Frédéric Nallet

12-Hydroxystearic acid lipid tubes under various experimental conditions

There is a growing interest for constructing supramolecular hollow tubes from amphiphilic molecules. Aqueous solutions of the ethanolamine salt of 12-hydroxystearic acid are known to form tubes of several tens of micrometers in length with a temperature-tunable diameter. However, the phase behavior of this system has not been fully studied. Herein, we report the variation of various physico-chemical parameters on the self-assembling properties of this system. The effects of the ionic strength, ethanol, doping with other lipids, pH, concentration, and the fatty acid/ethanolamine molar ratio R were investigated by both phase-contrast microscopy and DSC. We observed the formation of tubes in a wide range of parameters. For instance, the molar ratio R can be modified from 2/3 to 5/2 without altering the formation of tubes. In some but not all cases, the tube diameter still varied with temperature. These findings show that tubes form under various experimental conditions. This should increase the interest in producing such self-assemblies from low-cost fatty acids.

Morphology of open films of discotic hexagonal columnar liquid crystals as probed by grazing incidence X-ray diffraction

The structure and the orientation of thermotropic hexagonal columnar liquid crystals are studied by grazing incidence X-ray diffraction (GIXD) for different discotic compounds in the geometry of open supported thin films. Whatever the film deposition mode (either spin-coating or vacuum evaporation) and the film thickness, a degenerate planar alignment with the liquid crystalline columns parallel to the substrate is found. However, if a specific thermal process is applied to the liquid crystal film, homeotropic anchoring (columns normal to the interface) can be stabilized in a metastable state.

Superflexibility of graphene oxide

Significance Bending of a thin plate simultaneously involves contraction and stretching of matter relative to a neutral plane, and tensile rigidity dictates the ability of a thin platelet to be bent. If graphene or graphene oxide (GO) were actually behaving as thin platelets, they would display high bending rigidity. Bending measurements for atomic monolayers remain particularly challenging because of their difficult manipulation. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. Amazingly, the high stiffness of GO is associated with an unexpected low bending modulus. Graphene oxide (GO), the main precursor of graphene-based materials made by solution processing, is known to be very stiff. Indeed, it has a Young’s modulus comparable to steel, on the order of 300 GPa. Despite its very high stiffness, we show here that GO is superflexible. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. Characterizations are performed by the combination of synchrotron X-ray diffraction at small angles and in situ rheology (rheo-SAXS) experiments using the high X-ray flux of a synchrotron source. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. This superflexibility compares with the fluidity of self-assembled liquid bilayers. This behavior is discussed by considering the mechanisms at play in bending and stretching deformations of atomic monolayers. The superflexibility of GO is a unique feature to develop bendable electronics after reduction, films, coatings, and fibers. This unique combination of properties of GO allows for flexibility in processing and fabrication coupled with a robustness in the fabricated structure.

Sequestration of Proteins by Fatty Acid Coacervates for Their Encapsulation within Vesicles

Encapsulating biological materials in lipid vesicles is of interest for mimicking cells; however, except in some particular cases, such processes do not occur spontaneously. Herein, we developed a simple and robust method for encapsulating proteins in fatty acid vesicles in high yields. Fatty acid based, membrane-free coacervates spontaneously sequester proteins and can reversibly form membranous vesicles upon varying the pHu2005value, the precrowding feature in coacervates allowing for protein encapsulation within vesicles. We then produced enzyme-enriched vesicles and show that enzymatic reactions can occur in these micrometric capsules. This work could be of interest in the field of synthetic biology for building microreactors.

Multilamellar liposomes entrapping aminosilane-modified maghemite nanoparticles: “magnetonions”

4.6 nm-sized aminosilane-modified maghemite (γ-Fe(2)O(3)) nanoparticles (aMNPs) were synthesized and encapsulated into onion-type multilamellar vesicles of soybean phosphatidylcholine (90%mol) and monoolein (10%mol). The magnetic multilamellar vesicles were obtained by shearing lipids with an aqueous dispersion of the preformed aMNPs (ferrofluid). The influence of ferrofluid concentration on the stability of the constitutive lamellar phase and the resulting dispersed onions was analyzed by small-angle X-ray diffraction (SAXD) and cryo-TEM imaging, respectively. When [Fe(III)] <60 mM, stable, magnetic onions were produced with aMNPs inserted inside onion water compartments as isolated or aggregated particles. Encapsulation efficiencies were measured by EPR spectroscopy and magnetic measurements: much higher values (up to 75%) than unilamellar liposomes were found. The deduced aMNP-to-onion ratio increased with ferrofluid concentration before reaching a maximal value of ca. 45 as confirmed by cryo-TEM imaging. When [Fe(III)] >60 mM, uni- or oligolamellar vesicles in addition to onions formed, probably because of a two-phase separation between an aMNP-rich phase and an aMNP-containing lamellar phase as revealed by SAXD.

Confinement-induced phase transition in a DNA-lipid hydrated complex

We study the effect of the soft confinement by fluid lipid bilayers on the spatial organisation of DNA molecules in a DNA-zwitterionic lipid hydrated lamellar complex. The confinement is increased by dehydrating the complex in a controlled way, which leads to a decrease of the water channel thickness separating the periodically stacked bilayers. Using grazing-incidence small-angle X-ray scattering on an oriented thin film, we probe in situ as dehydration proceeds the structure of the DNA-lipid complex. A structural phase transition is evidenced, where an apparently disordered phase of DNA rods embedded within the one-dimensionally ordered lipid lamellar phase observed at high hydration is replaced by a 2D hexagonal structure of DNA molecules intercalated between the lipid bilayers.

Clouding in fatty acid dispersions for charge-dependent dye extraction

The clouding phenomenon in non-ionic surfactant systems is a common feature that remains rare for ionic detergents. Here, we show that fatty acid (negatively charged) systems cloud upon cooling hot dispersions depending on the concentration or when adding excess guanidine hydrochloride. The clouding of these solutions yields the formation of enriched fatty acid droplets in which they exhibit a polymorphism that depends on the temperature: upon cooling, elongated wormlike micelles transit to rigid stacked bilayers inside droplets. Above this transition temperature, droplets coalesce yielding a phase separation between a fatty acid-rich phase and water, allowing extraction of dyes depending on their charge and lipophilicity. Positively charged and zwitterionic dyes were sequestered within the droplets (and then in the fatty acid-rich upper phase) whereas the negatively charged ones were found in both phases. Our results show an additional case of negatively charged surfactant which exhibit clouding phenomenon and suggest that these systems could be used for extracting solutes depending on their charge and lipophilicity.

Nanoparticles of the poly([N-(2-hydroxypropyl)]methacrylamide)-b-poly[2-(diisopropylamino)ethyl methacrylate] diblock copolymer for pH-triggered release of paclitaxel

The potential of self-assembled nanoparticles (NPs) containing the fine tunable pH-responsive properties of the hydrophobic poly[2-(diisopropylamino)ethyl methacrylate] (PDPA) core and the protein repellence of the hydrophilic poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA) shell for in vitro cytostatic activity has been explored on cancer cells. The amphiphilic diblock copolymer poly[N-(2-hydroxypropyl)methacrylamide]-b-poly[2-(diisopropylamino)ethyl methacrylate] (PHPMA-b-PDPA) synthesized by a reversible addition–fragmentation chain transfer (RAFT) technique allows for excellent control of the polymer chain length for methacrylamides. The PHPMA-b-PDPA block copolymer dissolved in an organic solvent (ethanol/dimethylformamide) undergoes nanoprecipitation in phosphate buffer saline (PBS, pH ∼ 7.4) and self-assembles into regular spherical NPs after solvent elimination. The NPs’ structure was characterized in detail by dynamic (DLS), static (SLS) and electrophoretic (ELS) light scattering, small angle X-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM). The PHPMA chains prevented the fouling of proteins resulting in a remarkable stability of the NPs in serum. On decreasing pH the hydrophobic PDPA block becomes protonated (hydrophilised) in a narrow range of pH (6.51 < pH < 6.85; ΔpH ∼ 0.34) resulting in the fast disassembly of the NPs and chemotherapeutic drug release in a simulated acidic environment in endosomal and lysosomal compartments. A minimal amount of drug was released above the threshold pH of 6.85. The in vitro cytotoxicity studies showed an important increase in the activity of the NPs loaded with drug compared to the free drug. The particles size below the cut-off size of the leaky pathological vasculature (less than 100 nm), the excellent stability in serum and the ability to release a drug at the endosomal pH with concomitant high cytotoxicity make them suitable candidates for cancer therapy, namely for treatment of solid tumours exhibiting high tumor accumulation of NPs due to the Enhanced Permeability and Retention (EPR) effect.

Supramolecular polymorphism of DNA in non-cationic Lα lipid phases

The structure of a complex between hydrated DNA and a non-cationic lipid is studied, including its phase diagram. The complex is spontaneously formed by adding DNA fragments (ca. 150 base pairs in length) to non-cationic lipids and water. The self-assembly process often leads to highly ordered structures. The structures were studied by combining X-ray scattering, fluorescence and polarized microscopy, as well as freeze-fracture experiments with transmission electron microscopy. We observe a significant increase of the smectic order as DNA is incorporated into the water layers of the lamellar host phase, and stabilization of single phase domains for large amounts of DNA. The effect of confinement on DNA ordering is investigated by varying the water content, following three dilution lines. A rich polymorphism is found, ranging from weakly correlated DNA-DNA in-plane organizations to highly ordered structures, where transmembrane correlations lead to the formation of columnar rectangular and columnar hexagonal superlattices of nucleotides embedded between lipid lamellae. From these observations, we suggest that addition of DNA to the lamellar phase significantly restricts membrane fluctuations above a certain concentration and helps the formation of the lipoplex. The alteration of membrane steric interactions, together with the appearance of interfacial interactions between membranes and DNA molecules may be a relevant mechanism for the emergence of highly ordered structures in the concentrated regime.

Confined diffusion of hydrophilic probes inserted in lyotropic lamellar phases

Abstract.The dynamic behaviour of three hydrophilic probes (two dyes and one fluorescently-labelled protein) inserted in the water layers of lyotropic lamellar phases has been studied by confocal fluorescence recovery experiments. Two different, ionic (AOT/NaCl/ H2O and non-ionic ( C12E5 /hexanol/ H2O host systems were studied. The confinement effect has been carefully monitored using the swelling properties of the lamellar phases. In all cases, we measure the evolution of the probe diffusion coefficient in the layer plane D⊥versus the separation between the membranes dw. Depending on the composition of the lamellar phase, this distance can be continuously adjusted from 500Å to about 20Å. For all systems, we observe a first regime, called dilute regime, where the diffusion coefficient decreases almost linearly with 1/dw . In this regime, the Faxén theory for the friction coefficient of a spherical particle symmetrically dragged between two rigid walls can largely explain our results. More unexpectedly, when the membranes are non-ionic, and also quite flexible ( C12E5 /hexanol in water), we observe the existence of a second, concentrated (or confined) regime, where the diffusion coefficient is nearly constant and different from zero for membrane separations smaller than the particle size. This new regime can be heuristically explained by simple arguments taking into account the membrane fluidity.