Investigation of the thermal stability of Ni/C multilayers by X-ray methods

Microstructural properties of Ni/C multilayers prepared by PLD (pulsed laser deposition) have been investigated after heat treatment in vacuum at temperatures in the range of 50 degrees C to 500 degrees C. X-ray diffractometry, X-ray reflectometry, fluorescence EXAFS (extended X-ray absorption fine structure) and HREM (high resolution transmission electron microscopy) have been applied to characterize samples in the initial state and after annealing. The multilayer reflectivity remained unchanged or increased at temperatures below 400 degrees C due to sharpening of the interfaces caused by the formation of alpha-nickel and nickel carbide. The reflectivity decreased at temperatures above 400 degrees C because of the fragmentation of the nickel layers. It can be shown, that both chemical and mechanical driving forces are responsible for the observed modifications of the initial specimen state.     

Investigation of the thermal stability of Ni/C multilayers by X-ray methods

Microstructural properties of Ni/C multilayers prepared by PLD (pulsed laser deposition) have been investigated after heat treatment in vacuum at temperatures in the range of 50°C to 500°C. X-ray diffractometry, X-ray reflectometry, fluorescence EXAFS (extended X-ray absorption fine structure) and HREM (high resolution transmission electron microscopy) have been applied to characterize samples in the initial state and after annealing. The multilayer reflectivity remained unchanged or increased at temperatures below 400°C due to sharpening of the interfaces caused by the formation of -nickel and nickel carbide. The reflectivity decreased at temperatures above 400°C because of the fragmentation of the nickel layers. It can be shown, that both chemical and mechanical driving forces are responsible for the observed modifications of the initial specimen state.     

Formation of polyelectrolyte multilayers on fibres: influence on wettability and fibre/fibre interaction

Polydimethyldiallylammonium chloride (PDADMAC) and polystyrene sulfonate (PSS) have been used to build-up polyelectrolyte multilayers (PEMs) on chemical soft wood fibres and on SiO2 at various electrolyte concentrations. Adsorption onto SiO2 was studied using a stagnation point adsorption reflectometer (SPAR), and the adsorbed amount of PDADMAC and PSS on the fibres was determined using nitrogen analysis and Sch?niger burning, respectively. The adsorption onto the two substrates was then compared. Paper testing showed that the tensile index (TI) increased by about 90% when 11 layers had been adsorbed, and that there was a correlation between the adsorbed amount and the increase in TI. It was also shown that the particular polymer present in the outermost layer significantly influenced the TI, and that PDADMAC produced a higher TI. A correlation between the adsorbed amount and the TI was also found. Individual fibres were partly treated with a PEM and analysed using a dynamic contact angle analyser (DCA) and environmental scanning electron microscopy (ESEM).     

In vitro stability study of organosilane self-assemble monolayers and multilayers

The stability of self-assembled monolayers (SAMs) and multilayers formed on silicon surface by amino-terminated silanes and SAMs formed by alkyl and glycidyl terminated silanes were investigated in vitro with saline solution at 37 degrees C for up to 10 days. FTIR and XPS results indicated that amino-terminated SAMs and multilayers are very unstable if the alkyl chain is short ((CH2)3), while stable if the alkyl chain is long ((CH2)11). On the other hand, alkyl-terminated SAMs are very stable regardless of the alkyl chain length, and glycidyl terminated SAM retained approximately 77% of the organosilane molecules after 10 days. Hydrogen bonding between the organosilane monomer and silicon surface and among the organosilane monomers is believed to contribute to the instability of the SAM and multilayer formed by amino-terminated silane with a short alkyl chain ((CH2)3). Therefore, the widely used (3-aminopropyl) trimethoxysilane (APTMS) SAM and multilayer may not be suitable for implantable biomedical applications.     

Formation of weak polyelectrolyte multilayers studied by spin labeling

Multilayers of alternately adsorbing poly(allylamine) (PAH) and poly(acrylic acid) (PAA) of opposite charges on silica have been studied by the spin labeling technique, as a function of pH. The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops protruding in solution with a fast motion and trains adsorbed on the solid with a hindered motion. These two parts have been evaluated for each of the polymer layers separately, and the thickness of the coatings has been described more precisely by characterizing the four contributions existing, for example, for a bilayer. Complexation is demonstrated by the loss of loops and tails belonging to the first polyelectrolyte. The overall picture emerging from the data is explained in terms of compensation of charges and entropy of confinement.     

Formation of polyelectrolyte multilayers from polysaccharides at low ionic strength

Layer-by-layer deposition of sodium carboxymethylcellulose (NaCMC) and chitosan (CHI) was used to create polyelectrolyte multilayers on ellipsoidal beta-FeOOH particles at low ionic strength. Using electro-optics, we investigated the formation of films in dependence on the polyelectrolyte charge density by controlling pH of the dipping solutions. We found out a linear growth of the CMC/CHI films when they are constructed from highly charged CHI (at pH 4.0) and weakly charged NaCMC (at pH 4.0 and 5.5). The hydrodynamic thickness of the film constructed at pH 4.0/4.0 is unusually large for a linearly growing film (ca. 220 nm after deposition of 8 bilayers), but it strongly decreases (ca. 4 times) with increasing ionization of NaCMC (at pH 5.5). In both cases, the multilayer buildup proceeded through a series of adsorption-desorption steps. This was explained by a partial loss of CHI from the film surface on exposure to the solution of longer NaCMC molecules. The irregular film growth correlated quite well with the variations in the electrical polarizability of the polymer-coated particles. This correlation enabled us to conclude that the adsorption of both polymers occurs only on the film surface, with no diffusion in and out of the film bulk during deposition of each CMC/CHI bilayer.     

Formation and structure of polyelectrolyte and nanoparticle multilayers: effect of particle characteristics

The formation and structure of multilayer films containing a cationic polyelectrolyte and anionic silica nanoparticles were studied by means of ellipsometry and atomic force microscopy. Three types of silica particles of different sizes were examined. The density and thickness of the films as well as the adsorption kinetics appear to be strongly dependent on the choice of particle; smaller particles favor the formation of smooth and dense films with a higher content of the inorganic component.     

Pyrolysis/gas chromatography/mass spectrometry in the analysis of glycated poly-L-lysine

Pyrolysis/gas chromatography/mass specfrometry was applied to the study of products arising from the interaction between glucose and poly-L -lysine. The comparison of pyrograms of control poly-L -lysine and glycated poly-L -lysine led to the identification of six different compounds, four of which were furan derivatives that can be considered as markers for advanced glycation processes.     

Site-specific functionalization on individual colloids: size control, stability, and multilayers

Individual colloidal particles are locally functionalized with nanoscale control. Here we use the particle lithography technique to mask one region of a silica or polystyrene particle (size 3.0 mum down to 170 nm), while the remaining 95% or more of the particle is coated with various sized nanocolloids. The images and data show precise and predictable control over the size of the region, with fine-tuned patch size control obtainable by changing the ionic strength of the solution. The coating on the particles remains stable even when subjected to sonication for 5 min. Both single regions and multilayer annuluses are readily formed. Particle lithography provides a general, reliable, stable, controllable, and scalable method for placing site-specific functionalizations on individual particles, opening the way to more complex particle patterning and the bottom-up assembly of more complex structures.     

Morphology and toughness of coextruded PS/PMMA multilayers

The micromechanical behaviour of multilayered tapes made of two brittle incompatible amorphous polymers PS and PMMA was studied by means of an optical and a High Voltage Electron Microscope (HVEM). Microlayers of PS and PMMA were coextruded with varying number of layers: 64, 512 and 4096 layers. Mechanical properties of the PS/PMMA tapes were also examined. An increase in layer numbers was found to lead to a decrease in layer thickness that, in turn, resulted in: a) formation of thicker and longer crazes and, therefore, increased volume of the material involved in the plastic deformation; b) a noticeable increase in strength and strain at break (i.e. of toughness) of the samples. Enhanced toughness of the multilayered tapes is accounted for by massive cooperating crazing and yielding of both PS and PMMA phases.     

NASICON类固溶体的形成和稳定

用固相反应法合成了NASICON固溶体Na~1~+~2Zr~2~-~xM~x(PO~4)~3(M=Y、Yb或In)。测定了晶胞参数随着取代量x的变化, 并从结构和结晶化学的角度进行解释。用三价离子Y^3^+、Yb^3^+、In^3^+或二价离子Mg^2^+作为结晶化学稳定剂取代Na~3Zr~2Si~2PO~1~2中的Zr^4^+离子能生成固溶体, 并把NASICON的高温三方相(空间群R3c)稳定在室温, 阻止了在150℃左右时单斜三方的相转变。总结了高温相稳定在低温的条件。     

Formation and stability of gaseous tolyl ions

Collisional activation mass spectra confirm that tolyl ions can be produced from a variety of CH₃C₆H₄Y compounds. High purity o-, m- and p-tolyl ions are prepared by chemical ionization of the corresponding fluorides (Y=F) as proposed by Harrison. In electron ionization of CH₃C₆H₄Y formation of the more stable tropylium and benzyl ionic isomers usually accompanies that of the o-, m- and p-tolyl ions. Isomerization of low energy [CH₃C₆H₄Y]⁺? to [Y–methylenecyclohexadiene]⁺? is proposed to account for most [benzyl]⁺ formation, while the tropylium ion appears to arise from the isomerization of tolyl ions formed with higher internal energies, [o-, m-, p-tolyl]⁺→ [benzyl]⁺→ [tropylium]⁺, consistent with Dewar's predictions from MINDO/3 calculations.     

Nanocomposite-based lignocellulosic fibers 1. Thermal stability of modified fibers with clay-polyelectrolyte multilayers

The layer-by-layer (LbL) assembly process of creating highly structured thin films derived from layers of polyelectrolytes and nanoparticles was adopted in this study to modify the surface of lignocellulosic fibers. Aqueous dispersions of clay nanoplatelets were created with ultrasonication and characterized with dynamic light scattering and atomic force microscopy in which confirmed the presence of individual clay nanoplatelets. Film thickness of never-dried clay and poly(diallyldimethylammonium chloride) (PDDA) multilayers was studied with a quartz crystal microbalance with dissipation monitoring (QCM-D). Using identical LbL deposition parameters, a slurry of steam-exploded wood fibers was modified by alternate adsorption of PDDA and clay with multiple rinsing steps after each adsorption cycle. Zeta potential measurements were used to characterize the fiber surface charges after each adsorption step while SEM images revealed that the LbL film masked the cellulose microfibril structure. Using a thermogravimetric analyzer, LbL modified steam-exploded wood fibers were observed to attain increased thermal stability relative to the unmodified material tested in both air and nitrogen atmospheres. Significant char for the LbL clay coated steam-exploded wood suggests the multilayer film serves as a barrier creating an insulating layer to prevent further decomposition of the material. This nanotechnology may have a positive impact on the processing of lignocellulosic fibers in thermoplastic matrices, designing of paper-based overlays for building products, and modification of cellulosic fibers for textiles.     

Curcumin-loaded layer-by-layer folic acid and casein coated carboxymethyl cellulose/casein nanogels for treatment of skin cancer

Targeted drug delivery systems using natural polysaccharide/protein biopolymer for tumor cells are an attractive platform for enriching the therapeutic effects and reducing the side effects of the drug. Carboxymethyl cellulose (CMC) and casein (CA) nanogels (NGs) loaded with curcumin (CUR) were prepared by self-assembly method and fabricated with folic acid (FA) and casein using layer-by-layer (LbL) technique for skin cancer drug delivery. The prepared samples were characterized by techniques like zeta potential, FTIR, XRD, TGA and Cryo-SEM. Both the swelling and in vitro drug release was performed in acidic pH (4.5 and 6.8) and physiological pH 7.4. Hemolysis assays demonstrated that the drug carriers are hemocompatible. Confocal microscope studies indicate facilitated uptake of 2-FA/CA/CUR@CMC-CA NGs in MEL-39 melanoma cancer cell line, which in turn result in a higher potential for apoptosis. Compared to pure CUR and CUR@CMC-CA NGs, the 2-FA/CA/CUR@CMC-CA NGs has lower IC50 value and superior cytotoxicity in MEL-39 cells because of folate-receptor mediated endocytosis evaluated by the cellular viability quantification using MTT assay and optical microscope images. Finally from in vitro skin permeation experiments, 2-FA/CA/CUR@CMC-CA NGs showed 3.47 ± 0.03 to 4.15 ± 0.25 μg/ml CUR concentrations at the stratum corneum, epidermal and dermal layers. Overall, our results put forth 2-FA/CA/CUR@CMC-CA NGs as an aspiring candidate to achieve enhanced anticancer effects against melanoma skin cancer.     

Formation and dielectric properties of polyelectrolyte multilayers studied by a silicon-on-insulator based thin film resistor

The formation of polyelectrolyte multilayers (PEMs) is investigated using a silicon-on-insulator based thin film resistor which is sensitive to variations of the surface potential. The buildup of the PEMs at the silicon oxide surface of the device can be observed in real time as defined potential shifts. The influence of polymer charge density is studied using the strong polyanion poly(styrene sulfonate), PSS, combined with the statistical copolymer poly(diallyl-dimethyl-ammoniumchloride-stat-N-methyl-N-vinylacetamide), P(DADMAC-stat-NMVA), at various degrees of charge (DC). The multilayer formation stops after a few deposition steps for a DC below 75%. We show that the threshold of surface charge compensation corresponds to the threshold of multilayer formation. However, no reversion of the preceding surface charge was observed. Screening of polyelectrolyte charges by mobile ions within the polymer film leads to a decrease of the potential shifts with the number of layers deposited. This decrease is much slower for PEMs consisting of P(DADMAC-stat-NMVA) and PSS as compared to PEMs consisting of poly(allylamine-hydrochloride), PAH, and PSS. From this, significant differences in the dielectric constants of the polyelectrolyte films and in the concentration of mobile ions within the films can be derived.     

Formation and stability of phytate complexes in solution

1,2,3,4,5,6 hexakis (di-hydrogen phosphate) myo-inositol, best known as phytic acid, is a very important molecule from a biological, environmental and technological point of view. For a thorough understanding of phytate properties and the mechanisms involving this ligand, a careful study of its acid–base behavior and of the formation and stability of its complexes in solution is necessary. Unfortunately, regarding the thermodynamic data on phytate complexes in solution, some are lacking, while some others exhibit large discrepancies between different authors. This motivated a detailed evaluation of the literature on this topic, aimed at identifying the most accurate data on phytate coordination chemistry in solution. This review presents the results of this, reporting and analyzing the most significant thermodynamic parameters published for both phytate protonation and complex formation with several metal and organometal cations, as well as polyammonium ligands.     

Formation and stability of oxocarbenium ions from glycosides

Structural, protecting group and leaving group effects in the formation of oxocarbenium intermediates were studied in the gas phase. It is found that significant stabilization of oxocarbenium cations is achieved by protecting groups that interact with the cationic center via neighboring group participation despite the electron-withdrawing character of these moieties. On the other hand, ethereal protecting groups do not facilitate the formation of oxocarbenium intermediates. The experimental findings are supported by DFT calculations that show the following order of stabilization by the group adjacent to the cationic center: RCO > SiR(3) > R, where R is an alkyl group. This indicates that the SN1-like mechanism that is commonly proposed for this reaction is not always valid. Moderate leaving group effect is also detected in a series of thioaryl glucopyranosides.     

Film stability during postassembly morphological changes in polyelectrolyte multilayers due to acid and base exposure

The mechanism of the transition from a continuous morphology to a porous morphology within polyelectrolyte multilayers (PEMs) of linear poly(ethylene imine) (LPEI) and poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) and PAA assembled by the layer-by-layer (LbL) technique is examined. These morphological changes were created by both acidic and basic postassembly treatments. Basic postassembly treatment is shown to create different types of porosity than acidic postassembly treatment. The morphological variation from the introduction of porosity to the collapse of these porous structures and the dissolution of films under postassembly treatments was observed by AFM, optical microscopy, quartz crystal microbalance (QCM), and SEM. These morphological transitions which are a result of structural rearrangement of weak polyelectrolytes due to pH changes are closely related to the neutralization of the polycations and the ionization of polyanions. Results obtained from FTIR spectroscopy and QCM confirm that polyelectrolytes are being selectively or partially released from the polyelectrolyte multilayers thin films (PEMs) in response to the pH treatment as a function of exposure time. In conclusion, here new information is presented about the structural reorganization found in a number of weak polyelectrolyte systems. This information will be useful in designing functional materials based on polyelectrolytes.     

Polyelectrolyte/magnetite nanoparticle multilayers: preparation and structure characterization

Polyelectrolyte composite planar films containing a different number of iron oxide (Fe3O4) nanoparticle layers have been prepared using the layer-by-layer adsorption technique. The nanocomposite assemblies were characterized by ellipsometry, UV-vis spectroscopy, and AFM. Linear growth of the multilayer thickness with the increase of the layer number, N, up to 12 reflects an extensive character of this parameter in this range. A more complicated behavior of the refractive index is caused by changes in the multilayer structure, especially for the thicker nanocomposites. A quantitative analysis of the nanocomposite structure is provided comparing a classical and a modified effective medium approach taking into account the influence of light absorption by the Fe3O4 nanoparticles on the complex refractive index of the nanocomposite and contributions of all components to film thickness. Dominant influence of co-adsorbed water on their properties was found to be another interesting peculiarity of the nanocomposite film. This effect, as well as possible film property modulation by light, is discussed.