Ionic Strength Effects in Polyelectrolyte Brushes: The Counterion Correction

Synthesis of Poly (2-Acrylamido-2-methylpropanesulfnoinc Salt) Modified Carbon Spheres

The paper reports a facile synthesis of novel anionic spherical polymer brushes which was based on grafting sodium 2-acrylamido-2-methylpropane-1-sulfonate from the surface of 4,4'-Azobis (4-cyanopentanoyl chloride)-modified carbon spheres. Various characterization methods involving a scanning electron microscope, energy dispersive X-ray spectroscopy, Fourier transform infrared spectrum, and thermo-gravimetric analysis were utilized to analyze the morphology, chemical composition, bonding structure, and thermal stability, respectively. The molecular weight (Mw) and polydispersity (Mw/Mn) of brushes were 616,000 g/mol and 1.72 determined by gel permeation chromatography experiments. Moreover, the dispersibility of ASPB in water and in the presence of aqueous NaCl solutions of different concentrations was investigated. Results show that the dispersibility of carbon spheres has been enhanced owing to grafted polyelectrolyte chains, while the zeta potential of the particle decreases and its brush layer shrinks upon exposure to sodium ions (Na+).

Molecular weight and dispersity affect chain conformation and pH-response in weak polyelectrolyte brushes

The pH-dependence of the conformation of annealed polyelectrolyte brushes can be tuned by varying the molecular weight distribution, as characterized via weight-average molecular weight and dispersity.

Revisiting the strong stretching theory for pH-responsive polyelectrolyte brushes: effects of consideration of excluded volume interactions and an expanded form of the mass action law

In this paper, we develop a theory to account for the effect of excluded volume (EV) interactions in the strong stretching theory (SST) based description of pH-responsive polyelectrolyte (PE) brushes. The existing studies have considered the PE brushes to be present in a θ-solvent and hence have neglected the EV interactions; however, such a consideration cannot describe the situations where the pH-responsive brushes are in a "good" solvent. Secondly, we consider a more expanded form of the mass action law, governing the pH-dependent ionization of the PE molecules, in the SST description of the PE brushes. This expanded form of the mass action law considers different values of γa³ (γ is the density of chargeable sites on the PE molecule and a is the PE Kuhn length) and therefore is an improvement over the existing SST models of PE brushes as well as other theories involving pH-responsive PE molecules that always consider γa³ = 1. Our results demonstrate that the EV effects enhance the brush height by inducing additional PE inter-segmental repulsion. Similarly, the consideration of the expanded form of the mass action law would lead to a reduced (enhanced) brush height for γa³ < 1 (γa³ > 1). We also quantify variables such as the monomer density distribution, the distribution of the ends of the PE brush, and the EDL electrostatic potential and explain their differences with respect to those obtained with no EV interactions or γa³ = 1.

Counterion-Specific Protein Adsorption on Polyelectrolyte Brushes

Protein adsorption is an important issue in biorelated fields. We have investigated the protein adsorption on the poly(ionic liquid) (PIL) brushes in the presence of different types of counterions. The protein adsorption is driven by a decrease in osmotic pressure within the brushes with an increase in entropy via the release of counterions. Our study demonstrates that counterion specificity has a significant influence on protein adsorption on the PIL brushes. There have been two different regimes for counterion-specific protein adsorption. When the released counterions cannot bind to the protein surface, the counterion-specific protein adsorption is dominated by the ion-specific counterion condensation within the PIL brushes. If the released counterions can bind to the protein surface, then counterion-specific protein adsorption is dominated by the ion-specific rebinding of released counterions on the protein surface. This work opens up a new opportunity for controlling protein adsorption on polyelectrolyte brushes.

Tunable nanoparticle arrays at charged interfaces

Structurally tunable two-dimensional (2D) arrays of nanoscale objects are important for modulating functional responses of thin films. We demonstrate that such tunable and ordered nanoparticles (NP) arrays can be assembled at charged air-water interfaces from nanoparticles coated with polyelectrolyte chains, DNA. The electrostatic attraction between the negatively charged nonhybridizing DNA-coated gold NPs and a positively charged lipid layer at the interface facilitates the formation of a 2D hexagonally closed packed (HCP) nanoparticle lattice. We observed about 4-fold change of the monolayer nanoparticle density by varying the ionic strength of the subphase. The tunable NP arrays retain their structure reasonably well when transferred to a solid support. The influence of particle's DNA corona and lipid layer composition on the salt-induced in-plane and normal structural evolution of NP arrays was studied in detail using a combination of synchrotron-based in situ surface scattering methods, grazing incidence X-ray scattering (GISAXS), and X-ray reflectivity (XRR). Comparative analysis of the interparticle distances as a function of ionic strength reveals the difference between the studied 2D nanoparticle arrays and analogous bulk polyelectrolyte star polymers systems, typically described by Daoud-Cotton model and power law scaling. The observed behavior of the 2D nanoparticle array manifests a nonuniform deformation of the nanoparticle DNA corona due to its electrostatically induced confinement at the lipid interface. The present study provides insight on the interfacial properties of the NPs coated with charged soft shells.

BIOPHYSICAL PROPERTIES OF NUCLEIC ACIDS AT SURFACES RELEVANT TO MICROARRAY PERFORMANCE

Both clinical and analytical metrics produced by microarray-based assay technology have recognized problems in reproducibility, reliability and analytical sensitivity. These issues are often attributed to poor understanding and control of nucleic acid behaviors and properties at solid-liquid interfaces. Nucleic acid hybridization, central to DNA and RNA microarray formats, depends on the properties and behaviors of single strand (ss) nucleic acids (e.g., probe oligomeric DNA) bound to surfaces. ssDNA's persistence length, radius of gyration, electrostatics, conformations on different surfaces and under various assay conditions, its chain flexibility and curvature, charging effects in ionic solutions, and fluorescent labeling all influence its physical chemistry and hybridization under assay conditions. Nucleic acid (e.g., both RNA and DNA) target interactions with immobilized ssDNA strands are highly impacted by these biophysical states. Furthermore, the kinetics, thermodynamics, and enthalpic and entropic contributions to DNA hybridization reflect global probe/target structures and interaction dynamics. Here we review several biophysical issues relevant to oligomeric nucleic acid molecular behaviors at surfaces and their influences on duplex formation that influence microarray assay performance. Correlation of biophysical aspects of single and double-stranded nucleic acids with their complexes in bulk solution is common. Such analysis at surfaces is not commonly reported, despite its importance to microarray assays. We seek to provide further insight into nucleic acid-surface challenges facing microarray diagnostic formats that have hindered their clinical adoption and compromise their research quality and value as genomics tools.

Response of adsorbed polyelectrolyte monolayers to changes in solution composition

Reflectometry and quartz crystal microbalance are used to study the response of adsorbed polyelectrolyte monolayers to solutions of variable composition. These techniques respectively yield the dry and wet masses of the adsorbed layer, and by combing these results, one obtains the water content and the thickness of the polyelectrolyte films. The systems investigated are films of adsorbed poly(allyl amine) (PAH) and poly-L-lysine (PLL) on silica and films of poly(styrene sulfonate) (PSS) on amino-functionalized silica. When such films are adsorbed from concentrated polyelectrolyte solutions containing high levels of salt, they are found to swell reversibly up to a factor of 2 when incubated in solutions of low salt. This swelling is attributed to the strengthening of repulsive electrostatic interactions between the adsorbed polyelectrolyte chains. PAH films may also swell upon decrease of pH, and collapse upon a pH increase. This transition shows a marked hysteresis and can be rationalized by the competition of electrostatic repulsions between the chains and their attraction to the surface. The presently observed swelling phenomena are caused by a collective process driven by the electrostatic repulsion between the densely adsorbed polyelectrolyte chains. Such responsive layers are only obtained by adsorption from high polyelectrolyte and salt concentrations. Layers absorbed at low polyelectrolyte and salt concentrations show only minor swelling effects, since the adsorbed polyelectrolytes layers are dilute and the adsorbed polyelectrolyte chains interact only weakly.

Small angle neutron scattering study of polyelectrolyte brushes grafted to well-defined gold nanoparticle interfaces

Small angle neutron scattering (SANS) has been used to study the conformations, and response to added salt, of a polyelectrolyte layer grafted to the interfaces of well-defined gold nanoparticles. The polyelectrolyte layer is prepared at a constant coverage by grafting thiol-functionalized polystyrene (M(w) = 53k) to gold nanoparticles of well-defined interfacial curvature (R(c) = 26.5 nm) followed by a soft-sulfonation of 38% of the segments to sodium polystyrene sulfonate (NaPSS). The SANS profiles can be fit by Fermi-Dirac distributions that are consistent with a Gaussian distribution but are better described by a parabolic distribution plus an exponential tail, particularly in the high salt regime. These distributions are consistent with the predictions and measurements for osmotic and salted brushes at interfaces of low curvature. When the concentration of added salt exceeds the concentration of counterions inside the brush, there is a salt-induced deswelling, but even at the highest salt concentration the brush remains significantly swollen due to a short-ranged excluded volume interaction. This is responsible for the observed resistance to aggregation of these comparatively high concentration polyelectrolyte stabilized gold nanoparticle dispersions even in the presence of a high concentration of added salt.

Variations in the Nanomechanical Properties of Virulent and Avirulent Listeria monocytogenes

Atomic force microscopy (AFM) was used to quantify both the nanomechanical properties of pathogenic (ATCC 51776 & EGDe) and non-pathogenic (ATCC 15313 & HCC25) Listeria monocytogenes strains and the conformational properties of their surface biopolymers. The nanomechanical properties of the various L. monocytogenes strains were quantified in terms of Young's moduli of cells. To estimate Young's moduli, the classic Hertz model of contact mechanics and a modified version of it that takes into account substrate effects were used to fit the AFM nanoindentation-force measurements collected while pushing onto the bacterial surface biopolymer brush. When compared, the classic Hertz model always predicted higher Young's moduli values of bacterial cell elasticity compared to the modified Hertz model. On average, the modified Hertz model showed that virulent strains are approximately twice as rigid (88.1 ± 14.5 KPa) as the avirulent strains (47.3 ± 7.6 kPa). To quantify the conformational properties of L. monocytogenes' strains surface biopolymers, two models were used. First, the entropic-based, statistical mechanical, random walk formulation, the wormlike chain (WLC) model was used to estimate the elastic properties of the bacterial surface molecules. The WLC model results indicated that the virulent strains are characterized by a more flexible surface biopolymers as indicated by shorter persistence lengths (L(p) = 0.21 ± 0.08 nm) compared to the avirulent strains (L(p) = 0.24 ± 0.14 nm). Second, a steric model developed to describe the repulsive forces measured between the AFM tip and bacterial surface biopolymers indicated that the virulent strains are characterized by crowded and longer biopolymer brushes compared to those of the avirulent strains. Finally, scaling relationships developed for grafted polyelectrolyte brushes indicated L. monocytogenes strains' biopolymer brushes are charged. Collectively, our data indicate that the conformational properties of the bacterial surface biopolymers and their surface densities play an important role in controlling the overall bacterial cell elasticity.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.

Characterizations and properties of hairy latex particles

Industrial latex composed of a hydrophobic core surrounded by a charged hydrophilic layer exhibits excellent stability toward monovalent salt. That feature is classically attributed to a steric effect due to a loss of entropy during overlapping of coating materials. The so-called electrosteric stabilization is, however, not a straightforward function of the nature of the hydrophilic corona. This suspension was characterized in dilute solution by scattering and electrophoresis techniques. In contrast to spherical brushes the interface between the core and the corona is not well defined. The layer is more similar to a highly hydrated nonuniform gel with few longer strands that control the hydrodynamic properties than to a polyelectrolyte brush whose dependence on ionic strength reflects the concentration of counterions inside a well-defined structure. Thus the steric contribution to stabilization of these hairy particles appears to be insignificant in the range studied. The highly hydrated nature and the global charge of the layer are two predominant factors for the stability of the particles.

Effect of counterions on the swelling of spherical polyelectrolyte brushes

We investigate the swelling of colloidal spherical polyelectrolyte brushes in the presence of different counterions. The colloidal particles consist of a solid poly(styrene) core of ca. 100 nm diameter onto which linear polyelectrolyte chains are chemically grafted. Two types of polyelectrolyte chains have been used here: The cationic polyelectrolyte poly(2-(acryloyl)ethyltrimethylammonium chloride)) (PATAC) and the anionic poly(styrenesulfonate) (PSS). Both systems are dispersed in water and the degree of swelling of the surface layer is studied by dynamic light scattering. Adding more and more salt leads to a strong shrinking of the surface layer as expected for polyelectrolyte brushes. It is shown that data obtained at low ionic strength can be collapsed on suitable master curves for monovalent and divalent counterions, respectively. For some ions, however, high salt concentrations may lead to a re-swelling of the brush layer in case of the cationic systems. This points to specific interactions of the counterions with the PATAC chains. This strong specific interaction between the counterions and the attached polyelectrolyte may even lead to flocculation of the particles at intermediate salt concentration. Surprisingly, for iodide and magnesium counterions the solubility increases again if the salt concentration is raised to 1 mol/l. Hence, specific interaction leads to salting-out effects as well as to salting-in effects for these colloidal particles. All specific effects seen at high concentrations of added salt can be explained by the increase of the reduced excluded-volume parameter which is due to the adsorption of salt ions.

A Modified Box Model Including Charge Regulation for Protein Adsorption in a Spherical Polyelectrolyte Brush

Recent experiments showed significant adsorption of bovine serum albumin (BSA) in spherical polyelectrolyte brushes (SPB) consisting of polyacrylic acid, even for pH values above the isoelectric point of the protein, when both protein and polyion are negatively charged. To describe these experimental findings theoretically, we have constructed a spherical box model for an annealed brush consisting of a weak polyelectrolyte that includes the adsorption of BSA. At equilibrium the chemical potential of BSA in solution equals that at each location in the brush, while the net force on the polyions (including osmotic, stretching, and excluded volume terms) is zero at each location. Protein adsorption is predicted above the isoelectric point and--in agreement with experimental data--is a strong function of ionic strength and pH. Adsorption of protein in the brush is possible because the pH in the brush is below the isoelectric point and protein reverses its charge from negative to positive when it adsorbs.

Mechanism of protein binding to spherical polyelectrolyte brushes studied in situ using two-photon excitation fluorescence fluctuation spectroscopy

We used two-photon excitation fluorescence fluctuation spectroscopy with photon counting histogram (PCH) analysis as a new tool to study the binding of globular proteins to colloidal particles in situ. Whereas fluorescence fluctuations are traditionally evaluated by calculating the autocorrelation function (fluorescence correlation spectroscopy), a complementary PCH analysis has been performed in this study which is advantageous when particle concentrations of a multicomponent system are of interest and the particles can be distinguished through particle brightness differences. The binding of two proteins, staphylococcal nuclease (SNase) and bovine serum albumin (BSA), to spherical polyelectrolyte brushes (SPB) was measured as a function of protein concentration and ionic strength of the solution at pH-values where SNase and BSA are positively and negatively charged, respectively. It has been found that SNase and BSA strongly bind to the SPB regardless of the protein charge. When the ionic strength of the solution is raised to 100 mM, the SPB become resistant to both proteins. These findings provide further evidence for a binding mechanism where the proteins are mainly driven to the SPB by the "counterion evaporation" force, while Coulomb interactions play a minor role. The results of this study characterize the potential of SPB as a new class of carrier particles for proteins whose use in biotechnological applications appears to be rewarding.

Role of ionic strength on the relationship of biopolymer conformation, DLVO contributions, and steric interactions to bioadhesion of Pseudomonas putida KT2442

Biopolymers produced extracellularly by Pseudomonas putida KT2442 were examined via atomic force microscopy (AFM) and single molecule force spectroscopy. Surface biopolymers were probed in solutions with added salt concentrations ranging from that of pure water to 1 M KCl. By studying the physicochemical properties of the polymers over this range of salt concentrations, we observed a transition in the steric and electrostatic properties and in the conformation of the biopolymers that were each directly related to bioadhesion. In low salt solutions, the electrophoretic mobility of the bacterium was negative, and large theoretical energy barriers to adhesion were predicted from soft-particle DLVO theory calculations. The brush layer in low salt solution was extended due to electrostatic repulsion, and therefore, steric repulsion was also high (polymers extended 440 nm from surface in pure water). The extended polymer brush layer was "soft", characterized by the slope of the compliance region of the AFM approach curves (-0.014 nN/nm). These properties resulted in low adhesion between biopolymers and the silicon nitride AFM tip. As the salt concentration increased to > or =0.01 M, a transition was observed toward a more rigid and compressed polymer brush layer, and the adhesion forces increased. In 1 M KCl, the polymer brush extended 120 nm from the surface and the rigidity of the outer cell surface was greater (slope of the compliance region = -0.114 nN/nm). A compressed and more rigid polymer layer, as well as a less negative electrophoretic mobility for the bacterium, resulted in higher adhesion forces between the biopolymers and the AFM tip. Scaling theories for polyelectrolyte brushes were also used to explain the behavior of the biopolymer brush layer as a function of salt concentration.

Synthesis and aqueous solution properties of polyelectrolyte-grafted silica particles prepared by surface-initiated atom transfer radical polymerization

A range of polyelectrolyte-grafted silica particles have been prepared by grafting suitable initiators onto near-monodisperse, 304-nm-diameter silica particles using siloxane chemistry, followed by surface-initiated atom transfer radical polymerization (ATRP) of four ionic vinyl monomers, namely sodium 4-styrenesulfonate (SStNa), sodium 4-vinylbenzoate (NaVBA), 2-(dimethylamino)ethyl methacrylate (DAM), and 2-(diethylamino)ethyl methacrylate (DEA) in protic media. The resulting polyelectrolyte-grafted silica particles were characterized using dynamic light scattering (DLS), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), helium pycnometry, and diffuse reflectance infrared Fourier transfer spectroscopy (DRIFTS). The TGA results indicated that the polyelectrolyte contents of the silica particles could be varied from 0.6% to 6.0% in weight. SEM studies revealed several surface morphologies for the grafted polyelectrolytes and XPS analysis of the particle surface also provided good evidence for surface grafting. Combined aqueous electrophoresis and DLS studies confirmed that these polyelectrolyte-grafted silica particles had pH-dependent colloid stabilities, as expected. Cationic polyelectrolyte-grafted silica particles were colloidally stable at low or neutral pH, but became aggregated at high pH. Conversely, anionic polyelectrolyte-coated silica particles became unstable at low pH. It was found that the rate of surface-initiated ATRP was substantially slower than the analogous solution polymerization. Finally, there was some evidence to suggest that, at least in some cases, a significant fraction of polymer chains became detached from the silica particles during polymerization.

Cation-induced collapse of low-molecular-weight polyacrylic acid in the dispersion of barium titanate

Observations on the steric layers formed by the adsorption of low-molecular-weight polyacrylic acid (PAA) were taken using the colloidal probe method in an atomic force microscope. The effects of divalent barium ions and of monovalent potassium ions at varying concentrations were observed on the repulsive interaction profiles. High ionic concentrations screened double-layer forces to small distances, whereby the acting forces were reduced to steric interactions. De Gennes scaling theory was used to model the effect of electrolyte on an aqueous barium titanate system, which was stabilized with PAA. The brush model was found to represent the force curves better than the mushroom model. The collapse of PAA layers with increasing salt approximated a grafted polymer brush in monovalent electrolyte, but the addition of barium ions caused markedly less steric collapse. It is suggested that the formation of a Ba(2+)-PAA complex in the adsorbed layer increases its compressibility parameter.

Spherical polyelectrolyte brushes: comparison between annealed and quenched brushes

We report on a study of spherical polyelectrolyte brushes that consist of a solid core onto which linear polyelectrolyte chains are chemically grafted. The core particles are made up of solid poly(styrene) and have a radius R of ca. 50 nm. As polyelectrolyte chains the weak polyelectrolyte poly(acrylic acid) or the strong polyelectrolyte poly(styrenesulfonate) was used. These chains were generated directly on the surface of the core particles by a grafting-from technique. Hence, the chains are chemically bound to the surface but can be cleaved off and analyzed separately. The contour length L(c) and the number of grafted chains per unit area sigma can thus be determined accurately. The thickness L of the brush layer on the surface has been determined by dynamic light scattering. It is measured for different L(c)/R as a function of pH, ionic strength, and valency of counterions. Annealed brushes exhibit a transition with increasing pH in which the chains are stretched to nearly full length. This can be traced back to the building up of the osmotic pressure of the counterions. The brush height L decreases considerably with increasing ionic strength, most notably when adding divalent ions. The entire set of L as a function of R, L(c), and sigma can be fully explained in terms of a simple two-parameter theory developed by Hariharan et al. [Macromolecules 31, 7514 (1998)].

Polymer adsorption-driven self-assembly of nanostructures

Driven by prospective applications, there is much interest in developing materials that can perform specific functions in response to external conditions. One way to design such materials is to create systems which, in response to external inputs, can self-assemble to form structures that are functionally useful. This review focuses on the principles that can be employed to design macromolecules that when presented with an appropriate two-dimensional surface, will self-assemble to form nanostructures that may be functionally useful. We discuss three specific examples: (a) biomimetic recognition between polymers and patterned surfaces. (b) control and manipulation of nanomechanical motion generated by biopolymer adsorption and binding, and (c) creation of patterned nanostructuctures by exposing molten diblock copolymers to patterned surfaces. The discussion serves to illustrate how polymer sequence can be manipulated to affect self-assembly characteristics near adsorbing surfaces. The focus of this review is on theoretical and computational work aimed toward elucidating the principles underlying the phenomena pertinent to the three topics noted above. However, synergistic experiments are also described in the appropriate context.