Triazine-based covalent organic frameworks for photodynamic inactivation of bacteria as type-II photosensitizers

With the increase of antibiotic resistances in microorganisms, photodynamic inactivation (PDI) as a clinically proven antibacterial therapy is gaining increasing attention in recent years due to its high efficacy. Herein, we reported two covalent organic frameworks (COFs) materials, namely COFs-Trif-Benz and COF-SDU1, as effective type-II photosensitizers for photodynamic inactivation of bacteria. COFs-Trif-Benz and COF-SDU1 are synthesized through a facile solvothermal reaction between tri-(4-formacylphenoxy)-1,3,5-triazine (trif) and benzidine or p-phenylenediamine with high yield. Their highly ordered and porous structures were confirmed by Fourier transform infrared (FT-IR) spectra, solid state 13C CP/MAS NMR spectrum, powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) analyses. The electronic absorption spectra and electrochemical experiments revealed that the extensive π-conjugation over COFs-Trif-Benz and COF-SDU1 greatly enhance their absorbance capability for visible light and make them have a lower band gap. The photocatalytic antibacterial assay was studied against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli O86:B7 (E. coli O86) bacteria. Two materials can kill more than 90% bacteria at concentrations of 100 μg mL− 1 after 60–90 min of illumination. Thus, both COFs are effective photosensitizers. Mechanism investigation revealed the antibacterial characteristics of the COFs-Trif-Benz and COF-SDU1 can generate reactive oxygen species (ROS) by energy transfer to molecular oxygen (3O2) to produce a highly reactive singlet oxygen (1O2). Hence, the two materials during the photodynamic were mainly via mechanism type II.