Fluorescent determination of cysteine and homocysteine via adjustable synthesis of flower-shaped covalent organic frameworks

Detection of cysteine (Cys) and homocysteine (Hcy) is very important for the early diagnosis and prevention of related diseases. Herein, we propose a facile, green, room-temperature controllable approach for the synthesis of fluorescent covalent organic frameworks (COFs) with high specific surface areas, high crystallinity, good morphology, excellent chemical and thermal stability by using 2,5-dimethoxyterephthaldehyde and 1,3,5-tris(4-aminophenyl)benzene as precursors. Interestingly, we found that the size and morphology of COFs could be controlled by the amount of acetic acid. With increasing numbers of acetic acid (HAc) molecules, the morphology of COFs gradually changed from spherical to flower-shaped, and the particle size gradually decreased from micrometer to nanometer. Furthermore, we were surprised to find that the reaction time influenced the uniform dispersion and crystallinity of flower-shaped COFs. With increasing the reaction time, the boundary around COFs became blurred, hierarchical flower-shaped COFs gradually appeared, and the flower-shaped gradually became clear and uniform. On this basis, the flower-shaped COFs were further used for highly sensitive determination of Cys and Hcy, with detection limits as low as 11.4 nM and 6.7 nM, respectively. The possible quenching mechanism was deduced by the Dmol3 package provided by Materials Studio and DFT calculations of Gaussian 09 software. This strategy could provide us a new insight into the controllable preparation of flower-shaped fluorescent COFs and broaden the application range of COFs for biomarker detection.