As expected, the fluorescent emission of JUC-557-nanosheet (λ em = 514 nm) exhibited a blue-shift of approximately 50 nm compared to that of bulk JUC-557 (λ em = 564 nm, Fig. ). The observed emission colors were distinguishable on the Commission Internationale de L'Eclairage (CIE) chromaticity diagram, yellow for JUC-557 and green for JUC-557-nanosheet (Fig. ). As shown in Supplementary Table , the fluorescence absolute quantum yield of bulk JUC-557 in solid state was only 9.0%, which can be attributed to the contribution of PE groups as AIE chromophores. However, compared with its bulk counterpart, JUC-557-nanosheet in solid state showed a 2-fold increase of fluorescence absolute quantum yield (~ 21.5%, Supplementary Table ), which is better than those of COF materials composed of only ACQ fluorophores. This is because of an enhanced emission by weakening the luminescence quenching of carbazole knots and no significant change of the fluorescence of PE groups. Furthermore, JUC-557nanosheet dispersed in ethanol showed a more improved absolute quantum yield (up to 23.0%) because the nanosheets can be well scattered in solution, leading to further fluorescence enhancement (Supplementary Table ). Meanwhile, the TRPL curve of JUC-557-nanosheet showed an enhanced lifetime (τ 0, 6.68 ns) compared with its bulk material (τ 0, 4.95 ns, Supplementary Fig. ). Notably, JUC-557-nanosheet also maintained a stable fluorescence intensity in harsh chemical environments, such as in strong acidic (HCl, pH = 1) and alkali aqueous solution (NaOH, pH = 14, Supplementary Figs. 23-24). different concentrations (τ 0 /τ ≈1, Supplementary Fig. ), indicating a static nature of the fluorescence quenching. The apparent quenching constant K q (K q = K a /τ 0 ) was evaluated to be 3.53 × 10 12 M -1 s -1 , which is two orders of magnitude higher than those of conventional bimolecular quenching systems (~10 10 M -1 s -1 ). This result revealed that the fluorescence quenching of JUC-557-nanosheet for Fe 3+ could be attributed to the formation a non-emissive complex, which has also been proved by the shift of UV-vis absorption peaks of JUC-557-nanosheet after the addition of Fe 3+ ions (Supplementary Figs. and). Furthermore, the fluorescence spectrum of Fe 3+ in water obviously overlapped with that of JUC-557-nanosheet, which implies that the energy transfer also causes the fluorescence quenching (Supplementary Fig. ). Therefore, the synergistic mechanism of static quenching and energy transfer quenching brings forth an excellent recognition of JUC-557-nanosheet for Fe 3+ ion.