Covalent Organic Framework Nanosheets as an Enhancer for Light-Responsive Oxidase-like Nanozymes: Multifunctional Applications in Colorimetric Sensing, Antibiotic Degradation, and Antibacterial Agents

We herein fabricated poly(ethyleneimine)-stabilized covalent organic framework nanosheets (PEI-COF NSs) via hydrothermal treatment and showed their roles in enhancing light-responsive oxidase-like activity; the interaction of citrate-capped gold nanoparticles (C-AuNPs) with PEI-COF NSs, named as PEI-COF NSs@C-AuNPs, results in an increased contact with dissolved O2 due to large surface and porosity of PEI-COF NSs and abundant amino functional groups on their planar surface. As a result of these features, the conversion of dissolved oxygen into superoxide anions and hydroxyl radicals under visible light can also be effectively catalyzed. Thus, PEI-COF NSs@C-AuNPs under visible light can catalyze more efficiently than C-AuNPs. Subsequently, the surface modification of PEI-COF NSs@C-AuNPs with thiolate polyethylene glycol-functionalized folic acid allowed the colorimetric sensing of folic acid receptor-overexpressed MCF-7 cells with a limit of detection of 30 cells/mL. Successively, the treatment of PEI-COF NSs@C-AuNPs with NaBH4 produces more surface defects, providing almost complete photodegradation of tetracycline and ciprofloxacin. In addition, the citrate-capped silver nanoparticles (C-AgNPs) decorated with PEI-COF NSs (named PEI-COF NSs@C-AgNPs) showed excellent antibacterial activity against Escherichia coli and Staphylococcus aureus cells with 17 and 34 ng/mL minimum inhibitory concentrations and 2.1 and 1.8 g/mL inhibition zone diameters, respectively, under visible light; the bactericidal mechanism of PEI-COF NSs@C-AgNPs was further investigated using electron microscopy-related techniques.