Optimized ICPCVD-Based TiO2 for Photonics.

We propose obtaining TiO2 films by ICPCVD for the fabrication of low-loss waveguides. The challenge is to produce a dense and homogeneous layer with a high refractive index and low absorption in the visible range. Crystallized layers with features such as grains and amorphous layers have a rather low index for the application targeted, so we aimed for an intermediate state. We investigated the influence of plasma power, pressure, deposition time and annealing temperature on the structural, crystalline, and optical properties in order to tailor them. We showed that crystallization into rutile at the nanoscale occurred during deposition and under wisely chosen conditions, we reached a refractive index of 2.5 at 630 nm without creating interfaces or inhomogeneity in the layer depth. Annealing permits one to further increase the index, up to 2.6. TEM analysis on one sample before and after annealing confirmed the nano-polycrystallization and presence of both anatase and rutile phases and we considered that this intermediate state of crystallization was the best compromise for guided optics.

Real-Time Observation of "Soft" Magic-Size Clusters during Hydrolysis of the Model Metallodrug Bismuth Disalicylate.

Colloidal bismuth therapeutics have been used for hundreds of years, yet remain mysterious. Here we report an X-ray pair distribution function (PDF) study of the solvolysis of bismuth disalicylate, a model for the metallodrug bismuth subsalicylate (Pepto-Bismol). This reveals catalysis by traces of water, followed by multistep cluster growth. The ratio of the two major species, {Bi9O7} and {Bi38O44}, depends on exposure to air, time, and the solvent. The solution-phase cluster structures are of significantly higher symmetry in comparison to solid-state analogues, with reduced off-center Bi3+ displacements. This explains why such "magic-size" clusters can be both stable enough to crystallize and sufficiently labile for further growth.

Zinc distribution and speciation in Arabidopsis halleri x Arabidopsis lyrata progenies presenting various zinc accumulation capacities.

The purpose of this study was to investigate the relationship between the chemical form and localization of zinc (Zn) in plant leaves and their Zn accumulation capacity. An interspecific cross between Arabidopsis halleri sp. halleri and Arabidopsis lyrata sp. petrea segregating for Zn accumulation was used. Zinc (Zn) speciation and Zn distribution in the leaves of the parent plants and of selected F(1) and F(2) progenies were investigated by spectroscopic and microscopic techniques and chemical analyses. A correlation was observed between the proportion of Zn being in octahedral coordination complexed to organic acids and free in solution (Zn-OAs + Zn(aq)) and Zn content in the leaves. This pool varied between 40% and 80% of total leaf Zn depending on the plant studied. Elemental mapping of the leaves revealed different Zn partitioning between the veins and the leaf tissue. The vein : tissue fluorescence ratio was negatively correlated with Zn accumulation. The higher proportion of Zn-OAs + Zn(aq) and the depletion of the veins in the stronger accumulators are attributed to a higher xylem unloading and vacuolar sequestration in the leaf cells. Elemental distributions in the trichomes were also investigated, and results support the role of carboxyl and/or hydroxyl groups as major Zn ligands in these cells.

Formation of metallic copper nanoparticles at the soil-root interface.

Copper is an essential element in the cellular electron-transport chain, but as a free ion it can catalyze production of damaging radicals. Thus, all life forms attempt to prevent copper toxicity. Plants diminish excess copper in two structural regions: rare hyperaccumulators bind cationic copper to organic ligands in subaerial tissues, whereas widespread metal-tolerant plants segregate copper dominantly in roots by mechanisms thought to be analogous. Here we show using synchrotron microanalyses that common wetlands plants Phragmites australis and Iris pseudoacorus can transform copper into metallic nanoparticles in and near roots with evidence of assistance by endomycorrhizal fungi when grown in contaminated soil in the natural environment. Biomolecular responses to oxidative stress, similar to reactions used to abiotically synthesize Cu0 nanostructures of controlled size and shape, likely cause the transformation. This newly identified mode of copper biomineralization by plant roots under copper stress may be common in oxygenated environments.

Trichomes of tobacco excrete zinc as zinc-substituted calcium carbonate and other zinc-containing compounds.

Tobacco (Nicotiana tabacum L. cv Xanthi) plants were exposed to toxic levels of zinc (Zn). Zn exposure resulted in toxicity signs in plants, and these damages were partly reduced by a calcium (Ca) supplement. Confocal imaging of intracellular Zn using Zinquin showed that Zn was preferentially accumulated in trichomes. Exposure to Zn and Zn + Ca increased the trichome density and induced the production of Ca/Zn mineral grains on the head cells of trichomes. These grains were aggregates of submicrometer-sized crystals and poorly crystalline material and contained Ca as major element, along with subordinate amounts of Zn, manganese, potassium, chlorine, phosphorus, silicon, and magnesium. Micro x-ray diffraction revealed that the large majority of the grains were composed essentially of metal-substituted calcite (CaCO3). CaCO3 polymorphs (aragonite and vaterite) and CaC2O4 (Ca oxalate) mono- and dihydrate also were identified, either as an admixture to calcite or in separate grains. Some grains did not diffract, although they contained Ca, suggesting the presence of amorphous form of Ca. The presence of Zn-substituted calcite was confirmed by Zn K-edge micro-extended x-ray absorption fine structure spectroscopy. Zn bound to organic compounds and Zn-containing silica and phosphate were also identified by this technique. The proportion of Zn-substituted calcite relative to the other species increased with Ca exposure. The production of Zn-containing biogenic calcite and other Zn compounds through the trichomes is a novel mechanism involved in Zn detoxification. This study illustrates the potential of laterally resolved x-ray synchrotron radiation techniques to study biomineralization and metal homeostasis processes in plants.

Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallidurans CH34 exposed to selenite and selenate.

Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se(0)). We have studied the kinetics of selenite (Se(IV)) and selenate (Se(VI)) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se(0) and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se(0). Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se(0) was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. Se(IV) was detected as a transient species in the first 12 h after selenate introduction, Se(0) also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.

Zn speciation in the organic horizon of a contaminated soil by micro-X-ray fluorescence, micro- and powder-EXAFS spectroscopy, and isotopic dilution.

Soils that have been acutely contaminated by heavy metals show distinct characteristics, such as colonization by metal-tolerant plant species and topsoil enrichment in weakly degraded plant debris, because biodegradation processes are strongly inhibited by contamination. Such an organic topsoil, located downwind of an active zinc smelter and extremely rich in Zn (approximately 2%, dry weight), was investigated by X-ray diffraction, synchrotron-based X-ray microfluorescence, and powder- and micro-extended X-ray absorption fine structure (EXAFS) spectroscopy for Zn speciation and by isotopic dilution for Zn lability. EXAFS spectra recorded on size fractions and on selected spots of thin sections were analyzed by principal component analysis and linear combination fits. Although Zn primary minerals (franklinite, sphalerite, and willemite) are still present (approximately 15% of total Zn) in the bulk soil, Zn was found to be predominantly speciated as Zn-organic matter complexes (approximately 45%), outer-sphere complexes (approximately 20%), Zn-sorbed phosphate (approximately 10%), and Zn-sorbed iron oxyhydroxides (approximately 10%). The bioaccumulated Zn fraction is likely complexed to soil organic matter after the plants' death. The proportion of labile Zn ranges from 54 to 92%, depending on the soil fraction, in agreement with the high proportion of organically bound Zn. Despite its marked lability, Zn seems to be retained in the topsoil thanks to the huge content of organic matter, which confers to this horizon a high sorption capacity. The speciation of Zn in this organic soil horizon is compared with that found in other types of soils.

Molecular-scale speciation of Zn and Ni in soil ferromanganese nodules from loess soils of the Mississippi Basin.

Determining how environmentally important trace metals are sequestered in soils at the molecular scale is critical to developing a solid scientific basis for maintaining soil quality and formulating effective remediation strategies. The speciation of Zn and Ni in ferromanganese nodules from loess soils of the Mississippi Basin was determined by a synergistic use of three noninvasive synchrotron-based techniques: X-ray microfluorescence (microXRF), X-ray microdiffraction (microXRD), and extended X-ray absorption fine structure spectroscopy (EXAFS). We show that Ni is distributed between goethite (alpha-FeOOH) and the manganese oxide lithiophorite, whereas Zn is bound to goethite, lithiophorite, phyllosilicates, and the manganese oxide birnessite. The selective association of Ni with only iron and manganese oxides is an explanation for its higher partitioning in nodules over the soil clay matrix reported from soils worldwide. This could also explain the observed enrichment of Ni in oceanic manganese nodules. The combination of these three techniques provides a new method for determining trace metal speciation in both natural and contaminated environmental materials.