Iron oxide nanoparticles (IONPs) are frequently used in biomedical applications, yet their toxic potential is still a major concern. While most studies of biosafety focus on cellular responses after exposure to nanomaterials, little is reported to analyze reactions on the surface of nanoparticles as a source of cytotoxicity. Here we report that different intracellular microenvironment in which IONPs are located leads to contradictive outcomes in their abilities to produce free radicals. We first verified pH-dependent peroxidase-like and catalase-like activities of IONPs and investigated how they interact with hydrogen peroxide (H(2)O(2)) within cells. Results showed that IONPs had a concentration-dependent cytotoxicity on human glioma U251 cells, and they could enhance H(2)O(2)-induced cell damage dramatically. By conducting electron spin resonance spectroscopy experiments, we showed that both Fe(3)O(4) and γ-Fe(2)O(3) nanoparticles could catalyze H(2)O(2) to produce hydroxyl radicals in acidic lysosome mimic conditions, with relative potency Fe(3)O(4) > γ-Fe(2)O(3), which was consistent with their peroxidase-like activities. However, no hydroxyl radicals were observed in neutral cytosol mimic conditions with both nanoparticles. Instead, they decomposed H(2)O(2) into H(2)O and O(2) directly in this condition through catalase-like activities. Transmission electron micrographs revealed that IONPs located in lysosomes in cells, the acidic environment of which may contribute to hydroxyl radical production. This is the first study regarding cytotoxicity based on their enzyme-like activities. Since H(2)O(2) is continuously produced in cells, our data indicate that lysosome-escaped strategy for IONP delivery would be an efficient way to diminish long-term toxic potential.

The generation of reactive oxygen species (ROS) is an important mechanism of nanomaterial toxicity. We found that Prussian blue nanoparticles (PBNPs) can effectively scavenge ROS via multienzyme-like activity including peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) activity. Instead of producing hydroxyl radicals (•OH) through the Fenton reaction, PBNPs were shown to be POD mimetics that can inhibit •OH generation. We theorized for the first time that the multienzyme-like activities of PBNPs were likely caused by the abundant redox potentials of their different forms, making them efficient electron transporters. To study the ROS scavenging ability of PBNPs, a series of in vitro ROS-generating models was established using chemicals, UV irradiation, oxidized low-density lipoprotein, high glucose contents, and oxygen glucose deprivation and reperfusion. To demonstrate the ROS scavenging ability of PBNPs, an in vivo inflammation model was established using lipoproteins in Institute for Cancer Research (ICR) mice. The results indicated that PBNPs hold great potential for inhibiting or relieving injury induced by ROS in these pathological processes.

Semiconductor nanostructures with photocatalytic activity have the potential for many applications including remediation of environmental pollutants and use in antibacterial products. An effective way for promoting photocatalytic activity is depositing noble metal nanoparticles (NPs) on a semiconductor. In this paper, we demonstrated the successful deposition of Au NPs, having sizes smaller than 3 nm, onto ZnO NPs. ZnO/Au hybrid nanostructures having different molar ratios of Au to ZnO were synthesized. It was found that Au nanocomponents even at a very low Au/ZnO molar ratio of 0.2% can greatly enhance the photocatalytic and antibacterial activity of ZnO. Electron spin resonance spectroscopy with spin trapping and spin labeling was used to investigate the enhancing effect of Au NPs on the generation of reactive oxygen species and photoinduced charge carriers. Deposition of Au NPs onto ZnO resulted in a dramatic increase in light-induced generation of hydroxyl radical, superoxide and singlet oxygen, and production of holes and electrons. The enhancing effect of Au was dependent on the molar ratio of Au present in the ZnO/Au nanostructures. Consistent with these results from ESR measurements, ZnO/Au nanostructures also exhibited enhanced photocatalytic and antibacterial activity. These results unveiled the enhanced mechanism of Au on ZnO and these materials have great potential for use in water purification and antibacterial products.

We demonstrate that BiOCl single-crystalline nanosheets possess surface structure-dependent molecular oxygen activation properties under UV light. The (001) surface of BiOCl prefers to reduce O2 to ·O2(-) through one-electron transfer, while the (010) surface favors the formation of O2(2-) via two-electron transfer, which is cogoverned by the surface atom exposure and the situ generated oxygen vacancy characteristics of the (001) and (010) surfaces under UV light irradiation.

The hydrophobicity profiles across phosphatidylcholine (PC)-cholesterol bilayer membranes were estimated in both frozen liposome suspensions and fluid-phase membranes as a function of alkyl chain length, unsaturation, and cholesterol mole fraction. A series of stearic acid spin labels, with the probe attached to various positions along the alkyl chain, cholesterol-type spin labels (cholestane and androstane spin labels), and Tempo-PC were used to examine depth-dependent changes in local hydrophobicity, which is determined by the extent of water penetration into the membrane. Local hydrophobicity was monitored primarily by observing the z component of the hyperfine interaction tensor (Az) of the nitroxide spin probe in a frozen suspension of the membrane at -150 degrees C and was further confirmed in the fluid phase by observing the rate of collision of Fe(CN)6(3-) with the spin probe in the membrane using saturation recovery ESR. Saturated-PC membranes show low hydrophobicity (high polarity) across the membrane, comparable to 2-propanol and 1-octanol, even at the membrane center where hydrophobicity is highest. Longer alkyl chains only make the central hydrophobic regions wider without increasing the level of hydrophobicity. Introduction of a double bond at C9-C10 decreases the level of water penetration at all locations in the membrane, and this effect is considerably greater than the cis configuration than with the trans configuration. Incorporation of cholesterol (30 mol %) dramatically changes the profiles; it decreases hydrophobicity (increases water penetration) from the polar headgroup region to a depth of approximately C7 and C9 for saturated- and unsaturated-PC membranes, respectively, which is about where the bulky rigid steroid ring structure of cholesterol reaches in the membrane. Membrane hydrophobicity sharply increases at these positions from the level of methanol to the level of pure hexane, and hydrophobicity is constant in the inner region of the membrane. Thus, formation of effective hydrophobic barriers to permeation of small polar molecules requires alkyl chain unsaturation and/or cholesterol. The thickness of this rectangular hydrophobic barrier is less than 50% of the thickness of the hydrocarbon regions. Results obtained in dioleoyl-PC-cholesterol membranes in the fluid phase are similar to those obtained in frozen membranes. These results correlate well with permeability data for water and amino acids in the literature.

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Co3O4 nanoparticles (Co3O4 NPs), synthesized by the coprecipitation method, showed intrinsic catalase-like, peroxidase-like, and SOD-like activity. The catalytic activity of Co3O4 NPs was much higher than analogous Fe3O4 NPs. Co3O4's mechanisms of catalytic activity were analyzed in detail using the electron spin resonance (ESR) method, which confirmed that Co3O4 NPs don't follow the classical Fenton reactions with hydrogen peroxide the way Fe3O4 NPs do. The high redox potential of Co(3+)/Co(2+) was supposed to be the leading cause of the differences in both activity and mechanism with Fe3O4. Based on the high, peroxidase-like activity, a new immunohistochemical assay was designed in which the avastin antibody was conjugated onto the surface of Co3O4 NPs. The conjugates obtained were used to detect vascular endothelial growth factor (VEGF) that was overexpressed in tumor tissue. When the experimental and control groups were stained, there were clear distinctions between them. This study showed that there are many opportunities to improve the enzyme-like activities of nanomaterials and also to improve their potential applications for biocatalysis and bioassays, especially in relatively harsh conditions.

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