Formation of Polyelectrolyte Multilayers by Flexible and Semiflexible Chains

Magnetophoresis of Magnetic Pickering Emulsions Under Low Field Gradient: Macroscopic and Microscopic Motion

Monodispersed iron oxide nanoparticles (IONPs) coated with polystyrenesulfonate (PSS) and cetrimonium bromide (CTAB) have been used to stabilize magnetic Pickering emulsions (MPEs). Magnetophoresis of MPEs under the influence of a low gradient magnetic field (∇B < 100 T/m) was investigated at the macroscopic and microscopic scale. At the macroscopic scale, for the case of pH 7, the MPE achieved a magnetophoretic velocity of 70.9 μm/s under the influence of ∇B at 93.8 T/m. The magnetic separation efficiency of the MPE at 90% was achieved within 30 min for pH 3, 7, and 10. At pH 10, the colloidal stability of the MPE was the lowest compared to that for pH 3 and 7. Thus, MPE at pH 10 required the shortest time for achieving the highest separation efficiency, as the MPE experienced cooperative magnetophoresis at alkaline pH. The creaming rate of the MPE at all conditions was still lower compared to magnetophoresis and was negligible in influencing its separation kinetics profiles. At the microscopic scale, the migration pathways of the MPEs (with diameters between 2.5 and 7.5 μm) undergoing magnetophoresis at ∇B ∼ 13.0 T/m were recorded by an optical microscope. From these experiments, and taking into consideration the MPE size distribution from the dynamic light scattering (DLS) measurement, we determined the averaged microscopic magnetophoretic velocity to be 7.8 ± 5.5 μm/s. By making noncooperative magnetophoresis assumptions (with negligible interactions between the MPEs along their migration pathways), the calculated velocity of individual MPEs was 9.8 μm/s. Such a value was within the percentage error of the experimental result of 7.8 ± 5.5 μm/s. This finding allows for an easy and quick estimation of the magnetophoretic velocity of MPEs at the microscale by using macroscopic separation kinetics data.

Overcharging of polyelectrolyte complexes: an entropic phenomenon

Overcharging in complex coacervation, in which a polyelectrolyte complex coacervate (PEC) initially containing equal moles of the cationic and anionic monomers absorbs a large excess of one type of polyelectrolyte species, is predicted using a recently developed thermodynamic model describing complexation through a combination of reversible ion binding on the chains and long-range electrostatic correlations. We show that overcharging is favored roughly equally by the translational entropy of released counterions and the binding entropy of polyelectrolytes in the polyelectrolyte complex, thus helping resolve competing explanations for overcharging in the literature. We find that the extent of overcharging is non-monotonic in the concentration of added salt and increases with both strength of ion-pairing between polyions and chain hydrophobicity. The predicted extent of overcharging of the PEC is directly compared with that of multilayers made of poly(diallyldimethylammonium), PDADMA, and poly(styrene-sulfonate), PSS, overcompensated by the polycation in two different salts: KBr and NaCl. Accounting for the specificity of salt ion interactions with the polyelectrolytes, we find good qualitative agreement between theory and experiment.

Constructing self-adhesive and robust functional films on titanium resistant to mechanical damage during dental implanting

HYPOTHESIS

Osseointegration can be enhanced by introducing bioactive polyelectrolyte-multilayer films on implant surfaces. To guarantee films to function successfully in use, keeping structural integrity during implanting is necessary, which requires films with strong adhesion and cohesion to resist the mechanical damage. Catechol is considered as the origin of amazing adhesion of mussels. We hypothesize that catechol functionalization of polyelectrolytes enables film construction on implants in a non-aggressive way, and helps films resist mechanical damages during implanting.

EXPERIMENTS

With lipopolysaccharide-amine nanopolymersomes (NPs), catechol-functionalized hyaluronic acid and NPs (cHA, cNPs) as a polycation, polyanion and primer, respectively, catechol-functionalized polyelectrolyte-multilayer films (cPEMs) were constructed on substrates via Layer-by-layer self-assembly. Effects of catechol functionalization on construction, surface properties, assembly mechanisms, structural integrity, mechanical properties and cytotoxicity of cPEMs were studied.

FINDINGS

Self-adhesive cPEMs can be constructed on substrates, which grow exponentially and are driven by coordination, covalent bonding, electrostatic interactions, hydrogen bonding, etc. cPEMs with suitable catechol concentrations can resist mechanical damage to keep structural integrity in simulated clinical implantation, show stronger adhesion and cohesion than non-catechol-functionalized films in nanoscratch and nanoindentation tests, and are non-cytotoxic to MSCs. With excellent drug-loading and cytosolic-delivery capacity of NPs, cPEM is promising in improving osseointegration of implants.

Complex Aerogels Generated from Nano-Polysaccharides and Its Derivatives for Oil-Water Separation

The complex aerogel generated from nano-polysaccharides, chitin nanocrystals (ChiNC) and TEMPO-oxidized cellulose nanofibers (TCNF), and its derivative cationic guar gum (CGG) is successfully prepared via a facile freeze-drying method with glutaraldehyde (GA) as cross-linkers. The complexation of ChiNC, TCNF, and CGG is shown to be helpful in creating a porous structure in the three-dimensional aerogel, which creates within the aerogel with large pore volume and excellent compressive properties. The ChiNC/TCNF/CGG aerogel is then modified with methyltrichlorosilane (MTCS) to obtain superhydrophobicity/superoleophilicity and used for oil–water separation. The successful modification is demonstrated through FTIR, XPS, and surface wettability studies. A water contact angle of 155° on the aerogel surface and 150° on the surface of the inside part of aerogel are obtained for the MTCS-modified ChiNC/TCNF/CGG aerogel, resulting in its effective absorption of corn oil and organic solvents (toluene, n-hexane, and trichloromethane) from both beneath and at the surface of water with excellent absorption capacity (i.e., 21.9 g/g for trichloromethane). More importantly, the modified aerogel can be used to continuously separate oil from water with the assistance of a vacuum setup and maintains a high absorption capacity after being used for 10 cycles. The as-prepared superhydrophobic/superoleophilic ChiNC/TCNF/CGG aerogel can be used as a promising absorbent material for the removal of oil from aqueous media.

Interaction of Cationic, Anionic, and Nonionic Macroraft Homo- and Copolymers with Laponite Clay

The functionalization of Laponite RD platelets with different cationic, anionic, and nonionic homo- and copolymers synthesized by reversible addition-fragmentation chain transfer (RAFT) has been investigated. The effective interaction of the macromolecular RAFT agents (macroRAFTs) with the inorganic particles is known to be of crucial importance for the successful coating of minerals with polymers via RAFT-mediated emulsion polymerization to produce polymer-encapsulated inorganic particles. The macroRAFT agents synthesized in the present work contain carefully selected reinitiating R groups, which bear either ionizable tertiary amine or quaternary ammonium moieties (from 2-(dimethylamino)ethyl methacrylate, DMAEMA), negatively charged acrylic acid (AA) repeat units, or neutral polyethylene glycol (PEG) side chains, and are capable of interacting with Laponite via different adsorption mechanisms. The equilibrium adsorption of these RAFT (co)polymers was investigated by the plotting of adsorption isotherms, and either L-type or H-type curves were obtained. The hydrophobicity of the macroRAFT was shown to promote adsorption, as did the pending configuration of the PEG block. Charge repulsion between AA and the negatively charged surface of Laponite at pH 7.5, on the other hand, was prejudicial for adsorption, while the strong electrostatic interaction between the cationic DMAEMA molecules and the Laponite surface led to high-affinity-type curves.

Multifunctional Tannic Acid (TA) and Lysozyme (Lys) Films Built Layer by Layer for Potential Application on Implant Coating

A multifunctional (TA/Lys)_n film, featuring good antioxidant property, fast cell attachment at the initial stage, enhanced osteogenesis, and broad-spectrum antibacterial property, was constructed by the layer-by-layer (LBL) method. The building process was monitored by quartz crystal microbalance with dissipation (QCM-D); the physical properties, such as topography, stiffness in dry and liquid state, and conformation of Lys in the film, were thoroughly characterized. These physical properties were modulated by varying the salt concentration at which the film was constructed. The film not only allows for favorable cell attachment and proliferation of preosteoblasts Mc3t3-E1 but also provides antibacterial property against Gram-positive bacteria, S. aureus and M. lysodeikticus, and Gram-negative bacteria, E. coli. It also displays good antioxidant property, which plays a critical role on fast cell attachment at the initial stage.

Electrophoretic fabrication of an active and selective wrinkle surface on hydrogels

We developed a novel method for the fabrication of a wrinkle structure on the hydrogel surface in aqueous conditions by the electrophoretic formation of a polyion complex.

Polyelectrolyte multilayers under compression: concurrent osmotic stress and colloidal probe atomic force microscopy

Colloidal interactions have been characterised using both osmotic stress and surface forces. Here these methods are employed concurrently to measure the interaction forces of polyelectrolyte multilayers that when cross-linked form a dextran impermeable membrane. The force data, corrected for the thickness of the polyelectrolyte multilayer film, has been expressed as pressure versus separation enabling the interaction from osmotic stress measurements to be compared to the measured interaction from the colloid probe technique. The combined technique is valuable in evaluating the interaction forces associated with compression of polymer films at different rates and over a wide range of pressure and demonstrates features that are not revealed when just one technique is employed. The combination of the techniques allows both attractive forces and strongly repulsive forces to be measured and shows that the measured repulsion is greater in the force data than in the osmotic data. This is due to insufficient equilibration time in the AFM measurements, even at the slowest approach rates available, indicating that AFM force measurements between polyelectrolytes will always contain a dynamic component. That is we demonstrate that colloid probe measurements between polymer surfaces overestimate the equilibrium repulsive interaction due to the rate at which the measurement is performed.

Fouling-resistant membranes for separation of oil-in-water emulsions

Membranes coated with a polyelectrolyte multilayer can resist fouling during the separation of emulsions stabilized by ionic surfactants.

Layer-by-layer assembly of versatile nanoarchitectures with diverse dimensionality: a new perspective for rational construction of multilayer assemblies

Over the past few decades, layer-by-layer (LbL) assembly of multilayer thin films has garnered considerable interest on account of its ability to modulate nanometer control over film thickness and its extensive choice of usable materials for coating planar and particulate substrates, thus allowing for the fabrication of responsive and functional thin films for their potential applications in a myriad of fields. Herein, we provide elaborate information on the current developments of LbL assembly techniques including different properties, molecular interactions, and assembly methods associated with this promising bottom-up strategy. In particular, we highlight the principle for rational design and fabrication of a large variety of multilayer thin film systems including multi-dimensional capsules or spatially hierarchical nanostructures based on the LbL assembly technique. Moreover, we discuss how to judiciously choose the building block pairs when exerting the LbL assembly buildup which enables the engineering of multilayer thin films with tailor-made physicochemical properties. Furthermore, versatile applications of the diverse LbL-assembled nanomaterials are itemized and elucidated in light of specific technological fields. Finally, we provide a brief perspective and potential future challenges of the LbL assembly technology. It is anticipated that our current review could provide a wealth of guided information on the LbL assembly technique and furnish firm grounds for rational design of LbL assembled multilayer assemblies toward tangible applications.

Landing Dynamics of Swimming Bacteria on a Polymeric Surface: Effect of Surface Properties

Landing of bacteria for adhesion on a surface is a common phenomenon in our life. However, how surface properties are involved in this process remains largely unclear. Using digital holographic microscopy, we investigated the three-dimensional motions of flagellate Escherichia coli swimming near polymeric surfaces with different properties in aqueous solution before adhesion. We monitored the bacteria landing dynamics, which shows that the density distribution, the probability, and the orientation for collisions of the bacteria are determined by their motility but are slightly affected by the surface properties. However, surface hydrophobicity reduces the near-wall velocity of the bacteria through collisions and slightly increases the collision duration. This promotes the landing and adhesion of bacteria. By contrast, most bacteria collide with the surface using their flagella, which resist adhesion.

Layer-by-layer deposition on a heterogeneous surface: Effect of sorption kinetics on the growth of polyelectrolyte multilayers

Surface functionalization by means of controlled deposition of charged polymers or nanoparticles using the layer-by-layer (LbL) approach has been used to modify mostly engineered materials with well-defined surface chemistry and morphology. In this regard, natural and inhomogeneous interfaces have gained very little attention. Furthermore, natural substrates are susceptible to alterations by factors commonly used to control the growth of multilayers, such as pH, temperature and ionic strength. Here, we study the impact of sorption kinetics of a bilayer system (Poly(diallyldimethylammonium chloride) (PDDA) and Poly(sodium 4-styrenesulfonate) (PSS)) on a natural heterogeneous wood surface at neutral pH, without salt addition, on the multilayer buildup. To overcome analytical limitations we introduce a complementary approach based on UV reflectance spectroscopy, atomic force microscopy (AFM) and zeta potential measurements. Compared to immersion times used for ideal substrates, we found that a high surface coverage requires relatively long immersion, approximately 30min, into polyelectrolyte solutions, while a sufficient removal of polyelectrolyte excess during the washing step, requires even longer, about 100min. Based on these findings, we show that film growth can be controlled kinetically. Long immersion times provide well-defined and regular multilayers. The obtained data points to specific requirements to be considered when LbL treatments are applied to rough, porous and heterogeneous surfaces, and thereby sets a basis for a successful transfer of various surface functionalization approaches already shown on ideal surfaces.

Self-Assembly Behavior of Ultrahighly Charged Amphiphilic Polyelectrolyte on Solid Surfaces

The adsorption process of a geminized amphiphilic polyelectrolyte, comprising double elementary charges and double hydrophobic tails in each repeat unit (denoted as PAGC₈), was investigated and characterized by means of quartz crystal microbalance with dissipation (QCM-D), ellipsometry, and atomic force microscopy (AFM). By comparison, the self-assembly behaviors of a traditional polyelectrolyte without hydrophobic chains (denoted as PASC₁) and an amphiphilic polyelectrolyte with a single hydrophilic headgroup and hydrophobic tail in each repeat unit (denoted as PASC₈) at the solid/liquid interface were also investigated in parallel. A two-regime buildup was found in both amphiphilic systems of PASC₈ and PAGC₈, where the first regime was dependent on electrostatic interactions between polyelectrolytes and oppositely charged substrates, and the rearrangements of the preadsorbed chains and their aggregation behaviors on surface dominated the second regime. Furthermore, it was found that the adsorbed amount and conformation changed as a function of the charge density and bulk concentrations of the polyelectrolytes. The comparison of the adsorbed mass obtained from QCM-D and ellipsometry allowed calculating the coupling water content which reached high values and indicated a flexible aggregate conformation in the presence of PAGC₈, resulting in controlling the suspension stability even at an extremely low concentration. In order to provide an insight into the mechanism of the suspension stability of colloidal dispersions, we gave a further explanation with respect to the interactions between surfaces in the presence of the geminized polyelectrolyte.

Dynamics of the layer-by-layer assembly of a poly(acrylic acid)-lanthanide complex colloid and poly(diallyldimethyl ammonium)

Poly(acrylic acid) (PAA) and lanthanide (Ln) ions, such as Ce(3+), Eu(3+), and Tb(3+), were prepared as dispersed complex colloidal particles through three different protocols with rigorous control of the pH value and mixing ratio. The negatively charged PAA-Ln complex particles were layer-by-layer (LbL) assembled with positively charged poly(diallyldimethyl ammonium) (PDDA) to prepare a thin film. The film thickness growth is much quicker than PDDA/PAA film. Due to the incorporation of Ln(3+) ions, the film exhibits fluorescence. During LbL assembly, PDDA-PAA association based on electrostatic force and PAA-Ce association based on coordination are in competition, which leads to the LbL assembly of PDDA and PAA-Ln complex colloidal particles being a complicated dynamic process.

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 6.07-32.3 MPa 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.

Controllably local gene delivery mediated by polyelectrolyte multilayer films assembled from gene-loaded nanopolymersomes and hyaluronic acid

To explore a spatiotemporally controllable gene delivery system with high efficiency and safety, polyelectrolyte multilayer (PEM) films were constructed on titanium or quartz substrates via layer-by-layer self-assembly technique by using plasmid deoxyribonucleic acid-loaded lipopolysaccharide–amine nanopolymersomes (pNPs) as polycations and hyaluronic acid (HA) as polyanions. pNPs were chosen because they have high transfection efficiency (>95%) in mesenchymal stem cells (MSCs) and induce significant angiogenesis in zebrafish in conventional bolus transfection. The assembly process of PEM films was confirmed by analyses of quartz crystal microbalance with dissipation, X-ray photoelectron spectroscopy, infrared, contact angle, and zeta potential along with atomic force microscopy observation. Quartz crystal microbalance with dissipation analysis reveals that this film grows in an exponential mode, pNPs are the main contributor to the film mass, and the film mass can be modulated in a relatively wide range (1.0–29 μg/cm²) by adjusting the deposition layer number. Atomic force microscopy observation shows that the assembly leads to the formation of a patterned film with three-dimensional tree-like nanostructure, where the branches are composed of beaded chains (pNP beads are strung on HA molecular chains), and the incorporated pNPs keep structure intact. In vitro release experiment shows that plasmid deoxyribonucleic acid can be gradually released from films over 14 days, and the released plasmid deoxyribonucleic acid exists in a complex form. In vitro cell experiments demonstrate that PEM films can enhance the adhesion and proliferation of MSCs and efficiently transfect MSCs in situ in vitro for at least 4 days. Our results suggest that a (pNPs/HA)_n system can mediate efficient transfection in stem cells in a spatially and temporally controllable pattern, highlighting its huge potential in local gene therapy.

Stiff chains inhibit and flexible chains promote protein adsorption to polyelectrolyte multilayers

We tested the hypothesis that the level of protein adsorption onto polyelectrolyte multilayers (PEMs) is influenced by the chain stiffness of the polymers forming the multilayer. The implication being that by altering the chain stiffness, PEMs can be formed that promote or inhibit protein adsorption. Protein adsorption to PEMs consisting of flexible and semi-flexible polyelectrolytes was investigated. The flexible polyelectrolytes were poly(sodium 4-styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) and the semi-flexible polyelectrolytes were sulfated chitosan (SC) and cationic guar gum (CGG). Polyelectrolytes were used in pairs to produce four types of polyelectrolyte multilayer films. Moreover, each of these films could be terminated with either of the polyelectrolytes resulting in protein adsorption being studied on 8 systems. Protein adsorption was investigated by optical reflectometry and quartz crystal microbalance with dissipation using bovine serum albumin as the test protein. We found that when a pair of semi-flexible polyelectrolytes was used very little protein adsorption took place, irrespective of which polyelectrolyte was used to terminate the film. When the film was formed by flexible polyelectrolytes, significant protein adsorption took place and the degree of adsorption depended strongly on which polyelectrolyte was used to terminate the film. We explain these observations by considering the conformation of the polyelectrolyte in the outermost region of the film and relate this to the flexibility of the polyelectrolyte chains employed to produce the polyelectrolyte multilayer.

Layer-by-layer assembly of thin films containing exfoliated pristine graphene nanosheets and polyethyleneimine

A method for the modification of surface properties through the deposition of stabilized graphene nanosheets is described. Here, the thickness of the film is controlled through the use of the layer-by-layer technique, where the sequential adsorption of the cationic polyethyleneimine (PEI) is followed by the adsorption of anionic graphene sheets modified with layers of polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) surfactants. The graphene particles were prepared using the surfactant-assisted liquid-phase exfoliation technique, with the low residual negative charge arising from edge defects. The buildup of the multilayer assembly through electrostatic interactions was strongly influenced by the solution conditions, including pH, ionic strength, and ionic species. Thereby, not only could the thickness of the film be tailored through the choice of the number of bilayers deposited but the viscoelastic properties of the film could also be modified by changing solution conditions at which the different species were deposited. The quartz crystal microbalance was used to measure the mass of graphene and polyelectrolyte immobilized at the interface as well as to probe the energy dissipated in the adsorbed layer.

Ion specificity at a low salt concentration in water-methanol mixtures exemplified by a growth of polyelectrolyte multilayer

By use of a quartz crystal microbalance with dissipation (QCM-D), we have investigated the specific ion effect on the growth of poly(sodium 2-acrylamido-2-methylpropanesulfonate)/poly(diallyldimethylammonium chloride) multilayer at a salt concentration as low as 2.0 mM in water-methanol mixtures. QCM-D results demonstrate that specific ion effect can be observed in methanol and water-methanol mixtures though it is negligible in water. Moreover, the specific ion effect is amplified as the molar fraction of methanol (xM) increases from 0% to 75% but is weakened again with the further increase of xM from 75% to 100%. Nuclear magnetic resonance measurements reveal that the counterion-polyelectrolyte segment interactions may not account for the observed ion specificity. By extending the Collins' concept of matching water affinities to methanol and water-methanol mixtures, we suggest that the ion-solvent interactions and the resulted counterion-charged group interactions are responsible for the occurrence of the specific ion effect. The conductivity measurements indicate that water and methanol molecules may form complexes, and the change of relative proportion of complexes with the xM causes the amplification or weakening of the specific ion effect.