Figure 3.

Schematic illustration of antimicrobial photodynamic therapy mechanism for fungal keratitis. The ground-state photosensitizer (PS) absorbs photons and is excited to the first short-lived excited singlet state and either returns to the ground state or undergoes intersystem crossing to a long-lived triplet state. The triplet state PS exerts downstream function via a type I or type II photosensitization process. For type I reaction, charge is transferred from the excited PS to oxygen (O₂), and therefore leading to the formation of hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), and superoxide anion (O₂₋·). For type II reaction, the triplet PS undergoes energy exchange with triplet ground state oxygen, leading to the formation of singlet oxygen ¹O₂. Type I and type II reactions can occur at the same time during irradiation. Nevertheless, type II reaction is mainly involved in antimicrobial photodynamic action. The reaction depends most importantly on PS used and the concentration of O₂ in aPDT.