Polyelectrolyte Multilayer Films Research Articles

Permeabilty of silver cations through (PAH–PSS)m polyelectrolyte multilayer films to deposit silver in underlying (PAH–tannic acid)n film without external reducing agent at pH 5.0

Polyelectrolyte multilayer films made from the alternated deposition of poly(allylamine) as the polycation (PAH) and from poly(styrene-4-sulfonic acid) as the polyanion are known to be very compact and impermeable to multivalent ions and to display a strong selectivity for monovalent cations with respect to multilvalent ones. Herein, the particular case of the diffusion of silver cations at pH 5.0 through (PAH–PSS)m films of variable thickness is investigated using an original detection method of the silver cations. In this method, the (PAH–PSS)m multilayer films are deposited on films made from the alternated adsorption of PAH and tannic acid (TA). When those (PAH–TA)n films are put in contact with Ag+ cations, the TA allows for the reduction of Ag+ cations in silver particles easily detectable by means of UV–vis spectroscopy. The deposition of (PAH–PSS)m multilayers on top of (PAH–TA)6 films does not imped the deposition of silver up to m = 10 (PAH–PSS) bilayers, corresponding to a thickness of 25 nm. The high permeability of (PAH–PSS)m films is not due to a lack of homogeinity because hexacyanoferrate anions are not able to react with Fe3+ cations encapsulated in (PAH–TA)6 films when those films are capped with a (PAH–PSS)5 film known to be impermeable to Fe(CN)64− in the absence of defects in the film. It is also found that the reduction of Ag+ cations on a (PAH–TA)4 film is a slow process needing more than 1 h to be achieved.

Nanostructured multilayer polyelectrolyte films with silver nanoparticles as antibacterial coatings

Ultrathin polyelectrolyte films containing silver nanoparticles appear to be a promising material for antimicrobial coatings used in the medical area. The present work is focused on the formation of multilayer polyelectrolyte films using: polyethyleneimine (PEI) as polycation, Poly(sodium 4-styrenesulfonate) (PSS) as polyanions and negatively charged silver nanoparticles (AgNPs), which led to the polyelectrolyte-silver nanocomposite coatings. The film thickness and mass were measured by ellipsometry and quartz crystal microbalance with dissipation monitoring (QCM-D) and the structure and morphology of films were visualized using scanning electron microscopy (SEM). Systematic increase of the UV–Vis absorption confirmed formation of the consecutive layers of the film. The analysis of bacteria cell adhesion to films surface was done by the luminometry measurement. Three gram-negative bacterial strains with strong adhesive properties were used in this study: Escherichia coli, Aeromonas hydrophila, and Asaia lannenesis. It was found that nanocomposite films have antimicrobial properties, which makes them very interesting for a number of practical applications, e.g. for the prevention of microbial colonization on treated surfaces.

Evaluation of the bacterial anti-adhesive properties of polyacrylic acid, chitosan and heparin-modified medical grade Silicone rubber substrate

In this study, we aimed to develop polyelectrolyte multilayered films that could be absorbed onto a silicone rubber substrate; this composite material was then tested for its ability to inhibit bacterial adhesion. Polyacrylic acid (PAA) grafting onto the silicone rubber substrate was confirmed by attenuated total reflectance-infrared spectroscopy and the surface roughness of multilayered films was measured using an atomic force microscope and scanning electron microscopy. Immobilization of PAA, chitosan (CS) and heparin (HEP) films conferred antibacterial activity against Escherichia coli and Staphylococcus Aureus. The adhesion of both E. coli and S. aureus on PAA-CS-HEP multilayer film was reduced when compared to pristine, PAA, and PAA–CS films; this was due to effective complex formation between polyelectrolytes and cell adhesion proteins. Moreover, immobilization of HEP on the PAA-CS composite was efficiently reduced cell adhesion.

Graphene–polyelectrolyte multilayer film formation driven by hydrogen bonding

A method for preparing hydrogen bonded multilayer thin films comprised of layer pairs of surfactant stabilized graphene and an anionic polyelectrolyte is described. The films were constructed at low pH using the Layer-By-Layer (LbL) technique, where the adsorption of the cationic polyelectrolyte, polyethyleneimine (PEI) is followed by the sequential alternating adsorption of the anionic polyelectrolyte, polyacrylic acid (PAA) and anionic graphene sheets modified with Pluronic® F108, a polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) surfactant. Quartz Crystal Microbalance (QCM) measurements indicate that film formation was driven by hydrogen bonding between the carboxylic acid group of the PAA and ethylene oxide unit present in the surfactant. QCM measurements and Raman spectra showed evidence of non-linear and linear growth at low and high numbers of adsorbed layers respectively, suggesting overall superlinear film growth. Atomic Force Microscopy (AFM) Quantitative Nanomechanical Mapping (QNM) measurements of the films indicated that the reduced Young’s Modulus of the films decreased with increasing numbers of adsorbed layers, reaching a bulk value of 6.07–32.3MPa for samples with greater than 300 layers of surfactant stabilized graphene and PAA. The films were also shown to deteriorate partially with aqueous solutions at neutral and basic pH. The thin films exhibited features advantageous for use in coatings, such as pH responsiveness in addition to different mechanical properties, surface roughness, and internal structures based on the number of layers adsorbed.

Bioinspired Titanium Drug Eluting Platforms Based on a Poly-β-cyclodextrin-Chitosan Layer-by-Layer Self-Assembly Targeting Infections.

In the field of implantable titanium-based biomaterials, infections and inflammations are the most common forms of postoperative complications. The controlled local delivery of therapeutics from implants through polyelectrolyte multilayers (PEMs) has recently emerged as a versatile technique that has shown great promise in the transformation of a classical medical implant into a drug delivery system. Herein, we report the design and the elaboration of new biodegradable multidrug-eluting titanium platforms based on a polyelectrolyte multilayer bioactive coating that target infections. These systems were built up in mild conditions according to the layer-by-layer (L-b-L) assembly and incorporate two biocompatible polysaccharides held together through electrostatic interactions. A synthetic, negatively charged β-cyclodextrin-based polymer (PCD), well-known for forming stable and reversible complexes with hydrophobic therapeutic agents, was exploited as a multidrug reservoir, and chitosan (CHT), a naturally occurring, positively charged polyelectrolyte, was used as a barrier for controlling the drug delivery rate. These polyelectrolyte multilayer films were strongly attached to the titanium surface through a bioinspired polydopamine (PDA) film acting as an adhesive first layer and promoting the robust anchorage of PEMs onto the biomaterials. Prior to the multilayer film deposition, the interactions between both oppositely charged polyelectrolytes, as well the multilayer growth, were monitored by employing surface plasmon resonance (SPR). Several PEMs integrating 5, 10, and 15 bilayers were engineered using the dip coating strategy, and the polyelectrolyte surface densities were estimated by colorimetric titrations and gravimetric analyses. The morphologies of these multilayer systems, as well as their naturally occurring degradation in a physiological medium, were investigated by scanning electron microscopy (SEM), and their thicknesses were measured by means of profilometry and ellipsometry studies. Finally, the ability of the coated titanium multilayer devices to act as a drug-eluting system and to treat infections was validated with gentamicin, a relevant water-soluble antibiotic commonly used in medicine due to its broad bactericidal spectrum.

An interferon-γ-delivery system based on chitosan/poly(γ-glutamic acid) polyelectrolyte complexes modulates macrophage-derived stimulation of cancer cell invasion in vitro

Macrophages represent a large component of the tumour microenvironment and are described to establish interactions with cancer cells, playing crucial roles in several stages of cancer progression. The functional plasticity of macrophages upon stimulation from the environment makes them susceptible to the influence of cancer cells and also renders them as promising therapeutic targets.In this work, we describe a drug delivery system to modulate the phenotype of macrophages, converting them from the pro-tumour M2 phenotype to the anti-tumour M1 phenotype, based on the incorporation of a pro-inflammatory cytokine (interferon-γ) in chitosan (Ch)/poly(γ-glutamic acid) (γ-PGA) complexes. Ch is a biocompatible cationic polysaccharide extensively studied and γ-PGA is a biodegradable, hydrophilic and negatively charged poly-amino acid. These components interact electrostatically, due to opposite charges, resulting in self-assembled structures that can be designed to deliver active molecules such as drugs and proteins.Ch and γ-PGA were self-assembled into polyelectrolyte multilayer films (PEMs) of 371nm thickness, using the layer-by-layer method. Interferon-γ (IFN-γ) was incorporated within the Ch layers at 100 and 500ng/mL. Ch/γ-PGA PEMs with IFN-γ were able to modulate the phenotype of IL-10-treated macrophages at the cell cytoskeleton and cytokine profile levels, inducing an increase of IL-6 and a decrease of IL-10 production. More interestingly, the pro-invasive role of IL-10-treated macrophages was hindered, as their stimulation of gastric cancer cell invasion in vitro decreased from 4 to 2-fold, upon modulation by Ch/γ-PGA PEMs with IFN-γ. This is the first report proposing Ch/γ-PGA PEMs as a suitable strategy to incorporate and release bioactive IFN-γ with the aim of modulating macrophage phenotype, counteracting their stimulating role on gastric cancer cell invasion.

Retention of Cu(II) and Ni(II) ions by filtration through polymer-modified membranes

Organic polyethersulfone (PES) membranes were chemically modified by polyelectrolyte multilayer films of different nature. Amongst the polyelectrolyte assemblies used, poly(allylamine hydrochloride)/poly(sodium-4-styrene sulfonate) (PAH/PSS) film was found to be the most efficient system to remove metallic cations from contaminated aqueous solutions through ultrafiltration experiments. The growth of the PAH/PSS film on PES substrates was studied in situ using quartz crystal microbalance experiments. Total organic carbon analysis was also performed to demonstrate that the chemical modification of PES membranes by PAH and PSS polyelectrolytes was effective and to show the absence of polyelectrolyte desorption from the membranes when the modified membranes were immersed in aqueous solutions. The filtration parameters (immersion time of the membrane, pH of the metal solutions) affecting the removal of copper and nickel ions were also optimized. In addition, the filtration of a solution containing both copper and nickel ions was successfully achieved through PAH/PSS-modified membranes. The ageing of these latter was tested and it was shown that both the integrity of the modified-membranes and their ability to decontaminate aqueous solutions were not affected by successive filtrations.

Microcontact printing of polyelectrolyte multilayer thin films: Glass–viscous flow transition based effects and hydration methods

Abstract Micro and nano-patterned surfaces are important for many applications ranging from antibiofouling over tissue engineering to electronics. Often the incorporation of functional entities is of interest. Polymer coatings especially polyelectrolyte multilayer (PEM) films and patterns are materials offering a large variety of tuning and engineering. The PEM pattern printing quality bases not only on the surface force balance but also in the way the PEM is softened, which can be done by printing the PEM in water, using an ultrasound humidifier or by exposing the film to (hot) water vapor. In this publication it is shown, that cold water vapor from an ultrasound humidifier or direct printing in water is superior to steam evaporation onto PEM thin films as humidification method. In addition the capillary pressure of the patterns within the stamp and the glass–viscous flow transition point of the PEM thin film are the significant parameters for PEM printing. This is because the PEM can surpass the glass–viscous flow transition point due to the shear forces and be sucked into the stamp microwells (or holes) preventing a structure replication. Under high temperatures and in aqueous conditions, the PEM can be expelled from the microwells due to the osmotic pressure produced by the counter ions of PEM in glass–viscous flow state and dissolving polyelectrolyte if a PEM with counter ion based charge balance is used.

Hyaluronic Acid/Poly-l-Lysine Multilayers as Reservoirs for Storage and Release of Small Charged Molecules.

Polyelectrolyte multilayer films are nowadays very attractive for bioapplications due to their tunable properties and ability to control cellular response. Here we demonstrate that multilayers made of hyaluronic acid and poly-l-lysine act as high-capacity reservoirs for small charged molecules. Strong accumulation within the film is explained by electrostatically driven binding to free charges of polyelectrolytes. Binding and release mechanisms are discussed based on charge balance and polymer dynamics in the film. Our results show that transport of molecules through the film-solution interface limits the release rate. The multilayers might serve as an effective platform for drug delivery and tissue engineering due to high potential for drug loading and controlled release.

Incorporation of antimicrobial peptides on functionalized cotton gauzes for medical applications

A large group of low molecular weight natural compounds that exhibit antimicrobial activity has been isolated from animals and plants during the past two decades. Among them, peptides are the most widespread resulting in a new generation of antimicrobial agents with higher specific activity. In the present study we have developed a new strategy to obtain antimicrobial wound-dressings based on the incorporation of antimicrobial peptides into polyelectrolyte multilayer films built by the alternate deposition of polycation (chitosan) and polyanion (alginic acid sodium salt) over cotton gauzes. Energy dispersive X ray microanalysis technique was used to determine if antimicrobial peptides penetrated within the films. FTIR analysis was performed to assess the chemical linkages, and antimicrobial assays were performed with two strains: Staphylococcus aureus (Gram-positive bacterium) and Klebsiella pneumonia (Gram-negative bacterium). Results showed that all antimicrobial peptides used in this work have provided a higher antimicrobial effect (in the range of 4log–6log reduction) for both microorganisms, in comparison with the controls, and are non-cytotoxic to normal human dermal fibroblasts at the concentrations tested.

A FRAP-based evaluation of protein diffusion in polyelectrolyte multilayers

The evaluation of molecular diffusion by fluorescence recovery after photobleaching (FRAP) gives an option to examine properties of various soft materials aiming at material design and development. Biofunctional polyelectrolyte multilayer films can be fabricated using the layer-by-layer technique that offers easy tunability of the film chemical composition, thickness, and mechanical properties. Here the mobility of a model protein cytochrome C loaded into the multilayer film consisting of hyaluronic acid and poly-l-lysine is investigated. An approach based on analytical solution of Fick’s law is used for evaluation of the protein diffusion coefficient from FRAP measurements. The FRAP measurement procedure has been optimised to ensure a high quality of the recorded data and to improve sensitivity of the measurements. For this purpose, various settings of the confocal laser scanning microscope have been examined and changing of the pinhole size has been found the most appropriate way to reduce signal noise and improve sensitivity. This study opens up a possibility of the fundamental understanding of intermolecular interactions within the polyelectrolyte multilayers as well as applicability of FRAP for the evaluation of diffusion of various molecules.

Cationic β-cyclodextrin polymer applied to a dual cyclodextrin polyelectrolyte multilayer system

A polyelectrolyte multilayer film (PEM) based on cationic and anionic β-cyclodextrin polyelectrolytes was coated onto a textile substrate for future drug delivery purposes. We firstly synthesized a novel cationic β-cyclodextrin polymer (polyEPG-CD) by crosslinking β-cyclodextrin (βCD) with epichlorohydrin (EP) under basic conditions, in the presence of glycidyltrimetrylammonium chloride (GTMAC) as cationizing group. The influence of preparation conditions has been investigated in order to preferably obtain a water soluble fraction whose charge density and molecular weights were optimal for the layer-by-layer (LbL) deposition process. The different cationic cyclodextrin polymers obtained were characterized by FTIR, NMR, colloidal titration, conductimetry, thermogravimetric analysis and size exclusion chromatography. Besides, the counterpart polyelectrolyte was a β-cyclodextrin polymer crosslinked with citric acid, polyCTR-CD, whose synthesis and characterization have been previously reported. Finally we realized the Layer by Layer (LbL) build-up of the PEM coating onto the textile support, using the dip coating method, by alternatively soaking it in cationic polyEPG-CD and anionic polyCTR-CD solutions. This multilayer self-assembly was monitored by SEM, gravimetry and OWLS in function of both polyelectrolytes concentrations and ratios. Solutions parameters such as pH, ionic strenght were also discussed.

Effects of fluid shear stress on polyelectrolyte multilayers by neutron scattering studies.

The structure of layer-by-layer (LbL) deposited nanofilm coatings consists of alternating polyethylenimine (PEI) and polystyrenesulfonate (PSS) films deposited on a single crystal quartz substrate. LbL-deposited nanofilms were investigated by neutron reflectomery (NR) in contact with water in the static and fluid shear stress conditions. The fluid shear stress was applied through a laminar flow of the liquid parallel to the quartz/polymer interface in a custom-built solid-liquid interface cell. The scattering length density profiles obtained from NR results of these polyelectrolyte multilayers (PEM), measured under different shear conditions, showed proportional decrease of volume fraction of water hydrating the polymers. For the highest shear rate applied (ca. 6800 s(-1)) the water volume fraction decreased by approximately 7%. The decrease of the volume fraction of water was homogeneous through the thickness of the film. Since there were not any significant changes in the total polymer thickness, it resulted in negative osmotic pressures in the film. The PEM films were compared with the behavior of thin films of thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) deposited via spin-coating. The PEM and pNIPAM differ in their interactions with water molecules, and they showed opposite behaviors under the fluid shear stress. In both cases the polymer hydration was reversible upon the restoration of static conditions. A theoretical explanation is given to explain this difference in the effect of shear on hydration of polymeric thin films.

Modification of PEN and PET film surfaces by plasma treatment and layer-by-layer assembly of polyelectrolyte multilayer thin films

The surfaces of polyethylene naphthalate and polyethylene terephthalate films were modified by plasma treatment, layer-by-layer (LbL) assembly of polyelectrolyte multilayer thin films and subsequent deposition of SiO2 particles. Plasma treatment rendered the film surfaces hydrophilic and led to the formation of fibrillar patterns. The plasma-treated film surfaces were compatible with multilayers acquired via LbL assembly, and the outer adsorbed polyelectrolyte layer determined the charge of the multilayer surfaces. Negatively charged SiO2 particles were adsorbed to the positively charged surface of the multilayers. This process is potentially applicable to surface modifications of polymer materials by charged compounds.

Large-scale solvent driven actuation of polyelectrolyte multilayers based on modulation of dynamic secondary interactions.

Polyelectrolyte multilayers (PEMs), assembled from weak polyelectrolytes, have often been proposed for use as smart or responsive materials. However, such response to chemical stimuli has been limited to aqueous environments with variations in ionic strength or pH. In this work, a large in magnitude and reversible transition in both the swelling/shrinking and the viscoelastic behavior of branched polyethylenimine/poly(acrylic acid) multilayers was realized in response to exposure with various polar organic solvents (e.g., ethanol, dimethyl sulfoxide, and tetrahydrofuran). The swelling of the PEM decreases with an addition of organic content in the organic solvent/water mixture, and the film contracts without dissolution in pure organic solvent. This large response is due to both the change in dielectric constant of the medium surrounding the film as well as an increase in hydrophobic interactions within the film. The deswelling and shrinking behavior in organic solvent significantly enhances its elasticity, resulting in a stepwise transition from an initially liquid-like film swollen in pure water to a rigid solid in pure organic solvents. This unique and recoverable transition in the swelling/shrinking behaviors and the rheological performances of weak polyelectrolyte multilayer film in organic solvents is much larger than changes due to ionic strength or pH, and it enables large scale actuation of a freestanding PEM. The current study opens a critical pathway toward the development of smart artificial materials.

Electrocatalytic amplification of nanoparticle collisions at electrodes modified with polyelectrolyte multilayer films.

We report electrochemical catalytic amplification of individual collisions between ∼57 nm diameter Pt nanoparticles (Pt NPs) and 12.5 μm diameter Au ultramicroelectrodes modified with passivating, electrostatically assembled polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer deposition method. Two key findings are reported. First, despite the thicknesses of the insulating PEM films, which range up to 5 nm, electrons are able to tunnel from the Pt NPs to the electrode resulting in electrocatalytic N2H4 oxidation at the PEM film-solution interface. These single-particle measurements are in accord with prior reports showing that the electrochemical activity of passive PEM films can be reactivated by adsorption of metallic NPs. Second, it is possible to control the frequency of the collisions by manipulating the net electrostatic charge present on the outer surface of the PEM thin film. These results, which demonstrate that chemistry can be used to control electrocatalytic amplification, set the stage for future sensing applications.

Imprinting of metal receptors into multilayer polyelectrolyte films: fabrication and applications in marine antifouling.

Polymeric films constructed using the layer-by-layer (LbL) fabrication process were employed as a platform for metal ion immobilization and applied as a marine antifouling coating. The novel Cu2+ ion imprinting process described is based on the use of metal ion templates and LbL multilayer covalent cross-linking. Custom synthesized, peptide mimicking polycations composed of histidine grafted poly(allylamine) (PAH) to bind metal ions, and methyl ester containing polyanions for convenient cross-linking were used in the fabrication process. Two methods of LbL film formation have been investigated using alternate polyelectrolyte deposition namely non-imprinted LbLA, and imprinted LbLB. Both LbL films were cross linked at mild temperature to yield covalent bridging of the layers for improved stability in a sea water environment. A comparative study of the non-imprinted LbLA films and imprinted LbLB films for Cu2+ ion binding capacity, leaching rate and stability of the films was performed. The results reveal that the imprinted films possess enhanced affinity to retain metal ions due to the preorganization of imidazole bearing histidine receptors. As a result the binding capacity of the films for Cu2+ could be improved by seven fold. Antifouling properties of the resulting materials in a marine environment have been demonstrated against the settlement of barnacle larvae, indicating that controlled release of Cu ions was achieved.

Highly sensitive and colorimetric detection of hydrogen sulphide by in situ formation of Ag2S@Ag nanoparticles in polyelectrolyte multilayer film

Ag ion reacted with H2S gas in polyelectrolyte multilayer film to form Ag2S nanoparticles that catalyze the formation of Ag NPs.

Micro-contact printing of PEM thin films: effect of line tension and surface energies

A theory and method for calculating printing resolution limits for microcontact printing of a condensed polyelectrolyte multilayer thin film, based on surface energies and line tension is presented.

Naturally inspired polyelectrolyte multilayer composite films synthesised through layer-by-layer assembly and chemically infiltrated with CaCO3.

Naturally occurring composite structures like antler bone and nacre have a highly ordered structural design at the nanoscale. Nature's successful architecture has attracted widespread interest in mimicking such systems artificially, the goal being to design tough composite materials with adaptable mechanical properties. Here we report results on synthesis pathways towards fabricating such materials, including a chemical infiltration route where calcium carbonate particles nucleate and grow inside polyelectrolyte multilayers assembled via a layer-by-layer route. SEM analysis demonstrates a considerable change in the morphology of thin films upon chemical infiltration. The depth of mineralisation within the multilayer is confirmed by TOF-SIMS studies of both mineralised and non-mineralised thin films. TGA was used to calculate the overall content of CaCO3 within multilayer films. Infiltrated multilayers have shown up to 60% w/w of calcium carbonate which is comparable to structures like bones. X-ray diffraction to characterise the crystallographic structure and micromechanical testing involving nano-indentation have also been conducted. The Young's modulus of mineralised multilayer thin films significantly increased up to 10 GPa after infiltration in comparison to the non-mineralised multilayers with a modulus of only 3.8 GPa, while the increase in hardness is almost 50-fold. Thus, the synthetic composites can be compared with natural biomineralised tissues like nacre, ultimately replicating the natural strength of biomimetic materials on the nanoscale.