Benzothiadiazole-based covalent organic frameworks with red-light emission for visual detection of norfloxacin

In view of the widespread application of norfloxacin (NOR) and the potential environmental and health risks posed by its residues, a visual approach for the portable yet efficient detection of NOR was presented by using red-light covalent organic frameworks (COFs) based on benzothiadiazole (BTD) as the linkage center. The aggregation-caused quenching (ACQ) effect of BTD-COFs could be suppressed effectively by precisely designing another monomer's structure to enhance the fluorescence of BTD-COFs. Leveraging the red fluorescence of BTD-COFs and the intrinsic blue emission of NOR, ratiometric fluorescence sensors were constructed, achieving visual detection of NOR. Among these BTD-COFs, the COF_BTD-TP can enrich NOR well owing to its appropriate pores and numerous hydroxyl groups, showing best performance for NOR detection with a detection limit of 0.012 μM and a linear range of 0.036-18.08 μM. More importantly, it displayed a distinct and vivid multicolor fluorescence change from red to pink, purple, and finally blue under UV light during NOR detection, enabling visual discrimination by the naked eye. When combined with a smartphone APP, the sensor enabled portable and visual monitoring NOR, exhibiting a detection limit of 0.45 μM and a linear range of 1.35 μM to 30.1 μM. This study not only provides new ideas for designing COFs with red-light emission but also showcases their broad potential applications in environmental monitoring and food safety detection.