Langmuir-Blodgett Monolayers Research Articles

Interfacial Properties of B Phycoerythrin Extracted from the Red Microalga <I>Rhodosorus Marinus</I> at Hexadecane-Water and Air-Water Interfaces

In this work we report the effect of pH on the fluorescence spectra and on the interfacial behaviour at the air-water and hexadecane-water interfaces of the protein B phycoerythrin, extracted and purified from the red microalga Rhodosorus marinus. We performed isotherms and compressionexpansion cycles and demonstrate that the stability of B phycoreythrin monolayers at the air-water interface depends on its aggregation state conditioned by pH in the subphase. Stable monolayers were obtained for pH 4.5, 7 and 2.5 except for pH 10 were B phycoerythrin molecules showed to be dissociated. At the hexadecane-water interface and for pH 4.5 and 7, protein molecules were adsorbed producing a higher surface pressure in comparison with the surface pressure obtained with pH 2.5 and 10, probably due to the enhancement of hydrophobic interactions favoured by a globular conformation, also found at the air-water interface. Atomic force microscopy images of Langmuir-Blodgett monolayers showed higher monolayer homogeneity for pH 4.5 and 7. Fluorescence emission intensity was higher for pH 4.5 and 7 where B PE molecules remain in a more compact conformation. © 2011 American Scientific Publishers.

Nanomechanical Properties of Supported Lipid Bialyers

Lipid reorganization induced morphology alteration in asymmetric bilayers prepared by Langmuir-Blodgett (LB)/ Langmuir-Schaeffer (LS) method was directly visualized by means of the atomic force microscopy (AFM) and fluorescence imaging. The LB monolayer in the bilayer system was prepared using binary mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl -sn-glycero-3-phosphocholine (DPPC) in 1:1 molar ratio, with 0.2 mol% Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine triethylammonium salt (TR-DHPE dye). The shapes of the flower like DPPC-enriched domains were found to be dependent on the preparing temperatures. These domains were well maintained after the LS layer of DOPC was deposited. The contrast of these domains has changed from the original dark to bright, and turned dark again after the sample stayed in water for several hours. Along with the contrast reverse, a third phase was observed to separate out within the domains, exhibiting a much taller height than the outside-domain areas. Mechanical properties of these regions were detected by means of force spectroscopy. The increased heights within DPPC domains and the rapid contrast reverse suggest that not only DOPC but also DPPC lipids have reorganized in the LB/LS bilayer, stimulated the morphology changes.

The Effect of Point Mutations on the Secondary Structure and Membrane Interaction of Antimicrobial Peptide Anoplin

Anoplin (GLLKRIKTLL-NH(2)) is the smallest linear α-helical antimicrobial peptide found naturally to date. Antibacterial and hemolytic properties of anoplin depend strongly on physicochemical properties. Two anoplin derivatives, anoplin-8K (Ano8K, GLLKTIKKLL-NH(2)) and anoplin-1K5 V8K (Ano1K5 V8K, KLLKVIKLL-NH(2)), were found to have increased bacterial and low hemolytic activity. In the present work physicochemical properties of these three peptides were studied by UV resonance Raman (UVRR) spectroscopy, Langmuir-Blodgett monolayer technique, and carboxyfluorescein (CF) leakage assay. UVRR data indicated that all three peptides adopt predominantly unordered conformation in aqueous buffer solution. In membrane-mimicking trifluoroethanol, the α-helical content increases for all three peptides with Ano1K5 V8K having the highest α-helix percentage, followed by Ano8K and anoplin. Critical micelle concentrations were found to be similar for all three peptides, and the saturation pressure decreases in the sequence Ano1K5 V8K, anoplin, Ano8K. Critical pressure of insertion was found to be greater for anionic lipid monolayer DPPG than for zwitterionic lipid DPPC indicating preferential adsorption of all three peptides to DPPG. Finally, membrane lytic activities of all three peptides toward various model lipid vesicles were compared through CF leakage assay. Overall the data indicate that antimicrobial activity of anoplin increases with charge, whereas membrane lytic activity correlates with peptides helicity and amphipathicity.

AFM and KPFM Study of Effect of Cholesterol and Cortisol on Structure of Lipid Monolayers

The plasma membrane is a complex structure, composed primarily of phospholipids and other macromolecules, such as proteins, sterols and steroids, which define its complex functions. Phospholipid monolayers are widely used as convenient models to study plasma membrane. We investigated model lipid monolayers composed of dipalmitoylphosphatidylcholine (DPPC) in order to elucidate effect of cholesterol and cortisol on the structure and properties of these model systems. Supported DPPC monolayers were prepared with Langmuir-Blodgett monolayer technique and imaged by Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM). Frequency modulation KPFM technique (FM-KPFM) (1) based on electrostatic force gradient was used to measure local surface potential distribution in monolayers. This technique is performed by applying modulation frequency in addition to the resonance frequency of the cantilever and has been shown previously to have superior resolution for measuring surface potential in biomolecular films (2). We found that cholesterol and cortisol produced structural changes (nanoscale domains) both in topography and surface potential maps of the monolayers. In addition with Differential Scanning Calorimetry (DSC) we found that both steroids produce changes the phase transitions of the DPPC membrane. 1. U. Zerweck, C. Loppacher, T. Otto, S. Grafström, and L. M. Eng, Phys. Rev. B 71 (2005) 125424. 2. B. Moores, F.Hane, L. M. Eng, Z. Leonenko, Ultramicroscopy, (2010) pub. on line: http://dx.doi.org/10.1016/j.ultramic.2010.02.036.

Evaluation of Biotinylated PAMAM Dendrimer Toxicity in Models of the Blood Brain Barrier: A Biophysical and Cellular Approach

The interaction of biotinylated G4 poly(amidoamine) (PAMAM) dendrimer conjugates and G4 PAMAM dendrimers with in vitro models of the blood brain barrier (BBB) was evaluated using Langmuir Blodgett monolayer techniques, atomic force microscopy (AFM) and lactate dehydrogenase measures of cell membrane toxicity. Results indicate that both G4 and G4 biotinylated PAMAM dendrimers disrupt the composition of the liquid condensed (LC) and liquid expanded (LE) phases of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid monolayer. The disruption is concentration dependent and more marked for G4 biotinylated PAMAMs. Lactate dehydrogenase (LDH) assays using endothelial cell culture models of the BBB indicate that biotinylation results in higher levels of toxicity than non-biotinylation. This approach provides valuable information to assess nanoparticle toxicity for drug delivery to the brain.

Enhanced Optical and Nonlinear Optical Responses in a Polyelectrolyte Templated Langmuir−Blodgett Film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.

Preparation, luminescence and photoelectric properties of Langmuir–Blodgett films of alkynylplatinum(II)–zinc(II) porphyrinate/heteropolyoxometalate hybrid composites

A new class of organometallic/inorganic hybrid Langmuir–Blodgett (LB) films, consisting of rigid-rod alkynylplatinum(II)–zinc(II) porphyrinate complex (OMA) as the π-conjugated donor–acceptor-type molecule and tungsto(molybdo)phosphoric heteropolyacids (HPA) (HPA = H 3PMo 12O 40 and H 3PW 12O 40, abbreviated as HPMo 12 and HPW 12, respectively) of the Keggin structure as the inorganic component, were prepared and characterized by π–A isotherms, UV–vis absorption and luminescence spectra, low-angle X-ray diffraction, scanning tunneling microscopy and surface photovoltage spectroscopy. Our experimental results indicate that stable, well-defined and well-organized Langmuir and LB films have been formed in pure water and heteropolyacid subphase. They typically have a highly organized lamellar structure in which a monolayer of HPA is most likely embedded inside the OMA molecular space formed by long chains of PBu 3. Luminescence spectra of these hybrid LB films show that HPMo 12 and HPW 12 can enhance the emission of OMA to some extent. These LB composites show good photovoltage responses and a photovoltage of 79 μV can be obtained for the OMA/HPMo 12 system when it is excited by light. The monolayer LB films on ITO wafer can also display interesting electrical conductivity.

Photoelectric Response from D–π–A Dye's Langmuir-Blodgett Monolayer Film Modified Indium-Tin Oxide Electrode

Langmui-Blodgett (LB) monolayer films of two new amphiphilic D–π–A dyes were fabricated on both pure and acidic water subphases and transferred onto the ITO electrode by the LB) technique. The ultraviolet-visible absorption spectra and photocurrent properties of the ordered LB films were investigated.

Surface‐Enhanced Fluorescence with Shell‐Isolated Nanoparticles (SHINEF)

Heart of gold: Surface-enhanced fluorescence (SEF) is demonstrated with silica-coated gold nanoparticles (SHINEF) that act as “smart dust” on the surface of a Langmuir–Blodgett monolayer. Coating gold nanoparticles of different sizes and shapes opens a wide range of applications for SEF, where the shape of the core with an appropriate thickness of the coating can be tuned for specific tasks. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Nanostrand Formation of Block Copolymers at the Air/Water Interface

Langmuir-Blodgett monolayers consisting of a network of nanostrands have occasionally been reported in the literature, but are often coexistent with other morphologies, which is not useful for potential applications. With the use of PS-P4VP/PDP, a polystyrene-poly(4-vinyl pyridine) diblock copolymer of 12 mol % VP content mixed with 3-pentadecylphenol, it is shown that the disordered nanostrand network morphology can be obtained reproducibly and uniformly over large surface areas by spreading chloroform solutions of relatively high copolymer concentration. Use of a more slowly evaporating spreading solvent, 1,1,2,2-tetrachloroethane, and a low subphase temperature, 8-9 °C, results in much more densely aligned nanostrands. Poorly spreading solvents such as nitrobenzene produce the well-known fingerprint pattern often observed in spin- or dip-coated thin films of block copolymers. A mechanism for nanostrand network formation is proposed that involves the momentary formation of a fingerprint morphology in spreading drops followed by its breakup at the level of the mobile P4VP/PDP stripes as spreading continues, leaving P4VP-anchored PS nanostrands floating on the water surface.

Incorporation of a synthetic mycobacterial monomycoloyl glycerol analogue stabilizes dimethyldioctadecylammonium liposomes and potentiates their adjuvant effect in vivo

The combination of delivery systems such as cationic liposomes and immunopotentiating molecules is a promising approach for the rational design of vaccine adjuvants. In this study, a synthetic analogue of the mycobacterial lipid monomycoloyl glycerol (MMG), referred to as MMG- 1, was synthesized and combined with the cationic surfactant dimethyldioctadecylammonium (DDA). The purpose of the study was to provide a thorough pharmaceutical characterization of the resulting DDA/MMG- 1 binary system and to evaluate how incorporation of MMG- 1 affected the adjuvant activity of DDA liposomes. Thermal analyses demonstrated that MMG- 1 was incorporated into the DDA lipid bilayers, and cryo-transmission electron microscopy (TEM) confirmed that liposomes were formed. The particles had a polydisperse size distribution and an average diameter of approximately 400 nm. Evaluation of the colloidal stability indicated that at least 18 mol% MMG- 1 was required to stabilize the DDA liposomes as the average particle size remained constant during storage for 6 months. The improved colloidal stability is most likely caused by increased hydration of the lipid bilayer. This was demonstrated by studying Langmuir–Blodgett monolayers of DDA and MMG- 1 which revealed an increased surface pressure in the presence of high concentrations of MMG- 1 when the DDA/MMG- 1 monolayers were fully compressed, indicating an increased interaction with water due to enhanced hydration of the lipid head groups. Finally, immunization of mice with the tuberculosis fusion antigen Ag85B-ESAT-6 and DDA/MMG- 1 liposomes induced a strong cell-mediated immune response characterized by a mixed Th1/Th17 profile and secretion of IgG1 and IgG2c antibodies. The Th1/Th17-biased immunostimulatory effect was increased in an MMG- 1 concentration-dependent manner with maximal observed effect at 31 mol% MMG- 1. Thus, incorporation of 31 mol% MMG- 1 into DDA liposomes results in an adjuvant system with favorable physical as well as immunological properties.

An ensemble and single-molecule fluorescence microscopy investigation of phase-separated monolayer films stained with Nile Red

Phase-separated Langmuir–Blodgett monolayer films prepared from mixtures of arachidic acid (C 19H 39COOH) and perfluorotetradecanoic acid (C 13F 27COOH) were stained via spin-casting with the polarity sensitive phenoxazine dye Nile Red, and characterized using a combination of ensemble and single-molecule fluorescence microscopy measurements. Ensemble fluorescence microscopy and spectromicroscopy showed that Nile Red preferentially associated with the hydrogenated domains of the phase-separated films, and was strongly fluorescent in these areas of the film. These measurements, in conjunction with single-molecule fluorescence imaging experiments, also indicated that a small sub-population of dye molecules localizes on the perfluorinated regions of the sample, but that this sub-population is spectroscopically indistinguishable from that associated with the hydrogenated domains. The relative importance of selective dye adsorption and local polarity sensitivity of Nile Red for staining applications in phase-separated LB films as well as in cellular environments is discussed in context of the experimental results.

The influence of salinity on surfactant miscibility in mixed dipalmitoylphosphatidylcholine – perfluorooctadecanoic acid monolayer films

The miscibility, mechanical and morphological properties of mixed Langmuir and Langmuir-Blodgett monolayers prepared from the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and the perfluorinated fatty acid perfluorooctadecanoic acid have been studied as a function of film composition and subphase salinity. It was demonstrated here, for the first time, that the extent of surfactant miscibility in mixed phospholipid-perfluoroacid monolayers, and hence the resulting mechanical properties of the monolayer film, can be controlled by altering the concentration of sodium ions in the underlying subphase. Elevated Na(+) concentrations resulted in lower net attractive interactions between film components, likely through specific ion adsorption to the negatively-charged perfluoroacid, along with decreased film elasticities. These results differ significantly from conventional fatty-acid-carboxylate monolayer systems in which film cohesion is typically enhanced through adsorption of cations to surfactant headgroups. Atomic force microscope images of films deposited onto solid mica substrates revealed that the films deposited from pure water formed multimolecular aggregates of surfactant, which could be attributed to the highly cohesive nature of the films, but the use of salt in the subphase diminished aggregate formation and resulted in the production of homogeneous monolayer films.

Monolayer behavior of binary systems of betulinic acid and cardiolipin: Thermodynamic analyses of Langmuir monolayers and AFM study of Langmuir–Blodgett monolayers

Betulinic acid (BA, a natural pentacyclic triterpene) can induce mitochondrial membrane damage and trigger the mitochondrial pathway of apoptosis in tumor cells. The monolayer behavior of binary systems of BA and cardiolipin (CL, a unique phospholipid found only in mitochondria membrane in animals) was studied by surface pressure-area (π-A) measurements and analyses and Atomic force microscopy (AFM) observation. The miscibility analysis presents that in mixed monolayers BA takes both tilted and nearly perpendicular orientations at surface pressure below 30 mN/m but only nearly perpendicular orientation at 30 mN/m. The thermodynamic stability analysis indicates that phase separation and repulsion occur in mixed BA/CL monolayers. The compressibility analysis shows that at 30 mN/m, 20% addition of BA does markedly translate the liquid-condensed CL monolayer to mixed BA/CL monolayer with the coexistence of liquid-condensed and liquid-expanded phases. The AFM images of supported monolayers give direct evidence of the conclusions obtained from the analyses of π-A isotherms. These results confirm that at high surface pressure near to real biologic situations, BA orients nearly perpendicularly with hydroxyl group toward water, causes phase separation and changes the permeability of CL film, which correlates with the mitochondrial membrane damage induced by BA.

Formation of ionic bonds between a fatty-acid Langmuir–Blodgett monolayer and a zinc oxide substrate

A fatty-acid Langmuir-Blodgett (LB) monolayer ionically adsorbed to the substrate has been prepared by adopting ZnO-based conductor and pure water respectively as the substrate and subphase. Ionization of the fatty-acid molecules has been detected by infrared reflection-absorption spectroscopy (IR-RAS), which is enabled by the condensed free electrons in the doped ZnO substrate. On the contrary, IR-RAS measurements have indicated that fatty-acid molecules transferred onto Au and indium-tin-oxide substrates from a pure-water surface are not ionized. In addition, it has been demonstrated that these ionically bonded fatty-acid monolayers protect the ZnO transparent electrode from chemical attack by NH(3) gas. Since the LB technique is a simple method applicable to a wide variety of materials, the present results may lead to a new way of obtaining functional organic-inorganic nanosystems.

First‐Layer Effect in Graphene‐Enhanced Raman Scattering

Graphene as a substrate for enhancing Raman scattering, called graphene-enhanced Raman scattering (GERS), has been reported in previous work. Herein, it is found that the "first-layer effect", which is widely used to explain the chemical-enhanced mechanism in surface-enhanced Raman scattering (SERS), exists in the GERS system. The Langmuir-Blodgett (LB) technique is used to construct mono- and multilayer ordered aggregates of protoporphyrin IX (PPP). Raman spectra of PPP with different layer numbers of the LB film on graphene are collected. The Raman signal from the first monolayer LB film of PPP has a larger contribution to the Raman enhancement than that from subsequent monolayers. Meanwhile, the Raman enhancement is dependent on the molecular configuration in contact with graphene, in which the functional group of PPP in direct contact with graphene has a stronger enhancement than other groups. These results reveal that GERS is strongly dependent on the distance between graphene and the molecule, which is convincing evidence that the Raman enhancement effect based on graphene belongs to the chemical-enhanced mechanism. This discovery provides a convenient system for the study of the chemical-enhanced mechanism and will benefit further understanding of SERS.

Formation of One-Dimensional Capped ZnO Nanoparticle Assemblies at the Air/Water Interface

The self-assembling behavior and microscopic structure of zinc oxide nanoparticle Langmuir-Blodgett monolayer films were investigated for the case of zinc oxide nanoparticles coated with a hydrophobic layer of dodecanethiol. Evolution of nanoparticle film structure as a function of surface pressure (π) at the air-water interface was monitored in situ using Brewster's angle microscopy, where it was determined that π = 16 mN/m produced near-defect-free monolayer films. Transmission electron micrographs of drop-cast and Langmuir-Schaefer deposited films of the dodecanethiol-coated zinc oxide nanoparticles revealed that the nanoparticle preparation method yielded a microscopic structure that consisted of one-dimensional rodlike assemblies of nanoparticles with typical dimensions of 25 × 400 nm, encased in the organic dodecanethiol layer. These nanoparticle-containing rodlike micelles were aligned into ordered arrangements of parallel rods using the Langmuir-Blodgett technique.

High‐Performance Langmuir–Blodgett Monolayer Transistors with High Responsivity

Molekulare Feldeffekttransistoren mit Ladungsträgermobilitäten wie Standardtransistoren (bis 0.04 cm2 V−1 s−1), hohen An/Aus-Stromverhältnissen (>106) und hoher Ansprechempfindlichkeit entstehen durch Kombination von Langmuir-Blodgett-Techniken mit Mikro/Nanofertigung aus einheitlichen selbstorganisierten Kupferphthalocyanin(CuPc)-Halbleiter-Monoschichten und einzelnen Graphenschichten als planare Kontakte. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Annealing-Induced Changes in Double-Brush Langmuir−Blodgett Films of α-Helical Diblock Copolypeptides

The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) monolayers of diblock copolymers (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) with unidirectional helix orientation deposited on hydrophilic silicon substrates was characterized by means of small-angle X-ray reflectivity, transmission Fourier transform infrared spectroscopy, and atomic force microscopy. Upon annealing at 100 degrees C for 24 h, the alpha-helices became less tilted toward the substrate surface normal. Surface area shrinkage accompanied the change in tilt, indicated by an increase in both film thickness and electron density, resulting in more compact and uniform films. The enhancement of the helix orientation by thermal annealing was greater for the PMLGSLG block and for the diblock copolymers with the shorter block lengths. For these diblock copolymers, annealing resulted in postorientation of the PMLGSLG block helices almost perpendicular to the substrate surface. This effect originates from a considerable increase in intermolecular packing of the PLGA block caused by hydrogen bonds between the carboxylic groups upon annealing, as well as the high mobility of the PMLGSLG block helices for rearrangement favored by the melted side chain mantle at elevated temperatures.

High‐Performance Langmuir–Blodgett Monolayer Transistors with High Responsivity

Molecular field-effect transistors with bulk-like carrier mobility (as high as 0.04 cm2 V−1 s−1), high on/off current ratios (over 106), and high responsivity are formed by the integration of Langmuir–Blodgett techniques with sophisticated micro/nanofabrication. The transistors are formed from self-assembled uniform monolayers of copper phthalocyanine (CuPc) semiconductors and single-layer graphene as planar contacts.