Type I photodynamic therapy by organic–inorganic hybrid materials: From strategies to applications

Photodynamic therapy (PDT) is a promising approach for treatment of cancer and bacterial infection. Upon excitation by light, photosensitizers (PSs) produce reactive oxygen species (ROS), which could induce cell destruction. ROS could be produced via two kinds of photoreactions, which are type I (electron transfer) mechanism producing superoxide anions, hydrogen peroxides and hydroxyl radicals, and type II (energy transfer) mechanism generating singlet oxygen. Traditional type II PDT suffers from the problem of oxygen-dependent. Fortunately, type I mechanism provides a promising solution that makes PDT practically operated in hypoxic environment. In this review, we attempt to provide a systemic overview of a variety of approaches to generate and improve type I PDT. Inorganic PSs possess the intrinsic feature of generating electron-hole pairs under irradiation, resulting in a charge separated state which is favorable for type I pathway. Organic PSs are generally involved in type II PDT. Strategies of covalent modification and metal coordination are employed for transformation from type II to type I pathway. Provided examples focus on macrocycles and ruthenium(II) complex. We finally emphasize the potentiality of supramolecular assembly as a novel non-covalent strategy to promote type I PDT. It provides a facile method to fabricate nanomaterials with multiple functional building blocks, which could tune type I/II PDT without tedious synthesis. It is also involved in optimizing PSs delivery owing to their unique, nanoscale related properties.