Polysaccharide-based polyelectrolyte multilayers

In recent years, the layer-by-layer technique has grown in various fields. One of the emerging trends of bio-applications is the use of polysaccharides as main film components, which stems from their intrinsic physical, chemical and biological properties. These allow the simple formation, by self-assembly, of new kinds of mimics of extra-cellular matrices from plant and animal tissues. These assemblies, which possess specific properties arising from their hydration and internal composition, can indeed contain additional functionalities obtained by chemical modification of the biopolymers or film post-processing. They can be molded into different forms (films, membranes, and capsules).     

Recent developments in the properties and applications of polyelectrolyte multilayers

Complexity and innovation in the application of layer-by-layer adsorbed polyelectrolyte systems continues to grow. In this review, we will recap recent findings on the application of multilayers at the biological interface, and as thin films for controlling wetting properties. We also review recent experiments on determining the mechanical properties of polyelectrolyte multilayers.     

Metal-ion responsive redox polyelectrolyte multilayers

We present polyelectrolyte multilayer modified electrodes exhibiting novel chemically responsive redox behaviour due to the combination of both redox and metal-ion-ligand functionalities on the same sites.     

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.     

Conductivity spectra of polyphosphazene-based polyelectrolyte multilayers

Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ+) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ-). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be attributed to long-range ionic transport, are on the order of sigmadc = 10-10-10-7 S.cm-1 and strongly depend on relative humidity. For PAZ+/PAZ- multilayers sigmadc is consistently larger by one decade as compared to PSS/PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(sigmadc) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity.     

Micropatterning neuronal cells on polyelectrolyte multilayers

This paper describes an approach to adhere retinal cells on micropatterned polyelectrolyte multilayer (PEM) lines adsorbed on poly(dimethylsiloxane) (PDMS) surfaces using microfluidic networks. PEMs were patterned on flat, oxidized PDMS surfaces by sequentially flowing polyions through a microchannel network that was placed in contact with the PDMS surface. Polyethyleneimine (PEI) and poly(allylamine hydrochloride) (PAH) were the polyions used as the top layer cellular adhesion material. The microfluidic network was lifted off after the patterning was completed and retinal cells were seeded on the PEM/PDMS surfaces. The traditional practice of using blocking agents to prevent the adhesion of cells on unpatterned areas was avoided by allowing the PDMS surface to return to its uncharged state after the patterning was completed. The adhesion of rat retinal cells on the patterned PEMs was observed 5 h after seeding. Cell viability and morphology on the patterned PEMs were assayed. These materials proved to be nontoxic to the cells used in this study regardless of the number of stacked PEM layers. Phalloidin staining of the cytoskeleton revealed no apparent morphological differences in retinal cells compared with those plated on polystyrene or the larger regions of PEI and PAH; however, cells were relatively more elongated when cultured on the PEM lines. Cell-to-cell communication between cells on adjacent PEM lines was observed as interconnecting tubes containing actin that were a few hundred nanometers in diameter and up to 55 microm in length. This approach provides a simple, fast, and inexpensive method of patterning cells onto micrometer-scale features.     

Stimulation and binding properties of polyelectrolyte multilayers verified by ATR‐FTIR spectroscopy

We report on the deposition and properties of multilayers composed of reactive polymers on planar surfaces. As reactive polymers the poly cations poly(ethylene‐imine), poly(L‐lysine) (PLL), poly(allylamime) (PAA) and the polyanions poly(acrylate) (PAC), poly(vinylsulfate), poly(maleate‐co‐olefines) were used. ATR‐FTIR Spectroscopy was adopted to study deposition, binding and stimulation properties of polymer multilayers. The binding of charged species of different molecular size such as rhodanide anions and sodium oleate from solution was examined, whereby binding was found to be dependent on the charge of the outermost layer. For these two analytes a selectivity parameter Q, defined as the ratio between the adsorbed amount obtained at the negatively charged and that at the positively charged surface, respectively, was determined. Furthermore, swelling experiments on multilayer assemblies of PLL and PAC exposed to mixtures of ethanol/water (10–70% EtOH content) were carried out. Our experiments gave evidence, that the PLL layers showed a more significant increase in density than the PAC layers. The conformation of PLL incorporated into multilayers could be changed by pH variation.     

Lateral mobility of polyelectrolyte chains in multilayers

In this work, the lateral mobility of polyelectrolyte multilayers was investigated by means of the fluorescence recovery after photobleaching (FRAP) technique, with special attention to the effect of relevant parameters during and after preparation. Different polyelectrolytes with respect to charge density, stiffness, and hydrophilicity were compared. From the experimental results emerged that the density of charged sites along the polymer is the most important parameter controlling the formation of polymer complexes. At higher charge density, more complexes are formed, and the diffusion coefficient decreases. It was observed that the intrinsic backbone stiffness reduces the interpenetration of polyelectrolyte layers and the formation of complexes promoting the lateral mobility. In addition, the lateral mobility increases with increasing ionic strength and with decreasing hydration shell of the added anion in the polyelectrolyte solution. The effect of heating or annealing in electrolyte solution after preparation was also investigated along with the embedding of the probing layer at controlled distances to the multilayer surface.     

Surface viscoelastic properties of floating polyelectrolyte multilayers films: a capillary wave study

A capillary wave technique was used to study the viscoelastic properties of floating polyelectrolyte multilayers of (PSS/PAH)(n) at the air-water interface. Oppositely charged polyelectrolyte layers were adsorbed onto two different Langmuir monolayers, either the lipid dimethyldioctadecylammonium bromide (DODAB) or the block copolymer poly(styrene-b-sodium acrylate) (PS-b-PAA). The results allow to propose a schematic representation of the multilayers in three zones: Zone I as a precursor, representing the adhesion between the Langmuir monolayer and the bulk polyelectrolyte multilayer. Zone II forms a bulk or core zone of the multilayer. Zone III as an outer zone in direct contact with the aqueous phase. The results show an increase of the elasticity after the formation of four polyelectrolyte layers accompanied by an apparent negative viscosity. This behaviour was interpreted as a translation of elasticity dominance from zone I to zone II. The Young modulus of seven layers was in the same order of magnitude as observed for planar polyelectrolyte multilayer films.     

Self-assembly of S-layer-enveloped cytochrome c polyelectrolyte multilayers

We report a study of the electrostatic layer-by-layer self-assembly of electroactive polyelectrolyte multilayers incorporating the redox protein cytochrome c (cyt c) combined with recrystallization of the bacterial cell wall surface layer from Bacillus sphaericus CCM 2177 SbpA (S-layer). The polyelectrolyte multilayer assembly was prepared on flat gold electrodes with a nanometer-scale roughness that allowed monitoring of the film formation throughout all the assembly stages by atomic force microscopy measurements in liquid with respect to topography and forces. The deposition of alternating layers of sulfonated polyaniline and cyt c was carried out by adsorption from the corresponding solutions on a cyt c monolayer electrode. The electroactivity of cyt c within the assembly was confirmed by cyclic voltammetry. We showed that the surface properties of the electrode terminating layer change after each adsorption step accordingly. We also found that S-layer recrystallization on the top of the multilayer film was feasible while electroactivity of cyt c within a polyelectrolyte matrix was partially maintained. This approach offers a new strategy to design a biocompatible and permselective outer envelope of a polyelectrolyte multilayer, promising sensor applications.     

Ordered polyelectrolyte multilayers. Rules governing layering in organic binary multilayers

We study the growth and internal structure of polyelectrolyte multilayers obtained by combining three polyanions with nine polycations of the ionene family, of systematically varied chemical architecture. We find that, contrary to a generally held belief, ordered organic multilayers are by no way exceptional, provided one of the polyelectrolytes bears groups which induce structure in water, such as long hydrophobic segments or mesogenic groups. However, this condition is not sufficient, as order will or will not emerge in the multilayer depending on the specific pairing of the polyelectrolytes. The results support the notion that layering in the multilayer results from some degree of prestructuring of a water-swollen layer adsorbed during each step of deposition. These findings pave the way to new possible uses of polyelectrolyte multilayers, for example, for applications requiring preferential alignment or strong confinement of specific functional groups.     

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.     

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.     

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.     

Visco-elastic and adhesive properties of adsorbed polyelectrolyte multilayers determined in situ with QCM-D and AFM measurements

The build-up of multilayers constructed from polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) under different pH conditions was continuously monitored using the quartz crystal microbalance with dissipation. The adsorbed amount of polymer as well as the amount of coupled water was determined. Furthermore, from dissipation measurements, it was possible to determine the visco-elastic properties of the adsorbed multilayer. These properties were highly dependent on the polyelectrolyte present in the outermost layer. The multilayer was far more rigid and elastic with PAA as the outermost layer. Furthermore, a link has been established between the conformability or rigidity of a multilayer covered surface and the adhesion between such surfaces. Adhesion measurements using the atomic force microscope showed a greater pull-off force when the more viscous PAH was present in the outermost layer.     

From exponential to linear growth in polyelectrolyte multilayers

There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(L-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985) and later by Salom?ki et al. (Salom?ki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232-11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This "forbidden" zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.     

The Hofmeister anion effect and the growth of polyelectrolyte multilayers

The influence of a variety of counteranions on the properties of polyelectrolyte multilayers deposited by layer-by-layer technique is studied by using ellipsometry and AFM. We found out that in thin dry multilayers (20-90 nm) ofpoly(4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA), the thickness follows reasonably well the position of the counteranion in the Hofmeister series. The polyelectrolyte-counteranion interaction is studied by means of viscosity measurements of semidilute solutions of PDADMA in the presence of different anions. The dynamic viscosities follow the Hofmeister series of anions and correlate with the thickness of multilayers. Two parameters describing the interaction of ions with water, the Jones-Dole viscosity B coefficient and the hydration entropy, are used to explain the anion effect on the developing multilayer thickness. Reasonably smooth and monotonic functional dependence is observed between the layer thickness and these two parameters.     

Creation of functional membranes using polyelectrolyte multilayers and polymer brushes

Over the last 15 years, the layer-by-layer deposition of polyelectrolytes and the growth of polymer brushes from surfaces have become established techniques for the formation of a wide range of thin films. This article discusses the use of these techniques in creating the skin layer of nanofiltration or gas-separation membranes and in functionalizing the interior of membranes for protein adsorption or catalysis. In the case of separation membranes for nanofiltration, the minimal thickness of layer-by-layer films allows for high flux, and the wide range of available polyelectrolytes that can form these films permits the tailoring of membranes for separations such as water softening, the reduction of F (-) concentrations, and the removal of dyes from wastewater. For gas separation, polymers grown from surfaces are more attractive than layer-by-layer coatings because most polyelectrolyte films are not highly gas-selective. Cross-linked poly(ethylene glycol dimethacrylate) films grown from porous alumina exhibit CO(2)/CH(4) selectivities of around 20, and the careful selection of monomers should further improve the selectivity of similar membranes. Both layer-by-layer methods and polymer brushes can also be employed to modify the interior of membranes, and we have utilized these techniques to create catalysts, antibody arrays in membranes, and membrane absorbers for protein purification. Polymer brushes are particularly attractive because they allow the absorption of multilayers of protein to yield membranes with binding capacities as high as 150 mg protein/cm(3). Some challenges in the practical implementation of these systems, such as the economical formation of membranes using highly permeable polymeric supports, and future directions in research on membrane modification with multilayer films and polymer brushes are also discussed herein.