The Stern-Völmer plot exhibited a linear relationship (R 2 = 0.98) (Fig. ), which should be caused by the interactions of the carbazole sites with I 2 . Notably, the K a was estimated to be 2.10 × 10 5 M -1 (R 2 = 0.99), which is higher than those of fluorescence porous materials reported so far for the detection of I 2 , such as TTDATA (K a = 4.33 × 10 2 M -1 ), TS-TAD (K a = 5.76 × 10 3 M -1 ), and TDPDB (K a = 5.83 × 10 4 M - 1 , Fig. and Supplementary Table ). Furthermore, trace I 2 can be detected by the fluorescence quenching of JUC-557-nanosheet (Supplementary Figs. ). The LOD was measured to be 1.19 × 10 -9 M (302 ppb). This detection could be repeated ten times without reduced sensitivity, proving that the COF nanosheet has an excellent cycling stability (Fig. ). Moreover, the TRPL spectra of JUC-557-nanosheet were studied, and it showed a slight change after adding different amounts of I 2 with τ 0 /τ ≈1 (from 6.48-6.25 ns, Fig. ), indicating a static nature of the fluorescence quenching. The obvious change of UV-vis absorption band of JUC-557-nanosheet caused by the addition of I 2 was observed, indicating the formation of a nonemissive complex (Supplementary Fig. ). Meanwhile, the absorption band of I 2 in water was obviously overlapped with the excitation band of JUC-557-nanosheet, leading to a fluorescence quenching from resonance energy transfer of nanosheet with I 2 (Supplementary Fig. ). Like the detection of Fe 3+ , the synergistic mechanism of static quenching and energy transfer quenching renders an outstanding fluorescence sensing of JUC-557-nanosheet for I 2 .