Langmuir-Blodgett Transfer Research Articles

Langmuir-Blodgett Transfer of Nanocrystal Monolayers: Layer Compaction, Layer Compression, and Lattice Stretching of the Transferred Layer.

Grazing incidence small angle X-ray scattering (GISAXS) was used to study the structure and interparticle spacing of monolayers of organic ligand-stabilized iron oxide nanocrystals floating at the air-water interface on a Langmuir trough, and after transfer to a solid support via the Langmuir-Blodgett technique. GISAXS measurements of the nanocrystal arrangement at the air-water interface showed that lateral compression decreased the interparticle spacing of continuous films. GISAXS also revealed that Langmuir-Blodgett transfer of the nanocrystal layers to a silicon substrate led to a stretching of the film, with a significant increase in interparticle spacing.

Insights on the Molecular Orientation of Oligo(p-phenylene vinylene) Derivatives with Alkyl Chains in Langmuir Films

One of the oligo(p-phenylene vinylene) derivatives having alkyl chains, OPV-1, is known to form characteristic rod-shaped nanoaggregates on the water surface, and thin films with aligned nanoaggregates can be obtained by the Langmuir–Blodgett (LB) technique. Although the molecular aggregation and orientation of the “conjugated rings part” in the LB films have already been investigated, analysis of the detailed molecular structure involving the “alkyl chains” is left behind. In the present study, to reveal the role of the alkyl chains in the molecular aggregation in Langmuir films prior to the LB transfer, infrared (IR) external reflection spectroscopic measurements on the water surface are carried out. It has been revealed that not only the conjugated rings of OPV-1 but also the alkyl chains with an ordered conformation are lying on the water surface. The characteristic orientation is found to be kept on a solid substrate even after the LB transfer by IR p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). In addition, pMAIRS was first employed for analyses of the biaxial orientation in the LB films, and the molecular orientations for the conjugated rings and the alkyl chains were quantitatively evaluated from both in-plane and out-of-plane perspectives.

Atomic force microscopy study of the complexation of sterols and the glycoalkaloid α-tomatine in Langmuir-Blodgett monolayers.

Glycoalkaloids are secondary metabolites produced by plants that aid in their protection from pathogens and pests. They are known to form 1:1 complexes with 3β-hydroxysterols such as cholesterol causing membrane disruption. So far, the visual evidence showcasing the complexes formed between glycoalkaloids and sterols in monolayers has been mainly restricted to some earlier studies using Brewster angle microscopy which were of low resolution showing the formation of floating aggregates of these complexes. This study is aimed at using atomic force microscopy (AFM) for topographic and morphological analysis of the aggregates of these sterol-glycoalkaloid complexes. Langmuir-Blodgett (LB) transfer of mixed monolayers of the glycoalkaloid α-tomatine, sterols, and lipids in varying molar ratios onto mica followed by AFM examination was performed. The AFM method allowed visualization of the aggregation of sterol-glycoalkaloid complexes at nanometer resolution. While aggregation was observed in mixed monolayers of α-tomatine with cholesterol and in mixed monolayers with coprostanol, no sign of complexation was observed for the mixed monolayers of epicholesterol and α-tomatine, confirming their lack of interaction found in prior monolayer studies. Aggregates were observed in transferred monolayers of ternary mixtures of α-tomatine with cholesterol and the phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or egg sphingomyelin (egg SM). The formation of aggregates was found to be less prevalent for mixed monolayers of DMPC and cholesterol containing α-tomatine than it was for mixed monolayers containing egg SM and cholesterol with α-tomatine. The observed aggregates were generally elongated structures, of a width ranging from about 40-70nm.

Phase transitions of the pulmonary surfactant film at the perfluorocarbon-water interface

Pulmonary surfactant is a lipid-protein complex that forms a thin film at the air-water surface of the lungs. This surfactant film defines the elastic recoil and respiratory mechanics of the lungs. One generally accepted rationale of using oxygenated perfluorocarbon (PFC) as a respiratory medium in liquid ventilation is to take advantage of its low surface tensions (14–18 mN/m), which was believed to make PFC an ideal replacement of the exogenous surfactant. Compared with the extensive studies of the phospholipid phase behavior of the pulmonary surfactant film at the air-water surface, its phase behavior at the PFC-water interface is essentially unknown. Here, we reported the first detailed biophysical study of phospholipid phase transitions in two animal-derived natural pulmonary surfactant films, Infasurf and Survanta, at the PFC-water interface using constrained drop surfactometry. Constrained drop surfactometry allows in situ Langmuir-Blodgett transfer from the PFC-water interface, thus permitting direct visualization of lipid polymorphism in pulmonary surfactant films using atomic force microscopy. Our data suggested that regardless of its low surface tension, the PFC cannot be used as a replacement of pulmonary surfactant in liquid ventilation where the air-water surface of the lungs is replaced with the PFC-water interface that features an intrinsically high interfacial tension. The pulmonary surfactant film at the PFC-water interface undergoes continuous phase transitions at surface pressures less than the equilibrium spreading pressure of 50 mN/m and a monolayer-to-multilayer transition above this critical pressure. These results provided not only novel biophysical insight into the phase behavior of natural pulmonary surfactant at the oil-water interface but also translational implications into the further development of liquid ventilation and liquid breathing techniques.

Efficient tunability of size and optical properties of reduced graphene oxide-ZnO composite nanocrystallites on solid substrates

Growth of ultra-thin luminescent reduced graphene oxide (rGO)-zinc oxide (ZnO) composite nanostructures was accomplished by Langmuir-Blodgett (LB) transfer of GO-Zn composite layers onto solid substrates and subsequent oxidation. Layer structure of LB transferred composites and oxidation procedure determine the size distribution, density, structural order, optical quality of grown ZnO and the extent of GO reduction. Prolonged oxidation (120 min) of composite monolayers produce ZnO nanocrystallites nearly deprived of defects and exhibiting substantially enhanced band edge emission. Prior anchoring of Zn2+ ions on GO layers and oxidation facilitate controlled growth of ZnO nanocrystallites and nanoclusters (20–100 nm), offer tunability of absorption edge (325–355 nm) and assists in amendment of surface defects/band edge emission (376–384 nm). Conductivity of precursor and oxidized composite layers was mapped by recording STM images as well as STS spectra and, the obtained differential conductivity spectra substantiate the changes in GO conductivity and provide range of bandgap values of grown ZnO nanocrystallites (3.1–3.4 eV). The multiple GO-Zn compositing procedures and optimum oxidation conditions introduced in present work facilitate development of rGO-ZnO composite nanostructures with high optical, crystalline quality and conductive characteristics, which pave path for fabrication of such nanostructures for optoelectronic device applications.

Langmuir-Blodgett transfer from the oil-water interface

HypothesisAlmost all Langmuir-Blodgett (LB) films were prepared with the classical Langmuir film balance, developed more than a century ago. To date, the success of the classical Langmuir film balance and the LB transfer technique is primarily restricted to the study of self-assembled monolayers at the air-water surface. It is challenging to study self-assembled monolayers at the oil-water interface, since the Langmuir film balance requires stacked oil and water layers. We hypothesize that a newly developed experimental method, called constrained drop surfactometry (CDS), is capable of preparing and characterizing LB films from the oil-water interface. ExperimentsWe have developed a novel droplet-based LB transfer technique capable of preparing LB films from the oil-water interface. In conjunction with atomic force microscopy, we have demonstrated the capacity of the CDS in studying a natural pulmonary surfactant film self-assembled at the perfluorocarbon-water interface, and have compared to the LB films prepared from the air-water surface using the classical Langmuir film balance. FindingsOur findings have demonstrated a novel paradigm for studying self-assembled monolayers and for preparing LB films from the oil-water interface. The CDS holds great promise for expanding the applicability of the traditional LB transfer technique from the air-water surface to the oil-water interface.

Modulation of gastric lipase adsorption onto mixed galactolipid-phospholipid films by addition of phytosterols

The rapid and preferential adsorption of a gastric lipase recombinant dog gastric lipase (rDGL) in heterogeneous films of phospholipids and triacylglycerols has previously been unveiled using Langmuir films analyzed by tensiometry, ellipsometry and Langmuir-Blodgett transfer coupled to atomic force microscopy. Here we invest the adsorption behavior of rDGL in heterogeneous galactolipid and mixed galactolipid-phospholipid or galactolipid-phospholipid-phytosterol films representative of plant membrane. Again rDGL, preferentially got adsorbed at the expanded lipid phases of the films underlining the genericity of such adsorption behavior. The addition of phytosterols to these mixtures resulted in the creation of defects, favoring the adsorption of rDGL at the fluid phases, but also improving the adsorption capacities of the lipase at the phase boundaries and towards the defects in the condensed phase. rDGL, like all gastric lipases, does not show any activity on galactolipids and phospholipids but its adsorption impacts their lateral organization and may change the adsorption and activity of other lipolytic enzymes in the course of digestion.

Formation of Surface Wrinkles in Collapsed Langmuir Films of a Polyhedral Oligomeric Silsesquioxane Containing Diblock Copolymer.

Surface pressure versus mean molecular area isotherms of Langmuir films of a hybrid diblock copolymer of poly(ethylene glycol) (PEG) and poly(methacrylo polyhedral oligomeric silsesquioxane) P(MA-POSS) together with Brewster angle microscopy reveal details of the phase transitions. The formation of a periodic wrinkling pattern in collapsed films is observed by epifluorescence microscopy after applying several compression-expansion cycles above the surface pressure of ≈18 mN/m. The wrinkle formation is reversible upon compression and expansion of the Langmuir films. Two distinct orientations of POSS molecules are assumed in Langmuir films upon compression, vertically for chains close to the water surface and horizontally orientated upper layers with significant amounts of PEG in between them. Thus, the wrinkling forms mainly in the top stiffer MA-POSS blocks above a certain compressional stress. The wrinkles disappear during the Langmuir-Blodgett (LB) transfer. Nevertheless, atomic force microscopy and grazing incidence wide-angle X-ray scattering experiments reveal the formation of highly ordered POSS molecules in LB films.

Electrografting and Langmuir-Blodgett: Covalently Bound Nanometer-Thick Ordered Films on Graphite.

We present two different molecular organizations obtained from octadecylamine (ODA) molecules on a highly oriented pyrolytic graphite (HOPG) surface: (i) self-organized physisorbed ODA molecules lying flat on the surface and (ii) a strongly electrografted compact crystalline monolayer of ODA molecules standing up on the surface. This new structure is obtained by combining the Langmuir-Blodgett transfer of an ODA Langmuir film onto HOPG with oxidative electrografting. The presence of an organic film on HOPG is characterized by attenuated total reflectance-infrared spectroscopy and Raman spectroscopy, while atomic force microscopy and scanning tunneling microscopy allow the observation of the two molecular organizations with adsorbed molecules lying flat on HOPG or strongly grafted in an upright position on the HOPG surface. Interestingly, the second molecular organization preserves a hexagonal symmetry and its lattice parameters are intermediate between those of ODA Langmuir films and that of the HOPG underlying surface. The functionalization of surfaces with organic films is a major issue in the design of sensors with biomedical applications or organic electronics and energy storage devices and these structures may find applications in these fields.

Atomic Force Microscopy Imaging of Adsorbed Pulmonary Surfactant Films

Pulmonary surfactant (PS) is a lipid-protein complex that adsorbs to the air-water surface of the lung as a thin film. Previous studies have suggested that the adsorbed PS film is composed of an interfacial monolayer, plus a functionally attached vesicular complex, called the surface-associated surfactant reservoir. However, direct visualization of the lateral structure and morphology of adsorbed PS films using atomic force microscopy (AFM) has been proven to be technically challenging. To date, all AFM studies of the PS film have relied on the model of Langmuir monolayers. Here, we showed the first, to our knowledge, AFM imaging of adsorbed PS films under physiologically relevant conditions using a novel, to our knowledge, experimental methodology called constrained drop surfactometry. In conjunction with a series of methodological innovations, including subphase replacement, in situ Langmuir-Blodgett transfer, and real-time surface tension control using closed-loop axisymmetric drop shape analysis, constrained drop surfactometry allowed the study of lateral structure and topography of animal-derived natural PS films at physiologically relevant low surface tensions. Our data suggested that a nucleation-growth model is responsible for the adsorption-induced squeeze-out of the PS film, which likely results in an interfacial monolayer enriched in dipalmitoylphosphatidylcholine with the attached multilayered surface-associated surfactant reservoir. These findings were further supported by frequency-dependent measurements of surface dilational rheology. Our study provides novel, to our knowledge, biophysical insights into the understanding of the mechanisms by which the PS film attains low surface tensions and stabilizes the alveolar surface.

In Situ Spectroelectrochemical Investigation of Potential‐Dependent Changes in an Amphiphilic Imidazolium‐Based Ionic Liquid Film on the Au(111) Electrode Surface

AbstractThe stability of ionic liquids (ILs) at charged interfaces makes them very attractive in electrochemical applications. In this work, a Langmuir–Blodgett transfer is used to fabricate a monolayer of an amphiphilic 1‐methyl‐3‐octadecylimidazolium perchlorate IL on the Au(111) surface. The IL monolayer immersed in an aqueous electrolyte solution is very stable over a wide potential window. Polarization modulation infrared reflection absorption spectroscopy is used to monitor structural changes in the model of the electrical double layer of the IL in aqueous solution. The imidazolium cation is in direct contact with the Au(111) surface. The imidazolium ring adopts a rigid orientation, inclined toward the metal surface, in the monolayer on the Au(111) surface. The orientation of hydrocarbon chains responds to electric potentials. At low negative surface charge densities of the Au(111) electrode, the hydrocarbon chains in the amphiphilic cation have a large average tilt versus surface normal. A negative potential shift is accompanied by a reorientation of the hydrophobic hydrocarbon chains that adopt an up‐ward orientation, making the film permeable for counter‐ and/or co‐ions. This transition leads to the formation of an ordered, single‐component monolayer of the amphiphilic cation on the electrode surface.

Two-dimensional patterns in dip coating - first steps on the continuation path

We present a brief comparative investigation of the bifurcation structure related to the formation of two-dimensional deposition patterns as described by continuum models of Cahn–Hilliard type. These are, on the one hand a driven Cahn–Hilliard model for Langmuir–Blodgett transfer of a surfactant layer from the surface of a bath onto a moving plate and on the other hand a driven thin-film equation modelling the surface acoustic wave-driven coating of a plate by a simple liquid. In both cases, we present selected two-dimensional steady states corresponding to deposition patterns and discuss the main structure of the corresponding bifurcation diagrams.

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity.

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

Molecularly Designed Interfacial Viscoelasticity by Dendronized Polymers: From Flexible Macromolecules to Colloidal Objects.

The thermodynamic and rheological properties of densely packed dendronized polymers (DPs) at water-air interfaces were studied here for first- and fourth-generation DPs (PG1, PG4) with both small (Pn ≈ 50) and large (Pn ≈ 500) backbone degrees of polymerization. The excellent control over the structural characteristics of these polymers enabled us to investigate how the interfacial properties change as we go from thin, flexible macromolecules toward thicker molecular objects that display colloidal features. The effects of the dendron generation, affecting the persistence length, as well as the degree of polymerization and surface pressure on the formation of DP layers at the water-air interface were studied. Surface pressure measurements and interfacial rheology suggest the existence of significant attractive interactions between the molecules of the higher generation DPs. While all DPs featured reproducible Π-A diagrams, successive compression-expansion cycles and surface pressure relaxation experiments revealed differences in the stability of the formed films, which are consistent with the variations in shape persistence and interactions between the studied DPs. Atomic force microscopy after Langmuir-Blodgett transfer of the films displayed a nanostructuring that can be attributed to the increase in attractive forces with increasing polymer generation and anisotropy. The importance of such structures on the surface properties was probed by interfacial shear rheology, which validated the existence of strong albeit brittle structures for fourth-generation DPs. Ultimately, we demonstrate how in particular rod-like DPs can be used as robust foam stabilizers.

Effects of time-periodic forcing in a Cahn-Hilliard model for Langmuir-Blodgett transfer.

The influence of a temporal forcing on the pattern formation in Langmuir-Blodgett transfer is studied employing a generalized Cahn-Hilliard model. The occurring frequency-locking effects allow for controlling the pattern formation process. In the case of one-dimensional (i.e., stripe) patterns one finds various synchronization phenomena such as entrainment between the distance of deposited stripes and the forcing frequency. In two dimensions, the temporal forcing gives rise to the formation of intricate complex patterns such as vertical stripes, oblique stripes, and lattice structures. Remarkably, it is possible to influence the system in the spatial direction perpendicular to the forcing direction leading to synchronization in two spatial dimensions.

Synthesis and electrochemical characterization of 4-thio pseudo-glycolipids as candidate tethers for lipid bilayer models

Two 4-thio pseudo-glycolipids, a disulfide and a thiol, were synthesized as candidates to build tethered bilayer lipid membranes (tBLMs) on gold (111) surface. Monolayers of the disulfide and thiol were built on gold surfaces using the self-assembly and Langmuir-Blodgett (LB) transfer methods. Monolayers were prepared in several solvents and at various temperatures; their quality was assessed by differential capacitance. Best monolayers were achieved when the disulfide was self-assembled in ethanol. The quality of such monolayers was further improved by allowing the assembly to proceed under stirring of the solution at increased temperatures. The charge number per adsorbed molecule and surface concentration of disulfide on gold (111) surface were determined by chronocoulometry. A DPhPC/disulfide tBLM was then built by vesicle fusion of 1,2-diphytanyl-sn-glycero-3-phosphocholine (DPhPC) on top of the disulfide monolayer. The minimum capacitance of the gold electrode with tBLM (1.3 μF cm−2) was close to the value of a real cell membrane and the AFM force spectroscopy measurements showed that the DPhPC/disulfide tBLM had a thickness of 6.2 ± 0.6 nm consistent with the thickness expected for a bilayer. Finally, the potential of the tBLM to study transmembrane proteins was assessed by investigating the reconstitution of gramicidin A into the membrane by polarization modulation infrared absorption spectroscopy (PM-IRRAS). These results demonstrate that the disulfide is a good candidate to construct tBLMs on gold surface and to study transmembrane proteins.

Langmuir monolayers based on rigid wedge-shaped dendrons of benzenesulfonic acid

The structure formation of wedge-shaped monodendrons based on symmetric benzenesulfonic acid with different lengths of peripheral alkyl chains was studied in Langmuir monolayers and Langmuir–Blodgett (LB) films. A phase transition from the liquid-expanded state to the liquid-condensed state was observed on compression of the Langmuir monolayers of the dendrons containing dodecyl lateral chains. The transition is accompanied by the formation of star-shaped aggregates visualized by Brewster angle microscopy. The three-layer LB transfer results in the reorganization of the monolayer into regions of bi-, tetra-, and hexalayers on a solid substrate with a low coverage of the surface. Homogeneous liquid-condensed mono layers are formed for the dendrons with hexa- and octadecyl chains, and the film thickness achieved by the LB transfer corresponds to the monolayer alignment of the molecules with the surface coverage up to 90%. It was determined that varying the alkyl length of wedge-shaped dendrones based on symmetric benzenesulfonic acid leads to a change in phase behavior of Langmuir monolayers as well as Langmuir–Blodgett films formed by them.

Dip-coating with prestructured substrates: transfer of simple liquids and Langmuir–Blodgett monolayers

When a plate is withdrawn from a liquid bath, either a static meniscus forms in the transition region between the bath and the substrate or a liquid film of finite thickness (a Landau–Levich film) is transferred onto the moving substrate. If the substrate is inhomogeneous, e.g. has a prestructure consisting of stripes of different wettabilities, the meniscus can be deformed or show a complex dynamic behavior. Here we study the free surface shape and dynamics of a dragged meniscus occurring for striped prestructures with two orientations, parallel and perpendicular to the transfer direction. A thin film model is employed that accounts for capillarity through a Laplace pressure and for the spatially varying wettability through a Derjaguin (or disjoining) pressure. Numerical continuation is used to obtain steady free surface profiles and corresponding bifurcation diagrams in the case of substrates with different homogeneous wettabilities. Direct numerical simulations are employed in the case of the various striped prestructures.The final part illustrates the importance of our findings for particular applications that involve complex liquids by modeling a Langmuir–Blodgett transfer experiment. There, one transfers a monolayer of an insoluble surfactant that covers the surface of the bath onto the moving substrate. The resulting pattern formation phenomena can be crucially influenced by the hydrodynamics of the liquid meniscus that itself depends on the prestructure on the substrate. In particular, we show how prestructure stripes parallel to the transfer direction lead to the formation of bent stripes in the surfactant coverage after transfer and present similar experimental results.

Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers.

A self-assembled phospholipid monolayer at an air-water interface is a well-defined model system for studying surface thermodynamics, membrane biophysics, thin-film materials, and colloidal soft matter. Here we report a study of two-dimensional phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer at the air-water interface using a newly developed methodology called constrained drop surfactometry (CDS). CDS is superior to the classical Langmuir balance in its capacity for rigorous temperature control and leak-proof environments, thus making it an ideal alternative to the Langmuir balance for studying lipid polymorphism. In addition, we have developed a novel Langmuir-Blodgett (LB) transfer technique that allows the direct transfer of lipid monolayers from the droplet surface under well-controlled conditions. This LB transfer technique permits the direct visualization of phase coexistence in the DPPC monolayer. With these technological advances, we found that the two-dimensional phase behavior of the DPPC monolayer is analogous to the three-dimensional phase transition of a pure substance. This study has implications in the fundamental understanding of surface thermodynamics as well as applications such as self-assembled monolayers and pulmonary surfactant biophysics.

Monolayer kinetic model of formation of gold nanoparticles by reducing agents hexadecylaniline or bovine serum albumin

In the formation of metal nanoparticles (NPs) in aqueous solution two distinguishable processes take place: the nucleation and the NPs’ growth. In the present paper is proposed an experimental approach which aims to clarify whether the interfacial organization of the reduction reaction plays a role during the growth process. The gold nanoparticles were formed in hexadecylaniline (HDA) or bovine serum albumin (BSA) monolayers spread on aqueous chloroauric acid solution. The kinetics of the NPs formation was followed by measuring of the monolayer properties, namely the change of the surface area with time at constant surface pressure,ΔA(t)π, as well as the NPs’ size and shape by AFM after Langmuir-Blodgett (LB) transfer. The proposed simple monolayer system has few advantage for better control of the physicochemical parameters of the reduction reaction which occurs at the air/water interface, e.g. the surface concentration and the interfacial organization of both components- tetrachloroauric acid (HAuCl4) and HDA (or BSA), the degree of unfolding of BSA molecules, the number of reaction sites involved in interactions etc.A kinetic model of the interfacial reaction of NPs’ formation at the air/water interface was developed and the kinetic constants of the reduction reaction (kr), of nuclei formation (k1), and NPs’ growth (k2) were estimated. It was found that the rate constant of the reduction reaction is many order of magnitude higher than the rate constant of NPs’ formation. The formation of critical nuclei and their subsequent growth under the action of the reducing agent HDA were found to be 100 times faster in comparison with the action of BSA as a reductant.