Due to the highly-chemical stability of JUC-557-nanosheet, we further studied its fluorescence sensing and selective recognition for NB under harsh conditions, such as strong acidic (pH = 1) and strong alkali (pH = 14) solutions (H 2 O vs. EtOH = 9: 1). As shown in Supplementary Figs. 97-100, the fluorescence quenching percentages (100% in pH =1 and 98% in pH =14, respectively) and K a values (9.18 × 10 6 M - 1 at pH = 1 and 5.22 × 10 6 M -1 at pH = 14, respectively) were almost unchanged in such harsh chemical environment. On the contrary, the fluorescence intensity of most of imine-linked carbazole-based COFs, such as BCTB-PD COF, BCTB-BCTA COF, and BCTA-TP COF, declined significantly under these harsh conditions because of protonation or deprotonation of imine-linked frameworks. In addition, luminous MOFs are usually unstable in water, strong acidic and alkaline environments. Therefore, these results indicate that such functional COF materials would be an excellent candidate for chemically stable fluorescent indicators.