Fluorescent core-shell SiO2@vertical covalent organic frameworks nanosheets for sensing application

Fluorescent covalent organic frameworks (COFs) are attracting more and more attentions. However, it is very difficult to obtain COFs with strong fluorescence due to fluorescence quenching caused by interlaminar accumulation and intramolecular rotation. Here, the core-shell SiO2@COFTFP-TAPB (1,3,5-tri(4-aminophenyl)-benzene: TAPB, 1,3,5-triformylphloroglucinol: TFP) was prepared by assembling vertical COFTFP-TAPB through amine-aldehyde dehydration condensation reaction on -NH2-SiO2 nanospheres (NPs) for the first time. The 2D COFTFP-TAPB nanosheets with thickness of 25 nm and good crystallinity vertically stand on the surface of -NH2-SiO2 NPs based on the imine-linkage. Such core-shell structure results in larger BET surface area, effectively overcomes the interlaminar accumulation, and improves optical properties of COFTFP-TAPB. The core-shell SiO2@COFTFP-TAPB exhibits multiemission from the enol-imine form and keto-enamine form, which is sensitive to pH. After loading with glucose oxidase and urease, it could be used to detect glucose and urea successfully. The detection ranges are 0.23 μM-14 mM and 1.50 μM-90 mM, and the detection limits reaches 0.07 μM and 5.14 μM, respectively. The work provides a feasible idea to overcome the interlaminar accumulation and reaggregation of COFs during application.