Construction of covalent organic frameworks from nanospheres for Cu²⁺ detection

The preparation of functionalized covalent organic frameworks (COFs) from other kinds of materials plays an important role in the research field of COFs. However, most reports focus on the transformation process and ignore the functionalization of the COFs. Herein, we report a strategy to construct hydrazine-linkage functionalized COFs from imine-linkage nanosphere via linker exchange. The conversion process based on the reversible covalent bonds of the nanosphere and the COFs was investigated by powder X-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy. With the reaction time going by, the (100) diffraction peaks become strong, and the stretching vibrations of C=N and unsaturated carbon of the COFs replace those of the nanosphere; moreover, the uniform spherical morphology become linear, needle-like, and stick-like morphology, indicating the formation of the COFs. The COFs have great crystallinity, porosity, stability, and good luminescent properties, which makes them fluorescent sensors for the detection of Cu2+ with the detection limit as low as 0.082 mu mol L-1 due to the interaction between heteroatoms and Cu2+, leading to the fluorescence quenching of the COFs.