Influence of osmotic pressure on adhesion of lipid vesicles to solid supports

Exploring the Interfacial Dynamics of Unilamellar and Multilamellar Cationic Liposomes on SiO2 and Their Interactions with Membrane-Active Peptide

Understanding the interplay between lipid assemblies and solid supports is crucial for advancing model membrane systems and biomedical applications. This study investigates the interfacial behaviors of unilamellar and multilamellar cationic liposomes on silicon dioxide and their interactions with a membrane-active AH peptide. Using QCM-D monitoring, unilamellar liposomes were found to rapidly form SLBs through one-step adsorption kinetics, whereas multilamellar liposomes exhibited slower adsorption. Further addition of liposomes caused fusogenic interactions with SLBs, where multilamellar liposomes formed more rigid lipid membranes. Upon AH peptide exposure, unilamellar-based lipid membranes showed higher susceptibility to structural transformations, achieving complete SLB formation, while multilamellar-based lipid membranes displayed reduced sensitivity and retained residual viscoelastic components, indicative of incomplete SLB formation. These findings underscore the significant influence of liposome lamellarity on their interfacial dynamics and peptide interactions, crucial for designing effective lipid-based delivery and sensing systems.

Distance tuneable integral membrane protein containing floating bilayers via in situ directed self-assembly

Model membranes allow for structural and biophysical studies on membrane biochemistry at the molecular level, albeit on systems of reduced complexity which can limit biological accuracy. Floating supported bilayers offer a means of producing planar lipid membrane models not adhered to a surface, which allows for improved accuracy compared to other model membranes. Here we communicate the incorporation of an integral membrane protein complex, the multidomain β-barrel assembly machinery (Bam), into our recently developed in situ self-assembled floating supported bilayers. Using neutron reflectometry and quartz crystal microbalance measurements we show this sample system can be fabricated using a two-step self-assembly process. We then demonstrate the complexity of the model membrane and tuneability of the membrane-to-surface distance using changes in the salt concentration of the bulk solution. Results demonstrate an easily fabricated, biologically accurate and tuneable membrane assay system which can be utilized for studies on integral membrane proteins within their native lipid matrix.

Kinetics of Ganglioside-Rich Supported Lipid Bilayer Formation with Tracer Vesicle Fluorescence Imaging

Gangliosides, forming a class of lipids complemented by sugar chains, influence the lateral distribution of membrane proteins or membrane-binding proteins, act as receptors for viruses and bacterial toxins, and mediate several types of cellular signaling. Gangliosides incorporated into supported lipid bilayers (SLBs) have been widely applied as a model system to examine these biological processes. In this work, we explored how ganglioside composition affects the kinetics of SLB formation using the vesicle rupturing method on a solid surface. We imaged the attachment of vesicles and the subsequent SLB formation using the time-lapse total internal reflection fluorescence microscopy technique. In the early phase, the ganglioside type and concentration influence the adsorption kinetics of vesicles and their residence/lifetime on the surface before rupturing. Our data confirm that a simultaneous rupturing of neighboring surface-adsorbed vesicles forms microscopic lipid patches on the surface and it is triggered by a critical coverage of the vesicles independent of their composition. In the SLB growth phase, lipid patches merge, forming a continuous SLB. The propagation of patch edges catalyzes the process and depends on the ganglioside type. Our pH-dependent experiments confirm that the polar/charged head groups of the gangliosides have a critical role in these steps and phases of SLB formation kinetics.

Suspended phospholipid bilayers: A new biological membrane mimetic

HYPOTHESIS

The attractive interaction between a cationic surfactant monolayer at the air-water interface and vesicles, incorporating anionic lipids, is sufficient to drive the adsorption and deformation of the vesicles. Osmotic rupture of the vesicles produces a continuous lipid bilayer beneath the monolayer.

EXPERIMENTAL

Specular neutron reflectivity has been measured from the surface of a purpose-built laminar flow trough, which allows for rapid adsorption of vesicles, the changes in salt concentration required for osmotic rupture of the adsorbed vesicles into a bilayer, and for neutron contrast variation of the sub-phase without disturbing the monolayer.

FINDINGS

The neutron reflectivity profiles measured after vesicle addition are consistent with the adsorption and flattening of the vesicles beneath the monolayer. An increase in the buffer salt concentration results in further flattening and fusion of the adsorbed vesicles, which are ruptured by a subsequent decrease in the salt concentration. This process results in a continuous, high coverage, bilayer suspended 11 Åbeneath the monolayer. As the bilayer is not constrained by a solid substrate, this new mimetic is well-suited to studying the structure of lipid bilayers that include transmembrane proteins.

Formation of a Fully Anionic Supported Lipid Bilayer to Model Bacterial Inner Membrane for QCM-D Studies

Supported lipid bilayers (SLBs) on quartz crystals are employed as versatile model systems for studying cell membrane behavior with the use of the highly sensitive technique of quartz crystal microbalance with dissipation monitoring (QCM-D). Since the lipids constituting cell membranes vary from predominantly zwitterionic lipids in mammalian cells to predominantly anionic lipids in the inner membrane of Gram-positive bacteria, the ability to create SLBs of different lipid compositions is essential for representing different cell membranes. While methods to generate stable zwitterionic SLBs and zwitterionic-dominant mixed zwitterionic–anionic SLBs on quartz crystals have been well established, there are no reports of being able to form predominantly or fully anionic SLBs. We describe here a method for forming entirely anionic SLBs by treating the quartz crystal with cationic (3-aminopropyl) trimethoxysilane (APTMS). The formation of the anionic SLB was tracked using QCM-D by monitoring the adsorption of anionic lipid vesicles to a quartz surface and subsequent bilayer formation. Anionic egg L-α-phosphatidylglycerol (PG) vesicles adsorbed on the surface-treated quartz crystal, but did not undergo the vesicle-to-bilayer transition to create an SLB. However, when PG was mixed with 10–40 mole% 1-palmitoyl-2-hydroxy-sn-glycero-3-phospho-(1′-rac-glycerol) (LPG), the mixed vesicles led to the formation of stable SLBs. The dynamics of SLB formation monitored by QCM-D showed that while SLB formation by zwitterionic lipids followed a two-step process of vesicle adsorption followed by the breakdown of the adsorbed vesicles (which in turn is a result of multiple events) to create the SLB, the PG/LPG mixed vesicles ruptured immediately on contacting the quartz surface resulting in a one-step process of SLB formation. The QCM-D data also enabled the quantitative characterization of the SLB by allowing estimation of the lipid surface density as well as the thickness of the hydrophobic region of the SLB. These fully anionic SLBs are valuable model systems to conduct QCM-D studies of the interactions of extraneous substances such as antimicrobial peptides and nanoparticles with Gram-positive bacterial membranes.

Dissimilar Deformation of Fluid- and Gel-Phase Liposomes upon Multivalent Interaction with Cell Membrane Mimics Revealed Using Dual-Wavelength Surface Plasmon Resonance

The mechanical properties of biological nanoparticles play a crucial role in their interaction with the cellular membrane, in particular for cellular uptake. This has significant implications for the design of pharmaceutical carrier particles. In this context, liposomes have become increasingly popular, among other reasons due to their customizability and easily varied physicochemical properties. With currently available methods, it is, however, not trivial to characterize the mechanical properties of nanoscopic liposomes especially with respect to the level of deformation induced upon their ligand-receptor-mediated interaction with laterally fluid cellular membranes. Here, we utilize the sensitivity of dual-wavelength surface plasmon resonance to probe the size and shape of bound liposomes (∼100 nm in diameter) as a means to quantify receptor-induced deformation during their interaction with a supported cell membrane mimic. By comparing biotinylated liposomes in gel and fluid phases, we demonstrate that fluid-phase liposomes are more prone to deformation than their gel-phase counterparts upon binding to the cell membrane mimic and that, as expected, the degree of deformation depends on the number of ligand-receptor pairs that are engaged in the multivalent binding.

Modelling the adsorption of phospholipid vesicles to a silicon dioxide surface using Langmuir kinetics

Supported Lipid Bilayers (SLBs) are model biological membranes that have been developed to study the interactions between biomolecules in a cell membrane. Though forming SLBs is relatively easy, their formation mechanism remains a topic of debate. When buffered solutions containing phosphatidylcholine vesicles are flowed over a silicon dioxide (SiO₂) surface they adsorb intact to the surface to form a Supported Vesicle Layer (SVL) if the pH of the buffer is above 9. We have run experiments with buffers with a pH at or above 9 to study the kinetics of the adsorption of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles to an SiO₂ surface, which is the first step in the formation of an SLB. We used a quartz crystal microbalance (QCM) to monitor the real-time changes in the mass of the SVL as it formed from solutions with different lipid concentrations. Increases in the maximum frequency change with increasing lipid concentration indicated that both adsorption and desorption of DOPC vesicles were occurring, and that an equilibrium was established between the DOPC vesicles in the SVL and in the bulk solution. From the data acquired we were able to determine that the equilibrium constant for the adsorption and desorption of DOPC vesicles was 18 ± 1. The data was fitted to a Langmuir adsorption model from which the rate constants for the adsorption and desorption of DOPC vesicles were determined to be k_a = (0.0107 ± 0.0004) mL mg^-1 s^-1 and k_d = (5.8 ± 0.3) × 10^-4 s^-1. The best fit to the experimental data was achieved if a parameter (α = (0.035 ± 0.003) s^-1) was used to account for the time taken for the lipid concentration to reach its steady state value in the flow cell used in the experiments.

Nanoarchitectured air-stable supported lipid bilayer incorporating sucrose-bicelle complex system

Cell-membrane-mimicking supported lipid bilayers (SLBs) provide an ultrathin, self-assembled layer that forms on solid supports and can exhibit antifouling, signaling, and transport properties among various possible functions. While recent material innovations have increased the number of practically useful SLB fabrication methods, typical SLB platforms only work in aqueous environments and are prone to fluidity loss and lipid-bilayer collapse upon air exposure, which limits industrial applicability. To address this issue, herein, we developed sucrose-bicelle complex system to fabricate air-stable SLBs that were laterally mobile upon rehydration. SLBs were fabricated from bicelles in the presence of up to 40 wt% sucrose, which was verified by quartz crystal microbalance-dissipation (QCM-D) and fluorescence recovery after photobleaching (FRAP) experiments. The sucrose fraction in the system was an important factor; while 40 wt% sucrose induced lipid aggregation and defects on SLBs after the dehydration-rehydration process, 20 wt% sucrose yielded SLBs that exhibited fully recovered lateral mobility after these processes. Taken together, these findings demonstrate that sucrose-bicelle complex system can facilitate one-step fabrication of air-stable SLBs that can be useful for a wide range of biointerfacial science applications.

Formation of Supported Lipid Bilayers (SLBs) from Buffers Containing Low Concentrations of Group I Chloride Salts

Supported lipid bilayers (SLBs) are a useful tool for studying the interactions between lipids and other biomolecules that make up a cell membrane. SLBs are typically formed by the adsorption and rupture of vesicles from solution. Although it is known that many experimental factors can affect whether SLB formation is successful, there is no comprehensive understanding of the mechanism. In this work, we have used a quartz crystal microbalance (QCM) to investigate the role of the salt in the buffer on the formation of phosphatidylcholine SLBs on a silicon dioxide (SiO₂) surface. We varied the concentration of sodium chloride in the buffer, from 5 to 150 mM, to find the minimum concentration of NaCl that was required for the successful formation of an SLB. We then repeated the experiments with other group I chloride salts (LiCl, KCl, and CsCl) and found that at higher salt concentrations (150 mM) SLB formation was successful for all of the salts used, and the degree of deformation of the adsorbed vesicles at the critical vesicle coverage was cation-dependent. The results showed that at an intermediate salt concentration (50 mM) the critical vesicle coverage was cation-dependent and at low salt concentrations (12.5 mM) the cation used determined whether SLB formation was successful. We found that the successful formation of SLBs could occur at lower electrolyte concentrations for KCl and CsCl than it did for NaCl. To understand these results, we calculated the magnitude of the vesicle-surface interaction energy using the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended-DLVO theory. We managed to explain the results obtained at higher salt concentrations by including cation-dependent surface potentials in the calculations and at lower salt concentrations by the addition of a cation-dependent hydration force. These results showed that the way that different cations in solution affect the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-SiO₂ surface interaction energy depends on the ionic strength of the solution.

Mechanistic Aspects of the Evolution of 3D Cholesterol Crystallites in a Supported Lipid Membrane via a Quartz Crystal Microbalance with Dissipation Monitoring

The irreversible formation of cholesterol monohydrate crystals within biological membranes is the leading cause of various diseases, including atherosclerosis. Understanding the process of cholesterol crystallization is fundamentally important and could also lead to the development of improved therapeutic strategies. This has driven several studies investigating the effect of the environmental parameters on the induction of cholesterol crystallite growth and the structure of the cholesterol crystallites, while the kinetics and mechanistic aspects of the crystallite formation process within lipid membranes remain poorly understood. Herein, we fabricated cholesterol crystallites within a supported lipid bilayer (SLB) by adsorbing a cholesterol-rich bicellar mixture onto a glass and silica surface and investigated the real-time kinetics of cholesterol crystallite nucleation and growth using epifluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) monitoring. Microscopic imaging showed the evolution of the morphology of cholesterol crystallites from nanorod- and plate-shaped habits during the initial stage to mostly large, micron-sized three-dimensional (3D) plate-shaped crystallites in the end, which was likened to Ostwald ripening. QCM-D kinetics revealed unique signal responses during the later stage of the growth process, characterized by simultaneous positive frequency shifts, nonmonotonous energy dissipation shifts, and significant overtone dependence. Based on the optically observed changes in crystallite morphology, we discussed the physical background of these unique QCM-D signal responses and the mechanistic aspects of Ostwald ripening in this system. Together, our findings revealed mechanistic details of the cholesterol crystallite growth kinetics, which may be useful in biointerfacial sensing and bioanalytical applications.

From spherical compartments to polymer films: exploiting vesicle fusion to generate solid supported thin polymer membranes

Solid supported polymer membranes as scaffold for the insertion of functional biomolecules provide the basis for mimicking natural membranes. They also provide the means for unraveling biomolecule-membrane interactions and engineering platforms for biosensing. Vesicle fusion is an established procedure to obtain solid supported lipid bilayers but the more robust polymer vesicles tend to resist fusion and planar membranes rarely form. Here, we build on vesicle fusion to develop a refined and efficient way to produce solid supported membranes based on poly(dimethylsiloxane)-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA) amphiphilic triblock copolymers. We first create thiol-bearing polymer vesicles (polymersomes) and anchor them on a gold substrate. An osmotic shock then provokes polymersome rupture and drives planar film formation. Prerequisite for a uniform amphiphilic planar membrane is the proper combination of immobilized polymersomes and osmotic shock conditions. Thus, we explored the impact of the hydrophobic PDMS block length of the polymersome on the formation and the characteristics of the resulting solid supported polymer assemblies by quarz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). When the PDMS block is short enough, attached polymersomes restructure in response to osmotic shock, resulting in a uniform planar membrane. Our approach to rapidly form planar polymer membranes by vesicle fusion brings many advantages to the development of synthetic planar membranes for bio-sensing and biotechnological applications.

Ligand-receptor-mediated attachment of lipid vesicles to a supported lipid bilayer

The interaction of exosomes (cell-secreted [Formula: see text]100 nm-sized extracellular vesicles) or membrane-enveloped virions with cellular lipid membranes is often mediated by relatively weak ligand-receptor bonds. Interactions of this type can be studied using vesicles and observing their attachment to receptors located in a lipid bilayer formed at a solid surface. The contact region between a vesicle and the supported lipid bilayer and accordingly the number of ligand-receptor pairs there can be increased by deforming a vesicle. Herein, I (i) estimate theoretically the corresponding deformation energy assuming a disk-like or elongated shape of vesicles, (ii) present the equations allowing one to track such deformations by employing total internal reflection fluorescence microscopy and surface plasmon resonance, and (iii) briefly discuss some related experimental studies.

Osmotic Concentration-Controlled Particle Uptake and Wrapping-Induced Lysis of Cells and Vesicles

In vivo, high protein and ion concentrations determine the preferred volumes of cells, organelles, and vesicles. Deformations of their lipid-bilayer membranes by nanoparticle wrapping reduce the interior volumes available to solutes and thus induce large osmotic pressure differences. Osmotic concentration can therefore be an important control parameter for wrapping of nanoparticles. We employ a curvature-elasticity model of the membrane and contact interaction with spherical particles to study their wrapping at initially spherical vesicles. Although the continuous particle-binding transition is independent of the presence of solutes, the discontinuous envelopment transition shifts to higher adhesion strengths and the corresponding energy barrier increases with increasing osmotic concentration. High osmotic concentrations stabilize partial-wrapped, membrane-bound states for both, particle attachment to the inside and the outside. In this regime, wrapping of particles controls membrane tension, with power-law dependencies on osmotic concentration and adhesion strength. For high adhesion strengths, particle wrapping can lead to the opening of mechanosensitive channels in cell membranes and to lysis. Membrane tension-induced stabilization of partial-wrapped states as well as wrapping-induced lysis play important roles not only for desired mechano-bacteriocidal effects of engineered nanomaterials but may also determine viral burst sizes of bacteria and control endocytosis for mammalian cells.

Supported Lipid Bilayer Formation: Beyond Vesicle Fusion

Supported lipid bilayers (SLBs) are cell-membrane-mimicking platforms that can be formed on solid surfaces and integrated with a wide range of surface-sensitive measurement techniques. SLBs are useful for unravelling details of fundamental membrane biology and biophysics as well as for various medical, biotechnology, and environmental science applications. Thus, there is high interest in developing simple and robust methods to fabricate SLBs. Currently, vesicle fusion is a popular method to form SLBs and involves the adsorption and spontaneous rupture of lipid vesicles on a solid surface. However, successful vesicle fusion depends on high-quality vesicle preparation, and it typically works with a narrow range of material supports and lipid compositions. In this Feature Article, we summarize current progress in developing two new SLB fabrication techniques termed the solvent-assisted lipid bilayer (SALB) and bicelle methods, which have compelling advantages such as simple sample preparation and compatibility with a wide range of material supports and lipid compositions. The molecular self-assembly principles underpinning the two strategies and important experimental parameters are critically discussed, and recent application examples are presented. Looking forward, we envision that these emerging SLB fabrication strategies can be widely adopted by specialists and nonspecialists alike, paving the way to enriching our understanding of lipid membrane properties and realizing new application possibilities.

Characterizing the Supported Lipid Membrane Formation from Cholesterol-Rich Bicelles

Supported lipid bilayers (SLBs) are simplified model membrane systems that mimic the fundamental properties of biological cell membranes and allow the surface-sensitive tools to be used in numerous sensing applications. SLBs can be prepared by various methods including vesicle fusion, solvent-assisted lipid bilayer (SALB), and bicelle adsorption and are generally composed of phospholipids. Incorporating other biologically relevant molecules, such as cholesterol (Chol), into SLBs has been reported with the vesicle fusion and SALB methods, whereas it remains unexplored with the bicelle absorption method. Herein, using the quartz crystal microbalance-dissipation (QCM-D) and fluorescence microscopy techniques, we explored the possibility of forming SLBs from Chol-containing bicelles and discovered that Chol-enriched SLBs can be fabricated with bicelles. We also compared the Chol-enriched SLB formation of the bicelle method to that of vesicle fusion and SALB and discussed how the differences in lipid assembly properties can cause the differences in the adsorption kinetics and final results of SLB formation. Collectively, our findings demonstrate that the vesicle fusion method is least favorable for forming Chol-enriched SLBs, whereas the SALB and bicelle methods are more favorable, highlighting the need to consider the application requirements when choosing a suitable method for the formation of Chol-enriched SLBs.

Excitation of Fluorescent Lipid Probes Accelerates Supported Lipid Bilayer Formation via Photosensitized Lipid Oxidation

Fluorescent lipid probes are commonly used to label membranes of cells and model membranes like giant vesicles, liposomes, and supported lipid bilayers (SLB). Here, we show that excitation of fluorescent lipid probes with BODIPY-like conjugates results in a significant acceleration of the rupture and SLB formation process for unsaturated phospholipid vesicles on SiO₂ surfaces. The resulting SLBs also have smaller measured masses, which is indicative of a reduction in membrane thickness and/or membrane density. The excitation of fluorescent probes with NBD and Texas Red conjugates does not accelerate the SLB formation process. In the absence of fluorescent probes or light, the inclusion of oxidized phospholipids also accelerates SLB formation. The excitation-induced acceleration caused by BODIPY-like probes is eliminated when the probes are present with saturated phospholipids not susceptible to oxidation, and it is attenuated when a lipophilic antioxidant (α-tocopherol) is present. These results suggest that BODIPY-phospholipid conjugates are photosensitizers, and their excitation causes oxidation of lipid membranes, which significantly alters membrane properties.

Nanoparticles without and with protein corona: van der Waals and hydration interaction

The van der Waals (vdW) interaction between nanoparticles (NPs) in general, and especially between metal NPs, may be appreciable, and may result in nanoparticle aggregation. In biofluids, NPs become rapidly surrounded by a protein corona (PC). Here, the vdW and hydration interaction of NPs with and without PC are compared in detail. The focus is on two widely used types of NPs fabricated of SiO2 and Au and possessing weak and strong vdW interactions, respectively. For SiO2, the presence of PC increases the vdW interaction, but it remains relatively weak and insufficient for aggregation. For Au, the presence of PC decreases the vdW interaction, and in the case of small NPs (≤ 40 nm in diameter) it may become insufficient for aggregation as well while the larger NPs can aggregate.

Influence of NaCl Concentration on Bicelle-Mediated SLB Formation

The deposition of two-dimensional bicellar disks on hydrophilic surfaces is an emerging approach to fabricate supported lipid bilayers (SLBs) that requires minimal sample preparation, works at low lipid concentrations, and yields high-quality SLBs. While basic operating steps in the fabrication protocol mimic aspects of the conventional vesicle fusion method, lipid bicelles and vesicles have distinct architectural properties, and understanding how experimental parameters affect the efficiency of bicelle-mediated SLB formation remains to be investigated. Herein, using the quartz crystal microbalance-dissipation and localized surface plasmon resonance techniques, we investigated the effect of bulk NaCl concentration on bicelle-mediated SLB formation on silicon dioxide surfaces. For comparison, similar experiments were conducted with vesicles as well. In both cases, SLB formation was observed to occur rapidly provided that the NaCl concentration was sufficiently high (>50 mM). Under such conditions, the effect of NaCl concentration on SLB formation was minor in the case of bicelles and significant in the case of vesicles where it is expected to be related primarily to osmotic pressure. At lower NaCl concentrations, bicelles also formed SLBs but slowly, whereas adsorbed vesicles remained intact. These findings were complemented by time-lapsed fluorescence microscopy imaging and fluorescence recovery after photobleaching measurements that corroborated bicelle-mediated SLB formation across the range of tested NaCl concentrations. The results are discussed by comparing the architectural properties of bicelles and vesicles along with theoretical analysis of the corresponding adsorption kinetics.

Interaction of Giant Unilamellar Vesicles with the Surface Nanostructures on Dragonfly Wings

The waxy epicuticle of dragonfly wings contains a unique nanostructured pattern that exhibits bactericidal properties. In light of emerging concerns of antibiotic resistance, these mechano-bactericidal surfaces represent a particularly novel solution by which bacterial colonization and the formation of biofilms on biomedical devices can be prevented. Pathogenic bacterial biofilms on medical implant surfaces cause a significant number of human deaths every year. The proposed mechanism of bactericidal activity is through mechanical cell rupture; however, this is not yet well understood and has not been well characterized. In this study, we used giant unilamellar vesicles (GUVs) as a simplified cell membrane model to investigate the nature of their interaction with the surface of the wings of two dragonfly species, Austrothemis nigrescens and Trithemis annulata, sourced from Victoria, Australia, and the Baix Ebre and Terra Alta regions of Catalonia, Spain. Confocal laser scanning microscopy and cryo-scanning electron microscopy techniques were used to visualize the interactions between the GUVs and the wing surfaces. When exposed to both natural and gold-coated wing surfaces, the GUVs were adsorbed on the surface, exhibiting significant deformation, in the process of membrane rupture. Differences between the tensile rupture limit of GUVs composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine and the isotropic tension generated from the internal osmotic pressure were used to indirectly determine the membrane tensions, generated by the nanostructures present on the wing surfaces. These were estimated as being in excess of 6.8 mN m^-1, the first experimental estimate of such mechano-bactericidal surfaces. This simple model provides a convenient bottom-up approach toward understanding and characterizing the bactericidal properties of nanostructured surfaces.

Biomolecules Turn Self-Assembling Amphiphilic Block Co-polymer Platforms Into Biomimetic Interfaces

Biological membranes constitute an interface between cells and their surroundings and form distinct compartments within the cell. They also host a variety of biomolecules that carry out vital functions including selective transport, signal transduction and cell-cell communication. Due to the vast complexity and versatility of the different membranes, there is a critical need for simplified and specific model membrane platforms to explore the behaviors of individual biomolecules while preserving their intrinsic function. Information obtained from model membrane platforms should make invaluable contributions to current and emerging technologies in biotechnology, nanotechnology and medicine. Amphiphilic block co-polymers are ideal building blocks to create model membrane platforms with enhanced stability and robustness. They form various supramolecular assemblies, ranging from three-dimensional structures (e.g., micelles, nanoparticles, or vesicles) in aqueous solution to planar polymer membranes on solid supports (e.g., polymer cushioned/tethered membranes,) and membrane-like polymer brushes. Furthermore, polymer micelles and polymersomes can also be immobilized on solid supports to take advantage of a wide range of surface sensitive analytical tools. In this review article, we focus on self-assembled amphiphilic block copolymer platforms that are hosting biomolecules. We present different strategies for harnessing polymer platforms with biomolecules either by integrating proteins or peptides into assemblies or by attaching proteins or DNA to their surface. We will discuss how to obtain synthetic structures on solid supports and their characterization using different surface sensitive analytical tools. Finally, we highlight present and future perspectives of polymer micelles and polymersomes for biomedical applications and those of solid-supported polymer membranes for biosensing.

Molecular diffusion and nano-mechanical properties of multi-phase supported lipid bilayers

Understanding the properties of cell membranes is important in the fields of fundamental and applied biology.

Layer-by-Layer Assembly of Nanosized Membrane Fractions for the Assessment of Cytochrome P450 Xenobiotic Metabolism

Herein, we report the use of sequential layer-by-layer (LbL) assembly to design nanostructured films made of recombinant bacterial membrane fractions (MF), which overexpress cytochrome P450 (CYP) and cytochrome P450 reductase. The ability to incorporate MF in LbL multilayered films is demonstrated by an in situ quartz crystal microbalance with dissipation monitoring using poly-l-lysine or poly-l-ornithine as a polycation. Results show that MF preserve a remarkable CYP1A2 catalytic property in the adsorbed phase. Moreover, atomic force microscopy images reveal that MF mostly adopt a flattened conformation in the adsorbed phase with an extensive tendency to aggregate within the multilayered films, which is more pronounced when increasing the number of bilayers. Interestingly, this behavior seems to enhance the ability of embedded MF to remain active after repeated uses. The proposed strategy constitutes a practical alternative for the immobilization of active CYP enzymes. Besides their fundamental interest, MF-based multilayers are useful nano-objects for the creation of new biomimetic reactors for the assessment of xenobiotic metabolism.

Osmotic Shock-Triggered Assembly of Highly Charged, Nanoparticle-Supported Membranes

Spherical nanoparticle-supported lipid bilayers (SSLBs) combine precision nanoparticle engineering with biocompatible interfaces for various applications, ranging from drug delivery platforms to structural probes for membrane proteins. Although the bulk, spontaneous assembly of vesicles and larger silica nanoparticles (>100 nm) robustly yields SSLBs, it will only occur with low charge density vesicles for smaller nanoparticles (<100 nm), a fundamental barrier in increasing SSLB utility and efficacy. Here, through whole mount and cryogenic transmission electron microscopy, we demonstrate that mixing osmotically loaded vesicles with smaller nanoparticles robustly drives the formation of SSLBs with high membrane charge density (up to 60% anionic lipid or 50% cationic lipid). We show that the osmolyte load necessary for SSLB formation is primarily a function of absolute membrane charge density and is not lipid headgroup-dependent, providing a generalizable, tunable approach toward bulk production of highly curved and charged SSLBs with various membrane compositions.

Nanoarchitectonic-Based Material Platforms for Environmental and Bioprocessing Applications

The challenges of pollution, environmental science, and energy consumption have become global issues of broad societal importance. In order to address these challenges, novel functional systems and advanced materials are needed to achieve high efficiency, low emission, and environmentally friendly performance. A promising approach involves nanostructure-level controls of functional material design through a novel concept, nanoarchitectonics. In this account article, we summarize nanoarchitectonic approaches to create nanoscale platform structures that are potentially useful for environmentally green and bioprocessing applications. The introduced platforms are roughly classified into (i) membrane platforms and (ii) nanostructured platforms. The examples are discussed together with the relevant chemical processes, environmental sensing, bio-related interaction analyses, materials for environmental remediation, non-precious metal catalysts, and facile separation for biomedical uses.

Membrane Reconstitution of Monoamine Oxidase Enzymes on Supported Lipid Bilayers

Monoamine oxidase A and B (MAO-A and B) are mitochondrial outer membrane enzymes that are implicated in a number of human diseases, and the pharmacological inhibition of these enzymes is a promising therapeutic strategy to alleviate disease symptoms. It has been suggested that optimal levels of enzymatic activity occur in the membrane-associated state, although details of the membrane association process remain to be understood. Herein, we have developed a supported lipid bilayer platform to study MAO-A and B binding and evaluate the effects of known pharmacological inhibitors on the membrane association process. By utilizing the quartz crystal microbalance-dissipation (QCM-D) technique, it was determined that both MAOs exhibit tight binding to negatively and positively charged bilayers with distinct concentration-dependent binding profiles while only transiently binding to neutral bilayers. Importantly, in the presence of known inhibitors, the MAOs showed increased binding to negatively charged bilayers, although there was no effect of inhibitor treatment on binding to positively charged bilayers. Taken together, our findings establish that the membrane association of MAOs is highly dependent on membrane surface charge, and we outline an experimental platform to support the in vitro reconstitution of monoamine oxidases on synthetic membranes, including the evaluation of pharmacological drug candidates.

Spatiotemporal Kinetics of Supported Lipid Bilayer Formation on Glass via Vesicle Adsorption and Rupture

Supported lipid bilayers (SLBs) represent one of the most popular mimics of the cell membrane. Herein, we have used total internal reflection fluorescence microscopy for in-depth characterization of the vesicle-mediated SLB formation mechanism on a common silica-rich substrate, borosilicate glass. Fluorescently labeling a subset of vesicles allowed us to monitor the adsorption of individual labeled vesicles, resolve the onset of SLB formation from small seeds of SLB patches, and track their growth via SLB-edge-induced autocatalytic rupture of adsorbed vesicles. This made it possible to perform the first quantitative measurement of the SLB front velocity, which is shown to increase up to 1 order of magnitude with time. This effect can be classified as dramatic because in many other physical, chemical, or biological kinetic processes the front velocity is either constant or decreasing with time. The observation was successfully described with a theoretical model and Monte Carlo simulations implying rapid local diffusion of lipids upon vesicle rupture.

Formation of Alkanethiol Supported Hybrid Membranes Revisited

A phospholipid monolayer supported on an alkanethiol self-assembled monolayer (SAM) constitutes a supported hybrid membrane, a model of biological membranes optimized for electronic access through the underlying metal support surface. It is believed that phospholipids, when deposited from aqueous liposome suspension, spontaneously cover the alkanethiol-modified surface, owing to the reduction of surface free energy of the hydrophobic alkane surface exposed to the solution. However, the formation of the hybrid layer has to overcome significant energy barriers in rupturing the vesicle and "unzipping" the membrane leaflets; hence drivers of the spontaneous hybrid membrane formation are unclear. In this work, the authors studied the efficiency of the liposome deposition method to form hybrid membranes on octanethiol and hexadecanethiol SAMs in aqueous environment. Using quartz crystal microbalance to monitor the deposition process it was found that the hybrid membrane did not form spontaneously; the deposit was dominated by hemi-fused liposomes that can only be removed by applying osmotic stress. However, osmotic stress yielded a reproducible layer characterized by ≈-5Hz frequency change that is also confirmed by fluorescence microscopy imaging, irrespective of lipid concentration and the chain length of the SAMs. The frequency change is ≈20% of the frequency change expected for a tightly bound bilayer membrane, or 40% of a single leaflet, suggesting that the lipid layer is in a different conformation compared to a bilayer membrane: the acyl chains are most likely parallel to the SAM surface, likely due to strong hydrophobic interaction. Comparing these results to the literature it appears that the initial formation of hybrid membranes is inhibited by the ionic environment, while osmotic stress leads to the observed unique layer conformation.

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.

Investigating how vesicle size influences vesicle adsorption on titanium oxide: a competition between steric packing and shape deformation

Understanding the adsorption behavior of lipid vesicles at solid-liquid interfaces is important for obtaining fundamental insights into soft matter adsorbates as well as for practical applications such as supported lipid bilayer (SLB) fabrication. While the process of SLB formation has been highly scrutinized, less understood are the details of vesicle adsorption without rupture, especially at high surface coverages. Herein, we tackle this problem by employing simultaneous quartz crystal microbalance-dissipation (QCM-D) and localized surface plasmon resonance (LSPR) measurements in order to investigate the effect of vesicle size (84-211 nm diameter) on vesicle adsorption onto a titanium oxide surface. Owing to fundamental differences in the measurement principles of the two techniques as well as a mismatch in probing volumes, it was possible to determine both the lipid mass adsorbed near the sensor surface as well as the total mass of adsorbed lipid and hydrodynamically coupled solvent in the adsorbed vesicle layer as a whole. With increasing vesicle size, the QCM-D frequency signal exhibited monotonic behavior reaching an asymptotic value, whereas the QCM-D energy dissipation signal continued to increase according to the vesicle size. In marked contrast, the LSPR-tracked lipid mass near the sensor surface followed a parabolic trend, with the greatest corresponding measurement response occurring for intermediate-size vesicles. The findings reveal that the maximum extent of adsorbed vesicles contacting a solid surface occurs at an intermediate vesicle size due to the competing influences of vesicle deformation and steric packing. Looking forward, such information can be applied to control the molecular self-assembly of phospholipid assemblies as well as provide the basis for investigating deformable, soft matter adsorbates.

Interfacial Forces Dictate the Pathway of Phospholipid Vesicle Adsorption onto Silicon Dioxide Surfaces

The pathway of vesicle adsorption onto a solid support depends on the material composition of the underlying support, and there is significant interest in developing material-independent strategies to modulate the spectrum of vesicle-substrate interactions on a particular surface. Herein, using the quartz crystal microbalance-dissipation (QCM-D) technique, we systematically investigated how solution pH and membrane surface charge affect vesicle adsorption onto a silicon dioxide surface. While vesicle adsorption and spontaneous rupture to form complete supported lipid bilayer (SLBs) occurred in acidic conditions, it was discovered that a wide range of adsorption pathways occurred in alkaline conditions, including (i) vesicle adsorption and spontaneous rupture to form complete SLBs, (ii) vesicle adsorption and spontaneous rupture to form incomplete SLBs, (iii) irreversible adsorption of intact vesicles, (iv) reversible adsorption of intact vesicles, and (v) negligible adsorption. In general, SLB formation became more favorable with increasingly positive membrane surface charge although there were certain conditions at which attractive electrostatic forces were insufficient to promote vesicle rupture. To rationalize these findings, we discuss how solution pH and membrane surface charge affect interfacial forces involved in vesicle-substrate interactions. Taken together, our findings present a comprehensive picture of how interfacial forces dictate the pathway of phospholipid vesicle adsorption onto silicon dioxide surfaces and offer a broadly applicable framework to characterize the interactions between phospholipid vesicles and inorganic material surfaces.

Effect of Glucose on the Mobility of Membrane-Adhering Liposomes

Enclosed lipid bilayer structures, referred to as liposomes or lipid vesicles, have a wide range of biological functions, such as cellular signaling and membrane trafficking. The efficiency of cellular uptake of liposomes, a key step in many of these functions, is strongly dependent on the contact area between a liposome and a cell membrane, which is governed by the adhesion force w, the membrane bending energy κ, and the osmotic pressure Δp. Herein, we investigate the relationship between these forces and the physicochemical properties of the solvent, namely, the presence of glucose (a nonionic osmolyte). Using fluorescence microscopy, we measure the diffusivity D of small (∼50 nm radius), fluorescently labeled liposomes adhering to a supported lipid bilayer or to the freestanding membrane of a giant (∼10 μm radius) liposome. It is observed that glucose in solution reduces D on the supported membrane, while having negligible effect on D on the freestanding membrane. Using well-known hydrodynamic theory for the diffusivity of membrane inclusions, these observations suggest that glucose enhances the contact area between the small liposomes and the underlying membrane, while not affecting the viscosity of the underlying membrane. In addition, quartz crystal microbalance experiments showed no significant change in the hydrodynamic height of the adsorbed liposomes, upon adding glucose. This observation suggests that instead of osmotic deflation, glucose enhances the contact area via adhesion forces, presumably due to the depletion of the glucose molecules from the intermembrane hydration layer.

Membrane adaptation limitations inEnterococcus faecalisunderlie sensitivity and the inability to develop significant resistance to conjugated oligoelectrolytes

COEs are emerging antimicrobials to combat drug resistant infections and to which bacteria develop only limited resistance.

Membrane tension increases fusion efficiency of model membranes in the presence of SNAREs

The large gap in time scales between membrane fusion occurring in biological systems during neurotransmitter release and fusion observed between model membranes has provoked speculations over a large number of possible factors that might explain this discrepancy. One possible reason is an elevated lateral membrane tension present in the presynaptic membrane. We investigated the tension-dependency of fusion using model membranes equipped with a minimal fusion machinery consisting of syntaxin 1, synaptobrevin and SNAP 25. Two different strategies were realized; one based on supported bilayers and the other one employing sessile giant liposomes. In the first approach, isolated patches of planar bilayers derived from giant unilamellar vesicles containing syntaxin 1 and preassembled SNAP 25 (ΔN-complex) were deposited on a dilatable PDMS sheet. In a second approach, lateral membrane tension was controlled through the adhesion of intact giant unilamellar vesicles on a functionalized surface. In both approaches fusion efficiency increases considerably with lateral tension and we identified a threshold tension of 3.4 mN m⁻¹, at which the number of fusion events is increased substantially.

Influence of the environmental tonicity perturbations on the release of model compounds from large unilamellar vesicles (LUVs): A mechanistic investigation

In this work, the influence of environmental tonicity perturbations on the size and release kinetics of model markers from liposomes (calcein and rhodamine) was investigated. Large unilamellar vesicles (LUVs) were prepared from a mixture composed of organic solvents containing dissolved phosphatidylcholine and phosphate buffered saline (PBS, pH 7.4). Organic phase was removed by rotary evaporation and the obtained liposomal dispersions were extruded to reduce the liposomal sizes to approx. 400 nm. The LUVs were exposed to PBS of different tonicity to induce water migration, and consequently, generate an osmotic pressure on the vesicle membranes. The markers release kinetics were studied by the dialysis method employing Franz diffusion cells. LUVs appeared to be more susceptible to the osmotic swelling than the shrinking and the size changes were significantly more pronounced for calcein-loaded LUVs in comparison to rhodamine-loaded LUVs. The calcein release from LUVs was highly affected by the water influx/efflux, whereas rhodamine release was less affected by the tonicity perturbations. Mechanistically, it appeared that hydrophilic molecules (calcein) followed the water flux, whereas lipophilic molecules (rhodamine) seemed to be more affected by the changes in LUVs size and consequent alteration of the tightness of the phospholipid bilayer (where the lipophilic marker was imbedded in). These results demonstrate that the different tonicity (within the inner core and external environment of vesicles) can enhance/hamper the diffusion of a marker from LUVs and that osmotically active liposomes could be used as a novel controlled drug delivery system.

A model derived from hydrodynamic simulations for extracting the size of spherical particles from the quartz crystal microbalance

A model derived from hydrodynamic simulations is presented for extracting the size of adsorbed nanoparticles in QCM-D measurements, and is applicable to both low and high surface coverage regimes.

Quantitative Profiling of Nanoscale Liposome Deformation by a Localized Surface Plasmon Resonance Sensor

Characterizing the shape of sub-100 nm, biological soft-matter particulates (e.g., liposomes and exosomes) adsorbed at a solid-liquid interface remains a challenging task. Here, we introduce a localized surface plasmon resonance (LSPR) sensing approach to quantitatively profile the deformation of nanoscale, fluid-phase 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes contacting a titanium dioxide substrate. Experimental and theoretical results validate that, due to its high sensitivity to the spatial proximity of phospholipid molecules near the sensor surface, the LSPR sensor can discriminate fine differences in the extent of ionic strength-modulated liposome deformation at both low and high surface coverages. By contrast, quartz crystal microbalance-dissipation (QCM-D) measurements performed with equivalent samples were qualitatively sensitive to liposome deformation only at saturation coverage. Control experiments with stiffer, gel-phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes verified that the LSPR measurement discrimination arises from the extent of liposome deformation, while the QCM-D measurements yield a more complex response that is also sensitive to the motion of adsorbed liposomes and coupled solvent along with lateral interactions between liposomes. Collectively, our findings demonstrate the unique measurement capabilities of LSPR sensors in the area of biological surface science, including competitive advantages for probing the shape properties of adsorbed, nanoscale biological particulates.

Interpretation of amperometric kinetics of content release during contacts of vesicles with a lipid membrane

The exocytotic pathway of secretion of molecules from cells includes transport by vesicles, tether-mediated fusion of vesicles with the plasma membrane accompanied by pore formation, and diffusion-mediated release of their contents via a pore to the outside. In related basic biophysical studies, vesicle-content release is tracked by measuring corresponding amperometric spikes. Although experiments of this type have a long history, the understanding of the underlying physics is still elusive. The present study elucidates the likely contribution of line energy, membrane tension and bending, osmotic pressure, hydration forces, and tethers to the potential energy for fusion-related pore formation and evolution. The overdamped Langevin equation is used to describe the pore dynamics, which are in turn employed to calculate the kinetics of content release and to interpret the shape of amperometric spikes.

Controlling the Formation of Phospholipid Monolayer, Bilayer, and Intact Vesicle Layer on Graphene

Exciting progress has been made in the use of graphene for bio- and chemical sensing applications. In this regard, interfacing lipid membranes with graphene provides a high-sealing interface that is resistant to nonspecific protein adsorption and suitable for measuring biomembrane-associated interactions. However, a controllable method to form well-defined lipid bilayer coatings remains elusive, and there are varying results in the literature. Herein, we demonstrate how design strategies based on molecular self-assembly and surface chemistry can be employed to coat graphene surface with different classes of lipid membrane architectures. We characterize the self-assembly of lipid membranes on CVD-graphene using quartz crystal microbalance with dissipation, field-effect transistor, and Raman spectroscopy. By employing the solvent-assisted lipid bilayer (SALB) method, a lipid monolayer and bilayer were formed on pristine and oxygen-plasma-treated CVD-graphene, respectively. On these surfaces, vesicle fusion method resulted in formation of a lipid monolayer and intact vesicle layer, respectively. Collectively, these findings provide the basis for improved surface functionalization strategies on graphene toward bioelectronic applications.

Solid-Supported Lipid Multilayers under High Hydrostatic Pressure

In this work, the structure of solid-supported lipid multilayers exposed to increased hydrostatic pressure was studied in situ by X-ray reflectometry at the solid-liquid interface between silicon and an aqueous buffer solution. The layers' vertical structure was analyzed up to a maximum pressure of 4500 bar. The multilayers showed phase transitions from the fluid into different gel phases. With increasing pressure, a gradual filling of the sublayers between the hydrophilic head groups with water was observed. This process was inverted when the pressure was decreased, yielding finally smaller water layers than those in the initial state. As is commonly known, water has an abrasive effect on lipid multilayers by the formation of vesicles. We show that increasing pressure can reverse this process so that a controlled switching between multi- and bilayers is possible.