Generation of Singlet Oxygen by Persistent Luminescent Nanoparticle-Photosensitizer Conjugates: A Proof of Principle for Photodynamic Therapy without Light

The broader application of photodynamic therapy as a treatment procedure for cancer is hampered by the limited penetration of light through mammalian tissues. Since the photosensitized generation of cytotoxic singlet oxygen requires effective excitation of the tumor-localized photosensitizer, photodynamic action can only be guaranteed for the first few milli-meters of the irradiated tissues. In this work, we demonstrated that the phenomenon of persistent luminescence, that is, delayed emission from certain metal-ion excited states (with crystal defects acting as energy traps), can provide an alternative excitation possibility. Thus, persistent luminescent nanoparticles functionalized by FRET-matching Bodipy sensitizers (FRET=Forster resonance energy transfer) were excited in situ before administration into a cell culture or an organism. It was found that this system continues to produce singlet oxygen regardless of their location and without any need for continuous photonic excitation.