Deposited Langmuir-Blodgett Films Research Articles

Ultrasensitive Immunosensor Based on Langmuir-Blodgett Deposited Ordered Graphene Assemblies for Dengue Detection.

In this manuscript partially reduced graphene oxide (RGO) nanosheet-based electrodes have been utilized for quantification of the NS1 protein and subsequently for dengue detection. NS1 is the biomarker found circulating in the body of dengue-infected persons on or after first day of the appearance of disease symptoms. Graphene oxide (GO) has been synthesized using a modified Hummer's method, and its ordered nanostructured films have been electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates using Langmuir-Blodgett (LB) deposition. Deposited LB films of GO have been reduced with hydrazine vapors to obtain RGO-coated ITO electrodes. NS1 antibodies have been grafted onto the ordered thin films using covalent linking, and the bioelectrodes have been utilized for the specific detection of NS1 antigen. The electrochemical performance of the fabricated bioelectrodes for NS1 antigen detection has been explored in standard and spiked sera samples. The limit of detection for the standard samples and spiked serum samples is found to be 0.069 ng mL-1 and 0.081 ng mL-1, respectively, with a sensitivity of 8.41 and 36.75 Ω per ng mL, respectively, in the detection range of 101 to 107 ng mL-1.

Mass sensitivity of Langmuir-Blodgett monolayer film coated surface acoustic wave resonators to volatile organic solvents

Langmuir-Blodgett (LB) monolayer films deposited from a Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD), have been studied for their ability to be used as sensing layer for chemical gas sensors. DPPE-NBD molecules are known for their similarity to bio membrane molecules. Therefore, it is expected that they can serve as a matrix for immobilization of proteins, enzymes, aptamers, while preserving their function for selective reaction with organic analytes. In this study, LB monolayers of DPPE-NBD were deposited on Rayleigh Surface Acoustic Wave (RSAW) resonant devices working at 411 MHz and simultaneously on ultra-flat Si wafer substrates for Atomic Force Microscopy (AFM) inspection. Depositions were carried out both at a low surface pressure where the liquid-expanded phase dominates, and at a high surface pressure, where the liquid-condensed (solid) phase dominates. AFM topography reveals the liquid and solid phase coexistence as well as the formation of 3D pyramids of 3 to 30 nm height and 50 to 1000 nm in diameter. In a vapour sensing experiment, the RSAW devices were exposed subsequently to vapours of 6 volatile organic compounds (VOCs) and water. The most significant resonant frequency shift of 225 kHz which corresponds to 11.42 ng mass change was observed with chloroform vapours when the substrate with the solid phase LB monolayer was used as a sensing layer. Adsorption and desorption of the vapours was very fast (a few seconds) and completely reversible. The higher-pressure deposited LB film demonstrates higher sensitivity to all gases. Mechanisms of this behaviour are discussed.

Localization of the Wannier–Mott Exciton on a Langmuir-Film/CdS Organic Semiconductor Interface

The low-temperature (T = 2 K) light reflectance spectra of organic semiconductor structures produced by depositing Langmuir–Blodgett films on a cadmium sulfide (CdS) crystal surface are studied. The spectra were studied in the region of the resonant frequency of the exciton state An = 1 in CdS. The spectra were analyzed within a multilayer medium model with allowance for the spatial dispersion and an exciton-free “dead” layer near the crystal surface contacting with the film. A conclusion is made that, as a result of the deposition of an organic film on a semiconductor crystal surface, the Wanier–Mott exciton is spatially localized near the film–crystal interface.

Electrical Characterization of Polyaniline-ZnO nano-composite Langmuir-Blodgett thin films by Conductive Atomic Force Microscopy

ABSTRACTPolyaniline–Zinc oxide nano-composite material was prepared by chemical polymerization of aniline with ZnO nano-particles doping. Surface Pressure-Area (π-A) isotherms of Polyaniline (PANi) and Polyaniline–Zinc oxide nano-composite shows phase transformations of monolayer during compression process. Multiple isotherms indicate that the monolayer of the composite material can retain its configuration during compression-expansion cycles. The structural, topographical and electrical properties of these deposited Langmuir Blodgett films were studied and characterized by UV-Visible spectroscopy (UV), Atomic force microscopy (AFM), Conductive Atomic force microscopy (C-AFM) respectively. For detailed investigations of the LB film properties, Conductive AFM is used to measure the I-V relationship of a surface of Langmuir Blodgett (LB) films of Polyaniline and Polyaniline–Zinc oxide nano-composite. The contact size of the AFM cantilever tip can be as small as a few nanometers, so, the local variation of the electrical property, which is unseen in the macroscopic level, can be observed by the I-V curve. A current ranging up to 3 nA and 20 nA have been observed in the case of PANi and PANi-ZnO nano-composite LB film, respectively. Conductive data images of the ITO substrate, PANi and PANi-ZnO nano-composite LB film on the ITO substrate obtained with an applied bias voltage of 4V showed that the distribution of current on the whole surface is almost uniform and very less inhomogeneities have been observed in the surface conductance of the PANi and PANi-ZnO nano-composite LB film.

Hydrophilic-like wettability of Cl-passivated Ge(0 0 1) surface

The hydrophilic/hydrophobic nature of the Cl-passivated Ge(001) surface is investigated directly by contact angle (CA) measurement and indirectly by growing nickel arachidate Langmuir–Blodgett (LB) films on the Cl-passivated Ge(001) surface. Passivation of Ge(001) surface by Cl atoms is confirmed by X-ray photoelectron spectroscopy measurement. CA measurements show that the Cl-passivated Ge(001) surface has intermediate wettability, i.e., the surface has intermediate hydrophilic/hydrophobic behavior. Structural information obtained from the deposited LB films by using X-ray reflectivity and atomic force microscopy analysis shows that the surface is homogeneous and hydrophilic (∼85%), although very few effectively hydrophobic (∼15%) regions are present. Structural study in molecular level thus helps to identify the surface nature in nanometer level, which is not possible by simple macroscopic CA measurement. Specific electrostatic and dispersive effects of Cl atoms are possibly responsible for such hydrophilic-like nature of the Cl-passivated Ge(001) surface.

Controlled Deposition of Highly Oriented Type I Collagen Mimicking In Vivo Collagen Structures

The structural arrangement of type I collagen in vivo is critical for the normal functioning of tissues, such as bone, cornea, and blood vessels. At present, there are no low-cost techniques for fabricating aligned collagen structures for applications in regenerative medicine. Here, we report a straightforward approach to fabricate collagen films, with defined orientation of collagen fibrillar aggregates within a matrix of oriented collagen molecules. Langmuir-Blodgett (LB) technology was used to deposit thin films of oriented type I collagen onto substrates. It was found that collagen does not behave like classical LB materials, such as amphiphilic hydrocarbon acids or lipids. The thickness of the deposited collagen films and the area-pressure isotherms were found to depend on the amount of material spread. In addition, no film collapse was detected and the deposited LB films were thicker than the theoretical dimension of a collagen monolayer (1.5 nm) formed by triple helical collagen molecules. Individual LB films with thicknesses of up to 20 nm were obtained, and multiple depositions yielded overall film thicknesses of up to 100 nm. Films consisted of a matrix of collagen molecules containing thicker fibrillar aggregates of collagen (micrometers in length). These fibrillar aggregates were built up of shorter unit molecules forming "spun thread" structures, some of which exhibited a zigzag pattern. In addition to aligning collagen unidirectionally (similar for example to tendon), we performed a two-step deposition procedure, in which the substrate was turned 90 degrees between two consecutive collagen deposition steps. The resulting films showed orthogonally aligned collagen layers, mimicking the structure of cornea. Thus, this technique permits control of the thickness of individual layers, the orientation of successive layers, and the number of layers within the construct. Therefore, it may have widespread applicability for the engineering of collagen-rich tissues.

Stable Langmuir–Blodgett film deposition of polyaniline and arachidic acid

An experimental study was carried out to examine the stable Langmuir–Blodgett (LB) film deposition of mixtures of polyaniline (PA) and arachidic acid (AA) at different concentration ratios, with the aim of developing a systematic methodology for the production of quality PA/AA film at the highest deposition speed. The quality of LB film was examined before, during and after the film deposition process. Images of the PA/AA composite materials at air–liquid interface, based on the Brewster angle microscopy, revealed that a 50/50 concentration ratio of PA/AA composite layer produced the best LB film structure. The feasibility of LB film deposition was determined from dynamic contact angles which were measured by a flow visualization technique. The quality of the deposited film was judged by the transfer ratio and the atomic force microscopy images. The effect of pH was also examined, and it was found that the LB film deposition for the PA/AA system could only be carried out in a narrow range of pH. It was found that the deposited LB film of the PA/AA system had a high transfer ratio and improved surface roughness at the deposition speed 30 mm/min. This deposition speed is much higher than those previously reported.

Langmuir and Langmuir−Blodgett Films of Mixed Asphaltene and a Demulsifier

Asphaltene was extracted from Athabasca oil sands bitumen with n-heptane. The extracted maltene-free asphaltene was characterized for physicochemical properties through molecular weight, chemical composition, and functional group analysis. Monolayer characteristics of mixtures of asphaltene and a polymeric demulsifier were studied using a Langmuir interfacial trough at air−water and heptol−water interfaces through measurements of pressure−area and relaxation isotherms (“heptol” refers to a solvent mixture of heptane and toluene). Single-layer Langmuir−Blodgett (LB) films from monolayers of mixed asphaltene and demulsifier were deposited on silicon wafers at air−water and heptol−water interfaces. The deposited LB films were characterized by atomic force microscopy (AFM) imaging and contact angle measurements. Asphaltene forms nanoaggregates at both air−water and heptol−water interfaces. Asphaltene monolayers at a heptol−water interface are more compressible than those at an air−water interface and show little hysteresis. The presence of a demulsifier in a mixed monolayer renders the monolayer more compressible at both air−water and heptol−water interfaces. The asphaltene and demulsifier aggregates follow ideal mixing behavior at both air−water and heptol−water interfaces. AFM images of deposited monolayers reveal the presence of nanosize aggregates. The presence of demulsifier in an asphaltene monolayer can reduce the mechanical strength of the asphaltene monolayer as indicated by reduced rigidity with increasing demulsifier content in the compressibility plot.

Configuration and photochemical reaction of a bolaamphiphilic diacid with a diazo resin in monolayers and Langmuir–Blodgett films

The monolayer formation of a bolaamphiphile, 1,18-octadecanedicarboxylic acid (ODA), on pure water and the subphase containing a positively charged photoactive 2-nitro- N-methyldiphenylamine-4-diazoniumformaldehyde resin (NDR) have been investigated by π– A isotherms, π– t curves, and Brewster angle microscopy (BAM) measurements. It has been revealed that although an unstable monolayer was formed by ODA alone, a stable complex monolayer between ODA and NDR could be formed at the interface through electrostatic adsorption and hydrogen bonding. It has been shown that the ODA formed a U-shaped monolayer at a lower pressure and was converted to a stretched configuration upon compression to a higher surface pressure on the subphase containing NDR. Under UV irradiation at the interface photoreaction can occur in the complex monolayer, which causes shrinkage of the monolayer. Photochemical reactions can also occur in deposited Langmuir–Blodgett films. In reactions occurring at the air/water interface, the two ends of ODA can react with NDR to form an ester containing aromatic rings. This makes the compound more hydrophobic and can easily be stretched without any phase transition upon compression. When the film with U-shaped configuration was deposited onto solid substrates, the configuration could be kept even upon photoirradiation.

Molecular arrangement and photochemistry of a bolaamphiphilic anthracene derivative in Langmuir–Blodgett film

Molecular arrangement and photochemical reactions have been investigated in the Langmuir–Blodgett (LB) films of a novel bolaamphiphilic anthracene derivative (BISANS) in comparison with those of one-headed amphiphilic anthracene derivative (ANS). It has been found that ANS forms a stable monolayer at the air/water interface, while BISANS undergoes aggregation processes to form crystal film on the water surface, as evidenced from the surface pressure (π)–area (A) isotherm, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements. UV–VIS spectra reveal that both the H-aggregation and the J-aggregation exist in the deposited LB films of bolaamphiphilic BISANS; while only J-aggregation is formed in the LB film of ANS. The content of the H-aggregation in the LB films depends on the transfer surface pressure. The orientation of anthracyl chromophores has an effect on the photoreaction in the LB films. When both the ANS and the BISANS LB films were irradiated with UV light (365nm), photodimerization took place predominantly in nitrogen atmosphere, while photooxidation occurred preferentially in air.

Control of size of PbI 2 nanocrystals using Langmuir–Blodgett films of n-octadecyl succinic acid

Lead iodide (PbI 2) particulate films, composed of highly oriented nanocrystals have been generated in situ using an oriented inorganic extended solid within a deposited Langmuir–Blodgett film (LB film) of octadecyl lead succinate (ODSAPb). Solid (PbI 2) was formed by the reaction of gaseous HI with ODSAPb LB film, which acts as the template for building the inorganic solid. Mixed LB systems with 1:1 mol ratio of ODSAPb with octadecyl amine (ODAM) were also used to form the PbI 2. The films were characterized using attenuated total reflection FTIR (ATR-FTIR) to check whether HI had completely protonated the films. The observation of absorption features between 550 and 300 nm region in the UV-Visible spectra of these films strongly suggest sizes of PbI 2 ranging between 2.0 and 3.0 nm. The optical band gap estimated from LB films of these two systems showed a marked increase from 0.36 eV for the pure ODSAPb template to 0.47 eV for the ODSAPb+ODAM mixed film. The LB films were examined by small angle XRD and atomic force microscopy (AFM) and indicated that there is a variation in the structural organization in the monolayers and in the size of PbI 2 in the pure ODSAPb LB films as compared with the mixed films with ODAM. The average contraction in a axis and elongation in the c axis of the unit cell ( a= b=3.9 Å and c=7.0 Å) for PbI 2 in the template, compared with the values for PbI 2 in bulk confirm the presence of the nanocrystals in the LB films. The increase in the band gap is also reflected in the increase in the c value of the unit cell parameter that changes from 6.3 to 7.0 Å in going from the pure ODSAPb matrix to mixed film of ODSAPb and ODAM.

Experimental and molecular modelling studies of overbased detergent particles

Langmuir trough, Langmuir–Blodgett (LB) film and ellipsometry experiments, and molecular modelling, were carried out to characterise a series of overbased detergent (OD) systems made with several surfactant types, including the first application of these approaches to a new class based on calixarates. Systems were synthesised with low, mid and high levels of basicity, which on the TBN (total base number) scale were 150, 250 and 350, respectively. The diameters determined from the Langmuir trough measurements were 3.4, 3.5 and 3.6 nm, respectively. These are consistently higher than those of phenate particles with the same basicities. The phenate and calixarate particles self-assembled into multilayer LB films on a stearic acid-coated silicon wafer. Film thicknesses were determined by ellipsometry. Differences in the estimated particle diameter for the phenates and calixarates by these two methods suggest that the OD particles are, in general, and to varying extents, non-spherical. In the phenate case the deposited LB film thickness was a strong function of the Langmuir film deposition surface pressure in the region where the monolayer is essentially close-packed, supporting the discoid shape for these particles, which was first predicted by our previous molecular modelling studies. The molecular modelling results suggest that the sulfonate systems are the most spherical, followed probably by the salicylates. Preliminary results on the calix[6]arates suggest that they form prolate ellipsoidal particles, capped at both ends with the surfactant molecule. The experiments and simulations are consistent in indicating that the particle shape and internal atomistic structure inside the inorganic core region of the particle are quite sensitive to the surfactant type.

Characterization of Arachidate Langmuir−Blodgett Films by Variable Energy Positron Beams

Arachidate Langmuir−Blodgett (LB) films of different chemical composition and number of monomolecular layers deposited on silylated silica glass substrates were studied by means of positron annihilation spectroscopy. The applied methods included the measurement of the Doppler broadening of the annihilation photopeak with variable energy positron beams and bulk positron lifetime measurements. The studied samples were 58 monomolecular layers (MML) thick Mg- and Cd-arachidate, arachidic acid (50 MML) and a series of Pb-arachidate samples with 4, 10, 20, 40, and 58 MML. The investigation showed that the variable energy positron beam technique is capable of measuring the thickness of the deposited LB films. The measured positron annihilation parameters are sensitive to the chemical composition of the films and the behavior of the films in a vacuum. The results confirmed the stability of salt base LB films in high vacuum conditions and showed the desorption of pure acid films. These investigations have also sho...

Dynamic tension and adsorption behavior of aqueous lung surfactants

The dynamic tension behavior, at constant or at pulsating area conditions, of two commercial lung surfactants in saline is reported. The bubble method, at constant or pulsating area, at 37°C and the pendant drop method at 23°C were used. For Exosurf, a commercial synthetic lung surfactant consisting of dissolved tyloxapol and dispersed dipalmitoylphosphatidylcholine (or DPPC) and hexadecanol (H), the equilibrium and dynamic tensions are high (over 30 mN m −1) and similar to those of tyloxapol alone. Aqueous DPPC/H mixtures have lower tensions than Exosurf. Survanta, a commercial lung surfactant replacement drug consisting of DPPC, other lipids, and two hydrophobic lung surfactant proteins, produces dynamic surface tensions that are substantially lower than those of Exosurf. Diluted 10-fold, Survanta produces under pulsating area (at 20 cycles min −1) lower minimum tensions than undiluted Survanta (6 vs. 12 mN m −1), but higher maximum tensions. In addition, Survanta tension behavior is unusual, having three local maxima and three local minima per cycle, suggesting major variations of its surface composition in each cycle. Monolayer pressure-area isotherms and Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy results on deposited Langmuir–Blodgett films support this suggestion. They also provide direct evidence of the presence of phospholipids (DPPC or others) on the surface, but only indirect evidence of the presence of other components, on the surface of aqueous Exosurf or Survanta.

Optical ellipsometry (polarization) measurements on alkyl ammonium metal complexes

Spectroscopic ellipsometry techniques have been used to determine the refractive indices and thicknesses of thin-film Langmuir–Blodgett (LB) films. By using a spectrometer incorporating a charge-coupled detector (Ocean Optics Inc., S2000), extensive and rapid collection of data is possible. The data for each film are obtained by analysis of the ellipsometric reflection coefficient measured as a function of angle of the incident light. The experimental arrangement consisted of an intense white source (incorporating a projection lamp) providing a collimated beam of light approximately 1 mm in diameter that passed through a rotatable polarizer onto the film at an incident angle φ. The reflected beam was then picked up using an optic fiber and transmitted to the spectrometer. Thus, the plane of polarization was variable, as was the angle of incidence, and reflection intensities were scanned over the wavelength range 330–1010 nm using the maximum available instrumental collection (integration) time of 2552 ms. Measurements were made on alkyl ammonium metal complexes previously investigated for potential soft Xray mirrors [1, 2]. These materials deposit with the metal atoms aligned in layers between the alkyl ammonium chains, the appropriate length of which, in principle, can be manipulated. Whereas most of the work on depositing LB films had been carried out on glass substrates, the films themselves are highly transparent with refractive indices differing insufficiently from glass. Hence, films were deposited on silicon substrates (1 0 0 surface) 625μm in thickness supplied by WackerChemitronic. These were cut to 40 mm by 8 mm, an appropriate size for LB deposition. Although deposition of the LB films was more difficult on silicon than on glass, good deposition was obtained, particularly for the complex (C22H45NH3)2CuCl2(OH)2 and for (C18H37NH3)2ZnCl2(OH)2. These compounds are analogous to the well-known materials (CnH2n+1NH3)2MCl4 where M=Mn, Fe, Cu and Cd (see, for instance, [3]). Ellipsometry measurements have been reported previously on LB films (for example [4–6]) but not the technique of a charge-coupled detector to scan as a function of wavelength and angle and of curve fitting as used here. Samples were mounted in the vertical plane on a goniometer rotatable about a vertical axis. Measurements were usually made at 10◦ intervals of the angle of incidence, φ. A second rotation arm tracked the reflected

Temperature-independent conductivity of Langmuir–Blodgett films of a helical polymer and its salt complex

A helical polyether–lithium perfluorophenate complex bearing alkyl side-chains has been shown to form well-organised Langmuir–Blodgett (LB) assemblies. Y-type deposited LB films of the preformed complex (1:1) exhibit a conductivity (normal to the plane) ∼2 orders of magnitude higher than that of the bulk at 20°C and ∼9 orders of magnitude greater than the pure uncomplexed polymer LB film. Infrared spectroscopy has confirmed the presence of the salt in these deposited films. These results demonstrate that this preformed polymer complex does not lose its salt component to the aqueous subphase as a Langmuir film. The ionic conductivity of the polymer complex LB film is temperature independent over the region 20–90°C, above which partial disruption to the film occurs with a lowering of the conductivity.

Instrument for depositing Langmuir–Blodgett films composed of alternating monolayers using a protective layer of water

A novel instrument has been developed for depositing Langmuir–Blodgett (LB) films composed of alternating molecular monolayers of amphiphilic molecules and adsorbed layers of soluble compounds. The main feature of the deposition method is the possibility to prevent the transfer of the monolayer onto the substrate when dipping up. This can be achieved by keeping the external boundary of the deposited monolayer in contact with the aqueous solution by means of a protecting hydrophilic plate which covers the substrate with the film while still immersed in water. After that, the plate is fixed to the substrate parallel to its surface and both can be transferred to other compartments of the instrument for adsorption of soluble compounds onto the active hydrophilic surface or for deposition of monolayers of other types. Principal design elements, electronics, and software of the LB instrument based on this principle are described. Results on the deposition of LB films composed of alternating monolayers of amphiphilic donor molecules of hexadecylbis(ethylenedithio)tetrathiafulvalene and adsorbed layers of the soluble protein cytochrome P 450scc are reported as an example of the device functioning.

Evaluation of C15TCNQ Langmuir-Blodgett Ultrathin Films on Aluminum Thin Films by Attenuated Total Reflection Measurements

The attenuated total reflection (ATR) properties of C15TCNQ Langmuir-Blodgett (LB) ultrathin films deposited on the Al thin films about 20 nm thick were measured in order to evaluate the actual structure. The C15TCNQ LB ultrathin films were prepared by the vertical dipping method, using two deposition processes. The dielectric constants and the thicknesses of these deposited LB films were theoretically calculated from the ATR curves, taking into account the presence of the natural oxide layer on Al. It was considered that Z-type LB films were obtained when the deposition of each monolayer was carried out only as the substrates were withdrawn. In contrast, when monolayers were deposited during both upward and downward movements of the substrates, the resulting LB films were considered to be not perfect Y-type, but partially Z-type.

Preparation and thermostable properties of Langmuir-Blodgett films of copper tetra-tert-butyl phthalocyanine complex

Langmuir-Blodgett (LB) films of copper tetra-tert-butyl phthalocyanine (CuTTBPc) have been produced on SiO 2 substrates. The perfection of LB-film/substrate and LB-film/air interfaces has been evaluated by using X-ray reflectometry. The effect of surface pressure, deposition type (Z or Y) and preliminary heat treatment on the quality of the deposited LB films is reported. A study was made of thermostable properties of the LB films based on pure CuTTBPc and the latter 1:1 molar ratio mixture with arachidic acid.

Deposition of uniform fullerene films by LB technique

Deposition of uniform fullerene (C 60) films is carried out using the Langmuir—Blodgett (LB) technique. As an intermediate stage an LB film from a mixed monolayer of C 60 and a specially selected surfactant compound is deposited. The surfactant compound used forms itself into amorphous uniform LB films and C 60 molecules distribute homogeneously in this matrix. Another property of the surfactant compound is its solubility in hexane, in which C 60 does not dissolve. One-component C 60 film is obtained after treatment of the sample with hexane. To achieve this result mixtures of C 60 with several compounds were studied and finally one of them was selected. Surface pressure-area isotherms were recorded to evaluate the degree of C 60 dissolution in matrix monolayers. Deposited LB films were studied by optical microscopy, electron diffraction and transmission electron microscopy techniques to record changes of the structure and morphology under variation of the conditions and selection of the compounds. One-component C 60 monolayers at the air-water interface as well as C 60 films transferred with the horizontal lift technique were also investigated for interpretation of the data on mixed multilayers. Some features of the structure of the studied films are discussed.