Au Nanorod/ZnO Core–Shell Nanoparticles as Nano-Photosensitizers for Near-Infrared Light-Induced Singlet Oxygen Generation

Photosensitizer is the key element in photodynamic therapy to generate singlet oxygen to exert cytotoxic effects on cancerous or diseased tissues. Conventional organic photosensitizers generally have poor photostability. Nano-photosensitizers generally display high resistance to photodegradation. However, the high toxicity of quantum dots and the requirement of UV light activation for ZnO and TiO₂ limit their practical applications. Here, we developed Au nanorod (Au NR)/ZnO core–shell nanostructures that integrate Au nanorods (NRs) with ZnO to act as near-infrared light-activated photosensitizers. The core–shell nanostructure helps avoiding direct contact of Au NRs with cells to reduce their cytotoxicity. These Au NR/ZnO core–shell NPs were found to display effective singlet oxygen generation under continuous-wave near-infrared light irradiation. Au NR/ZnO nanoparticles with thicker ZnO shells were found to display higher singlet oxygen generation efficiency. The mechanism of singlet oxygen generation by these Au NR/ZnO NPs was ascribed to the injection of near-infrared light-excited hot electrons of Au NRs into the conduction band of ZnO. The injected electrons were subsequently scavenged by oxygen molecules to form superoxide radicals O₂^–•, which can be further oxidized by the holes on Au NRs to form singlet oxygen. The proposed mechanism has been confirmed by ultrafast transient absorption and pump-probe measurements. These Au NR/ZnO core–shell nanoparticles are promising nano-photosensitizers for near-infrared-activated photodynamic therapy