A novel ethylene linkage-based covalent organic framework for turn-on fluorescence sensing for Al 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 Al 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 Al, and the detection limit was 1.815 μM. Importantly, CN-COF exhibits a satisfactory recovery for detecting Al in drinking water and fish samples. CN-COF also showed the intuitive semi-quantitative detection ability for Al via the color change with the naked eyes. The special pore structure is conducive to allow Al enter to coordinate with O and N atoms on the wall of CN-COF scaffold. The revisable fluorescence change upon the selective addition of Al and XRD, EDTA, XPS and DFT results demonstrated the complex process. The inhibition of the photoinduced electron transition from O atoms to Al 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 Al.