Preparation and Application of a Fast, Naked-Eye, Highly Selective, and Highly Sensitive Fluorescent Probe of Schiff Base for Detection of Cu2+

A fluorescent probe, N′-((3-methyl-5-oxo-1-phenyl-4, 5-dihydro-1H-pyrazol-4-yl) methylene)-2-oxo-2H-chromene-3-carbohydrazide (MPMC), was synthesized and characterized. Characterizations of the synthetic MPMC were conducted via proton nuclear magnetic resonance (1HNMR) spectroscopy and carbon-13 nuclear magnetic resonance spectroscopy (13C NMR). The fluorescence emission behaviors of the MPMC probe towards diverse metal ions were detected, and the probe exhibited high sensitivity and selectivity towards Cu2+ over other metal ions via the quenching of its fluorescence. Furthermore, the existence of other metal actions made no apparent difference to the fluorescence intensity of the MPMC-Cu2+ system; that is, MPMC displayed a good anti-interference ability. Job’s plot of the MPMC and copper ions indicated that the detection limit was 10.23 nM (R2 = 0.9612) for the assayed actions, with a stoichiometric ratio of 1:1 for MPMC and Cu2+. Additionally, the color of the MPMC probe solution changed from nearly colorless to yellow in the presence of Cu2+ in visible light, and the color change could be observed by the naked eye. Similarly, the color resolved from bright yellow into blue in ultraviolet light. Moreover, reusability studies indicated that the MPMC probe was reusable. The pH effect of the MPMC probe on Cu2+ had a broad range of pH detection, i.e., from 4.0 to 11.0. The response time of the MPMC probe for determining Cu2+ was within 1 min. The recognition of Cu2+ via MPMC performed on pre-treated paper under sunlight and UV light both had a distinct colour change. Thus, the solid-state method for detecting Cu2+ with the naked eye was both economical and convenient.