Stable adhesion of phospholipid vesicles to modified gold surfaces

Recent Experimental Developments in Studying Passive Membrane Transport of Drug Molecules

The ability to measure the passive membrane permeation of drug-like molecules is of fundamental biological and pharmaceutical importance. Of significance, passive diffusion across the cellular membranes plays an effective role in the delivery of many pharmaceutical agents to intracellular targets. Hence, approaches for quantitative measurement of membrane permeability have been the topics of research for decades, resulting in sophisticated biomimetic systems coupled with advanced techniques. In this review, recent developments in experimental approaches along with theoretical models for quantitative and real-time analysis of membrane transport of drug-like molecules through mimetic and living cell membranes are discussed. The focus is on time-resolved fluorescence-based, surface plasmon resonance, and second-harmonic light scattering approaches. The current understanding of how properties of the membrane and permeant affect the permeation process is discussed.

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.

Quantitative analysis of molecular partition towards lipid membranes using surface plasmon resonance

Understanding the interplay between molecules and lipid membranes is fundamental when studying cellular and biotechnological phenomena. Partition between aqueous media and lipid membranes is key to the mechanism of action of many biomolecules and drugs. Quantifying membrane partition, through adequate and robust parameters, is thus essential. Surface Plasmon Resonance (SPR) is a powerful technique for studying 1:1 stoichiometric interactions but has limited application to lipid membrane partition data. We have developed and applied a novel mathematical model for SPR data treatment that enables determination of kinetic and equilibrium partition constants. The method uses two complementary fitting models for association and dissociation sensorgram data. The SPR partition data obtained for the antibody fragment F63, the HIV fusion inhibitor enfuvirtide, and the endogenous drug kyotorphin towards POPC membranes were compared against data from independent techniques. The comprehensive kinetic and partition models were applied to the membrane interaction data of HRC4, a measles virus entry inhibitor peptide, revealing its increased affinity for, and retention in, cholesterol-rich membranes. Overall, our work extends the application of SPR beyond the realm of 1:1 stoichiometric ligand-receptor binding into a new and immense field of applications: the interaction of solutes such as biomolecules and drugs with lipids.

Ultrasensitive Plasmonic Platform for Label-Free Detection of Membrane-Associated Species

Lipid membranes and membrane proteins are important biosensing targets, motivating the development of label-free methods with improved sensitivity. Silica-coated metal nanoparticles allow these systems to be combined with supported lipid bilayers for sensing membrane proteins through localized surface plasmon resonance (LSPR). However, the small sensing volume of LSPR makes the thickness of the silica layer critical for performance. Here, we develop a simple, inexpensive, and rapid sol-gel method for preparing thin conformal, continuous silica films and demonstrate its applicability using gold nanodisk arrays with LSPRs in the near-infrared range. Silica layers as thin as ∼5 nm are observed using cross-sectional scanning transmission electron microscopy. The loss in sensitivity due to the thin silica coating was found to be only 16%, and the biosensing capabilities of the substrates were assessed through the binding of cholera toxin B to GM1 lipids. This sensor platform should prove useful in the rapid, multiplexed detection and screening of membrane-associated biological targets.

Ionic strength dependent vesicle adsorption and phase behavior of anionic phospholipids on a gold substrate

The authors report on the effect of ionic strength on the formation of supported vesicle layers of anionic phospholipids 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) and dimyristoylphosphatidylserine (DMPS) onto gold. Using quartz crystal microbalance with dissipation monitoring the authors show that vesicle adsorption is mainly governed by NaCl concentration, reflecting the importance of electrostatic interactions in anionic lipids, as compared to zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine. At low ionic strength, low or no adsorption is observed as a result of vesicle-vesicle electrostatic repulsion. At medium ionic strength, the negative charges of DMPG and DMPS are screened resulting in larger adsorption and a highly dissipative intact vesicle layer. In addition, DMPS exhibits a peculiar behavior at high ionic strength that depends on the temperature of the process.

Soft landing of cell-sized vesicles on solid surfaces for robust vehicle capture/release

Based on a concept of a smooth and steady landing of fragile objects without destruction via a soft cushion, we have developed a model for the soft landing of deformable lipid giant unilamellar vesicles (GUVs) on solid surfaces. The foundation for a successful soft landing is a solid substrate with a two-layer coating, including a bottom layer of positively charged lysozymes and an upper lipid membrane layer. We came to a clear conclusion that anionic GUVs when sedimented on a surface, the vesicle rupture occurs upon the direct contact with the positively charged lysozyme layer due to the strong coulombic interactions. In contrast, certain separation distances was achieved by the insertion of a soft lipid membrane cushion between the charged GUVs and the lysozyme layer, which attenuated the coulombic force and created a mild buffer zone, ensuring the robust capture of GUVs on the substrate without their rupture. The non-covalent bonding facilitated a fully reversible stimuli-responsive capture/release of GUVs from the biomimetic solid surface, which has never been demonstrated before due to the extreme fragility of GUVs. Moreover, the controllable capture/release of cells has been proven to be of vital importance in biotechnology, and similarity the present approach to capture/release cells is expected to open the previously inaccessible avenues of research.

Biosensor based on lectin and lipid membranes for detection of serum glycoproteins in infected patients with dengue

In this work, we developed a biosystem based on Concanavalin A (ConA) and lipid membranes to recognize glycoproteins from the serum of patients contaminated with dengue serotypes 1, 2 and 3 (DENV1, DENV2 and DENV3). The modified gold electrode was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and atomic force microscopy. Morphological analyses of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), DPPC-ConA, DPPC-ConA-DENV1, DPPC-ConA-DENV2 and DPPC-ConA-DENV3 revealed the existence of a non-uniform covering and large globules. EIS and CV measurements have shown that redox probe reactions on the modified gold electrodes were partially blocked due to the adsorption of lipid-ConA system and reveal the interaction response of the immobilized ConA to the presence of glycoproteins of dengue serum. The biosystem exhibited a wide linear response to different concentrations of sera of dengue serotypes 1, 2 and 3. A higher impedimetric response to glycoproteins present in dengue serotype 3 was observed. Our results demonstrate the applicability of lectin and lipid membranes to the development of biosensors for dengue infections.

Trapping of vesicles on patterned surfaces by physisorption for potential biosensing applications

The pre-defined selective positioning of a controlled number of vesicles on a rigid substrate is crucial in many potential applications such as diagnostics, biosensors, lab-on-a chip, microanalyses and reaction chambers. In this paper, the vesicles made up of block copolymer using Poly [-(2-methyloxazoline) -poly- (dimethylsiloxane)-poly- (2-methyloxazoline)] (ABA) with dimensions of 100-200 nm are trapped by physisorption on hydrophilic surfaces. We discuss the protocols established for vesicle trapping. The optimum conditions obtained for physisorption is 15 minutes incubation followed by one cycle of DI water rinse. Trapping of 1-10 vesicles in lobe shape micro-wells fabricated by photo lithography using photoresist on UltraStick(™) slides was demonstrated. To overcome the issue of amalgamation of emitted light from optically sensitive photoresist and fluorescently tagged vesicles, an alternative approach of Si/SiO(2) microwell array coupled with APTES (3-AminoPropylTriEthoxySilane) treated bottom surfaces was developed.

Vesicle adsorption and phospholipid bilayer formation on topographically and chemically nanostructured surfaces

We have investigated the influence of combined nanoscale topography and surface chemistry on lipid vesicle adsorption and supported bilayer formation on well-controlled model surfaces. To this end, we utilized colloidal lithography to nanofabricate pitted Au-SiO(2) surfaces, where the top surface and the walls of the pits consisted of silicon dioxide whereas the bottom of the pits was made of gold. The diameter and height of the pits were fixed at 107 and 25 nm, respectively. Using the quartz crystal microbalance with dissipation monitoring (QCM-D) technique and atomic force microscopy (AFM), we monitored the processes occurring upon exposure of these nanostructured surfaces to a solution of extruded unilamellar 1-palmitolyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles with a nominal diameter of 100 nm. To scrutinize the influence of surface chemistry, we studied two cases: (1) the bare gold surface at the bottom of the pits and (2) the gold passivated by biotinamidocaproyl-labeled bovine serum albumin (BBSA) prior to vesicle exposure. As in our previous work on pitted silicon dioxide surfaces, we found that the pit edges promote bilayer formation on the SiO(2) surface for the vesicle size used here in both cases. Whereas in the first case we observed a slow, continuous adsorption of intact vesicles onto the gold surface at the bottom of the pits, the presence of BBSA in the second case prevented the adsorption of intact vesicles into the pits. Instead, our experimental results, together with free energy calculations for various potential membrane configurations, indicate the formation of a continuous, supported lipid bilayer that spans across the pits. These results are significantly important for various biotechnology applications utilizing patterned lipid bilayers and highlight the power of the combined QCM-D/AFM approach to study the mechanism of lipid bilayer formation on nanostructured surfaces.

Characterization of the Concentration-Dependence of Solute Diffusivity and Partitioning in a Model Dextran-Agarose Transport System

This study reports experimental measurements of solute diffusivity and partition coefficient for various solute concentrations and gel porosities, and proposes novel constitutive relations to describe these observed values. The longer-term aim is to explore the theoretical ramifications of accommodating variations in diffusivity and partition coefficient with solute concentration and tissue porosity, and investigate whether they might suggest novel mechanisms not previously recognized in the field of solute transport in deformable porous media. The study implements a model transport system of agarose hydrogels to investigate the effect of solute concentration and hydrogel porosity on the transport of dextran polysaccharides. The proposed phenomenological constitutive relations are shown to provide better fits of experimental results than prior models proposed in the literature based on the microstructure of the gel. While these constitutive models were developed for the transport of dextran in agarose hydrogels, it is expected that they may also be applied to the transport of similar molecular weight solutes in other porous media. This quantification can assist in the application of biophysical models that describe biological transport in deformable tissues, as well as the cell cytoplasm.

Multiplexed plasmonic sensing based on small-dimension nanohole arrays and intensity interrogation

We performed multiplexed sensing on nanohole array devices to simultaneously obtain information on molecular absorption, scattering, and refractive-index change, which were distinguished by using different array structures with distinct optical behavior. Up to 25 arrays were fabricated within a 65 microm x 50 microm area to provide real-time information of the local surface environment. The performance of multiplexed sensing was examined by flowing NaCl, Coomassie blue, bovine serum albumin, and liposome solutions that exhibit different visible light absorption/scattering properties and different refractive indices. Experimental artifacts from light source fluctuation, sample injections, and light scattering induced by aggregates in solutions were detected by monitoring superwavelength holes or nanohole arrays with different periodicity and hole diameters.

Biotin-containing phospholipid vesicle layer formed on self-assembled monolayer of a saccharide-terminated alkyl disulfide for surface plasmon resonance biosensing

We describe a technique to form a biotin-containing phospholipid vesicle layer on a self-assembled monolayer (SAM) deposited on a gold surface to immobilize biotinylated receptor proteins for a surface plasmon resonance (SPR) biosensor. The adsorption of vesicle of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) was examined by SPR on the SAMs of dithiobis(1-deoxy-glucitol-1-carbamoyl pentane) (DDGP), 11-mercaptoundecanoic acid, 11-mercaptoundecanol, 11-amino-1-undecanethiol, and 12-mercaptododecane, and it was found that the DOPC vesicle rapidly adsorbed on the DDGP SAM to achieve the highest coverage of the surface. By quartz crystal microbalance with dissipation monitoring (QCM-D), the DOPC layer formed on the DDGP SAM was shown to be a vesicle layer, in which intact DOPC vesicles physisorbed on the SAM surface. To immobilize a biotinylated receptor protein, one of three biotinylated phospholipids, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (biotin-DOPE), N-((6-(biotinoyl)amino)hexanoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (biotin-X-DHPE) and N-(biotinoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (biotin-DHPE), was mixed with DOPC to form a biotin-containing vesicle layer on the DDGP SAM. A comparative binding study of NeutrAvidin and the biotin-containing vesicle layers showed that the use of biotin-X-DHPE achieved the most rapid immobilization of NeutrAvidin on the vesicle layer at the highest surface density. Furthermore, biotinylated protein A, as a receptor protein, could be immobilized through NeutrAvidin on the vesicle layer containing DOPC and biotin-X-DHPE, and its reaction with immunoglobulin G, as an analyte, was successfully observed by SPR. The results demonstrate that the biotin-containing vesicle layer on the DDGP SAM must be a useful component for SPR biosensor surfaces.

Dissipation-enhanced quartz crystal microbalance studies on the experimental parameters controlling the formation of supported lipid bilayers

We report on the investigations of the transformation of spherically closed lipid bilayers to supported lipid bilayers in aqueous media in contact with SiO(2) surfaces. The adsorption kinetics of small unilamellar vesicles composed of dimyristoyl- (DMPC) and dipalmitoylphosphatidylcholine (DPPC) mixtures on SiO(2) surfaces were investigated using a dissipation-enhanced quartz crystal microbalance (QCM-D) as a function of buffer (composition and pH), lipid concentration (0.01-1.0 mg/mL), temperature (15-37 degrees C), and lipid composition (DMPC and DMPC/DPPC mixtures). The lipid mixtures used here possess a phase transition temperature (T(m)) of 24-33 degrees C, which is close to the ambient temperature or above and thus considerably higher than most other systems studied by QCM-D. With HEPES or Tris.HCl containing sodium chloride (150 mM) and/or calcium chloride (2 mM), intact vesicles adsorb on the surface until a critical density ((c)) is reached. At close vesicle contact the transformation from vesicles to supported phospholipid bilayers (SPBs) occurs. In absence of CaCl(2), the kinetics of the SPB formation process are slowed, but the passage through (c) is still observed. The latter disappears when buffers with low ionic strength were used. SPB formation was studied in a pH range of 3-10, yet the passage through (c) is obtained only for pH values above to the physiological pH (7.4-10). With an increasing vesicle concentration, (c) is reached after shorter exposure times. At a vesicle concentration of 0.01-1 mg/mL, vesicle fusion on SiO(2) proceeds with the same pathway and accelerates roughly proportionally. In contrast, the pathway of vesicle fusion is strongly influenced by the temperature in the vicinity of T(m). Above and around the T(m), transformation of vesicles to SPB proceeds smoothly, while below, a large number of nonruptured vesicles coexist with SPB. As expected, the physical state of the membrane controls the interaction with both surface and neighboring vesicles.

Covalently anchored lipid structures on amine-enriched polystyrene

The study of the adhesion of lipid vesicles on surfaces is of increasing interest in the field of medical implants and tissue engineering (protein-resistant surfaces), drug delivery, biosensors, and biochips. In this work, lipid coverage was developed from PEG-coated vesicles (with sizes from 100 to 300 nm) by covalently binding poly(ethylene glycol)-alpha-disteroylphosphatidylethanolamine-omega-benzotriazole carbonate (DSPE-PEG-BTC) molecules onto the surface amine groups by carbamate chemistry. Lipid surface density and the surface structure of multilamellar (MLVs) and extruded unilamellar (LUVs) vesicles deposited on three types of polystyrene (PS) well-plates were probed by fluorescence and atomic force microscopy (AFM) imaging. A significant difference in the vesicle surface coverage of PS substrates was observed with a substantial increase in lipid multilayers on the amine-enriched PS surface using both unilamellar and multilamellar vesicles.

Using the atomic force microscope to study the interaction between two solid supported lipid bilayers and the influence of synapsin I

To measure the interaction between two lipid bilayers with an atomic force microscope one solid supported bilayer was formed on a planar surface by spontaneous vesicle fusion. To spontaneously adsorb lipid bilayers also on the atomic force microscope tip, the tips were first coated with gold and a monolayer of mercapto undecanol. Calculations indicate that long-chain hydroxyl terminated alkyl thiols tend to enhance spontaneous vesicle fusion because of an increased van der Waals attraction as compared to short-chain thiols. Interactions measured between dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and dioleoyloxypropyl trimethylammonium chloride showed the electrostatic double-layer force plus a shorter-range repulsion which decayed exponentially with a decay length of 0.7 nm for dioleoylphosphatidylcholine, 1.2 nm for dioleoylphosphatidylserine, and 0.8 nm for dioleoyloxypropyl trimethylammonium chloride. The salt concentration drastically changed the interaction between dioleoyloxypropyl trimethylammonium chloride bilayers. As an example for the influence of proteins on bilayer-bilayer interaction, the influence of the synaptic vesicle-associated, phospholipid binding protein synapsin I was studied. Synapsin I increased membrane stability so that the bilayers could not be penetrated with the tip.

Temperature dependence of vesicle adhesion

The influence of thermal fluctuations on the adhesion behavior of fluid vesicles is investigated with the help of Monte Carlo simulations. The adhesion area A(ad) of a fluid vesicle adhering to a smooth attractive substrate is studied systematically for different values of temperature, adhesion strength, and potential range. For low temperatures T , the ratio A(ad) /A between the adhesion area and the total area A of the vesicle is a linear function of T/kappa , where kappa is the bending rigidity. Linear fits of the simulation data allow an extrapolation to T=0 which corresponds well with data obtained from a simplified analytic model. A new ansatz for A(ad) (T) which is based on the eigenmodes of the adhering vesicle explains the linear behavior of A(ad) (T) for low T and helps to define a fit function which reproduces the linear behavior of the obtained simulation data. This fit function may be used in order to determine the bending rigidity and the adhesion strength from the observed adhesion geometry.

Factors affecting responsivity of unilamellar liposomes to 20 kHz ultrasound

Ultrasound is commonly used in the preparation of unilamellar liposome dispersions and is often considered for cell membrane disruption for drug delivery or DNA transfection applications. To better understand the physical and chemical properties of lipid membranes that render them susceptible to ultrasonic permeabilization, the roles of temperature, lipid composition (cholesterol and PEG-lipid content), and liposome size have been studied. The results of these studies suggest that lipid packing is very important to ultrasound responsiveness; surprisingly, cohesive energy and tensile strength are not. Taken together, the experimental results implicate a defect-mediated permeabilization mechanism, rather than pore formation or membrane tearing. The implications of this work for drug release from liposomes and ultrasound-mediated DNA transfection are discussed.

Micropatterning of Nanoengineered Surfaces to Study Neuronal Cell Attachment in Vitro

Methods for producing protein patterns with defined spatial arrangement and micro- and nanoscale features are important for studying cellular-level interactions, including basic cell-cell communications, cell signaling, and mechanisms of drug action. Toward this end, a straightforward, versatile procedure for fabricating micropatterns of bioactive nanofilm coatings as multifunctional biological testbeds is demonstrated. The method, based on a combination of photolithography and layer-by-layer self-assembly (LbL), allows for precise construction of nanocomposite films of potentially complex architecture, and patterning of these films on substrates using a modified lift-off (LO) procedure. As a first step in evaluating nanostructures made with this process, "comparison chips," comprising two coexisting regions of square patterns with relevant proteins/polypeptides on a single substrate, were fabricated with poly(diallyldimethylammonium chloride) (PDDA) as a cell-repellent background. Using neuronal cells as a model biological system, comparison chips were produced with secreted phospholipase A2 (sPLA2), a known membrane-active enzyme for neurons, for direct comparison with gelatin, poly-l-lysine (PLL), or bovine serum albumin (BSA). Fluorescence microscopy, surface profilometry, and atomic force microscopy techniques were used to evaluate the structural properties of the patterns on these chips and show that the patterning technique was successful. Preliminary cell culture studies show that neurons respond and bind specifically to the sPLA2 enzyme embedded in the polyelectrolyte thin films and present as the outermost layer. These findings point to the potential for this method to be applied in developing test substrates for a broad array of studies aimed at identifying important biological structure-function relationships.