Preparation of Au/Ag multilayers via layer-by-layer self-assembly in spherical polyelectrolyte brushes and their catalytic activity

Spherical polyelectrolyte brushes(SPBs) consisting of polystyrene(PS) core and poly(2-aminoethyl methacrylate hydrochloride)(PAEMH) shell were prepared by photo-emulsion polymerization. Au nanoparticles(Au-NPs) with controlled size and size distribution were synthesized in situ using SPBs as nanoreactors. Via layer-by-layer deposition technique on the surface of SPBs, nano-composite particles with Au/Ag-NPs bilayer and Au/Ag/Au-NPs trilayer were prepared. The structures of the as-prepared Au/Ag multilayer SPBs were characterized by UV-Vis spectroscopy, TEM, ICP-AES and DLS. The charge reversal of the nano-composite particles observed by zeta potential confirmed the success of layer-by-layer assembly. The Au/Ag-NPs bilayer nano-composite particles showed high catalytic efficiency with an apparent activation energy of about 41.2 k J/mol in the reduction reaction of 4-nitrophenol to 4-aminophenol in the existence of sodium borohydride monitored. The catalytic activity of Au/Ag-NPs multilayer SPBs close to that of Au-NPs SPBs and much higher than that of Ag-NPs SPBs reveals its potential applications in cost-effective catalysts with high-performance.     

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.     

Interaction of dissolved proteins with spherical polyelectrolyte brushes

We consider the adsorption of bovine serum albumin (BSA) on spherical polyelectrolyte brushes (SPB). The SPB consist of a solid polystyrene core of 100nm diameter onto which linear polyelectrolyte chains (poly(acrylic acid), (PAA)) are grafted. The adsorption of BSA is studied at a pH of 6.1 at different concentrations of added salt and buffer (MES). We observe strong adsorption of BSA onto the SPB despite the effect that the particles as well as the dissolved BSA are charged negatively. The adsorption of BSA is strongest at low salt concentration and decreases drastically with increasing amounts of added salt. The adsorbed protein can be washed out again by raising the ionic strength. The various driving forces for the adsorption are discussed. It is demonstrated that the main driving force is located in the electrostatic interaction of the protein with the brush layer of the particles. All data show that the SPB present a new class of carrier particles whose interaction with proteins can be tuned in a well-defined manner.     

Counterion distributions and effective interactions of spherical polyelectrolyte brushes

We consider the electrosteric repulsion of colloidal particles whose surface carries a dense layer of long polyelectrolyte chains (spherical polyelectrolyte brushes). The theory of electrosteric repulsion of star polyelectrolytes developed recently is augmented to include particles with a finite core radius. It is shown that most of the counterions are confined within the brush layer. The strong osmotic pressure thus created within the brush layer dominates the repulsive interaction between two such particles. Because of this the pair interaction potential between spherical polyelectrolyte brushes can be given in terms of an analytic expression. The theoretical predictions are compared with available experimental data and semi-quantitative agreement between the two is found.     

Rheology of dilute and semidilute suspensions of spherical polyelectrolyte brushes

The viscosity, η, of dilute aqueous suspensions of spherical polyelectrolyte brushes is discussed. The spherical polyelectrolyte brushes consist of a solid polystyrene core of 100-nm diameter onto which linear poly(acrylic acid) chains are grafted. The relative viscosity, η/η_S (η_S is the viscosity of the solvent) of these suspensions is studied as a function of shear rate in the presence of different amounts of added salt. A marked dependence on shear rate is found, in particular when going to higher concentrations. Extrapolation to zero shear rate leads to the relative zero-shear viscosity, η₀/η_S, which can be described in terms of an effective volume fraction, φ_eff, for all salt concentrations under consideration. Moreover, the hydrodynamic radii derived from φ_eff coincide with data obtained by dynamic light scattering in the infinitely dilute regime. Data taken at higher concentrations point to a shrinking of the brush layer owing to mutual interaction.     

Electrostatic binding of oppositely charged surfactants to spherical polyelectrolyte brushes

We use Brownian dynamics (BD) simulations to investigate the formation and structural characteristics of the complex between a spherical polyelectrolyte brush (SPB) and oppositely charged surfactants. Increasing the amount of added surfactants leads to a collapsed conformation of the SPB and the number of adsorbed surfactants exhibits a linear dependence. Nevertheless, the surfactant uptake into the SPB does not increase with further addition of surfactants. It is found that the surfactant length has a strong influence on the SPB conformation and the adsorption properties of surfactant. Upon changing the surfactant length from 3 to 11, the SPB undergoes a swelling-deswelling-reswelling conformational transition. The brush deswelling is due to the increase in the surfactant uptake. The increasing size of adsorbed aggregates is a main reason for reswelling of the SPB. A non-linear relationship between the brush thickness and the grafting density is observed. Especially at intermediate grafting densities, increasing the number of grafted chains has a weak effect on the brush thickness. We also find that a completely collapsed brush conformation occurs at high surfactant/SPB charge ratios or large surfactant lengths, while the brush layer is in a partly collapsed or extended state at an intermediate charge ratio and surfactant length.     

Temperature-induced unfolding of ribonuclease A embedded in spherical polyelectrolyte brushes

We use Fourier Transform infrared spectroscopy (FT-IR) spectroscopy to study the thermal unfolding and refolding behavior of ribonuclease (RNase A) adsorbed to spherical polyelectrolyte brushes (SPB). The SPB consist of a solid poly(styrene) core of ca. 100 nm diameter onto which long chains of poly(styrene sulfonic acid), PSS have been densely attached. The particles bearing the adsorbed protein are dispersed in aqueous buffer solution at a pH close to the isoelectric point (9.6) of the protein. The secondary structure of the protein was analyzed by FT-IR spectroscopy and compared to the structure of the native protein before adsorption. The unfolding of the free RNase A in solution was found to be fully reversible with an unfolding temperature of 65 degrees C, in accordance to previous studies. However, after adsorption to the SPB, the unfolding temperature of the protein molecule is lowered by 10 degrees C and the Van't Hoff enthalpy of the unfolding process is significantly reduced. Moreover the unfolding of the adsorbed protein is irreversible. The phenomenon may be explained by an increase in binding sites due to unfolding of the globular structure. Protein adsorption to a spherical polyelectrolyte brush.     

Analysis of spherical polyelectrolyte brushes by small angle X-ray scattering

Spherical polyelectrolyte brushes （SPBs） with PS core and poly（acrylic acid） （PAA） brushes were prepared and analyzed by SAXS in this article. A radial electron density profile of SPB was brought up, which fits well with the SAXS result and shows a core-shell structure. The effect of pH on SPB form was represented by SAXS and it proves that the chains of SPB will stretch in response to increased pH owning to the increased electrostatic repulsion. SPBs immobilized with magnetic nanoparticles or bovine serum albumin （BSA） were prepared and analyzed by SAXS as well. SAXS could characterize the changes of electron density inside brushes of SPBs due to the immobilization of magnetic nanoparticles or BSA. This provides significant supports for further application of immobilized metal nanoparticles or proteins.     

Enrichment and distribution of counterions in spherical polyelectrolyte brushes probed by SAXS

The spatial correlation of counterions [Li⁺, Na⁺, Rb⁺, Cs⁺, NH₄⁺, (CH₃)₄N⁺] with spherical polyelectrolyte brushes (SPBs), which consist of a PS core and chemically grafted PSS chains, was comprehensively studied through a combination of SAXS, DLS, and Zeta potential. Results show that the SAXS intensity profiles of the brush appears to be “insensitive” to the concentration of Na⁺. By contrast, introducing salt ions with a density lower than sodium [NH₄⁺, (CH₃)₄N⁺ and Li⁺] into the brush layer will cause a decrease in the scattering intensity while introducing those with a higher density than sodium (Rb⁺ and Cs⁺) will cause the opposite result. As verified by the combined results of SAXS, DLS, and Zeta potential, the collapse of the brush and the enrichment of the counterions in the brush layer occur simultaneously. It was further demonstrated that the concentration of counterions enriched in the innermost layer of the brush shell can be enhanced up to hundreds of times compared with the bulk concentration, and the counterion distribution in SPB shell follows a radial attenuation distribution. SAXS is confirmed to be powerful in probing the enrichment and distribution of counterions within SPB. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 738–747     

High catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes

We present a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes that act as carriers. The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which long chains of poly(2-methylpropenoyloxyethyl) trimethylammonium chloride are grafted. These positively charged chains form a dense layer of polyelectrolytes on the surface of the core particles ("spherical polyelectrolyte brush") that tightly binds divalent PtCl6-(2) ions. The reduction of these ions within the brush layer leads to nearly monodisperse nanoparticles of metallic platinum. The average size of the particles is approximately 2 nm. The composite particles exhibit excellent colloidal stability. The catalytic activity is investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4 in the presence of the nanoparticles. The kinetic data could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. In all cases, a delay time t0 has been observed, after which the reactions start. This time is shorter when the catalyst has already been used. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles.     

Surface potential of spherical polyelectrolyte brushes in the presence of trivalent counterions

We consider the ζ-potential and the effective charge of spherical polyelectrolyte brushes (SPBs) in aqueous solution in the presence of trivalent europium ions. The SPB consists of a polystyrene core of ca. 250 nm diameter onto which long chains of the strong polyelectrolyte poly(styrene sulfonate) are grafted (contour length: 82 nm). At low concentration of EuCl₃ the chains are stretched to nearly full length. If the concentration of the trivalent ions is raised, the surface layer of the polyelectrolyte chains collapses. The ζ-potential of the SPB is calculated from the electrophoretic mobilities measured at different concentrations of EuCl₃. At the collapse, ζ decreases by the partial neutralization of the charges by the trivalent ions. The experimental ζ-potential thus obtained agrees with the theoretical surface potential Ψ_theo calculated for the effective shear plane by a variational free energy model of the SPB.     

Selective uptake of different proteins by annealed and quenched cationic spherical polyelectrolyte brushes

The selective uptake of bovine serum albumin (BSA) and β-glucosidase (β-G) by annealed and quenched cationic spherical polyelectrolyte brushes (SPB) was systematically studied by combining turbidimetric titration, dynamic light scattering and small angle X-ray scattering (SAXS). These two kinds of SPB consist of a same polystyrene core and a dense shell of poly (2-aminoethyl methacrylate hydrochloride) (PAEMH) and poly [2-(methacryloyloxy) ethyl] trimethylammonium chloride (PMAETA), respectively. Results reveal that the adsorption/desorption of proteins on SPB can be easily controlled by changing external conditions (pH and ionic strength). For a particular annealed or quenched SPB, there is a significant difference of the interaction pH regions between the brush and the two proteins, and this difference can be tuned by ionic strength. At low ionic strength, quenched brushes were more suitable for selective adsorption of BSA and β-G, while annealed brushes performed better at high ionic strength. SAXS analysis demonstrated that volume exclusion effect played a remarkable role in protein uptake by both SPB, and larger proteins were more likely to be adsorbed on the outer layer of the brush. The unique core-shell structure and controllable chain types make SPB an excellent candidate in selective adsorption/separation of proteins of different sizes.     

Adhesion of spherical polyelectrolyte brushes on mica: an in situ AFM investigation

We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.     

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).     

Distribution of magnetic nanoparticles in spherical polyelectrolyte brushes as observed by small-angle X-ray scattering

Magnetic nanoparticles (MNPs) with a size of about 2 nm are prepared in nanoreactors of spherical polyelectrolyte brushes (SPBs) consisting of a solid polystyrene (PS) core and a shell of linear poly(acrylic acid) (PAA) chains densely grafted onto the core by one end. The synthesized MNP are strongly adhered to PAA chains due to the intense interaction of chemical coordination with the carboxyl groups. The generation of MNP in SPB layer is legibly revealed by small-angle X-ray scattering (SAXS) due to the significant increase in electron density. The radial distribution of MNP in SPB is built by fitting SAXS data. Most of MNP are found to locate nearby the surface of PS core. Compared to dynamic light scattering and transmission electron microscopy, SAXS can observe the generation and distribution of MNP in SPB as well as the changes upon changing pH and salt concentration in real time. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1681–1688     

High elongation of polyelectrolyte chains in the osmotic limit of spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy

We investigate the conformation of long polyelectrolyte chains attached to colloidal latex particles by cryogenic transmission electron microscopy (cryo-TEM). The dense grafting of the polyelectrolyte chains ("polyelectrolyte brush") leads to a confinement of the counterions and a concomitantly high osmotic pressure within the polyelectrolyte layer attached to the core particles. Cryo-TEM has provided first model-independent direct proof for the strong stretching of the polyelectrolyte chains by direct visualization. If salt is added, cryo-TEM clearly shows how chains collapse because of the strong screening of the electrostatic interaction. Moreover, the analysis of interacting particles by cryo-TEM shows that the polyelectrolyte chains retract at close contact. Hence, we demonstrate how cryo-TEM can be used to analyze directly the spatial structure of polyelectrolyte brushes in situ.     

Formation of surface-attached microstructured polyelectrolyte brushes

Surface-attached micropattemed polyelectrolyte brushes on planar solid surfaces are generated using free radical polymerization photo-initiated by self-assembled initiator monolayers. It is shown that the formed patterns can be either negative or positive with different patterning processes.     

Controlled growth of triblock polyelectrolyte brushes

We have achieved a significant breakthrough in the synthesis of polyelectrolyte brushes of controlled thickness and density, which has been demonstrated by the synthesis of triblock copolymer brushes composed of cationic, neutral, and anionic segments.