Gold nanostructures are among the noble metal nanomaterials being intensely studied due to their good biocompatibility, tunable localized surface plasmon resonance (SPR), and ease of modification. These properties give gold nanostructures many potential chemical and biomedical applications. Herein, we demonstrate the critical role of oxygen activation during the decomposition of hydrogen peroxide (H₂O₂) in the presence of photoexcited gold nanorods (AuNRs) by using electron spin resonance (ESR) techniques. Upon SPR excitation, O₂ is activated first, and the resulting reactive intermediates further activate H₂O₂ to produce?OH. The reactive intermediates exhibit singlet oxygen-like (¹O₂-like) reactivity, indicated by ¹O₂-specific oxidation reaction, quenching behaviors, and the lack of the typical ¹O₂ ESR signal. In addition, by using the antioxidant sodium ascorbate (NaA) as an example, we show that hydroxyl radicals from H₂O₂ activation can induce much stronger NaA oxidation than that in the absence of H₂O₂. These results may have significant biomedical implications. For example, as oxidative stress levels are known to influence tumorigenesis and cancer progression, the ability to control redox status inside tumor microenvironments using noble metal nanostructures may provide new strategies for regulating the metabolism of reactive oxygen species and new approaches for cancer treatment.

Platinum nanoparticles (NPs) are reported to mimic various antioxidant enzymes and thus may produce a positive biological effect by reducing reactive oxygen species (ROS) levels. In this manuscript, we report Pt NPs as an enzyme mimic of ferroxidase by depositing platinum nanodots on gold nanorods (Au@Pt NDRs). Au@Pt NDRs show pH-dependent ferroxidase-like activity and have higher activity at neutral pH values. Cytotoxicity results with human cell lines (lung adenocarcinoma A549 and normal bronchial epithelial cell line HBE) show that Au@Pt NDRs are taken up into cells via endocytosis and translocate into the endosome/lysosome. Au@Pt NDRs have good biocompatibility at NDR particle concentrations lower than 0.15 nΜ. However, in the presence of H₂O₂, lysosomelocated NDRs exhibit peroxidase-like activity and therefore increase cytotoxicity. In the presence of Fe²⁺, the ferroxidase-like activity of the NDRs protects cells from oxidative stress by consuming H₂O₂. Thorough consideration should be given to this behavior when employing Au@Pt NDRs in biological systems.