Oppositely Charged Polyelectrolytes Research Articles

Investigation of Silk Fibroin/Poly(Acrylic Acid) Interactions in Aqueous Solution.

Silk fibroin (SF) is a protein with many outstanding properties (superior biocompatibility, mechanical strength, etc.) and is often used in many advanced applications (epidermal sensors, tissue engineering, etc.). The properties of SF-based biomaterials may additionally be tuned by SF interactions with other (bio)polymers. Being a weak amphoteric polyelectrolyte, SF may form polyelectrolyte complexes (PECs) with other polyelectrolytes of opposite charge, such as poly(acrylic acid) (PAA). PAA is a widely used, biocompatible, synthetic polyanion. Here, we investigate PEC formation between SF and PAA of two different molecular weights (MWs), low and high, using various techniques (turbidimetry, zeta potential measurements, capillary viscometry, and tensiometry). The colloidal properties of SF isolated from Bombyx mori and of PAAs (MW, overlap concentration, the influence of pH on zeta potential, adsorption at air/water interface) were determined to identify conditions for the SF-PAA electrostatic interaction. It was shown that SF-PAA PEC formation takes place at different SF:PAA ratios, at pH 3, for both high and low MW PAA. SF-PAA PEC's properties (phase separation, charge, and surface activity) are influenced by the SF:PAA mass ratio and/or the MW of PAA. The findings on the interactions contribute to the future development of SP-PAA PEC-based films and bioadhesives with tailored properties.

SELF-ASSEMBLED MULTILAYERS OF WATER GLUCOSE MODIFIED-CHITOSAN AND GLUCOSE OXIDASE FOR DETECTION OF GLUCOSE IN MILK SAMPLES

Background: A crucial aspect of electrochemical enzymatic biosensor development is the immobilization of the enzymes, as it directly influences the sensitivity of the bioelectrode. Among the different methods used to incorporate enzymes on the surface of the transducers, layer-by-layer (LbL) self-assembly based on electrostatic interaction with polyelectrolytes of opposite charge stands out due to its simplicity and reproducibility. Aims: The aim of the work was to develop an electrochemical glucose biosensor by LbL assembly of a new functionalized chitosan polycation and the enzyme glucose oxidase (GOx). Methods: Chitosan was chemically functionalized with glucose by the Maillard reaction. The resulting polycation, named G-Chit, is soluble in the medium compatible with the enzyme. The bioelectrode was obtained by alternating adsorption of G-Chit and GOx onto carbon paste electrodes. By selecting the number of bilayer of G-Chit/GOX, the enzyme concentration, and the pH, the electroanalytical performance of the biosensor was optimized. The electrochemical responses were characterized by cyclic voltammetry and chronoamperometry. Results: Under optimized experimental conditions, the biosensor exhibited a sensitivity of (0.81 ± 0.03) µA mM-1 in a glucose concentration range of (0.18 to 1.75) mM. Discussion: Results indicated that catalytic response increases both with the number of G-Chit/GOx bilayers and the enzyme concentration, obtaining the best responses for 3 bilayers and 2 mg mL-1, respectively, while the optimum working pH value was 7.0. Conclusions: The analytical response of the biosensor was tested in milk samples with negligible matrix effects, suggesting a potential application in other dairy products. Results show that G-Chit appears promising for the immobilization of enzymes.

Phase Behaviors of a Binary Blend of Oppositely Charged Polyelectrolytes: A Weak Segregation Approach

In our quest for a better understanding of phase behaviors regarding polyelectrolyte systems, we develop a Landau free energy for a stoichiometric blend of weakly charged polyanions and polycations. Its free energy functional is first formulated as that for Gaussian chains subject to effective interaction fields. The local interaction part, which necessitates excluded volume and charged chain connectivity, is given by a locally equilibrated excess equation of state for the charged hard-sphere chain blend with mean-field dispersion interactions. The long-ranged part of Coulomb interactions is included through an electric potential field generated inside. The resultant functional is then expanded as a series in two proper order parameters to yield the desired Landau free energy. Unfavorable energetics makes the blend phase segregated but only on a nanoscopic scale due to global electroneutrality. The interplay of self- and mutual dispersion interactions produces a variety of phase behaviors ranging from conventional order–disorder-type transitions to complicated ordering transitions in carefully selected blends. Critical compositions are revealed to depend on asymmetry in chain sizes and disparity in self-dispersion interactions between the constituents. Furthermore, it is argued that nanostructured materials with drastically different responses to pressure can be designed by blending polyelectrolytes.

Complexation between Oppositely Charged Polyelectrolytes in Dilute Solution: Effects of Charge Asymmetry

We use dissipative particle dynamics to study polyelectrolyte complexation in dilute solutions under conditions of either chain-length or concentration asymmetry between the polycation and the polyanion, characterized by the macromolecular charge ratio of the minor component to the major component. Our results show that generally the systems form clusters carrying net macromolecular charges. The stoichiometry of these net-charged macromolecular clusters depends on the overall charge asymmetry, and clusters with certain stoichiometry can dominate the cluster population under a wide range of chain-length and concentration asymmetry. The morphology and stability of these net-charged clusters are closely related to their stoichiometry. When the charge ratio reaches a threshold value, the polyions condense into a single large coacervate cluster, signaling the onset of macroscopic coacervation. The threshold value is less for concentration asymmetry systems than for chain-length asymmetry systems at the same salt concentration. This threshold value decreases with added salt, as salt ions cause merging of small clusters into larger ones, eventually leading to the dominance of a single large cluster. This “salting-out” phenomenon is followed by a “salting-in” behavior, as further addition of salt dissolves the large cluster. The preferred size and composition of the net-charged clusters observed in the simulation highlight the significant role of length and concentration asymmetry on polyelectrolyte complex coacervation and hint at the possibility of a microstructured liquid.

Polymer/silica core–shell nanoparticles with temperature-dependent stability properties

This work aimed at the synthesis of hydrogel-based composite core/shell nanoparticles and their subsequent surface modification with thermoresponsive copolymers. Submicron hydrogel-based nanoparticles were obtained from the layer-by-layer coating of silica nanoparticles with two natural oppositely-charged polyelectrolytes, chitosan and sodium alginate. Further modifications with a PVAm-b-PNIPAM copolymer synthetized by RAFT polymerization was achieved by the “grafting to” approach. First, the optimum feed weight ratio (fwr) was determined by a combined approach of zeta potential and T2 relaxation time measurements. Then, diblock grafting at this optimum fwr was performed and characterized by XPS. XPS analysis confirmed the presence of copolymer at the particles’ surface with the increase of C and N atomic percentage. The quantification study was carried out by spectrofluorimetry using the fluorescently labeled PVAm-b-P(NIPAM-stat-NVC) copolymer and revealed that the grafting efficiency could reach 60 %. Finally, a study of thermosensivity properties confirmed that our smart system allowed a temperature-induced destabilization of the particles suspension at 45 °C. This work has promising prospects in the field of controlled drug delivery.

The development and application of nanocomposites with pH-sensitive “gates” to control the release of active agents: Extending the shelf-life of fresh wheat noodles

Controlled release of antimicrobial agents is an efficient and promising technology for food preservation. Halloysite nanotubes (HNTs) are a novel material that act as carriers of active agents, while also providing a pathway for sustained release. By using coatings that responded to their environment, we were able to obtain controlled release such that the desired amounts of active agents were delivered to the packaged food at the desired time. In this study, we loaded cinnamaldehyde into HNTs lumen, and used opposite charged polyelectrolytes to cap the ends of the HNTs to control the release of cinnamaldehyde. Fourier transform infrared spectroscopy (FTIR), Brunauer, Emmett, Teller's test (BET) thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscope (TEM), cumulative release rate, and fumigant antibacterial activity were conducted to evaluate the performance of the encapsulated samples. Tube-like morphology of the HNTs, and the location of cinnamaldehyde and polyelectrolytes were identified. The results showed that the stimuli-responsive layer formed by polyelectrolytes exhibited different release rate at different pH values. At higher pH, the polyelectrolytes layer restrained transportation of active molecules, while at lower pH, the coating assisted the release of cinnamaldehyde. Additionally, results of antimicrobial test in vitro illustrated significant fumigant antimicrobial activity of cinnamaldehyde/polyelectrolytes functionalized HNTs. To further learn the preservative effect of samples in food packaging, we conducted a pilot study of packed fresh wheat noodles with the designed nanocomposites, and tested total plate count, pH, color change and environmental-SEM of noodles. Results indicated that the nanocomposites with controlled pH-responsive cinnamaldehyde release is a promising packaging technology for extending the shelf-life of fresh wheat noodles.

Temporal Changes of Adsorbed Layer Thickness and Electrophoresis of Polystyrene Sulfate Latex Particles after Long Incubation of Oppositely Charged Polyelectrolytes with Different Charge Densities.

The different desorption concepts of the two polyelectrolytes PTMA5M and PTMC5M, which have similar molecular weights and differ in the charge density on the polystyrene sulfate latex (PSL) particles by 25 times, and with various charge densities in a long incubation, were systematically investigated based on hydrodynamic adsorbed layer thickness (δH) and electrophoretic mobility (EPM) under two ionic strengths in the present study. Herein, in the case of highly charged polyelectrolyte PTMA5M, desorption continued for 4 h and re-adsorbing proceeded after a longer incubation time higher than 4 h. Meanwhile, in the case of lowly charged polyelectrolyte PTMC5M, an adsorption–desorption equilibrium was suggested to take into account the unchanging of both δH and EPM.

Dynamic Coupling in Unentangled Liquid Coacervates Formed by Oppositely Charged Polyelectrolytes.

We develop a scaling theory that predicts the dynamics of symmetric and asymmetric unentangled liquid coacervates formed by solutions of oppositely-charged polyelectrolytes. Symmetric coacervates made from oppositely-charged polyelectrolytes consist of polycations and polyanions with equal and opposite charge densities along their backbones. These symmetric coacervates can be described as mixtures of polyelectrolytes in the quasi-neutral regime with a single correlation length. Asymmetric coacervates are made from polycations and polyanions with unequal charge densities. The difference in charge densities results in a double semidilute structure of asymmetric coacervates with two correlation lengths, one for the high-charge-density and the other for the low-charge-density polyelectrolytes. We predict that the double-semidilute structure in asymmetric coacervates results in a dynamic coupling which increases the friction of the high-charge-density polyelectrolyte. This dynamic coupling increases the contribution to the zero-shear viscosity of the high-charge-density polyelectrolyte. The diffusion coefficient of the high-charge-density polyelectrolyte is predicted to depend on the concentration and degree of polymerization of the low-charge-density polyelectrolyte in the coacervate if the size of the low-charge-density polymer is smaller than the correlation length of the high-charge-density polymer. We also predict a non-monotonic salt concentration dependence of the zero-shear viscosity of asymmetric coacervates.

Structure and properties of polyelectrolyte complexes of various type (chitosan chloride – polyacrilic acid) and triple polyelectrolyte-metalic complexe with cations Cu2+

Using FT-IR-spectroscopy, X-ray diffraction and thermomechanical analysis structure and thermomechanical properties of two nonstoichiometry and one stoichiometry polyelectrolyte complexes (PEC) based on opposite charged polyelectrolytes – strong cationic polyelectrolyte (chitosan chloride) and weak anionic polyelectrolyte (polyacrylic aсid) and triple polyelectrolyte-metal complexes (TPMC) based on stoichometry polyelectrolyte complexes and cations Cu(II) were investigated. It was shown, that chitosan chloride has amorphous-cristallinity structure, which is significantly different from the structure of neat chitosan, and polyacrylic acid posses amorphous structure. Meantime, all PEC samples have amorphous structure, differing from the structure of weak anionic polyelectrolyte, at the same time amorphous structure of nonstoichiometric PECs insignificantly different from that of stoichiometric polyelectrolyte complexes. Additionally, amorphous structure of TPMC has another structure, compared to all PEC. According to thermomechanical analysis, all PECs have one temperature transition from glassy to highly elastic state (from 77 to 84 °C). The deformation value of the samples of nonstoichiometric PEC is similar and somewhat less than the deformation of the stoichiometric PEC. The TPMC sample has two glass transitions (81 and 226 °C), and his high-temperature transition characterizes the segmental mobility of fragments of macromolecules of one stoichiometric PEC, the polar groups of which form chelate circles with Cu (II) cations. Deformation parameter of the TPMC is higher in comparison with the stoichiometric polyelectrolyte complexes. Keywords: structure, properties, deformation, ionic force, cationic Cu(II), polyelectrolyte, polyelectrolyte-metal complexes, chitosan chloride, polyacrylic acid, X-ray diffraction, thermomechanical analysis.

On the Mechanism of Shear-Thinning in Viscous Oppositely Charged Polyelectrolyte Surfactant Complexes (PESCs).

Semidilute mixtures of the cationically modified cellulose-based polyelectrolyte JR 400 and the anionic surfactant sodium dodecyl sulfate (SDS) form highly viscous solutions if a slight excess of charges from the polyelectrolyte is present. The reason for this is the formation of mixed rodlike aggregates in which the surfactant cross-links several polyelectrolyte chains. The same solutions also show shear-thinning behavior. In this paper, we use rheoSANS to investigate the structural evolution of the rodlike aggregates under steady shear and thereby elucidate the mechanism of shear-thinning in these viscous oppositely charged polyelectrolyte surfactant complexes.

Electrostatically Stabilized Microphase Separation in Blends of Oppositely Charged Polyelectrolytes

A possibility of microphase separation in a blend of oppositely charged polyelectrolytes, being immiscible when uncharged, is studied theoretically and by means of dissipative particle dynamics (DP...

Synthesis and Characterization of a Polymer Matrix of Carrageenan, Chitosan and Hyaluronan on Stainless Steel 316L

A novel bioactive coating matrix consisting of three biopolymers – carrageenan, chitosan, and hyaluronan, was synthesized via alternate layer-by-layer (L-b-L) self-assembly of the oppositely-charged polyelectrolytes on a 316 L SS scaffold. The L-b-L deposition was carried out by substrate immersion in each polyelectrolyte solution for various length of time with 80 mins as the optimum time based on morphological non-homogeneity of coating evaluated through SEM. Co-60 γ-irradiation of the coating was done to enhance cross-linking. To test the coating matrix’s resitstance to shear stress, the coated scaffolds were subjected to agitation of 500 to 2000 rpm in phosphate buffer solution (PBS) for 24 hours. The biopolymer surface subjected to 500 rpm agitation did not exhibit noticeable surface changes and appeared similar to that of the control sample. The optimum coating was then evaluated for hemocompatibility and demonstrated low cytotoxicity with average live cell count of 77.54% after 48 hours of incubation.

Layer-By-Layer Biofunctionalization As a Novel Route for High-Sensitivity and High-Specificity Label-Free Affinity Biosensing with Nanostructured Materials

Nanostructured materials hold the promise to revolutionize the label-free biosensing of analytes at concentrations down to those required for clinical applications, beyond what microstructured materials did so far, by leveraging the deeper interaction between materials and analytes with comparable size. On the other hand, when the characteristic dimension of the materials gets down to the nanoscale, the biofunctionalization chemistry commonly used to promote the binding of bioreceptors and enable, in turn, the specific detection of target analytes becomes less effective. Here, we propose an effective and robust route for the surface biofunctionalization of nanostructured materials based on the layer-by-layer electrostatic nano-assembling of oppositely-charged polyelectrolytes, which were engineered with bioreceptors covalently linked to the polymer chain. LbL biofunctionalization allows stability, density, and distribution of bioreceptors available on the transducer surface to be carefully controlled, thus solving most of the drawbacks of the common biofunctionalization chemistry on nanostructured materials. The proof-of-concept demonstration of advantages of LbL biofunctionalization is given on nanostuctured porous silicon (PSi) interferometers, which are biofunctionalized for the affinity detection of streptavidin in raw saliva via LbL nano-assembling of a positively-charged poly(allylamine hydrochloride) (PAH) with a negatively-charged biotinylated poly(methacrylic acid) (b-PMAA). LbL-biofunctionalizion of PSi interferometers results to be very effective and highly robust, enabling high-sensitivity and high-specificity detection of streptavidin with a detection limit of 600 fM. This represents a 100000-fold improvement with respect to control PSi interferometers biofunctionalized using common silane-based chemistry and a 300-fold improvement with respect to best PSi label-free biosensors reported in the current literature, pushing PSi optical biosensors to performance comparable to those of best label-free plasmonic and photonic platforms. [1] Layer-by-layer biofunctionalization of nanostructured porous silicon for high-sensitivity and high-selectivity label-free affinity biosensing, S Mariani, V Robbiano, LM Strambini, A Debrassi, G Egri, L Dähne, G. Barillaro, Nature Communications 9 (1), 5256 (2018)

Self-coacervation of ampholyte polymer chains as an efficient encapsulation strategy

Coacervation is a phase separation process involving two aqueous phases, one solute-phase and one solute-poor phase. It is frequently observed among oppositely-charged polyelectrolyte systems. In this study, we focus on self-coacervation involving a single polymer chain and investigate its potential for encapsulation applications.Negatively charged polyacrylic acid polymer chains were partially cationized using diamine and carbodiimide chemistry affording ampholytes, named PAA-DA, with tunable charge ratio. When dispersed in water, at pH 7, PAA-DA was soluble but a phase separation occurs when decreasing pH close to the isoelectric point. Coacervation is found only for a given amine-to-acid ratio otherwise precipitation is observed. Increasing the pH above 4 yielded progressive destruction of the coacervates droplets via the formation of vacuoles within droplets and subsequent full homogeneous redispersion of PAA-DA in water. However, addition of calcium allowed increasing the coacervate droplet stability upon increasing the pH to 7 as the divalent ion induced gelation within droplets.Moreover, the coacervate droplets present the ability to spontaneously sequestrate a broad panel of entities, from small molecules to macromolecules or colloids, with different charges, size and hydrophobicity. Thanks to the reversible character of the coacervates, triggered-release could be easily achieved, either by varying the pH or by removing calcium ions in the case of calcium-stabilized coacervates. Self-coacervation presents the advantage of pathway-independent preparation, offering a real output interest in pharmacy, water treatment, food science or diagnostics.

Influence of charge sequence on the adsorption of polyelectrolytes to oppositely-charged polyelectrolyte brushes.

When a solution of polyanionic chains is placed in contact with a polycationic brush, the polyanions adsorb into the brush. We investigate the influence of the charge sequences of the free and bound species on the thermodynamics of polyelectrolyte adsorption. As model systems, we consider free and brush polyelectrolytes with either block or alternating charge sequences, and study the adsorption process using coarse-grained Langevin dynamics with implicit solvent, explicit counterions, and excess salt. Free energy, internal energy, and entropy of adsorption are computed using umbrella sampling methods. When the number of polyanions exceed the number of polycations, the brush becomes overcharged. Free chains adsorb most strongly when both free and tethered chains have a block charge sequence, and most weakly when both species have an alternating sequence. Adsorption is stronger when the free polyanion has a block sequence and the tethered polycation is alternating than in the reverse case of an alternating free polymer and a tethered block copolymer. Sequence-dependent effects are shown to be largely energetic, rather than entropic, in origin.

Self-curing super-stretchable polymer/microgel complex coacervate gels without covalent bond formation.

Elastic physical gels are highly desirable because they can be conveniently prepared and readily shaped. Unfortunately, many elastic physical gels prepared in water require in situ free-radical polymerization during the gel formation stage. In contrast, complex coacervate gels are physical gels that can be prepared by simply mixing two pre-formed oppositely-charged polyelectrolytes. However, as far as we are aware, highly elastic complex coacervate gels have not yet been reported. Herein, we combine polyanionic microgel particles with a well-known commercially-available cationic polyelectrolyte to prepare polymer/microgel complex coacervate (PMCC) physical gels. This new family of gels requires annealing at only 37 °C and behaves like a covalent gel but does not form covalent bonds. Thermal reconfiguration of the dynamic ionic bonds transforms the shapeable pre-gel into a highly elastic gel that is super-stretchable, adhesive, self-healing, highly swellable and can be further toughened using Ca2+ as an ionic crosslinker. Our PMCC gels have excellent potential for applications as engineering gels and structural biomaterials, as well as for wound healing and water purification.

Layer-by-layer biofunctionalization of nanostructured porous silicon for high-sensitivity and high-selectivity label-free affinity biosensing

Nanostructured materials premise to revolutionize the label-free biosensing of analytes for clinical applications, leveraging the deeper interaction between materials and analytes with comparable size. However, when the characteristic dimension of the materials reduces to the nanoscale, the surface functionalization for the binding of bioreceptors becomes a complex issue that can affect the performance of label-free biosensors. Here we report on an effective and robust route for surface biofunctionalization of nanostructured materials based on the layer-by-layer (LbL) electrostatic nano-assembly of oppositely-charged polyelectrolytes, which are engineered with bioreceptors to enable label-free detection of target analytes. LbL biofunctionalization is demonstrated using nanostructured porous silicon (PSi) interferometers for affinity detection of streptavidin in saliva, through LbL nano-assembly of a bi-layer of positively-charged poly(allylamine hydrochloride) (PAH) and negatively-charged biotinylated poly(methacrylic acid) (b-PMAA). High sensitivity in streptavidin detection is achieved, with high selectivity and stability, down to a detection limit of 600 fM.

Effects of reaction pH on self-crosslinked chitosan-carrageenan polyelectrolyte complex gels and sponges

Macromolecular biomaterials often require covalent crosslinking to achieve adequate stability for their given application. However, the use of auxiliary chemicals may be associated with long-term toxicity in the body. Oppositely-charged polyelectrolytes (PEs) have the advantage that they can self-crosslink electrostatically and those derived from marine organisms such as chitosan (CS) and carrageenan (CRG) are inexpensive non-toxic alternatives to glycosaminoglycans present in the extracellular matrix of human tissues. The aim of this study was to explore the properties of crosslinker-free PEC gels and freeze-dried PEC sponges based on CS and CRG precursors. We offer new insights into the optimisation of conditions and mechanisms involved in the process and offer a systematic study of property changes across a full range of pH values. Zeta-potential measurements indicated that the PECs produced at pH 2–6 had a high strength of electrostatic interaction with the highest being at pH 4–5. This resulted in strong intra-crosslinking in the PEC gels which led to the formation of higher yield, viscosity, fibre content and lower moisture content. The weaker interaction between CS and CRG at pH 7–12 resulted in higher levels of CS incorporated into the complex and the formation of more inter-crosslinking through entanglements and secondary interactions between PEC units. This resulted in the production of stable PEC sponge materials compared with the PEC materials produced at pH 6 and below. From the range of samples tested, the PECs produced at pH 7.4 appeared to show the optimum combination of yield, stability and homogeneity.

Structure of Liquid Coacervates formed by Oppositely Charged Polyelectrolytes.

We develop a scaling theory and perform molecular dynamic simulations of weakly interacting coacervates with electrostatic interaction energy per charge less than thermal energy kT. Such liquid coacervates formed by oppositely charged polyelectrolytes can be asymmetric in charge density and number of charges per chain. We predict that these coacervates form interpenetrating solutions with two correlation lengths and two qualitatively different types of conformations of polyelectrolytes with lower and higher charge densities, which are analogous to chain conformations in quasi-neutral and in polyelectrolyte solutions, respectively. Weaker charged chains are attracted to and adsorbed on stronger charged chains forming a screening "coat" around the stronger charged polyelectrolytes. Salt added at lower concentrations screens the repulsion between stronger charged chains, thereby reducing the thickness of the screening coat and resulting in the non-zero net polymer charge in the coacervate. At higher salt concentrations salt screens the attraction between oppositely charged chains, decreasing the coacervate concentration and its polymeric charge density. Thus, we predict a non-monotonic salt concentration dependence of polymeric charge density for asymmetric coacervates. Phase diagram for a mixture of oppositely charged polyelectrolytes at various compositions is proposed for different salt concentrations.

Electrically-responsive deformation of polyelectrolyte complex (PEC) fibrous membrane

This study aims at the effect of electro-osmotic flow on a fibrous membrane comprised of assembled oppositely-charged polyelectrolytes and submerged in aqueous ionic solution. As electric field is applied across an edge-clamped circular membrane, it deforms under the action of a transverse force, the origin of which is in focus. Fibers were electrospun from a blend of poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA). A model is presented, which explains and describes the membrane deformation in terms of the ionic strength and pH level of the solution, the membrane geometry and porosity, the ζ-potential and elastic modulus of the membrane, as well as the applied cross-membrane electric potential difference. The model is corroborated by a set of experiments. The dependence of the membrane deformation on the electric potential was found to be rather weak, while the elastic modulus was found to be strongly dependent on the solution pH.