Covalent organic framework-based surface plasmon-enhanced fluorescence sensing for real-time monitoring of cell apoptosis

The complex physiological environment of living organisms is a major hurdle for in situ monitoring of vital cellular activities. Here, we propose that the surface plasmon-coupled emission (SPCE) biointerface sensing system prepared by modifying covalent organic frameworks (COFs) on metal substrates, can be a powerful tool for biointerface sensing. We have successfully developed a novel pH-responsive fluorescent COF nanoprobe, where fluorophores were precisely post-modified into intrinsically enriched chemically reactive sites within the nanoporous structure. A graphene oxide-assisted assembly strategy was employed to facilitate the robust integration of COFs onto the Ag film. Remarkably, the resulting COF-modified biosensing platform achieves a 40-fold directional fluorescence enhancement in directional fluorescence, attributed to the synergistic coupling between the near-field excited fluorophore dipole, Ag nanofilm and π-conjugated graphene oxide. By precisely controlling the penetration depth of the evanescent through angular modulation of incident light, selective detection of the extracellular and intracellular information can be realized. This allows us to construct a stable, fluorescence-enhanced biosensor chip based on surface plasmon coupling for accurate in situ monitoring of extracellular pH changes during apoptosis.