Disassembly of Hydrophobic Photosensitizer by Biodegradable Zeolitic Imidazolate Framework-8 for Photodynamic Cancer Therapy

Photodynamic therapy (PDT), an alternative to conventional cancer therapeutics, has gained increasing attention for its noninvasive advantage and simultaneous fluorescence imaging property. PDT is a tripartite process that functions in the simultaneous presence of a photosensitizer (PS), light, and available oxygen molecules. However, many highly efficient PSs are hydrophobic and highly tend to self-aggregate in aqueous solution, leading to quick quenching of the PDT effect. Here we construct zeolitic imidazolate framework-8 (ZIF-8) containing water-insoluble photosensitizer zinc(II) phthalocyanine (ZnPc), a typical hydrophobic PS, by one-step coprecipitation process, named as ZnPc@ZIF-8. The micropores of ZIF-8 act as molecular cages to separate and maintain hydrophobic ZnPc in the monomeric state and protect it against self-aggregation, which enables the encapsulated ZnPc to generate cytotoxic singlet oxygen (1O2) under light irradiation (650 nm) in aqueous condition. The formed nanosystem of ZnPc@ZIF-8 can be endocytosed by cancer cells and exhibits red fluorescent emission with excellent photodynamic activity for cancer treatment in vitro. In addition, ZnPc@ZIF-8 is acid-sensitive and would completely degrade after PDT, which can be monitored by the self-quenching of fluorescence emission of ZnPc. This work paves a facile way for resolving the problem of solubility and bioavailability of hydrophobic PS by utilizing metal-organic frameworks as nanocarriers.