Supported Ir and Pt Clusters: Reactivity with Oxygen Investigated by Extended X-ray Absorption Fine Structure Spectroscopy

Corrosion-Activated Chemotherapeutic Function of Nanoparticulate Platinum as a Cisplatin Resistance-Overcoming Prodrug with Limited Autophagy Induction

Despite nanoparticulate platinum (nano-Pt) has been validated to be acting as a platinum-based prodrug for anticancer therapy, the key factor in controlling its cytotoxicity remains to be clarified. In this study, it is found that the corrosion susceptibility of nano-Pt can be triggered by inducing the oxidization of superficial Pt atoms, which can kill both cisplatin-sensitive/resistance cancer cells. Direct evidence in the oxidization of superficial Pt atoms is validated to observe the formation of platinum oxides by X-ray absorption spectroscopy. The cytotoxicity is originated from the dissolution of nano-Pt followed by the release of highly toxic Pt ions during the corrosion process. Additionally, the limiting autophagy induction by nano-Pt might prevent cancer cells from acquiring autophagy-related drug resistance. With such advantages, the possibility of further autophagy-related drug resistance could be substantially reduced or even eliminated in cancer cells treated with nano-Pt. Moreover, nano-Pt is demonstrated to kill cisplatin-resistant cancer cells not only by inducing apoptosis but also by inducing necrosis for pro-inflammatory/inflammatory responses. Thus, nano-Pt treatment might bring additional therapeutic benefits by regulating immunological responses in tumor microenvironment. These findings support the idea that utilizing nano-Pt for its cytotoxic effects might potentially benefit patients with cisplatin resistance in clinical chemotherapy.

Effect of particle size on the oxidizability of platinum clusters

The catalytic properties of transition metal particles often depend crucially on their chemical environment, but so far, little is known about how the effects of the environment vary with particle size, especially for clusters consisting of only a few atoms. To gain insight into this topic, we have studied the oxygen affinity of free Pt(x) clusters as a function of cluster size (x = 1, 2, 3, 4, 5, and 10) using density functional theory (DFT) calculations (GGA-PW91). DFT-based Nosé-Hoover molecular dynamics has been used to explore the configuration space of the Pt(x)O(x) and Pt(x)O(2x) clusters, leading to the discovery of several novel Pt-oxide structures. The formation of small Pt-oxide clusters by oxidizing the corresponding Pt(x) clusters is found to be significantly more exothermic than the formation of bulk Pt-oxides from Pt metal. The exothermicity generally increases as cluster size decreases but exhibits strongly nonlinear dependence on the cluster size. The nanoclusters are also structurally distinct from the bulk oxides and prefer one- and two-dimensional chain and ringlike shapes. These findings help elucidate the oxidation behavior of Pt nanoclusters and lay the foundation for understanding the reactivity of Pt nanoclusters in oxidizing chemical environments.

Effects of Adsorbates on Supported Platinum and Iridium Clusters: Characterization in Reactive Atmospheres and during Alkene Hydrogenation Catalysis by X-ray Absorption Spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.