Langmuir-Blodgett Monolayers Research Articles

Graphene metal oxide composite supercapacitor electrodes

This study presents composite electrode materials based on graphene oxide (GO) and transition metal oxide nanostructures for supercapacitor applications. Electrophoretic deposition of GO on a conductive substrate was used to form reduced graphene oxide (rGO) films through chemical reduction. The specific capacitance of the rGO was calculated up to 117 F/g at 100 mV/s scan rate from KOH (1 M) electrolyte using an Ag/AgCl reference electrode. The strong interaction of GO with Co3O4 and MnO2 nanostructures was demonstrated in the self-assembled Langmuir–Blodgett monolayer composite, showing the potential to fabricate thin film supercapacitor electrodes without using binder materials. This two-step process is nontoxic and scalable and holds promise for improved energy density from redox capacitance in comparison with the conventional double layer supercapacitors.

Nanopatterned Cadmium Selenide Langmuir–Blodgett Platform for Leukemia Detection

We present results of the studies relating to preparation of Langmuir-Blodgett (LB) monolayers of tri-n-octylphosphine oxide-capped cadmium selenide quantum dots (QCdSe) onto indium-tin oxide (ITO) coated glass substrate. The monolayer behavior has been studied at the air-water interface under various subphase conditions. This nanopatterned platform has been explored to fabricate an electrochemical DNA biosensor for detection of chronic myelogenous leukemia (CML) by covalently immobilizing the thiol-terminated oligonucleotide probe sequence via a displacement reaction. The results of electrochemical response studies reveal that this biosensor can detect target DNA in the range of 10(-6) to 10(-14) M within 120 s, has a shelf life of 2 months, and can be used about 8 times. Further, this nucleic acid sensor has been found to distinguish the CML-positive and the control negative clinical patient samples.

Minimizing Defects in Polymer-Based Langmuir–Blodgett Monolayers and Bilayers via Gluing

Polymeric surfactants were prepared by quaternization of poly(4-chloromethylstyrene) with N,N-dimethyl-N-n-dodecylamine and N,N-dimethyl-N-n-octylamine to give 1 and 2, respectively. Each of these polymers formed stable monolayers at the air/water interface. Injection of poly(acrylic acid) (PAA) beneath the surface of these films led to a substantial increase in their cohesiveness (i.e., "gluing"), as evidenced by a dramatic increase in their surface viscosity. Examination of monolayers of 1 by atomic force microscopy, after being transferred to silicon wafers that were surface-modified with n-octadecyltrichlorosilane, showed that the presence of PAA leads to intact film. In contrast, transfer of unglued monolayers resulted in poor coverage. Comparison of the barrier properties of single glued and unglued LB bilayers formed in the presence and in the absence of PAA have shown that PAA minimizes defect formation within these ultrathin assemblies.

Effect of an Upward and Downward Interface Dipole Langmuir–Blodgett Monolayer on Pentacene Organic Field-Effect Transistors: A Comparison Study

Effect of an Upward and Downward Interface Dipole Langmuir–Blodgett Monolayer on Pentacene Organic Field-Effect Transistors: A Comparison Study

Effect of an Upward and Downward Interface Dipole Langmuir–Blodgett Monolayer on Pentacene Organic Field-Effect Transistors: A Comparison Study

We studied carrier behaviors of pentacene organic field-effect transistors (OFETs) with an upward and a downward orientation dipole monolayer, inserted between the organic active layer and gate insulator by the Langmuir–Blodgett technique. The OFETs with an upward orientation of dipole monolayer showed large negative threshold voltage and high contact resistance compared with the reference OFETs without dipole monolayer, while the OFETs with a downward orientation dipole monolayer exhibited positive threshold voltage and low contact resistance. Based on the findings from this comparison study, we argued that using interface dipole monolayer is a useful way to design OFET performance.

Synchrotron FTIR Microscopy of Langmuir–Blodgett Monolayers and Polyelectrolyte Multilayers at the Solid–Solid Interface

Synchrotron FTIR microscopy has been used to probe the structure of model boundary lubricant layers confined at the solid-solid interface. The combination of high brightness of the IR source and a novel contact geometry that uses a hemispherical internal reflection element as the means for light delivery has enabled the detection of <2.5 nm thin monolayer lubricant layers in the solid-solid contact, in addition to allowing for spectral acquisition from specific regions of the contact. Spectra of hydration water from within a confined polyelectrolyte multilayer film have also been acquired, highlighting the altered hydrogen bonding environment within the polymer layer.

Average Orientation of a Molecular Rotor Embedded in a Langmuir–Blodgett Monolayer

A molecular rotor in which a naphthalene rotator is attached through a silicon atom to three fatty acid chains has been synthesized, and Langmuir-Blodgett techniques were used to deposit on silica surfaces monolayers of its calcium salt, both neat and diluted with stearic acid salts. The monolayer films have been characterized by ellipsometry and Fourier transform infrared (FT-IR) grazing-incidence attenuated total internal reflection (GATR) spectroscopy on Si-SiO(2) and by UV-vis absorption spectroscopy on SiO(2). The measurements were combined with calculations of the electronic (INDO/S) and vibrational (DFT) transition moment directions to deduce the average orientation of the rotor molecules, including the naphthalene ring, relative to the surface. In both neat and mixed films, the naphthalene ring is found to preferentially tilt toward the surface, enough that its rotation is most likely hindered. A comparable picture was obtained from molecular mechanics calculations on a mixed film of the naphthalene rotor and stearic acid.

Amyloid-Beta 1-42 Binds Preferentially to Nanoscale Electrostatic Domains in Cholesterol-Enriched DOPC Lipid Membrane

The plasma membrane is a complex structure, composed primarily of phospholipids and other macromolecules, such as proteins, sterols and steroids. It plays a vital role in cell motility and acts as a barrier for extracellular materials. Amyloid fibrils are linked to multiple neurodegenerative diseases, such as Alzheimer’s. Although fibril plaque formation is associated with biological membranes in vivo, the role of the lipid membrane in fibril formation and toxicity is not well understood. We investigated the interaction of model lipid membranes with Aβ 1-42 peptide. Using Atomic force microscopy, we demonstrated that binding of Aβ 1-42 peptide to DOPC bilayers with 20% cholesterol is non-uniform and resulted in the formation of nanoscale islands loaded with A-beta. We attribute this effect to the presence of electrostatic nanoscale domains induced by cholesterol in the DOPC bilayer. These domains were resolved by AFM imaging of the lipid bilayer and by frequency modulated Kelvin probe force microscopy in the monolayer samples prepared with the Langmuir-Blodgett monolayer technique.

Single molecule probes of membrane structure: Orientation of BODIPY probes in DPPC as a function of probe structure

Single molecule fluorescence measurements have recently been used to probe the orientation of fluorescent lipid analogs doped into lipid films at trace levels. Using defocused polarized total internal reflection fluorescence microscopy (PTIRF-M), these studies have shown that fluorophore orientation responds to changes in membrane surface pressure and composition, providing a molecular level marker of membrane structure. Here we extend those studies by characterizing the single molecule orientations of six related BODIPY probes doped into monolayers of DPPC. Langmuir-Blodgett monolayers transferred at various surface pressures are used to compare the response from fluorescent lipid analogs in which the location of the BODIPY probe is varied along the length of the acyl chain. For each BODIPY probe location along the chain, comparisons are made between analogs containing phosphocholine and smaller fatty acid headgroups. Together these studies show a general propensity of the BODIPY analogs to insert into membranes with the BODIPY probe aligned along the acyl chains or looped back to interact with the headgroups. For all BODIPY probes studied, a bimodal orientation distribution is observed which is sensitive to surface pressure, with the population of BODIPY probes aligned along the acyl chains increasing with elevated surface pressure. Trends in the single molecule orientations for the six analogs reveal a configuration where optimal placement of the BODIPY probe within the acyl chain maximizes its sensitivity to the surrounding membrane structure. These results are discussed in terms of balancing the effects of headgroup association with acyl chain length in designing the optimal placement of the BODIPY probe.

Miscibility Behavior and Nanostructure of Monolayers of the Main Phospholipids of Escherichia coli Inner Membrane

We report a thermodynamic study of the effect of calcium on the mixing properties at the air-water interface of two phospholipids that mimic the inner membrane of Escherichia coli: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol. In this study, pure POPE and POPG monolayers and three mixed monolayers, χ(POPE) = 0.25, 0.5, and 0.75, were analyzed. We show that for χ(POPE) = 0.75, the values of the Gibbs energy of mixing were negative, which implies attractive interactions. We used atomic force microscopy to study the structural properties of Langmuir-Blodgett monolayers that were transferred onto mica substrate at lateral surface pressures of 25 and 30 mN m(-1). The topographic images of pure POPE and POPG monolayers exhibited two domains of differing size and morphology, showing a step height difference within the range expected for liquid-condensed and liquid-expanded phases. The images captured for χ(POPE) = 0.25 were featureless, and for χ(POPE) = 0.5 small microdomains were observed. The composition that mimics quantitatively the proportions found in the inner membrane of E. coli , χ(POPE) = 0.75, showed large liquid condensed domains in the liquid expanded phase. The extension of each domain was quantitatively analyzed. Because calcium is used in the formation of supported bilayers of negatively charged phospholipids, the possible influence of the nanostructure of the apical on the distal monolayer is discussed.

Characteristic Fragmentation of Polysiloxane Monolayer Films by Bombardment with Monatomic and Polyatomic Primary Ions in TOF-SIMS

This study reports the characteristic fragmentation patterns from two polysiloxane polymers that form ordered overlayer on silver substrates. Results are compared for the bombardment of various monatomic and polyatomic projectiles of Cs(+), C(60)(+) (10 keV), Bi(1)(+), and Bi(3)(+) (25 keV) in the high mass range time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra. Results are reported from sub-monolayer (solution cast) coverages of poly(dimethylsiloxane)s with the number average molecular weights (M(n)) of 2200 and 6140 Da, respectively, and Langmuir-Blodgett monolayers of poly(methylphenylsiloxane) with molecular weights (MW) from 600 and 1000 Da. For each film, Bi projectiles resulted in the emission of positive silver cluster ions from the substrate under the polymer overlayer and peaks corresponding to silver cluster ions with larger mass were observed by impact of polyatomic 25 keV Bi(3)(+) projectiles. In addition, depending on the change of energy of Bi (3) (+) , a different pattern of fragments was observed. With Cs(+) and C(60)(+) impact, however, the emission of silver cluster ions was not detected. In the case of C(60)(+) impact for PDMS-6140, peaks corresponding to silver-cationized intact oligomers were not observed. In this paper, these results are explained by the possible bombardment mechanism for each projectile, based on its mass, energy, and split trajectories of the component atoms under the polyatomic impact.

Healing of Defects at the Interface of Nematic Liquid Crystals and Structured Langmuir-Blodgett Monolayers

We use Langmuir-Blodgett molecular monolayers and nematic liquid crystals as model two- and three-dimensional orientationally ordered systems to study the stability and healing of topological defects at their contact interfaces. Integer-strength defects at the monolayer induce disclinations of similar strength in the nematic that, however, do not propagate deep into the bulk, but rather form single- or double-split arch-shaped loops pinned to the interface. This behavior is qualitatively independent of the far-field director orientation and involves either half-integer singular or twist-escaped unity-strength nonsingular nematic disclinations. These two defect configurations can be selected by varying sample preparation given their comparable free energy, consistently with direct probing by use of laser tweezers.

Thermodynamic and structural characterization of a mixed perfluorocarbon–phospholipid ternary monolayer surfactant system

Pulmonary lung surfactant is a mixture of surfactants that reduces surface tension during respiration. Perfluorinated surfactants have potential applications for artificial lung surfactant formulations, but the interactions that exist between these compounds and phospholipids in surfactant monolayer mixtures are poorly understood. We report here, for the first time, a detailed thermodynamic and structural characterization of a minimal pulmonary lung surfactant model system that is based on a ternary phospholipid–perfluorocarbon mixture. Langmuir and Langmuir–Blodgett monolayers of binary and ternary mixtures of the surfactants 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) and perfluorooctadecanoic acid (C18F) have been studied in terms of miscibility, elasticity and film structure. The extent of surfactant miscibility and elasticity has been evaluated via Gibbs excess free energies of mixing and isothermal compressibilities. Film structure has been studied by a combination of atomic force microscopy and fluorescence microscopy. Combined thermodynamic and microscopy data indicate that the ternary monolayer films were fully miscible, with the mixed films being more stable than their pure individual components alone, and that film compressibility is minimally improved by the addition of perfluorocarbons to the phospholipids. The importance of these results is discussed in context of these mixtures’ potential applications in pulmonary lung surfactant formulations.

Plasmon-Enhanced Fluorescence and Spectral Modification in SHINEF

Shell-isolated gold nanoparticles provide a reliable, portable, and efficient substrate for luminescence enhancement giving surface-enhanced fluorescence, termed SHINEF. The objectives of the present work are 2-fold: first, tuning the size of the gold core and the shell thickness for maximum SHINEF enhancement, and second, to use the shell isolated nanoparticles (SHINs) to demonstrate the spectral profile modification of the fluorescence spectrum. It is shown, using Langmuir–Blodgett monolayers of a dye with two distinct emissions (monomer and excimer), that the far field radiation of the nanostructure modulate the observed emission from the target fluorophore located in the near field zone of the nanostructure sustaining localized surface plasmon resonances (LSPR).

Deposition and Photopolymerization of Phase-Separated Perfluorotetradecanoic Acid–10,12-Pentacosadiynoic Acid Langmuir–Blodgett Monolayer Films

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.

Oxide nanoparticle arrays for sensors of CO and NO2 gases

Metal oxide sensors with active films from Fe2O3 and CoFe2O4 hexagonally assembled nanoparticle (NP) arrays were studied. NPs were synthesized by high-temperature solution phase reaction. Sensing NP layers were deposited by Langmuir–Blodgett (LB) technique. LB layers were characterized by XRD, SEM and magnetic measurements. NPs are monodomain and superparamagnetic at RT. Sensor active films are formed from 1 or 7 LB monolayers deposited on the alumina substrates equipped with heating meander and interdigitated contacts. LB layers were heat-treated or UV irradiated to remove the insulating surfactant. Sensing properties were studied in a test chamber containing reducing (CO) or oxidizing (NO2) gas in concentrations between 5 and 100ppm in the mixture with dry air. Response current signal (Igas/Iair) vs. temperature and response vs. gas concentration calibration curves were measured. Best response values were obtained with CoFe2O4 devices between 300 and 400°C, being 3 and 10 for 100ppm of CO and 5ppm of NO2, respectively. The response and recovery times of sensors are between 3 and 30min. The Fe2O3 and CoFe2O4 sensors with response of 8 or 10 to 5ppm of NO2 might be of practical applications in the detection of explosives.

Structural characterization and photocatalytic activity of ultrathin TiO 2 films fabricated by Langmuir–Blodgett technique with octadecylamine

TiO 2 thin films with nanometer-scale thicknesses were prepared by the hydrolysis of titanium potassium oxalate using octadecylamine (ODA) Langmuir–Blodgett (LB) films as templates. After optimizing conditions in immersion process, the amount of TiO 2 generated in the ODA LB film was found to be precisely controlled by the number of deposited ODA layers. Morphological measurements showed that uniform TiO 2 film with surface roughness of less than 1.3 nm could be prepared from the monolayer LB films through subsequent heat-treatment process, while generation of cracks became less noticeable on the 5-layer film after heat-treatment at lower holding temperature with slow heating rate. In addition, photocatalytic activities of the TiO 2 films were examined from the decomposition of cadmium stearate (CdSt) LB films and stearic acid (SA) cast films for different time intervals of irradiation with UV light. Atomic force microscopy measurements showed that an almost flat surface of the CdSt LB film changes to a moth-eaten appearance as a result of decomposition under UV light irradiation. Furthermore, the post-heat-treatment at higher temperatures resulted in decreased photocatalytic activity of the TiO 2 film for the decomposition of SA cast film.

Supported Phospholipid Monolayers. The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy

The molecular structure, packing properties, and hydrating water of Langmuir-Blodgett monolayers of the phospholipids 1,2-distearoyl-sn-glyercophosphatidylcholine (DSPC, 18:0 PC), its deuterated an ...

Orientation of Fluorescent Lipid Analogue BODIPY-PC to Probe Lipid Membrane Properties: Insights from Molecular Dynamics Simulations

Single-molecule fluorescence measurements have been used to characterize membrane properties, and recently showed a linear evolution of the fluorescent lipid analogue BODIPY-PC toward small tilt angles in Langmuir-Blodgett monolayers as the lateral surface pressure is increased. In this work, we have performed comparative molecular dynamics (MD) simulations of BODIPY-PC in DPPC (dipalmitoylphosphatidylcholine) monolayers and bilayers at three surface pressures (3, 10, and 40 mN/m) to explore (1) the microscopic correspondence between monolayer and bilayer structures, (2) the fluorophore's position within the membrane, and (3) the microscopic driving forces governing the fluorophore's tilting. The MD simulations reveal very close agreement between the monolayer and bilayer systems in terms of the fluorophore's orientation and lipid chain order, suggesting that monolayer experiments can be used to approximate bilayer systems. The simulations capture the trend of reduced tilt angle of the fluorophore with increasing surface pressure, as seen in the experimental results, and provide detailed insights into fluorophore location and orientation, not obtainable in the experiments. The simulations also reveal that the enthalpic contribution is dominant at 40 mN/m, resulting in smaller tilt angles of the fluorophore, and the entropy contribution is dominant at lower pressures, resulting in larger tilt angles.

Nanopatterning in Langmuir-Blodgett Monolayers of a Thermoresponsive Double Hydrophilic Block Copolymer Studied by Atomic Force Microscopy

The microphase-separation of Langmuir-Blodgett (LB) monolayers of a rhodamine B (RhB) end-labeled double hydrophilic block copolymer (DHBC), RhB-Poly(N,N-dimethylacrylamide)-b-poly(N,N-diethylacryl-amide) (RhB-PDMA(207)b-PDEA177) and the 1:1 segmental mixture of PDEA and RhB-PDMA homopolymers was followed by AFM. The DHBC LB films revealed a loose distribution of nano-aggregates with variable geometries below the lower critical solution temperature (LCST) of PDEA (32 degrees C) and low surface pressure (3 mN m(-1)). By increasing either the temperature above the LCST of PDEA or the surface pressure beyond the immersion regime of PDMA in the subphase (7 mN m(-1)) a dense nanopatterning was obtained. The absence of a corresponding regular nanopatterning in LB films of mixed homopolymers with the same composition highlights the role of the covalent bonding between PDEA and PDMA on the self-segregation of the two blocks at the air-water interface.