Recent advances in nanoparticle carriers for photodynamic therapy

Photodynamic therapy (PDT) is a clinical treatment based on the activation of light-absorbing molecules, or photosensitizers (PSs). Upon light irradiation at a specific wavelength, PSs generate reactive oxygen species (ROS), which are toxic to the targeted disease cells. PSs are initially in a singlet state at the ground state energy level (S0) and they change to the excited state (S1) after absorbing photons of light (1). Because this excited state is unstable and short-lived, PSs return to their ground state by providing fluorescence or transferring to the excited triplet state (T1), which is relatively more stable than S1. Then, the PSs at T1 can transfer hydrogen, electron, or direct energy to the surrounding oxygen and generate ROS. Typically, near-infrared (NIR) lasers within 600- to 800-nm wavelength have been used for PDT to prevent interference by endogenous chromophores in tissues (2). Currently, PDT is being widely tested or administered in clinics for cancer therapy of the skin, prostate, head and neck, pancreas, breast, and lung (3)