Liposomes tethered to a biopolymer film through the hydrophobic effect create a highly effective lubricating surface

Hydrophobically modified chitosan biopolymer connects halloysite nanotubes at the oil-water interface as complementary pair for stabilizing oil droplets

The integration of cationic and hydrophobic functionalities into hydrophobically modified chitosan (HMC) biopolymer facilitates complementary emulsion stabilization with negatively charged halloysite clay nanotubes (HNT). Oil-in-water emulsions with smaller droplet sizes and significantly improved interfacial resistance to droplet coalescence are obtained on complementary emulsion stabilization by HNT and HMC compared to the individual emulsifiers alone. Contact angle measurements shows that the adsorption of the cationic HMC onto the negatively charged HNT modifies the surface wettability of the nanotubes, facilitating the attachment of the nanotubes to the oil-water interface. High resolution cryo-SEM imaging reveals that free HMC chains locks the nanotubes together at the oil-water interface, creating a high barrier to droplet coalescence. The emulsion stability is an order of magnitude higher for conditions in which the aqueous HNT dispersion is stabilized by the HMC compared to conditions where the negatively charged HNT is strongly flocculated by the cationic HMC. The hydrophobic interaction between HMC chains, insertion of HMC hydrophobes into the oil phase and electrostatic interactions between HMC and HNT are proposed as key mechanisms driving the increased emulsion stability. For potential application as a dispersant system for crude oil spill treatment, the nanotubular morphology of HNT was further exploited for the encapsulation of the water-insoluble surfactant, sorbitan monooleate (Span 80). The HMC and HNT sterically strengthens the oil-water interfacial layer while release of the Span 80 surfactant from the HNT lumen lowers the oil-water interfacial tension. The concepts advanced here are relevant in the development of environmentally-benign dispersants for oil spill remediation.

Spontaneous Formation of Stable Vesicles and Vesicle Gels in Polar Organic Solvents

The self-assembly of lipids into nanoscale vesicles (liposomes) is routinely accomplished in water. However, reports of similar vesicles in polar organic solvents like glycerol, formamide, and ethylene glycol (EG) are scarce. Here, we demonstrate the formation of nanoscale vesicles in glycerol, formamide, and EG using the common phospholipid lecithin (derived from soy). The samples we study are simple binary mixtures of lecithin and the solvent, with no additional cosurfactants or salt. Lecithin dissolves readily in the solvents and spontaneously gives rise to viscous fluids at low lipid concentrations (∼2-4%), with structures ∼200 nm detected by dynamic light scattering. At higher concentrations (>10%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology confirms the elastic response of gels, with their elastic modulus being ∼20 Pa at ∼10% lipid. Images from cryo-scanning electron microscopy (cryo-SEM) indicate that concentrated samples are "vesicle gels," where multilamellar vesicles (MLVs, also called "onions"), with diameters between 50 and 600 nm, are close-packed across the sample volume. This structure can explain both the elastic rheology as well as the static birefringence of the samples. The discovery of vesicles and vesicle gels in polar solvents widens the scope of systems that can be created by self-assembly. Interestingly, it is much easier to form vesicles in polar solvents than in water, and the former are stable indefinitely, whereas the latter tend to aggregate or coalesce over time. The stability is attributed to refractive index-matching between lipid bilayers and the solvents, i.e., these vesicles are relatively "invisible" and thus experience only weak attractions. The ability to use lipids (which are "green" or eco-friendly molecules derived from renewable natural sources) to thicken and form gels in polar solvents could also prove useful in a variety of areas, including cosmetics, pharmaceuticals, and lubricants.

Boundary Lubrication, Hemifusion, and Self-Healing of Binary Saturated and Monounsaturated Phosphatidylcholine Mixtures ⧫

A wide range of phosphatidylcholine (PC) lipids with different degrees of unsaturation has been identified in the human synovial fluid and on the cartilage surface. The outstanding lubricity of the articular cartilage surface has been attributed to boundary layers comprising complexes of such lipids, though to date, only lubrication by single-component PC-lipid-based boundary layers has been investigated. As distinguishable lubrication behavior has been found to be related to the PC structures, we herein examined the surface morphology (on mica) and the lubrication ability of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs) prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good boundary lubrication (friction coefficients ≤ ca. 10^-4) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ± 1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Antimicrobial sponge prepared by hydrophobically modified chitosan for bacteria removal

Hydrophobically modified chitosan (HMCS) prepared by reacting chitosan with dodecyl aldehyde can generate very stable foam when dissolved in mild acidic condition under vigorous mechanical stirring. A durable and lightweight (density of 32 mg/ml) sponge was obtained by freeze-drying the stably formed HMCS foam. In addition to the cationic nature of chitosan, the grafted C₁₂ alkyl chains were also able to help HMCS sponge for capturing E. coli cells (∼4.0 × 10⁸ cells/mg sponge) by intercalating into the outer membrane of E. coli cells. E. coli cells captured on HMCS sponge were found to be mostly dead and easily released into the bulk solution so that the active surface could be continuously regenerated for capturing and killing the rest of alive cells. In comparison with its counterpart (chitosan sponge), HMCS sponge maintained a higher operational stability for the removal of E. coli cells. After 5 repeated cells removal operation, the removal capacity of HMCS sponge could be regenerated back to >90% by thorough washing with ethanol.

Cells capture and antimicrobial effect of hydrophobically modified chitosan coating on Escherichia coli

Hydrophobically modified chitosan (HMCS), prepared by reacting alkyl aldehyde with chitosan was demonstrated to be an effective antimicrobial and transparent coating. The grafted alkyl chains exist as protruded hydrophobic tails on the HMCS coating surface. In contact with E. coli cells, HMCS coating captured and immobilized the cells via these hydrophobic tails. The hydrophobic tails could also kill the cells captured on the coating surface as visualized by fluorescence microscope. More than 50% of the initially loaded cells (2.5×10⁴ CFU) could be killed after 2h contact with HMCS coating. The cells capture and killing effects of the coating surface could be completely neutralized by treating with α-cyclodextrin to sequester the protruded hydrophobic tails. The facile coating of antimicrobial HMCS on surface also enabled the easy fabrication of patterned E. coli cells arrays.

Spatially directed vesicle capture in the ordered pores of breath-figure polymer films

This work describes a new method to selectively capture liposomes and other vesicle entities in the patterned pores of breath-figure polymer films. The process involves the deposition of a hydrophobe containing biopolymer in the pores of the breath figure, and the tethering of vesicles to the biopolymer through hydrophobic interactions. The process is versatile, can be scaled up and extended to the deposition of other functional materials in the pores of breath figures.