A stable luminescent covalent organic framework nanosheet for sensitive molecular recognition

Despite the rapid development of fluorescence detectors over the past decade, it still remains a considerable challenge to exploit a highly stable, sensitive, and selective fluorescence platform for molecular recognition. In this study, we report a stable carbazole-based sp2 carbon fluorescence covalent organic framework (COF) nanosheet, termed JUC-557-nanosheet. Owing to the synergistic effect of AIE- and ACQ-based chromophores in JUC-557-nanosheet, this architecture shows high absolute quantum yields (up to 23.0%) in solid state and the dispersed in various solvents and excellent sensing performance toward specific analytes, such as iodine (Ka: 2.10  105 M-1 and LOD: 302 ppb), 2,4,6-trinitrotoluene (Ka: 4.38  105 M-1 and LOD: 129 ppb), and especially nitrobenzene (Ka: 6.18  106 M-1 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.