Influence of Divalent Cations on Deformation and Rupture of Adsorbed Lipid Vesicles

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive "steric-hydration" forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Engineering phosphatidylinositol-4,5-bisphosphate model membranes enriched in endocytic cargo: A neutron reflectometry, AFM and QCM-D structural study

The combination of in vitro models of biological membranes based on solid-supported lipid bilayers (SLBs) and of surface sensitive techniques, such as neutron reflectometry (NR), atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D), is well suited to provide quantitative information about molecular level interactions and lipid spatial distributions. In this work, cellular plasma membranes have been mimicked by designing complex SLB, containing phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) lipids as well as incorporating synthetic lipo-peptides that simulate the cytoplasmic tails of transmembrane proteins. The QCM-D results revealed that the adsorption and fusion kinetics of PtdIns4,5P2 are highly dependent of Mg2+. Additionally, it was shown that increasing concentrations of PtdIns4,5P2 leads to the formation of SLBs with higher homogeneity. The presence of PtdIns4,5P2 clusters was visualized by AFM. NR provided important insights about the structural organization of the various components within the SLB, highlighting that the leaflet symmetry of these SLBs is broken by the presence of CD4-derived cargo peptides. Finally, we foresee our study to be a starting point for more sophisticated in vitro models of biological membranes with the incorporation of inositol phospholipids and synthetic endocytic motifs.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC₁₁S) and dodecanethiolate (CH₃C₁₁S) or hydroxylundecanethiolate (HOC₁₁S) comprising different surface mole fractions of ferrocene (χ_Fc^surf). An increase in the surface hydrophilicity and surface free energy of the FcC₁₁S/HOC₁₁S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χ_Fc^surf. SLBs of ≳80% area coverage form on the FcC₁₁S/HOC₁₁S SAM for all the phospholipid types down to χ_Fc^surf of at least 0.2, composition yielding a water contact angle (θ_W) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC₁₁S/CH₃C₁₁S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χ_Fc^surf ≳ 0.6 (θ_W = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

Accurate Metal-Imidazole Interactions

Modeling the interaction between a metal ion and small molecules can provide pivotal information to bridge and close the gap between two types of simulations: metal ions in water and metal ions in metalloproteins. As previously established, the 12-6-4 Lennard-Jones (LJ)-type nonbonded model, because of its ability to account for the induced dipole effect, has been highly successful in simulating metal ion systems. Using the potential of mean force (PMF) method, the polarizability of the metal-chelating nitrogen from two types of imidazole molecules, delta nitrogen protonated (HID) and epsilon nitrogen protonated (HIE), has been parametrized against experiment for 11 metals (Ag(I), Ca(II), Cd(II), Co(II), Cu(I), Cu(II), Fe(II), Mg(II), Mn(II), Ni(II), and Zn(II)) in conjunction with three commonly used water models (TIP3P, SPC/E, and OPC). We show that the standard 12-6 and unmodified 12-6-4 models are not able to accurately model these interactions and, indeed, predict that the complex should be unstable. The resultant parameters further establish the flexibility and the reliability of the 12-6-4 LJ-type nonbonded model, which can correctly describe three-component interactions between a metal, ligand, and solvent by simply tuning the polarizability of the chelating atom. Also, the transferability of this model was tested, showing the capability of describing metal-ligand interactions in various environments.

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

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

Unraveling How Multivalency Triggers Shape Deformation of Sub-100 nm Lipid Vesicles

Multivalent ligand-receptor interactions are critical to the function of membrane-enveloped biological and biomimetic nanoparticles, yet resulting nanoparticle shape changes are rarely investigated. Using the localized surface plasmon resonance (LSPR) sensing technique, we tracked the attachment of biotinylated, sub-100 nm lipid vesicles to a streptavidin-functionalized supported lipid bilayer (SLB) and developed an analytical model to extract quantitative details about the vesicle-SLB contact region. The experimental results were supported by theoretical analyses of biotin-streptavidin complex formation and corresponding structural and energetic aspects of vesicle deformation. Our findings reveal how varying the surface densities of streptavidin receptors in the SLB and biotin ligands in the vesicles affects the extent of nanometer-scale vesicle deformation. We also identify conditions, i.e., a critical ligand density, at which appreciable vesicle deformation began, which provides insight into how the membrane bending energy partially counterposes the multivalent binding interaction energy. These findings are generalizable to various multivalent ligand-receptor systems.

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.

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

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

Biologically interfaced nanoplasmonic sensors

Understanding biointerfacial processes is crucial in various fields across fundamental and applied biology, but performing quantitative studies via conventional characterization techniques remains challenging due to instrumentation as well as analytical complexities and limitations. In order to accelerate translational research and address current challenges in healthcare and medicine, there is an outstanding need to develop surface-sensitive technologies with advanced measurement capabilities. Along this line, nanoplasmonic sensing has emerged as a powerful tool to quantitatively study biointerfacial processes owing to its high spatial resolution at the nanoscale. Consequently, the development of robust biological interfacing strategies becomes imperative to maximize its characterization potential. This review will highlight and discuss the critical role of biological interfacing within the context of constructing nanoplasmonic sensing platforms for biointerfacial science applications. Apart from paving the way for the development of highly surface-sensitive characterization tools that will spur fundamental biological interaction studies and improve the overall understanding of biological processes, the basic principles behind biointerfacing strategies presented in this review are also applicable to other fields that involve an interface between an inorganic material and a biological system.

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

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

Systematic Parametrization of Divalent Metal Ions for the OPC3, OPC, TIP3P-FB, and TIP4P-FB Water Models

Divalent metal ions play important roles in biological and materials systems. Molecular dynamics simulation is an efficient tool to investigate these systems at the microscopic level. Recently, four new water models (OPC3, OPC, TIP3P-FB, and TIP4P-FB) have been developed and better represent the physical properties of water than previous models. Metal ion parameters are dependent on the water model employed, making it necessary to develop metal ion parameters for select new water models. In the present work, we performed parameter scanning for the 12-6 Lennard-Jones nonbonded model of divalent metal ions in conjunction with the four new water models as well as four previous water models (TIP3P, SPC/E, TIP4P, and TIP4P-Ew). We found that these new three-point and four-point water models provide comparable or significantly improved performance for the simulation of divalent metal ions when compared to previous water models in the same category. Among all eight water models, the OPC3 water model yields the best performance for the simulation of divalent metal ions in the aqueous phase when using the 12-6 model. On the basis of the scanning results, we independently parametrized the 12-6 model for 24 divalent metal ions with each of the four new water models. As noted previously, the 12-6 model still fails to simultaneously reproduce the experimental hydration free energy (HFE) and ion-oxygen distance (IOD) values even with these new water models. To solve this problem, we parametrized the 12-6-4 model for the 16 divalent metal ions for which we have both experimental HFE and IOD values for each of the four new water models. The final parameters are able to reproduce both the experimental HFE and IOD values accurately. To validate the transferability of our parameters, we carried out benchmark calculations to predict the energies and geometries of ion-water clusters as well as the ion diffusivity coefficient of Mg²⁺. By comparison to quantum chemical calculations and experimental data, these results show that our parameters are well designed and have excellent transferability. The metal ion parameters for the 12-6 and 12-6-4 models reported herein can be employed in simulations of various biological and materials systems when using the OPC3, OPC, TIP3P-FB, or TIP4P-FB water model.

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.

Temperature-Dependent Dynamics of Giant Vesicles Composed of Hydrolysable Lipids Having an Amide Linkage

Various amphiphiles including surfactants and lipids have been designed and synthesized to improve and create new functionalities. In particular, the emergence of cell-like behaviors of giant vesicles (GVs) composed of synthetic lipids has drawn much attention in the development of chemical models for cells. The aim of this study was to measure temperature-dependent morphological changes of GVs induced by fragmentation and subsequent growth using hydrolysable cationic lipids having an amide linkage. Results from differential scanning calorimetry, fluorescence spectroscopy using an environment-responsive probe, and confocal Raman microscopy showed that the dynamics observed were due to changes in the vesicle membrane, including variation in the lipid composition, induced by thermal stimulation.

Immobilization of natural lipid biomembranes and their interactions with choline carboxylates. A nanoplasmonic sensing study

The cell membrane is mainly composed of lipid bilayers with inserted proteins and carbohydrates. Lipid bilayers made of purified or synthetic lipids are widely used for estimating the effect of target compounds on cell membranes. However, the composition of such biomimetic membranes is much simpler than the composition of biological membranes. Interactions between compounds and simple composition biomimetic membranes might not demonstrate the effect of target compounds as precisely as membranes with compositions close to real organisms. Therefore, the aim of our study is to construct biomimetic membrane closely mimicking the state of natural membranes. Liposomes were prepared from lipids extracted from L-α-phosphatidylcholine, Escherichia coli, yeast (Saccharomyces cerevisiae) and bovine liver cells through agitation and sonication. They were immobilized onto silicon dioxide (SiO₂) sensor surfaces using N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid buffer with calcium chloride. The biomimetic membranes were successfully immobilized onto the SiO₂ sensor surface and detected by nanoplasmonic sensing. The immobilized membranes were exposed to choline carboxylates. The membrane disruption effect was, as expected, more pronounced with increasing carbohydrate chain length of the carboxylates. The results correlated with the toxicity values determined using Vibrio fischeri bacteria. The yeast extracted lipid membranes had the strongest response to introduction of choline laurate while the bovine liver lipid extracted liposomes were the most sensitive towards the shorter choline carboxylates. This implies that the composition of the cell membrane plays a crucial role upon interaction with choline carboxylates, and underlines the necessity of testing membrane systems of different origin to obtain an overall image of such interactions.

Interactions of Monovalent and Divalent Cations at Palmitoyl-Oleoyl-Phosphatidylcholine Interface

Li⁺ is a biologically active and medically important cation. Experiments show that Li⁺ modulates some phospholipid bilayer properties in a manner similar to divalent cations, rather than other monovalent cations. We previously performed a comparative simulation study of the interaction of several monovalent cations with palmitoyl-oleoyl-phosphatidylcholine bilayers and reported that Li⁺ exhibited the highest association with lipids and formed a unique tetrahedral coordinated structure with lipid head groups. Here we extend these studies to two biologically important divalent cations, Mg²⁺ and Ca²⁺, and observe that, just like monovalent cations, Mg²⁺ and Ca²⁺ reduce bilayer areas and increase chain order. Bilayer area changes induced by cations are strongly correlated with the amount of charge inside the headgroup region; however, Mg²⁺ and Li⁺ are clear outliers. At the same time though, Mg²⁺ adsorption in the bilayer is the smallest among all cations, which is in contrast to Li⁺ that binds strongly to lipids. In fact, in contrast to all other cations, Mg²⁺ remains fully hydrated in the lipid headgroup region. However, Li⁺ and Mg²⁺ share high overlap between their inner-shell coordination topologies. This suggests that Li⁺ can structurally replace Mg²⁺, which is bound to other biomolecules with up to fourfold coordination, provided such replacement is energetically feasible. We compute structural topologies and compare them quantitatively using a new weighted-graphs-based method. Finally, we find that the specificity of cation interaction with lipid head groups exhibit consistent trend with the solvation shell energetics of ions in lipid headgroup and bulk water regions.

Quartz Crystal Microbalances as Tools for Probing Protein-Membrane Interactions

Extensive studies on the spontaneous collapse of phospholipid vesicles into supported lipid bilayers (SLBs) have led to procedures which allow SLB formation on a wealth of substrates and lipid compositions. SLBs provide a widely accepted and versatile model system which mimics the natural cell membrane separating the extracellular and intracellular fluids of the living cell. The quartz crystal microbalance with dissipation monitoring (QCM-D) has been central in both the understanding of vesicle collapse into SLBs on various substrates but also in probing the kinetics and mechanisms of biomolecular interactions with SLBs in real time. We describe a robust procedure to form SLBs of zwitterionic and charged lipids on SiO₂ sensor crystals which subsequently can be exploited to probe the interaction between proteins and peptides with the SLB.

Determination of the Main Phase Transition Temperature of Phospholipids by Nanoplasmonic Sensing

Our study demonstrates that nanoplasmonic sensing (NPS) can be utilized for the determination of the phase transition temperature (T_m) of phospholipids. During the phase transition, the lipid bilayer undergoes a conformational change. Therefore, it is presumed that the T_m of phospholipids can be determined by detecting conformational changes in liposomes. The studied lipids included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Liposomes in gel phase are immobilized onto silicon dioxide sensors and the sensor cell temperature is increased until passing the T_m of the lipid. The results show that, when the system temperature approaches the T_m, a drop of the NPS signal is observed. The breakpoints in the temperatures are 22.5 °C, 41.0 °C, and 55.5 °C for DMPC, DPPC, and DSPC, respectively. These values are very close to the theoretical T_m values, i.e., 24 °C, 41.4 °C, and 55 °C for DMPC, DPPC, and DSPC, respectively. Our studies prove that the NPS methodology is a simple and valuable tool for the determination of the T_m of phospholipids.

Temperature-Induced Denaturation of BSA Protein Molecules for Improved Surface Passivation Coatings

Bovine serum albumin (BSA) is the most widely used protein for surface passivation applications, although it has relatively weak, nonsticky interactions with hydrophilic surfaces such as silica-based materials. Herein, we report a simple and versatile method to increase the stickiness of BSA protein molecules adsorbing onto silica surfaces, resulting in up to a 10-fold improvement in blocking efficiency against serum biofouling. Circular dichroism spectroscopy, dynamic light scattering, and nanoparticle tracking analysis showed that temperature-induced denaturation of BSA proteins in bulk solution resulted in irreversible unfolding and protein oligomerization, thereby converting weakly adhesive protein monomers into a more adhesive oligomeric form. The heat-treated, denatured BSA oligomers remained stable after cooling. Room-temperature quartz crystal microbalance-dissipation and localized surface plasmon resonance experiments revealed that denatured BSA oligomers adsorbed more quickly and in larger mass quantities onto silica surfaces than native BSA monomers. We also determined that the larger surface contact area of denatured BSA oligomers is an important factor contributing to their more adhesive character. Importantly, denatured BSA oligomers were a superior passivating agent to inhibit biofouling on silica surfaces and also improved Western blot application performance. Taken together, the findings demonstrate how temperature-induced denaturation of BSA protein molecules can lead to improved protein-based coatings for surface passivation applications.

Effects of mono- and di-valent metal cations on the morphology of lipid vesicles

Lipid vesicles are an attractive model membrane experimental platform that is widely used in a biological context. The stability of vesicles can affect their performance and depends on various experimental conditions. How bio-related ions affect vesicle morphology is poorly understood in some cases. Herein, we investigated changes in vesicle morphology influenced by cation in the static and flowing environments. The effects of different mono- and di-valent metal cations on the morphology of lipid vesicles were systematically studied using the various techniques. The results showed that divalent cations caused significant aggregation or fusion of lipid vesicles, but monovalent cations had little effect on the vesicle morphology. Cation binding increased the net surface potential of vesicles, leading to changes in the zeta potential. The same qualitative kinetics were observed for cations that had the same valence at the same ionic strength. However, different types of cations gave different quantitative effects. The order of the ability to destroy the vesicle morphology was Cu²⁺ > Mg²⁺ > Ca²⁺ > Na⁺ > K⁺. These results are of practical value in the use of lipid vesicles as a bionic model, and help to shed light on the role of ions at membrane surfaces and interfaces.

Surface Plasmon Resonance Study of the Binding of PEO-PPO-PEO Triblock Copolymer and PEO Homopolymer to Supported Lipid Bilayers

Poloxamer 188 (P188), a poly(ethylene oxide)- b-poly(propylene oxide)- b-poly(ethylene oxide) triblock copolymer, protects cell membranes against various external stresses, whereas poly(ethylene oxide) (PEO; 8600 g/mol) homopolymer lacks protection efficacy. As part of a comprehensive effort to elucidate the protection mechanism, we used surface plasmon resonance (SPR) to obtain direct evidence of binding of the polymers onto supported lipid bilayers. Binding kinetics and coverage of P188 and PEO were examined and compared. Most notably, PEO exhibited membrane association comparable to that of P188, evidenced by comparable association rate constants and coverage. This result highlights the need for additional mechanistic understanding beyond simple membrane association to explain the differential efficacy of P188 in therapeutic applications.

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

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

Nanoplasmonic Sensing and Capillary Electrophoresis for Fast Screening of Interactions between Phosphatidylcholine Biomembranes and Surfactants

Nanoplasmonic sensing (NPS), based on localized surface plasmon resonance, with sensors composed of glass covered with golden nanodisks and overlaid with a SiO2 coating was applied in this study. Egg phosphatidylcholine (eggPC), being an easily accessible membrane-forming lipid, was used for preparation of biomimicking membranes. Small unilamellar vesicles with an approximate hydrodynamic diameter of 30 nm, formed by sonication in 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid buffer, were adsorbed within 10 min on the sensor surface either as intact vesicles or as a planar bilayer. The adsorbed biomembrane systems were further utilized for interaction studies with four different well-known surfactants (negatively and positively charged, zwitterionic, and nonionic) and each surfactant was tested at concentrations below and above the critical micelle concentration (CMC). Our results allowed the evaluation of different NPS patterns for every particular supported membrane system, surfactant, and its concentration. The most significant effect on the membrane was achieved upon the introduction of zwitterionic surfactant micelles, which in fact completely solubilized and removed the lipid membranes from the sensor surface. Other surfactant micelles interacted with the membranes and formed mixed structures remaining on the sensor surface. The studies performed at the concentrations below the CMCs of the surfactants showed that different mixed systems were formed. Depending on the supported membrane system and the type of surfactant, the mixed systems indicated different formation kinetics. Additionally, the final water rinse revealed the stability of the formed systems. To investigate the effect of the studied surfactants on the overall surface charge of the biomembrane, capillary electrophoresis (CE) experiments were carried out in parallel with the NPS analysis. The electroosmotic flow mobility of an eggPC-coated fused silica capillary was used to measure the total surface charge of the biomembrane after its treatment with the surfactants. Our results indicated in general good correlation between CE and NPS data. However, some discrepancies were seen while applying either zwitterionic or positively charged surfactants. This confirmed that CE analysis was able to provide additional data about the investigated systems. Taken together, the combination of NPS and CE proved to be an efficient way to describe the nature of interactions between biomimicking membranes and amphiphilic molecules.

Nanoplasmonic sensors for biointerfacial science

In recent years, nanoplasmonic sensors have become widely used for the label-free detection of biomolecules across medical, biotechnology, and environmental science applications. To date, many nanoplasmonic sensing strategies have been developed with outstanding measurement capabilities, enabling detection down to the single-molecule level. One of the most promising directions has been surface-based nanoplasmonic sensors, and the potential of such technologies is still emerging. Going beyond detection, surface-based nanoplasmonic sensors open the door to enhanced, quantitative measurement capabilities across the biointerfacial sciences by taking advantage of high surface sensitivity that pairs well with the size of medically important biomacromolecules and biological particulates such as viruses and exosomes. The goal of this review is to introduce the latest advances in nanoplasmonic sensors for the biointerfacial sciences, including ongoing development of nanoparticle and nanohole arrays for exploring different classes of biomacromolecules interacting at solid-liquid interfaces. The measurement principles for nanoplasmonic sensors based on utilizing the localized surface plasmon resonance (LSPR) and extraordinary optical transmission (EOT) phenomena are first introduced. The following sections are then categorized around different themes within the biointerfacial sciences, specifically protein binding and conformational changes, lipid membrane fabrication, membrane-protein interactions, exosome and virus detection and analysis, and probing nucleic acid conformations and binding interactions. Across these themes, we discuss the growing trend to utilize nanoplasmonic sensors for advanced measurement capabilities, including positional sensing, biomacromolecular conformation analysis, and real-time kinetic monitoring of complex biological interactions. Altogether, these advances highlight the rich potential of nanoplasmonic sensors and the future growth prospects of the community as a whole. With ongoing development of commercial nanoplasmonic sensors and analytical models to interpret corresponding measurement data in the context of biologically relevant interactions, there is significant opportunity to utilize nanoplasmonic sensing strategies for not only fundamental biointerfacial science, but also translational science applications related to clinical medicine and pharmaceutical drug development among countless possibilities.

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.

Nanoplasmonic sensors for detecting circulating cancer biomarkers

The detection of cancer biomarkers represents an important aspect of cancer diagnosis and prognosis. Recently, the concept of liquid biopsy has been introduced whereby diagnosis and prognosis are performed by means of analyzing biological fluids obtained from patients to detect and quantify circulating cancer biomarkers. Unlike conventional biopsy whereby primary tumor cells are analyzed, liquid biopsy enables the detection of a wide variety of circulating cancer biomarkers, including microRNA (miRNA), circulating tumor DNA (ctDNA), proteins, exosomes and circulating tumor cells (CTCs). Among the various techniques that have been developed to detect circulating cancer biomarkers, nanoplasmonic sensors represent a promising measurement approach due to high sensitivity and specificity as well as ease of instrumentation and operation. In this review, we discuss the relevance and applicability of three different categories of nanoplasmonic sensing techniques, namely surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS), for the detection of different classes of circulating cancer biomarkers.

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

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

Effect of Glucose on the Mobility of Membrane-Adhering Liposomes

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

Formation of supported lipid bilayers of charged E. coli lipids on modified gold by vesicle fusion

We describe a simple way of fusing E. coli lipid vesicles onto a gold surface. Supported lipid bilayers on metal surfaces are interesting for several reasons: transducing a biological signal to an electric readout, using surface analytical tools such as Surface Plasmon Resonance (SPR), Infrared Reflection Absorption Spectroscopy, Neutron Reflectivity or Electrochemistry. The most widely used method to prepare supported lipid membranes is fusion of preexisting liposomes. It is quite efficient on hydrophilic surfaces such as glass, mica or SiO₂, but vesicle fusion on metals and metal oxide surfaces (as gold, titanium oxide or indium tin oxide), remains a challenge, particularly for vesicles containing charged lipids, as is the case of bacterial lipids. We describe a simple method based on modifying the gold surface with a charged mercaptopropionic acid self-assembled monolayer and liposomes partially solubilized with detergent. The formed bilayers were characterized using a Quartz Crystal Microbalance with dissipation (QCM-D) and Atomic Force Microscopy (AFM). Some advantages of this protocol are that the stability of the self-assembled monolayer allows for repeated use of the substrate after detergent removal of the bilayer and that the amount of detergent required for optimal fusion can be determined previously using the lipid-detergent solubility curve.

Surface Charge and Overlayer pH Influence the Dynamics of Supported Phospholipid Films

Understanding the thermodynamics and kinetics of interactions between model lipid bilayers and planar supports is of critical importance in the furtherance of biosensing and the creation of biomimetic devices. Evaluating these properties can be accomplished through understanding the diffusional properties of the bilayer constituents. In this report, the dynamics of a model DMPC bilayer supported on a phosphorylated silica surface are studied in the presence and absence of interfacial Ca²⁺ as a function of pH of the aqueous overlayer. The data for this system reveal the importance of the balance of ionic interactions between the interfacial species, and the dependence of the diffusional, kinetic and thermodynamic properties of the system on pH. The thermodynamic data suggest that interactions between the bilayer and surface are mediated enthalpically rather than entropically.

Detection of Amphipathic Viral Peptide on Screen-Printed Electrodes by Liposome Rupture Impact Voltammetry

Detection of infectious viruses and disease biomarkers is of utmost importance in clinical screening for effective identification and treatment of diseases. We demonstrate here the use of liposome rupture impact voltammetry for the qualitative detection of model amphipathic viral peptide on a screen-printed electrode. This novel, proof-of-concept method was proposed for the quick and reliable detection of viruses by nonfaradaic liposome rupture impact voltammetry with the aid of 1,2-dioleoyl-sn-glycero-3-phosphocholine liposomes. This provides an avenue for the development of future on-site, point-of-care detection devices for medical and biological applications.

Cholesterol affects the interaction between an ionic liquid and phospholipid vesicles. A study by differential scanning calorimetry and nanoplasmonic sensing

The present work aims at studying the interactions between cholesterol-rich phosphatidylcholine-based lipid vesicles and trioctylmethylphosphonium acetate ([P₈₈₈₁][OAc]), a biomass dissolving ionic liquid (IL). The effect of cholesterol was assayed by using differential scanning calorimetry (DSC) and nanoplasmonic sensing (NPS) measurement techniques. Cholesterol-enriched dipalmitoyl-phosphatidylcholine vesicles were exposed to different concentrations of the IL, and the derived membrane perturbation was monitored by DSC. The calorimetric data could suggest that the binding and infiltration of the IL are delayed in the vesicles containing cholesterol. To clarify our findings, NPS was applied to quantitatively follow the resistance of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine incorporating 0, 10, and 50mol% of cholesterol toward the IL exposure over time. The membrane perturbation induced by different concentrations of IL was found to be a concentration dependent process on cholesterol-free lipid vesicles. Moreover, our results showed that lipid depletion in cholesterol-enriched lipid vesicles is inversely proportional to the increasing amount of cholesterol in the vesicles. These findings support that cholesterol-rich lipid bilayers are less susceptible toward membrane disrupting agents as compared to membranes that do not incorporate any sterols. This probably occurs because cholesterol tightens the phospholipid acyl chain packing of the plasma membranes, increasing their resistance and reducing their permeability.

Probing the Interaction of Dielectric Nanoparticles with Supported Lipid Membrane Coatings on Nanoplasmonic Arrays

The integration of supported lipid membranes with surface-based nanoplasmonic arrays provides a powerful sensing approach to investigate biointerfacial phenomena at membrane interfaces. While a growing number of lipid vesicles, protein, and nucleic acid systems have been explored with nanoplasmonic sensors, there has been only very limited investigation of the interactions between solution-phase nanomaterials and supported lipid membranes. Herein, we established a surface-based localized surface plasmon resonance (LSPR) sensing platform for probing the interaction of dielectric nanoparticles with supported lipid bilayer (SLB)-coated, plasmonic nanodisk arrays. A key emphasis was placed on controlling membrane functionality by tuning the membrane surface charge vis-à-vis lipid composition. The optical sensing properties of the bare and SLB-coated sensor surfaces were quantitatively compared, and provided an experimental approach to evaluate nanoparticle-membrane interactions across different SLB platforms. While the interaction of negatively-charged silica nanoparticles (SiNPs) with a zwitterionic SLB resulted in monotonic adsorption, a stronger interaction with a positively-charged SLB resulted in adsorption and lipid transfer from the SLB to the SiNP surface, in turn influencing the LSPR measurement responses based on the changing spatial proximity of transferred lipids relative to the sensor surface. Precoating SiNPs with bovine serum albumin (BSA) suppressed lipid transfer, resulting in monotonic adsorption onto both zwitterionic and positively-charged SLBs. Collectively, our findings contribute a quantitative understanding of how supported lipid membrane coatings influence the sensing performance of nanoplasmonic arrays, and demonstrate how the high surface sensitivity of nanoplasmonic sensors is well-suited for detecting the complex interactions between nanoparticles and lipid membranes.

Optimizing the Formation of Supported Lipid Bilayers from Bicellar Mixtures

Supported lipid bilayers (SLBs) are widely studied model membrane platforms that are compatible with various surface-sensitive measurement techniques. SLBs are typically formed on silica-based materials, and there are numerous possible fabrication routes involving either bottom-up molecular self-assembly or vesicle adsorption and rupture. In between these two classes of fabrication strategies lies an emerging approach based on depositing quasi-two-dimensional lamellar, bicellar disks composed of a mixture of long-chain and short-chain phospholipids to promote the formation of SLBs. This approach takes advantage of the thermodynamic preference of long-chain phospholipids to form planar SLBs, whereas short-chain phospholipids have brief residence times. Although a few studies have shown that SLBs can be formed on silica-based materials from bicellar mixtures, outstanding questions remain about the self-assembly mechanism as well as the influence of the total phospholipid concentration, ratio of the two phospholipids (termed the "q-ratio"), and process of sample preparation. Herein, we address these questions through comprehensive quartz crystal microbalance-dissipation, fluorescence microscopy, and fluorescence recovery after photobleaching experiments. Our findings identify that optimal SLB formation occurs at lower total concentrations of phospholipids than previously used as short-chain phospholipids behave like membrane-destabilizing detergents at higher concentrations. Using lower phospholipid concentrations, we also discovered that the formation of SLBs proceeds through a two-step mechanism involving a critical coverage of bicellar disks akin to vesicle fusion. In addition, the results indicate that at least one cycle of freeze-thaw-vortexing is useful during the sample preparation process to produce SLBs. Taken together, the findings in this work identify optimal routes for fabricating SLBs from bicellar mixtures and reveal mechanistic details about the bicelle-mediated SLB formation process, which will aid further exploration of bicellar mixtures as tools for model membrane fabrication.

The Influence of Metal Ions on the Dynamics of Supported Phospholipid Langmuir Films

The translational diffusion dynamics of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at a planar phosphorylated support surface containing metal ions (Mg²⁺, Ca²⁺, Ba²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Zr⁴⁺) was investigated using X-ray photoelectron spectroscopy (XPS) and fluorescence recovery after photobleaching (FRAP). Fluorescence recovery curves yielded diffusion constants on the order of 2-5 μm²/s for the chromophore-tagged 12:0 NBD-Lyso-PC. Ionic interactions between the zwitterionic headgroup and metal ions were found to play a secondary role in mediating lipid fluidity. This work provides quantitative insight into the extent to which the fluidity of a supported lipid film is mediated by the ionic interactions between headgroup and surface versus that of the lipid-lipid tailgroup interactions.

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.

Unraveling Interactions between Ionic Liquids and Phospholipid Vesicles Using Nanoplasmonic Sensing

Owing to their unique properties and unlimited structural combinations, the ubiquitous use of ionic liquids (ILs) is steadily increasing. The objective of the present work is to shed light onto the effects of amidinium- and phosphonium-based ILs on phospholipid vesicles using a nanoplasmonic sensing measurement technique. A new and relatively simple method was developed for the immobilization of large unilamellar vesicles on two different hydrophilic surfaces composed of titanium dioxide and silicon nitride nanolayers. Among the pretreatment conditions studied, vesicle attachment on both substrate materials was achieved with HEPES buffer in the presence of sodium hydroxide and calcium chloride. To get an understanding of how ILs interact with intact vesicles or with supported lipid bilayers, the ILs 1,5-diazabicyclo(4.3.0)non-5-enium acetate ([DBNH][OAc]), tributyl(tetradecyl)phosphonium acetate ([P₁₄₄₄₄][OAc]), and tributylmethylphosphonium acetate ([P₄₄₄₁][OAc]) were introduced into the biomimetic system, and the characteristics of their interactions with the immobilized vesicles were determined. Depending on the IL, in situ real-time IL binding and/or phospholipid removal processes were observed. Although [DBNH][OAc] did not have any significant effect on the phospholipid vesicles, the strongest and the most significant effect was observed with [P₁₄₄₄₄][OAc]. The latter caused clear changes in the phospholipid bilayer: the ILs interacted with the bilayers, resulting in deformation of the vesicles most probably due to the formation of vesicle-IL aggregates. Only a mild effect was observed when [P₄₄₄₁][OAc], at a very high concentration, was exposed to the intact vesicles. In general, these results led to new insights into the effects of ILs on phospholipid vesicles, which are of great importance to the overall understanding of the harmfulness of ILs on biomembranes and biomimicking systems. In addition, the present work highlights the pivotal role of this highly surface-sensitive indirect biosensing technique in scrutinizing and dissecting the integrity and architecture of phospholipid vesicles in the nanoscale range.

A phenomenological model of the solvent-assisted lipid bilayer formation method

The mechanism of solvent-assisted lipid bilayer assembly at the solid–liquid interface is elucidated by matching an adsorption model to quartz crystal microbalance data.