Investigation of the interface in silica-encapsulated liposomes by combining solid state NMR and first principles calculations

In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {(1)H-(29)Si}(1)H and {(1)H-(31)P}(1)H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that corresponds to a realistic model of an amorphous silica slab. These data combined with first principles NMR parameters calculations by GIPAW (Gauge Included Projected Augmented Wave) show that the phosphate moieties are not directly interacting with silanols. The stabilization of the interface is achieved through the presence of water molecules located in-between the head groups of the phospholipids and the silica surface forming an interfacial H-bonded water layer. A detailed study of the (31)P chemical shift anisotropy (CSA) parameters allows us to interpret the local dynamics of DPPC in liposils. Finally, the VASP/solid state NMR/GIPAW combined approach can be extended to a large variety of organic-inorganic hybrid interfaces.     

Bio-dissolution of colloidal-size clay minerals entrapped in microporous silica gels

Four colloidal-size fractions of strongly anisotropic particles of nontronite (NAu-2) having different ratios of basal to edge surfaces were incubated in the presence of heterotrophic soil bacteria to evaluate how changes in mineral surface reactivity influence microbial dissolution rate of minerals. To avoid any particle aggregation, which could change the reactive surface area available for dissolution, NAu-2 particles were immobilized in a biocompatible TEOS-derived silica matrix. The resulting hybrid silica gels support bacterial growth with NAu-2 as the sole source of Fe and Mg. Upon incubation of the hybrid material with bacteria, between 0.3% and 7.5% of the total Fe included in the mineral lattice was released with a concomitant pH decrease. For a given pH value, the amount of released Fe varied between strains and was two to twelve-fold higher than under abiotic conditions. This indicates that complexing agents produced by bacteria play an important role in the dissolution process. However, in contrast with proton-promoted NAu-2 dissolution (abiotic incubations) that was negatively correlated with particle size, bacterial-enhanced dissolution was constant for all size fractions used. We conclude that bio-dissolution of nontronite particles under acidic conditions seems to be controlled by bacterial metabolism rather than by the surface reactivity of mineral.     

High dilution of anionic vacancies in Sr(0.8)Ba(0.2)Fe(O,F)(~2.5)

The (Ba,Sr)FeO(3-δ) system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)cubic compounds (generally δ < 0.27, Fe(3/4+)). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (δ ≈ 0.5, Fe(3+)) within the room-temperature stable tetragonal (pseudocubic) Sr(0.8)Ba(0.2)FeF(~0.1)(O,F)(~2.5.) The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/□ constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr(0.8)Ba(0.2)FeO(~2.7) shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe(3+) of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe(3+) cations (M(Fe) ≈ 2.64(1) μB at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.     

3,5-Bis(ethynyl)pyridine and 2,6-bis(ethynyl)pyridine spanning two Fe(Cp*)(dppe) units: role of the nitrogen atom on the electronic and magnetic couplings

The role of the nitrogen atom on the electronic and magnetic couplings of the mono-oxidized and bi-oxidized pyridine-containing complex models [2,6-{Cp(dpe)Fe-C≡C-}(2)(NC(5)H(3))](n+) and [3,5-{Cp(dpe)Fe-C≡C-}(2)(NC(5)H(3))](n+) is theoretically tackled with the aid of density-functional theory (DFT) and multireference configuration interaction (MR-CI) calculations. Results are analyzed and compared to those obtained for the reference complex [1,3-{Cp*(dppe)Fe-C≡C-)}(2)(C(6)H(4))](n+). The mono-oxidized species show an interesting behavior at the borderline between spin localization and delocalization and one through-bond communication path among the two involving the central ring, is favored. Investigation of the spin state of the dicationic complexes indicates ferromagnetic coupling, which can differ in magnitude from one complex to the other. Very importantly, electronic and magnetic properties of these species strongly depend not only upon the location of the nitrogen atom in the ring versus that of the organometallic end-groups but also upon the architectural arrangement of one terminus, with respect to the other and/or vis-à-vis the central ring. To help validate the theoretical results, the related families of compounds [1,3-{Cp*(dppe)Fe-C≡C-)}(2)(C(6)H(4))](n+), [2,6-{Cp*(dppe)Fe-C≡C-}(2)(NC(5)H(3))](n+), [3,5-{Cp*(dppe)Fe-C≡C-}(2)(NC(5)H(3))](n+) (n = 0-2) were experimentally synthesized and characterized. Electrochemical, spectroscopic (infrared (IR), M?ssbauer), electronic (near-infrared (NIR)), and magnetic properties (electron paramagnetic resonance (EPR), superconducting quantum interference device (SQUID)) are discussed and interpreted in the light of the theoretical data. The set of data obtained allows for many strong conclusions to be drawn. A N atom in the long branch increases the ferromagnetic interaction between the two Fe(III) spin carriers (J > 500 cm(-1)), whereas, when placed in the short branch, it dramatically reduces the magnetic exchange in the di-oxidized species (J = 2.14(5) cm(-1)). In the mixed-valence compounds, when the N atom is positioned on the long branch, the intermediate excited state is higher in energy than the different ground-state conformers and the relaxation process provides exclusively the Fe(II)/Fe(III) localized system (H(ab) ≠ 0). Positioning the N atom on the short branch modifies the energy profile and the diabatic mediating state lies just above the reactant and product diabatic states. Consequently, the LMCT transition becomes less energetic than the MMCT transition. Here, the direct coupling does not occur (H(ab) = 0) and only the coupling through the bridge (c) and the reactant (a) and product (b) diabatic states is operating (H(ac) = H(bc) ≠ 0).     

Heterogeneous reactivity of chlorine atoms with sodium chloride and synthetic sea salt particles

The uptake of chlorine atoms on sodium chloride (NaCl) and synthetic sea salt (SSS) particles was studied using a discharge flow reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients of Cl˙ on NaCl particles are γ(NaCl)(Cl) ≈ 2 × 10(-2) for size-selected particles or γ(NaCl)(Cl) ≈ 5 × 10(-2) for thin films and for SSS particles γ(SSS)(Cl) ≈ 4 × 10(-3). Heterogeneous recombination of Cl atoms to Cl(2) molecules was observed for the two solid surfaces. The study was performed over the temperature range 258 to 353 K. The temperature dependence of the uptake was observed and the heat of adsorption of Cl˙ on NaCl particles was estimated at Q(ads) = 63 kJ mol(-1) assuming an Eley-Rideal mechanism. The role of surface adsorbed water has also been shown. The atmospheric implications of these findings are discussed briefly.     

Rotation set for maps of degree 1 on the graph sigma

For a continuous map on a topological graph containing a unique loop S it is possible to define the degree and, for a map of degree 1, rotation numbers. It is known that the set of rotation numbers of points in S is a compact interval and for every rational r in this interval there exists a periodic point of rotation number r. The whole rotation set (i.e., the set of all rotation numbers) may not be connected and it is not known in general whether it is closed.     

Determination of the enantiomeric excess of α-hydroxy acids

This work describes a convenient and accurate method for optical purity determination of α-hydroxy acids. Their derivatization with commercially available valine methyl ester affords diastereoisomers easily separable by HPLC using achiral C₁₈ columns.     

Stability of Semi-Discrete Shock Profiles by Means of an Evans Function in Infinite Dimensions

Semi-discrete shock profiles are traveling wave solutions of hyperbolic systems of conservation laws under discretization in space. The existence of semi-discrete shocks has been investigated in earlier papers. Here the spectral stability of those nonlinear waves is addressed, and formulated in terms of a variational delay differential operator. Constructing a generalized Evans function, in infinite dimensions, it is shown how to derive stability criteria. Some examples are given when the criterion is fully explicit, e.g., for extreme Lax shocks. Additionally, connection is made with the alternative approach proposed by Chow, Mallet-Paret, and Shen (Journal of Differential Equations 1998), regarding the stability of traveling waves in general Lattice Dynamical Systems.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Lead zirconate titanate (PZT) films deposited by a hydrothermal method

We present a hydrothermal method to deposit PZT films. It uses a single-step hydrothermal process and user-friendly precursors. Pb(Zr_0.52,Ti_0.48)O₃ polycrystalline films, 5–10 μm thick, were produced in an aqueous basic solution (4M KOH) on a titanium substrate in a closed autoclave at 180 °C. A time-depending study to see the evolution of the films' formation is described. Some actuation testing is also presented.