Assembled Photosensitizers Applied for Enhanced Photodynamic Therapy

Photodynamic therapy (PDT) has emerged as an efficient method for cancer therapy. However, traditional photosensitizers (PSs) always have low bioavailability. For example, hydrophobic PSs tend to aggregate in cells and lead to aggregation-induced quenching; while hydrophilic PSs that have good solubility in water systems can hardly penetrate into cells whose membranes are lipophilic. To overcome these drawbacks, suitable PSs that meet the requirements of PDT are needed. Numerous investigations have been introduced, especially the molecular-assembly technique that can increase the bioavailability of PSs during the tumor therapy process. Besides, increasing the quantum yield of reactive oxygen species (ROS) by adjusting the PS triplet state lifetime as well as developing aggregation-induced emission (AIE) agents can also improve the PDT effect. This review summarizes the molecular-assembly technique to obtain intelligent PSs to achieve high PDT efficiency. First, increasing the quantum yield of ROS by decreasing the energy gap between S1 and T1 states or increasing the spin-orbit coupling Hamiltonian are introduced. Second, we present the bioavailability of traditional PSs by improving the amphiphilicity of the PSs or using intelligent nanostructures. Then, the AIE PSs that can form ROS in the aggregated state under irradiation are described. Finally, the perspective and challenges of PDT are discussed.