Linker engineering to regulate the fluorescence of hydrazone-linked covalent organic frameworks for the real-time visual detection of norfloxacin and multiple information encryption

The misuse and improper handling of norfloxacin (NOR) can cause serious harm; therefore, the sensitive and visual detection of NOR is of great significance. However, it is still a huge challenge to establish the multiplex fluorescence detection of NOR. Herein, four hydrazone-linked covalent organic frameworks (COFs) with different luminescence were prepared using 2,5-bis(2-methoxyethoxy)terephthalohydrazide as the nucleus and aldehyde monomers containing methoxy and different numbers of hydroxyl groups as linkers, respectively, and the precise modulation of the emission of hydrazone-linked COFs was achieved. The intrinsic blue emission of NOR was combined with hydrazone-linked COFs to construct ratiometric fluorescence sensors to monitor NOR. Among them, a two-signal change ratiometric fluorescence sensor based on COFBMTH-HB showed the highest sensitivity, the lowest detection limit (51 ng mL−1) and the widest linear range (153 ng mL−1 to 48 μg mL−1), originating from the lowest background fluorescence and the largest output signal. Based on the obvious color changes in the detection process, a novel test paper was also proposed with simultaneous multi-chroma and was combined with a smartphone App as a real-time detection tool to successfully monitor NOR in real fish samples. This portable paper-based sensor provides support for rapid and simple NOR visual detection. In addition, a two-dimensional code with multiple information encryption was developed by taking advantage of the obvious color changes as well as the excitation dependence of COFBMTH-HB during the detection of NOR, which further broadens the application of two-dimensional codes in the field of anti-counterfeiting.