Introduction
Gold (Au) can be deposited as nanoparticles (NPs) smaller than 10 nm in diameter on a variety of metal oxide (MOx) NPs (Au/MOx) (see Fig. S1 as an example). Au/MOx have attracted much attention due to their high catalytic performance for such as room temperature CO oxidation and selective oxidations in liquid phase1,2. The catalytic activity of Au strongly depends on the kind of support. For example, although TiO2 is almost inactive for CO oxidation, the deposition of Au NPs onto TiO2 enables to catalyze CO oxidation at room temperature3,4. The selective oxidation of glucose by Au supported on activated carbon or metal oxides has been a very active research area, as the transformation of readily available glucose to valuable gluconic acid is of great importance5,6.
These data in the field of heterogeneous catalysis have firmly established that deposition of Au on MOx NPs increases dramatically the intrinsic catalytic activity and this could also be the case for the biological activity. For example, Menchon et al.7 reported that Au/CeO2 exhibits antioxidant activity against reactive oxygen species (ROS) in Hep3B and HeLa cell related lines due to a peroxidase activity. Since the inflammatory reaction is highly dependent on oxidative stress8 one can hypothesize that, in addition to their antioxidant properties, Au/MOx can have anti-inflammatory effects. Such an effect could have important implications in terms of medical utilization of Au/MOx. However, to the best of our knowledge no data on this effect is available in the current literature.
Therefore, in the present study we examined the effect of Au/MOx on cytotoxic and inflammatory response of macrophages, a key cell type involved in the inflammatory reaction. We used murine macrophages and investigated the roles of the size of Au NPs, the chemical nature of the supporting MOx NPs, and the kinetics of Au/MOx interference with the inflammatory reaction.
Results
Physicochemical properties of Au/MOx
Physicochemical properties of Au/MOx are summarized in Table 1. Sizes of primary particles of MOx (TiO2, ZrO2, and CeO2) were in the range of 7–26 nm but MOx NPs are generally agglomerated to form secondary micrometer-sized (2.4–9.4 μm) particles. Actual gold loading amounts of Au/MOx were ca. 1 wt% for all samples. The average sizes of Au NPs on TiO2 and ZrO2 prepared by deposition–precipitation were both of 3.3 nm. To examine the size effect of Au NPs, Au NPs with an average size of 8 nm were deposited on TiO2 by solid grinding. High-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) images of Au/MOx are shown in Fig. 1. For Au/TiO2 and Au/ZrO2, Au NPs were highly dispersed on MOx NPs. On CeO2, the most of Au species is atomically dispersed but only small amount Au NPs with ca. 20 in diameter was observed (Fig. 1d).
References and Figures can be found in the full paper.