Covalent organic frameworks for fluorescent sensing: Recent developments and future challenges

Covalent-organic frameworks (COFs) is composed of organic units connected by strong covalent bonds. COFs have diverse structure, high porosity, high thermal and mechanical stability and controllable pore, good flexible and highly conjugated crystalline structure. Two-dimensional (2D) COFs nanosheet is similar to graphene, but it has periodic pores and lots of designable functional groups. The 2D COFs nanosheet also has a large specific surface area and good electrical conductivity owing to its highly conjugated crystalline structure. At the same time, COFs have the ability to replicate multiple identical binding sites through the whole extended frameworks. This ability shows that the signal can be effectively transduced and amplified through the framework to achieve high sensitivity when a single binding event occurs at one of the binding sites. Plenty of interesting investigations on the fluorescent sensors based on COFs have been reported through fluorescence enhancement or quenching resulted from this ability. So, the flexible 2D COFs nanosheets should be an excellent candidate to construct fluorescent sensors. This review first introduces the classification of COFs according to different connection units, and then analyzes and summarizes the structure and characterization methods of COFs. The application of COFs in fluorescent sensors is highlighted and the major challenges and limitations are further explored. The review in this paper is helpful to study the wide application of COFs in fluorescent sensors.