A novel ethylene linkage-based covalent organic framework for turn-on fluorescence sensing for Al3+ with excellent selectivity and sensitivity

Covalent organic Framework (COFs) has become a new platform for functional research and material design. A novel covalent organic framework (CN-COF) was first synthesized with p-xylylene dicyanide and 2-hydroxy-1,3,5-benzenetrialdehyde through the Knoevenagel condensation reaction. CN-COF is a porous crystal material with strong thermal and chemical stability. CN-COF exhibits a selective "turn-on" fluorescence response to Al3+ in ethanol with blue-shifted emission spectra over the other tested metal ions. The color changes from pink to earth yellow, and the fluorescence effect is clearly visible. The fluorescence intensity of CN-COF was linearly related to the concentration of Al3+, and the detection limit was 1.815 μM. Importantly, CN-COF exhibits a satisfactory recovery for detecting Al3+ in drinking water and fish samples. CN-COF also showed the intuitive semi-quantitative detection ability for Al3+ via the color change with the naked eyes. The special pore structure is conducive to allow Al3+ enter to coordinate with O and N atoms on the wall of CN-COF scaffold. The revisable fluorescence change upon the selective addition of Al3+ and XRD, EDTA, XPS and DFT results demonstrated the complex process. The inhibition of the photoinduced electron transition from O atoms to Al3+ induced the fluorescence enhancement. This study not only presents a synthesis idea for a new structural organic framework, but also offers a highly selective and sensitive fluorescence chemical sensor for the identification and detection of Al3+.