Solvent-assisted lipid self-assembly at hydrophilic surfaces: factors influencing the formation of supported membranes

Nanoelectrospray based synthesis of large, transportable membranes with integrated membrane proteins

Membrane proteins tend to be difficult to study since they need to be integrated into a lipid bilayer membrane to function properly. This study presents a method to synthesize a macroscopically large and freely transportable membrane with integrated membrane proteins which is useful for studying membrane proteins and protein complexes in isolation. The method could serve as a blueprint for the production of larger quantities of functionalised membranes for integration into technical devices similar to the MinION DNA sequencer. It is possible to self-assemble larger biological membranes on solid surfaces. However, they cannot be removed from their solid support without destroying them. In transportable form, self-assembled membranes are limited to sizes of about 17 nm in nanodiscs. Here we electrospray a series of molecular layers onto the liquid surface of a buffer solution which creates a flat, liquid environment on the surface that directs the self-assembly of the membrane. This method enables us to experimentally control the membrane composition and to succeed in producing large membranes with integrated OmpG, a transmembrane pore protein. The technique is compatible with the assembly of membrane based protein complexes. Listeriolysin O and pneumolysin efficiently assemble into non-covalent membrane pore complexes of approximately 30 units or more within the surface layer.

Chaotropic Agent-Assisted Supported Lipid Bilayer Formation

Supported lipid bilayers (SLBs) are useful structures for mimicking cellular membranes, and they can be integrated with a variety of sensors. Although there are a variety of methods for forming SLBs, many of these methods come with limitations in terms of the lipid compositions that can be employed and the substrates upon which the SLBs can be deposited. Here we demonstrate the use of an all-aqueous chaotropic agent exchange process that can be used to form SLBs on two different substrate materials: SiO2, which is compatible with traditional SLB formation by vesicle fusion, and Al2O3, which is not compatible with vesicle fusion. When examined with a quartz crystal microbalance with dissipation monitoring, the SLBs generated by chaotropic agent exchange (CASLBs) have similar frequency and dissipation shifts to SLBs formed by the vesicle fusion technique. The CASLBs block nonspecific protein adsorption on the substrate and can be used to sense protein-lipid interactions. Fluorescence microscopy was used to examine the CASLBs, and we observed long-range lateral diffusion of fluorescent probes, which confirmed that the CASLBs were composed of a continuous, planar lipid bilayer. Our CASLB method provides another option for forming planar lipid bilayers on a variety of surfaces, including those that are not amenable to the widely used vesicle fusion method.

Assessing the Quality of Solvent-Assisted Lipid Bilayers Formed at Different Phases and Aqueous Buffer Media: A QCM-D Study

Supported lipid bilayers (SLBs) are low-complexity biomimetic membranes, serving as popular experimental platforms to study membrane organization and lipid transfer, membrane uptake of nanoparticles and biomolecules, and many other processes. Quartz crystal microbalance with dissipation monitoring has been utilized to probe the influence of several parameters on the quality of SLBs formed on Au- and SiO2-coated sensors. The influence of the aqueous medium (i.e., buffer type) and the adsorption temperature, above and below the lipid melting point, is neatly explored for SLBs of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine formed by a solvent exchange. Below the lipid melting temperature, quality variations are observed upon the formation on Au and SiO2 surfaces, with the SLBs being more homogeneous for the latter. We further investigate how the buffer affects the detection of lipid melting in SLBs, a transition that necessitates high-sensitivity and time-consuming surface-sensitive techniques to be detected.

Cell-free expression with a quartz crystal microbalance enables rapid, dynamic, and label-free characterization of membrane-interacting proteins

Integral and interacting membrane proteins (IIMPs) constitute a vast family of biomolecules that perform essential functions in all forms of life. However, characterizing their interactions with lipid bilayers remains limited due to challenges in purifying and reconstituting IIMPs in vitro or labeling IIMPs without disrupting their function in vivo. Here, we report cell-free transcription-translation in a quartz crystal microbalance with dissipation (TXTL-QCMD) to dynamically characterize interactions between diverse IIMPs and membranes without protein purification or labeling. As part of TXTL-QCMD, IIMPs are synthesized using cell-free transcription-translation (TXTL), and their interactions with supported lipid bilayers are measured using a quartz crystal microbalance with dissipation (QCMD). TXTL-QCMD reconstitutes known IIMP-membrane dependencies, including specific association with prokaryotic or eukaryotic membranes, and the multiple-IIMP dynamical pattern-forming association of the E. coli division-coordinating proteins MinCDE. Applying TXTL-QCMD to the recently discovered Zorya anti-phage system that is unamenable to labeling, we discovered that ZorA and ZorB integrate within the lipids found at the poles of bacteria while ZorE diffuses freely on the non-pole membrane. These efforts establish the potential of TXTL-QCMD to broadly characterize the large diversity of IIMPs.

Cholesterol-Enriched Hybrid Lipid Bilayer Formation on Inverse Phosphocholine Lipid-Functionalized Titanium Oxide Surfaces

Hybrid lipid bilayers (HLBs) are rugged biomimetic cell membrane interfaces that can form on inorganic surfaces and be designed to contain biologically important components like cholesterol. In general, HLBs are formed by depositing phospholipids on top of a hydrophobic self-assembled monolayer (SAM) composed of one-tail amphiphiles, while recent findings have shown that two-tail amphiphiles such as inverse phosphocholine (CP) lipids can have advantageous properties to promote zwitterionic HLB formation. Herein, we explored the feasibility of fabricating cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces with the solvent exchange and vesicle fusion methods. All stages of the HLB fabrication process were tracked by quartz crystal microbalance-dissipation (QCM-D) measurements and revealed important differences in fabrication outcome depending on the chosen method. With the solvent exchange method, it was possible to fabricate HLBs with well-controlled cholesterol fractions up to ~65 mol% in the upper leaflet as confirmed by a methyl-β-cyclodextrin (MβCD) extraction assay. In marked contrast, the vesicle fusion method was only effective at forming HLBs from precursor vesicles containing up to ~35 mol% cholesterol, but this performance was still superior to past results on hydrophilic SiO2. We discuss the contributing factors to the different efficiencies of the two methods as well as the general utility of two-tail CP SAMs as favorable interfaces to incorporate cholesterol into HLBs. Accordingly, our findings support that the solvent exchange method is a versatile tool to fabricate cholesterol-enriched HLBs on CP SAM-functionalized TiO2 surfaces.

Lipidic Formulations Inspired by COVID Vaccines as Smart Coatings to Enhance Nanoparticle-Based Cancer Therapy

Recent advances in nanomedicine have led to the introduction and subsequent establishment of nanoparticles in cancer treatment and diagnosis. Nonetheless, their application is still hindered by a series of challenges related to their biocompatibility and biodistribution. In this paper, we take inspiration from the recently produced and widely spread COVID vaccines, based on the combinational use of ionizable solid lipid nanoparticles, cholesterol, PEGylated lipids, and neutral lipids able to incorporate mRNA fragments. Here, we focus on the implementation of a lipidic formulation meant to be used as a smart coating of solid-state nanoparticles. The composition of this formulation is finely tuned to ensure efficient and stable shielding of the cargo. The resulting shell is a highly customized tool that enables the possibility of further functionalizations with targeting agents, peptides, antibodies, and fluorescent moieties for future in vitro and in vivo tests and validations. Finally, as a proof of concept, zinc oxide nanoparticles doped with iron and successively coated with this lipidic formulation are tested in a pancreatic cancer cell line, BxPC-3. The results show an astonishing increase in cell viability with respect to the same uncoated nanoparticles. The preliminary results presented here pave the way towards many different therapeutic approaches based on the massive presence of highly biostable and well-tolerated nanoparticles in tumor tissues, such as sonodynamic therapy, photodynamic therapy, hyperthermia, and diagnosis by means of magnetic resonance imaging.

Tailoring a Solvent-Assisted Method for Solid-Supported Hybrid Lipid-Polymer Membranes

Combining amphiphilic block copolymers and phospholipids opens new opportunities for the preparation of artificial membranes. The chemical versatility and mechanical robustness of polymers together with the fluidity and biocompatibility of lipids afford hybrid membranes with unique properties that are of great interest in the field of bioengineering. Owing to its straightforwardness, the solvent-assisted method (SA) is particularly attractive for obtaining solid-supported membranes. While the SA method was first developed for lipids and very recently extended to amphiphilic block copolymers, its potential to develop hybrid membranes has not yet been explored. Here, we tailor the SA method to prepare solid-supported polymer-lipid hybrid membranes by combining a small library of amphiphilic diblock copolymers poly(dimethyl siloxane)-poly(2-methyl-2-oxazoline) and poly(butylene oxide)-block-poly(glycidol) with phospholipids commonly found in cell membranes including 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl). The optimization of the conditions under which the SA method was applied allowed for the formation of hybrid polymer-lipid solid-supported membranes. The real-time formation and morphology of these hybrid membranes were evaluated using a combination of quartz crystal microbalance and atomic force microscopy. Depending on the type of polymer-lipid combination, significant differences in membrane coverage, formation of domains, and quality of membranes were obtained. The use of the SA method for a rapid and controlled formation of solid-supported hybrid membranes provides the basis for developing customized artificial hybrid membranes.

Effect of Membrane Curvature Nanoarchitectonics on Membrane-Disruptive Interactions of Antimicrobial Lipids and Surfactants

Single-chain lipid amphiphiles such as fatty acids and monoglycerides along with structurally related surfactants have received significant attention as membrane-disrupting antimicrobials to inhibit bacteria and viruses. Such promise has motivated deeper exploration of how these compounds disrupt phospholipid membranes, and the membrane-mimicking, supported lipid bilayer (SLB) platform has provided a useful model system to evaluate corresponding mechanisms of action and potency levels. Even so, it remains largely unknown how biologically relevant membrane properties, such as sub-100 nm membrane curvature, might affect these membrane-disruptive interactions, especially from a nanoarchitectonics perspective. Herein, using the quartz crystal microbalance-dissipation (QCM-D) technique, we fabricated intact vesicle adlayers composed of different-size vesicles (70 or 120 nm diameter) with varying degrees of membrane curvature on a titanium oxide surface and tracked changes in vesicle adlayer properties upon adding lauric acid (LA), glycerol monolaurate (GML), or sodium dodecyl sulfate (SDS). Above their critical micelle concentration (CMC) values, LA and GML caused QCM-D measurement shifts associated with tubule- and bud-like formation, respectively, and both compounds interacted similarly with small (high curvature) and large (low curvature) vesicles. In marked contrast, SDS exhibited distinct interactions with small and large vesicles. For large vesicles, SDS caused nearly complete membrane solubilization in a CMC-independent manner, whereas SDS was largely ineffective at solubilizing small vesicles at all tested concentrations. We rationalize these experimental observations by taking into account the interplay of the headgroup properties of LA, GML, and SDS and curvature-induced membrane geometry, and our findings demonstrate that membrane curvature nanoarchitectonics can strongly influence the membrane interaction profiles of antimicrobial lipids and surfactants.

Streamlined Fabrication of Hybrid Lipid Bilayer Membranes on Titanium Oxide Surfaces: A Comparison of One- and Two-Tail SAM Molecules

There is broad interest in fabricating cell-membrane-mimicking, hybrid lipid bilayer (HLB) coatings on titanium oxide surfaces for medical implant and drug delivery applications. However, existing fabrication strategies are complex, and there is an outstanding need to develop a streamlined method that can be performed quickly at room temperature. Towards this goal, herein, we characterized the room-temperature deposition kinetics and adlayer properties of one- and two-tail phosphonic acid-functionalized molecules on titanium oxide surfaces in various solvent systems and identified optimal conditions to prepare self-assembled monolayers (SAMs), upon which HLBs could be formed in select cases. Among the molecular candidates, we identified a two-tail molecule that formed a rigidly attached SAM to enable HLB fabrication via vesicle fusion for membrane-based biosensing applications. By contrast, vesicles adsorbed but did not rupture on SAMs composed of one-tail molecules. Our findings support that two-tail phosphonic acid SAMs offer superior capabilities for rapid HLB coating fabrication at room temperature, and these streamlined capabilities could be useful to prepare durable lipid bilayer coatings on titanium-based materials.

Inkjet-Printed Phospholipid Bilayers on Titanium Oxide Surfaces: Towards Functional Membrane Biointerfaces

Functional biointerfaces hold broad significance for designing cell-responsive medical implants and sensor devices. Solid-supported phospholipid bilayers are a promising class of biological materials to build bioinspired thin-film coatings, as they can facilitate interactions with cell membranes. However, it remains challenging to fabricate lipid bilayers on medically relevant materials such as titanium oxide surfaces. There are also limitations in existing bilayer printing capabilities since most approaches are restricted to either deposition alone or to fixed microarray patterning. By combining advances in lipid surface chemistry and on-demand inkjet printing, we demonstrate the direct deposition and patterning of covalently tethered lipid bilayer membranes on titanium oxide surfaces, in ambient conditions and without any surface pretreatment process. The deposition conditions were evaluated by quartz crystal microbalance-dissipation (QCM-D) measurements, with corresponding resonance frequency (Δf) and energy dissipation (ΔD) shifts of around −25 Hz and <1 × 10−6, respectively, that indicated successful bilayer printing. The resulting printed phospholipid bilayers are stable in air and do not collapse following dehydration; through rehydration, the bilayers regain their functional properties, such as lateral mobility (>1 µm2/s diffusion coefficient), according to fluorescence recovery after photobleaching (FRAP) measurements. By taking advantage of the lipid bilayer patterned architectures and the unique features of titanium oxide’s photoactivity, we further show how patterned cell culture arrays can be fabricated. Looking forward, this work presents new capabilities to achieve stable lipid bilayer patterns that can potentially be translated into implantable biomedical devices.

Thermodynamic Modeling of Solvent-Assisted Lipid Bilayer Formation Process

The solvent-assisted lipid bilayer (SALB) formation method provides a simple and efficient, microfluidic-based strategy to fabricate supported lipid bilayers (SLBs) with rich compositional diversity on a wide range of solid supports. While various studies have been performed to characterize SLBs formed using the SALB method, relatively limited work has been carried out to understand the underlying mechanisms of SALB formation under various experimental conditions. Through thermodynamic modeling, we studied the experimental parameters that affect the SALB formation process, including substrate surface properties, initial lipid concentration, and temperature. It was found that all the parameters are critically important to successfully form high-quality SLBs. The model also helps to identify the range of parameter space within which conformal, homogeneous SLBs can be fabricated, and provides mechanistic guidance to optimize experimental conditions for lipid membrane-related applications.

Observation of Liquid-Liquid-Phase Separation and Vesicle Spreading during Supported Bilayer Formation via Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables the real-time visualization of nanoscale dynamics in solution. This technique has been used to study the formation and transformation mechanisms of organic and inorganic nanomaterials. Here, we study the formation of block-copolymer-supported bilayers using LP-TEM. We observe two formation pathways that involve either liquid droplets or vesicles as intermediates toward supported bilayers. Quantitative image analysis methods are used to characterize vesicle spread rates and show the origin of defect formation in supported bilayers. Our results suggest that bilayer assembly methods that proceed via liquid droplet intermediates should be beneficial for forming pristine supported bilayers. Furthermore, supported bilayers inside the liquid cells may be used to image membrane interactions with proteins and nanoparticles in the future.

Interactions of hydrophilic quantum dots with defect-free and defect containing supported lipid membranes

Quantum dots (QDs) are semiconductor nanoparticles with unique optical and electronic properties, whose interest as potential nano-theranostic platforms for imaging and sensing is increasing. The design and use of QDs requires the understanding of cell-nanoparticle interactions at a microscopic and nanoscale level. Model systems such as supported lipid bilayers (SLBs) are useful, less complex platforms mimicking physico-chemical properties of cell membranes. In this work, we investigated the effect of topographical homogeneity of SLBs bearing different surface charge in the adsorption of hydrophilic QDs. Using quartz-crystal microbalance, a label-free surface sensitive technique, we show significant differences in the interactions of QDs onto homogeneous and inhomogeneous SLBs formed following different strategies. Within short time scales, QDs adsorb onto topographically homogeneous, defect-free SLBs is driven by electrostatic interactions, leading to no layer disruption. After prolonged QD exposure, the nanomechanical stability of the SLB decreases suggesting nanoparticle insertion. In the case of inhomogeneous, defect containing layers, QDs target preferentially membrane defects, driven by a subtle interplay of electrostatic and entropic effects, inducing local vesicle rupture and QD insertion at membrane edges.

Influence of Substrate Hydrophilicity on Structural Properties of Supported Lipid Systems on Graphene, Graphene Oxides, and Silica

Pristine graphene, a range of graphene oxides, and silica substrates were used to investigate the effect of surface hydrophilicity on supported lipid bilayers by means of all-atom molecular dynamics simulations. Supported 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers were found in close-contact conformations with hydrophilic substrates with as low as 5% oxidation level, while self-assembled monolayers occur on pure hydrophobic graphene only. Lipids and water at the surface undergo large redistribution to maintain the stability of the supported bilayers. Deposition of bicelles on increasingly hydrophilic substrates shows the continuous process of reshaping of the supported system and makes intermediate stages between self-assembled monolayers and supported bilayers. The bilayer thickness changes with hydrophilicity in a complex manner, while the number of water molecules per lipid in the hydration layer increases together with hydrophilicity.

Expanding the Potential of the Solvent-Assisted Method to Create Bio-Interfaces from Amphiphilic Block Copolymers

Artificial membranes, as materials with biomimetic properties, can be applied in various fields, such as drug screening or bio-sensing. The solvent-assisted method (SA) represents a straightforward method to prepare lipid solid-supported membranes. It overcomes the main limitations of established membrane preparation methods, such as Langmuir-Blodgett (LB) or vesicle fusion. However, it has not yet been applied to create artificial membranes based on amphiphilic block copolymers, despite their enhanced mechanical stability compared to lipid-based membranes and bio-compatible properties. Here, we applied the SA method on different amphiphilic di- and triblock poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PDMS-b-PMOXA) copolymers and optimized the conditions to prepare artificial membranes on a solid support. The real-time membrane formation, the morphology, and the mechanical properties have been evaluated by a combination of atomic force microscopy and quartz crystal microbalance. Then, selected biomolecules including complementary DNA strands and an artificial deallylase metalloenzyme (ADAse) were incorporated into these membranes relying on the biotin-streptavidin technology. DNA strands served to establish the capability of these synthetic membranes to interact with biomolecules by preserving their correct conformation. The catalytic activity of the ADAse following its membrane anchoring induced the functionality of the biomimetic platform. Polymer membranes on solid support as prepared by the SA method open new opportunities for the creation of artificial membranes with tailored biomimetic properties and functionality.

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.

Solid-supported lipid bilayers - A versatile tool for the structural and functional characterization of membrane proteins

The cellular membrane is central to the development of single-and multicellular life, as it separates the delicate cellular interior from the hostile environment. It exerts tight control over entry and exit of substances, is responsible for signaling with other cells in multicellular organisms and prevents pathogens from entering the cell. In the case of bacteria and viruses, the cellular membrane also hosts the proteins enabling invasion of the host organism. In a very real sense therefore, the cellular membrane is central to all life. The study of the cell membrane and membrane proteins in particular has therefore attracted significant attention. Due to the enormous variety of tasks performed by the membrane, it is a highly complex and challenging structure to study. Ideally, membrane components would be studied in isolation from this environment, but unlike water soluble proteins, the amphiphilic environment provided by the cellular membrane is key to the structure and function of the cell membrane. Therefore, model membranes have been developed to provide an environment in which a membrane protein can be studied. This review presents a set of tools that enable the comprehensive characterization of membrane proteins: electrochemical tools, surface plasmon resonance, neutron scattering, the surface forces apparatus and atomic force microscopy are discussed, with a particular focus on experimental technique and data evaluation.

Lipid Bicelle Micropatterning Using Chemical Lift-Off Lithography

Supported lipid membranes are versatile biomimetic coatings for the chemical functionalization of inorganic surfaces. Developing simple and effective fabrication strategies to form supported lipid membranes with micropatterned geometries is a long-standing challenge. Herein, we demonstrate how the combination of chemical lift-off lithography (CLL) and easily prepared lipid bicelle nanostructures can yield micropatterned, supported lipid membranes on gold surfaces with high pattern resolution, conformal character, and biofunctionality. Using CLL, we functionalized gold surfaces with patterned arrays of hydrophilic and hydrophobic self-assembled monolayers (SAMs). Time-lapse fluorescence microscopy imaging revealed that lipid bicelles adsorbed preferentially onto the hydrophilic SAM regions, while there was negligible lipid adsorption onto the hydrophobic SAM regions. Functional receptors could be embedded within the lipid bicelles, which facilitated selective detection of receptor-ligand binding interactions in a model streptavidin-biotin system. Quartz crystal microbalance-dissipation measurements further identified that lipid bicelles adsorb irreversibly and remain intact on top of the hydrophilic SAM regions. Taken together, our findings indicate that lipid bicelles are useful lipid nanostructures for reproducibly assembling micropatterned, supported lipid membranes with precise pattern fidelity.

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.

Molecular Lipid Films on Microengineering Materials

In this study, we have systematically investigated the formation of molecular phospholipid films on a variety of solid substrates fabricated from typical surface engineering materials and the fluidic properties of the lipid membranes formed on these substrates. The surface materials comprise of borosilicate glass, mica, SiO₂, Al (native oxide), Al₂O₃, TiO₂, ITO, SiC, Au, Teflon AF, SU-8, and graphene. We deposited the lipid films from small unilamellar vesicles (SUVs) by means of an open-space microfluidic device, observed the formation and development of the films by laser scanning confocal microscopy, and evaluated the mode and degree of coverage, fluidity, and integrity. In addition to previously established mechanisms of lipid membrane-surface interaction upon bulk addition of SUVs on solid supports, we observed nontrivial lipid adhesion phenomena, including reverse rolling of spreading bilayers, spontaneous nucleation and growth of multilamellar vesicles, and the formation of intact circular patches of double lipid bilayer membranes. Our findings allow for accurate prediction of membrane-surface interactions in microfabricated devices and experimental environments where model membranes are used as functional biomimetic coatings.

Understanding How Membrane Surface Charge Influences Lipid Bicelle Adsorption onto Oxide Surfaces

The adsorption of two-dimensional bicellar disks onto solid supports is an emerging fabrication technique to form supported lipid bilayers (SLBs) that is efficient and requires minimal sample preparation. To date, nearly all relevant studies have focused on zwitterionic lipid compositions and silica-based surfaces, and extending the scope of investigation to other lipid compositions and surfaces would improve our understanding of application possibilities and underpinning formation processes. Herein, using the quartz crystal microbalance-dissipation technique, we systematically investigated the adsorption of charged lipid bicelles onto silicon dioxide, titanium oxide, and aluminum oxide surfaces. Depending on the lipid composition and substrate, we observed different adsorption pathways, including (i) SLB formation via one- or two-step adsorption kinetics, (ii) monotonic adsorption without SLB formation, and (iii) negligible adsorption. On each substrate, SLB formation could be achieved with particular lipid compositions, whereas the trend in adsorption pathways varied according to the substrate and could be controlled by adjusting the bicelle?substrate interaction strength. To rationalize these findings, we discuss how electrostatic and hydration forces affect bicelle?substrate interactions on different oxide surfaces. Collectively, our findings demonstrate the broad utility of lipid bicelles for SLB formation while revealing physicochemical insights into the role of interfacial forces in controlling bicelle adsorption pathways.

Microfluidic liquid cell chamber for scanning probe microscopy measurement application

In this paper, we present a universal microfluidic liquid chamber device platform for atomic force microscopy (AFM), which enables to fabricate the uniform lipid bilayer on the hydrophilic surface using the solvent-assisted lipid bilayer formation method. Using this device enables us to acquire the various properties of delicate soft matter, including morphological data, and mechanical property measurements, using high-resolution AFM systems. The proposed technology is expected to provide an understanding of complicated biological materials.

Characterizing the Membrane-Disruptive Behavior of Dodecylglycerol Using Supported Lipid Bilayers

Monoglycerides are esterified adducts of fatty acid and glycerol molecules that disrupt phospholipid membranes, leading to a wide range of biological functions such as antimicrobial activity. Among monoglycerides, glycerol monolaurate (GML) exhibits particularly high antimicrobial activity, although enzymatic hydrolysis of its ester group can diminish potency. Consequently, there have been efforts to identify more chemically stable versions of GML, most notably its alkylglycerol ether equivalent called dodecylglycerol (DDG). However, despite high structural similarity, biological studies indicate that DDG and GML are not functionally equivalent and it has been speculated that the two compounds might have different interaction profiles with phospholipid membranes. To address this outstanding question, herein, we employed supported lipid bilayer (SLB) platforms to experimentally characterize the interactions of DDG with phospholipid membranes. Quartz crystal microbalance-dissipation experiments identified that DDG causes concentration-dependent membrane morphological changes in SLBs and the overall extent of membrane remodeling events was greater than that caused by GML. In addition, time-lapsed fluorescence microscopy imaging experiments revealed that DDG causes extensive membrane tubulation that is distinct from how GML induces membrane budding. We discuss how differences in the head group properties of DDG and GML contribute to distinct membrane interaction profiles, offering insight into how the molecular design of DDG not only improves chemical stability but also enhances membrane-disruptive activity.

Dynamic Control of Intramolecular Rotation by Tuning the Surrounding Two-Dimensional Matrix Field

The intramolecular rotation of 4-farnesyloxyphenyl-4,4-difluoro-4-bora-3a,4a-diaza- s-indacene (BODIPY-ISO) was controlled by tuning its local physical environment within a mixed self-assembled monolayer at an air-water interface. Intramolecular rotation was investigated by considering the twisted intramolecular charge transfer (TICT) fluorescence of BODIPY-ISO, which increases in intensity with increasing viscosity of the medium. In situ fluorescence spectroscopy was performed on mixed monolayers of BODIPY-ISO with several different lipids at the air-water interface during in-plane compression of the monolayers. Depending on the identity of the lipid used, the fluorescence of the mixed monolayers could be enhanced by mechanical compression, indicating that the rotation of BODIPY-ISO can be controlled dynamically in mixtures with lipids dispersed at the air-water interface. Taken together, our findings provide insight into strategies for controlling the dynamic behavior of molecular machines involving mechanical stimuli at interfaces.

Charge and Coordination Directed Liposome Fusion onto SiO2 and TiO2 Nanoparticles

TiO₂ and SiO₂ are very useful materials for building biointerfaces. A particularly interesting aspect is their interaction with lipid bilayers. Many past research efforts focused on phosphocholine (PC) lipids, which form supported lipid bilayers (SLB) on SiO₂ at physiological conditions but are adsorbed as intact liposomes on TiO₂. Low pH was required to form PC SLBs on TiO₂. This work intends to understand the surface forces and chemistry responsible for such differences. Two charge neutral lipids: 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC) and 2-((2,3-bis(oleoyloxy)propyl)dimethylammonio)ethyl ethyl phosphate (DOCPe) and two negatively charged lipids: 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS) and 2-((2,3-bis(oleoyloxy)propyl)dimethylammonio)ethyl hydrogen phosphate (DOCP) were used. Using calcein leakage assays, adsorption measurement, cryo-TEM, and washing, we concluded that charge is the dominating factor on SiO₂. The two neutral lipids form SLB on SiO₂ at pH 3 and 7, but the two negatively charged ones cannot form. On TiO₂, both charge and coordination chemistry are important. The two anionic lipids formed SLB from pH 3 to 10. DOCP had stronger affinity than DOPS likely due to the tighter terminal phosphate binding of the former. The two neutral liposomes formed SLB only at pH 3, where phosphate interaction and van der Waals force are deemed important. The pH 3 prepared TiO₂ DOPC SLBs are destabilized at neutral pH, indicating the reversible nature of the interaction. This work has provided new insights into two important materials interacting with common liposomes, which are important for reproducible biosensing, device fabrication, and drug delivery applications.

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.

Characterizing How Acidic pH Conditions Affect the Membrane-Disruptive Activities of Lauric Acid and Glycerol Monolaurate

Fatty acids and monoglycerides are single-chain lipid amphiphiles that interact with phospholipid membranes as part of various biological activities. For example, they can exhibit membrane-disruptive behavior against microbial pathogens on the human skin surface. Supported lipid bilayers (SLBs) provide a useful experimental platform to characterize these membrane-disruptive behaviors, although related studies have been limited to neutral pH conditions. Herein, we investigated how lauric acid (LA) and glycerol monolaurate (GML) interact with SLBs and cause membrane morphological changes under acidic pH conditions that are representative of the human skin surface. Although LA induces tubule formation under neutral pH conditions, we discovered that LA causes membrane phase separation under acidic pH conditions. By contrast, GML induced membrane budding in both pH environments, although there was more extensive membrane remodeling under acidic pH conditions. We discuss these findings in the context of how solution pH affects the ionization states and micellar aggregation properties of LA and GML as well as its effect on the bending stiffness of lipid bilayers. Collectively, the findings demonstrate that solution pH plays an important role in modulating the interaction of fatty acids and monoglycerides with phospholipid membranes, and hence influences the scope and potency of their membrane-disruptive activities.

Topography design in model membranes: Where biology meets physics

IMPACT STATEMENT

Artificial membranes with complex topography aid the understanding of biological processes where membrane geometry plays a key regulatory role. In this review, we highlight how emerging material and engineering technologies have been employed to create minimal models of cell signaling pathways, in vitro. These artificial systems allow life scientists to answer ever more challenging questions with regards to mechanisms in cellular biology. In vitro reconstitution of biology is an area that draws on the expertise and collaboration between biophysicists, material scientists and biologists and has recently generated a number of high impact results, some of which are also discussed in this review.

Practical guide to characterize biomolecule adsorption on solid surfaces (Review)

The control over the adsorption or grafting of biomolecules from a liquid to a solid interface is of fundamental importance in different fields, such as drug delivery, pharmaceutics, diagnostics, and tissue engineering. It is thus important to understand and characterize how biomolecules interact with surfaces and to quantitatively measure parameters such as adsorbed amount, kinetics of adsorption and desorption, conformation of the adsorbed biomolecules, orientation, and aggregation state. A better understanding of these interfacial phenomena will help optimize the engineering of biofunctional surfaces, preserving the activity of biomolecules and avoiding unwanted side effects. The characterization of molecular adsorption on a solid surface requires the use of analytical techniques, which are able to detect very low quantities of material in a liquid environment without modifying the adsorption process during acquisition. In general, the combination of different techniques will give a more complete characterization of the layers adsorbed onto a substrate. In this review, the authors will introduce the context, then the different factors influencing the adsorption of biomolecules, as well as relevant parameters that characterize their adsorption. They review surface-sensitive techniques which are able to describe different properties of proteins and polymeric films on solid two-dimensional materials and compare these techniques in terms of sensitivity, penetration depth, ease of use, and ability to perform "parallel measurements."

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.

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.

Hybrid Biomimetic Interfaces Integrating Supported Lipid Bilayers with Decellularized Extracellular Matrix Components

This paper describes the functionalization of solid supported phospholipid bilayer with decellularized extracellular matrix (dECM) components, toward the development of biomimetic platforms that more closely mimic the cell surface environment. The dECM was obtained through a combination of chemical and enzymatic treatments of mouse adipose tissue that contains collagen, fibronectin, and glycosaminoglycans (GAGs). Using amine coupling chemistry, the dECM components were attached covalently to the surface of a supported lipid bilayer containing phospholipids with reactive carboxylic acid headgroups. The bilayer formation and the kinetics of subsequent dECM conjugation were monitored by quartz crystal microbalance with dissipation (QCM-D). Fluorescence recovery after photobleaching (FRAP) confirmed the fluidity of the membrane after functionalization with dECM. The resulting hybrid biomimetic interface supports the attachment and survival of the human hepatocyte Huh 7.5 and maintains the representative hepatocellular function. Importantly, the platform is suitable for monitoring the lateral organization and clustering of cell-binding ligands and growth factor receptors in the presence of the rich biochemical profile of tissue-derived ECM components.

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.

Engineering the interface between lipid membranes and nanoporous gold: A study by quartz crystal microbalance with dissipation monitoring

Nanoporous gold (np-Au) is a nanostructured metal with many desirable attributes. Despite the growing number of applications of nanoporous materials, there are still open questions regarding their fabrication and subsequent surface functionalization. For example, the hydrophobic nature of gold surfaces makes the formation of planar supported lipid layers challenging. Here, the authors engineer the interface between np-Au and 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid layers using well-differentiated approaches based on vesicle adsorption and solvent exchange methods. The results reveal that the nanotopography of the np-Au surface plays a clear role in the vesicle adsorption process. Compared to vesicle adsorption, the solvent exchange method proves successful in the formation of planar supported lipid bilayers in both np-Au and planar Au surfaces, being less sensitive to the surface morphological features. The influence of nanostructured surfaces on lipid layer formation is determined by the driving mechanisms behind each process, i.e., the balance of adhesion and cohesion forces in vesicle adsorption and lyotropic lipid phase transitions in solvent exchange, respectively. A better understanding of such interactions will contribute to the development of a variety of applications, from electrochemical biosensors to drug screening and delivery systems, using nanoporous gold coated with stimuli-responsive lipid layers.

Influence of natural organic matter (NOM) coatings on nanoparticle adsorption onto supported lipid bilayers

As the worldwide usage of nanoparticles in commercial products continues to increase, there is growing concern about the environmental risks that nanoparticles pose to biological systems, including potential damage to cellular membranes. A detailed understanding of how different types of nanoparticles behave in environmentally relevant conditions is imperative for predicting and mitigating potential membrane-associated toxicities. Herein, we investigated the adsorption of two popular nanoparticles (silver and buckminsterfullerene) onto biomimetic supported lipid bilayers of varying membrane charge (positive and negative). The quartz crystal microbalance-dissipation (QCM-D) measurement technique was employed to track the adsorption kinetics. Particular attention was focused on understanding how natural organic matter (NOM) coatings affect nanoparticle-bilayer interactions. Both types of nanoparticles preferentially adsorbed onto the positively charged bilayers, although NOM coatings on the nanoparticle and lipid bilayer surfaces could either inhibit or promote adsorption in certain electrolyte conditions. While past findings showed that NOM coatings inhibit membrane adhesion, our findings demonstrate that the effects of NOM coatings are more nuanced depending on the type of nanoparticle and electrolyte condition. Taken together, the results demonstrate that NOM coatings can modulate the lipid membrane interactions of various nanoparticles, suggesting a possible way to improve the environmental safety of nanoparticles.

Optimizing the Performance of Supported Lipid Bilayers as Cell Culture Platforms Based on Extracellular Matrix Functionalization

Strategies to fabricate biofunctionalized surfaces are essential for many biotechnological applications. Zwitterionic lipid bilayer coatings doped with lipids with chemically selective headgroups provide a robust platform for immobilization of biomolecules in an antifouling, protein resistant background. Herein, we assess the biological activity of two important components of the extracellular matrix (ECM), collagen type I (Col I) and fibronectin (FN), which are covalently attached to a supported lipid bilayer (SLB), and compare their activity with the same proteins, nonspecifically adsorbed onto a SiO₂ surface. The characterization of protein coatings by quartz crystal microbalance with dissipation revealed that Col I and FN attached to SLB are less dense and have higher structural flexibility than when adsorbed onto SiO₂. Cell adhesion, proliferation, and function, as well as Col I-FN interactions, were more efficient on the ECM-functionalized SLB, making it a promising platform for cell-based diagnostics, tissue engineering, medical implants, and biosensor development.

Dynamic Cellular Interactions with Extracellular Matrix Triggered by Biomechanical Tuning of Low-Rigidity, Supported Lipid Membranes

The behavior of cells in a tissue is regulated by chemical as well as physical signals arising from their microenvironment. While gel-based substrates have been widely used for mimicking a range of substrate rigidities, there is a need for the development of low rigidity substrates for mimicking the physical properties of soft tissues. In this study, the authors report the development of a supported lipid bilayer (SLB)-based low rigidity substrate for cell adhesion studies. SLBs are functionalized with either collagen I or fibronectin via covalent, amine coupling to a carboxyl group-modified lipid molecule. While the lipid molecules in the bilayer show long-range lateral mobility, the covalently functionalized extracellular matrix (ECM) proteins are immobile on the bilayer surface. Specific adhesion of cells results in an enrichment of the protein on the bilayer and the appearance of a zone of depletion around the cells. Further, the lateral reorganization of the ECM proteins is controlled by altering the fluidity of lipid molecules in the substrate. Thus, the experimental platform developed in this study can be utilized for addressing basic questions related to cell adhesion on low rigidity substrates as well as biomedical applications requiring adhesion of cells to low rigidity substrates.

Probing Spatial Proximity of Supported Lipid Bilayers to Silica Surfaces by Localized Surface Plasmon Resonance Sensing

On account of high surface sensitivity, localized surface plasmon resonance (LSPR) sensors have proven widely useful for studying lipid membrane configurations at solid-liquid interfaces. Key measurement capabilities include distinguishing adsorbed vesicles from supported lipid bilayers (SLBs) as well as profiling the extent of deformation among adsorbed vesicles. Such capabilities rely on detecting geometrical changes in lipid membrane configuration on a length scale that is comparable to the decay length of the LSPR-induced electromagnetic field enhancement (∼5-20 nm). Herein, we report that LSPR sensors are also capable of probing nanoscale (∼1 nm) variations in the distance between SLBs and underlying silica-coated surfaces. By tuning the electrostatic properties of lipid membranes, we could modulate the bilayer-substrate interaction and corresponding separation distance, as verified by simultaneous LSPR and quartz crystal microbalance-dissipation (QCM-D) measurements. Theoretical calculations of the expected variation in the LSPR measurement response agree well with experimental results and support that the LSPR measurement response is sensitive to subtle variations in the bilayer-substrate separation distance.

Correlating Membrane Morphological Responses with Micellar Aggregation Behavior of Capric Acid and Monocaprin

The interaction of single-chain lipid amphiphiles with phospholipid membranes is relevant to many scientific fields, including molecular evolution, medicine, and biofuels. Two widely studied compounds within this class are the medium-chain saturated fatty acid, capric acid, and its monoglyceride derivative, monocaprin. To date, most studies about these compounds have involved in vitro evaluation of their biological activities, while mechanistic details of how capric acid and monocaprin interact with phospholipid bilayers remain elusive. Herein, we investigated the effect of these two compounds on the morphological and fluidic properties of prefabricated, supported lipid bilayers (SLBs). The critical micelle concentration (CMC) of each compound was determined by fluorescence spectroscopy measurements. At or above its CMC, capric acid induced the formation of elongated tubules protruding from the SLB, as determined by quartz crystal microbalance-dissipation and fluorescence microscopy experiments. By contrast, monocaprin induced the formation of elongated tubules or membrane buds below and above its CMC, respectively. Fluorescence recovery after photobleaching (FRAP) experiments indicated that capric acid increased bilayer fluidity only above its CMC, whereas monocaprin increased bilayer fluidity both above and below its CMC. We discuss these findings in the context of the two compounds' structural properties, including net charge, molecular length and hydrogen-bonding capacity. Collectively, the findings demonstrate that capric acid and monocaprin differentially affect the morphological and fluidic properties of SLBs, and that the aggregation state of the compounds plays a critical role in modulating their interactions with phospholipid membranes.

DPPTE Thiolipid Self-Assembled Monolayer: A Critical Assay

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au-S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 ± 0.4 °C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

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.

Fabrication of charged membranes by the solvent-assisted lipid bilayer (SALB) formation method on SiO2 and Al2O3

In this study, we employed the solvent-assisted lipid bilayer (SALB) formation method to fabricate charged membranes on solid supports. The SALB formation method exploits a ternary mixture of lipid-alcohol-aqueous buffer to deposit lamellar phase structures on solid supports upon gradual increase of the buffer fraction. Using the quartz crystal microbalance with dissipation (QCM-D) technique, we investigated the formation of negatively and positively charged membranes via the SALB formation method and directly compared with the vesicle fusion method on two different oxide films. Bilayers containing an increasing fraction of negatively charged DOPS lipid molecules were successfully formed on both SiO2 and Al2O3 substrates using the SALB formation method at physiological pH (7.5). In contrast, the vesicle fusion method did not support bilayer formation on Al2O3 and those containing more than 10% DOPS ruptured on SiO2 only under acidic conditions (pH 5). Characterization of the fraction of negatively charge DOPS by in situ annexin 5A binding assay revealed that the fraction of DOPS lipid molecules in the bilayers formed on Al2O3 is significantly higher than that formed on SiO2. This suggests that the SALB self-assembly of charged membranes is predominantly governed by the electrostatic interaction. Furthermore, our findings indicate that when multicomponent lipid mixtures are used, the relative fraction of lipids in the bilayer may differ from the fraction of lipids in the precursor mixture.

Size and shape trump charge in interactions of oxovanadates with self-assembled interfaces: application of continuous shape measure analysis to the decavanadate anion

Using ⁵¹V NMR spectroscopy, dynamic light scattering and continuous shape analysis to characterize two polyoxometalate-encapsulation in reverse micelles.

Supported lipid bilayer repair mediated by AH peptide

High quality and complete supported lipid bilayers are formed on silicon oxide by employing an AH peptide mediated repair step.