Synthesis of a “Turn-On” Mg2+ fluorescent probe and its application in hydrogel adsorption

Abstract

A compound with a Schiff base structure, (E)-3-hydroxy-N'-(2-hydroxybenzaldehyde) (HMN), was synthesized using 3‑hydroxy-2-naphthoic acid hydrazine and 2-hydroxy-4-methoxybenzaldehyde as raw materials. The structure of probe HMN was characterized by 1H NMR, 13C NMR, HR-MS, IR and X-ray single crystal diffraction. This study found that the probe HMN can rapidly and highly selectively recognize Mg2+ in a water system with DMSO=1:9 (water system: 20% triethanolamine + 0.1 mol•L−1 L-cysteine). We used fluorescence spectroscopy and ultraviolet-visible spectroscopy to study the spectral properties of the probe HMN. The results showed that the combination of probe HMN and Mg2+ produced an obvious fluorescence enhancement effect which was negligibly affected by other metals and anions. After that, we used Job's curve, NMR titration, MS and DFT calculations to explore the binding mechanism of probe HMN and Mg2+, and found that they were coordinated in a 1:1 manner. We fitted a linear equation to different Mg2+ concentrations and fluorescence intensities, and the calculated detection limit (LOD) was 8.4802×10−8 mol/L. The standard addition and recovery experiments showed that the probe can be used for the qualitative detection of Mg2+ in water samples. A hydrogel (PAM-HMN) was prepared by doping HMN into polyacrylamide (PAM). We found that PAM-HMN had an efficient absorption performance for Mg2+, and the removal rate of Mg2+ from water reached 97.99%. The color change before and after adsorption of Mg2+ was easily identified by the naked eye under UV light. The microstructure and composition of PAM-HMN before and after adsorption were analysed by SEM-EDS. The probe and hydrogel derivatives have potential application value in environmental detection and pollution prevention. The experimental study of standard addition recovery showed that probe HMN could be used for the quantitative detection of Mg2+in actual water samples.