Mechanistic and Thermodynamic Insights into Anion Exchange by Green Rust.

Fougerite is a naturally occurring green rust, that is, a layered double hydroxide (LDH) containing iron (Fe). Fougerite was identified in natural settings such as hydromorphic soils. It is one of the few inorganic materials with large anion adsorption capacity that stems from the presence of isomorphic substitutions of Fe2+ by Fe3+ in its layers. The importance of anion adsorption in the interlayer of LDH has often been highlighted, but we are still missing a mechanistic understanding and a thermodynamic framework to predict the anion uptake by green rust. We combined laboratory and in operando synchrotron X-ray diffraction and scattering experiments with geochemical modeling to contribute to filling this gap. We showed that the overall exchange process in green rusts having nanometer and micrometer sizes can be seen as a simple anion exchange mechanism without dissolution-recrystallization or interstratification processes. A thermodynamic model of ion exchange, based on the Rothmund and Kornfeld convention, made it possible to predict the interlayer composition in a large range of conditions. This multiscale characterization can serve as a starting point for the building of robust and mechanistic geochemical models that will allow predicting the role of green rust on the geochemical cycle of ions, including nutrients, in soils.

Water Mobility within Compacted Clay Samples: Multi-Scale Analysis Exploiting 1H NMR Pulsed Gradient Spin Echo and Magnetic Resonance Imaging of Water Density Profiles.

1H NMR pulsed gradient spin echo attenuation and water density profile analysis by magnetic resonance imaging are both used to determine the mobility of water molecules confined within a porous network of compacted kaolinite clay sample (total porosity of ∼50%). These two complementary experimental procedures efficiently probe molecular diffusion within time scales varying between milliseconds and few hours, filling the gap between the time scale of diffusion dynamics measured by traditional quasi elastic neutron scattering and through-diffusion methods. Furthermore, magnetic resonance imaging is a nondestructive investigation tool that is able to assess the effect of the local structure on the macroscopic mobility of the diffusing probe.

Thin Films with Perpendicular Tetragonally Packed Rectangular Rods Obtained from Blends of Linear ABC Block Terpolymers.

A binary blend of poly(isoprene-block-styrene-block-(2-vinylpyridine)) (ISP) triblock terpolymers, having the same chain length but different compositions, was used to achieve an ordered lattice with 4-fold symmetry of rectangular-shaped rods of poly(isoprene) (I) and poly(2-vinylpyridine) (P). In given conditions, the I and P domains were oriented perpendicularly to the substrate, providing an appealing type of templates for nanopatterning. Thin films were prepared by spin coating, exposed to solvent vapor (providing morphological reorganization), and then characterized by atomic force microscopy, transmission electron microscopy, and grazing-incidence small-angle X-ray scattering. Selective I and P identifications were carried out by AFM and TEM on a model ISP, as well as development of a technique of electronic contrast enhancement to better assign the self-assembly structure in GISAXS.

Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms.

BACKGROUND: Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO6)(8-) octahedra connected through their edges and frequently contain vacancies and  (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO2) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy. RESULTS: Migration of Mn(3+) from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50-150 nm in the ab plane and ~10-50 nm along c*, that is a tenfold increase compared to fresh samples. CONCLUSION: The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.

Nonionic organoclay: a 'Swiss Army knife' for the adsorption of organic micro-pollutants?

A Na exchanged montmorillonite (Mt) was used as a starting layered material for the preparation of two organoclays synthesized with benzyl decyltrimethyl ammonium (BDTA) cationic surfactant and the tri-ethylene glycol mono n-decyl ether(C10E3), a nonconventional nonionic surfactant. The adsorption of the surfactants was performed at an amount of 0.7 times the cation exchange capacity (CEC) for BDTA and below the critical micelle concentration (cmc) where C10E3 is in a monomer state, leading to the intercalation of a lateral monolayer surfactant arrangement within the interlayer space and about 5-7% organic carbon content in organoclays. The environmental properties of both nonionic (C10E3Mt) and cationic (BDTAMt) organoclays were compared to those of the starting Mt clay with the sorption of three micro-pollutants: benzene, dimethyl-phthalate and paraquat. The adsorption isotherms and the derivative data determined through the fitting procedure by using Langmuir, Freundlich and Dubinin-Radushkevitch equation models explicitly highlighted the importance of the chemical nature of the micropollutants, which play on the adsorbents efficiency. The adsorption data combined with FTIR and XRD supplementary results suggest that C10E3Mt nonionic organoclay, although being less efficient for the retention of the different micropollutants, turned out to be the most polyvalent adsorbent since such hybrid material could adsorb the entire studied organic compounds.

A novel and easy chemical-clock synthesis of nanocrystalline iron-cobalt bearing layered double hydroxides.

A novel synthesis of cobalt-iron layered double hydroxide (LDH) with interlayer chlorides was investigated. The method consists in mixing concentrated solutions of hexaamminecobalt(III) trichloride with ferrous chloride at room temperature and in anoxic conditions. Four initial Fe/Co atomic ratios have been tried out (0.12, 0.6, 1.2 and 1.8). Neither heating nor addition of alkali was employed for adjusting the pH and precipitating the metal hydroxides. Still, each mixture led to the spontaneous precipitation of a LDH-rich solid having a crystal-chemistry that depended on the initial solution Fe/Co. These LDHs phases were carefully characterized by mean of X-ray diffraction, (57)Fe Mössbauer spectrometry, transmission electron microscopy and chemical analysis (total dissolution and phenanthroline method). Solution Eh and pH were also monitored during the synthesis. Increasing initial Fe/Co ratio impacted the dynamic of the observed stepwise reaction and the composition of the resulting product. Once the two solutions are mixed, a spontaneous and abrupt color change occurs after an induction time which depends on the starting Fe/Co ratio. This makes the overall process acting as a chemical clock. This spontaneous generation of CoFe-LDH arises from the interplay between redox chemistries of iron and cobalt-ammonium complexes.

Toward theranostic nanoparticles: CB[7]-functionalized iron oxide for drug delivery and MRI.

Iron oxide (γ-Fe2O3) nanoparticles (NPs) were efficiently coated with the water soluble macrocycle cucurbit[7]uril (CB[7]) by microwave heating. Density functional theory (DFT) calculations support a binding model in which the carbonyl oxygens of CB[7] coordinate directly to surface Fe3+ ions. The modified particles (CB[7]NPs) are stable under a wide pH range (2-12) and have a transverse relaxivity, R2, of 113 s-1 mM-1. Nile red (NR) dye was loaded into the cavities of the surface-adsorbed CB[7]s, and intracellular delivery of the dye to HCT116 cells was observed by confocal laser scanning microscopy. The dye-loaded particles (CB[7]NPs⊃NR) have a R2 of 172 s-1 mM-1. The stability, biocompatibility, and dual purpose functionality (drug delivery and magnetic resonance imaging) of the CB[7]NPs herald the theranostic potential of this system.

Uptake of functionalized mesoporous silica nanoparticles by human cancer cells.

Mesoporous silica nanoparticles (MSN) were functionalised by aminofluorescein (AMF) with diethylenetriaminepentaacetic acid spacer molecules which provide free carboxylic groups for binding cell-specific ligands such as folate. AMF allowed the exploration of cellular uptake by HeLa cells using confocal microscopy and flow cytometry. The functionalized nanoparticles (MSN-AMF) penetrated efficiently into HeLa cell cytoplasm through a clathrin dependent endocytosis mechanism. The number of endocytosed MSN-AMF was enhanced when using folate as a targeting molecule. Uptake kinetics revealed that most of MSN-AMF were internalized within 4 h of incubation. Moreover, we found that MSN-AMF were capable of escaping the acidic endolysosomal vesicles of HeLa cells. Cytotoxicity studies suggested that these nanoparticles are non-toxic to HeLa cells up to a dose level of 50 microg/ml.

Poly(ethylene glycol)-stabilized silver nanoparticles for bioanalytical applications of SERS spectroscopy.

The present work depicts the efficient one-step synthesis and detailed evaluation of stable aqueous colloids of silver nanoparticles (NPs) coated with poly(ethylene glycol) (PEG) covalently attached to their surface. Due to steric repulsion between polymer-modified surfaces, the stability of the nanoparticle suspension was preserved even at high ionic strength (0.1 M NaCl). At the same time, the PEG coating remains sufficiently permeable to allow surface-enhanced Raman scattering (SERS) from micromolar concentrations of small molecules such as the anticancer drug mitoxantrone (MTX). The enhancement efficiency of the hot spot-free Ag-PEG was compared to that of citrate-stabilized Ag colloids used after pre-aggregation. The potential of the polymer-stabilized colloids developed in this study is discussed in terms of bioanalytical applications of SERS spectroscopy.

Internally structured pickering emulsions stabilized by clay mineral particles.

The present study aims to describe emulsion particles containing a dispersed phase composed of nanostructured lipid mesophases and stabilized by montmorillonite and/or Laponite clay platelets. The size distributions of these emulsion particles were found independent of the clay mineral content and of the initial internal composition that determines the internal structure. The stabilization of the droplets by a shell of smectite layers was found possible even by montmorillonite which has a length of the same order or more than the droplets to stabilize. The clay platelets give a local flatness to the droplets that may influence the internal structure. In this paper, we describe the conditions to obtain such soft particles of about 220 nm, and we show by direct visualization the internal mesophase complexity and the shape of the particles. In particular, TEM analysis showed elongated particles with bent-back channels at their center but a different morphology at the periphery due to flat border conditions imposed by the presence of the clay minerals.

Ordered structures in carboxymethylcellulose-cationic surfactants-copper ions precipitated phases: in situ formation of copper nanoparticles.

Small-angle X-ray scattering has been used to investigate the nanostructures in precipitated phases of carboxymethylcellulose/oppositely charged C(n)TAB surfactants mixtures in interaction with bivalent copper ions. In copper-free precipitates, a transition from a 2D hexagonal phase H(I) to a micellar cubic surfactant structure Pm3n was found on increasing the polymer concentration (n = 14, 16). By addition of small amounts of copper ions, the internal structure also changes from H(I) to Pm3n cubic and finally turns into a less-ordered phase. The concentration at which this transition occurs strongly depends on the surfactant tail length. The presence of copper ions in the precipitated phase leads at the same time to a disorganization of the surfactant micelles' order and to the formation of a lamellar ordering, revealed by the presence of additional diffraction peaks above 10 mM copper. We used the polymer/surfactant networks as template systems, controlled by the copper ion content, for preparing embedded copper nanoparticles via in situ reduction of copper sulfate by sodium borohydride. The zerovalent copper particles formed in the cubic lattice are well-defined in size (less than 10 nm) and have a homogeneous distribution, whereas they are very polydisperse when the reduction is performed in the unorganized phase because the lack of order in the matrix does not protect them from aggregation.

Formation of assembled silver nanowires by reduction of silver thiolate in polyol/toluene medium.

Reduction of silver nitrate in polyol/toluene biphasic medium containing dodecanethiol led to organised silver nanowires, results of an interaction between silver nanoparticles and the layered phase AgSC12H25.