and LOD: 5 ppb), which is superior to those of fluorescence detection materials reported so far, including porous materials, small molecule probes and inorganics. Furthermore, its fluorescence quenching mechanism has been demonstrated to be a synergistic effect of static quenching and energy transfer quenching by a combined theoretical and experimental study, including time-resolved photoluminescence measurements, UV-vis absorption spectroscopy, and density functional theory calculations. As a chemically stable material, JUC-557-nanosheet preserves strong luminescence and sensitive recognition even under harsh conditions (such as strong acid with pH = 1 or strong base with pH = 14), and allows trace detection of various analytes via a handheld UV lamp. Therefore, these findings pave the way for developing stable ultrathin COF nanomaterials for highly sensitive and selective molecular detection. chemistry, making them easily be modified to create ideal functionalized materials. Theoretically, the high designability of construction in COFs is more accessibly designed as a monitor of molecular recognition via increasable adsorption sites and adjustable fluorescence for targeted analytes. For instance, Wang group reported a thioether-functionalized COF with multifunctionality, COF-LZU8, for the selective detection and facile removal of mercury(II). Qiu et al. developed a sp 2 carbon-conjugated fluorescent COF, termed TFPT-BTAN-AO, which shows a fast response time and a low detection limit suitable for detection of UO 2 2+ in H 2 O. Nevertheless, because of the scarcity of a suitable linkage that combines fluorescence with stability, most fluorescent COFs are chemically fragile, and stable COFs are less illuminating. Therefore, exploring highly stable and fluorescent COF materials for chemical detection still remains a great challenge.