Dramatic fluorescence enhancement of PCN-224 and its application in “turn off” immunoassay for sensitive detection of E. coli O157:H7 in milk

•FE-PCN-224 is synthesized with the dramatic fluorescence enhancement by 20.4 times.•Ag+ ions are reduced on FE-PCN-224 to achieve high-efficiency fluorescence quenching.•The proposed immunoassay reaches a lower LOD by 29.7 times than the classical ELISA. In this study, a type of luminescent porous coordination network-224 (PCN-224) in alkaline conditions was synthesized with the dramatic fluorescence enhancement by 20.4 times, which was explained by the fact that the decrease of Zr4+ content in alkaline conditions resulted in the partial recovery of the electron cloud density of 4,4′,4′′,4′′′-(Porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid) (TCPP). Given the large overlap between the excitation spectrum of PCN-224 and the absorption band of Ag nanoparticles (Ag NPs), the coating of the Ag layer on PCN-224 triggered the fluorescence quenching effect, which was applied to “turn off” fluorescence immunoassay for sensitive detection of Escherichia coli O157:H7 (E. coli O157:H7) in milk. The proposed immunoassay reached a low limit of detection (LOD) of 3.3 × 102 CFU mL−1, 29.7 times more sensitive than the conventional ELISA. It will provide a novel alternative strategy for sensitively detecting pathogenic bacteria in the field of food safety.