Stearic Acid Langmuir-Blodgett Films Research Articles

Silver nano-colloid particles embedded on Langmuir-Blodgett film matrix of stearic acid serving as a SERS active sensor for detecting the herbicide ‘Paraquat’

The herbicide Paraquat, widely used for efficient weed control, poses significant health risks to humans viz., severe toxicity to vital organs and induction of neurodegenerative disorder like Parkinson’s disease, underscoring the urgent need for developing sensitive detection methods for the herbicide. This study aims at fabricating a novel SERS-active substrate SA-LB/Ag (silver nano-colloids adsorbed on Langmuir-Blodgett film of stearic acid), as a SERS based sensor having high sensitivity, uniformity, and reproducibility to detect ultra-trace amounts of paraquat. The sensor was built up by immersing bilayer LB films of SA in silver nano-colloid suspension, prepared by mixing silver nitrate and sodium borohydride with vigorous stirring. Quantitative evaluations establish a significant linear regression across the concentration spectrum (100 ppm to 75 ppb) of paraquat, with a remarkable 15 ppb limit of detection (LOD) value. In practical terms, this SERS based sensor exhibits notable proficiency in detecting paraquat residues in (a) agricultural water from rice and vegetable fields and environmental water from adjoining water-logged areas, and (b) soil extracts of the agricultural fields. Moreover, the minimum toxic concentration of paraquat at the developmental stage of zebrafish embryos and larvae is found to be approximately 20 ppb, implying that the SA-LB/Ag sensor is sensitive enough to determine the limiting toxic level of paraquat for animal systems. Thus, the designed sensor holds far-reaching implications in ensuring food safety, monitoring the environment, and framing regulatory measures for paraquat use.

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.

Insertion of carbon nanotubes in Langmuir-Blodgett films of stearic acid and asparaginase enhancing the catalytic performance

In this paper, carbon nanotubes (CNT) were adsorbed on stearic acid (SA) Langmuir monolayers to serve as matrices for the incorporation of asparaginase. The interaction between the components at the air-water interface was evaluated by surface pressure-area isotherms, surface potential-area isotherms, polarization-modulation reflection absorption infrared spectroscopy (PM-IRRAS), and Brewster angle microscopy (BAM). The enzyme expanded the monolayers and changed the thermodynamic and electrical properties of the SA-CNT monolayers, as detected with the isotherms. PM-IRRAS spectra showed that the enzyme keeps its secondary structure when adsorbed at the monolayers and also alters the morphology of the air-water interface, as identified with BAM. The hybrid floating films were transferred to solid supports through the Langmuir-Blodgett (LB) technique, and the cotransfer of the enzyme was confirmed with fluorescence spectroscopy. The catalytic activity of asparaginase in the LB films was studied with UV–vis spectroscopy, which showed that the presence of CNT in the enzyme-lipid LB film not only tuned the catalytic activity, but also helped conserve its enzyme activity after weeks, showing higher persisting values of activity. UV–vis spectroscopy also showed that the catalytic activity is dependent basically on the enzyme molecules present on the surface of the LB films since multilayer films did not provide a proportional increase of enzyme activity. These results are related to the synergism between the compounds on the active layer, leading to a molecular architecture that allowed the adequate molecular accommodation of the analyte with the catalytic sites of the enzyme, which also preserved the asparaginase activity. This work then demonstrates the feasibility of employing LB films composed of fatty acids, CNT, and enzymes as devices for biosensing applications.

Conjugated Polymers Blended with Lipids and Galactosidase as Langmuir-Blodgett Films To Control the Biosensing Properties of Nanostructured Surfaces.

The structure of enzymes must be conserved when incorporated in nanoelectronic devices because their activity determines the function of the device as sensors. Among the systems that can retain their conformational structures, Langmuir-Blodgett (LB) films can be useful to exploit the construction of bioelectronic devices organized at the molecular level because biological and polymeric materials can be coupled as ultrathin films for biosensors and actuators. In this paper, we immobilized a β-galactosidase enzyme in the LB films of stearic acid and the conjugated polymer poly[(9,9-dioctylfluorene)- co-thiophene]. After the characterization of the floating films using tensiometry, vibrational spectroscopy, and Brewster angle microscopy, they were transferred to solid supports as LB films, and the catalytic activity of the enzyme could be preserved as analyzed using UV-vis spectroscopy. We noted that the presence of a supramolecular structure formed in the LB films not only conserved the enzyme activity but also exhibited regular and distinctive output signals in all molecular architectures employed in this work. These results are related to the synergism between the compounds on the active layer associated with a surface morphology that facilitated the analyte diffusion because of an adequate molecular accommodation of all components. This work then demonstrates the viability of employing LB films composed of lipids, enzymes, and synthetic polymers as devices for biosensing applications.

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.

Effects of surface topography on SERS response: Correlating nanoscopy with spectroscopy

This paper reports for the first time the hidden correlation between the topographical features of the bilayer Langmuir-Blodgett (LB) film substrates of stearic acid (SA) incubated in Au@Ag nanocolloids over various dipping times (DTs) with their corresponding SERS responses. The topographies of the as prepared substrates are investigated from the statistical considerations in terms of lateral correlation length, interface width, Hurst and Lyapnov exponents. The real space of the substrates are mapped directly from the FESEM and AFM images of the bilayer LB film of SA immersed in Au@Ag nanocolloids over various DTs ranging between 6 and 72 h. The SERS spectra of the Rhodamine 6G molecules adsorbed on the as prepared substrates have been reported. The statistical parameters of the substrates that exhibit maximum SERS efficacy have been suggested. The far field distributions in presence and in absence of Raman dipole together with spatial distribution of the near field from the hottest spot of the as prepared substrate have also been reported. To our knowledge, this is the first report that links nanoscopy with SERS spectroscopy from statistical considerations and is expected to open a new window towards the fabrication of more efficient and reproducible SERS active substrates in future endeavours.

Langmuir–Blodgett films of stearic acid deposited on substrates at different orientations relative to compression direction: alignment layer for nematic liquid crystal

ABSTRACTThe Langmuir monolayer at an air–water interface shows remarkably different surface pressure (π)–area (A) isotherm, when measured with the surface normal of a Wilhelmy plate parallel or perpendicular to the direction of compression of the monolayer. Such difference arises due to difference in stress exerted by the monolayer on the plate in different direction. In this article, we report the effect of changing the direction of substrate normal with respect to the compression of the monolayer during Langmuir–Blodgett (LB) film deposition on the morphology of the films. The morphology of the LB film of stearic acid is studied using an atomic force microscope. The morphology of the LB films is found to be different due to difference in the stress in different directions. The role of such surface morphology on the alignment of a nematic liquid crystal (LC) in LC cells is studied. The granular texture of LB films of stearic acid supports the homogeneous alignment of the LC whereas the uniform texture supports the homeotropic alignment of the LC.

Dynamic Analysis of the Removal of Fatty Acid from a Pet Surface Using a Quartz Crystal Microbalance

AbstractA quartz crystal microbalance (QCM) was used to study the removal of stearic acid from poly(ethylene terephthalate) (PET) substrate in aqueous alkali and surfactant solutions. The PET substrate was prepared on a QCM gold electrode by spin‐coating; its properties were examined by scanning electron microscopy, X‐ray photoelectron spectroscopy, and water contact angle measurements. As a result, the QCM electrode was found to be completely covered with a smooth PET film. To investigate the effect of soil deposition state on its removal, stearic acid (an oily soil model) was deposited onto the PET substrate with the Langmuir–Blodgett (LB) technique or by spraying. The QCM frequency was recorded during the removal of stearic acid from the PET substrate in aqueous solutions. In NaOH solution, stearic acid deposited by the LB method was more rapidly removed than when deposited by the spraying method. However, the LB films of stearic acid were difficult to be removed in surfactant solutions. With respect to removal behavior of spray‐deposited stearic acid in surfactant solutions, the results determined by the QCM method were compared with those from microscopic image analysis. The removal efficiencies in the sodium dodecyl sulfate solution were in good agreement between the two methods. However, in the alkyl ethoxylate solution, the removal efficiencies obtained by the QCM method were larger than those from microscopic image analysis. These experimental results were explained by the removal mechanism of stearic acid by the alkali and surfactant solutions.

Silver coated gold nanocolloids entrapped in organized Langmuir–Blodgett Film of stearic acid: Potential evidence of a new SERS active substrate

SERS active substrate containing silver coated gold (Au@Ag) nanocolloids entrapped in the Langmuir–Blodgett (LB) film matrix of stearic acid (SA) has been reported. The SERS efficacy of the as prepared substrate has been tested with trace concentrations of Rhodamine 6G (R6G) molecules. Enhancement factors ranging from 104–1013 orders of magnitude have been estimated for the characteristic vibrational signatures of R6G molecule. The colossal enhancement factors also signify the superiority of the as prepared substrate in comparison to Au@Ag nanocolloids. The optical responses and the morphological features of the substrates are estimated with aid of UV–vis absorption spectra and FESEM, AFM images respectively. Correlations between the surface morphologies, fractal dimensions and roughness features of the as prepared substrates are also drawn. The electric field distributions around the aggregated nanocolloids entrapped in the SA matrix have been envisaged with the aid of three dimensional finite difference time domain (3D-FDTD) simulations. Tuning the interparticle localized surface plasmon (LSP) coupling between the aggregated nanocolloids may be achieved by lifting the LB film of SA at different surface pressures.

Self‐assembly of silver nanocolloids in the Langmuir–Blodgett Film of stearic acid: Evidence of an efficient SERS sensing platform

Highly sensitive surface‐enhanced Raman scattering active substrate obtained by self‐assembly of silver nanocolloids (AgNCs) in the bilayer Langmuir–Blodgett (LB) film of stearic acid (SA) has been reported. Rhodamine 6G (R6G) has been used as the probe molecule to test the efficacy of the as prepared substrate. Gigantic enhancement factors ~1012 orders of magnitude have been estimated from the surface‐enhanced resonance Raman scattering [SER(R) S] spectrum of R6G, which proves that the as prepared substrate is superior or comparable with silver nanoparticle as dried AgNC solutions on microscopic slides. The optical properties of the as prepared substrates have been envisaged by ultraviolet‐visible absorption spectra, while their morphological features are mapped through field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) images. A correlation has been drawn between the SER(R) S efficacy and the corresponding FESEM and AFM images of the as prepared substrates. Electric field distributions around the aggregated AgNCs have been estimated with the aid of three‐dimensional finite difference time domain simulation studies. Localized surface plasmon coupling between the nanoaggregated geometries may be altered by lifting the LB film of SA at various surface pressures. Copyright © 2015 John Wiley & Sons, Ltd.

Salt Dependent Stability of Stearic Acid Langmuir–Blodgett Films Exposed to Aqueous Electrolytes

We use contact angle goniometry, imaging ellipsometry, and atomic force microscopy to study the stability and wettability of Langmuir-Blodgett (LB) monolayers of stearic acid on silica substrates, upon drying and exposure to aqueous solutions of varying salinity. The influences of Ca(2+) and Na(+) ions are compared by varying their concentrations, both in the subphase before the LB transfer, and in the droplets to which the dried LB layers are exposed. Ca(2+) ions in the subphase are found to enhance the stability, leading to contact angles up to 100°, as compared to less than 5° for Na(+). Consistent with the macroscopic wettability, AFM images show almost intact films with few holes exposing bare substrate when prepared in the presence of Ca(2+), while subphases containing Na(+) result in large areas of bare substrate after exposure to aqueous drops. The observations on varying the composition of the droplets corroborate the stabilizing effect of Ca(2+). We attribute these findings to the cation-bridging ability of Ca(2+) ions, which can bind the negatively charged stearate groups to the negatively charged substrates. We discuss the relevance of our findings in the context of enhanced oil recovery.

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.

A new evaluation method of surface energy of ultra-thin film

A new method is proposed for evaluating the dispersive component of the effective surface energy of an ultra-thin film. The dispersive component is obtained by introducing a cut-off distance and integrating the corrected van der Waals pressure equation for a symmetric multilayer system. The cut-off distance is calculated using the experimentally determined surface energy of the substrate without an ultra-thin film. A stearic-acid Langmuir–Blodgett film on a glass plate was used as an ultra-thin film sample on a solid substrate. The dependences of the surface energy on the ultra-thin film thickness were investigated experimentally and theoretically, and the effectiveness of this method is discussed.

Nucleation and growth of CuSO 4·5H 2O crystals on the liquid state stearic acid Langmuir–Blodgett film

Oriented crystallization of CuSO 4·5H 2O on a Langmuir–Blodgett (LB) film of stearic acid has been studied in the temperature ranges of 73–68 °C and 53–20 °C, respectively. This is the first time that the LB film at temperature above its melting point has been served as a template to induce nucleation and growth of crystals. The experimental results demonstrated that the LB film in the liquid state has the ability of directing the nucleation and growth of crystals. Moreover, X-ray diffraction patterns of the as-prepared crystals revealed that the orientation of the attached crystals on the LB film is affected by temperature greatly. The (1¯ 2 2) peak is very strong in crystals grown at higher temperature but absent at lower temperature. These results implied that structural changes in the LB film tuned by temperature determine the orientation of the attached crystals. The morphologies of the crystals showed that the orientation changes of the attached crystals influence the shapes and colors of CuSO 4·5H 2O crystals strongly.

Nucleation and Growth of Glycine Crystals with Controllable Sizes and Polymorphs on Langmuir–Blodgett Films

Control of crystal polymorph and size is very important in many application fields. Herein we demonstrate that Langmuir–Blodgett (LB) films of stearic acid (SA) and octadecylamine (ODA) can serve as templates and generate different polymorphs of glycine crystals. In the neutral aqueous solutions, γ-glycine crystallizes on LB films of ODA while the polymorphic outcome becomes the α-form on LB films of SA. These observed results could be explained by the electrostatic interactions and geometric lattice matching at the LB film/crystal interfaces, respectively. By keeping the appropriate supersaturation, we have successfully controlled the number of crystals grown on LB films; for example, in some certain cases, only one piece of crystal was grown on LB films in solution. Therefore, large crystals of centimeter size could be prepared. These experimental results suggest a new approach to produce an organic crystal with bulk scale.

Self-Assembly Effect during the Adsorption of Polynucleotides on Stearic Acid Langmuir−Blodgett Monolayer

Interaction of polyadenylic acid, poly(A), with stearic acid Langmuir-Blodgett (LB) monolayer was studied in different electrolyte surroundings. For this purpose LB films of stearic acid, transferred on the mica substrate from poly(A) containing subphase, were analyzed with atomic force microscopy (AFM). The density of polynucleotides surface coverage is ruled by the monovalent electrolyte concentration in the subphase that is in good agreement with previous results. Divalent cations in the subphase are needed to stabilize poly(A) molecules on the surface through formation of "salt bridges". At the very low divalent electrolyte concentration polynucleotides adsorb on the LB film to domains in which the effect of self-assembly is observed. Increase of divalent electrolyte concentration leads to the loss of this orientation effect. The explanation of this effect is proposed.

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.

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.