Water-Soluble Polymer Nanoparticles Constructed by Three-Component Self-Assembly: An Efficient Theranostic Agent for Phosphorescent Imaging and Photodynamic Therapy

Water-soluble polymer nanoparticles NP-1 and NP-2 were prepared by using a three-component self-assembly of an iridium complex, poly(4-vinyl pyridine-b-ethylene oxide) (P4VP-b-PEO), and methane sulfonic acid (MSA). Due to the unique metal-ligand charge-transfer transition property and the heavy-atom effect of the iridium atom, NP-1 and NP-2 showed bright phosphorescence and generated the singlet oxygen (1O2) species effectively under visible-light irradiation (λ>400 nm) with a power density of 300 mW cm−2. NP-2, an example of the polymer nanoparticles, showed minimal cytotoxic activity in the dark. Laser confocal fluorescence and flow-cytometry microscopy experiments demonstrated that NP-2 could be taken up by a model cancer-cell line of 4T1 cells, which could be used as a phosphorescent probe for cell imaging by preferentially staining the cytoplasm. After 6 hours of incubation with NP-2 (100 μg mL−1), the cell viability of 4T1 cells decreased to approximately 10 % upon visible-light irradiation (λ>400 nm, 300 mW cm−2) only for 10 minutes as a result of the generation of the 1O2 species, thus indicating a proof of concept for effective photodynamic therapy for cancer cells.