On the Operational Aspects of Measuring Nanoparticle Sizes.

Nanoparticles are defined as elementary particles with a size between 1 and 100 nm for at least 50% (in number). They can be made from natural materials, or manufactured. Due to their small sizes, novel toxicological issues are raised and thus determining the accurate size of these nanoparticles is a major challenge. In this study, we performed an intercomparison experiment with the goal to measure sizes of several nanoparticles, in a first step, calibrated beads and monodispersed SiO2 Ludox®, and, in a second step, nanoparticles (NPs) of toxicological interest, such as Silver NM-300 K and PVP-coated Ag NPs, Titanium dioxide A12, P25(Degussa), and E171(A), using commonly available laboratory techniques such as transmission electron microscopy, scanning electron microscopy, small-angle X-ray scattering, dynamic light scattering, wet scanning transmission electron microscopy (and its dry state, STEM) and atomic force microscopy. With monomodal distributed NPs (polystyrene beads and SiO2 Ludox®), all tested techniques provide a global size value amplitude within 25% from each other, whereas on multimodal distributed NPs (Ag and TiO2) the inter-technique variation in size values reaches 300%. Our results highlight several pitfalls of NP size measurements such as operational aspects, which are unexpected consequences in the choice of experimental protocols. It reinforces the idea that averaging the NP size from different biophysical techniques (and experimental protocols) is more robust than focusing on repetitions of a single technique. Besides, when characterizing a heterogeneous NP in size, a size distribution is more informative than a simple average value. This work emphasizes the need for nanotoxicologists (and regulatory agencies) to test a large panel of different techniques before making a choice for the most appropriate technique(s)/protocol(s) to characterize a peculiar NP.

Development of an Integrated Thermocouple for the Accurate Sample Temperature Measurement During High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) Experiments.

We have developed two integrated thermocouple (TC) crucible systems that allow precise measurement of sample temperature when using a furnace associated with an environmental scanning electron microscope (ESEM). Sample temperatures measured with these systems are precise (±5°C) and reliable. The TC crucible systems allow working with solids and liquids (silicate melts or ionic liquids), independent of the gas composition and pressure. These sample holder designs will allow end users to perform experiments at high temperature in the ESEM chamber with high precision control of the sample temperature.

Incorporation of uranium into a biomimetic apatite: physicochemical and biological aspects.

Bone is the main target organ for the storage of several toxic metals, including uranium. But the mode of action of uranium on bones remains poorly understood. To better assess the impact of uranium on bone cells, synthetic biomimetic apatites encompassing a controlled amount of uranium were prepared and analyzed. This study revealed the physicochemical impact of uranium on apatite mineralization: the presence of the metal induces a loss of crystallinity and a lower mineralization rate. The prepared samples were then used as substrates for bone cell culture. Osteoblasts were not sensitive to the presence of uranium in the support, whereas previous results showed a deleterious effect of uranium introduced into a cell culture solution. This work should therefore have some original prospects within the context of toxicological studies concerning the effect of metallic cations on bone cell systems.

Alternate dipping preparation of biomimetic apatite layers in the presence of carbonate ions.

The classical simulated body fluids method cannot be employed to prepare biomimetic apatites encompassing metallic ions that lead to very stable phosphates. This is the case for heavy metals such as uranium, whose presence in bone mineral after contamination deserves toxicological study. We have demonstrated that existing methods, based on alternate dipping into calcium and phosphate ions solutions, can be adapted to achieve this aim. We have also especially studied the impact of the presence of carbonate ions in the medium as these are necessary to avoid hydrolysis of the contaminating metallic cations. Both the apatite-collagen complex method and a standard chemical (STD) method employing only mineral solutions lead to biomimetic apatites when calcium and carbonate ions are introduced simultaneously. The obtained materials were fully characterized and we established that the STD method tolerates the presence of carbonate ions much better, and this leads to homogeneous samples. Emphasis was set on the repeatability of the method to ensure the relevancy of further work performed on series of samples. Finally, osteoblasts cultured on these samples also proved a similar yield and standard-deviation in their adenosine triphosphate content when compared to commercially available substrates designed to study of such cell cultures.

Vapour pressure dependence and thermodynamics of cylindrical metal-organic framework mesoparticles: an ESEM study.

Self-assembly of neodymium nitrate and 2,5-dihydroxyl-1,4-benzoquinone (DHBQ) leads to the formation of a metal organic framework (MOF) of formula [Nd2(DHBQ)3(H2O)6]·18H2O. X-ray diffraction studies show that its crystalline structure is that of a two-dimensional coordination polymer packed in parallel sheets, with organised clusters of water molecules lying between the sheets and bridging them via a dense H-bond network. However, instead of forming faceted crystals, this MOF assembles into unusually shaped cylindrical particles of micrometre size. Scanning electron microscopy revealed that the particles are indeed mesoparticles from aggregated MOF crystalline nano-grains. The mesoparticles are stimuli-responsive and shrink in size upon exposure to reduced water vapour pressure. The shrinkage is isotropic and depends on temperature, which allows measuring the coexistence curve of water inside the particles and in the gas phase. Owing to an elaborated environmental scanning-electron microscopy (ESEM) study, it was possible to determine the association energy of water in the mesoparticles. We found a value of 16 ± 6.5 kJ mol(-1). Since the only water present in the particles is the lattice water in the nano-grains, this association energy is the lattice energy of water in the nano-sized MOF crystals. This value allowed us to draw a model for the building process of these originally shaped cylindrical mesoparticles. This is the first example of determination of a thermodynamic value by ESEM.

Mechanism of RuO2 crystallization in borosilicate glass: an original in situ ESEM approach.

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO(2) particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochemical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO(2) particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds formed by reactions at high temperature between a simplified RuO(2)-NaNO(3) precursor and a sodium borosilicate glass (SiO(2)-B(2)O(3)-Na(2)O). The key parameter in the formation of acicular or polyhedral RuO(2) crystals is the chemistry of the ruthenium compound under oxidized conditions (Ru(IV), Ru(V)). The precipitation of needle-shaped RuO(2) crystals in the melt might be associated with the formation of an intermediate Ru compound (Na(3)Ru(V)O(4)) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO(2) crystals in the melt followed by an Ostwald ripening mechanism.

Influence of crystallization state and microstructure on the chemical durability of cerium-neodymium mixed oxides.

To underline the potential links between the crystallization state and the microstructure of powdered cerium-neodymium oxides and their chemical durability, several Ce(IV)(1-x)Nd(III)(x)O(2-x/2) mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity...). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions (with x = 0.09 and 0.16). For CeO(2), either the partial order related to the proton activity (n = 0.63) or the activation energy (E(A) = 37 kJ·mol(-1)) suggested that the dissolution was mainly driven by surface reactions occurring at the solid-liquid interface. The chemical durability of the cerium-neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface.