Upconverting nanoparticle-based multi-functional nanoplatform for enhanced photodynamic therapy: Promises and perils

Photodynamic therapy (PDT) is a noninvasive technique that selectively targets cancer cells. The photosensitizer absorbs light of a specific wavelength and generates highly reactive oxygen species (ROS) such as singlet oxygen, from molecular oxygen in the surrounding tissue. However, PDT is limited by the penetration depth and scattering of the visible excitation light in tissue. Near infrared absorbing upconversion nanoparticles (UCNPs) are used to extend the penetration depth as most tissues absorb weakly in the infrared. UCNPs are anti-Stokes type materials in which rare earth atoms are embedded in a crystalline matrix. Upconversion emission occurs via a two-photon process that is efficiently excited by low-cost CW diode laser sources. In PDT, these nanoparticles are coupled to sensitizers that absorb the upconverted emission, resulting in singlet oxygen formation. We further discuss the benefits of using UCNPs in PDT, the potential challenges, and also the potential pitfalls of PDT in general.