Advances in Colloid and Interface Science (v.158, #1-2)

Contents (vii).

Preface by Björn Lindman; Alberto Pais (1).

Polymeric microcapsules with light responsive properties for encapsulation and release by Matthieu F. Bédard; Bruno G. De Geest; Andre G. Skirtach; Helmuth Möhwald; Gleb B. Sukhorukov (2-14).
This review is dedicated to recent developments on the topic of light sensitive polymer-based microcapsules. The microcapsules discussed are constructed using the layer-by-layer self-assembly method, which consists in absorbing oppositely charged polyelectrolytes onto charged sacrificial particles. Microcapsules display a broad spectrum of qualities over other existing microdelivery systems such as high stability, longevity, versatile construction and a variety of methods to encapsulate and release substances. Release and encapsulation of materials by light is a particularly interesting topic. Microcapsules can be made sensitive to light by incorporation of light sensitive polymers, functional dyes and metal nanoparticles. Optically active substances can be inserted into the shell during their assembly as a polymer complex or following the shell preparation. Ultraviolet-addressable microcapsules were shown to allow for remote encapsulation and release of materials. Visible- and infrared- addressable microcapsules offer a large array of release strategies for capsules, from destructive to highly sensitive reversible approaches. Besides the Introduction and Conclusions, this review contains in four sections reviewing the effects of light 1) on polymer-based microcapsules, 2) microcapsules containing metal nanoparticles and 3) functional dyes, as well as a fourth section that revisits the implications of light addressable polymeric microcapsules as a microdelivery system for biological applications.
Keywords: Microcapsules; Polyelectrolytes; Layer-by-layer; Drug delivery; Metal nanoparticles; Encapsulation; UV; IR; Porphyrin; Azobenzene;

Synthesis and properties of polyelectrolyte microgel particles by H. Nur; V.T. Pinkrah; J.C. Mitchell; L.S. Benée; M.J. Snowden (15-20).
A series of cationic poly(N-isopropylacrylamide/4-vinylpyridine) [poly(NIPAM/4-VP)] polyelectrolyte co-polymer microgels have been prepared by surfactant free emulsion polymerization (SFEP) with varying compositions of 4-VP and NIPAM. The compositions of 4-VP were 15, 25, 35, 45, 55 wt.% relative to NIPAM. The temperature and pH responsive swelling–deswelling properties of these microgels have been investigated using dynamic light scattering (DLS) and electrophoretic mobility measurements. DLS results have shown that the particle diameter of the poly(NIPAM/4-VP) microgels decreases with increasing concentration (wt.%) of 4-VP over the 20–60 °C temperature range due to the increased amount of hydrophobic group. The particle size of all poly(NIPAM/4-VP) microgel series increases with decreasing pH, as the 4-VP units become more protonated at low pH below the pKa (5.39) of the monomer 4-VP. Electrophoretic mobility results have shown that electrophoretic mobility increases as the temperature/pH increases at a constant background ionic strength (1 × 10− 4  mol dm− 3 NaCl). These results are in good agreement with DLS results. The temperature/pH sensitivity of these microgels depends on the ratio of NIPAM/4-VP concentration in the co-polymer microgel systems. The combined temperature/pH responsiveness of these polyelectrolyte microgels can be used in applications where changes in particle size with small change in pH or temperature is of great consequence.
Keywords: Polyelectrolyte; Microgels; DLS; Electrophoretic mobility; pH and temperature responsive swelling;

Swelling properties of cross-linked DNA gels by Diana Costa; M. Graça Miguel; Björn Lindman (21-31).
This work represents our contribution to the field of physical chemistry of DNA gels, and concerns the synthesis and study of novel chemically cross-linked DNA gels. The use of covalent DNA gels is a very promising way to study DNA–cosolute interactions, as well as the dynamic behaviour of DNA and cationic compacting agents, like lipids, surfactants and polycations. Manipulating DNA in new ways, like DNA networks, allows a better understanding and characterization of DNA–cosolute complexes at the molecular level, and also allows us to follow the assembly structures of these complexes.The use of responsive polymer gels for targeted delivery of toxic and/or labile drugs has, during the past few years, shown to be a promising concept. The features found in the proposed system would find applications in a broader field of gel/drug interaction, for the development of controlled release and targeted delivery devices.

Cation-induced polyelectrolyte–polyelectrolyte attraction in solutions of DNA and nucleosome core particles by Nikolay Korolev; Alexander P. Lyubartsev; Lars Nordenskiöld (32-47).
The paper reviews our current studies on the experimentally induced cation compaction and aggregation in solutions of DNA and nucleosome core particles and the theoretical modelling of these processes using coarse-grained continuum models with explicit mobile ions and with all-atom molecular dynamics (MD) simulations. Recent experimental results on DNA condensation by cationic oligopeptides and the effects of added salt are presented. The results of MD simulations modelling DNA–DNA attraction due to the presence of multivalent ions including the polyamine spermidine and fragments of histone tails, which exhibit bridging between adjacent DNA molecules, are discussed. Experimental data on NCP aggregation, using recombinantly prepared systems are summarized. Literature data and our results of studying of the NCP solutions are compared with predictions of coarse-grained MD simulations, including the important ion correlation as well as bridging mechanisms. The importance of the results to chromatin folding and aggregation is discussed.
Keywords: DNA condensation; Ligand–DNA interaction; Chromatin; Ion correlations; Computer simulations; Molecular Dynamics; Histone tail modifications; Histone tail "bridging";

In this review we discuss recent results from computer simulations based on coarse-grained polyion models representing aqueous solutions of polyelectrolytes. The focus will be directed to the conformation of the polyions and, in particular, their condensation in bulk, induced by multivalent ions and oppositely charged polyelectrolytes, at responsive surfaces and under confinement.
Keywords: Monte Carlo simulations; Trivalent ions; Polycations; Responsive surfaces; Translocation; DNA;

The effective charge of poly(styrenesulfonate) has been investigated by diffusion and electrophoresis nuclear magnetic resonance (NMR). While the electrophoretic mobility is determined in the electrophoresis NMR experiment, the hydrodynamic friction is determined from diffusion NMR using Einstein's formula. On the timescale of the NMR experiment a steady state is reached, which results from the force balance between the electric field and the hydrodynamic friction from that the effective charge is calculated without any further model.For the monomer and short polymers the effective charge is equal to the nominal charge, the difference increases with an increasing degree of polymerisation. Increasing the ionic strength of the solution leads to enhanced counterion condensation. If the dielectric constant of the solution is lowered, condensation of counterions is enhanced as well. A lowered effective charge results in reduced repelling forces along the polymer chain and thus in a more compact conformation of the polymer as reflected in the hydrodynamic size. The effective charge of poly(styrenesulfonate) has been studied experimentally as a function of the degree of polymerisation, of the ionic strength and the dielectric constant of the solution.
Keywords: Polyelectrolyte; Effective charge; Electrophoresis NMR; Counterion condensation; PFG NMR;

Investigations dealing with association behaviour and structure formation in oppositely charged polyelectrolyte–surfactant mixtures in aqueous solutions are reviewed. Discussion is limited to a selection of vinyl based anionic polyelectrolytes that, when completely ionized, posses the same structural value of the linear charge density parameter. Particular emphasis is placed on the role of polymer chain properties in aggregates with surfactants. Chain characteristics are varied by changing the nature of the charged group, its ionization degree — when possible, the spatial distribution of these groups along the chain, i.e. the tacticity, and the hydrophobic character of other substituents attached to the chain. Quantitative information on the degree of binding in the form of binding isotherms is obtained using surfactant-sensitive membrane electrodes and microstructures of polyelectrolyte–surfactant complexes are determined by synchrotron small angle X-ray scattering. Considerable differences in the degree of binding (including the critical association concentration, CAC, values) and in structures are found. It is concluded that strong interactions in these systems arise from the electrostatic attraction, but this only forms the basis for initial extensive accumulation (anchoring) of surfactant ions in the vicinity of the polyion chain. When this is accomplished, additional specific interactions and effects may come into play. The most powerful of these interactions, the hydrophobic association between the chain and the micelle core, were found in polystyrenesulfonate, PSS, solutions. Other properties are less influential but still lead to CAC values that differ by more than one order of magnitude. These differences are explained by taking into account the chain conformation, flexibility, and hydrophobic character. Specific interactions between PSS and cetylpyridinium, CP, cations result in a soluble non-stoichiometric PSS–CP complex that could be characterized by measuring various solution properties as a function of polymer concentration and degree of complexation. The review is supplemented by including studies of complexation between the spherical fullerene hexamalonate anion and cationic surfactants, which demonstrate a high association tendency with characteristics similar to those found in binding of surfactants by linear polyelectrolytes.
Keywords: Polyelectrolyte–surfactant mixtures; Association; Structure formation; Vinyl-based polyelectrolytes; Cationic surfactants; Fullerene derivatives;

From triple interpolyelectrolyte-metal complexes to polymer-metal nanocomposites by Alexander B. Zezin; Valentina B. Rogacheva; Vladimir I. Feldman; Pavel Afanasiev; Alexey A. Zezin (84-93).
Nanocomposite polymer materials containing metal or metal oxide particles attract growing interest due to their specific unique combination of physical and electric behavior. Stoichometric triple interpolyelectrolyte-metal complexes (TIMC) are insoluble in water and in aqueous organic media and may include high content of metal ions; concentration of ions is easy to vary in such polymeric systems. Reduction of metal ions is a common method for obtaining nanoparticles. Interpolyelectrolyte complexes reveal high permeability for polar low-molecular substances and salts. Such swelling behavior is important for the reduction of metal ions included in these solids. The properties of triple interpolyelectrolyte-metal complexes and preparation of nanocomposites from these materials using various methods of metal ion reduction are discussed in this work.
Keywords: Interpolyelectrolyte-metal complexes; Polyelectrolytes; Polymer-metal nanocomposite; Preparation of nanoparticles; Reduction of metal ions;

Binding of polynucleotides to conjugated polyelectrolytes and its applications in sensing by Maria J. Tapia; Maria Montserín; Artur J.M. Valente; Hugh D. Burrows; Ricardo Mallavia (94-107).
We provide a brief overview of the structural characteristics of the main groups of conjugated polyelectrolytes (CPEs) as well as the methods of synthesis and their behaviour in solution. Their tendency to form aggregates in solution, which is one of the key points to be taken into account for them to be used in polynucleotide sensing, is also considered and the various strategies adopted to avoid it will be discussed. These include the synthetic one (with the incorporation of charged and/or bulky substituents), the use of organic co-solvents and the addition of surfactants.The main physical chemical changes (optical, photophysical, electrical conductivity and viscosity) observed upon direct binding between polynucleotide and CPE, the kind of interactions involved and their applicability in sensing are considered as a function of the CPE structural rigidity. Moreover, more complex devices developed in CPE–polynucleotide sensing with the involvement of additional spectroscopic probes to induce Förster resonant energy transfer processes (FRET) or superquenching phenomena are reviewed. Finally, the main CPE applications in biosensing and the potential use of these systems in understanding DNA compaction and possible extension to the construction of supramolecular oligonucleotide structures are summarized.
Keywords: Conjugated polyelectrolytes; Polynucleotides; Biosensor applications; Supramolecular oligonucleotide structures; Compaction;

Characterization of polyelectrolyte features in polysaccharide systems and mucin by Bo Nyström; Anna-Lena Kjøniksen; Neda Beheshti; Atoosa Maleki; Kaizheng Zhu; Kenneth D. Knudsen; Ramón Pamies; José G. Hernández Cifre; José García de la Torre (108-118).
This review elucidates several aspects on the behavior of charged polysaccharides and mucin. Viscosification of dilute aqueous solutions of hyaluronan (HA) occurs in the course of time at low shear flow, whereas shear thinning as time evolves is found at moderate shear rates. Hydrogen bonds and electrostatic interaction play an important role for the emergence of these features. No time effect of the viscosity is observed for semidilute HA solutions. A degradation of HA is observed at low and high pH and this effect continues over long times, and it is only in the approximate interval 5 < pH < 10 that HA is stable. Small angle neutron scattering (SANS) measurements on semidilute aqueous solutions of mucin at pH = 7 reveal a fractal dimension of 1.4, and the effect of temperature is insignificant on the fractal structure. This suggests that the mucin chains on a semi-local dimensional scale are rod-like. From various experimental methods on solutions of mucin it was found that at pH values around 2 (uncharged polymer), the intensive hydrophobic interactions lead to large association complexes, whereas at pH ≫ 2 the negative charges suppress the tendency of forming associations. At pH < 2, the mucin chains are compressed and they are decorated by some positive charges. In the semidilute regime, a fragmented network is developed. The intense association in semidilute solutions of mucin at pH = 2 is further supported by the results from rheo-small angle light scattering measurements. Effects of ionic strength on the radius of gyration (R g) for dilute solutions of HA (pH = 7) and positively charged hydroxyethylcellulose (HEC(+)) are studied with the aid of Monte Carlo simulations, and essential features of the polyelectrolyte effect on R g are captured in the computer simulation. Strong interactions are observed in aqueous mixtures of an anionic polysaccharide (HEC(−)) and an oppositely charged surfactant (cetyltrimethylammonium bromide; CTAB); this gives rise to extensive associations and macroscopic phase separation is approached. The massive association complexes are disclosed in the SANS experiments by a pronounced upturn in the scattered intensity at low values of the wave vector.
Keywords: Polyelectrolytes; Hyaluronan; Polysaccharides; Charged polysaccharide and oppositely charged surfactant; Structure; Viscosity; Monte Carlo simulation;

Glycosaminoglycans as polyelectrolytes by Emek Seyrek; Paul Dubin (119-129).
One of the barriers to understanding structure–property relations for glycosaminoglycans has been the lack of constructive interplay between the principles and methodologies of the life sciences (molecular biology, biochemistry and cell biology) and the physical sciences, particularly in the field of polyelectrolytes. To address this, we first review the similarities and differences between the physicochemical properties of GAGs and other statistical chain polyelectrolytes of both natural and abioitic origin. Since the biofunctionality and regulation of the structures of GAGs is intimately connected with interactions with their cognate proteins, we particularly compare and contrast aspects of protein binding, i.e. effects of both GAGs and other polyelectrolytes on protein stability, protein aggregation and phase behavior. The protein binding affinities and their dependences on pH and ionic strength for the two groups are discussed not only in terms of observable differences, but also with regard to contrasting descriptions of the bound state and the role of electrostatics. We conclude that early studies of the heparin–Antithromin system, proceeding to a large extent through the methods and models of protein chemistry and drug discovery, established not only many enabling precedents but also constraining paradigms. Current studies on heparan sulfate and chondroitin sulfate seem to reflect a more ecumenical view likely to be more compatible with concepts from physical and polymer chemistry.

Brush layers of polyelectrolytes, ionized chains of poly(glutamic acid) (PLGA) and poly(lysine) (PLL), prepared by the Langmuir–Blodgett method, were characterized using the surface forces measurements at various pHs, salt concentrations and chain densities. This paper reviews the major results: (1) the effective charge density of the brush layer calculated from the force profiles was much lesser than the density of the ionized groups of the polyelectrolyte brushes, indicating that nearly all the ionized groups were neutralized by the counterions in the brush layer; (2) the thickness of the brush layers agreed with the length of the extended polyelectrolytes and was practically independent of the salt concentrations studied (0.43–10 mM). The thickness was proportional to the polymerization degree of polyelectrolytes; (3) the initial elastic compressibility modulus of the brush layer of PLGA or PLL increased with increasing ionization degree, while it decreased with increasing salt concentration because of a decrease in the osmotic pressure of the counterions; (4) stress profiles between the brush layers were scaled for polyelectrolytes of various polymerization degrees according to the contour length of the polyelectrolyte. Similar scaling was also found for stress profiles obtained at various salt concentrations (0.43–10 mM) and pHs; (5) the “osmotic pressure of counterion” model reproduced well the steric components of the stress profiles, thus supporting that the steric repulsion was mainly due to the osmotic pressure of the counterions; and (6) a density-dependent jump in the properties of polyelectrolyte brushes such as transfer ratio, compressibility and surface potential has been found, indicating the existence of the density (interchain distance)-dependent transition of polyelectrolytes in solutions. We have proposed a counterion model to account for this transition.
Keywords: Polyelectrolyte brush; Surface forces; Langmuir–Blodgett film; Poly(glutamic acid); Poly(lysine); Counterion condensation;