Langmuir-Blodgett Monolayers Research Articles

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.

A Study on the Electrochemical Properties of Langmuir-Blodgett Monolayer Film Mixed with Polyamic Acid and Sphingomyelin

We investigated an electrochemical properties for Langmuir-Blodgett (LB) monolayer films of sphingomyelin and polyamic acid(1:1 molar ratio) mixture. LB monolayer films of mixture was deposited by the LB method on the indium tin oxide(ITO) glass. The electrochemical properties measured by cyclic voltammetry with three-electrode system in KClO4 solution. The current of reduction and oxidation range was measured from 1650 mV to -1350 mV, continuously. The scan rates were 50, 100, 150, 200 and 250 mV/s, respectively. As a result, LB monolayer films of sphingomyelin and polyamic acid mixture was appeared on irreversible process caused by the reduction current from the cyclic voltammogram. Diffusion coefficient (D) in the sphingomyelin and polyamic acid mixture was calculated 2.67 cms×10, 5.23 cms×10 at 0.1 N and 0.2 N KClO4 solutions, respectively.

Partitioning of doxorubicin into Langmuir and Langmuir–Blodgett biomimetic mixed monolayers: Electrochemical and spectroscopic studies

The partitioning of doxorubicin, a potent anticancer drug, was examined in relation to the composition and structural properties of the biomimetic molecular films. We have systematically studied the passive permeation of doxorubicin across the two-component monolayers of different organization, following the drug interactions with the hydrophilic part of the monolayer and then, with its hydrophobic moiety, separately. We investigated independently these two types of the interactions, likely competing in a native membrane, respectively for the Langmuir monolayers on an aqueous subphase and for Langmuir–Blodgett monolayers on gold, exposed to the drug in an aqueous solution. This gave us a unique possibility to monitor the drug interactions with different regions of biomimicking film. The overall picture emerging from these results suggests that the hydrophobicity of monolayer interior allows for the penetration and accumulation of the drug, while its organization controls the rate and amount of doxorubicin molecules through such films.

Thermodynamic Aspects of Cholesterol Effect on Properties of Phospholipid Monolayers: Langmuir and Langmuir–Blodgett Monolayer Study

Cholesterol is an important component of lipid rafts in mammalian cell membranes. Studies of phospholipid monolayers containing cholesterol provide insight into the role of cholesterol in regulating the properties of animal cells, raft stability, and organization. In this contribution, a study of the characteristics of binary Langmuir monolayers consisting of phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), and cholesterol (Chol), was conducted on the basis of the surface pressure-area per molecule (π-A) isotherms. Analysis of the results obtained provided information on the mean molecular area, the excess Gibbs energy of mixing, and condensation in the monolayer. The mixed monolayers were also deposited onto the mica plates and investigated by the contact angle measurements of water, formamide, and diiodomethane. The contact angles allowed calculating surface free energy of the films from the van Oss et al. approach. It was found that cholesterol determines the molecular packing and ordering of the monolayers closely connected with the kind of phospholipid. This is reflected in the values of surface free energy of the model membranes. From the thermodynamic analysis of phospholipid/cholesterol/liquid interactions, one may draw conclusions about the most favorable composition (stoichiometry) of the binary film which is especially important in view of the lipid rafts formation.

Tribological behavior of composite Langmuir–Blodgett films of triacontanoic acid

Tribological behavior of composite Langmuir–Blodgett monolayers of triacontanoic acid (TA) with molybdenum disulfide (MoS2) particles was studied. It was found that the highest wear stability against mechanical influence of steel indenter was observed for the composite coating formed from suspension of MoS2 particles and TA in equivalent weight ratio.

Understanding and Controlling Morphology Formation in Langmuir–Blodgett Block Copolymer Films Using PS-P4VP and PS-P4VP/PDP

This contribution offers a comprehensive understanding of the factors that govern the morphologies of Langmuir-Blodgett (LB) monolayers of amphiphilic diblock copolymers (BCs). This is achieved by a detailed investigation of a wide range of polystyrene-poly(4-vinyl pyridine) (PS-P4VP) block copolymers, in contrast to much more limited ranges in previous studies. Parameters that are varied include the block ratios (mainly for similar total molecular weights, occasionally other total molecular weights), the presence or not of 3-n-pentadecylphenol (PDP, usually equimolar with VP, with which it hydrogen bonds), the spreading solution concentration ("low" and "high"), and the LB technique (standard vs "solvent-assisted"). Our observations are compared with previously published results on other amphiphilic diblock copolymers, which had given rise to contradictory interpretations of morphology formation. Based on the accumulated results, we re-establish early literature conclusions that three main categories of LB block copolymer morphologies are obtained depending on the block ratio, termed planar, strand, and dot regimes. The block composition boundaries in terms of mol % block content are shown to be similar for all BCs having alkyl chain substituents on the hydrophilic block (such as PS-P4VP/PDP) and are shifted to higher values for BCs with no alkyl chain substituents (such as PS-P4VP). This is attributed to the higher surface area per repeat unit of the hydrophilic block monolayer on the water surface for the former, as supported by the onset and limiting areas of the Langmuir isotherms for the BCs in the dot regime. 2D phase diagrams are discussed in terms of relative effective surface areas of the two blocks. We identify and discuss how kinetic effects on morphology formation, which have been highlighted in more recent literature, are superposed on the compositional effects. The kinetic effects are shown to depend on the morphology regime, most strongly influencing the strand and, especially, planar regimes, where they give rise to a diversity of specific structures. Besides film dewetting mechanisms, which are different when occurring in structured versus unstructured films (the latter previously discussed in the literature), kinetic influences are discussed in terms of chain association dynamics leading to depletion effects that impact on growing aggregates. These depletion effects particularly manifest themselves in more dilute spreading solutions, with higher molecular weight polymers, and in composition regimes characterized by equilibrium degrees of aggregation that are effectively infinite. It is by understanding these various kinetic influences that the diversity of structures can be classified by the three main composition-dependent regimes.

Structure, morphology and interfacial behaviour of ethylene/methacrylate copolymers

Two alpha-diimine monometallic Ni catalysts, catalyst 1 and catalyst 2, were studied in the copolymerization reaction of ethylene (E) with methyl methacrylate (MMA). The reaction products of these reactions were characterized using not only the usual techniques such as NMR, GPC and DSC but also the Langmuir balance and AFM. 1H and 13C NMR spectra revealed that both copolymers and mixtures of E and MMA (in a range of 20–70 mol% of MMA) could be obtained with these catalysts. A better insight of the products was possible with 1H DOSY NMR. Since p(E-MMA) copolymers comprising hydrophobic PE blocks and surface active PMMA blocks are amphiphilic, the Langmuir monolayer technique provided further information on the two-dimensional phase behavior of copolymer monolayers at the air-water interface. Additionally, AFM topographic images of the Langmuir-Blodgett (LB) monolayers deposited on mica substrates clearly showed that the morphology of the copolymer LB monolayer is quite different from the corresponding mixture of PE and PMMA homopolymers. These techniques together with molecular modeling calculations allowed us to conclude that with catalyst 2 it was possible to obtain a true block copolymer by a mechanism involving a cationic ester-enolate metal complex as the active species.

Enhanced photocurrent generation from an electrostatically self-assembled film of sandwich-type tetracadmium(II) tungstophosphorate/hemicyanine

A photoelectric conversion film consisting of sandwich-type tetracadmium(II) tungstophosphorate [P2W18Cd4(H2O)2O68]10− (P2W18Cd4) and a bichromophore hemicyanine of (E)-1,1′-(hexane-1,6-diyl)bis(4-((E)-2-(4-(dimethylamino)naphthalen-1-yl)vinyl)pyridinium) bromide (N6) was prepared by electrostatic self-assembly technique, and characterized by UV–vis spectroscopy, cyclic voltammetry and photoelectrochemistry. The UV–visible spectra showed that the film was uniformly deposited and N6 molecules formed J-aggregations in the films. The cyclic voltammetry showed that the P2W18Cd4 and N6 in the films were electrochemically active with surface-confined characteristics. As irradiated with white light, the (P2W18Cd4/N6) monolayer film gave stable cathodic photocurrent that is 2.4- and 6.7-fold as great as electrostatically self-assembled monolayer film of (P2W18Cd4/H6) {H6=(E)-1,1′-(hexane-1,6-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide} and Langmuir–Blodgett monolayer film of analogous hemicyanine (E)-4-(2-(4-(dimethylamino)naphthalen-1-yl)vinyl)-1-octadecylpyridinium iodide. The photocurrent action spectrum indicated that the cathodic photocurrent was generated based on charge transfer excitation of the N6 in the film. The effects of applied bias voltages, electron acceptor, and layer numbers of the (P2W18Cd4/N6)n films on photocurrent generation of (P2W18Cd4/N6)n film were examined.

Relations between the structure, symmetry and the energy mechanism of the polar-organic molecule ultra-films during the tribology

The molecular dynamics simulation has been used to study the tribological properties of the seven kinds mixed Langmuir-Blodgett (LB) monolayers, which composed of the fatty acids CnH2n+1COOH and C17H31COOH (n=12,13,14,15,16,17) by the ratio of 1:1, and the monolayers compoesd by C16H33COOH. The results showed that: (1) the shear pressure of mixed LB monolayers decreased as the chain-length of molecules Cn increased. The friction force main come from the coulombic interaction between the monolayers. (2) the arrangement of the two kinds of tails of the molocules influence the friction character. The shear pressure is smaller when all the tails are isotactic-arrangement than anisotactic-arrangemnet, espacilly when the differrence of the chain-length is one C-C bondlength. (3) The shear pressure decreseased as the chainlength decreased, and the friction froce main come from the coulombic interaction, when tails of the up and down isotactic monolayers which are composed of one kind molecule are mirror symmetry, or the friction froce main come from the van der wasls (VDW) interaction when tails are central symmtry.

Face- and edge-on orientations of octa-acid and -alcohol substituted tetraazaporphyrins in Langmuir and Langmuir–Blodgett monolayers

Polyaromatic dyes, such as phthalocyanines and tetraazaporphyrins (TAPs), arrange with an edge-on orientation of the macrocycle in almost all Langmuir and Langmuir–Blodgett (LB) monolayers, especially when condensed at higher surface pressures. Presented here is a new amphiphilic TAP that contains eight terminal carboxylic acid groups attached to the macrocycle via decyl spacers, which forms stable Langmuir and LB monolayers with a flat-on orientation of the macrocycle. A spider-like molecular conformation is proposed based on the measured surface area per molecule and film thickness determined by ellipsometry and AFM. This conformation allows all eight terminal carboxylic acid and carboxylate groups to attach to the polar interface while the aromatic macrocycle sits on top of the orthogonal decyl chains to generate a micro-segregated layer structure. Octa-acid TAPs with shorter alkyl spacers form edge-on structures in Langmuir monolayers upon compression and are not free of 3-dimensional aggregates based on observations by Brewster angle microscopy (BAM). All octa-acid TAPs are prone to spontaneous self-assembly that is attributed to strong interactions between carboxylic acid groups. Aggregation and spontaneous self-assembly can be minimized by deprotonation of the carboxylic acid groups but all octa-acid TAPs become water soluble when more than four carboxylic acid groups are deprotonated at a pH higher than 8. Cu(II) chelates of the octa-acid TAPs were also prepared but are unsuitable for Langmuir monolayer formation because of insufficient solubility in spreading solvents. Langmuir and LB monolayers of octa-hydroxy TAPs with propyl and undecyl spacers were also studied for comparison. An edge-on orientation of the macrocycle is obtained at higher surface pressures for both TAPs, although a flat-on orientation of the macrocycle in a spider-like confirmation may be present at low surface pressure for the derivative with undecyl spacers. No mesophases are formed by any of the octa-acid and -hydroxy TAPs as confirmed by polarized optical microscopy and thermal analysis measurements. In fact, of the octa-acid TAPs, only the derivative with the longest spacer (decyl) melts at a temperature below the onset of thermal decomposition.

Identification of molecular recognition of Langmuir–Blodgett monolayers using surface-enhanced Raman scattering spectroscopy

A facile and effective approach for SERS identification of molecular recognition in Langmuir-Blodgett monolayers on smooth substrates was developed by spreading Ag nanoparticles on ordered alkyl chains in the monolayers, which acted as a spacer layer to separate analytes of interest from direct contact with active substrates.

Photonic Characteristics of Langmuir-Blodgett Self-Assembled Monolayers of Colloidal Silica Particles

Monodispersed colloidal crystals based on silica sub-micrometric particles were synthesized using the Stober–Fink–Bohn process. The control of nucleation and coalescence result in improved characteristics such as high sphericity and very low size dispersion. The resulting silica particles show characteristics suitable for self-assembling across large areas of closely-packed 2D crystal monolayers by an accurate Langmuir-Blodgett deposition process on glass, fused silica and silicon substrates. Due to their special optical properties, colloidal films have potential applications in fields including photonics, electronics, electro-optics, medicine (detectors and sensors), membrane filters and surface devices. The deposited monolayers of silica particles were characterized by means of FESEM, AFM and optical transmittance measurements in order to analyze their specific properties and characteristics. We propose a theoretical calculation for the photonic band gaps in 2D systems using an extrapolation of the photonic behavior of the crystal from 3D to 2D. In this work we show that the methodology used and the conditions in self-assembly processes are decisive for producing high-quality two-dimensional colloidal crystals by the Langmuir-Blodgett technique.

Interfacial behavior of pH responsive ampholytic heteroarm star block terpolymers

We report the pH-controlled surface behavior of amphiphilic PSn(P2VP-b-PAA)n heteroarm star block terpolymers, (n = number of arms, 9 and 22), bearing amphoteric diblock arms with varying polyvalent ionic charges (i.e. negative, positive, and zwitterionic) at the air–water and air–solid interfaces. We investigated the assembly of these pH-sensitive star terpolymers in Langmuir and Langmuir–Blodgett monolayers under different pH conditions of the subphase. The π-A isotherms acquired at variable pH conditions revealed a distinct aggregation behavior of surface micelles which is dependent on the ionization of the polyelectrolyte blocks and the number of arms. The star block copolymer with a small number of arms (n = 9) was found to exhibit a strong pH-dependent phase transformation under compression. The pH responsive (zwitterionic) behavior results in changes in surface morphologies from circular micelles to complex labyrinth structures. In contrast, star polymers with a larger number of arms (22) and a crowded branched architecture show stable circular domain morphology without the internal reorganization under variable conditions. The observed variety of surface behaviors is attributed to the fine balance of intra molecular interactions caused by the highly branched chain architecture composed of both acidic and basic blocks. This study proposes a novel strategy for pH induced tunable surface assembly of responsive ultrathin films with multicompartments. We indeed demonstrated that the enhanced responsiveness of star polymers to wide pH range and their diverse morphology at the interface is enabled by coupling amphoteric concept and tailored molecular architecture.

Mechanical Control of Molecular Aggregation and Fluorescence Switching/Enhancement in an Ultrathin Film

Optical responses of molecular aggregates and assemblies are often different from that of the individual molecules. Self-assembly approaches provide little physical control on the extent of aggregation. Mechanical compression of amphiphilic molecules (with chromophore/fluorophore head groups) at the air-water interface, followed by transfer as Langmuir-Blodgett (LB) films, should prove to be an elegant route to molecular assemblies with systematically tunable aggregation and optical responses. This concept is demonstrated using monolayer LB films of a diaminodicyanoquinodimethane (DADQ)-based amphiphile fabricated at different surface pressures. Films deposited above a threshold pressure exhibit a strong blue-shift in the absorption and fluorescence relative to those deposited below; computational investigations suggest that this is due to the formation of 2-dimensional close-packed assemblies. Significantly, the blue emission of the films deposited above the threshold pressure increases with compaction, demonstrating aggregation-induced fluorescence enhancement in ultrathin films, a phenomenon well-established in crystals and nanocrystals of selected classes of molecules including the DADQs. The sharp contrast with aggregation-induced fluorescence quenching observed with most dye molecules is illustrated by a parallel investigation of LB films of a hemicyanine-based amphiphile. The present study illustrates the efficacy of simple mechanical compression and the LB technique in fabricating ultrathin films with tailored supramolecular assembly and optical responses.

Growth and time dependent alignment of KCl crystals in Hemoglobin LB monolayer

Nature and organism often use the biomineralization technique to build up various highly regular structures such as bone, teeth, kidney stone etc., and recently this becomes the strategy to design and synthesis of novel biocomposite materials. We report here the controlled crystallization of KCl in Langmuir and Langmuir Blodgett (LB) monolayer of Hemoglobin (Hb) at ambient condition. The nucleation and growth of KCl crystals in Hb monolayer has temporal and KCl concentration dependency. The growth of KCl crystals in LB film of Hb has distinct behavior in the alignment of crystals from linear to fractal like structures depending on growth time. The crystallographic identity of the biomineralized KCl crystal is confirmed from HR-TEM, XRD, and from powder diffraction simulation. Our results substantiated that the template of Langmuir monolayer of proteins plays a crucial role in biomineralization as well as in designing and synthesizing of novel biocomposite materials.

Synthesis of Donor-σ-Perylenebisimide-Acceptor Molecules Having PEG Swallowtails and Sulfur Anchors

Donor-σ-Acceptor (D-σ-A) molecules, arrayed in a monolayer between electrodes, can serve as molecular rectifiers. Using perylene-3,4,9,10-tetracarboxylic bisimide (PBI) as the acceptor allows the attachment of the donor group to one imide nitrogen and a solubilizing swallowtail, normally a long (e.g., C(19)) alkane connected at midchain, on the other. Such an alkyl tail facilitates the formation of Langmuir-Blodgett (LB) monolayers. We have employed several modified swallowtails to make new D-σ-A molecules: poly(ethylene glycol) (PEG) swallowtails with 6 ether oxygens or with 4 ether oxygens to promote hydrophilicity in orienting LB monolayers, and alkyl swallowtails ending with sulfur anchors (thioacetate, thiol, or methyl disulfide) to stabilize attachment of the D-σ-A molecules to gold electrodes. The preparation and characterization of D-σ-A molecules containing combinations of these swallowtails with pyrene, ferrocene, and tetramethylphenylenediamine donor groups is described.

Preparation of porous monolayer film by immersing the stearic acid Langmuir–Blodgett monolayer on mica in salt solution

Porous materials have drawn attention from scientists in many fields such as life sciences, catalysis and photonics since they can be used to induce some materials growth as expected. Especially, porous Langmuir–Blodgett (LB) film is an ideal material with controlled thickness and flat surface. In this paper, stearic acid (SA), which has been extensively explored in LB film technique, is chosen as the template material with known parameters to prepare the LB film, and then the porous SA monolayer film is obtained by means of etching in salt solution. The main etching mechanism is suggested that the cations in the solution block the electrostatic interaction between the polar carboxyl group of SA and the electronegative mica surface. The influencing factors (such as concentration of salt solution, valence of cation and surface pressure) of the porous SA film are systematically studied in this work. The novel method proposed in this paper makes it convenient to prepare porous monolayer film for designed material growth or cell culture.

Various Nanostructures on Macroscopically Large Areas Prepared by Tunable Ion-Swelling

Various nanostructures were fabricated by ion irradiation on large area (100) Si surfaces covered by colloidal Langmuir-Blodgett films as nanolithographic masks. The ordered structure of the Langmuir-Blodgett monolayer composed from spherical Stöber silica particles of 200 nm and 450 nm diameter offer the possibility to form local surface swelling patterns during the ion bombardment step. Utilizing the dependence of the surface morphology on the irradiation parameters the tunability of nanostructuring was studied for 40 keV Ar+ and 500 keV Xe2+ ions. We show that the periodicity of the resulted surface pattern is determined by the size of the masking particles, while the height of nanostructures can be tuned by the ion fluence. The quality of projection of the nanomask contours to the substrate-the contrast of masking-can be set by choosing appropriate ion energy, thereby determining the curvature of the surface pattern. Moreover, deformation of the nanomask due to ion-nanoparticle interactions should be taken into account since these effects can be also utilized for tailoring various structures. The silica masking layers before and after ion irradiation and the resulting Si surface patterns were investigated by field emission scanning electron microscopy and atomic force microscopy analysis.

Growth of CdS nanocrystallites on graphene oxide Langmuir–Blodgett monolayers

Large area GO–Cd composite Langmuir–Blodgett monolayers were transferred onto Si substrate by introducing Cd2+ ions into the subphase. The changes in the behaviour of the Langmuir monolayer isotherm in the presence of Cd2+ ions are attributed to changes in the microstructure and density of the GO sheets on the subphase surface. The uptake of Cd onto the GO monolayers and the effect of subsequent sulphidation were investigated by AFM, FTIR, Raman, XPS and HRTEM techniques. The incorporation of Cd into the GO monolayers causes some overlapping of sheets and extensive formation of wrinkles. Sulphidation of the GO–Cd sheets results in the formation of uniformly distributed CdS nanocrystallites on the entire basal plane of the GO monolayers. The de-bonding of Cd with oxygen functional groups results in a reduction of the wrinkles. The GO sheets function primarily as a platform for the interaction of metal ions with oxygen functionalities and their structure and characteristic features are not affected by either uptake of Cd or formation of CdS.

Regular ZnO nanopillar arrays by nanosphere photolithography

Highly regular vertical ZnO nanopillar arrays were hydrothermally grown through a nucleation window pattern generated by nanosphere photolithography. The in-plane intensity modulation of the exposing ultraviolet light in the photoresist was performed by Stöber silica or polystyrene nanospheres in the masking Langmuir–Blodgett monolayer. By comparing six different nanosphere diameters in the 180–700nm range only those with diameter above the exposure wavelength of 405nm generate a pattern in the thin photoresist layer. The pattern quality is improving with increasing diameter, therefore, the masking for nanopillar growth was demonstrated with 700nm polystyrene nanospheres. The results of the nanosphere photolithography were supported by finite-difference time-domain calculations. This growth approach was shown to have the potential for low-cost, low-temperature, large area fabrication of ZnO pillars or nanowires enabling a precise engineering of geometry.