How Metal–Organic Framework Topology Affects Singlet Oxygen Production

Photodynamic sterilization uses light of a specific wavelength, oxygen, and a photosensitizer to generate reactive oxygen species such as singlet oxygen (¹O₂), which damages microorganisms. A photosensitizer with excellent photosensitivity is important for this process. Porphyrinic metal-organic frameworks (MOFs) are promising photosensitizers. These frameworks avoid the self-aggregation of porphyrins that usually occurs in aqueous solution. However, it is unclear how the topology of porphyrinic MOFs affects the generation of singlet oxygen.

Li-Jian Chen, Xiu-Ping Yan, Jiangnan University, Wuxi, China, and colleagues have studied four known porphyrinic zirconium MOFs, i.e. MOF-525, MOF-545, PCN-223, and PCN-224 (pictured below), which have different topologies (ftw, csq, shp and she, respectively). These MOFs are composed of Zr₆O₈ clusters connected by different numbers of meso-tetra(4-carboxyphenyl)porphyrin (TCPP) units. The team investigated the influence of the MOFs’ topologies on ¹O₂ generation and photodynamic antibacterial performance.

The ability of the MOFs the generate ¹O₂was evaluated by measuring the absorbance of anthracene-9,10-diyl-bis(methylene) dimalonic acid. This compound can be oxidized by the generated ¹O₂, resulting in a decrease in absorbance. The antibacterial ability of the four MOFs was investigated by comparing morphological and viability changes in E. coli and S. aureus bacteria after incubation with the MOFs.

The team found that the ¹O₂ generation and the photodynamic antibacterial performance decreased in the order MOF-545 > MOF-525 > PCN-224 > PCN-223. The researchers found that the number of porphyrin units per Zr₆O₈ cluster, the pore size, and the distance between inter-porphyrin active sites play important roles in ¹O₂ production. This work could provide guidance for designing new MOFs for efficient photodynamic sterilization and therapy.