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 millimeters 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=Fçrster 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.