.040-0.47 mu M by fluorescence data, which is lower than the permissible value of Ni2+ (1.2 mu M) 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).

Automated summary

Highly selective and sensitive 2,7-naphthyridine-based colorimetric and fluorescence Turn Off chemosensors (L1-L4) are reported for detection of Ni2+ in aqueous media. The receptors showed distinct color change from yellow to red with reversible and pH independent changes. The detection limits for Ni2+ by L1-L4 are lower than the permissible value in drinking water, and the binding stoichiometries were found to be 2:1, supporting the experimentally determined complexation ratios.