Gas transfer in supported Langmuir-Blodgett films of polymeric lipids

Asymmetric membranes were fabricated by depositing Langmuir-Blodgett (LB) films of polymeric lipids on porous supports. Three polymeric lipids were used for deposition. Up to 50 Y-type layers were deposited on one side of the porous supports of polypropylene (Celgard) and polytetrafluoroethylene (GORE-TEX) membranes. A marked decrease in gas transfer with increasing number of LB layers was observed for the asymmetric membranes fabricated with the porous polypropylene. Gas transfer in the asymmetric membranes using porous polytetrafluoroethylene remained unchanged with the number of polymeric layers. The LB multilayers on polypropylene showed no evidence for pores or cracks as determined by scanning electron microscopy (at a magnification of 40 000x), but the LB multilayers on polytetrafluoroethylene revealed large cracks. The gas permeabilities of nitrogen, methane and carbon dioxide in the polypropylene-based asymmetric membranes were a function of the molecular weight of the gas for two of the three polymers which suggests that micropores are present in the LB films. Heat annealing decreased the gas permeabilities from 0% to 40%, but only one polymer showed a change in selectivity.     

Magnetic properties of ferrocenylmethylacrylate-N-dodecylacryl-amide copolymer Langmuir–Blodgett films

Langmuir–Blodgett (LB) films of the copolymer with ferrocenyl-methyl-acrylate (FcMA) and N-dodecyl-acrylamide (DDA) were fabricated. The magnetic moment of these LB films became about twenty times larger than that of the powder. In addition, the LB films had a magnetic anisotropy depending on the dipping direction. The FTIR results suggested that the polymer chains in the LB films were preferably aligned with the perpendicular of the dipping direction. These results were discussed by the intermolecular electron transfer.     

Langmuir–Blodgett multilayers of imidazoles

Langmuir–Blodgett (LB) multilayers have been formed from a wide variety of compounds but the basic studies from which others have developed have involved a long hydrocarbon chain terminated at one end by a hydrophilic group. The quaternary ammonium ion has a greater attraction for water than do the other groups which have been used and is so hydrophilic as to preclude the formation of LB layers of straight chain molecules terminated by this group. Certainly all such molecules up to and including octadecyl trimethyl ammonium bromide form micelles in preference to monolayers at the air/water interface though metastable monolayers of this compound have been reported. Quaternized imidazoles have, like quaternary ammonium ions, a positive charge but seem more likely to form LB films. We have thus explored the properties of several of these compounds with a view to producing one which has the desired properties. In order to combat the highly hydrophilic nature of the imidazole group it is necessary to employ a highly hydrophobic ‘tail group’. Thus our most successful compound contains two octadecyl alkyl chains. We have studied the behaviour of this compound at the air/water interface and have found that the nature of the subphase has a very marked influence on the shape of the isotherms. With nitric acid added to the subphase to bring about a pH of 3.5 it is possible to obtain good Y deposition with deposition ratios equal to one in both the upward and downward directions. However we have not been able to achieve LB deposition using any other acid. On heating, the compound in question it changes from a crystalline structure to a smectic structure at 55°C.     

Preparation and characterization of new Ca–Al–polycarboxylate layered double hydroxides

The synthesis and characterization of composite materials based on calcium–aluminum layered double hydroxides (Ca–Al-LDHs), intercalated with organic polycarboxylate (PC) polymers, is described. The PC polymers, which exhibit a “comb-like” architecture, contain ethylene oxide side chains of different lengths. They are intercalated between the cationic Ca–Al–OH-layers when tricalcium aluminate is rehydrated in the presence of PC polymers. The intercalation compounds are characterized by X-ray diffraction (XRD), elemental analysis, IR spectroscopy and TEM. The basal spacings of the PC-LDH composite materials, determined by XRD, are dependent on the number of the ethylene oxide units (EOUs) in the side chain. Accordingly, first TEM micrographs reveal approximately similar layer distances. Elemental analysis as well as IR-spectroscopy proves the existence of the intercalated organic material.     

Electrical manifestation of dielectric non-centrosymmetry using Langmuir-Blodgett multilayers of ω-tricosenoic acid

Centrosymmetric (Y-type) and non-centrosymmetric (Z-type) Langmuir-Blodgett (LB) multilayers of ω-tricosenoic acid have been deposited as part of an M¦LB¦M structure and the electrical characteristics measured. The symmetry of the LB film was manifested in the electrical properties of the junction, with the behaviour of centrosymmetric LB films being independent of the applied bias sense and non-centrosymmetric LB films possessing a distinct bias sense dependence.     

Charge incorporation in ω-tricosenoic acid Langmuir-Blodgett multilayers

In order to investigate the amount of charge introduced into Langmuir- Blodgett (LB) films, multilayers of ω-tricosenoic acid have been deposited onto a number of silicon substrates under different dipping conditions. The effects of moisture and storage conditions on film properties have been examined, and a new mathematical model for the charge distribution within an LB film multilayer will be proposed.     

Structural characterization of Langmuir–Blodgett films by X-ray diffraction in transmission geometry

In this paper, we present a method which employs X-ray diffraction in transmission geometry for structural determination of Langmuir–Blodgett (LB) films. The LB multilayers are deposited on the outer surface of a thin glass capillary and diffraction pattern was recorded using a pin-hole flat camera. The remarkable advantage of this method is that both the multilayer reflections and in-plane reflections are recorded in one diffraction pattern which provides the following information about the film: layer thickness, electron density profile normal to the layers, in-plane packing of molecules and tilt angle of molecular chains with respect to the plane. As an example, we have determined the lattice parameters and the electron density profile of a lead stearate LB film according to one diffraction pattern where 15 orders of layer reflections and three in-plane reflections were recorded.     

Optical constants of 22-tricosenoic acid multilayers

Multilayers of the organic material 22-tricosenoic acid were made using the Langmuir-Blodgett (LB) technique. Optical constants of these multilayers were studied by looking at light reflected from them in an attenuated total reflection geometry. Such a geometry enabled incident light to be coupled to guided modes in the LB layer, resulting in a series of minima in the reflected intensity. Reflected light for both transverse electric and transverse magnetic incident polarizations was analysed, giving information on the optical anisotropy of the multilayers. Experiment was compared with theory to determine the dielectric constants and film thickness of the multilayers.     

Displacement Current of Nanodendrimer

In the Langmuir-Blodgett (LB) technique, a monolayer on the water surface is transferred onto a substrate, which is raised and dipped through the surface. From this, multilayers can be obtained in which constituent molecules are periodically arranged. The LB technique has attracted considerable interest in the fabrication of electrical and electronic devices. Many researchers have investigated the electrical properties of monolayer and multiplayer films. Dendrimers represent a new class of synthetic macromolecules characterized by a regularly branched tree-like structure. Multiple branching yields a large number of chain ends that distinguish dendrimers from conventional star-like polymers and microgels. The azobenzene dendrimer is one of the dendritic macromolecules that include the azo-group exhibiting a photochromic character. Due to the presence of the charge transfer element of the azo-group and its rod-shaped structure, these compounds are expected to have potential interest in electronics and photoelectronics, especially in nonlinear optics. In the present paper, we give pressure stimulation to organic thin films and detect the induced displacement current.     

Langmuir–Blodgett membranes for separation and sensing

The current status of Langmuir–Blodgett (LB) films is described with regard to biomimetic applications as separating membranes and sensor layers. In the first part, recent research activities on the utility of LB membranes for gas separation, liquid permeation, hyper- and ultrafiltration are reviewed. The morphological aspects of materials separation in LB films are discussed. In the second part, recent progress in the rapidly growing field of LB-film-based chemical sensors and blosensors is reviewed. The structure and function of gas and ion sensors, and enzyme and immunosensors is briefly discussed. Perspectives for future activities in the fields of LB membranes and sensors are outlined.     

Depth resolution during C60+ profiling of multilayer molecular films

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.     

Langmuir–Blodgett films of substituted bis(naphthalocyaninato) rare earth complexes: structure and interaction with nitrogen dioxide

Ordered molecular assemblies of novel alkylthio-substituted bis(naphthalocyaninato) rare earth complexes M[2,3-Nc(SC₁₂H₂₅)₈]₂ (M=Gd, Er) have been fabricated by Langmuir–Blodgett (LB) technique and characterized by surface pressure-area isotherm measurements, electronic absorption and polarized electronic absorption spectroscopy and low-angle X-ray diffractometry. The interaction between the thin films and NO₂ gas was investigated. The experimental results indicate that the bis(naphthalocyaninato) rare earth molecules form well-ordered stable monolayers with face-to-face orientation for the macrocycles and edge-on configuration for the molecules at the air–water interface and lamellar multilayers on solid substrates. The orientation angles of naphthalocyanine rings with respect to the quartz plate surface in the LB films for M[2,3-Nc(SC₁₂H₂₅)₈]₂ with M of Gd and Er have been determined based on the polarized absorbance and found to be 50° and 54°, respectively. The adsorption–desorption process of NO₂ on LB films of M[2,3-Nc(SC₁₂H₂₅)₈]₂ has been monitored by electronic absorption spectroscopy. A process of gas desorption was found to obey the Elovich equation.     

Hybrid Organic–Inorganic Materials—In Search of Synergic Activity

This review surveys the work developed in the field of functional hybrid materials, especially those containing conducting organic polymers (COPs), in combination with a variety of inorganic species, from molecular to extended phases, including clusters and nano‐sized inorganic particles. Depending on the dominating structural matrix, we distinguish and analyze organic–inorganic (OI) hybrids, nanocomposite materials, and inorganic–organic (IO) phases. These materials have been used in a wide variety of applications, including energy‐storage applications, electrocatalysis, the harnessing of electrochromic and photoelectrochromic properties, application in display devices, photovoltaics, and novel energy‐conversion systems, proton‐pump electrodes, sensors, or chemiresistive detectors, which work as artificial “noses”.     

Thermal behaviour of polymeric Langmuir-Blodgett multilayers

Langmuir-Blodgett multilayers of hydrocarbon and fluorocarbon polymers with hydrophilic spacer, lipid-polyelectrolyte complexes and mesogenic polymers have been prepared. The thermal behaviour of the multilayers was studied by small angle X-ray scattering, IR and UV—visible spectroscopy. Good thermal stabilities were found for the various classes of polymers. In addition, for both complexed multilayers and mesogenic polymer films, reorientation processes were observed.     

Amperometric cholesterol biosensors based on hybrid organic–inorganic Langmuir–Blodgett films

Preparation and characterization of a thin-film cholesterol biosensor employing an organic–inorganic hybrid system of cholesterol oxidase (ChOx) and Prussian blue (PB) is described. ChOx was immobilized in Langmuir–Blodgett (LB) films consisting of positively charged octadecyltrimethylammonium (ODTA) and nano-sized PB clusters. Immobilization was performed by simple immersion of ODTA/PB LB films into an aqueous solution of ChOx. Subsequent ChOx absorption into LB films was confirmed by infrared reflection–absorption spectroscopy. Obtained ODTA/PB/ChOx LB films clearly exhibited a response current to cholesterol under an applied potential of 0.0 V (vs. Ag/AgCl). The linearity of current density versus cholesterol concentration was confirmed for the range 0.2–1.2 mmol/L. The present study indicates that simple immersion of ODTA/PB LB films into an enzyme solution is a promising method to produce many types of thin-film biosensors comprising a hybrid system of an oxidative enzyme and PB nano-clusters that work at a very low potential range.     

New side-chain polyurethanes with highly conjugated push–pull chromophores for second order NLO applications

Six new side-chain polyurethanes with large molecular hyperpolarizabilities were synthesized by reaction of ethylenic and/or azo bridged push–pull chromophoric monomers with tolylene 2,4-diisocyanate. The NLO-phores show different conjugation pathway but analogous donor/withdrawing substituents. The chemical and thermal properties of the chromophores and of the polymers were examined. Evaluation of the second order NLO properties was performed by EFISH determination of μβ on the monomers, and by SHG measurements on some amorphous thin films obtained from polymers through spin-coating technique.     

Effect of subphase divalent ions on optical second harmonic properties of hemicyanine Langmuir-Blodgett films interleaved with compatible spacer

The surface pressure-area (π-A) isotherms of 1.10-bistearyl-4.6.13.15-tetraene-8-nitrogen-crown-6 (NC) monolayers at the air-water interface, the UV-visible absorption spectra and optical nonlinear properties of Y-type hemicyanine/NC interleaving Langmuir-Blodgett (LB) multilayers containing divalent ions in the subphase (such as Cd²⁺, Ba²⁺ and Co²⁺) were investigated in the paper. It was found that the π-A isotherms of the NC monolayer, the peak position of absorption spectra and the optical second harmonic intensity in the alternating LB multilayers strongly depend on the divalent ions in the subphase. The 2ω resonance effect of molecular hyperpolarizability β was responsible for the change of the observed second order polarizability χ⁽²⁾ of LB multilayers.     

Stability and characterization of phospholipid Langmuir-Blodgett films

The regularity of the bilayer structure was compared between dipalmitoylphosphatidic acid (DPPA) Langmuir-Blodgett (LB) films and “oriented multilayers” of DPPA, using X-ray diffraction and electron spectroscopy for chemical analysis (ESCA). The angular variation of the (001) Bragg spot due to the irregularity of the lamellar repeat was remarkable in the system of oriented multilayers. In contrast, the spot of DPPA LB films exhibited only a slight angular variation, suggesting a regular structure of this film. The take-off angle dependence of the phosphorus:carbon ratio obtained by ESCA indicates that the regular layered structure in DPPA LB film is maintained after 49 layers are deposited.     

An air-stable biomimetic Langmuir–Blodgett bilayer

The long-chain amphiphiles in biomembranes have their hydrophilic headgroups oriented outwards towards the aqueous environment, and their long aliphatic chains innermost. In Langmuir–Blodgett (LB) films the hydrophobic chains are normally outermost. LB films of long-chain derivatives can be prepared under water with the headgroups outermost and are stable while kept there, but the act of removing them from the aqueous environment removes the top monolayer. We demonstrate an adaptation of the LB technique which is appropriate for the preparation of biocompatible surfaces. It produces essentially bilayer films of long-chain derivatives on standard substrates with a hydrophilic outer surface. The films are stable in air. The films show large contact angle hysteresis with water, but the receding angle is in most cases less than 10°. The films can be immersed and withdrawn from water many times before wettability or average thickness change significantly. Compared to previously reported air-stable surfaces with a structure mimicking biological membranes, the substrate for the present surface is experimentally more convenient.     

Soft Langmuir–Blodgett Technique for Hard Nanomaterials

Materials and their assemblies of dimensions down to a few nanometers have attracted considerable scientific interest in physical, chemical, and biological sciences because of unique properties not available in their bulk counterparts. The Langmuir–Blodgett (LB) technique allows rigid nanomaterials to be aligned in particular structures through a flexible assembly process at liquid interfaces. In this review, we summarize the development of assembly of hard nanomaterials using soft LB techniques. An initial summary of the basic features of nanomaterials will include dimension‐related effects, synthesis, characterization, and analysis, and will be followed by examples of LB assemblies of nanomaterials described according to their morphology: nanoparticles, nanorods, nanowires, nanotubes, and nanosheets. Some of the nanomaterials have been fabricated in orientation‐controlled morphologies, and have been incorporated into prototype devices for gas sensing and photocurrent transport. In the final part of this review, the challenges remaining for LB techniques of hard nanomaterials will be overviewed, and will include a comparison with the widely‐used LB technique involving soft materials.