Langmuir-Blodgett Monolayers Research Articles

Reversible in situ Control over Monolayer Organization.

Cr2+ and Cr3+ ions are shown to mediate the formation, morphology, and organization of arachidic acid (AA) Langmuir-Blodgett (LB) monolayers. This finding, based on cyclic voltammetry (CV), linear sweep voltammetry (LSV) and fluorescence recovery after photobleaching (FRAP) measurements, show that Langmuir monolayer formation depends on subphase pH and metal ion concentration. Following monolayer deposition on indium tin oxide (ITO), the LB monolayer organization can be modified reversibly through control of the Cr oxidation state, which has not been shown before by other monolayers formed with other divalent metal ions. The dynamics and the mobility of a chromophore (perylene) incorporated into the monolayer sense changes in Cr oxidation state-dependent organization of the LB monolayer. Demonstrating reversible changes in monolayer organization provides an opportunity to control chemical and electron access to the interface support.

Evaluation of the antitumor activity of albendazole using Langmuir-Blodgett monolayers as surface mediated drug delivery system

This study demonstrates the application of Langmuir and Langmuir-Blodgett films as biomimetic drug reservoirs and delivery systems to investigate the effect of an anthelmintic on cancer cell culture. The repurposing of benzimidazole anthelmintics for cancer therapy due to their microtubule-inhibiting properties has gained attention, showing promising anticancer effects and tumor-suppressive properties. Although widely used in medicine, the low aqueous solubility of benzimidazole compounds poses challenges for studying their effects on cancer cells, requiring incorporation into various formulations. Our study demonstrates that incorporating albendazole into stable Palmitic Acid Langmuir monolayers, forming Langmuir-Blodgett films, significantly affects the proliferation of liver carcinoma cells. This report presents the initial findings of the effect of an antitumoral drug on cancer cell culture using a simple and repeatable methodology.

Molecular Templates on Surfaces by Exploiting Supramolecular Chemistry in Langmuir–Blodgett Monolayers

AbstractAttaining precise control over molecular arrangements is of paramount importance for numerous applications in nanotechnology, particularly in constructing molecular templates to accurately immobilize target materials on surfaces. A strategic combination of supramolecular and interfacial chemistry may serve to build a well‐organized molecular network, enabling the subsequent location of target molecules on specific positions of a surface. A supramolecular complex (compound 1) comprised of a melamine unit forming hydrogen bonds with dendritic arms terminated in a coumarin unit is utilized, which readily undergoes photodimerization. The research demonstrates the formation of well‐organized Langmuir films of compound 1 which can be transferred on substrates at low surface pressures adopting a lying‐flat orientation. Upon irradiation of the pristine films at 365 nm the coumarin units undergo photo‐cross linking, leading to the formation of a compact photo‐crosslinked film. Incubation of these photo‐crosslinked films in a solution containing 1‐hexanethiol results in the withdrawal of the melamine and the chemisorption of two thiol molecules per each melamine unit. The nanopores created by the removal of the melamine core are attributed to the disruption of hydrogen bonds in compound 1 by the thiols. This precisely defined molecular network holds significant promise as a template for orchestrating the arrangement of functional materials on surfaces.

Crystallization of Poly(l-lactic acid) on Water Surfaces via Controlled Solvent Evaporation and Langmuir-Blodgett Films.

Solvent evaporation is one of the most fundamental processes in soft matter. Structures formed via solvent evaporation are often complex yet tunable via the competition between solute diffusion and solvent evaporation time scales. This work concerns the polymer evaporative crystallization on the water surface (ECWS). The dynamic and two-dimensional (2D) nature of the water surface offers a unique way to control the crystallization pathway of polymeric materials. Using poly(l-lactic acid) (PLLA) as the model polymer, we demonstrate that both one-dimensional (1D) crystalline filaments and two-dimensional (2D) lamellae are formed via ECWS, in stark contrast to the 2D Langmuir-Blodgett monolayer systems as well as polymer solution crystallization. Results show that this filament-lamella biphasic structure is tunable via chemical structures such as molecular weight and processing conditions such as temperature and evaporation rate.

Langmuir-Blodgett organic films deposition for the formation of conformal 2D inorganic-organic heterostructures

2D inorganic materials offer the opportunity to design and build heterostructures with unprecedented atomic control down to a monolayer. Integrating inorganic 2D layered materials, with the unlimited variety of organic molecules, provides a good platform for basic research and the formation of functional 2D organic-inorganic heterostructures, with tailored optoelectronic properties. In this work, we propose to employ a Langmuir-Blodgett (LB) monolayer deposition technique for the large-scale assembly of 2D inorganic-organic heterostructures. First, the transition-metal dichalcogenidemonolayers (TMDCs), MoS2 and WSe2, are grown by a chemical vapor deposition (CVD)-based method. This is followed by the LB deposition of a monolayer(s) of Zn(II)-5-(4-aminophenyl)-10,15,20-(triphenyl)porphyrin (Zn-TPP(1f)). The morphology, optical and electrical properties were systematically studied for the two TMDC systems. Despite the similarities between both TMDCs, their heterostructures with Zn-TPP(1f) monolayers show a very different optical response; a strong quenching of the photoluminescence intensity is measured for the Zn-TPP(1f)/MoS2 system, an opposite trend, PL enhancement, is obtained for the WSe2 based heterostructure. These results are explained by the differences within their electronic band diagrams, as elucidated using a wide variety of spectroscopic methodologies. The orthogonal impact on the PL emission may serve as a general platform for the formation of 2D organic-inorganic heterostructures, with controlled optical properties.

In Situ Atomic Force Microscopy Observation of Folded-Chain Crystallization of Single Isolated Isotactic Poly(methyl methacrylate) Chains in Langmuir–Blodgett Monolayers

In Situ Atomic Force Microscopy Observation of Folded-Chain Crystallization of Single Isolated Isotactic Poly(methyl methacrylate) Chains in Langmuir–Blodgett Monolayers

Efficient triplet exciton phosphorescence quenching from a rhenium monolayer on silicon

Using phosphorescence lifetime image microscopy (PLIM) measurements we report up to 95% efficient triplet exciton energy transfer from a rhenium(i) fac-tricarbonyl bipyridine complex deposited as a Langmuir–Blodgett monolayer to crystalline silicon.

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production.

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a β-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

Enhanced luminescence and electrical conductivity of polymer ultrathin films doped with TiO2 nanoparticles

The chemical polymerization method was used to prepare nanocomposite materials of polyaniline (PANi) and Titanium dioxide (TiO2) nanoparticles. Ultrathin films were deposited by using the Langmuir Blodgett (LB) Technique. Surface Pressure vs mean molecular area isotherms of nanocomposite materials showed an increase in maximum attained surface pressure with variation in nanoparticle doping concentration. Photoluminescence (PL) spectra were taken for LB multilayers deposited on a quartz substrate, an increase in the intensity of PL peaks with increasing doping concentration of TiO2 nanoparticles. The topography of LB monolayer film under atomic force microscopy (AFM) revealed that films were uniformly and densely deposited on the substrate. Phase imaging ensured the presence of nanoparticles in the polymer matrix. Conductive atomic force microscopy (C-AFM) was used to investigate the local electrical properties of nanocomposite LB films. Current-voltage characteristics showed variation in current with changing doping concentrations of TiO2 nanocomposite in PANi matrix. The work presents new insights in analyzing uniformly deposited ultrathin films of the composites which can be probed by C-AFM to reveal local electric properties by understanding topography-current correlation at the nanoscale. The conductive data analysis could be useful for understanding the current flow through the nanostructure surface.

Atomic force microscopy study of the complexation of sterols and the glycoalkaloid α-tomatine in Langmuir-Blodgett monolayers.

Glycoalkaloids are secondary metabolites produced by plants that aid in their protection from pathogens and pests. They are known to form 1:1 complexes with 3β-hydroxysterols such as cholesterol causing membrane disruption. So far, the visual evidence showcasing the complexes formed between glycoalkaloids and sterols in monolayers has been mainly restricted to some earlier studies using Brewster angle microscopy which were of low resolution showing the formation of floating aggregates of these complexes. This study is aimed at using atomic force microscopy (AFM) for topographic and morphological analysis of the aggregates of these sterol-glycoalkaloid complexes. Langmuir-Blodgett (LB) transfer of mixed monolayers of the glycoalkaloid α-tomatine, sterols, and lipids in varying molar ratios onto mica followed by AFM examination was performed. The AFM method allowed visualization of the aggregation of sterol-glycoalkaloid complexes at nanometer resolution. While aggregation was observed in mixed monolayers of α-tomatine with cholesterol and in mixed monolayers with coprostanol, no sign of complexation was observed for the mixed monolayers of epicholesterol and α-tomatine, confirming their lack of interaction found in prior monolayer studies. Aggregates were observed in transferred monolayers of ternary mixtures of α-tomatine with cholesterol and the phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or egg sphingomyelin (egg SM). The formation of aggregates was found to be less prevalent for mixed monolayers of DMPC and cholesterol containing α-tomatine than it was for mixed monolayers containing egg SM and cholesterol with α-tomatine. The observed aggregates were generally elongated structures, of a width ranging from about 40-70nm.

Probing the effect of nitro-substituents in the modulation of LUMO energies for directional electron transport through 4d6 ruthenium(II)-based metallosurfactants.

Electron-withdrawing nitro-substituents were installed onto terpyridine- and phenanthroline-based metallosurfactants with 4d6 ruthenium(II), which were deposited as Langmuir-Blodgett monolayers aiming to study the feasibility of charge transport in Au|LB|Au junctions. The nitro groups are intended to modulate the energy of the frontier molecular orbitals to near to, or match that of Fermi levels in the gold electrodes. A series of heteroleptic metallosurfactants [RuII(C18OPh-terpy)(X-terpy)](PF6)2 and [RuII(C18OPh-terpy)(X-phen)Cl]PF6 were synthesized, where C18OPh-terpy is the 4'-[4-(octadecyloxy)phenyl]-2,2':6',2''-terpyridine amphiphile common to all species, X-terpy is a terpyridine with-H (1) or-phenyl-NO2 (2) and X-phen is a phenanthroline with-H (3) or-NO2 (4) groups. These metallosurfactants were characterized by experimental and computational methods, and the presence of nitro groups affect more affordable reductions at less negative potentials, as well as slightly more positive oxidations, these changes are less pronounced in species 2 than in 4. Species 1 and 2 showed limited Pockels-Langmuir and Langmuir-Blodgett film formation with lower collapse pressure of 27 mN m-1. In contrast, metallosurfactants 3 and 4 showed enhanced hydrophilicity indicated by higher collapse pressures of ca. 36 mN m-1. The LB monolayers of 3 and 4 were deposited on gold electrodes to form Au|LB|Au junctions and electron transport was measured as I/V curves. The NO2-bearing species 4 showed asymmetric curves associated with directional electron transport with amplitudes up to -2.0 nA and rectification ratios from 5 to 26 between -1 to +1 V and from 3 to 14 between -3 to +3 V.

A Thermodynamic Study on the Interaction between RH-23 Peptide and DMPC-Based Biomembrane Models.

Investigation of the interaction between drugs and biomembrane models, as a strategy to study and eventually improve drug/substrate interactions, is a crucial factor in preliminary screening. Synthesized peptides represent a source of potential anticancer and theragnostic drugs. In this study, we investigated the interaction of a novel synthesized peptide, called RH-23, with a simplified dimyristoylphosphatidylcholine (DMPC) model of the cellular membrane. The interaction of RH-23 with DMPC, organized either in multilamellar vesicles (MLVs) and in Langmuir-Blodgett (LB) monolayers, was assessed using thermodynamic techniques, namely differential scanning calorimetry (DSC) and LB. The calorimetric evaluations showed that RH-23 inserted into MLVs, causing a stabilization of the phospholipid gel phase that increased with the molar fraction of RH-23. Interplay with LB monolayers revealed that RH-23 interacted with DMPC molecules. This work represents the first experimental thermodynamic study on the interaction between RH-23 and a simplified model of the lipid membrane, thus providing a basis for further evaluations of the effect of RH-23 on biological membranes and its therapeutic/diagnostic potential.

Aggregation behaviors of gradient and diblock copoly(2-oxazoline) monolayers at the air-water interface

This study reveals the different behavior of amphiphilic gradient and diblock copolymers based on aliphatic and aromatic 2-oxazolines. The copolymers were synthesized by the polymerization of hydrophilic 2-ethyl-2-oxazoline with hydrophobic 2-(4-octyloxyphenyl)-2-oxazoline. The aggregation behavior of gradient and diblock copolymers with different ratios between hydrophilic and hydrophobic units was explored. Surface pressure (Π)–surface area ( A ) isotherms were measured at the air-water interface. The gradient copolymers produced higher surface pressures and smaller limiting areas, while the diblock copolymers exhibit relatively flexible behaviors. Langmuir–Blodgett monolayers were transferred on the hydrophilic silicon wafer substrates at various surface pressure levels, and the nanostructure of the films was investigated by using atomic force microscopy and X-ray reflectivity measurements. It was observed that the morphologies of gradient and diblock copolymers were similar, indicating that the films form a uniform monolayer. The diblock copolymers produced relatively small changes in thickness upon the compression, compared to the changes in gradient copolymer monolayer films. The thickness of the gradient copolymers which have a high hydrophilic ratio at low surface pressure increases by increasing surface pressure and hydrophobic ratio. But the diblock copolymers do not change the thickness by increasing the surface pressure and hydrophobic ratio. The results indicate that the gradient copolymer monolayers have relatively rigid domains where hydrophilic and hydrophobic parts are smeared. On the other hand, the diblock copolymer has a flexible hydrophilic cushioning effect. This study provides insights into the development of novel materials based on hydrophilic-hydrophobic interactions. • The different behavior of amphiphilic gradient and diblock copolymers based on aliphatic and aromatic 2-oxazolines. • The aggregation behavior of gradient and diblock copolymers with different ratios between hydrophilic and hydrophobic units was explored. • The gradient copolymer monolayers have relatively rigid domains where hydrophilic and hydrophobic parts are smeared. • The diblock copolymer has a flexible hydrophilic cushioning effect.

Characteristics of Phospholipid-Immunosuppressant-Antioxidant Mixed Langmuir-Blodgett Films.

Hemocompatibility is one of the major criteria for the successful cardiovascular applicability of novel biomaterials. In this context, monolayers of certain biomolecules can be used to improve surface biocompatibility. To this end, biocoatings incorporating a phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC), an immunosuppressant (cyclosporine A, CsA), and an antioxidant material (lauryl gallate, LG) were fabricated by depositing Langmuir films onto gold or mica substrates using the Langmuir–Blodgett (LB) technique. These LB monolayers were thoroughly characterized by means of quartz crystal microbalance (QCM), atomic force microscopy (AFM), cyclic voltammetry (CV), and contact angle (CA) measurements. The obtained results indicate that the properties of these LB films are modulated by the monolayer composition. The presence of LG in the three-component systems (DOPC–CsA–LG) increases the molecular packing and the surface coverage of the substrate, which affects the wettability of the biocoating. From the different compositions studied here, we conclude that DOPC–CsA–LG monolayers with a DOPC/CsA ratio of 1:1 and LG molar fractions of 0.50 and 0.75 exhibit improved surface biocompatible characteristics. These results open up new perspectives on our knowledge and better understanding of phenomena at the biomaterial/host interface.

Chemical Imaging of Lipid Segregation: Determining Different Length Scales of Heterogeneity with Compressive-Sensing Sum Frequency Generation Microscopy and Brewster Angle Microscopy

Membranes of various phospholipids may separate into different domains at micrometer length scales at the air-water interface. A significant challenge is to visualize the molecular organization and obtain chemical information on this surface. Langmuir-Blodgett monolayers of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) on fused silica were investigated with compressive sensing sum frequency generation (SFG) microscopy. Before and after mixing, SFG spectra for DSPC and DLPC revealed significant differences, indicating a structural change. Brewster angle microscopy images show phase separations that directly correlate the morphology of phospholipid mixtures with SFG images. Exploiting vibrational contrast in SFG images, exchange between the two phases was discovered, and quantitative thermodynamic analysis of lipid compositions in liquid-condensed and liquid-expanded (LE) phases was provided. Local SFG spectra reveal significant differences from one another, indicating the heterogeneity resulting from domain areas with distinct molecular orientation and conformation. Further heterogeneity across the domain boundary was presented on a finer scale, revealing an effect on DLPC due to the condensed phase DSPC, and the terminal methyl of DLPC perturbs the ordering of DSPC in the LE phase. This work demonstrates the heterogeneity of a two-dimensional binary lipid system mainly due to the aliphatic chain length and transition temperature difference.

(Digital Presentation) Photopolymerization of Tetraazaporphyrin Based Surfactants and Mesogens

Intermolecular interactions between tetraazaporphyrin (TAP) macrocycles are less dominated by π-π stacking interactions than interactions between their larger phthalocyanine derivatives. Consequently, the self-organization and -assembly of TAPs is more widely altered by changes to the type of complexed metal ion and substitution pattern. Presented here are different molecular designs for mesomorphic and amphiphilic TAPs that could be cross-linked by “click” chemistry and by the photo and thermal polymerization of diacetylene groups. All compounds were studied by polarized optical microscopy, thermal analysis, and X-ray diffraction to probe their (meso)phase structures and transitions and by the Langmuir-Blodget method to quantify their interfacial properties. Potential applications for these compounds include their use as active materials in sensors, functional coatings for metal oxide nanoparticles, and building blocks for dye nanoparticles.Topochemical Polymerization of a Nematic Tetraazaporphyrin Derivative to Generate Soluble Polydiacetylene Nanowires. M. Nazir Tahir, et al., 2019, Langmuir, 35(47) 15158−15167.Self-organization of Porphyrins and Phthalocyanines in two and three dimensions by S. Holger Eichhorn and Elmahdy Abdulhamied, in “Handbook of Porphyrin Science: Towards Tuned Properties of Porphyrinoids”, Vol. 42, Karl M. Kadish, Kevin M. Smith, Roger Guilard, Eds., World Scientific, Singapore, 2016, 173-232.Cross-linking of discotic tetraazaporphyrin dyes in 2 and 3 dimensions by “click” chemistry. Himadri Kayal, et al., 2013, J. Mater. Chem. C, 1(42), 7064-7072.Face- and edge-on orientations of octa-acid and -alcohol substituted tetraazaporphyrins in langmuir and Langmuir-Blodgett monolayers. M. M. Ahmida, et al., 2013, Soft Matter 9 (3), 811-9.

Quantitating the Binding Energy of Metal Ions to Langmuir-Blodgett Monolayers: The Copper(II)-Octadecylphosphonic Acid System.

We report on the formation and organization of a Cu2+-complexed octadecylphosphonic acid (ODPA) monolayer formed by Langmuir-Blodgett deposition. The formation of the Cu-complexed monolayer is seen to depend sensitively on the subphase pH and Cu2+ concentration, and it is possible to form a monolayer containing the regions of complexed and free ODPA. From the pressure-area isotherm data for these monolayers, we determine the equilibrium constant and free energy of formation for the Cu2+-ODPA complex, ΔG = -22.5 kJ/mol.

Triptycene-Based Molecular Rods for Langmuir-Blodgett Monolayers.

Herein we introduce fully modular synthesis leading to three representative examples of rigid molecular rods that are intended to form sturdy monolayers on various surfaces. These molecules contain two triptycene units that are designed to interlock into a compact "double-decker" structure. Two of the three final products provided suitable crystals for X-ray diffraction (analyzed on synchrotron), allowing deeper insight into packing in the 3-D crystal lattice. The acidity of all three compounds were determined by capillary electrophoresis, and the pKa values ranged between 2.06-2.53. All three rigid rods easily formed Langmuir-Blodgett monolayers (LBMs) on the water-air interfaces, with the area per molecule equal to 55-59 Å2 /molecule, suggesting tight intermolecular packing. The thickness of all three films reached ∼19 Å after transfer to a gold (111) surface, meaning that individual molecules are tilted maximally 38° from the axis perpendicular to the surface. The structure of one of these films on a gold (111) surface was visualized by AFM. These geometrically unique molecules represent promising platforms with a wide scope of applicability in the supramolecular architecture.

Protective Langmuir–Blodgett coatings based on long-chain fatty acids

The morphology and tribotechnical properties of Langmuir–Blodgett (LB) monolayers and multilayers of some fatty acids, such as nervonic (NA), cerotic (CA) and montanic acids (OCA), was investigated. It was established that the wear resistance of silicon surface coated by LB monolayer of OCA is greater by 11 and 1.3 times in comparison with the LB monomolecular films of NA and CA, respectively. The multilayer of OCA, formed by 140 layers, increases the stability of steel surface by 35 times.

Morphology of Mixed Langmuir and Langmuir-Schaefer Monolayers with Covered CdSe/CdS/ZnS Quantum Dots and Arachidic Acid.

The process of formation of a Langmuir-Schaefer (LS) matrix based on a mixed monolayer of arachidic acid (AA) and 8 nm CdSe/CdS/ZnS quantum dots (QDs) stabilized by molecules of trioctylphosphine oxide (TOPO) was investigated. The change in the morphology, monolayer compressibility, and area per elementary cell of the created mixed monolayers, depending on the ratio of the components, was studied. It is shown that the change in the morphology of Langmuir-Blodgett (LB) monolayers begins to occur at a ratio between the number of QDs and AA molecules of 1:24. Dendrimeric structures with a thickness of the order of 30-40 nm appear in the mixed monolayer when LB film deposition was carried out above the collapse surface pressure of a Langmuir film from only TOPO-covered QDs. Information on the dependence of the morphology of such structures on the molar ratio of the components is necessary for the production of ordered 2D nanostructures containing 0D and 1D objects with quantum bonds. Such nanostructures can be used in nanoelectronic and optoelectronic devices as a sensitive sensor element. The obtained results would be relevant for any type of spherical shape nanoparticles.