Modulation of Aggregation of Covalent Organic Framework Nanosheets with Aggregation-Induced Emission Activity for Ultrasensitive No-Wash Fluorescent Immunosensor

No-wash fluorescence immunosensors are favored because of their simple operation and precise results. However, cleverly designing two fluorescent materials so that their interaction through antigen-antibody specific binding to produce significant fluorescence changes poses a considerable challenge to the design of immunosensors. Herein, covalent organic framework nanosheets (CONs) with aggregation-induced emission (AIE) luminogens (AIEgens) were synthesized to regulate the aggregation of AIEgens through antigen-antibody binding for an ultrasensitive no-wash fluorescence immunosensor. The two-dimensional (2D) CONTFBE-PDAN exhibited excellent AIE activity. A Pt nanoparticle (PtNP)-modified AIE-CONTFBE-PDAN was designed as a signal amplifier, in which the PtNPs were used to bind antibody (Ab) via Pt-N/Pt-S bonds. After the sandwich immunocomplexes were formed by the antigen-antibody interaction, a more compact stacking assembly maximizes the restriction of intramolecular rotation, restriction of intramolecular vibration, restriction of intramolecular motion, and conformational planarization. This optimizes the spatial alignment required for efficient AIE, thereby yielding a strong fluorescence response. Using carcinoembryonic antigen 19-9 (CA 19-9) as a model tumor marker, a no-wash sandwich-type fluorescent immunosensor was constructed. Since a crystalline 2D AIE-CONTFBE-PDAN contains lots of AIEgens, the sensitivity of the fluorescence immunosensor is expected to improve greatly. The no-wash sandwich-type immunosensor showed good performance for CA 19-9 with a linear range of 0.01 mU/mL to 10 U/mL and a detection limit of 3.3 μU/mL (S/N = 3), which was satisfactory in real samples. To the best of our knowledge, this is the first time that AIE-CONs have been used for fluorescence immunosensing, which provides a new perspective for the trace detection of tumor markers.