Björn Lindman

Vesicle-Templated Layer-by-Layer Assembly for the Production of Nanocapsules

Hollow structures on the submicrometer scale (nm) are obtained with the assembly of polyelectrolytes according to the layer-by-layer (LbL) technique. Following the LbL procedure, polymers alginate and chitosan were alternatively adsorbed on a vesicular template made of didodecyldimethylammonium bromide (DDAB). Evidence for the removal of the vesicular template entrapped in the alginate/chitosan film is presented. The removal of the vesicular template was achieved through interactions between a nonionic surfactant (Triton X100) and the double-chained surfactant forming the vesicles. The application of this approach allowed the production of hollow nanospheres with a mild procedure, avoiding the use of strong acids or other extreme working conditions that can modify the shell integrity. The obtained nanostructures were characterized by means of dynamic light scattering (DLS), the zeta potential, and scanning electron microscopy (SEM). The SEM analysis demonstrated the presence, after the core removal process, of nanocapsules indistinguishable in size and shape from the parent core-shell system. The analysis of the surface charge of the hollow nanocapsules, after the core dissolution, by zeta potential measurements, indicates good aggregate stability. DLS measurements showed that the size of the nanocapsules is on the order of hundreds of nanometers. Moreover, the size of both the core-shell and the hollow particles did not appear to be perturbed by variations in temperature or ionic strength.

Fluorescence studies of polymer–surfactant association

Abstract Fluorescence spectroscopy has been successfully used for the study of central issues of solutions of surfactants and associating polymers. Different fluorescence techniques and methods are uniquely adapted to investigate problems in this field and can, by using extrinsic or intrinsic probes, provide information on molecular association, microstructure and molecular dynamics. This constitutes an important contribution to the understanding and control of macroscopic properties, as well as to their biological functions and technical applications. Important aspects of these mixed systems, related to their self-assembly, are: formation of micelles and hydrophobic microdomains in general; size and shape of surfactant molecular aggregates; formation and stability of vesicles; intra- vs. intermolecular association in polymers; conformational changes in polymers as affected by polymer–surfactant association; surfactant organization in adsorbed layers; kinetic aspects of the formation and disintegration of self-assembly structures; residence times of molecules in microdomains and migration of active molecules. Some of these issues will be addressed in this paper.

Phase behavior of polymer-surfactant systems in relation to polymer-polymer and surfactant-surfactant mixtures

Novel phase diagrams of systems of water and two cosolutes of colloidal size, either macromolecules or surfactant micelles, are presented. For a mixture of two oppositely charged surfactants, a complex phase diagram is obtained with several liquid crystalline phases and equilibrium vesicles. There is a strong tendency for two surfactants to mix and form a range of structures governed by geometrical packing and electrostatic interactions. In recent years, surfactant self-assembly in the presence of different polymers has attracted a great interest, both from fundamental and applied aspects. Attractive or repulsive interactions are observed depending on the system. For the former case, dilute solutions may be analysed in terms of a binding of the surfactant to the polymer or a depression of the critical micelle concentration of the surfactant by the polymer. An important feature of these solutions is thus that the surfactant molecules, also when interacting intimately with a polymer, give micellar-type structures. The phase behavior of polymer-surfactant systems has only recently attracted greater attention but has been shown most significant for the understanding of the interactions involved. Different types of phase separation phenomena are encountered including segregative and associative types. For systems of a polyelectrolyte and an oppositely charged surfactant, an associative interaction is observed leading to phase separation into one solution concentrated in both polymer and surfactant and one very dilute solution. In the presence of an electrolyte, phase separation may be eliminated and, at higher concentrations, a polymer incompatibility type of phase separation may result. It is found fruitful to analyse the phase diagrams of polymer-surfactant systems with those of polymer-polymer and surfactant-surfactant mixtures as a basis. Analogies and differences are discussed and it is found that polymer-surfactant systems show basic similarities to polymer-polymer systems, while surfactant mixtures are different, which is due to the exchange of surfactant molecules between micelles and the formation of mixed micelles and other aggregates. Surfactant mixtures are, therefore, not displaying a segregative type of phase separation.

Role of linker groups between hydrophilic and hydrophobic moieties of cationic surfactants on oligonucleotide-surfactant interactions.

The interaction between DNA and amino-acid-based surfactants with different linker groups was investigated by gel electrophoresis, ethidium bromide exclusion assays, circular dichroism, and melting temperature determinations. The studies showed that the strength of the interaction between the oligonucleotides and the surfactants is highly dependent on the linker of the surfactant. For ester surfactants, no significant interaction was observed for surfactant-to-DNA charge ratios up to 12. On the other hand, amide surfactants were shown to interact strongly with the oligonucleotides; these surfactants could displace up to 75% of the ethidium bromide molecules bound to the DNA and induced significant changes in the circular dichroism spectra. When comparing the headgroups of the surfactants, it was observed that surfactants with more hydrophobic headgroups (proline vs alanine) interacted more strongly with the DNA, in good agreement with previous studies.

Novel Biocompatible DNA Gel Particles

Surfactants with the cationic functionality based on an amino acid structure have been used to prepare novel biocompatible devices for the controlled encapsulation and release of DNA. We report here the formation of DNA gel particles mixing DNA (either single- (ssDNA) or double-stranded (dsDNA)) with two different single-chain amino acid-based surfactants: arginine-N-lauroyl amide dihydrochloride (ALA) and N(alpha)-lauroyl-arginine-methyl ester hydrochloride (LAM). The degree of DNA entrapment, the swelling/deswelling behavior, and the DNA release kinetics have been studied as a function of both the number of charges in the polar head of the amino acid-based surfactant and the secondary structure of the nucleic acid. Analysis of the data indicates a stronger interaction of ALA with DNA, compared with LAM, mainly attributed to the double charge carried by the former surfactant compared to the singly charged headgroup of the latter species. The stronger interaction with amphiphiles for ssDNA compared with dsDNA suggests the important role of hydrophobic interactions in DNA. Data on the microstructure of the complexes obtained from small-angle X-ray scattering (SAXS) of the particles strongly suggests a hexagonal packing. It was found that, the shorter the lattice parameter, the stronger the surfactant-DNA interaction and the slower the DNA release kinetics. Complexation and neutralization of DNA on the DNA gel particles was confirmed by agarose gel electrophoresis measurements.

Mixed Protein Carriers for Modulating DNA Release

Aqueous mixtures of oppositely charged polyelectrolytes undergo associative phase separation, resulting in coacervation, gelation, or precipitation. This phenomenon has been exploited in forming DNA gel particles by interfacial diffusion. We report here the formation of DNA gel particles by mixing solutions of double-stranded DNA with aqueous solutions containing two cationic proteins, lysozyme and protamine sulfate. The effect of the lysozyme/protamine ratio on the degree of DNA entrapment, surface morphology, swelling-deswelling behavior, and kinetics of DNA release has been investigated. By mixing the two proteins, we obtain particles that display higher loading efficiency and loading capacity values, in comparison to those obtained in single-protein systems. Examination of the release profiles has shown that in mixed protein particles, complex, dual-stage release kinetics is obtained. The overall release profile is dependent on the lysozyme/protamine ratio. The obtained profiles, or segments of them, are accuratelly fitted using the zero-order and first-order models, and the Weibull function. Fluorescence microscopy studies have suggested that the formation of these particles is associated with the conservation of the secondary structure of DNA. This study presents a new platform for controlled release of DNA from DNA gel particles formed by interfacial diffusion.

Controlling the Morphology in DNA Condensation and Precipitation

This work addresses the influence of solution inhomogeneity on conformation, aggregation, and coil/globule and bundle/single chain coexistence of T4 DNA molecules. The inhomogeneity is induced by mixing two solutions containing, respectively, protamine and DNA, with different relative concentrations, but aiming at producing the same final concentrations. The study was conducted by means of fluorescence microscopy (FM), complemented with scanning electron microscopy (SEM). It is shown that the degree of precipitation, the structures formed, and the relative population of compacted and unfolded structures are highly dependent on the method of preparation of the mixtures that contain the DNA/protamine complexes. Most of the structures reported in the literature, that is, overcharged/undercharged globules, toroids, chains internally segregated, and bundles composed of several chains were observed in our different mixtures of fixed final concentration.

The effect of postadded ethylene glycol surfactants on DNA-cationic surfactant/water mesophases.

The addition of amphiphiles grafted with polyethylene glycol units to constructs of DNA-amphiphiles is of most relevance for applications demanding colloidal stability. In this work, we study the self-assembly behavior of a true ternary mixture comprising (i) an electroneutral complex of DNA and a cationic surfactant (dodecyltrimethylammonium, DTA), (ii) water, and (iii) nonionic surfactant (dodecyl tetraethylene glycol, C12EO4; and dodecyl octaethylene glycol, C12EO8). The phase diagrams of the two systems: DNA-DTA/C12EO4/water and DNA-DTA/C12EO8/water were carried out using 2H NMR, and small-angle X-ray scattering (SAXS). In both mixtures, the DNA-DTA complex incorporates the postadded nonionic surfactant and large liquid crystalline regions were found. The supramolecular assemblies evolve from a 2D hexagonal structure of the normal type to a lamellar phase as the content of nonionic surfactant is increased. The effect of ethylene glycol unit size in the phase behavior is discussed. We suggest that when longer ethylene glycol units (C12EO8 vs C12EO4) are used, the DNA-DTA aggregate gets saturated with the nonionic surfactant and there exists a coexistence of a fully swollen mesophase phase of C12EO8 alone presumably of the normal hexagonal type with the lamellar and hexagonal phases of DNA-DTA/C12EO8/water.

Macroscopic phase separation in a polyelectrolyte gel interacting with oppositely charged surfactant : correlation between anomalous deswelling and microstructure

The volume transitions in cross-linked sodium polyacrylate gels following the absorption of cetyltrimethylammonium bromide from aqueous solutions is studied. The microstructure of the gels is investigated by means of time-resolved fluorescence quenching, small-angle X-ray scattering, and optical anisotropy. In general, the volume of the pre-swelled gels decreases linearly with the number of absorbed surfactant molecules. The shrinking is due to a collapse of the exterior parts of the gels where essentially all the surfactant is located. The surface phase is very dense and is made up from collapsed micelle/polyelectrolyte complexes arranged on a cubic lattice. The surfactant aggregation number is between 110 and 170. At sufficiently high surfactant-to-polymer ratios the entire gel is in a collapsed single phase state with the same micro-structure as the surface phase. It is observed that during the transition from the swollen to the fully collapsed state the gels may be trapped in a semi-swollen state, where the gels have a balloon-like shape due to a pressure across the surface phase. There is a strong correlation between this anomalous behavior and the growth of the micelles from globular to long cylinders and a transition from cubic to hexagonal packing. The observations are compared with the macroscopic surface phases observed during phase transitions in related systems.

Interactions between DNA and Nonionic Ethylene Oxide Surfactants are Predominantly Repulsive

In the present work, the interactions between double-stranded (ds) or single-stranded (ss) DNA and nonionic ethylene oxide (EO) surfactants, with special attention to the possible contributions from hydrophobic interactions, have been investigated using a multitechnique approach. It was found that the presence of ss as well as dsDNA induces a slight decrease of the cloud point of pentaethylene glycol monododecyl ether (C(12)E(5)). Assessment of the partitioning of DNA between the surfactant-rich and surfactant-poor phases formed above the cloud point showed that the polymer was preferably located in the surfactant-poor phase. Surface tensiometry experiments revealed that neither of the DNA forms induced surfactant micellization. Finally, it was shown by DNA melting measurements that another EO surfactant (C(12)E(8)) did not affect the relative stabilities of ss and dsDNA. To summarize, all experiments suggest that the net interaction between DNA and nonionic surfactants of the EO type is weakly repulsive, which can be attributed mainly to steric effects. In general, the results were practically identical for the ds and ss forms of DNA, except those from the cloud point experiments, where the decrease of the cloud point was less pronounced with ssDNA. This finding indicates the presence of an attractive component in the interaction, which can reasonably be ascribed to hydrophobic effects.