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Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was employed to investigate the interaction of cholesterol with the headgroups of dimyristoylphosphatidycholine (DMPC) molecules under a static electric field. DMPC/cholesterol (7:3 molar ratio) mixtures form a bilayer on a Au(111) electrode surface by fusion and spreading of small unilamellar vesicles. PM-IRRAS experiments provided detailed information concerning the conformation and hydration of headgroups of DMPC bilayers in the presence and absence of 30% cholesterol. The presence of 30% cholesterol increases the space between the headgroups of DMPC molecules and hence increases the hydration of the DMPC/cholesterol mixed bilayer. The conformational state of the headgroups of DMPC molecules in the mixed bilayer is also significantly changed. The phosphate group is closer to the surface compared with the pure DMPC bilayer. The conformation of the -O-C-C-N moiety changes from gauche to trans in the presence of cholesterol. [ABSTRACT FROM AUTHOR]

The scaffolded vesicle has been employedas an alternative meansof developing natural model membranes and envisioned as a potentialnutraceutical transporter. Furthering the research of the scaffoldedvesicle system, a nucleophilic substitution reaction was implementedto form an ester linkage between palmitate and terminal hydroxyl groupsof dextran in order to hydrophobically modify the hydrogel scaffold.An average tilt angle of 38° of the hydrophobic palmitate modifyinglayer on the surface of the hydrogel was determined from dichroicratios obtained from infrared spectra collected in the attenuatedtotal reflection (ATR) configuration. ATR-IR studies of the DMPC-coatedacylated hydrogel demonstrated that the hydrocarbon chains of theDMPC coating was similar to those of the DMPC bilayers and that theunderlying palmitate layer had a negligible effect on the averagetilt angle (26°) of the DMPC coating. The permeability of thisacylated hydrogel was investigated with fluorescence spectroscopyand the terbium/dipicolinic acid assay. The hydrophobic modificationon the surface of the hydrogel bead allowed for an efficient depositionof a DMPC layer that served as an impermeable barrier to terbium efflux.About 72% of DMPC-coated acylated hydrogel beads showed ideal barrierproperties. The remaining 28% were leaking, but the half-life of terbiumefflux of the DMPC-coated acylated hydrogel was increasing, and thetotal amount of leaked terbium was decreasing with the incubationtime. The half-life time and the retention were considered a markedimprovement relative to past scaffolded vesicle preparations. Theprocess of acylating hydrogel beads for efficient DMPC depositionhas been identified as another viable method for controlling the permeabilityof the scaffolded vesicle. [ABSTRACT FROM AUTHOR]

Langmuir–Blodgett and Langmuir–Schaeffer methods were employed to deposit a mixed bilayer consisting of 90% of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 10% of gramicidin (GD), a short 15 residue ion channel forming peptide, onto a Au(111) electrode surface. This architecture allowed us to investigate the effect of the electrostatic potential applied to the electrode on the orientation and conformation of DMPC molecules in the bilayer containing the ion channel. The charge density data were determined from chronocoulometry experiments. The electric field and the potential across the membrane were determined through the use of charge density curves. The magnitudes of potentials across the gold-supported biomimetic membrane were comparable to the transmembrane potential acting on a natural membrane. The information regarding the orientation and conformation of DMPC and GD molecules in the bilayer was obtained from photon polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) measurements. The results show that the bilayer is adsorbed, in direct contact with the metal surface, when the potential across the interface is more positive than −0.4 V and is lifted from the gold surface when the potential across the interface is more negative than −0.4 V. This change in the state of the bilayer has a significant impact on the orientation and conformation of the phospholipid and gramicidin molecules. The potential induced changes in the membrane containing peptide were compared to the changes in the structure of the pure DMPC bilayer determined in earlier studies. [ABSTRACT FROM AUTHOR]

Combined Langmuir−Blodgett vertical withdrawing and Langmuir−Schaefer horizontal touch (LB−LS) methods were employed to transfer DMPC bilayers onto a Au(111) electrode surface. Charge density measurements and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate electric field induced changes in the structure of the bilayer. The results show that the physical state and the molecular arrangement found in the monolayer at the air−water interface is to a large extent preserved in the bilayer formed by the LB−LS method. This approach provides an opportunity to produce supported bilayers with a well-designed architecture. The properties of the bilayer formed by the LB−LS method were compared to the properties of the bilayer produced by spontaneous fusion of unilamellar vesicles investigated in an earlier study (Bin, X.; Zawisza, I.; Lipkowski, J.Langmuir2005, 21, 330−347). The tilt angles of the acyl chains are much smaller in the bilayer formed by the LB−LS method and are closer to the angles observed for vesicles and stacked hydrated bilayers. The tilt angles of the phosphate and choline groups are also smaller and are characteristic of an orientation in which the area per DMPC molecule is small. The electric field induced changes of these angles are also less pronounced in the bilayer formed by the LB−LS method. We have shown that these differences are a result of the higher packing density of the phospholipid molecules in the bilayer formed by the LB−LS method. [ABSTRACT FROM AUTHOR]

Preparation of a nanoparticle modifiedgold substrate designedfor characterization of hydrophilic self-assembled monolayers (SAMs)of 1-thio-β-d-glucose (TG) with electrochemical surface-enhancedRaman spectroscopy (EC-SERS) is presented. Citrate stabilized goldnanoparticles were deposited on a polycrystalline gold electrode andsubjected to an electrochemical desorption procedure to completelyremove all traces of adsorbed citrate. Complete desorption of citratewas confirmed by recording cyclic voltammetry curves and SERS spectra.The citrate-free nanoparticle modified gold electrode was then incubatedin a 1 mg mL–1aqueous solution of TG for 16 h priorto being characterized by EC-SERS. The SERS spectra confirmed thatat potentials more negative than −0.10 V vs SCE thioglucoseforms a monolayer in which the majority of the molecules preservetheir lactol ring structure and only a small fraction of moleculesappear to be oxidized. At potentials more positive than −0.10V, the oxidation of TG molecules becomes prominent, and at potentialsmore positive than 0.20 V vs SCE, the monolayer of TG consists chieflyof oxidized product. The SERS spectra collected in the double layerregion suggest the SAM of TG is well hydrated and hence can be usedfor hydrophilic modifications of a gold surface. [ABSTRACT FROM AUTHOR]

Shell-isolated gold nanoparticles (SHINs) were employed to record shell-isolated nanoparticle-enhanced Raman spectra (SHINERS) of a passive layer formed at a gold surface during gold leaching from thiosulfate solutions. The (3-aminopropyl)triethoxysilane (APTES) and a sodium silicate solution were used to coat gold nanoparticles with a protective silica layer. This protective silica layer prevented interactions between the thiosulfate electrolyte and the gold core of the SHINs when the SHINs-modified gold electrode was immersed into the thiosulfate lixiviant. The SHINERS spectra of the passive layer, formed from thiosulfate decomposition, contained bands indicative of hydrolyzed APTES. We have demonstrated how to exploit the presence of these APTES bands as an internal standard to compensate for fluctuations of the surface enhancement of the electric field of the photon. We have also developed a procedure that allows for removal of the interfering APTES bands from the SHINERS spectra. These methodological advancements have enabled us to identify the species forming the passive layer and to determine that the formation of elemental sulfur, cyclo-S8, and polymeric sulfur chains is responsible for inhibition of gold dissolution in oxygen rich thiosulfate solutions. [ABSTRACT FROM AUTHOR]

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-β-d-glucose (β-Tg), producing a hydrophilic surface. A monolayer of β-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the β-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of β-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of β-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S8rings, indicating the S–C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of β-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer. [ABSTRACT FROM AUTHOR]

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-d-glucose formed by spontaneous self-assembly. The Langmuir–Blodgett/Langmuir–Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a “floating” BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force–distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) was used to characterize a phospholipid bilayer composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, at a Au(111) electrode surface. Results indicate that addition of cholesterol relaxes membrane elastic stress, increases membrane thickness, and reduces defect density. The thickness and thermotropic properties of the mixed DMPC−cholesterol bilayer supported at the gold electrode surface are quite similar to the properties of the mixed membrane in unilamellar vesicles. The stability of the supported membrane at potentials negative to the potential of zero charge Epzcwas investigated. This study demonstrates that the bilayer supported at the gold electrode surface is stable provided the applied potential (E− Epzc) is less than −0.3 V. At larger polarizations, swelling of the membrane is observed. Polarizations larger than −1 V cause electrodewetting of the bilayer from the gold surface. At these negative potentials, the bilayer remains in close proximity to the metal surface, separated from it by a ∼2 nm thick layer of electrolyte. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl- sn-glycero-3-phosphocholine (DMPC) at a Au(111) electrode surface. The bilayer was formed by one of two methods: fusion of lamellar vesicles or by the combination of Langmuir−Blodgett (LB) and Langmuir−Schaefer (LS) deposition. Results indicate that phospholipid vesicles rapidly adsorb and fuse to form a film at the electrode surface. The resulting film undergoes a very slow structural transformation until a characteristic corrugated phase is formed. Force−distance curve measurements reveal that the thickness of the corrugated phase is consistent with the thickness of a bilayer lipid membrane. The formation of the corrugated phase may be explained by considering the elastic properties of the film and taking into account spontaneous curvature induced by the asymmetric environment of the bilayer, in which one side faces the gold substrate and the other side faces the solution. The effect of temperature and electrode potential on the stability of the corrugated phase has also been described. [ABSTRACT FROM AUTHOR]

A combination of the Langmuir−Blodgett and Langmuir−Schaefer techniques has been used to build a 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayer at a Au(111) electrode surface with hydrogen-substituted acyl chains in the top leaflet (solution side) and deuterium-substituted acyl chains in the bottom leaflet (gold side). Polarization modulation infrared reflection−absorption spectroscopy was used to determine changes in the conformation and orientation of the acyl chains of DMPC caused by the incorporation of two selected perfluorinated compounds, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), into the top leaflet of the bilayer. The incorporation of perfluorinated compounds into the DMPC bilayer caused a broadening of the methylene peaks and a shift in the methylene band positions toward higher frequencies. In addition, the tilt angle of the acyl chains decreased in comparison to the tilt angle of a pure DMPC bilayer. The reported tilt angles were smaller upon insertion of PFOS (∼24°) than in the presence of PFOA (∼30°). Overall, the results show that the incorporation of the perfluorinated acids has an effect on the bilayer similar to that of cholesterol by increasing the membrane fluidity and thickness due to a decrease in the tilt angle of the acyl chains. [ABSTRACT FROM AUTHOR]

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface. [ABSTRACT FROM AUTHOR]

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to describe the electric-field-controlled surface aggregation of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS), a model zwitterionic surfactant, at a Au(111) electrode surface. At concentrations below the critical micelle concentration (CMC), the monomer adsorbs and aggregates at the surface. The charge on the metal (M) controls the orientation of adsorbed molecules and consequently the film structure. At high negative (M< −5 C cm-2) charge densities, a spongy, disordered film is formed in which the polar heads are turned toward the solution. At high positive (M> 5 C cm-2) charge densities, a planar film with “blisters” is observed with the polar heads of DDAPS turned to the metal. Hemicylindrical aggregates are observed in the intermediate charge density range (−5 < M< 5 C cm-2). At bulk concentrations higher than the CMC, micelles adsorb and the structure of these films is controlled by the fusion of the adsorbed micelles. STM and AFM images provided direct visualization of this field-driven surface aggregation of the zwitterionic surfactant. [ABSTRACT FROM AUTHOR]

We have applied a recently developed method (Langmuir2006, 22, 5509−5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number. [ABSTRACT FROM AUTHOR]

The adsorption behavior of the cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) on the Au(111) electrode surface was characterized using cyclic voltammetry, differential capacity, and chronocoulometry. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DeTATf has a multistate character. At low bulk DeTATf concentrations, the adsorption state is consistent with the formation of an adsorbed film of nearly flat molecules. At higher concentrations this film may represent a three-dimensional aggregated state. At negative potentials and charge densities close to 0 C cm-2, the data suggest the formation of a film of tilted molecules oriented with the hydrocarbon tail toward the metal surface and the polar head toward the solution. A surprising result of this study is that DeTATf displays adsorption characteristics of a zwitterionic rather than a cationic surfactant. This behavior indicates that the adsorbed species is an ion pair. [ABSTRACT FROM AUTHOR]