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At least 775 records · Page 43Linked to original sources
PubMed2005

Urea photosynthesis inside polyelectrolyte capsules: effect of confined media.

The influence of the restricted volume of poly(styrene sulfonate)/poly(allylamine hydrochloride) capsules of different size (2.2, 4.2, and 8.1 microm) on the TiO2-assisted photosynthesis of urea from inorganic precursors (CO2 and NO(3-)) in aqueous solution was demonstrated. Poly(vinyl alcohol) was employed as electron donor to facilitate the photosynthetic process. Decreasing the size of the confined microvolume of polyelectrolyte capsules accelerates the NO(3-) photoreduction, which is a limiting stage of the urea photosynthesis and, correspondingly, increases the efficiency of urea production. The highest yield of urea photosynthesis (37%) was achieved for Cu-modified TiO2 nanoparticles encapsulated inside 2.2 microm poly(styrene sulfonate)/poly(allylamine hydrochloride) capsules.

Journal Article↗
PubMed2005

Thermally stable holographic surface relief gratings and switchable optical anisotropy in films of an azobenzene-containing polyelectrolyte.

In a search for effective polymer film material for holographic surface patterning, commercially available azobenzene polyelectrolyte has been employed. Films of good optical quality in a wide range of thickness were produced. Optical dichroism up to 0.19 was induced upon irradiation with linearly polarized light. Surface relief gratings with amplitudes up to 630 nm and diffraction efficiency of 37% were inscribed by holographic exposure to the light of 488 nm. Due to the ionic nature of the material, the relief was stable at least up to the temperature of decomposition (ca. 200 degrees C) but could be erased and inscribed again by light.

Journal Article↗
PubMed2005

Surface analysis of erodible multilayered polyelectrolyte films: nanometer-scale structure and erosion profiles.

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) coupled with ellipsometry have been used to characterize the microscale and nanoscale structures of erodible multilayered films fabricated from degradable polyamine 1 and either sodium poly(styrene sulfonate) (SPS) or plasmid DNA. Striking differences were found in the topography, structures, and erosion profiles of these two materials upon incubation in PBS buffer at 37 degrees C. For films fabricated from SPS, AFM data are consistent with an erosion process that occurs uniformly without the generation of holes or pits over large, micrometer-scale areas. By contrast, films fabricated from plasmid DNA undergo structural rearrangements to present surface-bound particles ranging in size from 50 to 400 nm. Additional characterization of these particulate structures by SEM suggested that they are interpenetrated with or fused to underlying polyelectrolyte layers on the silicon surface, providing a potential mechanism to manipulate the adhesive forces with which these particles are bound to the surface. The erosion profile observed for polymer 1/SPS films suggests that it may be possible to design assemblies that release two film components with well-defined release kinetics. In the context of gene delivery, the presentation of condensed DNA as nanoparticles at these surfaces may be advantageous with respect to stimulating the internalization and processing of DNA by cells. A quantitative understanding of the factors influencing the fabrication, structure, and erosion profiles of these materials will be useful for the design of multilayered assemblies for specific applications in which controlled film erosion or the release of therapeutic materials is desired.

Kinetics↗
PubMed2005

Oil-in-water emulsions stabilized by highly charged polyelectrolyte-grafted silica nanoparticles.

Fully sulfonated poly(styrenesulfonate) brushes were grown from the surface of colloidal silica particles and used to prepare stable trichloroethylene-in-water and heptane-in-water Pickering emulsions. These particles were highly charged and colloidally stable in water but could not be dispersed in trichloroethylene or heptane. Both two-phase (emulsion plus neat water) and three-phase (emulsion separating neat oil and water phases) systems were observed, with water-continuous emulsion phases in all cases. Emulsion phases containing as much as 83% (v/v) oil were stable for over six months. Poly(styrenesulfonate)-grafted particles were very efficient emulsifiers; stable emulsion phases were prepared when using as little as 0.04 wt% particles. The emulsifying effectiveness of the poly(styrenesulfonate)-grafted silica particles can be attributed to the hydrophobicity of the vinylic polymer backbone that makes this highly charged polyelectrolyte unusually surface active at the oil/water interface.

Journal Article↗
PubMed2005

Electrochemical impedance spectroscopy of polyelectrolyte multilayer modified gold electrodes: influence of supporting electrolyte and temperature.

Electrochemical impedance spectroscopy and cyclic voltammetry are employed to characterize poly(styrenesulfonate)/poly(allylamine hydrochloride) multilayers assembled onto cysteamine-modified gold surfaces. The influence of the supporting electrolyte and temperature on the impedance response is studied because of both its practical interest and the need to test further the capillary membrane model recently developed by Barreira et al. [J. Phys. Chem. B 2004, 108, 17973]. The results obtained are interpreted quite satisfactorily in terms of this model, thus providing additional support to its usefulness for the description of ionic transport through polyelectrolyte multilayers. It is observed that the nature of the supporting electrolyte affects the film resistance and the electrode coverage. The temperature dependence of the diffusion coefficient is shown to follow the Arrhenius law, and the activation energy is estimated as 61 kJ/mol. Experiments with a large number of layers are also included to show that the impedance response of the multilayer then resembles that of a homogeneous membrane.

Biosensing Techniques↗
PubMed2005

Interaction and adhesion properties of polyelectrolyte multilayers.

The growth, morphology, and interaction/adhesion properties of supported poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) and DNA/PAH multilayers were investigated by means of surface plasmon resonance spectroscopy, atomic force microscope (AFM) imaging, and AFM-related force measurements. Multilayers were assembled on a prelayer of poly(ethylenimine) (PEI) both with and without drying. SPR results showed a linear growth of the assembly in the case of PSS/PAH multilayers and nonlinear growth for DNA/PAH multilayers. Measurements of forces acting between a bare glass sphere and a multilayer-coated surface indicated repulsive or attractive forces, depending on surface charge, which suggests that, on approach, electrostatic forces dominate. On separation, we observed large pull-off forces in the case of positively charged multilayers and weak pull-off forces in the case negatively charged multilayers. Multiple adhesions and plateau regions observed on separation were interpreted in terms of a bridging of multiple polymer chains between the glass particle and the multilayer and a stretching of the polyelectrolyte loops. The dependence of the pull-off force on the number of deposited layers shows regular oscillations.

Journal Article↗
PubMed2005

Formation of microcapsules from polyelectrolyte and covalent interactions.

A new approach combining electrostatic and covalent bonds was established for the formation of resistant capsules with long-term stability under physiological conditions. Three kinds of interactions were generated in the same membrane: (1) electrostatic bonds between alginate and poly-L-lysine (PLL), (2) covalent bonds (amides) between propylene-glycol-alginate (PGA) and PLL, and (3) covalent bonds (amides) between BSA and PGA. Down-scaling of the capsules size (< or =1 mm diameter) with a jet break-up technology was achieved by modifying the rheological properties of the polymer solution. Viscosity of the PGA solution was reduced by 95% with four successive pH stabilizations (pH 7), while filtration (0.2 microm) and sterilization was possible. Covalent bond formation was initiated by addition of NaOH (pH 11) using a transacylation reaction. Kinetics of the chemical reaction (pH 11) were simulated by two mathematical models and adapted in order to preserve immobilization of animal cells. It was demonstrated that diffusion of NaOH in the absence of BSA resulted in gelation of 94% of the bead and death of 94% of the cells after 10 s reaction. By addition of BSA only 46% of the cells were killed within the same reaction time (10 s). Mechanical resistance of this new type of capsule could be increased 5-fold over the standard polyelectrolytic system (PLL-alginate). Encapsulated CHO cells were successfully cultivated for 1 month in a repetitive batch mode, with the mechanical resistance of the capsules decreasing by only 10% during this period. The combination of a synthetic and natural protein resulted in enhanced stability toward culture medium and proteolytic enzymes (250%).

Journal Article↗
PubMed2006

Structure and mechanism of the deposition of multilayers of polyelectrolytes and nanoparticles.

MD simulation of the layer-by-layer assembly of polyelectrolytes (PEs) and nanoparticles (NPs) revealed that the assembly process is electrostatically driven with alternating charge reversal and an overcompensation mechanism. Layers were observed to grow in the lateral direction as well as in a direction normal to the surface. Weakly adsorbed PE molecules were observed to desorb from the flat and NP surfaces. Those molecules are attracted by suspended NPs in solution. PE molecules do not only pull NPs toward the surface but bridge NPs both in solution and on the surface, forming agglomerates and islands. The first double layer differs in structure from the second double layer as a result of strong adsorption of the PE molecules to the rigid surface.

Journal Article↗
PubMed2005

Polyelectrolyte modified solid surfaces: the consequences for ionic and mixed ionic/nonionic surfactant adsorption.

This paper describes how the cationic polyelectrolyte, polyDMDAAC (poly(dimethyl diallylammonium chloride)), is used to manipulate the adsorption of the anionic surfactant SDS and the mixed ionic/nonionic surfactant mixture of SDS (sodium dodecyl sulfate)/C(12)E(6) (monododecyl hexaethylene glycol) onto the surface of hydrophilic silica. The deposition of a thin robust polymer layer from a dilute polymer/surfactant solution promotes SDS adsorption and substantially modifies the adsorption of SDS/C(12)E(6) mixtures in favor of a surface relatively rich in SDS compared to the solution composition. Different deposition conditions for the polyDMDAAC layer are discussed. In particular, at higher solution polymer concentrations and in the presence of 1 M NaCl, a thicker polymer layer is deposited and the reversibility of the surfactant adsorption is significantly altered.

Journal Article↗
PubMed2005

Mechanically responsive films of variable hydrophobicity made of polyelectrolyte multilayers.

Mechanically responsive surfaces that allow to switch reversibly from a hydrophobic to a hydrophilic substrate are reported. The surfaces are constituted of polyelectrolyte multilayers deposited on modified charged silicone sheets. n bilayers of poly(allylamine)-Nafion (PAH-Naf) and m bilayers of poly(allylamine)-poly(acrylic acid) (PAH-PAA) composed the multilayers. A (PAH-Naf)(n) film possesses a water contact angle of around 105 degrees, whereas the contact angle of a (PAH-Naf)(4)-(PAH-PAA)(m) multilayer is around 50 degrees. When such a film with m < 5 and terminated by PAA is stretched out, its water contact angle increases up to around 100 degrees. Successive elongation/retraction cycles allow the water contact angle to alternate reversibly between 100 and 57 degrees indicating the reversible mechanical responsive nature of the film.

Letter↗
PubMed2006

Differences between tethered polyelectrolyte chains on bare mica and hydrophobically modified mica.

This study investigates the structures of layers of amphiphilic diblock copolymers of poly(t-butyl styrene)-poly(styrene sulfonate) (PtBS-PSS) adsorbed on both the bare mica surface (hydrophilic) and an octadecyltriethoxysilane (OTE)-modified mica surface (hydrophobic). When the surface is rendered hydrophobic, the nonsoluble block exhibits stronger interaction with the surface and higher adsorbed masses are achieved. Interaction forces between two such adsorbed layers on both substrates were measured using the surface forces apparatus. The effect of salt concentration (Cs) and molecular weight (N) on the height of the self-assembled layers (L0) was examined in each case. The resulting scaling relationship is in good agreement with predictions of the brush model, L0 proportional to N(1.0) in the low-salt limit and L0N(-1) proportional to (Cs/sigma)(-0.32) in the salted regime, when adsorption takes place onto the hydrophobized mica surface. For adsorption on the bare mica surface, L0N(-0.7) proportional, variant Cs(-0.17) agrees with the scaling prediction of the sparse tethering model. The results suggest that, on the hydrophilic bare mica surface, the adsorbed amount is not high enough to form a brush structure and only very little intermolecular stretching of the tethered chains occurs; in contrast, the presence of the hydrophobic OTE layer increases the tethering density such that the polyelectrolyte chains adopt a brush conformation.

Journal Article↗
PubMed2006

Layer-by-layer self-assembled polyelectrolyte multilayers with embedded liposomes: immobilized submicronic reactors for mineralization.

The development of chemical reactions in nanospaces is of paramount importance for the development of active nanodevices, particularly in nanofluidics. It has been shown in a previous paper that phospholipid vesicles can be incorporated without spontaneous bilayer rupture into poly-L-glutamic acid/poly(allylamine) (PGA/PAH) multilayered polyelectrolyte films. The aim of the present study was to use such a system as an "embedded submicronic reactor" able to trigger precipitation of calcium phosphates within closed spaces through an enzymatic reaction, the enzyme also being encapsulated in the vesicle interior. To this aim, large unilamellar vesicles (LUVs) were produced containing calcium ions as active ions in the mineralization process, spermine as an activator of crystal growth, and alkaline phosphatase as a catalyst to convert phosphate esters into phosphates. After stabilization by adding a layer of poly-(D-lysine), these vesicles were embedded in a (PGA-PAH)n film. A paranitrophenyl phosphate containing solution was then put in contact with this film. It is shown by means of infrared spectroscopy in the attenuated total reflection mode that, consecutively to this contact, calcium phosphates are growing inside the embedded vesicles. By using scanning near-field fluorescence microscopy, it is demonstrated that the alkaline phosphatase enzymes are most probably located inside the vesicles after their embedding. In addition, atomic force microscopy was used to show, after chemical removal of the organic top layer of the film, that the inorganic platelets produced after the precipitation reaction are localized in volumes of similar size and shape as that of the vesicles into which the phosphate ester hydrolysis and subsequent precipitation reaction did occur.

Journal Article↗
PubMed2006

Facilitating the culture of mammalian nerve cells with polyelectrolyte multilayers.

When neuron-like cells (NLCs) derived from pluripotent embryonal carcinoma cells (P19) are cultured on bare tissue culture substrates, they require a monolayer of fibroblast cells to exhibit normal neurite outgrowth, behavior typical of neuronal cultures. However, substrate treatment with polyelectrolyte multilayers (PEMs) composed of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonic acid) (PSS) significantly improved these cultures. Cell morphology was more spread, indicative of healthy cells, and direct attachment of neuronal cell bodies to the treated surface was observed. Neuronal outgrowth across the surface was not dependent on an underlying fibroblast monolayer with the PEMs surface treatment. Additionally, the PEMs surface treatment can be used to condition various surfaces, facilitating neuronal cultures on surfaces which are natively hydrophilic (tissue culture polystyrene) or hydrophobic (poly(dimethylsiloxane), PDMS). Microfluidic networks were used to micropattern the PEMs onto PDMS, resulting in confined regions of cellular attachment and directed neuronal outgrowth. The ability of PEMs to encourage NLC attachment without supporting cells to a variety of surfaces and surface geometries greatly simplifies neuronal culture methodology and enables neuronal investigations in new environments.

Animals↗
PubMed2006

Anionic polyelectrolyte adsorption on mica mediated by multivalent cations: a solution to DNA imaging by atomic force microscopy under high ionic strengths.

Adsorption of DNA molecules on mica, a highly negatively charged surface, mediated by divalent or trivalent cations is considered. By analyzing atomic force microscope (AFM) images of DNA molecules adsorbed on mica, phase diagrams of DNA molecules interacting with a mica surface are established in terms of concentrations of monovalent salt (NaCl) and divalent (MgCl2) or multivalent (spermidine, cobalt hexamine) salts. These diagrams show two transitions between nonadsorption and adsorption. The first one arises when the concentration of multivalent counterions is larger than a limit value, which is not sensitive to the monovalent salt concentration. The second transition is due to the binding competition between monovalent and multivalent counterions. In addition, we develop a model of polyelectrolyte adsorption on like-charged surfaces with multivalent counterions. This model shows that the correlations of the multivalent counterions at the interface between DNA and mica play a critical role. Furthermore, it appears that DNA adsorption takes place when the energy gain in counterion correlations overcomes an energy barrier. This barrier is induced by the entropy loss in confining DNA in a thin adsorbed layer, the entropy loss in the interpenetration of the clouds of mica and DNA counterions, and the electrostatic repulsion between DNA and mica. The analysis of the experimental results provides an estimation of this energy barrier. We then discuss some important issues, including DNA adsorption under physiological conditions.

Adsorption↗
PubMed2006

Microcapsules made of weak polyelectrolytes: templating and stimuli-responsive properties.

Hollow microcapsules composed of the weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) are templated on silicon oxide particles using the layer-by-layer adsorption. The colloidal template is removed with a buffer system of hydrofluoric acid and ammonium fluoride. With this buffer system, the template can be dissolved in mild pH conditions, where the polymeric layers are still stable. The morphology and the thickness of the resulting capsules are investigated with atomic force microscopy. The resulting hollow capsules show pH-dependent properties. The shells are stable over a broad pH range and swell and immediately dissolve for pH values below 2.3 and above 11. If the molecular weight of the poly(methacrylic acid) is increased, the enhanced entanglement of the polymers results in a reversible swelling of the capsules at low and at high pH. The swelling degree is probed with confocal laser scanning microscopy. In addition to the pH-dependent size variations, the different ionization degree of poly(methacrylic acid) as a function of pH is used for the selective binding of calcium ions.

Binding Sites↗
PubMed2006

Amplified fluorescence quenching of a conjugated polyelectrolyte mediated by Ca2+.

The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers. This effect has been referred to as amplified quenching. In the present communication, we demonstrate that aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) induced by Ca2+ has a pronounced effect on the amplified quenching of the polymer by the dication methyl viologen (MV2+). In particular, absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of a low concentration of Ca2+ induces aggregation of the polymer chains. The range of MV2+ concentrations within which linear Stern-Volmer quenching behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear Stern-Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains.

Journal Article↗
PubMed2006

Surfactant and polyelectrolyte gel particles that swell reversibly.

Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase is a viscous liquid, gel, or precipitate. In recent years, this phenomenon has been exploited to form gel-like particles, ranging from approximately 100 to 4000 microm in diameter, whose stability depends on equilibrium phase behavior. As the sample composition is varied, these particles either remain stable (in a two-phase mixture) or dissolve over time. Here, we present the formation of reversibly swelling gel particles from mixtures of N,N,N-trimethylammonium-derivatized hydroxyethyl cellulose (JR-400) and sodium dodecyl sulfate (SDS), whose swelling is controlled by the ambient solution conditions. The effects of cross-linking density and surfactant concentration are investigated by gravimetry and confocal microscopy. The resulting particles have a core/shell morphology and undergo reversible swelling/collapse transitions which, depending on the cross-link density, can be either gradual or abrupt with changing SDS concentration.

Journal Article↗
PubMed2006

Polyelectrolyte brushes as efficient ultrathin platforms for site-selective copper electroless deposition.

Ion-exchange in surface-initiated polyelectrolyte brushes provides a versatile route to the formation of catalytically active surfaces for electroless deposition of Cu. The advantage of this procedure is the covalent anchoring of the catalyst support layer, eliminating delamination of the metal film, even when deposited onto PDMS substrates. Furthermore, by tuning the concentration of PdCl4(2-) ions in the brushes, the rate of deposition and hence the thickness of the overall film can be controlled easily.

Journal Article↗