Stearic Acid Langmuir-Blodgett Research Articles

Enzyme activity preservation for galactose oxidase immobilized in stearic acid Langmuir-Blodgett films

The adsorption of the enzyme galactose oxidase onto stearic acid Langmuir monolayers was investigated with surface pressure-area isotherms, Brewster Angle Microscopy and polarization-modulated infrared reflection-absorption spectroscopy. The enzyme was incorporated in the monolayer at low surface pressures, expanding the lipid films, and further monolayer compression expelled it from the contact with the air phase and repositioned the enzyme below the lipid polar heads. The enzyme-lipid system was then stabilized by electrostatic interactions, leading the monolayer to condense at higher surface pressures, reducing the number of interfacial aggregates. The lipid-enzyme interactions at the interface made the monolayer less viscoelastic and more disordered as pointed by the reduced compressional modulus and decreased trans/gauche ratio for the lipid acyl chains. Also, the secondary structures of the adsorbed enzyme were kept as demonstrated with infrared spectroscopy. The condensed enzyme-lipid monolayer was transferred to solid supports as Langmuir-Blodgett films and the enzyme activity was detected, being preserved for one month. These results show how the molecular organization provided by the ultrathin films can help maintain the catalytic activity for immobilized enzymes, which is important, general speaking, for nanotechnology and biosensing manufacturing.

Formation and UV-Vis Studies of Lead Sulfide Nanoparticles

An elegant technique of synthesizing semiconductor lead sulfide nanoparticles (PbS-NPS) embedded in Langmuir-Blodgett (LB) films is presented here. Stearic acids have been used as a matrix to form and grow the quantum-sized PbS. Pure Y-type Pb-salts of stearic acid (SA) LB films were prepared at a dipping speed of 15 mm/min and surface pressure of 28 mN/m. PbS-NPs were formed inside LB films of SA by exposure the LB films to hydrogen sulfide (H2S) gas. The preparation of PbS-NPs is discussed in terms of particle formation and growth. The presence of PbS-NPs was investigated by UV-visible absorption. It was found that the PbS-NPs within SA LB films indicated a large blue shift of the optical absorption edge. The estimation of the size distribution performed by optical measurements allows one to attribute the particle range a few nanometers. This figure seems to depend upon the reaction conditions as well as the quality of the initial LB films.

Preparation of patterned biological film for nanolithography

I is a great challenge to fabricate the biological films with patterned structures or ultra-thin thickness for nanolithography in the field of bioelectronics. Many techniques such as optical lithography, electron beam lithography, dip-pen and nanoimprint lithography have been developed and applied to prepare the bio-films. However, most organic molecules are so fragile that the fabrication process of their films requires physiologic and gentle conditions. Here, in our lab we introduce new bio-film possessing methods and the prepared films are of well-patterned structures with ultra-thin thickness. (I) Two-dimension (2D) Xanthan scaffold layers with nanofibril structures were build up through annealing the xanthan solution. In this experiment, temperature plays a critical role to affect the scaffold formation during the annealing process. (II) Porous stearic acid (SA) monolayer films were prepared by means of etching the SA Langmuir-Blodgett (LB) film in salt solution. The pores size and coverage area can be easily adjusted by controlling the salt concentration and immersion duration time. Moreover, the cationic valence in solution and the surface pressure of SA LB film will affect the etching result as well. (III) Uniform ultra-thin chitosan films were fabricated using mechanical manipulating by atomic force microscopy (AFM). The quality of the film is determined by the diameter of chitosan nanoparticles spin-coated on the substrate, the viscoelastic property of the chitosan particles, humidity and temperature of the surrounding enviroment. Adopting these three novel and simple methods to fabricate the bio-films will path a promising way to the nanolithography, biosensors, bioelectronics etc.

Influence of Cd 2+ ions on electrical properties of stearic acid Langmuir–Blodgett films

Stearic acid incorporating cadmium ions prepared as an Langmuir–Blodgett (LB) thin films were grown onto aluminum coated glass substrate using different thicknesses to investigate the influence of cadmium ions on the electrical properties of the stearic acid. Electrical properties were investigated by measuring C– V and I– V curves at room temperature. The I– V curves show highly non-linear behavior with Schottky conduction mechanism in a high voltage regime for LB film samples. Barrier potential heights were calculated for the stearic acid LB films with and without cadmium ions. It was found that the Schottky barrier is lowered due to the presence of Cd 2+ ions.

Efficient fluorescence quenching near crystalline silicon from Langmuir–Blodgett dye films

The distance dependence of the fluorescence efficiency from Langmuir Blodgett (LB) dye layers deposited on top of silicon substrates was studied for different silicon crystal orientations (<100> and <111>). The distance to the silicon surface was varied with stearic acid LB layers (SA). Spectroscopic Ellipsometry (SE) provided accurate measurements of the thickness of the separation steps and the refractive index of the dye layer. It was found that the fluorescence efficiency of the overlying LB dye layers was quenched significantly by the presence of the semiconductor at close distances to the surface. No significant difference in the fluorescence quenching for the LB dye between the two crystal orientations was observed. The results obtained are compared with previous work from fluorescence time and intensity measurements. The importance of interference effects is stressed at high semiconductor-dye distances but the fluorescence quenching observed at small distances is due to efficient energy transfer to the semiconductor.

Preparation and Properties of Conducting Polymeric Ultrathin Films

A multilayer ultrathin film of poly(3,4-ethylene dioxythiophene) (PEDOT) was first prepared by a modified Langmuir- Blodgett (LB) technique. The polymerization of PEDOT monomer occurred in a multilayer as-prepared LB film by exposing the film to EDOT vapor. The formation of PEDOT particles in the LB film was confirmed by UV-Vis-near IR (UV-Vis-NIR) absorption spectrum and X-ray photoelectron spectroscopy (XPS). By using scanning electron microscopy (SEM), investigation on the film surface exhibited small PEDOT particles distributed in stearic acid (SA) LB films. Secondary ion mass spectrometry (SIMS) was employed to analyze the location of PEDOT formed and the results showed that PEDOT was well confirmed in the multilayer structure. A 60-layer PEDOT LB film had higher conductivity (2.6 S·cm −1) than a conventional PEDOT film, and exhibited excellent doping/dedoping characteristics. An investigation on the relationship between film conductivity and exposure time on EDOT ambience revealed that the film obtained steady conductivity after 120 min and it was concluded that lower oxidizer density in LB film might slow the polymerization speed of EDOT. In a study of sensitive characteristics, the PEDOT film showed faster and nonlinear responses to analyte gas in a lower concentration (ϕ<30×10 −6), and exhibited a linear response to analyte in a higher concentration (ϕ=(30–120)×10 −6). The result of gas sensitivity also showed that the film had excellent reversible and reproducible response to HCl vapor. The mechanism of conductivity and gas sensitivity of composite LB films was also included.

Tribological Properties of LB films Measured under Zero Applied Load

We examined the pull-off and friction forces between an atomic force microscope (AFM) probe and submicron-scale asperities covered with Langmuir Blodgett (LB) films. First, we used a focused ion beam to produce several kinds of asperity arrays on two silicon substrates. The average radius of curvature was 95 to 800 nm, depending on the asperity array. An LB film of stearic acid (C17H35COOH) or fluoro-carboxylic acid (C6F13C11H22COOH) was deposited on each substrate. The friction force was measured at each asperity array under a zero applied load. The pull-off force was measured at each asperity array and also at controlled relative humidities of 3 to 73%. The results showed that the pull-off and friction forces were almost proportional to the radius of the curvature of the asperity peak with both stearic acid and fluoro-carboxylic acid LB films. The gradient of the friction force against the pull-off force, which corresponded to the friction coefficient, was 0.021 and 0.14 for the stearic acid and fluoro-carboxylic acid respectively. The pull-off force increased with higher relative humidity on the fluoro-carboxylic acid. We concluded that differences in the pull-off and friction forces between the two kinds of LB films were mainly caused by differences in the stiffness of the LB film.

Formation and Electrooptic Characteristics of M-ttbPc LB Films

Ultrathin films of tetra-tert-butyl-phthalocyaninato-iron (FettbPc) mixed with promoting stearic acid (SA) were deposited on clean and Al-coated glass substrates by the Langmuir-Blodgett (LB) method. The LB films were investigated by X-ray diffraction (XRD) analysis, UV-visible spectroscopy and FT-IR to determine their structures and to analyze the optical and electrical properties. The photovoltaic efficiencies of solar cells composed of Al/FettbPc:SA LB film/Au and Al/CuttbPc:SA LB film/Au were 1.65×10-5% and 3.20×10-6%, respectively. The junctions at Al/Pc seem to be disrupted by interfacial layers of Al2O3. The magnitudes of the efficiencies were smaller than those of Al/In/M-Pc/Au cells, fabricated by an evaporation method, by a factor of 2 or 3.

Particulate assemblies of CdS and TiO 2 prepared by Langmuir–Blodgett technique with octadecylamine/methylstearate mixed films

Langmuir–Blodgett (LB) technique has been applied to fabrications of various nanoparticulate assemblies such as monolayer and multilayer of CdS particles, in-plane mixed monolayer and alternate multilayer of CdS/TiO 2 particles on the solid substrate. We examined the most favorable conditions for yielding uniform and dense packing of the particles in a 2-dimensional arrangement, followed by structural characterization of the as-deposited particulate layers. As a result, it was found for the CdS particulate films that charge density of cationic Langmuir monolayer markedly influences the amount of particles embedded on the substrate and that multilayer LB deposition enables stepwise control of thickness for the particulate assemblies. For the composite CdS/TiO 2 particulate films, desired in-plane mixing ratio and alternating layer structure of two kinds of particles were achieved, on which basis the possibilities of fabricating a variety of heteroparticulate layers are pointed out. In addition, photocatalytic activities of TiO 2 and CdS/TiO 2 particulate films fabricated by the LB technique were evaluated in terms of decomposition of both oleic acid cast film and stearic acid (SA) LB film by irradiating with UV light. The results exhibited that when the bilayer of TiO 2 particulate film is employed in place of the monolayer film, the photodecomposition rate of SA is sufficiently accelerated and that the alternate CdS/TiO 2 particulate layer is less active for the oxidative decomposition.

Spectroscopic study of non-amphiphilic 9-phenyl fluorene assembled in Langmuir–Blodgett films in two different matrices: dimer and excimer formation

Multilayered Langmuir–Blodgett (LB) films at different mole fractions of 9-phenyl fluorene (9PF) have been prepared using two different matrices viz. stearic acid (SA) and polymethyl methacrylate (PMMA). The spectroscopic properties of the mixed LB films, solution and microcrystal have been compared using absorption, steady-state fluorescence, and excitation spectroscopy. The spectroscopic studies give evidence of formation of excimeric sites in 9PF–SA LB films, while formation of dimeric species along with the excimeric sites have been evidenced in case of 9PF–PMMA LB films.

Microstructure and Ion Exchange in Stearic Acid Langmuir–Blodgett Films Studied by Fourier Transform Infrared-Attenuated Total Reflection Spectroscopy

Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectra have been recorded of 11-layer Langmuir–Blodgett (LB) films of stearic acid deposited at various surface pressures (0.1, 15, and 35 mN/m), and the molecular orientation angles were evaluated quantitatively, which supplied insight into the molecular order with the alkyl chains tightly packed like crystal in the LB films deposited at the zero and higher surface pressures. These experimental results indicate that, in the Langmuir film as the precursor of LB films, stearic acid molecules self-aggregate to form two-dimensional crystalline domains already even at the zero surface pressure, which results in the inhomogeneity of monolayer. The analysis of dependence of νC=O intensity on the surface pressure, surface density, and subphase temperature leads to the conclusion that the defects in LB films originate from the Langmuir film and be conserved upon deposition. Annealing below 50°C and cooling could improve the monolayer homogeneity, and thus a defect-free or low-defect LB films can be deposited. Furthermore, ion exchange conducted in the LB films, on the other hand, confirms the existence of structure defects in LB films of stearic acid. The polar plane microstructure, lateral transport along the polar planes and the coordination types of stearic acid/cation system may be the rate-limiting process. The results have implication on the possible uses of stearic acid LB films as ion-exchange materials or sensors.

Preparation and structure of quantum-sized cadmium sulfide grown in amphiphilic oligomer Langmuir–Blodgett films

An amphiphilic oligomer, polymaleic acid with octadecanol ester (PMAO), with ratios of 2:1, 3.7:1 and 6:1 between the carboxylic groups and the hydrocarbon chains, was synthesized and has been used as a matrix to form and grow quantum dot-sized CdS particles. The formation of quantum-sized CdS particles during exposure of Langmuir–Blodgett (LB) films of CdPMAO to hydrogen sulfide has been studied using Fourier transform infrared spectroscopy and UV-vis absorption spectra. This showed that hydrogen bonding of the PMAO molecules were replaced by the metal ion (Cd 2+) bridges to form the bridging complex structures before the reaction with H 2S. The intensities of the asymmetric ν a(COO −) and symmetric ν s(COO −) stretching modes were reduced after the reaction. But the presence of the asymmetric ν a(COO −) and symmetric ν s(COO −) stretching mode in spectra implied that not all the CdPMAO oligomers in the film were converted to PMAO after exposure to H 2S gas. The result from the UV-vis absorption spectra indicated that the CdS within PMAO LB films showed a larger blue shift of the optical absorption edge with the decrease of the ratio between the carboxylic groups and the hydrocarbon chains. Moreover, it could be deduced that the size of the aggregate of the sulfides within PMAO LB films was smaller than that formed in stearic acid LB films.

Resonance mechanisms in electron energy loss spectra of stearic acid Langmuir-Blodgett monolayers revealed by isotopic effects

In theoretical study, a distinction is made between the direct and the resonant collisions which govern electron energy loss spectroscopy of vibrational excitations in molecular media. Direct scattering takes place via long-range Coulomb and short-range vibronic interactions; the corresponding cross sections depend upon the reduced mass of the vibrational mode as M−12. On the other hand, the resonant collisions, involving the formation of a transient negative ion, have cross sections that scale as M14. Notable isotope effects have been found for the CH and CD stretching vibrations in organic compounds and were investigated by considering the high resolution electron energy loss spectra obtained with Langmuir-Blodgett films of mixtures of hydrogenated and deuterated stearic acid. From the ratio of the excitation cross sections for the CD and CH stretching vibrations, measured as a function of incident electron energies up to 10 eV, the relative importance of the direct and resonance scattering processes was estimated. For incident energies below a critical value between 4 and 5 eV, only direct scattering is operative; at the critical value, a resonance mechanism sets in, which rapidly contributes at the higher energies, at least equally with the direct collisions. Arguments are given indicating that the lowest-energy resonance has the lowest σ∗ (HCH) configuration.

Preparation and Characterization of Quantum-Sized PbS Grown in Amphiphilic Oligomer Langmuir−Blodgett Monolayers

Three kinds of amphiphilic oligomers, poly(maleic acid) with octadecanol ester (PMAO) with 2.0:1, 3.7:1, and 6.0:1 between the carboxylic groups and the hydrocarbon chains, were synthesized and have been used as a matrix to form and grown the quantum-sized PbS. Pure Y-type PbPMAO Langmuir−Blodgett (LB) films were prepared at a dipping speed of 4.0 cm/min and surface pressure of 18 mN/m. The results from Fourier tranform IR spectroscopy and X-ray diffraction measurements showed that an ordered structure was obtained from the PbPMAO LB films. The quantum-sized PbS particles were prepared by exposure of PbPMAO LB films to H2S and nanoparticulate PbS monolayers were formed without destroying the layered structure. The result from structured UV−vis absorption spectra shows that the PbS particles within the LB matrix are relatively monodispersed. It was found that the PbS within PMAO LB films indicated a larger blue shift of the optical absorption edge with the decrease of the ratio between the carboxylic groups and the hydrocarbon chains. Meanwhile, it can be deduced that the size of the aggregate of the sulfides within PMAO LB films is smaller than that formed in stearic acid LB films. The observed absorption peaks showed similarity with the monomolecular PbS.

Ag clusters electrochemically reduced in stearic acid Langmuir-Blodgett films and their structural characterizations

Silver ions confined in the silver stearate Langmuir-Blodgett (L-B) films of 8–14 layers were electrochemically reduced into Ag clusters, which made the observation of well-ordered scanning tunneling microscope (STM) images of the films with molecular resolution. A hexagonal packing of the hydrophobic chains was obtained. which was superimposed on the two-dimensional Ag clusters of 2–3 nm in diameter. Furthermore, a (2×1) structure of the hexagonal packing was observed. It was attributed to a self-assembled structure of the hydrophilic surface of the L-B films through hydrogen bonding. The strong surface enhanced Raman scattering (SERS) effect of Ag clusters made it possible to record Raman spectra of twoiayer stearic acid L-B films in the region of C—C stretching vibration frequencies. The appearance of the bands at 1 128.0 and 1 100. 7 cm−1, which correspond to the antisymmetric C—C stretching model of the acyl chains in theall-trans conformation and the C—C stretching model of the acyl chains in thegauche conformations, respectively, means that the methylene chains are mainly inall-trans conformation, but some of them become disordered.

Controlled growth of the size-quantized CdS at the interface of the stearic acid Langmuir–Blodgett films

A pure Y-type transfer form for cadmium stearate (CdSt 2) Langmuir–Blodgett (LB) films was obtained on a subphase of which the pH is suitable (6.4) as a result of using a higher surface pressure. The pressure dependence of the structure of the CdS produced within the LB matrix was observed. Monolayers of CdS were prepared by the exposure of CdSt 2 LB films deposited under higher surface pressure (37.5 mN/m) to H 2S gas at a pressure of 0.25 Torr. If the LB films deposited under lower surface pressure were used as the matrix, CdS would aggregate to the particles within the stearic acid (SA) LB films. And these phenomena suggest a novel means to control the microstructure of aggregates formed. We suggested that it should be ascribed to the larger interaction between the hydrocarbon chains of the LB films in the case of the higher surface pressure. This larger interaction can offer a sufficient position control for the guest sulphide monolayer. At the same time, the reaction speed of the LB films with the H 2S gas, as well as the form speed of CdS, was dependent on the layer number of the LB films and the surface pressure under which the LB films were deposited.

A Study on Inorganic Nanoparticles/Amphiphilic Oligomer in Nanoordered Multilayer

An amphiphilic oligomer, polymaleic acid with octadecanol monoester (PMAO), and its cadmium, lead and copper salts Langmuir-Blodgett (LB) films were prepared. These films possess an ordered structure as determined by X-ray diffraction measurements and FTIR spectroscopy. The long spacing of the films is 5.7 nm. After reaction with H2S, the long spacing increased to 5.8 nm, but an ordered structure was still kept. The nanoparticulate PbS, CdS and CuS multilayers were obtained. From the UV-vis spectrum, it was found that the nanoparticles within PMAO LB films show a larger blue shift of the optical absorption edge than that formed within the stearic acid LB film.

Fabrication and properties of carboran clusters contained in stearic acid LB films and possible applications for single electron electronics

Correlated discrete transfer of single electrons in ultrasmall tunnel junctions is of great interest for applications in molecular electronics. We report single-electron tunnelling at 300 K in the double tunnel junction system “graphite substrate-carboran cluster-STM tip”. The problem of reliably fixing clusters on the graphite surface has been solved by means of cluster incorporation into a stearic acid LB (Langmuir-Blodgett) monolayer matrix. We obtained compression isotherms of mixed monolayers with varying molar ratio [stearic acid]:[clusters]. STM images of deposited mixed films revealed that the clusters formed a regular structure.

STM observations of a self-assembled structure of stearic acid Langmuir-Blodgett films with molecular resolution

Silver ions confined in the silver stearate Langmuir-Blodgett (LB) films of 8–14 layers were electrochemically reduced, which enabled the observation of well-ordered scanning tunneling microscopy images of the films with molecular resolution. A hexagonal packing of the hydrophobic chains was obtained, which was superimposed on the two-dimensional Ag clusters of 2–3 nm diameter. Furthermore, a (2× 1) structure of the hexagonal packing was observed. It was attributed to a self-assembled structure of the hydrophilic surface of the LB films through hydrogen bonding.

Mechanism of Iron Inhibition by Stearic Acid Langmuir-Blodgett Monolayers

Abstract Using the Langmuir-Blodgett (LB) technique, stearic acid (SA) monolayers were deposited onto the surface of an iron (Fe) electrode to study the inhibition effect and the mechanism of SA in...