Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

Titanium oxide is a biocompatible material that supports vesicle adhesion. Depending on experimental parameters, adsorbed vesicles remain intact or rupture spontaneously. Vesicle rupture has been attributed to electrostatic attraction between vesicles and titanium oxide, although the relative contribution of various interfacial forces remains to be clarified. Herein, we investigated the influence of vesicle surface charge on vesicle adsorption onto titanium oxide and observed that electrostatic attraction is insufficient for vesicle rupture. Following this line of evidence, a continuum model based on the DLVO forces and a non-DLVO hydration force was applied to investigate the role of different interfacial forces in modulating the lipid-substrate interaction. Within an experimentally significant range of conditions, the model shows that the magnitude of the repulsive hydration force strongly influences the behavior of adsorbed vesicles, thereby supporting that the hydration force makes a strong contribution to the fate of adsorbed vesicles on titanium oxide. The findings are consistent with literature reports concerning phospholipid assemblies on solid supports and nanoparticles and underscore the importance of the hydration force in influencing the behavior of phospholipid films on hydrophilic surfaces.

An amphipathic α-helical (AH) peptide was recently discovered that can rupture the lipid envelope of many viruses including HIV, hepatitis C, dengue, and herpes simplex. Despite its broad-spectrum activity, the AH peptide specifically targets small viruses only and does not affect large viruses. Indirect observations of virus size-specific targeting have been confirmed in a model system comprised of intact lipid vesicles on a gold substrate. Depending on vesicle size, AH peptide can promote vesicle rupture, but the mechanism by which vesicle size influences the rupture process remains to be elucidated. Herein, using the dynamic light scattering and quartz crystal microbalance with dissipation techniques, we have combined experiment and theory to understand the effects of vesicle size on the interaction between the AH peptide and vesicles. We identified that the AH peptide-binding interaction can induce a structural rearrangement of the vesicle's lipid bilayer, which occurs independently of vesicle size. Kinetic analysis also revealed that AH peptide-binding occurs cooperatively for small vesicles only. Binding cooperativity is consistent with pore formation leading to vesicle rupture. By contrast, for large vesicles, AH peptide-binding is noncooperative and does not cause vesicle rupture, suggesting that the binding interaction occurs via a different mechanism. Compared to previous estimates that AH peptide is most effective against viruses with a diameter of less than 70 nm, our evidence validates that AH peptide may target a wider size range of enveloped viruses up to 160 nm in diameter. Taken together, our findings provide a quantitative rationale to understand the targeting specificity of AH peptide as a broad-spectrum antiviral drug candidate.

Tunable nanoplasmonic biosensing for lipid and protein applications is reported based on controlling lipid membrane architecture on surfaces. The interaction of a peptide with lipid membranes is highly sensitive to the membrane architecture on top of plasmonic nanodisks, and the measurement response varies in a manner which is consistent with the surrounding lipid environment.

Planar lipid bilayers on solid supports provide a controllable platform to mimic biological membranes. Adsorption and spontaneous rupture of vesicles is the most common method to form planar bilayers. While many substrates support vesicle adsorption, vesicles rupture spontaneously on only a few materials. In order to form planar bilayers on materials intractable to conventional vesicle fusion, an amphipathic, α-helical (AH) peptide has been identified that can rupture adsorbed vesicles and form planar bilayers on previously intractable materials. Most studies using AH peptide have employed zwitterionic lipid compositions only, and the range of suitable lipid compositions remains to be elucidated. Herein, using quartz crystal microbalance-dissipation and ellipsometry, we investigated the effects of membrane surface charge on AH peptide-mediated bilayer formation via the rupture of surface-adsorbed vesicles on titanium oxide. Our findings demonstrate that AH peptide can promote the formation of positively and negatively charged bilayers. Importantly, the kinetics of vesicle rupture by AH peptide are strongly influenced by the membrane surface charge. Although the titanium oxide surface is negatively charged, the formation of negatively charged bilayers was quickest among the tested lipid compositions. Taken together, the experimental data supports that the effects of membrane surface charge on the rupture kinetics are related to variations in the extent of vesicle destabilization prior to vesicle rupture. Given the wide range of lipid compositions amenable to AH peptide-mediated vesicle rupture, this work further suggests that AH peptide is largely unique among membrane-active peptides, thereby substantiating its position as a promising broad-spectrum antiviral agent.

Vesicle adhesion and fusion to interfaces are frequently used for the construction of biomimetic surfaces in biosensors and drug delivery. Ubiquitous in cell biology, vesicle fusion involves the transformation of two separate membranes into one contiguous lipid bilayer. In distinction, the deposition of vesicle membranes to hydrophobic surfaces requires the transformation of a lipidic bilayer into a monomolecular layer – a topologically distinct process termed hemifusion. Here, we used hydrophobically terminated self-assembled monolayers (SAMs) on solid surfaces to track the hemifusion of fluorescently labeled giant unilamellar vesicles (GUVs) at the single vesicle level with video time resolution (≈53 ms). We observed that a dilute monolayer, consisting of lipid extracted from the outer GUV leaflet, spreads outward across the hydrophobic surface from the vesicle adhesion site. Subsequently, bilayer hemifusion occurs by vesicle rupture near the hydrophobic surface, followed by spreading of lipid in a dense monolayer. GUV lipids thus transfer to the SAM surface in two concentric zones: an outer hemifusion zone comprises lipids drawn from the outer GUV leaflet and an inner hemifusion zone comprises lipids from both the inner and outer GUV leaflets and grows at a rate of ≈1000 µm2 s−1 (dA/dt = 970 ± 430 µm2 s−1 in n = 22 independent experiments). This growth rate is quantitatively consistent with the assumption that the spreading of the monolayer is entirely driven by the difference in surface energies of the hydrophobic and the lipid-covered SAM surfaces, which is dissipated by friction of the spreading monolayer on the SAM. Lipid transfer between the inner and outer GUV leaflets occurs via a hemifusion pore that forms early in the process near the membrane contact site. This pore also permits expulsion of water from the GUV interior as the vesicle contracts onto the contact site.

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A biosensing platform is presented by N.‐J. Cho and co‐workers that utilizes plasmonic nanodisk sensors for detecting lipids and proteins. The image shows circular gold nanodisks deposited on a glass substrate, as directly observed by atomic force microscopy. On page 4828, the nanodisks appear as dark yellow spots and the glass substrate underneath is blue. The platform is suitable for the fabrication of different types of lipid membrane architecture, enabling tunable measurement responses. Platforms of this kind have strong potential for lab‐on‐a‐chip and diagnostic applications.