Recent Progress in Upconversion Photodynamic Therapy.

Photodynamic therapy (PDT) is a minimally invasive cancer modality that combines a photosensitizer (PS), light, and oxygen. Introduction of new nanotechnologies holds potential to improve PDT performance. Upconversion nanoparticles (UCNPs) offer potentially advantageous benefits for PDT, attributed to their distinct photon upconverting feature. The ability to convert near-infrared (NIR) light into visible or even ultraviolet light via UCNPs allows for the activation of nearby PS agents to produce singlet oxygen, as most PS agents absorb visible and ultraviolet light. The use of a longer NIR wavelength permits light to penetrate deeper into tissue, and thus PDT of a deeper tissue can be effectively achieved with the incorporation of UCNPs. Recent progress in UCNP development has generated the possibility to employ a wide variety of NIR excitation sources in PDT. Use of UCNPs enables concurrent strategies for loading, targeting, and controlling the release of additional drugs. In this review article, recent progress in the development of UCNPs for PDT applications is summarized.