Single-component layer-by-layer weak polyelectrolyte films and capsules: Loading and release of functional molecules

Poly(carboxylic acid) hydrogel films and hollow capsules undergo reversible size changes in response to variations in pH and/or ionic strength. The films and capsules were obtained from hydrogenbonded poly-N-vinylpyrrolidone/poly(carboxylic acid) layer-by-layer films by chemical crosslinking of the polyacid, followed by pH-induced removal of poly-N-vinylpyrrolidone. Surface-attached hydrogel films present attractive matrices for reversible pH-stimulated loading and/or controlled release of large amounts of synthetic or natural macromolecules including proteins. By varying acidity of poly(carboxylic acids), the hydrogel swelling and the corresponding values of pH for encapsulation/release of functional molecules could be tuned in a wide range from pH 5 to 10. In addition, the capsules are capable of entrapping macromolecules by “locking” the capsule wall with an electrostatically associating polycation, followed by the release of the encapsulated macromolecules at high salt concentrations. The text was submitted by the authors in English.     

Effect of dipping solution pH values on electrostatic layer-by-layer self-assembly of side-chain azo polyelectrolyte

The effect of pH value on the electrostatic layer-by-layer self-assembly and the photo-responsive behavior of Poly{2-[4-(4-ethoxyphenylazo)phenoxy]ethyl acrylate-co-acrylic acid} (PEAPE) was studied. Results show that in the studied pH value range, the lower the pH value is, the higher is the UV-vis absorbance and the larger is the thickness of the multilayer films. FTIR studies indicate that the azo polyelectrolyte exhibits a different ionization degree in solutions with different pH values. The higher absorbance and the larger thickness of the layer-by-layer films can be attributed to the low ionization degree and the shrinkage conformation of PEAPE in the solution with low pH values. FTIR analysis also confirms that the driving force for layer-by-layer self-assembly of PEAPE and PDAC is the electrostatic interaction. __________ Translated from Acta Polymerica Sinica, 2007, 5: 440–445 [译自: 高 分子学报]     

Dependence of fluorescence quenching of a poly(p-phenyleneethynylene) polyelectrolyte on the electrostatic and hydrophobic properties of the quencher

This study investigates the fluorescence quenching of a poly(p-phenyleneethynylene) (1) based polyelectrolyte by positively charged and neutral macromolecules. This work shows that the change in the fluorescence yield of 1 depends on a number of factors, including electrostatic, hydrophobic, and energy transfer interactions with the quencher and also changes in the solution conditions such as concentration and ionic strength. The fluorescence quenching is attributed to the formation of aggregates that form upon addition of different quenchers to a solution of 1 and/or the solution conditions. The extent of 1's aggregation is shown to depend on the type of interaction between the polymer and the quencher, the concentration of the polymer, and the ionic strength of the solution.     

Thermoelasticity of weak polyelectrolyte networks

The thermoelastic behavior of poly(vinyl alcohol)–poly(acrylic acid) networks was evaluated in pure water and CaCl₂ solution. The ratio f_e/f of the energy component of the force to the total force, evaluated without taking into account polymer–diluent specific interactions, ranged from ?0.75 for networks swollen in pure water to ?5.7 in 0.1M CaCl₂. However, an analysis based on Flory's theory of polyelectrolyte gels yields f_e/f constant at ?1.32 when specific interactions are accounted for. In addition, the variation of In (r²)₀ with CaCl₂ concentration is 2000 times that with water. In neither pure water nor CaCl₂ solution can specific interactions be neglected.     

Salt softening of polyelectrolyte multilayer capsules

The changes in the morphology and the mechanical properties of hollow polyelectrolyte multilayer capsules made from poly(styrenesulfonate)/poly(allylamine hydrochloride) in response to added salt were investigated. We found that capsules shrink in response to salt exposure. The effect depends strongly on the nature of the salt added and follows trends of the Hoffmeister series, with weakly hydrated cations inducing the strongest shrinking. For NaCl, we have investigated additional effects on capsule mechanical properties that are occurring above a 3 M salt concentration and we found that the morphological changes are accompanied by a pronounced softening of the capsule wall material, which we can quantify by analyzing the force response of capsules in the prebuckling regime. This shows that salts can act as plasticizers in the multilayers and induce annealing effects.     

Human serum albumin self-assembly on weak polyelectrolyte multilayer films structurally modified by pH changes

Adsorption of proteins onto film surfaces built up layer by layer from oppositely charged polyelectrolytes is a complex phenomenon, governed by electrostatic forces, hydrogen bonds, and hydrophobic interactions. The amounts of the interacting charges, however, both in polyelectrolytes and in proteins adsorbed on such films are a function of the pH of the solution. In addition, the number and the accessibility of free charges in proteins depend on the secondary structure of the protein. The subtle interplay of all these factors determines the adsorption of the proteins onto the polyelectrolyte film surfaces. We investigated the effect of these parameters for polyelectrolyte films built up from weak "protein-like" polyelectrolytes (i.e., polypeptides), poly(L-lysine) (PLL), and poly(glutamic acid) (PGA) and for the adsorption of human serum albumin (HSA) onto these films in the pH range 3.0-10.5. It was found that the buildup of the polyelectrolyte films is not a simple function of the pure charges of the individual polyelectrolytes, as estimated from their respective pKa values. The adsorption of HSA onto (PLL/PGA)n films depended strongly on the polyelectrolyte terminating the film. For PLL-terminated polyelectrolyte films, at low pH, repulsion, as expected, is limiting the adsorption of HSA (having net positive charge below pH 4.6) since PLL is also positively charged here. At high pH values, an unexpected HSA uptake was found on the PGA-ending films, even when both PGA and HSA were negatively charged. It is suggested that the higher surface rugosity and the decrease of the alpha-helix content at basic pH values (making accessible certain charged groups of the protein for interactions with the polyelectrolyte film) could explain this behavior.     

Simulations of surface forces in polyelectrolyte solutions

We have simulated interactions between charged surfaces in the presence of oppositely charged polyelectrolytes by coupling perturbations in the isotension ensemble to a free energy variance minimization scheme. For polymeric systems, this method completely outperforms configurationally biased versions of grand canonical simulations. Proper diffusive equilibrium between bulk and slit has been established for polyelectrolytes with up to 60 monomers per chain. A consequence of imposing diffusive equilibrium conditions, in contrast to previous more restricted models, is the possibility of surface charge inversion; ion-ion correlation and the cooperativity of monomer adsorption drive the formation of a polyion layer close to the surface, that overcompensates the nominal surface charge. This is observed even at modest surface charge densities, and leads to a build up of a long ranged electrostatic barrier. In addition, the onset of charge inversion requires very low bulk polymer densities. Due to screening effects, this leads to a higher and more long-ranged free energy barrier at low, compared to high, bulk densities. Oscillatory forces, reminiscent of those found in simple hard sphere systems, are resolved in the high concentration regime. As a consequence of a second surface charge inversion, the system "stratifies" to form a stable polyelectrolyte layer in the central part of the slit, stabilized by the adsorbed surface layers.     

From the 2-dimensional unstable polyelectrolyte multilayer to the 3-dimensional stable dry polyelectrolyte capsules

Polystyrene-poly(acrylic acid)/poly(allylamine hydrochloride) polyelectrolyte multilayer was found to be instable and apt to reconstruct in the pure water. By depositing polystyrene-poly(acrylic acid)/poly(allylamine hydrochloride) multilayer on the polystyrene-poly(acrylic acid) hybrid CaCO(3) templates, novel polyelectrolyte capsules could be prepared after the removal of the templates. The resultant capsules could keep their three-dimensional (3D) spherical shape after being dried at room temperature, dramatically different from the conventional polyelectrolyte capsules based on nonhybrid templates by layer-by-layer procedure. The instable polyelectrolyte multilayer, hybrid templates, and assembly cycles were demonstrated to be three indispensable factors responsible for the formation of this type of 3D stable capsules. The formation mechanism was also discussed in this study.     

Behavior of temperature-sensitive PNIPAM confined in polyelectrolyte capsules.

Layer-by-layer assembled polyelectrolyte microcapsules are of great interest because they can possibly be used as microcontainers and they show interesting stimuli-responsive properties, which have been recently investigated. Here, we exploit capsules which are made temperature-sensitive by encapsulating poly(N-isopropylacrylamide) (PNIPAM). PNIPAM has a cloud point in water at about 32 degrees C, above which it collapses and is insoluble in water. Further this temperature responsiveness can be tuned by addition of various ions at various concentrations. Here, we present the encapsulation of PNIPAM inside polyelectrolyte microcapsules, and describe the dependence of the lower critical solution temperature (LCST) on the nature and the amount of different salts added. With this information, we demonstrate the ability to tune and finely control the collapse of encapsulated PNIPAM. In this light, this system could be used as a microsensor or drug- delivery system.     

Influence of weak groups on polyelectrolyte mobilities

The ionization of dissociable groups in weak polyelectrolytes does not occur in a homogenous fashion. Monomer connectivity imposes constraints on the localization of the dissociated (charged) monomers that affect the local electric potential. As a result, the mean bare charge along a weak polyelectrolyte can vary depending on the proximity to topological features (e.g. presence of crosslinks or dangling ends). Using reaction‐ensemble Monte‐Carlo simulations we calculate the dissociation inhomogeneities for a few selected PE configurations, linear, rod‐like, flexible four‐arm star, and a star with stiff arms. An ensemble preaverage is used to obtain the annealed bare charge profile for these different polymer configurations. Using molecular dynamics simulations within a Lattice‐Boltzman fluid, we investigate how the electrophoretic mobility is affected by the bare charge inhomogeneities arising from the annealed weak polyelectrolytes. Surprisingly, the mobility obtained for the situations corresponding to the predicted charge profile for annealed weak polyelectrolytes are not significantly different than the mobility obtained when all the monomers have an identical charge (under the constraint that the total polyelectrolyte bare charge is the same). This is also true for the stiff rod‐like variants where conformational changes induced from the localization of the monomer charges are negligible. In salty solutions, we find that counterions are affected by the electric potential modulations induced by the topological features. Since the counterions crowd in regions where the electric potential caused by the dissociated monomers is highest, they wash‐out the bare charge inhomogeneities and contribute to a more uniform effective backbone charge.     

Effect of shear stress on adhering polyelectrolyte capsules

A parallel plate flow chamber was implemented to study the deformation and adhesion of individual spherical hollow polyelectrolyte multilayered shells adhering to a coated surface. The device provides a well-defined laminar flow allowing the determination of the shear stress to which the capsules are being exposed up to 15 N/m(2). The results of the investigations indicate a strong dependence of the adhesion and mechanical resistance on the capsule size and wall thickness. Thin walled capsules, constituted of 8 polyelectrolyte layers (thickness congruent with 12 nm), are immediately deformed when exposed to flow while thick capsules, constituted of 16 layers (thickness congruent with 24 nm), of equal dimensions are detached from the surface for drag forces below 50 nN. It was observed that adhering capsules exposed to flow undergo an increase in their adhesion area in the direction of flow, resulting in rolling of the capsules. It was also found that the resistance of the capsules decreases after acetone treatment, indicating a weakening of the polyelectrolyte multilayer structure in the presence of this solvent.     

Enhanced dewatering of polyelectrolyte nanocomposites by hydrophobic polyelectrolytes

We demonstrate that increasing the hydrophobic environment around the charge center of a polyelectrolyte (PE) not only decreases the water content of an adsorbed PE layer but can even dewater up to ~50% of an initially hydrated substrate. The results of this work are expected to yield new stratagies to dewater PE systems and have potential applications in mineral recovery, paper manufacturing, and biomedical materials. Adsorption of a series of cationically derivatized dextran polyelectrolytes onto sulfated nanocrystalline cellulose (SNC) has been studied using quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). Synthesized samples of (N,N-dimethylamino)ethyldextran (DMAE-Dex), (N,N-diethylamino)ethyldextran (DEAE-Dex), and (N,N-diisopropylamino)ethyldextran (DIAE-Dex) had degrees of substitution (DS) ranging from 0.05 to 0.82. DMAE-Dex, DEAE-Dex, and DIAE-Dex all showed decreasing adsorption onto SNC and decreasing water content of the adsorbed film with increasing DS. Additionally, DEAE-Dex and DIAE-Dex films adsorbed onto SNC contained less water than DMAE-Dex films with the same DS. Interestingly, QCM-D results for high DS DIAE-Dex adsorbed onto SNC revealed mass loss, whereas SPR results clearly showed DIAE-Dex adsorbed. These observations were consistent with dehydration of the SNC substrate. This study indicates that the water content of the substrate could be tailored by controlling the DS and hydrophobic character of the adsorbed polyelectrolytes.     

Balance of forces in simulated bilayers

Two kinds of simulated bilayers are described, and the results are reported for lateral tension and for partial contributions of intermolecular forces to it. Data for the widest possible range of areas per surfactant head, from tunnel formation through tensionless state, transition to floppy bilayer, and its disintegration, are reported and discussed. The significance of the tensionless state is discussed.     

Encapsulation, release and applications of LbL polyelectrolyte multilayer capsules

Ever since their invention in 1998, polyelectrolyte multilayer micro- and nano-capsules have impacted various areas of biology, chemistry and physics. Here we highlight progress achieved since the millennium in the areas of encapsulation in and release from microcapsules, describe various structures including multicompartment and anisotropic constructs, and provide examples of several applications in biology. We also describe application areas such as drug delivery, intracellular trafficking, enzyme-catalyzed reactions, mechano-biology which benefited from recent developments in the area of polyelectrolyte multilayer capsules.     

Controlling ionic conductivity in lipid polyelectrolyte composite capsules by cholesterol

The effect of cholesterol on the formation and properties of bilayer lipid membranes deposited on polyelectrolyte multilayered capsules was studied. The permeability of lipid/cholesterol coated capsules for NaCl was derived from osmotic response experiments and ranged from 1.45 x 10(-8) to 2.9 x 10(-8) m.s(-1), which corresponds to a lipid layer conductivity of (0.7-1.4) x 10(-8) S.m(-1). These conductivity values were in good agreement with the value of 0.8 x 10(-8) S.m(-1) obtained by electrorotation and were by 3 orders of magnitude lower than those found earlier for lipid layers on polyelectrolyte capsules in the absence of cholesterol.     

Balance of enthalpy and entropy in depletion forces

Solutes added to solutions often dramatically impact molecular processes ranging from the suspension or precipitation of colloids to biomolecular associations and protein folding. Here we revisit the origins of the effective attractive interactions that emerge between and within macromolecules immersed in solutions containing cosolutes that are preferentially excluded from the macromolecular interfaces. Until recently, these depletion forces were considered to be entropic in nature, resulting primarily from the tendency to increase the space available to the cosolute. However, recent experimental evidence indicates the existence of additional, energetically-dominated mechanisms. In this review we follow the emerging characteristics of these different mechanisms. By compiling a set of available thermodynamic data for processes ranging from protein folding to protein–protein interactions, we show that excluded cosolutes can act through two distinct mechanisms that correlate to a large extent with their molecular properties. For many polymers at low to moderate concentrations the steric interactions and molecular crowding effects dominate, and the mechanism is entropic. To contrast, for many small excluded solutes, such as naturally occurring osmolytes, the mechanism is dominated by favorable enthalpy, whereas the entropic contribution is typically unfavorable. We review the available models for these thermodynamic mechanisms, and comment on the need for new models that would be able to explain the full range of observed depletion forces.     

Cylindrical cell model for the electrostatic free energy of polyelectrolyte complexes

Associative phase separation (complex coacervation) in a mixture of oppositely charged polyelectrolytes can lead to different types of (inter-)polyelectrolyte complexes (soluble micelles, macroscopic precipitation). In a previous report [Langmuir 2004, 20, 2785-2791], we presented a model for the electrostatic free energy change when (weakly charged) polyelectrolyte forms a homogeneous complex phase. The influence of ionization of the polymer on the electrostatic free energy of the complex was incorporated but the influence of complex density neglected. In the present effort, cylindrical cells are assumed around each polyelectrolyte chain in the complex, and on the basis of the Poisson-Boltzmann equation, the electrostatic free energy is calculated as a function of the complex density. After combination with Flory-Huggins mixing free energy terms and minimization of the total free energy, the equilibrium complex density is obtained, for a given ratio of polycations to polyanions in the complex. The analysis is used in an example calculation ofpolyelectrolyte film formation by alternatingly applying a polycation and a polyanion solution. The calculation suggests that the often observed exponential growth of a polyelectrolyte film when the polymer is weakly charged has a thermodynamic origin: the polyelectrolyte complex shifts repeatedly between two equilibrium states of different densities and compositions. However, when the polyelectrolytes are strongly charged the difference in the compositions between the two equilibrium states is very small, and exponential growth by an absorption mechanism is no longer possible.     

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.     

A facile route to synthesize silver nanoparticles in polyelectrolyte capsules

We are reporting a novel green approach to incorporate silver nanoparticles (NPs) selectively in the polyelectrolyte capsule shell for remote opening of polyelectrolyte capsules. This approach involves in situ reduction of silver nitrate to silver NPs using PEG as a reducing agent (polyol reduction method). These nanostructured capsules were prepared via layer by layer (LbL) assembly of poly(allylamine hydrochloride) (PAH) and dextran sulfate (DS) on silica template followed by the synthesis of silver NPs and subsequently the dissolution of the silica core. The size of silver nanoparticles synthesized was 60±20 nm which increased to 100±20 nm when the concentration of AgNO(3) increased from 25 mM to 50 mM. The incorporated silver NPs induced rupture and deformation of the capsules under laser irradiation. This method has advantages over other conventional methods involving chemical agents that are associated with cytotoxicity in biological applications such as drug delivery and catalysis.