Stability and structure of polyelectrolyte multilayers deposited from salt free solutions

Molecular-dynamics (MD) simulation results show that polyelectrolyte multilayers deposited from salt free solutions on charged planar surfaces are thermodynamically stable structures that form spontaneously regardless of the method of deposition. The simulation also shows that the polyelectrolyte multilayers are "fuzzy" in nature and molecules in one layer interpenetrate other layers. The influence of chain length, surface charge, and polymer charge is also investigated. Layer thickness was found to be independent of chain length. The ratio of surface to chain charge was found to influence the thickness of the first layer and the amount of polymer absorbed in the first few layers. The thickness of the subsequent layers was found to be independent of the charge ratio.     

Granular structure of self-assembled PAA/PAH and PSS/PAH nascent films imaged in situ by LC-AFM

Liquid cell atomic force microscopy (LC-AFM) is used to image self-assembled polyelectrolyte films eliminating any drying effects on the film structure. Weak/weak and strong/weak polyelectrolyte films are formed by the alternated deposition of poly(acrylic acid) [PAA]/poly(allylamine hydrochloride) [PAH], and poly(sodium 4-styrene sulfonate) [PSS]/PAH, respectively, forming a granular surface structure. Number and area of grains (GN, GA) are used to characterize the surface of these films during their build up process. We show that hydrophilic PAA increases GA and decreases GN, while these parameters follow an opposite behavior with PSS. In both cases, GA and GN always have a simple inverse relationship, and then grain surface coverage (GS=GNGA) is nearly constant and independent of polyelectrolyte nature and the substrates used here, but also in the published data as well. The drying of the weak/weak film was also imaged after natural and forced solvent evaporation, and the surface structure is strongly affected, although the GS values keep roughly the same value found for wet films. The set of these results indicates that GS may be considered as a constant parameter during the build-up for the self-nascent assembled polyelectrolytes. The granular structure is still maintained after glucose oxidase adsorption on these films with comparable GS values.     

Influence of solvent quality on the growth of polyelectrolyte multilayers

The effect of solvent conditions on the growth of polyelectrolyte (PE) multilayer films comprising poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) on planar substrates was investigated by means of surface plasmon resonance spectroscopy (SPRS), quartz crystal microbalance (QCM), and atomic force microscopy techniques. The solvent quality was varied by the addition of ethanol to the PE solutions used for deposition of the layers, thus tuning the relative strength of electrostatic and secondary intermolecular and intramolecular interactions. Experiments were performed with PE solutions both without added electrolyte and containing 0.5 M NaCl. Decreasing the solvent quality (i.e., increasing the amount of ethanol in the adsorption solution) resulted in a marked increase of both the multilayer film thickness and mass loading, as determined from the SPRS spectra and QCM frequency shifts, respectively. With the solution composition approaching the precipitation point, thick PAH/PSS films were formed due to the screening of the electrostatic intra- and interchain repulsions and enhanced hydrophobic interactions between the polyelectrolyte chains. However, the films formed from water/ethanol mixtures remained stable upon subsequent exposure to water or salt-containing solutions: no significant film desorption occurred after up to 24 h of exposure to water or 0.5 M NaCl solutions. In addition, the effect of postdeposition exposure to water/ethanol mixtures was investigated for PE multilayers assembled from aqueous solutions. In this case, the optical thickness of the films was determined during exposure to water/ethanol mixtures, and instead of swelling, the polyelectrolyte films collapse to the surface as a result of the unfavorable segment-solvent interactions.     

Influence of polyelectrolyte and salt on the structure of dispersions containing charged colloidal particles

An integral equation theory has been used as the basis for studying the structure of dispersions containing charged colloidal particles: globular protein molecules with a nonzero dipole moment, a polyelectrolyte and a low-molecular salt. It is demonstrated that there is an effective attraction between charged colloidal particles, which increases in the presence of charged polymer chains. The influence of the length of polyelectrolyte chains and of salt concentration on the partial structure factor of colloidal particles was studied.     

Adsorption of IgG on/in a PAH/PSS multilayer film: Layer structure and cell response

The binding of immunogloblulins (IgG) (mouse monoclonal recognizing IFNγ) on precoated polystyrene or silica surfaces by the layer-by-layer technique has been investigated with QCM-D and DPI. The aim of the work was to increase the sensitivity of the conventional enzyme-linked immunosorbent spot (ELISpot) assay. The polyelectrolytes used to build the multilayers were poly(allylamine hydrochloride) (PAH)/poly(sodium 4-styrenesulfonate) (PSS) alternately adsorbed from 150mM NaCl. The multilayer build up is linear and the internal structure of the PAH/PSS multilayer is compact and rigid as observed by low relative water content (20-25%) and high layer refractive index (n～1.5) after the formation of five bilayers. Incorporation of IgG within the PAH/PSS multilayer did not give rise to overcharging and did not affect the linear build up. ELISpot test on PAH/PSS multilayer modified polystyrene wells showed that the cytokine response was significantly smaller than on the regular PVDF backed polystyrene wells. This may be due to the compact and rigid nature of the PAH/PSS multilayer, which does not allow formation of the kind of three dimensional support needed to achieve bioactive IgG binding to the surface. Immunological tests of the polyelectrolyte multilayers in the absence of IgG showed that PSS terminated PAH/PSS multilayer did not induce any cytokine response whereas PAH terminated did, which suggests that PSS totally covers the surface from the cells point of view.     

Cytochrome c/polyelectrolyte multilayers investigated by E-QCM-D: effect of temperature on the assembly structure

Protein multilayers, consisting of cytochrome c (cyt c) and poly(aniline sulfonic acid) (PASA), are investigated by electrochemical quartz crystal microbalance with dissipation monitoring (E-QCM-D). This technique reveals that a four-bilayer assembly has rather rigid properties. A thickness of 16.3 ± 0.8 nm is calculated with the Sauerbrey equation and is found to be in good agreement with a viscoelastic model. The electroactive amount of cyt c is estimated by the deposited mass under the assumption of 50% coupled water. Temperature-induced stabilization of the multilayer assembly has been investigated in the temperature range between 30 and 45 °C. The treatment results in a loss of material and a contraction of the film. The electroactive amount of cyt c also decreases during heating and remains constant after the cooling period. The contraction of the film is accompanied by the enhanced stability of the assembly. In addition, it is found that cyt c and PASA can be assembled at higher temperatures, resulting in the formation of multilayer systems with less dissipation.     

Influence of assembly pH on compression and Ag nanoparticle synthesis of polyelectrolyte multilayers

Influence of assembly pH on compression and Ag nanoparticle synthesis of polyelectrolyte multilayers was studied using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid, 1:1SS:MA) sodium salt (PSSMA 1:1) as the building blocks. The thickest multilayers turned out at pH 4. A homogeneous compression by a silicone rubber stamp increased significantly the water contact angle to a same value which was independent on the original assembly pH anymore. The multilayers assembled at pH 4 could be maximumly compressed to a ratio of 70% by a silicone rubber stamp with linear patterns, which was considerably larger than those assembled at other pHs (the compression ratio ~50%). The Ag nanoparticles were then synthesized inside the multilayers either flat compressed or not. The results showed that the compression reduced significantly the amount of Ag nanoparticles for the multilayers assembled at pH 2 and pH 4. The particle amount was also decreased significantly when the multilayers were assembled at higher pH, pH 6, for example, regardless of the compression. Substantial alteration of the multilayers in terms of the surface morphology, thickness and refractive index was found during the reduction of Ag(+) containing multilayers by NaBH(4) solution.     

Hydrothermal-induced structure transformation of polyelectrolyte multilayers: from nanotubes to capsules

The assembled polyelectrolyte nanotubes composed of poly(styrenesulfonate) and poly(allylamine hydrochloride) multilayers by using the layer-by-layer assembly combined with the porous template method can be transformed into capsules by a high-temperature treatment. Scanning electron microscopy and confocal laser scanning microscopy images revealed the whole transition process. The structure transformation of polyelectrolyte multilayers after annealing can be initiated by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged polyelectrolyte groups. The transition process from tubes to capsules is supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes.     

Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers

This study addresses the effect of ionic strength and type of ions on the structure and water content of polyelectrolyte multilayers. Polyelectrolyte multilayers of poly(sodium-4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared at different NaF, NaCl and NaBr concentrations have been investigated by neutron reflectometry against vacuum, H(2)O and D(2)O. Both thickness and water content of the multilayers increase with increasing ionic strength and increasing ion size. Two types of water were identified, "void water" which fills the voids of the multilayers and does not contribute to swelling but to a change in scattering length density and "swelling water" which directly contributes to swelling of the multilayers. The amount of void water decreases with increasing salt concentration and anion radius while the amount of swelling water increases with salt concentration and anion radius. This is interpreted as a denser structure in the dry state and larger ability to swell in water (sponge) for multilayers prepared from high ionic strengths and/or salt solution of large anions. No exchange of hydration water or replacement of H by D was detected even after eight hours incubation time in water of opposing isotopic composition.     

Effect of temperature on the buildup of polyelectrolyte multilayers

The effect of temperature on the buildup of polyelectrolyte multilayers consisting of poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium) (PDADMA), and poly(allylamine) (PAH) was studied by using a quartz crystal microbalance. The increase of temperature in the deposition process was shown to have a considerable effect on the rate of the layer-by-layer buildup. The effect of temperature on the PDADMA/PSS deposition was found to be stronger than on the PAH/PSS deposition. The increasing temperature was found to extend the exponential buildup regime in all of the studied systems. A buildup model was created to simulate the buildup and to explain the effect of temperature. The model is based on the assumption that each deposition step leads to a quasi-equilibrium between the concentration of the polymer repeating unit in solution and the composition of the layer. According to the model, the layer-by-layer buildup is inherently exponential, becoming linear whenever diffusion is not fast enough to carry the polymer within the entire thickness of the film. This buildup model is discussed jointly with the earlier published three-zone model of the polyelectrolyte multilayers. The rate of the buildup is characterized by growth exponent beta. The temperature dependence of the growth exponent is discussed in connection with the thermodynamic parameters of the deposition.     

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.     

Salt-regulated attraction and repulsion of spherical polyelectrolyte brushes towards polyelectrolyte multilayers

Adsorption of colloidal particles presents an interesting alternative to the modification of surfaces using covalent coupling or physisorption of molecules. However, to tailor the properties of these materials full control over the effective particle-substrate interactions is required. We present a systematic investigation of the adsorption of spherical polyelectrolyte brushes (SPB) onto polyelectrolyte multilayers (PEM). A brush layer grafted from colloidal particles allows the incorporation of various functional moieties as well as the precise adjustment of their adsorption behaviour. In the presence of oppositely charged surfaces the amount of adsorbed SPB monotonically increases with the ionic strength, whereas equally charged substrates efficiently prevent colloidal attachment below a threshold salt concentration. We found that the transition from the osmotic to the salted brush regime at approximately 100 mM coincided with a complete loss of substrate selectivity. In this regime of high ionic strength, attractive secondary interactions become dominant over electrosteric repulsion. Due to the soft polyelectrolyte corona a surface coverage exceeding the theoretical jamming limit could be realized. Both the adsorption kinetics and the resulting thin film morphologies are discussed. Our study opens avenues for the production of two-dimensional arrays and three-dimensional multilayered structures of SPB particles.     

Two orientations and photochromism of polyelectrolyte/bacteriorhodopsin alternative deposition multilayers

A series of organized multilayers have been formed by the alternative adsorption of positively charged poly(dimethyldiallylammonium chloride) (PDAC) and purple membrane (PM) fragments in suspensions at pH = 4—11. Both UV-vis spectrophotometry and quartz crystal micro-balance (QCM) technique were used to monitor the deposition process of PDAC/bacteriorhodopsin (bR) multilayers, suggesting that PM fragments and PDAC are deposited alternatively on the substrate uniformly. Upon illumination, all these multilayers generate photovoltages with defined signs. The negative sign of photovoltage accompanying the formation of M-state at pH <7 indicates that the extracellular side of PM fragments is directed toward the substrate; and the positive sign at pH≥7 indicates that the cytoplasmic side of PM fragments is directed toward the substrate. In addition, the long-lived multiple M-state has been observed in all multilayer films. Moreover, M-state at high pH, which shows the longer lifetime than that at low pH, de     

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.     

Electrochemical characterization of polyelectrolyte/gold nanoparticle multilayers self-assembled on gold electrodes

Polyelectrolyte/gold nanoparticle multilayers composed of poly(l-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+). This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN)(6)](3-/4-) species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH(3))(6)](3+/2+) pair.     

Activity of an enzyme immobilized on polyelectrolyte multilayers

The immobilization of α-chymotrypsin on the surface of boron silicate glass microspheres is conducted via the technique of multilayer adsorption of polyelectrolytes. It is shown that the enzyme is adsorbed on both positively and negatively charged surfaces and its activity is partially preserved relative to that in solution. The activity of the enzyme depends on the number of polyectrolyte layers preliminarily adsorbed on glass microspheres and on the charge of the surface. The activity of α-chymotrypsin adsorbed on the negatively charged surface is four times higher than the activity of this enzyme adsorbed on a positively charged surface.     

Effect of precursor-layer surface charge on the layer-by-layer assembly of polyelectrolyte/nanoparticle multilayers

In this Article, we investigate the effect of a precursor layer, which is composed of four bilayers of polyethyleneimine (PEI) and poly(sodium styrene sulfonate) (PSS), on the subsequent LBL assembly of hybrid films composed of indium tin oxide (ITO) nanoparticles and PSS. A precursor polyelectrolyte layer is usually deposited to minimize interference by the substrate. It is shown here that the "effective" surface charge of the precursor layer can significantly affect the subsequent assembly behavior of [ITO/PSS](9.5) hybrid thin films. Depending on the surface charge of the precursor layer, the subsequent LbL assembly of [ITO/PSS](9.5) hybrid films can exhibit either one or two regimes. When two growth regimes are present, the first one consists of a "recovery regime", and the second is the expected "linear growth regime." The length of the "recovery regime" is dependent on how much positive charge the precursor layer possesses and how fast this surface charge can be compensated. This work reveals for the first time that changes in the surface charge of the precursor layer can have a significant effect on the subsequent LBL assembly process. The surface charge of the precursor layer was investigated using ζ-potential measurements on model silica microspheres. These experiments showed that the surface charge of the precursor layer, [PEI/PSS](4), is dependent on the pH of the solution in which it is immersed, and that it can reverse from a negatively charged surface to a positively charged one, at sufficiently low pH due to the protonation of PEI, despite having the negatively charged PSS layer as the outermost layer.     

Correlating the compliance and permeability of photo-cross-linked polyelectrolyte multilayers

Photo-cross-linkable polyelectrolyte multilayers were made from poly(allylamine) (PAH) and poly(acrylic acid) (PAA) modified with a photosensitive benzophenone. Nanoindentation, using atomic force microscopy (AFM) of these and unmodified PAH/PAA multilayers, was used to assess their mechanical properties in situ under an aqueous buffer. Under the conditions employed (and a 20 nm radius AFM tip), reliable nanoindentations that appeared to be decoupled from the properties of the silicon substrate were obtained for films greater than 150 nm in thickness. A strong difference in the apparent modulus was observed for films terminated with positive as compared to negative polyelectrolytes. Films terminated with PAA were more glassy, suggesting better charge matching of polyelectrolytes. Multilayers irradiated for up to 100 min showed a smooth, controlled increase in the modulus with little change in the water contact angle. The permeability to iodide ion, measured electrochemically, also decreased in a controlled fashion.     

The effect of support material and conditioning on wettability of PAH/PSS multilayer films

An efficient method for characterizing wetting properties of heterogeneous surfaces produced by sequential adsorption of polyelectrolytes was developed. Three types of polyelectrolytes were used: polyallylamine hydrochloride (PAH), polyethyleneimine (PEI), both of a cationic type, and polysodium 4-styrenesulfonate (PSS), of an anionic type. Multilayer films were prepared by 'layer-by-layer' (LbL) deposition technique. Natural ruby mica, glass, titanium foil and silicon wafers were used as the support material for PE films. Wetting of polyelectrolyte films was determined experimentally by contact angle measurements, using technique of direct image analysis of shape of sessile drops. Periodic oscillations in contact angle values were observed for multilayers terminated by polycation and polyanion, respectively, and the variations in contact angle values strongly depended on the conditions of adsorption and multilayer treatment after deposition. Therefore, the influence of ionic strength of polyelectrolyte solution used for deposition on wetting of multilayer films was considered and also the effect of conditioning in different environments was investigated. It is usually assumed that film properties and stability strongly depend on the first layer which is used to anchor a multilayer at the surface of support material. To investigate influence of the first layer, PAH/PSS films were compared with more complex ones having PEI as the first layer with a sequence of PSS/PAH deposited on top of it.     

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).