Cerium oxide (CeO2) nanoparticles, which are used in a variety of products including solar cells, gas sensors, and catalysts, are expected to increase in industrial use. This will subsequently lead to additional occupational exposures, making toxicology screenings crucial. Previous toxicology studies have presented conflicting results as to the extent of CeO2 toxicity, which is hypothesized to be due to the ability of Ce to exist in both a +3 and +4 valence state. Thus, to study whether valence state and oxygen vacancy concentration are important in CeO2 toxicity, CeO2 nanoparticles were doped with gadolinium to adjust the cation (Ce, Gd) and anion (O) defect states. The hypothesis that doping would increase toxicity and decrease antioxidant abilities as a result of increased oxygen vacancies and inhibition of +3 to +4 transition was tested. Differences in toxicity and reactivity based on valence state were determined in RLE-6TN rat alveolar epithelial and NR8383 rat alveolar macrophage cells using enhanced dark field microscopy, electron paramagnetic resonance (EPR), and annexin V/propidium iodide cell viability stain. Results from EPR indicated that as doping increased, antioxidant potential decreased. Alternatively, doping had no effect on toxicity at 24 h. The present results imply that as doping increases, thus subsequently increasing the Ce3+/Ce4+ ratio, antioxidant potential decreases, suggesting that differences in reactivity of CeO2 are due to the ability of Ce to transition between the two valence states and the presence of increased oxygen vacancies, rather than dependent on a specific valence state.Electronic supplementary materialThe online version of this article (doi:10.1007/s12011-015-0297-4) contains supplementary material, which is available to authorized users.

The role of particle size distribution inherently present in magnetic nanoparticles (NPs) is examined in considerable detail in relation to the measured magnetic properties of oleic acid-

Experimental results and a model are presented to explain the observed unusual enhancement of the effective magnetic anisotropy K eff with decreasing particle size D from 15 nm to 2.5 nm in 2There continues to be world-wide interest in the size dependent magnetic properties of magnetic nanoparticles (NPs) and their various applications in diverse fields such as catalysis, ferrofluids, sensors, magnetic storage media, and biomedicine [1][2][3][4][5][6]. Based on the nature of magnetic exchange coupling, two classes of NPs may be distinguished: magnetic NPs of metals such as Co [7,8] and Fe [9] and those of oxides such as antiferromagnetic CuO [10] and NiO[11] and ferrimagnetic magnetite (Fe 3 O 4 ) [12,13] and maghemite (γ-Fe 2 O 3 ) [14][15][16][17]. In the oxides, the exchange coupling between the transition-metal ions is facilitated by super-exchange via the intermediate O 2-ions. It is now generally recognized that magnetic properties of NPs depend on several factors such as particle size D (or volume V), size distribution, morphology, interparticle interactions (IPI), and interaction between surface spins and ligands. To unravel all these different effects even in structurally well-characterized samples requires detailed measurements as a function of particle size, temperature, magnetic field strength and measurement frequency.An important parameter for magnetic NPs is their magnetic anisotropy energy E a = K eff V which keeps the particle magnetic moment aligned in a particular direction. Therefore, how this energy and the anisotropy constant K eff varies with particle size D is important in relation to the stability of the stored information, e.g. in recording media. In addition to the finite size-effects, spins on the surface experience different anisotropy because of the reduced dimensionality and broken exchange bonds. The latter is particularly valid for oxides where presence of oxygen vacancies on the surfaces disrupts the super-exchange coupling between the cations [18]. In this paper, we present detailed magnetic measurements in structurally well-characterized γ-Fe 2 O 3 NPs of diameter D = 2.5, 3.4, 6.3 and 7.0 nm coated with oleic-acid (OA), with the analysis of the results focusing on the D-dependence of the effective anisotropy K eff and energy barriers to magnetic moment rotation. By including similar data available from literature on other sizes of γ-Fe 2 O 3 NPs with D up to 15 nm, it is shown that an extension of the often used Eq.: The theoretical basis for determining the effective anisotropy K eff from experimental data is first outlined here. For non-interacting single-domain magnetic NPs, each with volume V, K eff 3 is related to the experimentally observed blocking temperature T B by the relation:Here f o ~10 10 Hz is the system-dependent attempt frequency [20,21], f m is the measurement frequency and k B is the Boltzmann constant. In the presence of IPI characterized by an effective temperature T o < T B , Eq. (1) is replaced by [22,23]:To determine T o , T B is measured ...

In magnetic nanoparticle systems, the variation of the blocking temperature with the measuring frequency is often used to determine the strength of the interparticle interactions (IPI) through a parameter or the Vogel-Fulcher temperature . Presence of IPI is inferred if > 0 and = ∆ [ ∆ log 10 ] ⁄ < 0.13 where Δ signifies changes in and . Here it is shown that these two parameters are related by the Eq. = [1 − ( ⁄ )] where ≈0.11 to 0.15 is a constant of the system depending on the magnitudes of measuring frequency and the attempt frequency of the Néel relaxation. Experimental verification of this relationship is also presented using data on a variety of nanoparticle systems.