Organization Of Monolayers Research Articles

States of Adsorbed Dodecyl Amine and Water at a Silica Surface As Revealed by Vibrational Spectroscopy

States of adsorbed dodecylamine (DDA) at a silica surface have been studied as a function of pH by vibrational spectroscopy (SFVS, FTIR), contact angle measurements, and molecular dynamics (MD) simulations. The results show that the state of adsorbed DDA at a silica surface varies significantly at different pH values. At pH 6.30, there is no pronounced adsorption of DDA cations and the surface is hydrophilic. At pH 10.0, the vibrational spectroscopy results together with contact angle measurements and MD simulations suggest that amine is adsorbed as a well organized monolayer, the hemimicelle structure. Under these conditions, dehydration occurs based on SFVS analysis and the silica surface becomes hydrophobic. In the case of pH 12.3, it has been confirmed that continued adsorption of DDA neutral molecules occurs with the amine surface state changing from a monolayer to a bilayer or a micellar surface state as revealed both from SFVS analysis and MD simulations. At this high pH, extensive surface hydration is evident from SFVS results and the silica surface becomes hydrophilic.

Electrochemical characterization of BSA/11-mercaptoundecanoic acid on Au electrode

Abstract Recently, it has becoming increasingly important to control the organization of self-assembled monolayers (SAMs) of functionalized thiols and to bind various proteins on gold/silicon substrates for their potential integration in nanoscale sensors/biosensors and optical devices. The biomolecule immobilization on the surfaces by covalent chemistry allows fabrication of reproducible, protein-modified surfaces and became also a model to investigate the electrochemical response induced by protein binding. In this study, we report different nanostructured gold substrates and the adsorption of a protein, bovine serum albumin (BSA) on the 11-mercaptoundecanoic acid (MUA) layer for further biomedical applications. Nanostructured gold layers of 200 nm thickness have been prepared on both, flat and macroporous silicon (macroPS) substrates. The X-ray diffraction analyses emphasized a dominant (1 1 1) crystallographic orientation of nanostructured Au substrates, which is preferred orientation for binding and detection of organic molecules on the gold surface. Impedance spectroscopy measurements performed in specific frequency ranges show that the binding of protein to a single monolayer of MUA can be easily detected. The impedance changes were also corroborated with cyclic voltammetry and Raman spectroscopy analysis for further development of the biosensor transducer for converting of the specific molecular recognition events into either an optical or electrical signal.

Electrochemical and spectroscopic study of C 12H 25X molecules adsorption on copper sheets, X (–SH, –S–S–, –SeH and –Se–Se–)

In this contribution, we explored the possibility of using selenol and selenide molecules to form self-assembled monolayers (SAMs) on copper, in order to check the influence of anchoring groups on SAMs quality and compared it to well-known thiolate assemblies (formed with thiol and disulfide molecules). Precisely, monolayers of pure alkane chains have been self-assembled on electroreduced bulk copper. The different selected molecules present the following reactive anchoring groups: thiol (R–SH), disulfide (R–S–S–R), selenol (R–SeH) and diselenide (R–Se–Se–R), where R = C 12H 25–. Electrochemical (cyclic voltammetry and scanning electrochemical microscopy) techniques and spectroscopic (X-ray photoelectron and polarization modulation infrared reflection absorption spectroscopy) have been used to characterize the surface composition and monolayer organization. Atomic force microscopy (AFM) measurements complete this study. All molecules analyzed have been shown to form monolayers of variable quality. The R–SH and R–SeH monolayers seem to lead to better organized and insulating layers than the R–S–S–R and R–Se–Se–R monolayers. However, the case of the diselenide is more complex and could lead to some interesting properties.

Sequence-Directed Organization of β-Peptides in Self-Assembled Monolayers

The sequence-directed organization of self-assembled monolayers of amphiphilic beta-peptides adsorbed on gold surfaces is studied using Monte Carlo simulations. A phenomenological model is presented in which each (helical) molecule is represented by a rigid nanorod; side groups are placed at appropriate locations. This model can distinguish between globally amphiphilic (GA) and nonglobally amphiphilic (iso-GA) sequence isomers. The simulations show that the GA isomers have a high degree of orientational order that is not exhibited by the iso-GA isomers, which is consistent with experiment (Pomerantz et al. Chem. Mater. 2007, 19, 4436). The effect of surface coverage and relative strength of electrostatic, hydrophilic, and hydrophobic interactions on the self-assembly of beta-peptides is quantified.

Supramolecular Organization of the Main Photosynthetic Antenna Complex LHCII: A Monomolecular Layer Study

The light-harvesting pigment-protein complex LHCII is a main antenna complex of the photosynthetic apparatus of plants, responsible for collecting light energy and also for photoprotection against overexcitation-induced damage. Realization of both functions depends on molecular organization of the complex. Monolayer technique has been applied to address the problem of supramolecular organization of LHCII. Analysis of the isotherms of compression of monomolecular films formed at the argon-water interface shows that LHCII appears in two phases: one characterized by the specific molecular area characteristic of trimeric and one of monomeric organization of LHCII. Monolayers of LHCII were deposited by means of the Langmuir-Blodgett technique to solid supports and examined by means of AFM, FTIR, fluorescence spectroscopy, and fluorescence lifetime imaging microscopy (FLIM). FTIR analysis shows that organization of the trimers of LHCII within a monolayer is associated with formation of intermolecular hydrogen bonds between neighboring polypeptides. The linear-dichroism FTIR analysis reveals that polypeptide fragments involved in intermolecular interactions are oriented at an angle of 67 degrees with respect to the normal axis to the plane of the layer. Fluorescence and fluorescence lifetime analysis reveal that the organization of LHCII within monolayers is associated with formation of the low-lying excitonic energy levels that can be potentially responsible for excess excitation quenching. FLIM and AFM reveal heterogeneous organization of LHCII monolayers, in particular, formation of ring-like structures. The potential of LHCII to form molecular structures characterized by pigment excitonic interactions is discussed in terms of regulation of the photosynthetic accessory function and photoprotection against overexcitation-induced damage.

Comparative Characterization of Lateral Organization and Packing Properties of Lipids in Pulmonary Surfactant Membranes and Interfacial Films

The main function of the lipid-protein complex pulmonary surfactant is to stabilize the respiratory surface by formation of a surface active interfacial film on top of the thin aqueous layer that covers the alveoli. From synthesis in the pulmonary epithelium, until formation of the functional film, surfactant complexes adopt different lamellar structures during its storage and secretion. The functional structure of surfactant makes Langmuir monolayers especially useful for the analysis of structure-function correlations in the pulmonary surfactant system, although they are also widely used as a model to obtain structural information about biological membranes. However, the exact correspondence between lateral organization and molecular packing in bilayers and monolayers is still a matter of controversy.The fluorescent probe laurdan (6-dodecanoyl-2-dimethylaminonaphthalene) is a lipophilic dye used to analyze the structure of membranes due to its spectral characteristics. Once inserted in membranes, the fluorescence emission of laurdan is very sensitive to the level of hydration of the phospholipid headgroups. Changes in packing or lateral organization of the membrane produce a shift of the emission maximum of the label from 440 nm in ordered membranes to 490 nm in disordered membrane phases. Taking advantage of the properties of this probe, films from pulmonary surfactant lipids containing laurdan were prepared and compressed to obtain surface pressure-area isotherms. Surface pressure and area occupied per molecule along compression were obtained in parallel with the generalized polarization function (GPF) of laurdan -as calculated from its interfacial fluorescence emission spectra- and compared with the fluorescence of laurdan in multilamellar suspensions of the same surfactant lipids at different temperatures.

Probing the Organization of Charged Self-Assembled Monolayers by Using the Effects of pH, Time, Electrolyte Anion, and Temperature, on the Charge Transfer of Electroactive Probes

The effect of the solution pH and the electrolyte anion on the capacitive behavior observed for a self-assembled monolayer (SAM) of the ionizable 11-amino-1-undecanethiol (AUT) deposited on polycrystalline Au was checked by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using pure electrolyte solutions. The double layer capacitance (Cdl) decreased with increasing the pH and was shown to be dependent on the electrolyte anion, suggesting that the equilibria controlling the degree of charge at the monolayer may affect the order of the thiol chains at the outermost part of the film. The effect of these parameters, plus the temperature and the time of electrode−solution contact, were also checked by monitoring the electron transfer (ET) process of electroactive Ru(NH3)6+3 and Fe(CN)6−3 redox probes in solution. The results obtained at short contact times showed that while the structure of the monolayer was not shown to affect significantly the charge transfer process for attractive pr...

Electrochemical characterization of a 1,8-octanedithiol self-assembled monolayer (ODT-SAM) on a Au(1 1 1) single crystal electrode

Recently, it has becoming increasingly important to control the organization of self-assembled monolayers (SAMs) of ω-functionalized thiols for its potential applications in the construction of more complex molecular architectures. In this paper, we report on the spontaneous formation of a SAM of octanedithiol (ODT) as a function of the modification time. Electrochemical techniques such as cyclic voltammetry, double layer capacitance and electrochemical impedance spectroscopy are used for the characterization of this monolayer. The increase in modification time brings about changes in the octanedithiol self-assembled monolayer (ODT-SAM) reductive desorption voltammograms that indicate an evolution toward a more ordered and compact monolayer. This trend has also been found by following the changes in the electron transfer processes of the redox probe K 3Fe(CN) 6. In fact, the ODT-SAM formed at low-modification time does not significantly perturb the electrochemical response as it is typical of either a low coverage or of the presence of large defects in the layer. Upon increasing the modification time, the voltammograms of the redox probe adopt a sigmoidal shape indicating the existence of pinholes in the monolayer distributed as an array of microelectrodes. The surface coverage as well as the size and distribution of these pinholes have been determined by the impedance technique that gives a more reliable evaluation of these monolayer structural parameters.

Spontaneous formation of DPPC monolayers at aqueous/vapor interfaces and the impact of charged surfactants

Surface tensiometry and vibrational sum-frequency spectroscopy were used to examine the structure and organization in phospholipid monolayers at the aqueous/vapor interface in the absence and in the presence of simple, charged surfactants. 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was the phospholipid employed in these studies and surfactants included sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB). DPPC spontaneously spreads on a pure water (pH = 5.5) surface to form monolayers as evidenced by an equilibrium spreading pressure (ESP) of 7.9 ± 2.3 mN/m and a clearly resolved vibrational spectrum. Low concentrations of surfactants inhibit the spreading of DPPC and result in significantly lower ESP values. Anionic and cationic surfactants at higher concentrations have opposite effects on monolayer organization; SDS creates well-organized monolayers while DTAB leads to poor organization of lipid molecules. Surface-specific vibrational spectra showed that high concentrations of charged surfactants (≥ 100 µM) lead to accumulation of net surface charges as evidenced by destructive and constructive interferences. Selectively deuterating surfactants results in changes in vibrational band intensities and phases enabling assignment of relative orientations of equivalent functional groups belonging to the lipid and surfactant.

Two-Dimensional Miscibility Studies of Alamethicin and Selected Film-Forming Molecules

Alamethicin (ALM), a 20-amino acid antibiotic peptide (peptaibol) from fungal sources, was mixed in Langmuir monolayers with six different surfactants: semifluorinated (F6H18, F10H19, F8H10OH, F6H10SH) and hydrogenated (C18SH and DODAC), aimed at finding appropriate molecules for ALM incorporation for nanodevice construction. Alamethicin-containing mixed monolayers were investigated by means of surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM). Our results show that only semifluorinated alkanes can serve as an appropriate material since they form miscible and homogeneous monolayers with ALM within the whole concentration range. All the remaining surfactants, possessing polar groups, were found to demix with ALM. This effect was explained as being due to the existence of strong polar interactions between vertically oriented surfactant molecules, which tend to separate from horizontally oriented alpha-helices of the peptide. On the contrary, semifluorinated alkanes, lacking any polar group in their structure and bearing a large dipole moment, interact with ALM, also possessing a huge cumulative dipole moment. These dipole-dipole interactions between ALM and SFAs are more attractive than those between SFA molecules in their pure monolayers, causing the large ALM molecule, situated parallel to the interface, to be surrounded by SFA molecules in perpendicular orientation, leading to the formation of a highly organized binary mixed monolayer. BAM images of the ALM monolayer indicate that this peptide collapses with the nucleation and growth mechanism, like the majority of surfactants, which contradicts the model of ALM collapse by desorption, previously published in the literature.

Structure and electrochemical reactivity of new sulphur–silicon podands adsorbed on silver or gold surfaces

By the self-assembly monolayer (SAM) organization, three new podands belonging to silylpropanethiols have been tested as to their ability to form nanolayers protecting the noble metal surface (gold or silver) and to form complexes with monovalent metal cations on the metal surfaces. The stable self-assembled chemisorbed layers, providing protection to metal surface against electrooxidation and capable of blocking propylene carbonate (PC) electroreduction and Li electrodeposition were produced. Reflection-absorption infrared spectroscopy (RAIRS) indicated cleavage of the S–H bond upon adsorption of species 1–3 with the formation of S–Ag bonds on the metal surface. By cyclic voltammetry, it was found that the primary adsorbate formed on a Au electrode at Ead (between −0.2 and −1.2 V vs. SCE) underwent reductive desorption at E < −1.3 V vs. SCE. The structures of 1–3 and their complexes with Na+ cations on the Ag surfaces were calculated and visualized by the AM1d semi-empirical method.

Molecular interactions of cord factor with dipalmitoylphosphatidylcholine monolayers: Implications for lung surfactant dysfunction in pulmonary tuberculosis

Molecular interactions between mycobacterial cell wall lipid, cord factor (CF) and the abundant surfactant lipid, dipalmitoylphosphatidylcholine (DPPC) were investigated using Langmuir monolayers at physiological temperatures (37 °C). Surface topography of the films was visualized by atomic force microscopy (AFM). Thermodynamic behavior of the mixed monolayers was evaluated by investigating the molecular area excess, excess Gibbs free energy of mixing and maximum compressibility modulus (SCM max). Cord factor formed immiscible and thermodynamically unstable monolayers with DPPC. Monolayer presence of cord factor altered the physical state of DPPC monolayers from liquid condensed to liquid expanded with the lowering of SCM max from 160 to 40 mN/m, respectively. AFM imaging exhibited smooth homogenous surface topography of DPPC films which in the presence of cord factor was markedly altered with the appearance of aggregates and increased surface roughness. The results highlight the capacity of cord factor to disturb DPPC monolayer organization and structure. Interfacial presence of cord factor results in DPPC monolayer fluidization. Lung surfactant function is attributed to its ability to form well packed low compressibility films. Such molecular interactions suggest a dysfunction of lung surfactant in pulmonary tuberculosis due to surfactant monolayer fluidization.

Impact of the anchoring groups X (–SH, –S–S–, –SeH and –Se–Se–) of CF 3(CF 2) 3(CH 2) 11X molecules self-assembled on oxidised electroplated copper

In this contribution, monolayers of a partially fluorinated alkyl chains have been self-assembled on electroplated copper oxide. The molecules bear the following reactive groups: thiol (R–SH), disulfide (R–S–S–R), selenol (R–SeH) and diselenide (R–Se–Se–R), where R = F 3C–(CF 2) 3–(CH 2) 11–. Spectroscopic and electrochemical techniques have been used to characterise the surface composition and the monolayer organisation. All molecules have been shown to form monolayers of good quality except for the disulfide which leads to poor adsorption. While the copper oxide film is easily reduced by thiol, selenol and diselenide, this is less evident for disulfide. Additional to the anchoring group, the structure of the molecule is also found to have a determinant effect. The stability of the resulting molecular film and the copper protection efficiency ability is in close relation with the molecule structure and the interface stability.

Probing peptide–membrane interactions using AFM

AbstractAtomic force microscopy (AFM) has become a powerful addition to the range of instruments available to probe the organization of lipid monolayers and bilayers. Currently, AFM is the only tool that can provide nanoscale topographic images of supported lipid membranes under physiological conditions, enabling researchers to resolve their detailed structure and to monitor their interaction with drugs, peptides and proteins. Here, we survey recent data obtained by our research groups that demonstrate the power of the technique for exploring peptide–membrane interactions, with an emphasis on microbial lipopeptides and on tilted peptides. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Star-Shaped Oligo(p-phenylenevinylene) Substituted Hexaarylbenzene: Purity, Stability, and Chiral Self-assembly†

An oligo(p-phenylenevinylene) (OPV)-substituted hexaarylbenzene has been synthesized and fully characterized. Recycling gel permeation chromatography appeared to be a powerful technique to obtain the OPV molecules in a very pure form. X-ray analysis and polarization optical microscopy revealed that the OPV molecule is plastic crystalline at room temperature with an ordered columnar superstructure. In apolar solvents, the molecules self-assemble via a highly cooperative fashion into right-handed chiral superstructures, which are stable even at high temperatures and low concentration. Atomic force microscopy revealed right-handed fibers with a diameter of 6 nm, indicating pi-stacked aggregates; on a silicon oxide substrate, supercoiled chiral structures were observed. STM studies on a liquid-solid interface showed that the star-shaped OPV molecule forms an organized monolayer having a chiral hexagonal lattice.

The effect of self-assembled monolayers on polarization-dependent two-photon photoemission and on the angular distribution of the photoelectrons

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p-polarized versus s-polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.

Electrochemical Study of Benzodiazepines Derivatives Applied to the Analysis of Structure-Activity Relationships and Interactions with Membrane Models

Cyclic voltammetry was applied at the water/1,2 - dichloroethane interface to study the transfer of a series of benzodiazepines. Transfer potentials were determined with the aim to correlate the partition coefficient of these compounds with the presence of different substituents and its biological activity. On the other hand, the effect of Flunitrazepam (a benzodiazepine) on the organization and permeability of lipidics monolayers adsorbed at the interface was analyzed by cyclic voltammetry and electrochemical spectroscopy impedance. These results contribute to studies of quantitative structure - activity relationships and also to those related with to non - specific interaction of these drugs with biological membranes.

Differential effects of human SP-A1 and SP-A2 variants on phospholipid monolayers containing surfactant protein B

Surfactant protein A (SP-A), the most abundant protein in the lung alveolar surface, has multiple activities, including surfactant-related functions. SP-A is required for the formation of tubular myelin and the lung surface film. The human SP-A locus consists of two functional SP-A genes, SP-A1 and SP-A2, with a number of alleles characterized for each gene. We have found that the human in vitro expressed variants, SP-A1 (6A 2) and SP-A2 (1A 0), and the coexpressed SP-A1/SP-A2 (6A 2/1A 0) protein have a differential influence on the organization of phospholipid monolayers containing surfactant protein B (SP-B). Lipid films containing SP-B and SP-A2 (1A 0) showed surface features similar to those observed in lipid films with SP-B and native human SP-A. Fluorescence images revealed the presence of characteristic fluorescent probe-excluding clusters coexisting with the traditional lipid liquid-expanded and liquid-condensed phase. Images of the films containing SP-B and SP-A1 (6A 2) showed different distribution of the proteins. The morphology of lipid films containing SP-B and the coexpressed SP-A1/SP-A2 (6A 2/1A 0) combined features of the individual films containing the SP-A1 or SP-A2 variant. The results indicate that human SP-A1 and SP-A2 variants exhibit differential effects on characteristics of phospholipid monolayers containing SP-B. This may differentially impact surface film activity.

The study on the interaction between phytosterols and phospholipids in model membranes

Sterols are one of the major components of cellular membranes. Although in mammalian membranes cholesterol is a predominant sterol, in the human organism plant sterols (phytosterols) can also be found. Phytosterols, especially if present in concentrations higher than normal (phytosterolemia), may strongly affect membrane properties. In this work, we studied phytosterol–phospholipid interactions in mixed Langmuir monolayers serving as model membranes. Investigated were two phytosterols, β-sitosterol and stigmasterol and a variety of phospholipids, both phosphatidylethanolamines and phosphatidylcholines. The phospholipids had different polar heads, different length and saturation of their hydrocarbon chains. The interactions between molecules in mixed sterol/phospholipid films were characterized with the mean area per molecule ( A 12) and the excess free energy of mixing (Δ G Exc). The effect of the sterols on the molecular organization of the phospholipid monolayers was analyzed based on the compression modulus values. It was found that the incorporation of the phytosterols into the phospholipid monolayers increased their condensation. The plant sterols revealed higher affinity towards phosphatidylcholines as compared to phosphatidylethanolamines. The phytosterols interacted more strongly with phospholipids possessing longer and saturated chains. Moreover, both the length and the saturation of the phosphatidylcholines influenced the stoichiometry of the most stable complexes. Our results, compared with those presented previously for cholesterol/phospholipid monolayers, allowed us to draw a conclusion that the structure of sterol (cholesterol, β-sitosterol, stigmasterol) does not affect the stoichiometry of the most stable complexes formed with particular phospholipids, but influences their stability. Namely, the strongest interactions were found for cholesterol/phospholipids mixtures, while the weakest for mixed systems containing stigmasterol.

Imaging Photoelectron Transmission through Self-Assembled Monolayers: The Work-Function of Alkanethiols Coated Gold

In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolayers. Using this method, we measure the relative work-function of alkanethiols of different length on gold substrate, and we are able to follow the changes occurring when the surface coverage is varied. We also computed the work-function of model alkanethiols using a plane-wave density functional theory approach, in order to demonstrate the correlation between changes in the work-function with the monolayer organization and density.