Naked Eye Turn-On Fluorometric Sensor for Ni²⁺ Detection in Aqueous Solution Based on Schiff Base

In recent years, the design and synthesis of fluorescent sensors have drawn considerable attention for the specific and sensitive detection of biologically and environmentally important metal ions in living cells and aqueous solutions. Herein, we reported the photophysical properties of a Schiff base fluorescent sensor N’-(2-hydroxy-3-methoxybenzylidene)-3-methoxybenzohydrazide (RN5) that possesses a high affinity for the Ni²⁺ at a nanomolar concentration. Through fluorescence titration analysis at 600 nm, we confirmed that the sensor RN5 exhibited a colorimetric (from colorless to yellow) and fluorometric turn-on response (fluorescence enhancement) in CH₃CN:H₂O (2/8, v/v) medium in the presence of Ni²⁺, whereas no noticeable emission was observed in the case of other examined metals ion. The complexation reaction of Ni²⁺ with RN5 has been investigated by the colorimetric, absorption spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The 1:1 binding mode was confirmed between the sensor RN5 and Ni²⁺ by job plot evaluation and theoretically confirmed by DFT studies. The detection limit for Ni²⁺ was found to be 12.8 nM estimated by the titration method, which was lower than the guideline for drinking water, 1.2 μM by the Environmental Protection Agency (EPA), USA. The sensor RN5 exhibited the excellent fluorescence sensing behavior in a wide pH range of 5–12. The turn-on fluorescence behavior of the Ni²⁺ with the sensor RN5 was fast, so that it could be employed for quantitative and qualitative determination. Test kits, containing RN5 were fabricated which could perform as the suitable and complete assay “in-the-field” detection of Ni²⁺. Finally, the sensor RN5 was applied successfully for the detection of Ni²⁺ in the real water samples.