Also, we proved the unique iodine recognition ability of the nanosheet in the seawater (from the Bohai Sea, China). As shown in Supplementary Fig. , the seawater with the trace iodine caused an apparent fluorescence quenching of the nanosheet. The K a value of JUC-557-nanosheet with I 2 in seawater was estimated to be 1.27 × 10 5 M -1 , which is slightly lower than that from pure I 2 because organics in seawater could partially react with iodine. Furthermore, I 2 vapor can be quickly detected based on the fluorescence quenching of JUC-557-nanosheet, and the quenching percentage was almost up to 90% within only 30 s (Fig. ). These results demonstrated that JUC-557-nanosheet had a promising practical application for the detection of harmful I 2 as a sensitive sensor.