Domain Localization by Graphene Oxide in Supported Lipid Bilayers

The gel-phase domains in a binary supported lipid bilayer (SLB) comprising dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) were localized on graphene oxide (GO) deposited on a SiO₂/Si substrate. We investigated the distribution of the gel-phase domains and the liquid crystalline (L_α) phase regions in DOPC+DPPC-SLB on thermally oxidized SiO₂/Si substrates with GO flakes to understand the mechanism of the domain localization on GO. Fluorescence microscopy and atomic force microscopy revealed that the gel-phase domains preferably distributed on GO flakes, whereas the fraction of the L_α-phase increased on the bare SiO₂ surface which was not covered with the GO flakes. The gel-phase domain was condensed on GO more effectively at the lower cooling rate. We propose that nucleation of the gel-phase domain preferentially occurred on GO, whose surface has amphiphilic property, during the gel-phase domain formation. The domains of the liquid ordered (L_o) phase were also condensed on GO in a ternary bilayer containing cholesterol that was phase-separated to the L_o phase and the liquid disordered phase. Rigid domains segregates on GO during their formation process, leaving fluid components to the surrounding region of GO.

Mechanistic elucidation of freezing-induced surface decomposition of aluminum oxyhydroxide adjuvant

The freezing-induced aggregation of aluminum-based (Alum) adjuvants has been considered as the most important cause of reduced vaccine potency. However, the intrinsic properties that determine the functionality of Alum after freezing have not been elucidated. In this study, we used engineered aluminum oxyhydroxide nanoparticles (AlOOH NPs) and demonstrated that cryogenic freezing led to the mechanical pressure-mediated reduction of surface hydroxyl. The sugar-based surfactant, octyl glucoside (OG), was demonstrated to shield AlOOH NPs from the freezing-induced loss of hydroxyl content and the aggregation through the reduction of recrystallization-induced mechanical stress. As a result, the antigenic adsorption property of frozen AlOOH NPs could be effectively protected. When hepatitis B surface antigen (HBsAg) was adjuvanted with OG-protected frozen AlOOH NPs in mice, the loss of immunogenicity was inhibited. These findings provide insights into the freezing-induced surface decomposition of Alum and can be translated to design of protectants to improve the stability of vaccines.

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The freezing stress led to the destruction of surface hydroxyl group on AlOOH NPs

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Octyl glucoside protected AlOOH NPs from freezing-induced surface decomposition

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Octyl glucoside protected vaccines from freezing-induced loss of immunogenicity

Comparative Study of Lipid- and Polymer-Supported Membranes Obtained by Vesicle Fusion

We compare the fusion of giant lipid and block-copolymer vesicles on glass and poly(dimethylsiloxane) substrates. Both types of vesicles are similar in their ability to fuse to hydrophilic substrates and form patches with distinct heart or circular shapes. We use epifluorescence/confocal microscopy and atomic force microscopy on membrane patches to (i) characterize bilayer fluidity and patch-edge stability and (ii) follow the intermediate stages in the formation of continuous supported bilayers. Polymer membranes show much lower membrane fluidity and, unlike lipids, an inability of adjacent patches to fuse spontaneously into continuous membranes. We ascribe this effect to hydration repulsion forces acting between the patch edges, which can be diminished by increasing the sample temperature. We show that large areas of supported polymer membranes can be created by fusing giant vesicles on glass or poly(dimethylsiloxane) substrates and annealing their edges.

Quenching Efficiency of Quantum Dots Conjugated to Lipid Bilayers on Graphene Oxide Evaluated by Fluorescence Single Particle Tracking

A single particle observation of quantum dots (QDs) was performed on lipid bilayers formed on graphene oxide (GO). The long-range fluorescence quenching of GO has been applied to biosensing for various biomolecules. We demonstrated the single particle observation of a QD on supported lipid bilayers in this study, aiming to detect the quenching efficiency of lipid and protein molecules in a lipid bilayer by fluorescence single particle tacking (SPT). A single lipid bilayer or double lipid bilayers were formed on GO flakes deposited on a thermally oxidized silicon substrate by the vesicle fusion method. The QDs were conjugated on the lipid bilayers, and single particle images of the QDs were obtained under the quenching effect of GO. The quenching efficiency of a single QD was evaluated from the fluorescence intensities on the regions with and without GO. The quenching efficiency reflecting the layer numbers of the lipid bilayers was obtained.

Revealing local molecular distribution, orientation, phase separation, and formation of domains in artificial lipid layers: Towards comprehensive characterization of biological membranes

Lipids, together with molecules such as DNA and proteins, are one of the most relevant systems responsible for the existence of life. Selected lipids are able to assembly into various organized structures, such as lipid membranes. The unique properties of lipid membranes determine their complex functions, not only to separate biological environments, but also to participate in regulatory functions, absorption of nutrients, cell-cell communication, endocytosis, cell signaling, and many others. Despite numerous scientific efforts, still little is known about the reason underlying the variability within lipid membranes, and its biochemical significance. In this review, we discuss the structural complexity of lipid membranes, as well as the importance to simplify studied systems in order to understand phenomena occurring in natural, complex membranes. Such systems require a model interface to be analyzed. Therefore, here we focused on analytical studies of artificial systems at various interfaces. The molecular structure of lipid membranes, specifically the nanometric thickens of molecular bilayer, limits in a major extent the choice of highly sensitive methods suitable to study such structures. Therefore, we focused on methods that combine high sensitivity, and/or chemical selectivity, and/or nanometric spatial resolution, such as atomic force microscopy, nanospectroscopy (tip-enhanced Raman spectroscopy, infrared nanospectroscopy), phase modulation infrared reflection-absorption spectroscopy, sum-frequency generation spectroscopy. We summarized experimental and theoretical approaches providing information about molecular structure and composition, lipid spatial distribution (phase separation), organization (domain shape, molecular orientation) of lipid membranes, and real-time visualization of the influence of various molecules (proteins, drugs) on their integrity. An integral part of this review discusses the latest achievements in the field of lipid layer-based biosensors.

Solid-Supported Assembly Composed of n-Octyl-β-D-glucopyranoside and 1,2-Dioleoyl-sn-glycero-3-phosphocholine in Equilibrium with Its Ambient Aqueous Solution System Including Dispersed Assembly

In the last few decades, the preparation of solid-supported lipid bilayers by immersing a solid substrate in an aqueous solution where the lipid is dissolved with the aid of a surfactant, followed by dilution of the solution, has been reported. In this study, we attempted to interpret the evolution of supported surfactant/lipid assemblies towards the supported lipid bilayer in terms of a phase equilibrium between the supported assembly phase and its ambient solution system consisting of the dispersed surfactant/lipid assembly phase and the bulk solution phase comprising monomeric surfactant and lipid. We characterized the supported assembly formed on hydrophilized Ge or mica substrates in equilibrium with aqueous solutions containing various concentrations of the nonionic surfactant, n-octyl-β-D-glucopyranoside (OG) and the amphoteric phospholipid, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), using interaction-force-profile measurements by atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). We also investigated the ambient solution system using equilibrium dialysis to obtain the partition equilibrium profile of OG between the bulk solution and dispersed assembly phases in the micellar or vesicular states. These studies indicate that the properties of the supported assembly depend on the composition of the dispersed assembly and concentration of monomerically dissolved OG. Further, a type of micellar-bilayer state transition occurs in the supported assembly, roughly synchronized with that in the dispersed assembly.

A Study of the Effects of Plasma Surface Treatment on Lipid Bilayers Self-Spreading on a Polydimethylsiloxane Substrate under Different Treatment Times

Plasma-treated poly(dimethylsiloxane) (PDMS)-supported lipid bilayers are used as functional tools for studying cell membrane properties and as platforms for biotechnology applications. Self-spreading is a versatile method for forming lipid bilayers. However, few studies have focused on the effect of plasma treatment on self-spreading lipid bilayer formation. In this paper, we performed lipid bilayer self-spreading on a PDMS surface with different treatment times. Surface characterization of PDMS treated with different treatment times is evaluated by AFM and SEM, and the effects of plasma treatment of the PDMS surface on lipid bilayer self-spreading behavior is investigated by confocal microscopy. The front-edge velocity of lipid bilayers increases with the plasma treatment time. By theoretical analyses with the extended-DLVO modeling, we find that the most likely cause of the velocity change is the hydration repulsion energy between the PDMS surface and lipid bilayers. Moreover, the growth behavior of membrane lobes on the underlying self-spreading lipid bilayer was affected by topography changes in the PDMS surface resulting from plasma treatment. Our findings suggest that the growth of self-spreading lipid bilayers can be controlled by changing the plasma treatment time.

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.

Pore-Spanning Plasma Membranes Derived from Giant Plasma Membrane Vesicles

Giant plasma membrane vesicles (GPMVs) are a highly promising model system for the eukaryotic plasma membrane. The unresolved challenge, however, is a path to surface-based structures that allows accessibility to both sides of the plasma membrane through high-resolution techniques. Such an approach would pave the way to advanced chip-based technologies for the analysis of complex cell surfaces to study the roles of membrane proteins, host-pathogen interactions, and many other bioanalytical and sensing applications. This study reports the generation of planar supported plasma membranes and for the first-time pore-spanning plasma membranes (PSPMs) derived from pure GPMVs that are spread on activated solid and highly ordered porous silicon substrates. GPMVs were produced by two different vesiculation agents and were first investigated with respect to their growth behavior and phase separation. Second, these GPMVs were spread onto silicon substrates to form planar supported plasma membrane patches. PSPMs were obtained by spreading of pure GPMVs on oxygen-plasma activated porous substrates with pore diameters of 3.5 μm. Fluorescence micrographs unambiguously showed that the PSPMs partially phase separate in a mobile ordered phase surrounded by a disordered phase, which was supported by cholesterol extraction using methyl-β-cyclodextrin.

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.

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.

Impact of Pore Size and Surface Chemistry of Porous Silicon Particles and Structure of Phospholipids on Their Interactions

By exploiting its porous structure and high loading capacity, porous silicon (PSi) is a promising biomaterial to fabricate protocells and biomimetic reactors. Here, we have evaluated the impact of physicochemical properties of PSi particles [thermally oxidized PSi, TOPSi; annealed TOPSi, AnnTOPSi; (3-aminopropyl) triethoxysilane functionalized thermally carbonized PSi, APTES-TCPSi; and thermally hydrocarbonized PSi, THCPSi] on their surface interactions with different phospholipids. All of the four phospholipids were similarly adsorbed by the surface of PSi particles, except for TOPSi. Among four PSi particles, TOPSi with hydrophilic surface and smaller pore size showed the weakest adsorption toward phosphatidylcholines. By increasing the pore size from roughly 12.5 to 18.0 nm (TOPSi vs AnnTOPSi), the quantity of phosphatidylcholines adsorbed by TOPSi was enhanced to the same level of hydrophilic APTES-TCPSi and hydrophobic THCPSi. The 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) exhibited the highest release ratio of phospholipids from all four PSi particles, and phosphatidylserine (DPPS) showed the lowest release ratio of phospholipids from PSi particles, except for TOPSi, which adsorbed less phospholipids due to the small pore size. There is consistency in the release extent of phospholipids from PSi particles and the isosteric heat of adsorption. Overall, our study demonstrates the importance of pore size and surface chemistry of PSi particles as well as the structure of phospholipids on their interactions. The obtained information can be employed to guide the selection of PSi particles and phospholipids to fabricate highly ordered structures, for example, protocells, or biomimetic reactors.

Host Cell Prediction of Exosomes Using Morphological Features on Solid Surfaces Analyzed by Machine Learning

Exosomes are extracellular nanovesicles released from any cells and found in any body fluid. Because exosomes exhibit information of their host cells (secreting cells), their analysis is expected to be a powerful tool for early diagnosis of cancers. To predict the host cells, we extracted multidimensional feature data about size, shape, and deformation of exosomes immobilized on solid surfaces by atomic force microscopy (AFM). The key idea is combination of support vector machine (SVM) learning for individual exosome particles and their interpretation by principal component analysis (PCA). We observed exosomes derived from three different cancer cells on SiO₂/Si, 3-aminopropyltriethoxysilane-modified-SiO₂/Si, and TiO₂ substrates by AFM. Then, 14-dimensional feature vectors were extracted from AFM particle data, and classifiers were trained in 14-dimensional space. The prediction accuracy for host cells of test AFM particles was examined by the cross-validation test. As a result, we obtained prediction of exosome host cells with the best accuracy of 85.2% for two-class SVM learning and 82.6% for three-class one. By PCA of the particle classifiers, we concluded that the main factors for prediction accuracy and its strong dependence on substrates are incremental decrease in the PCA-defined aspect ratio of the particles with their volume.

Amphiphobic Septa Enhance the Mechanical Stability of Free-Standing Bilayer Lipid Membranes

Artificial bilayer lipid membranes (BLMs) provide well-defined systems for investigating the fundamental properties of membrane proteins, including ion channels, and for screening the effect of drugs that act on them. However, the application of this technique is limited due to the low stability and low reconstitution efficiency of the process. We previously reported on improving the stability of BLM based on the fabrication of microapertures having a tapered edge in SiO₂/Si₃N₄ septa and efficient ion channel incorporation based on vesicle fusion accelerated by a centrifugal force. Although the BLM stability and incorporation probability were dramatically improved when these approaches were used, some BLMs were ruptured when subjected to a centrifugal force. To further improve the BLM stability, we investigated the effect of modifying the surface of the SiO₂/Si₃N₄ septa on the stability of BLM suspended in the septa. The modified surfaces were characterized in terms of hydrophobicity, lipophobicity, and surface roughness. Diffusion coefficients of the lipid monolayers formed on the modified surfaces were also determined. Highly fluidic lipid monolayers were formed on the amphiphobic substrates that had been modified with long-chain perfluorocarbons. Free-standing BLMs formed in amphiphobic septa showed a much higher mechanical stability, including tolerance to water movement and applied centrifugal forces with and without proteoliposomes, than those formed in the septa that had been modified with a short alkyl chain. These results demonstrate that highly stable BLMs are formed when the surface of the septa has amphiphobic properties. Because highly fluidic lipid monolayers that are formed on the septa seamlessly connect with BLMs in a free-standing region, the high fluidity of the lipids contributes to decreasing potential damage to BLMs when mechanical stresses are applied. This approach to improve the BLM stability increases the experimental efficiency of the BLM systems and will contribute to the development of high-throughput platforms for functional assays of ion channel proteins.

Adsorption and binding dynamics of graphene-supported phospholipid membranes using the QCM-D technique

We report on the adsorption dynamics of phospholipid membranes on graphene-coated substrates using the quartz crystal microbalance with dissipation monitoring (QCM-D) technique. We compare the lipid vesicle interaction and membrane formation on gold and silicon dioxide QCM crystal surfaces with their graphene oxide (GO) and reduced (r)GO coated counterparts, and report on the different lipid structures obtained. We establish graphene derivative coatings as support surfaces with tuneable hydrophobicity for the formation of controllable lipid structures. One structure of interest formed is lipid monolayer membranes which were formed on rGO, which are otherwise challenging to produce. We also demonstrate and monitor biotin-avidin binding on such a membrane, which will then serve as a platform for a wide range of biosensing applications. The QCM-D technique could be extended to both fundamental studies and applications of other covalent and non-covalent interactions in 2-dimensional materials.

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.

Substrate-Induced Structure and Molecular Dynamics in a Lipid Bilayer Membrane

The solid-substrate-dependent structure and dynamics of molecules in a supported lipid bilayer (SLB) were directly investigated via atomic force microscopy (AFM) and single particle tracking (SPT) measurements. The appearance of either vertical or horizontal heterogeneities in the SLB was found to be strongly dependent on the underlying substrates. SLB has been widely used as a biointerface with incorporated proteins and other biological materials. Both silica and mica are popular substrates for SLB. Using single-molecule dynamics, the fluidity of the upper and lower membrane leaflets was found to depend on the substrate, undergoing coupling and decoupling on the SiO₂/Si and mica substrates, respectively. The anisotropic diffusion caused by the locally destabilized structure of the SLB at atomic steps appeared on the Al₂O₃(0001) substrate because of the strong van der Waals interaction between the SLB and the substrate. Our finding that the well-defined surfaces of mica and sapphire result in asymmetry and anisotropy in the plasma membrane is useful for the design of new plasma-membrane-mimetic systems. The application of well-defined supporting substrates for SLBs should have similar effects as cell membrane scaffolds, which regulate the dynamic structure of the membrane.

A theoretical study on the morphological phase diagram of supported lipid bilayers

A morphological phase diagram is constructed using classical density function theory (CDFT).

Septin Interferes with the Temperature-Dependent Domain Formation and Disappearance of Lipid Bilayer Membranes

Domain formation or compartmentalization in a lipid bilayer membrane has been thought to take place dynamically in cell membranes and play important roles in the spatiotemporal regulation of their physiological functions. In addition, the membrane skeleton, which is a protein assembly beneath the cell membrane, also regulates the properties as well as the morphology of membranes because of its role as a diffusion barrier against constitutive molecules of the membrane or as a scaffold for physiological reactions. Therefore, it is important to study the relationship between lipid bilayer membranes and proteins that form the membrane skeleton. Among cytoskeletal systems, septin is unique because it forms arrays on liposomes that contain phosphoinositides, and this property is thought to contribute to the formation of the annulus in sperm flagellum. In this study, a supported lipid bilayer (SLB) was used to investigate the effect of septin on lipid bilayers because SLBs rather than liposomes are suitable for observation of the membrane domains formed. We found that SLBs containing phosphatidylinositol (PI) reversibly form domains by decreasing the temperature and that septin affects both the formation and the disappearance of the cooling-induced domain. Septin inhibits the growth of cooling-induced domains during decreases in temperature and inhibits the dispersion and the disappearance of those domains during increases in temperature. These results indicate that septin complexes, i.e., filaments or oligomers assembling on the surface of lipid bilayer membranes, can regulate the dynamics of domain formation via their behavior as an anchor for PI molecules.

Organic bioelectronics probing conformational changes in surface confined proteins

The study of proteins confined on a surface has attracted a great deal of attention due to its relevance in the development of bio-systems for laboratory and clinical settings. In this respect, organic bio-electronic platforms can be used as tools to achieve a deeper understanding of the processes involving protein interfaces. In this work, biotin-binding proteins have been integrated in two different organic thin-film transistor (TFT) configurations to separately address the changes occurring in the protein-ligand complex morphology and dipole moment. This has been achieved by decoupling the output current change upon binding, taken as the transducing signal, into its component figures of merit. In particular, the threshold voltage is related to the protein dipole moment, while the field-effect mobility is associated with conformational changes occurring in the proteins of the layer when ligand binding occurs. Molecular Dynamics simulations on the whole avidin tetramer in presence and absence of ligands were carried out, to evaluate how the tight interactions with the ligand affect the protein dipole moment and the conformation of the loops surrounding the binding pocket. These simulations allow assembling a rather complete picture of the studied interaction processes and support the interpretation of the experimental results.

Nanopore formation process in artificial cell membrane induced by plasma-generated reactive oxygen species

We investigated morphological change of an artificial lipid bilayer membrane induced by oxygen radicals which were generated by non-equilibrium atmospheric pressure plasma. Neutral oxygen species, O((3)Pj) and O2((1)Δg), were irradiated of a supported lipid bilayer existing under a buffer solution at various conditions of dose time and distances, at which the dose amounts of the oxygen species were calculated quantitatively. Observation using an atomic force microscope and a fluorescence microscope revealed that dose of the neutral oxygen species generated nanopores with the diameter of 10-50 nm in a phospholipid bilayer, and finally destructed the bilayer structure. We found that protrusions appeared on the lipid bilayer surface prior to the formation of nanopores, and we attributed the protrusions to the precursor of the nanopores. We propose a mechanism of the pore formation induced by lipid oxidation on the basis of previous experimental and theoretical studies.

Skin lipids: localization of ceramide and fatty acid in the unit cell of the long periodicity phase

The lipid matrix of the skin's stratum corneum plays a key role in the barrier function, which protects the body from desiccation. The lipids that make up this matrix consist of ceramides, cholesterol, and free fatty acids, and can form two coexisting crystalline lamellar phases: the long periodicity phase (LPP) and the short periodicity phase (SPP). To fully understand the skin barrier function, information on the molecular arrangement of the lipids in the unit cell of these lamellar phases is very desirable. To determine this arrangement in previous studies, we examined the molecular arrangement of the SPP. In this study, neutron diffraction studies were performed to obtain information on the molecular arrangement of the LPP. The diffraction pattern reveals nine diffraction orders attributed to the LPP with a repeating unit of 129.4 ± 0.5 Å. Using D2O/H2O contrast variation, the scattering length density profiles were calculated for protiated samples and samples that included either the perdeuterated acyl chain of the most abundant ceramide or the most abundant perdeuterated fatty acid. Both perdeuterated chains are predominantly located in the central part of the unit cell with substantial interdigitation of the acyl chains in the unit cell center. However, a fraction of the perdeuterated chains is also located near the border of the unit cell with their acyl chains directing toward the center. This arrangement of lipids in the LPP unit cell corresponds with the location of their lipid headgroups at the border and also inside of the unit cell at a well-defined position (±21 Å from the unit cell center), indicative of a three-layer lipid arrangement within the 129.4 ± 0.5 Å repeating unit.

A diffusive ink transport model for lipid dip-pen nanolithography

Despite diverse applications, phospholipid membrane stacks generated by dip-pen nanolithography (DPN) still lack a thorough and systematic characterization that elucidates the whole ink transport process from writing to surface spreading, with the aim of better controlling the resulting feature size and resolution. We report a quantitative analysis and modeling of the dependence of lipid DPN features (area, height and volume) on dwell time and relative humidity. The ink flow rate increases with humidity in agreement with meniscus size growth, determining the overall feature size. The observed time dependence indicates the existence of a balance between surface spreading and the ink flow rate that promotes differences in concentration at the meniscus/substrate interface. Feature shape is controlled by the substrate surface energy. The results are analyzed within a modified model for the ink transport of diffusive inks. At any humidity the dependence of the area spread on the dwell time shows two diffusion regimes: at short dwell times growth is controlled by meniscus diffusion while at long dwell times surface diffusion governs the process. The critical point for the switch of regime depends on the humidity.

High-Throughput Universal DNA Curtain Arrays for Single-Molecule Fluorescence Imaging

Single-molecule studies of protein-DNA interactions have shed critical insights into the molecular mechanisms of nearly every aspect of DNA metabolism. The development of DNA curtains-a method for organizing arrays of DNA molecules on a fluid lipid bilayer-has greatly facilitated these studies by increasing the number of reactions that can be observed in a single experiment. However, the utility of DNA curtains is limited by the challenges associated with depositing nanometer-scale lipid diffusion barriers onto quartz microscope slides. Here, we describe a UV lithography-based method for large-scale fabrication of chromium (Cr) features and organization of DNA molecules at these features for high-throughput single-molecule studies. We demonstrate this approach by assembling 792 independent DNA arrays (containing >900,000 DNA molecules) within a single microfluidic flowcell. As a first proof of principle, we track the diffusion of Mlh1-Mlh3-a heterodimeric complex that participates in DNA mismatch repair and meiotic recombination. To further highlight the utility of this approach, we demonstrate a two-lane flowcell that facilitates concurrent experiments on different DNA substrates. Our technique greatly reduces the challenges associated with assembling DNA curtains and paves the way for the rapid acquisition of large statistical data sets from individual single-molecule experiments.

Scanning tunneling microscope observation of the phosphatidylserine domains in the phosphatidylcholine monolayer

A mixed monolayer of 1,2-dihexanoyl-sn-glycero-3-phospho-l-serine (DHPS) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) on an 1-octanethiol-modified gold substrate was visualized on the nanometer scale using in situ scanning tunneling microscopy (STM) in aqueous solution. DHPS clusters were evident as spotty domains. STM enabled us to distinguish DHPS molecules from DHPC molecules depending on their electronic structures. The signal of the DHPS domains was abolished by neutralization with Ca(2+). The addition of the PS + Ca(2+)-binding protein of annexin V to the Ca(2+)-treated monolayer gave a number of spots corresponding to a single annexin V molecule.

Self-assembly formation of lipid bilayer coatings on bare aluminum oxide: overcoming the force of interfacial water

Widely used in catalysis and biosensing applications, aluminum oxide has become popular for surface functionalization with biological macromolecules, including lipid bilayer coatings. However, it is difficult to form supported lipid bilayers on aluminum oxide, and current methods require covalent surface modification, which masks the interfacial properties of aluminum oxide, and/or complex fabrication techniques with specific conditions. Herein, we addressed this issue by identifying simple and robust strategies to form fluidic lipid bilayers on aluminum oxide. The fabrication of a single lipid bilayer coating was achieved by two methods, vesicle fusion under acidic conditions and solvent-assisted lipid bilayer (SALB) formation under near-physiological pH conditions. Importantly, quartz crystal microbalance with dissipation (QCM-D) monitoring measurements determined that the hydration layer of a supported lipid bilayer on aluminum oxide is appreciably thicker than that of a bilayer on silicon oxide. Fluorescence recovery after photobleaching (FRAP) analysis indicated that the diffusion coefficient of lateral lipid mobility was up to 3-fold greater on silicon oxide than on aluminum oxide. In spite of this hydrodynamic coupling, the diffusion coefficient on aluminum oxide, but not silicon oxide, was sensitive to the ionic strength condition. Extended-DLVO model calculations estimated the thermodynamics of lipid-substrate interactions on aluminum oxide and silicon oxide, and predict that the range of the repulsive hydration force is greater on aluminum oxide, which in turn leads to an increased equilibrium separation distance. Hence, while a strong hydration force likely contributes to the difficulty of bilayer fabrication on aluminum oxide, it also confers advantages by stabilizing lipid bilayers with thicker hydration layers due to confined interfacial water. Such knowledge provides the basis for improved surface functionalization strategies on aluminum oxide, underscoring the practical importance of surface hydration.

Contribution of the hydration force to vesicle adhesion on titanium oxide

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

Substrate Effects on the Formation Process, Structure and Physicochemical Properties of Supported Lipid Bilayers

Supported lipid bilayers are artificial lipid bilayer membranes existing at the interface between solid substrates and aqueous solution. Surface structures and properties of the solid substrates affect the formation process, fluidity, two-dimensional structure and chemical activity of supported lipid bilayers, through the 1–2 nm thick water layer between the substrate and bilayer membrane. Even on SiO₂/Si and mica surfaces, which are flat and biologically inert, and most widely used as the substrates for the supported lipid bilayers, cause differences in the structure and properties of the supported membranes. In this review, I summarize several examples of the effects of substrate structures and properties on an atomic and nanometer scales on the solid-supported lipid bilayers, including our recent reports.

Gold surface supported spherical liposome-gold nano-particle nano-composite for label free DNA sensing

Immobilization of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) liposome-gold nano-particle (DOPE-AuNP) nano-composite covalently on 3-mercaptopropionic acid (MPA) on gold surface is demonstrated for the first time for electrochemical label free DNA sensing. Spherical nature of the DOPE on the MPA monolayer is confirmed by the appearance of sigmoidal voltammetric profile, characteristic behavior of linear diffusion, for the MPA-DOPE in presence of [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+) redox probes. The DOPE liposome vesicle fusion is prevented by electroless deposition of AuNP on the hydrophilic amine head groups of the DOPE. Immobilization of single stranded DNA (ssDNA) is made via simple gold-thiol linkage for DNA hybridization sensing in the presence of [Fe(CN)(6)](3-/4-). The sensor discriminates the hybridized (complementary target hybridized), un-hybridized (non-complementary target hybridized) and single base mismatch target hybridized surfaces sensitively and selectively without signal amplification. The lowest target DNA concentration detected is 0.1×10(-12)M. Cyclic voltammetry (CV), electrochemical impedance (EIS), differential pulse voltammetry (DPV) and quartz crystal microbalance (QCM) techniques are used for DNA sensing on DOPE-AuNP nano-composite. Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Atomic Force Microscopy (AFM), Dynamic Light Scattering (DLS) and Ultraviolet-Visible (UV) spectroscopic techniques are used to understand the interactions between the DOPE, AuNP and ssDNA. The results indicate the presence of an intact and well defined spherical DOPE-AuNP nano-composite on the gold surface. The method could be applied for fabrication of the surface based liposome-AuNP-DNA composite for cell transfection studies at reduced reagents and costs.

PHOSPHOLIPID ASSEMBLY ON SUPERPARAMAGNETIC NANOPARTICLES FOR THERMORESPONSIVE DRUG DELIVERY APPLICATIONS

Thermoresponsive nanocomposites were prepared by immobilizing a 2–3 nm thick phospholipid layer on the surface of superparamagnetic Fe3O4 nanoparticles via high-affinity avidin/biotin interactions. Morphological and physicochemical surface properties were assessed using transmission electron microscopy, confocal laser scanning microscopy, differential scanning calorimetry, and attenuated total reflectance Fourier transform infrared spectroscopy. The zeta potential of Fe3O4 colloids in phosphate buffered saline (PBS) decreased from -23.6 to -5.0 mV as a consequence of phospholipid immobilization. Nevertheless, heating properties of these superparamagnetic nanoparticles within an alternating magnetic field were not significantly affected. Hyperthermia-relevant temperatures > 40°C were achieved within 10–15 min using a 7-mT magnetic field alternating at a frequency of 1 MHz. Loading of the surface-associated phospholipid layer with the hydrophobic dye dansylcadaverine was accomplished at an efficiency of 479 ng/mg Fe3O4 . Release of this drug surrogate was temperature-dependent, resulting in a 2.5-fold greater release rate when nanoparticles were exposed to a temperature above the experimentally determined melting temperature of 39.7°C. These data underline the feasibility of preparing novel, stimulus-induced drug delivery systems where payload release from a colloid-immobilized phospholipid assembly is triggered by hyperthermia.

Anomalous diffusion in supported lipid bilayers induced by oxide surface nanostructures

Hierarchic structure and anomalous diffusion on submicrometer scale were introduced into an artificial cell membrane, and the spatiotemporal dependence of lipid diffusion was visualized on nanostructured oxide surfaces. We observed the lipid diffusion in supported lipid bilayers (SLBs) on step-and-terrace TiO(2)(100) and amorphous SiO(2)/Si surfaces by single molecule tracking (SMT) method. The SMT at the time resolution of 500 μs to 30 ms achieved observation of the lipid diffusion over the spatial and temporal ranges of 100 nm/millisecond to 1 μm/second. The temporal dependence of the diffusion coefficient in the SLB on TiO(2)(100) showed that the crossover from anomalous diffusion to random diffusion occurred around 10 ms. The surface fine architecture on substrates will be applicable to induce hierarchic structures on the order of 100 nm or less, which correspond to the microcompartment size in vivo.

Disposition of ceramide in model lipid membranes determined by neutron diffraction

The lipid matrix present in the uppermost layer of the skin, the stratum corneum, plays a crucial role in the skin barrier function. The lipids are organized into two lamellar phases. To gain more insight into the molecular organization of one of these lamellar phases, we performed neutron diffraction studies. In the diffraction pattern, five diffraction orders were observed attributed to a lamellar phase with a repeat distance of 5.4 nm. Using contrast variation, the scattering length density profile could be calculated showing a typical bilayer arrangement. To obtain information on the arrangement of ceramides in the unit cell, a mixture that included a partly deuterated ceramide was also examined. The scattering length density profile of the 5.4-nm phase containing this deuterated ceramide demonstrated a symmetric arrangement of the ceramides with interdigitating acyl chains in the center of the unit cell.

Intrinsic heterogeneity in liposome suspensions caused by the dynamic spontaneous formation of hydrophobic active sites in lipid membranes

The spontaneous, dynamic formation of hydrophobic active sites in lipid bilayer membranes is studied and characterized. It is shown that the rates of formation and consumption of these active sites control at least two important properties of liposomes: their affinity for hydrophobic surfaces and the rate by which they spontaneously release encapsulated molecules. The adhesion and spreading of liposomes onto hydrophobic polystyrene nanoparticles and the spontaneous leakage of an encapsulated fluorescent dye were monitored for different liposome compositions employing Cryo-TEM, DLS, and fluorescence measurements. It was observed that an apparently homogeneous, monodisperse liposome suspension behaves as if composed by two different populations: a fast leaking population that presents affinity for the hydrophobic substrate employed, and a slow leaking population that does not attach immediately to it. The results reported here suggest that the proportion of liposomes in each population changes over time until a dynamic equilibrium is reached. It is shown that this phenomenon can lead to irreproducibility in, for example, spontaneous leakage experiments, as extruded liposomes leak much faster just after preparation than 24 h afterward. Our findings account for discrepancies in several experimental results reported in the literature. To our knowledge, this is the first systematic study addressing the issue of an existing intrinsic heterogeneity of liposome suspensions.

Understanding protein adsorption phenomena at solid surfaces

Protein adsorption at solid surfaces plays a key role in many natural processes and has therefore promoted a widespread interest in many research areas. Despite considerable progress in this field there are still widely differing and even contradictive opinions on how to explain the frequently observed phenomena such as structural rearrangements, cooperative adsorption, overshooting adsorption kinetics, or protein aggregation. In this review recent achievements and new perspectives on protein adsorption processes are comprehensively discussed. The main focus is put on commonly postulated mechanistic aspects and their translation into mathematical concepts and model descriptions. Relevant experimental and computational strategies to practically approach the field of protein adsorption mechanisms and their impact on current successes are outlined.

Phospholipid bilayer formation on a variety of nanoporous oxide and organic xerogel films

Lipid bilayers supported by nanoporous xerogel materials are being explored as models for cell membranes. In order to better understand and characterize the nature of the surface-bilayer interactions, several oxide and organic nanoporous xerogel films (alumina, titania, iron oxide, phloroglucinol-formaldehyde, resorcinol-formaldehyde and cellulose acetate) have been investigated as a scaffold for vesicle-fused 1,2-dioleoyl-glycero-3-phosphocholine (DOPC) lipid bilayer formation and mobility. The surface topography of the different substrates was analyzed using contact and tapping-mode atomic force microscopy and the surface energy of the substrates was determined using contact angle goniometry. Lipid bilayer formation has been observed with fluorescence microscopy and lateral lipid diffusion coefficients have been determined using fluorescence recovery after photobleaching. Titania xerogel films were found to be a robust and convenient support for formation of a two-phase DOPC/1,2-distearoyl-glycero-3-phosphocholine bilayer and domains were observed with this system. It was found that the cellulose acetate xerogel film support produced the slowest lipid lateral diffusion.

Evolution of supported planar lipid bilayers on step-controlled sapphire surfaces

Self-organized step/terrace structures on a sapphire surface were used to investigate interface properties between a solid surface and a supported planar lipid bilayer (SPB). We prepared random-stepped, single-stepped and multistepped sapphire surfaces. Some multistepped surfaces covered with crossing steps exhibit phase-separation into hydrophilic and hydrophobic domains. We studied evolution of self-spreading lipid bilayers that are subject to the atomic structures and chemical states on the surfaces. The growth direction of SPBs in the self-spreading method is regulated by the atomic steps. While the SPBs were apparently uniform after a 1 h self-spreading, a density gradient of the lipid molecules was observed even after 24 h spreading. We found that various patterns of the SPBs that depend on the density of the lipid molecules are self-assembled on the phase-separated surfaces. Although the SPB is supported on the sapphire surface via an about 1 nm water layer, the self-spreading direction and the morphology of the SPBs are affected by the atomic steps, whose height is much smaller than that of the water layer.

Influence of phase separating lipids on supported lipid bilayer formation at SiO2 surfaces

The importance of the lipid phase on the formation of supported lipid bilayers (SLBs) via vesicle fusion and on the resulting SLB homogeneity at SiO(2) surfaces has been studied by the quartz crystal microbalance with dissipation (QCM-D) monitoring technique. Physiologically relevant lipid compositions were chosen to correspond to different regions (l(d), l(o) and coexistence of phases) in established phase diagrams of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), N-palmitoyl-D-erythro-sphingosylphosphorylcholine (PSM) and cholesterol. For most compositions, SLBs formed through vesicle rupture in a critical-surface-coverage dependent manner. Inclusion of PSM and cholesterol into POPC vesicles significantly impaired the vesicle rupture process such that a higher critical concentration of vesicles on the surface was needed before the rupture process started. When increasing the cholesterol content the vesicles formed SLBs containing more defects in the form of intact vesicles adsorbed on the surface up to a point (l(o) phase) where vesicles did not break at all but formed supported vesicular layers. The hampering of vesicle rupture is interpreted in terms of the ability of cholesterol to accommodate vesicle deformation. Experiments using elevated temperatures to alter the lipid membrane into a more fluid phase significantly improved the quality of the SLB showing the importance of both cholesterol content and the lipid phase on SLB homogeneity.

Lipid bilayer membrane with atomic step structure: supported bilayer on a step-and-terrace TiO2(100) surface

The formation of a supported planar lipid bilayer (SPLB) and its morphology on step-and-terrace rutile TiO 2(100) surfaces were investigated by fluorescence microscopy and atomic force microscopy. The TiO 2(100) surfaces consisting of atomic steps and flat terraces were formed on a rutile TiO 2 single-crystal wafer by a wet treatment and annealing under a flow of oxygen. An intact vesicular layer formed on the TiO 2(100) surface when the surface was incubated in a sonicated vesicle suspension under the condition that a full-coverage SPLB forms on SiO 2, as reported in previous studies. However, a full-coverage, continuous, fluid SPLB was obtained on the step-and-terrace TiO 2(100) depending on the lipid concentration, incubation time, and vesicle size. The SPLB on the TiO 2(100) also has step-and-terrace morphology following the substrate structure precisely even though the SPLB is in the fluid phase and an approximately 1-nm-thick water layer exists between the SPLB and the substrate. This membrane distortion on the atomic scale affects the phase-separation structure of a binary bilayer of micrometer order. The interaction energy calculated including DLVO and non-DLVO factors shows that a lipid membrane on the TiO 2(100) gains 20 times more energy than on SiO 2. This specifically strong attraction on TiO 2 makes the fluid SPLB precisely follow the substrate structure of angstrom order.