Langmuir-Blodgett Bilayers Research Articles

Macroscopic conductance in Langmuir–Blodgett bilayers of charge-transfer salts

Abstract Macroscopic conductance was observed in Langmuir–Blodgett (LB) bilayers deposited by two versions of the Langmuir–Schaefer (LS) technique. Bilayers of several compositions on the basis of donor compounds and donor–acceptor mixtures doped in ferric chloride solutions after the deposition were studied. Relatively high conductivity for such super-thin films was obtained due to the application of specific methods of substrate surface preparation and optimized procedure of doping. The conductivity values were in the range of 0.2–1.0 S cm −1 for bilayers and 10–30 S cm −1 for the films composed of 12 monolayers. This difference can be caused by the formation of bilayers of heterogeneous X-type/Y-type structure, which is in a good agreement with the results of surface potential measurements. Bilayers deposited by the LB technique either are non-conductive or possess very low conductivity. The most probable reason of such a behaviour is their discontinuity at the microscopic level. This conclusion is based on the data obtained by FT-IR spectroscopy, surface potential, and optical microscopy techniques as well as by transfer ratio measurements.

Second-Harmonic Generation from Alternate-Layer Langmuir-Blodgett Films of Optically Non-Linear Materials in which the Interleaving Species is a Complementary Dye or Poly(t-butyl methacrylate)

Non-centrosymmetric Langmuir–Blodgett (LB) films of optically non-linear dyes have been fabricated by alternating layers of complementary molecules, where the attachment of the hydrophobic alkyl chain, relative to the donor–(π -bridge)–acceptor chromophore, is reversed. The second-harmonic intensity increases quadratically with the number of LB bilayers, but the normalized intensity, I 2ω(N) /N 2 , is suppressed because the intramolecular and interlayer dipoles (D–π –A....D–π –A) are opposed. The second-order susceptibility, thickness and chromophore tilt angle, relative to the substrate normal, are X(2)zzz = 110 pm V–1 at 1.064 µm, l= 3.4 nm and φ = 31° for E-N-octadecyl-4-{2-[4-(N,N-dimethylaminophenyl)]ethenyl}pyridinium iodide (D–π–A-C 18 H 37 ) and X(2)zzz = 170 pm V –1 ,l = 2.5 nm and φ = 26° for E-N-methyl-4-[2-(4-N-methyl-N-octadecylaminophenyl))ethenyl]pyridinium iodide (C 18 H 37–D–π–A), the data corresponding to monolayer films deposited at 35 mN m –1 . The susceptibility of the alternate-layer structure is only modest (X(2)zzz = 20 pm V -1) but may be increased to c.70 pm V –1 by interleaving the active layers with passive spacers. The harmonic wavelength overlaps the charge-transfer band, whereas films of the naphthyl analogue, E-N-octadecyl-4-{2-[4-(N,N-dimethylaminonaphthyl)]ethenyl}pyridinium octadecylsulfate, interleaved with poly(t-butyl methacrylate), have a very slight absorbance of c.4.7 × 10 –4bilayer–1 at 532 nm and are transparent at 1.064 µm. The second-order susceptibility is 36 pm V –1 .

Atomic Force Microscope Image Contrast Mechanisms on Supported Lipid Bilayers

This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.

Blistering of Langmuir-Blodgett Bilayers Containing Anionic Phospholipids as Observed by Atomic Force Microscopy

Asymmetric bilayers of different phospholipid compositions have been prepared by the Langmuir-Blodgett (L-B) method, and imaged by atomic force microscopy (AFM). Such bilayers can function as a model for biological membranes. The first leaflet consisted of zwitterionic phospholipids phosphatidylcholine (PC) or phosphatidylethanolamine (PE). The second leaflet consisted of the anionic phospholipid phosphatidylglycerol (PG), in either the condensed or liquid phase or, for comparison, of PC. Different bilayers showed different morphology. In all bilayers defects in the form of holes were present. In some bilayers with a first leaflet consisting of PC, polygonal line-shaped defects were observed, whereas when the first leaflet consisted of PE, mainly round defects were seen. Not only the shape, but also the amount of defects varied, depending on the condition and the composition of the second leaflet. In most of the PG-containing systems the defects were surrounded by elevations, which reversibly disappeared in the presence of divalent cations. This is the first time that such elevations have been observed on phospholipid bilayers. We propose that they are induced by phospholipid exchange between the two leaflets around the defects, leading to the presence of negatively charged phospholipids in the first leaflet. Because the substrate is also negatively charged, the bilayer around the edges is repelled and lifted up. Since it was found that the elevations are indeed detached from the substrate, we refer to this effect as bilayer blistering.

Organic/inorganic Langmuir–Blodgett films based on metal phosphonates 2: zirconium phosphonate-based alternating layer films

Langmuir–Blodgett (LB) films of zirconium salts of the functionalized phosphonic acids 4-octadecyloxyphenylphosphonic acid (P0), 4-octadecyloxybiphenylphosphonic acid (B0), and 4-(4-octadecyloxyphenyldiazenyl)phenylphosphonic acid (A0) have been prepared and characterized. Layers of P0, B0, or A0 can be deposited onto zirconated octadecylphosphonic acid layers using a three-step deposition procedure to form alternating zirconium phosphonate LB bilayers. Film composition and molecular orientations are characterized through X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR).

Effect of substrate anchoring on the mechanical strength of Langmuir–Blodgett bilayers

Using the surface-force apparatus (SFA), we have made out-of-contact force and in-contact adhesion measurements between composite Langmuir–Blodgett (LB) bilayers. The outer layer is a crystalline monolayer of a novel glycine-containing amphiphile, whose headgroups make a strong, hydrogen-bond-aided adhesion when in normal contact with an identical monolayer. Two different inner layers were studied, with different affinities for the mica substrate. One, a crystalline monolayer of the phospholipid dipalmitoylphosphatidyl-ethanolamine (DPPE), is held onto mica by comparatively weak interactions. Another monolayer, a covalently polymerized network of octadecyltriethoxysilane (OTE), is covalently anchored to plasma-treated mica through the thermal hydrolysis of silanol groups on both substrate and monolayer. We observe that separation of two DPPE/glycine-amphiphile bilayers in hydrogen-bonded contact leads to fracture at the mica/bilayer interface, with a pull-off force of F/ R=−75 mN/m. Bonded OTE/glycine-amphiphile bilayers fracture at the OTE/glycine-amphiphile interface, with a pull-off force of F/ R=−142 mN/m. These results demonstrate that the mechanisms of LB film fracture cannot be explained considering only the breaking of hydrophobic contacts between neighboring amphiphile tails.

Direct atomic force microscopic evidence of hydrogen bonding interaction in phosphatidic acid Langmuir-Blodgett bilayer

Molecularly resolved atomic force microscopic images of phosphatidic acid Langmuir-Blodgett bilayers show that phosphate groups in polar region of the films are packing in a distorted hexagonal organization with long-range orientational and positional order. Intermolecular hydrogen bonding interactions, which should be responsible for the ordering and stability of bilayers, are visualized directly between adjacent phosphate groups in the polar region of the bilayer. Some adjacent phosphatidic acid molecules link each other through the formation of intermolecular hydrogen bonds between phosphate groups in polar region to form local supramolecules, which provide the bilayer's potential as a functionized film in the investigation on the lateral conductions of protons in the biological bilayers.

Structure and Forces in Transfection Related Surfactant Systems

AbstractExperiments have been performed on the cationic bilayer and liposome-forming surfactant DODAB, a successful DNA transfection agent. The surface forces apparatus (SFA) was utilized to measure the long-range colloidal and short-range adhesion forces between Langmuir-Blodgett bilayers exposing DODAB in the outer monolayers. Forces were measured in aqueous solutions of varying salt concentrations between 1 and 10 mM, with and without single-stranded DNA added to the solution. These represent the first measurements of the forces between surfactant/DNA assemblies. At low salt concentrations without DNA, the interbilayer forces are repulsive due to their electrostatic interaction, in agreement with DLVO theory. In contrast, in the presence of DNA which adsorbs to the bilayer surfaces, the forces between the bilayers changed to an enhanced and longer ranged repulsive steric-electrostatic interaction. At shorter range. the DNA can be excluded from the gap leading to an enhanced adhesion at contact accompanied by lipid rearrangement.

A novel structure observed on the phospholipid Langmuir-Blodgett bilayer by atomic force microscopy

A novel structure consisting of roundish objects formed on dimyristoyl-phosphatidylcholine (DMPC) Langmuir-Blodgett bilayer films was observed by atomic force microscopy (AFM). Such a structure may have resulted from the collapse of DMPC liposomes formed at the interface of water/organic phase when they have lost the water support.

Vectorial PET in LB bilayers and in vesicles: nanosecond fluorescence and laser flash-photolysis investigation

Vectorial photoinduced electron transfer was investigated in Langmuir-Blodgett bilayers and in vesicles with the same phospholipid composition. The donor was covalently bound to one of the Hydrophobie chains of the phospholipid in the mimetic membrane, whereas the acceptor was adsorbed at the polar head group surface. The location of the redox couple was preliminarily determined studying the photophysical properties of the donor in the phospholipid bilayer as well as the process of adsorption of the acceptor at the phospholipid-water interface. The photoinduced electron transfer reaction was studied measuring the quenching of pyrene fluorescence in the presence of increasing concentrations of acceptor, the process was studied both with steady-state and time-resolved fluorescence emission in the nanosecond time scale. The reaction transient species formed upon photoexcitation were identified performing nanosecond laser-flash photolysis experiments in vesicle systems. The dynamics of the charge recombination process following photoinduced electron transfer, was investigated studying the kinetic of decay of the absorption signal of the transient species.

Polysiloxane-based biomembranes

Natural phosphatidylcholine was attached to a polysiloxane backbone. The Langmuir film, Langmuir-Blodgett deposition and Montal-Mueller properties were studied. The material forms exceptionally stable Langmuir films. Montal-Mueller bilayers have a resistance of about 2 × 10 4 Ω cm −2 and a breakdown voltage of about 350 mV. Langmuir-Blodgett bilayer films have a low-voltage resistance of about 2 × 10 5 Ω cm −2, a transition to a square-law dependence at around 0.1 V, and a breakdown voltage greater than 1 V. There is evidence that bacteriorhodopsin in readily incorporated into such films.

Spectroscopic and gas adsorption properties of thulium bisphthalocyanine Langmuir-Blodgett films

Langmuir monolayers of thulium bisphthalocyanine (TmPc 2) were formed on a water subphase with a well defined π-A isotherm and Langmuir-Blodgett (LB) films were fabricated on solid substrates. The spectroscopy of the material and LB films, UV-visible, near-infrared, infrared, Raman, resonance Raman, and surface enhanced Raman scattering, is presented. The tilted molecular orientation of the TmPc 2 molecules in the floating layer, indicated by the limiting area of the π-A isotherm, is also confirmed in the LB films by transmission and reflection-absorption infrared spectroscopy (RAIRS). The characterization includes the assignment of characteristic vibrational frequencies observed in the Raman and infrared spectra. The adsorption of NO x (NO 2/N 2O 4) gas on a bilayer LB film of TmPc 2 has also been studied. The LB film was deposited onto an interdigitated gold electrode and the absorption-desorption process was monitored by the change in electrical conductivity during exposure to NO x gas seeded in N 2. The thermal conductivity of the molecular semiconductor and the kinetics of the gas absorption and desorption processes at different temperatures have been investigated.

Atomic force microscope measurements and manipulation of Langmuir-Blodgett films with modified tips

A simple method for rendering atomic force microscope tips and cantilevers hydrophilic or hydrophobic through glow discharge in an appropriate gas atmosphere is introduced. Force curves at different humidities of these modified cantilevers were taken on freshly cleaved mica (hydrophilic surface) and on a monolayer of dipalmitoylphosphatidylethanolamine transferred onto mica (hydrophobic surface) to characterize the behavior of the cantilevers on hydrophilic and hydrophobic surfaces. Furthermore, Langmuir-Blodgett bilayers, with a dipalmitoylphosphatidylethanolamine bottom layer and a dipalmitoylphosphatidylcholine top layer, were imaged in the constant force mode in a multimode atomic force microscope in air under controlled humidity conditions. The friction and elasticity signal were recorded parallel to the topography. By varying the force exerted by the tip on the sample, different layers of the Langmuir-Blodgett system could be removed or flattened. Removal exposed underlying layers that exhibited a different friction and elasticity behavior. Furthermore, force scans with tips rendered hydrophobic were taken on the different layers of the sample to characterize the hydrophilic/hydrophobic nature of the layers. Only by combining the results obtained by the different methods can the structure of the lipid layer systems be identified.

The structure and stability of phospholipid bilayers by atomic force microscopy

Atomic force microscopy (AFM) was used to investigate the structure, stability, and defects of the hydrophilic surfaces of Langmuir-Blodgett bilayer films of distearoylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylethanolamine (DPPE) in the solid phase, and dilinoleoylphosphatidylethanolamine (DLPE) in the fluid phase. Their relative resilience to external mechanical stress by the scanning tip and by fluid exchange were also investigated. DPPE monolayers showed parallel ridges at the surface with a period of 0.49 nm, corresponding to the rows of aligned headgroups consistent with the known crystallographic structure. DSPC and DLPE monolayers did not show any periodic order. The solid DSPC and DPPE monolayers were stable to continued rastering by the AFM tip; however, the stability of DLPE monolayers depended on the pH of the aqueous environment. Structural defects in the form of monolayer gaps and holes were observed after fluid exchange, but the defects in DLPE monolayer at pH 11 were stable during consecutive scanning. At pH 9 and below, the defects induced by fluid exchange over DLPE monolayers were more extensive and were deformed easily by consecutive scanning of the AFM tip at a force of 10 nN. The pH dependence of resilience was explained by the increasing bending energy or frustration due to the high spontaneous curvature of DLPE monolayers at low pH. The tangential stress exerted by the AFM tip on the deformable monolayers eventually produced a ripple pattern, which could be described as a periodic buckling known as Shallamach waves.

Assembly and molecular organization of self-assembled lipid bilayers on solid substrates monitored by surface plasmon resonance spectroscopy

The structural properties of lipid films, made from a squalene/butanol solution containing varying amounts (0–15 mg/ml) of egg phosphatidylcholine and deposited on a thin metallic silver layer, were investigated using surface plasmon resonance (SPR) spectroscopy. Optical parameters (thickness, refractive index and extinction coefficient) of such supported self-assembled lipid membranes were obtained from a theoretical analysis of the experimental SPR curves. The mass of the lipid membrane and the area and volume occupied by one lipid molecule were also calculated. The results were consistent with the formation of durable and homogeneous lipid bilayers on the solid substrate, and indicated similarities in structural properties between the present lipid bilayers and freely suspended and Langmuir-Blodgett bilayer membranes. Such bilayers represent a simple model for biological membranes, as well as providing a means of immobilizing proteins for various practical applications, including receptor-based sensors and molecular devices. The results confirm the value of the SPR technique for investigating the properties of thin biomolecular dielectric films deposited on a metal surface.

Vectorial photoinduced electron transfer in phospholipid vesicles and LB bilayers

Photoinduced electron transfer (PET) was studied in phospholipid vesicles and in Langmuir-Blodgett bilayers in the attempt to produce a model for electron transfer processes in biological media. Spatial organization of the reaction centers in lipid membranes needs to be controlled in order to provide high efficiency of light-to-chemical energy conversion. We designed a phospholipid system where the donor is localized in the inner bilayer whereas the acceptor is at the polar groups-water interface. We used dipalmitoylphosphatidic acid vesicles containing low molar fractions of dipalmitoylphosphatidylcholine with pyrene (donor) bound to one of the alkyl chains. Methylviologen (acceptor) was added to the external aqueous phase; upon photoexcitation of the donor we observed the electron transfer to take place in a unidirectional manner from the inside of the bilayer to the interface. Information about the location of the donor was obtained studying the photophysical properties of the pyrene chromophore in vesicles and in LB layers. The photoinduced electron transfer reaction was evidenced by quenching of pyrene fluorescence in the presence of increasing concentrations of acceptor, the process was studied both with steady-state and time-resolved fluorescence emission. Fluorescence intensity was found to decrease with increasing concentration of methylviologen, similar results were obtained for vesicles and LB layers of analog composition immersed in a methylviologen solution. Lifetimes of the excited species were found to be of the same order of magnitude in vesicle and LB-layer systems.

Molecular Dynamics of Langmuir-Blodgett Films: II. Bilayers

Abstract We have simulated Langmuir Blodgett bilayers of stearic acid at 25°C for two head-group areas using the molecular dynamics method. At Am = 21.2 A2 the molecules are tilted at 18° to the surface normal with a next-nearest-neighbour azimuthal orientation. The head-group dipoles are parallel to the plane of the surface and antiparallel to each other. The precise ordering of the dipoles depends on the density of the bilayer. There is a strong correlation between the translational and orientational ordering of the molecules in the two layers due to strong inter-layer electrostatic coupling between the head-groups. The head-groups exhibit a strong crystalline structure in the plane of the bilayer at both dessities. Increasing the density to 20.6 A2 does not significantly alter the tilt angle, this contrast to the energy minimisation results and the molecular dynamics results for the monolayer at the same temperature and head-group area. It produces a distortion of the translational ordering of the mole...

Crystallization process of Langmuir–Blodgett films of octadecylthiobenzoquinone

Atomic force microscopy images of the electron acceptor 2-octadecylthio-1,4-benzoquinone prepared as 2–4 Y-type Langmuir–Blodgett films revealed the formation of nanoscale crystals one week after deposition. The crystals have a layered structure with a total height from 3 to 15 nm and they each covered an area of <1 μm2 on the substrate. In the plane of a layer, the molecules formed a two molecular rectangular unit cell with sides of 0.51±0.01 nm and 1.35±0.03 nm. The measured step height of a layer was 3.6±0.2 nm and gives a tilt of the alkyl chains of 54±3° relative to the normal to the interface between layers. The packing density of 0.34±0.1 nm2 per molecules in the plane of a layer is significantly different from the packing density on the water subphase during deposition and the packing density found on the flat Langmuir–Blodgett bilayer of about 0.26 nm2. This means that a transition from a two-dimensional packing to a three-dimensional bulk crystal has taken place. This transition is more easily induced on a hydrophobic silicon substrate than on a bilayer of hydrophobic fatty acid. Six months after preparation, the crystal had grown in size, and now covered a typical area of ≊15 μm2. The mobility of molecules necessary for this growth is discussed.

Unexpected Square Symmetry Seen by Atomic Force Microscopy in Bilayer Films of Disk-Like Molecules

Thin films of disk-shaped molecules are expected to display anisotropic optical and transport properties, leading to applications in optical display or sensor technologies. Bilayer Langmuir-Blodgett films of monomeric triphenylene mesogens have been studied by atomic force microscopy. The triphenylene cores of the constituent molecules tend to promote the formation of columnar structures in the plane of the substrate and along the direction of deposition of the film. Atomic force microscopy images of bilayer Langmuir-Blodgett films revealed two types of structure, one corresponding to an aligned columnar structure and the other to an unusual square lattice, which may result from the superposition of columnar structures in adjacent layers that intersect at near right angles. Annealing such bilayers near the melting point of the bulk compound improved the structural ordering by reducing the angular spread of orientations associated with the well-developed columnar structure in some areas and by producing a more distinct square lattice in other areas of the sample.

Stability and Modification of Polyglutamate Langmuir-Blodgett Bilayer Films

Langmuir-Blodgett bilayers of a polyglutamate (PG) statistical copolymer have been investigated using atomic force microscopy (AFM) and X-ray reflectivity. PG ordered bilayer films on silicon surfaces possess an «expanded» thickness of 4.12±0.05 nm and an average roughness of 0.7±0.2 nm. Possibilities for surface modification by the AFM tip were explored. Formation of an «abrasion» surface morphology was related to a stick-slip mode of the AFM tip movement. Using the AFM tip, several surface modifications were investigated: fabrication of large (several hundreds of nanometers) and small (several nanometers) holes; «writing» of grooves of predictable geometry; fabrication of holes of single layer and bilayer depth; cleaning of the surface and fabrication of holes inside this new area