A novel Schiff base derivative as a fluorescent probe for selective detection of Cu2+ ions in buffered solution at pH 7.5: Experimental and quantum chemical calculations

Abstract

Herein, we report a novel Schiff base receptor (Z)-N'-((1H-indol-3-yl) methylene) nicotinohydrazide (NF1) as a fluorescent chemosensor for the selective detection of Cu2+ in buffer solution (20 mM HEPES, pH 7.5, 1:4 (v:v) water: DMSO) in the presence of other competitive metal ions (Na+, Mg2+ Cd2+, Ni2+, Co2+). The probe NF1 was synthesized and characterized by spectroscopic and single crystal X-ray diffraction studies. The fluorescence binding studies was performed by adding different concentration of Cu2+ metal ions to the solution of NF1 (6.4 × 10−4 M). The successive addition of Cu2+ ions (0-20 mM), results in a decrease in the fluorescence intensity of NF1 due to chelation enhanced fluorescence quenching, which is attributed to the paramagnetic nature of transition metal ion Cu2+. The fluorescence quenching of NF1 after adding different concentrations of Cu2+ ions (0 to 20 equivalents) along with a large blue shift of 65 nm in emission wavelength observed are the evidence for the formation of the NF1-Cu2+ complex. The results of Job's plot and mass spectroscopic analysis, altogether indicated the 1:1 stoichiometry of the NF1-Cu2+ complex. The shift in vibrational modes of C=O and N-C modes was observed in FT-IR studies, and also these could be ascribed for the complex formation. The association and dissociation constants of the NF1-Cu2+ complex were calculated by using the Stern-Volmer plot. The photophysical properties such as quantum yield and fluorescence lifetimes were calculated for NF1 and NF1 with 1 equivalent of Cu2+. The proposed complex structure was optimized by applying the DFT method. The highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap was calculated for probe NF1 and NF1-Cu2+ complex. Theoretical calculations were performed using the TD-DFT method to calculate the excited state properties of the molecule NF1 and NF1-Cu2+ complex and compared with experimental results. All the theoretical and experimental findings strongly suggest that NF1 can be a good candidate for copper sensing applications.