Development and prospects of covalent organic framework-based ratiometric fluorescent sensors

Ratiometric fluorescence sensors attract more and more attention in the field of fluorescence sensors due to their excellent properties. The advanced material with high luminescent properties is the key to developing ratiometric fluorescence sensors. The covalent organic frameworks (COFs) stand out as an exceptional candidate for fluorescence sensing due to their expansive specific surface area and abundant active sites. However, the design, preparation, and application of COF-based ratiometric fluorescent sensors still represent a promising area for further development. This review collects the design principle of COFs and summarizes the design idea, sensing mode, and mechanism of COF-based ratiometric fluorescent sensors. Furthermore, the approaches to achieving a more comprehensive application of ratiometric fluorescent sensors with two emission centers are discussed. This review presents the research progress, challenges, and future directions in the field of COFs for fluorescent sensors, which is important for the development of high-performance ratiometric fluorescence sensors and the expansion of the application of COFs.