Strong repulsive interactions in polyelectrolyte-liposome clusters close to the isoelectric point: a sign of an arrested state

Electrostatic complexation of polyelectrolyte and magnetic nanoparticles: from wild clustering to controllable magnetic wires

We present the electrostatic complexation between polyelectrolytes and charged nanoparticles. The nanoparticles in solution are γ-Fe2O3 (maghemite) spheres with 8.3 nm diameter and anionic surface charges. The complexation was monitored using three different formulation pathways such as direct mixing, dilution, and dialysis. In the first process, the hybrids were obtained by mixing stock solutions of polymers and nanoparticles. A 'destabilization state' with sharp and intense maximum aggregation was found at charges stoichiometry (isoelectric point). While on the two sides of the isoelectric point, 'long-lived stable clusters state' (arrested states) were observed. Dilution and dialysis processes were based on controlled desalting kinetics according to methods developed in molecular biology. Under an external magnetic field (B = 0.3 T), from dialysis at isoelectric point and at arrested states, cationic polyelectrolytes can 'paste' these magnetic nanoparticles (NPs) together to yield irregular aggregates (size of 100 μm) and regular rod-like aggregates, respectively. These straight magnetic wires were fabricated with diameters around 200 nm and lengths comprised between 1 μm and 0.5 mm. The wires can have either positive or negative charges on their surface. After analyzing their orientational behavior under an external rotating field, we also showed that the wires made from different polyelectrolytes have the same magnetic property. The recipe used a wide range of polyelectrolytes thereby enhancing the versatility and applied potentialities of the method. This simple and general approach presents significant perspective for the fabrication of hybrid functional materials.

Double charge inversion in polyethylenimine-decorated liposomes

The study of the interaction of a cationic polymer as PEI with phospholipids membranes is of special relevance for gene therapy because the PEI is a potential nonviral vector to transfer DNA in living cells. We used light scattering, zeta potential, and electron transmission microscopy to characterize the interaction between DMPG and DOPC liposomes with PEI as a function of the charge molar ratio, pH, temperature, initial size of the liposomes, and headgroup of the lipids. Unexpectedly, a double charge inversion and two different ranges of PEI-liposome concentrations where an aggregation occurs were found, when the proper pH and initial size of the liposomes were chosen. The interaction is analyzed in terms of the interaction potential proposed by Velegol and Thwar for colloidal particles with a nonuniform surface charge distribution. Results show a remarkable dependence of the stability on pH and the initial size of the liposomes, which explains the low reproducibility of the experiments if no special care is taken in preparing the samples. Comparatively small changes in the pH or in the liposomes size lead to a completely different stability behavior.

PLGA-based nanoparticles: effect of chitosan in the aggregate stabilization. A dielectric relaxation spectroscopy study

Chitosan-modified polylactic-co-glycolic acid (PLGA) nanoparticles with average diameter of 200 nm in PBS buffer solution have been investigated by means of dielectric relaxation spectroscopy measurements in the frequency range (1 MHz-2 GHz) where interfacial polarizations occur. PLGA-based nanoparticles offer remarkable advantages in different biotechnological fields, such as their biocompatibility, easiness of administration and rather complete biodegradation. However, despite the use of these drug delivery systems is increasing, little is known about the basic process involved in the formation of complexes and in the subsequent release kinetics. In the present work, we have characterized the colloidal behavior of PLGA-based nanoparticles in the presence of oppositely charged chitosan polyelectrolyte by means of dynamic light scattering, electrophoretic mobility and radiowave dielectric relaxation measurements. In particular, we have emphasized how the presence of a coating layer at the nanoparticle surface could exert a marked slowing-down in the drug release. The consequence of this finding is briefly discussed at the light of some biological implications.

Interactions between adsorbed hydrogenated soy phosphatidylcholine (HSPC) vesicles at physiologically high pressures and salt concentrations

Using a surface force balance, we measured normal and shear interactions as a function of surface separation between layers of hydrogenated soy phosphatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion at physiologically high salt concentrations (0.15 M NaNO₃). Cryo-scanning electron microscopy shows that each surface is coated by a close-packed HSPC-SUV layer with an overlayer of liposomes on top. A clear attractive interaction between the liposome layers is seen upon approach and separation, followed by a steric repulsion upon further compression. The shear forces reveal low friction coefficients (μ = 0.008-0.0006) up to contact pressures of at least 6 MPa, comparable to those observed in the major joints. The spread in μ-values may be qualitatively accounted for by different local liposome structure at different contact points, suggesting that the intrinsic friction of the HSPC-SUV layers at this salt concentration is closer to the lower limit (μ = ~0.0006). This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.

Synthesis and physicochemical characterization of new twin-tailed N-oxide based gemini surfactants

New gemini surfactants (GSs) constituted by two double alkyl chain (from 7 to 17 methylenic units) N-oxide monovalent surfactants joined by a PEG spacer of different length (from 3 to 21 ethylene glycol units), thus combining the properties of both N-oxide and GS surfactants, were synthetized and characterized. The different hydrophilic/hydrophobic balance of the molecular structure strongly influences the morphology and the electrical features of the aggregates. Despite the zwitterionic nature of the polar head groups, all the aggregates are characterized by positive potential thus suggesting protonation at the interface; however, the extent of protonation was shown to strongly depend on the length of the alkyl chain and of the spacer.

Interaction of a charged polymer with zwitterionic lipid vesicles

The interaction between polyethylenimine (PEI) and phospholipid bilayers plays an important role in several biophysical applications such as DNA transfection of target cells. Despite considerable investigation into the nature of the interaction between PEI and phospholipid bilayers, the physical process remains poorly understood. In this paper, we study the impact of PEI on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles as a function of salt concentration using several techniques including dynamic (DLS) and static (SLS) light scattering, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). At low salt concentration, vesicles aggregate, leading to the formation of stable clusters whose final size depends on the PEI concentration. At high salt concentration the system does not aggregate; DSC and NMR data reveal that the PEI penetrates into the bilayer, and SLS measurements are consistent with PEI crossing the bilayer. The transfectional ability of PEI is discussed in terms of these results.

Polyelectrolyte-induced aggregation of liposomes: a new cluster phase with interesting applications

Different charged colloidal particles have been shown to be able to self-assemble, when mixed in an aqueous solvent with oppositely charged linear polyelectrolytes, forming long-lived finite-size mesoscopic aggregates. On increasing the polyelectrolyte content, with the progressive reduction of the net charge of the primary polyelectrolyte-decorated particles, larger and larger clusters are observed. Close to the isoelectric point, where the charge of the adsorbed polyelectrolytes neutralizes the original charge of the particles' surface, the aggregates reach their maximum size, while beyond this point any further increase of the polyelectrolyte-particle charge ratio causes the formation of aggregates whose size is progressively reduced. This re-entrant condensation behavior is accompanied by a significant overcharging. Overcharging, or charge inversion, occurs when more polyelectrolyte chains adsorb on a particle than are needed to neutralize its original charge so that, eventually, the sign of the net charge of the polymer-decorated particle is inverted. The stability of the finite-size long-lived clusters that this aggregation process yields results from a fine balance between long-range repulsive and short-range attractive interactions, both of electrostatic nature. For the latter, besides the ubiquitous dispersion forces, whose supply becomes relevant only at high ionic strength, the main contribution appears due to the non-uniform correlated distribution of the charge on the surface of the polyelectrolyte-decorated particles ('charge-patch' attraction). The interesting phenomenology shown by these system has a high potential for biotechnological applications, particularly when the primary colloidal particles are bio-compatible lipid vesicles. Possible applications of these systems as multi-compartment vectors for the simultaneous intra-cellular delivery of different pharmacologically active substances will be briefly discussed.

Effect of temperature on the reentrant condensation in polyelectrolyte-liposome complexation

Interactions of oppositely charged macroions in aqueous solution give rise to intriguing aggregation phenomena, resulting in finite-size, long-lived clusters, characterized by a quite narrow size distribution. Particularly, the adsorption of highly charged linear polyelectrolytes on oppositely charged colloidal particles is strongly correlated and some short-range order arises from competing electrostatic interactions between like-charged polymer chains (repulsion) and between polymer chains and particle surface (attraction). In these systems, in an interval of concentrations around the isoelectric point, relatively large clusters of polyelectrolyte-decorated particles form. However, the mechanisms that drive the aggregation and stabilize, at the different polymer/particle ratios, a well-defined size of the aggregates are not completely understood. Nor is clear the role that the correlated polyion adsorption plays in the aggregation, although the importance of "patchy interactions" has been stressed as the possible source of attractive interaction term between colloidal particles. Different models have been proposed to explain the formation of the observed cluster phase. However, a central question still remains unanswered, i.e., whether the clusters are true equilibrium or metastable aggregates. To elucidate this point, in this work, we have investigated the effect of the temperature on the cluster formation. We employed liposomes built up by DOTAP lipids interacting with a simple anionic polyion, polyacrylate sodium salt, over an extended concentration range below and above the isoelectric condition. Our results show that the aggregation process can be described by a thermally activated mechanism.