Spread Monolayers Research Articles

Highly sensitive gas sensor using hierarchically self-assembled thin films of graphene oxide and gold nanoparticles

In this study, we fabricated hierarchically self-assembled thin films composed of graphene oxide (GO) sheets and gold nanoparticles (Au NPs) using the Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) techniques and investigated their gas-sensing performance. First, a thermally oxidized silicon wafer (Si/SiO2) was hydrophobized by depositing the LB films of cadmium arachidate. Thin films of ligand-capped Au NPs and GO sheets of the appropriate size were then sequentially transferred onto the hydrophobic silicon wafer using the LB and the LS techniques, respectively. Several different films were prepared by varying the ligand type, film composition, and surface pressure of the spread monolayer at the air/water interface. Their structures were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and their gas-sensing performance for NH3 and CO2 was assessed. The thin films of dodecanethiol-capped Au NPs and medium-sized GO sheets had a better hierarchical structure with higher uniformity and exhibited better gas-sensing performance.

Polymeric Nanoparticles Limit the Collective Migration of Cellular Aggregates.

Controlling the propagation of primary tumors is fundamental to avoiding the epithelial to mesenchymal transition process leading to the dissemination and seeding of tumor cells throughout the body. Here we demonstrate that nanoparticles (NPs) limit the propagation of cell aggregates of CT26 murine carcinoma cells used as tumor models. The spreading behavior of these aggregates incubated with NPs is studied on fibronectin-coated substrates. The cells spread with the formation of a cell monolayer, the precursor film, around the aggregate. We study the effect of NPs added either during or after the formation of aggregates. We demonstrate that, in both cases, the spreading of the cell monolayer is slowed down in the presence of NPs and occurs only above a threshold concentration that depends on the size and surface chemistry of the NPs. The density of cells in the precursor films, measured by confocal microscopy, shows that the NPs stick cells together. The mechanism of slowdown is explained by the increase in cell–cell interactions due to the NPs adsorbed on the membrane of the cells. The present results demonstrate that NPs can modulate the collective migration of cells; therefore, they may have important implications for cancer treatment.

Tailoring plasmonic response by Langmuir–Blodgett gold nanoparticle templating for the fabrication of SERS substrates

Nanoparticle self-assembly is a robust and versatile strategy for the development of functional nanostructured materials, offering low-cost and scalable methods that can be fine-tuned for many different specific application. In this work, we demonstrate a pathway for the fabrication of tailorable quasi-two-dimensional lattices of gold nanoparticles to be used in Surface Enhanced Raman Scattering (SERS) detection of biomolecules. As a first step, nanoparticles are spread as a monolayer at the water/air interface, compressed to a target lateral density in a Langmuir–Blodgett technique, and transferred to a properly functionalized substrate surface. Once firmly adhered to the substrate, the lattice of nanoparticles can be directly used or be further processed using electroless gold deposition to let the nanoparticle grow thus tuning the plasmonic response and SERS enhancement. Compared to direct deposition or self-assembly methods, our protocol enables to obtain consistent results and much higher coverage of Au nanoparticles thanks to the active control of the surface pressure of the spread monolayer.

Quantum-chemical analysis of condensed monolayer phases of N-alkanoyl-substituted alanine at the air/water interface

The thermodynamic and structural parameters of monolayers of N-alkanoyl-substituted alanine with 8–17 methylene fragments in the alkyl chain are calculated on the basis of the quantum chemical semiempirical PM3 method. Four optimized structures of monomers are obtained. The energetically most preferred conformer is used for construction of dimers with “sequential” and “parallel” orientation of the hydrophilic parts of the monomers and tetramers with square structure. The hydrophobic chains of N-alkanoyl-substituted alanine are found to be inclined in the dimers to the normal of the spread monolayer: δ = 10° and φ = 43°.Comparison of clusterization Gibbs’ energy for small clusters suggests that 2D film formation takes place by formation of linear associates comprised of dimers with “parallel” orientation of the hydrophilic groups. The temperature increase causes growth of linear 1D clusters leading to the dendritic structure of the monolayer. The parameters of the unit cell of 2D film are calculated: a = 4.65 Ǻ, b = 6.20 Ǻ, the angle between the sides of the cell θ = 105˚. The tilt angle of amphiphilic molecules to the normal of the air/water interface is t = 47°. The calculated parameters of the unit cell are in good agreement with the existing experimental data. The spontaneous clusterization threshold for N-alkanoyl-substituted alanine at the air/water interface is calculated to be 15 carbon atoms in the chain at 288 K. This is only 2 carbon atoms more than the experimental value and within the margin of error of the model used for the calculation.

Dilational surface elasticity of spread monolayers of pulmonary lipids in a broad range of surface pressure

A new approach to the analysis of nonlinear surface dilational rheological properties was developed with the aim to enlarge the accessible range of surface pressures. The dilational surface elasticity of spread monolayers of DPPC, cholesterol, DMEA and their mixtures was determined in the region of low surface tensions (less than 10 mN/m) corresponding to the state of pulmonary surfactants at the lung interface. The dilational surface elasticity of pure DPPC monolayer proved to be high (∼200 mN/m) up to the collapse. The addition of cholesterol, DMEA or their mixture to the DPPC monolayer increased the dynamic surface elasticity, but could decrease significantly the surface pressure of the monolayer collapse if the concentration of additions exceeds an optimal value.

Spreading rate and dispersion behavior of evaporation-suppressant monolayer on open water surfaces: Part 1 – At zero wind stress

Effective evaporation suppression by a monolayer film is largely dependent on the maintenance of that film over as much of the water surface as practically possible. Hence, an autonomous system capable of adaptive re-application of monolayer according to the prevailing wind conditions in order to satisfy this requirement is highly desirable. Key to the design and functioning of a system of this nature is a fundamental understanding of the spatial movement/distribution characteristics of the monolayer material. To ‘bridge’ between centimeter-scale, clean room laboratory experimentation (e.g. with a Petri dish) and field conditions (i.e. hectare-scale open water storages), the spreading rate and dispersion behavior of different application quantities of octadecanol monolayer (in water-emulsion) was investigated on 0.3m, 2m and 6m diameter laboratory water tanks under zero wind stress. A standard formula for spreading is shown to be applicable to ‘bulk’ monolayer from a center point on circular, open water surfaces of scale 0.3m, 2m and 6m diameter. Under still conditions, monolayer spread (as expected) in a uniform circular pattern outwards, centered on the point of application. ‘Bulk’ application involved the placement of enough material to form layers 1×, 3× or 6× monomolecular. Hitherto this has not been demonstrated with either these ‘overdosed’ (3×, 6×) material quantities or at these scales, scales which come closer to those required for monolayer application for the evaporation mitigation of practical open water storages. An empirical relationship for spreading rate has been developed as a function of both time and storage size.

Spreading rate and dispersion behavior of evaporation-suppressant monolayer on open water surfaces: Part 2 – Under wind stress

Wind causes migration and eventual removal (dispersal or beaching) of evaporation-suppressing monolayer on open-water storages. Hence, an autonomous system capable of adaptive re-application of monolayer according to the prevailing wind conditions is highly desirable. Key to the design and functioning of such a system is a fundamental understanding of the spatial movement/distribution characteristics of the monolayer material. To ‘bridge’ between centimeter-scale, clean room laboratory experimentation (e.g. Petri dish–scale in a wind tunnel) and field conditions (i.e. hectare-scale open-water storages), the drift velocity and spreading behavior of C18OH monolayer (in water-emulsion), applied continuously during constant wind stress, were investigated on a 6m-diameter indoor water tank for wind speeds in the range 4–8m/s. Monolayer was found to spread in a teardrop shape initially, which evolved into a wedge shape whose close-to-straight edges were detectable visually due to the wave-damping effect of the monolayer. The internal angle of the wedge decreased with increasing wind speed, consistent with the force equilibrium between the lateral force of the monolayer spreading outwards and the increasing shear imposed with increasing wind speed. The relationship between internal angle of the wedge and wind velocity was a power law. The widely-accepted spreading kinetics formula was used to derive an empirical relationship for the drift velocity that is a power law with respect to the wind speed. This model was compared with the experimental data, with a modest degree of agreement.

Inferring single-cell behaviour from large-scale epithelial sheet migration patterns.

Cell migration plays an important role in a wide variety of biological processes and can incorporate both individual cell motion and collective behaviour. The emergent properties of collective migration are receiving increasing attention as collective motion's role in diseases such as metastatic cancer becomes clear. Yet, how individual cell behaviour influences large-scale, multi-cell collective motion remains unclear. In this study, we provide insight into the mechanisms behind collective migration by studying cell migration in a spreading monolayer of epithelial MCF10A cells. We quantify migration using particle image velocimetry and find that cell groups have features of motion that span multiple length scales. Comparing our experimental results to a model of collective cell migration, we find that cell migration within the monolayer can be affected in qualitatively different ways by cell motion at the boundary, yet it is not necessary to introduce leader cells at the boundary or specify other large-scale features to recapitulate this large-scale phenotype in simulations. Instead, in our model, collective motion can be enhanced by increasing the overall activity of the cells or by giving the cells a stronger coupling between their motion and polarity. This suggests that investigating the activity and polarity persistence of individual cells will add insight into the collective migration phenotypes observed during development and disease.

A study of the interaction between polyelectrolyte-coated nanostructured CaCO3 particles and a stearic acid monolayer spread at the water/air interface

The influence of nanostructured CaCO3 particles, both uncoated and coated with a polyelectrolyte (poly(diallyldimethylammonium chloride), polyethyleneimine, fluorescein-5-isothiocyanate-labeled poly(allylamine hydrochloride), or sodium polystyrene sulfonate), on a stearic acid monolayer spread on the surface of an aqueous subphase has been studied. The interaction of the particles present in the subphase with the monolayer as depending on the presence and composition of a polymer coating has been estimated with the help of compression isotherms and the Brewster angle microscopy. The monolayers were transferred from the aqueous subphase onto a solid substrate and studied by scanning electron microscopy. Strong interaction has been revealed between the calcium carbonate particles and the stearic acid monolayer. It has been shown that the transfer of the monolayer from the aqueous suspension surface onto the solid substrate may be accompanied by the detachment of the polymer coating from the surface of CaCO3 particles or their transfer together with the monolayer.

Astrocytic Calcium Waves Signal Brain Injury to Neural Stem andProgenitorCells.

SummaryBrain injuries, such as stroke or trauma, induce neural stem cells in the subventricular zone (SVZ) to a neurogenic response. Very little is known about the molecular cues that signal tissue damage, even over large distances, to the SVZ. Based on our analysis of gene expression patterns in the SVZ, 48 hr after an ischemic lesion caused by middle cerebral artery occlusion, we hypothesized that the presence of an injury might be transmitted by an astrocytic traveling calcium wave rather than by diffusible factors or hypoxia. Using a newly established in vitro system we show that calcium waves induced in an astrocytic monolayer spread to neural stem and progenitor cells and increase their self-renewal as well as migratory behavior. These changes are due to an upregulation of the Notch signaling pathway. This introduces the concept of propagating astrocytic calcium waves transmitting brain injury signals over long distances.

The L-Gamma Phase of Pulmonary Surfactant

When compressed, alveolar films of pulmonary surfactant resist collapse from the air/water interface. This physiologically essential characteristic remains unexplained. Recent studies show that self-assembled films of pulmonary surfactant formed by adsorption are more resistant to collapse than spread monolayers containing the same constituents. One candidate for the self-assembled stable structure is the Lγ phase. The unit cell of this structure contains two lamellar bilayers. Half of the monolayers are ordered, and half are disordered.

Interfacial behavior of PAMAM-PCL dendrimers and in situ spontaneous formation of gold nanoparticles at the toluene-water and air-water interfaces: Experimental and theoretical studies

Using pendant drop and Langmuir techniques, the interfacial behavior of hydroxyl-terminated poly(amido amine) (PAMAM-OH) dendrimers of generation 4, modified with poly(ε-caprolactone) (PAMAM-PCL), were analyzed and characterized. The study was performed at the toluene-water and air-water interfaces using two different molecular weights of PAMAM-PCL. Using this method, the interfacial activity of the dendrimers is shown to depend on the chain length of the PCL blocks. Using the Gibbs equation, the driving force for the adsorption process at the toluene-water interface was found to be of an enthalpic nature. In addition, stable monolayers of PAMAM-PCL at the air-water interface were obtained. For this analysis, the elasticity and critical exponent of the excluded volume, ν, for the PAMAM-PCL spread monolayers were determined. Alternatively, when the pure water phase was changed to an AuCl4− aqueous solution for both interfaces, different behavior in the interfacial activity of PAMAM-PCL was detected. Moreover, the spontaneous formation of gold nanoparticles at room temperature, at both interfaces, was observed. This was confirmed by UV–vis spectra and atomic force microscopy (AFM) images. Finally, molecular simulations and electronic structure calculations helped us to gain insights on the behavior of the dendrimers at the toluene-water interface and the PAMAM-PCL-gold nanoparticle interactions, respectively.

Interactions of a Tetrazine Derivative with Biomembrane Constituents: A Langmuir Monolayer Study.

Tetrazine (Tz) is expected to be used for bioimaging and as an analytical reagent. It is known to react very fast with trans-cyclooctene under water in organic chemistry. Here, to understand the interaction between Tz and biomembrane constituents, we first investigated the interfacial behavior of a newly synthesized Tz derivative comprising a C18-saturated hydrocarbon chain (rTz-C18) using a Langmuir monolayer spread at the air-water interface. Surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms were measured for monolayers of rTz-C18 and biomembrane constituents such as dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), dipalmitoyl phosphatidylethanolamine (DPPE), palmitoyl sphingomyelin (PSM), and cholesterol (Ch). The lateral interaction between rTz-C18 and the lipids was thermodynamically elucidated from the excess Gibbs free energy of mixing and two-dimensional phase diagram. The binary monolayers except for the Ch system indicated high miscibility or affinity. In particular, rTz-C18 was found to interact more strongly with DPPE, which is a major constituent of the inner surface of cell membranes. The phase behavior and morphology upon monolayer compression were investigated by using Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM). The BAM and FM images of the DPPC/rTz-C18, DPPG/rTz-C18, and PSM/rTz-C18 systems exhibited a coexistence state of two different liquid-condensed domains derived mainly from monolayers of phospholipids and phospholipids-rTz-C18. From these morphological observations, it is worthy to note that rTz-C18 is possible to interact with a limited amount of the lipids except for DPPE.

Influence of an Additive-Free Particle Spreading Method on Interactions between Charged Colloidal Particles at an Oil/Water Interface.

The assembly and manipulation of charged colloidal particles at oil/water interfaces represent active areas of fundamental and applied research. Previously, we have shown that colloidal particles can spontaneously generate unstable residual charges at the particle/oil interface when spreading solvent is used to disperse them at an oil/water interface. These residual charges in turn affect the long-ranged electrostatic repulsive forces and packing of particles at the interface. To further uncover the influence arising from the spreading solvents on interfacial particle interactions, in the present study we utilize pure buoyancy to drive the particles onto an oil/water interface and compare the differences between such a spontaneously adsorbed particle monolayer to the spread monolayer based on solvent spreading techniques. Our results show that the solvent-free method could also lead particles to spread well at the interface, but it does not result in violent sliding of particles along the interface. More importantly, this additive-free spreading method can avoid the formation of unstable residual charges at the particle/oil interface. These findings agree well with our previous hypothesis; namely, those unstable residual charges are triboelectric charges that arise from the violently rubbing of particles on oil at the interface. Therefore, if the spreading solvents could be avoided, then we would be able to get rid of the formation of residual charges at interfaces. This finding will provide insight for precisely controlling the interactions among colloidal particles trapped at fluid/fluid interfaces.

Hematopoietic Stem and Progenitor Cell Expansion in Contact with Mesenchymal Stromal Cells in a Hanging Drop Model Uncovers Disadvantages of 3D Culture.

Efficient ex vivo expansion of hematopoietic stem cells with a concomitant preservation of stemness and self-renewal potential is still an unresolved ambition. Increased numbers of methods approaching this issue using three-dimensional (3D) cultures were reported. Here, we describe a simplified 3D hanging drop model for the coculture of cord blood-derived CD34+ hematopoietic stem and progenitor cells (HSPCs) with bone marrow-derived mesenchymal stromal cells (MSCs). When seeded as a mixed cell suspension, MSCs segregated into tight spheroids. Despite the high expression of niche-specific extracellular matrix components by spheroid-forming MSCs, HSPCs did not migrate into the spheroids in the initial phase of coculture, indicating strong homotypic interactions of MSCs. After one week, however, HSPC attachment increased considerably, leading to spheroid collapse as demonstrated by electron microscopy and immunofluorescence staining. In terms of HSPC proliferation, the conventional 2D coculture system was superior to the hanging drop model. Furthermore, expansion of primitive hematopoietic progenitors was more favored in 2D than in 3D, as analyzed in colony-forming assays. Conclusively, our data demonstrate that MSCs, when arranged with a spread (monolayer) shape, exhibit better HSPC supportive qualities than spheroid-forming MSCs. Therefore, 3D systems are not necessarily superior to traditional 2D culture in this regard.

Nanopatterning of mobile lipid monolayers on electron-beam-sculpted Teflon AF surfaces.

Direct electron-beam lithography is used to fabricate nanostructured Teflon AF surfaces, which are utilized to pattern surface-supported monolayer phospholipid films with 50 nm lateral feature size. In comparison with unexposed Teflon AF coatings, e-beam-irradiated areas show reduced surface tension and surface potential. For phospholipid monolayer spreading experiments, these areas can be designed to function as barriers that enclose unexposed areas of nanometer dimensions and confine the lipid film within. We show that the effectiveness of the barrier is defined by pattern geometry and radiation dose. This surface preparation technique represents an efficient, yet simple, nanopatterning strategy supporting studies of lipid monolayer behavior in ultraconfined spaces. The generated structures are useful for imaging studies of biomimetic membranes and other specialized surface applications requiring spatially controlled formation of self-assembled, molecularly thin films on optically transparent patterned polymer surfaces with very low autofluorescence.

The Role of Cellular Mechanical Properties in Microenvironment-Dependent Behavior

Understanding the role of mechanics in cell behavior is necessary for effective development of tissue engineering therapies. This study observed how cellular mechanical phenotype influenced cell morphology and actin organization on substrates coated with different extracellular matrix ligands. Inherent cell stiffness, represented by Young's modulus, for MG-63 (osteosarcoma) and SH-SY5Y (neuroblastoma) cells was mechanically determined using atomic force microscopy. These cells have dissimilar moduli when probed over the nucleus (1.3±0.5 and 0.26±0.15 kPa for MG-63 and SH-SY5Y cells, respectively). To investigate the effect of substrate compliance, cells were cultured on polyacrylamide gels ranging from 0.3 to 12 kPa and functionalized with fibronectin, laminin, or collagen-1. We hypothesized that cells would spread and attach more successfully to substrates with an elastic modulus equal to or greater than that of the cell, provided physiologically relevant ligands are present. Cellular responses were captured daily over 4 days using fluorescent microscopy. MG-63 cells showed two different organizational phenotypes: a multi-cell, spheroid-like structure and a spread monolayer. The threshold substrate compliance required for MG-63 cells to organize into either phenotype was designated by the cells’ own stiffness, most clearly observed on collagen-1 coated gels. While SH-SY5Y cells exhibited similar phenotypic organizations when plated on varying substrates, no mechanical compliance threshold was observed. However, SH-SY5Y cells exhibited greater tendencies toward forming spheroids when plated on fibronectin. These morphological changes show a possible correlation between the cells’ characteristic mechanical phenotypes and their relationship to the microenvironment. Cells exhibiting less compliant phenotypes may have more substrate stiffness-dependent behavior, whereas more compliant cell types may have more ligand-dependent behavior. These findings indicate that cellular mechanosensing is influenced not only by the mechanical and biological characteristics of the microenvironment but also by the inherent mechanical phenotype of cells comprising a population.

Dense suspension splat: monolayer spreading and hole formation after impact.

We use experiments and minimal numerical models to investigate the rapidly expanding monolayer formed by the impact of a dense suspension drop against a smooth solid surface. The expansion creates a lacelike pattern of particle clusters separated by particle-free regions. Both the expansion and the development of the spatial inhomogeneity are dominated by particle inertia and, therefore, are robust and insensitive to details of the surface wetting, capillarity, and viscous drag.

Dilational surface elasticity of spread monolayers of polystyrene microparticles.

The dependence of the dilational surface elasticity on the surface pressure of the spread monolayers of polystyrene microparticles is studied at the water-air interface. The surface rheological measurements together with the data from optical methods allow the division of the whole range of surface pressures into three zones characterized by different monolayer structures. The extremely high surface elasticity (∼500 mN m(-1)) at surface pressures close to 30 mN m(-1) is similar to the results for the adsorption layer of the complexes formed between silica particles and surfactant molecules and is probably caused by strong hydrophobic attraction between the particles. At the same time, some other characteristic features of the viscoelasticity of the monolayers of polysterene microparticles differ strongly from the properties of previously studied systems.

An interfacial and comparative in vitro study of gastrointestinal lipases and Yarrowia lipolytica LIP2 lipase, a candidate for enzyme replacement therapy

Lipolytic activities of Yarrowia lipolytica LIP2 lipase (YLLIP2), human pancreatic (HPL) and dog gastric (DGL) lipases were first compared using lecithin-stabilized triacylglycerol (TAG) emulsions (Intralipid) at various pH and bile salt concentrations. Like DGL, YLLIP2 was able to hydrolyze TAG droplets covered by a lecithin monolayer, while HPL was not directly active on that substrate. These results were in good agreement with the respective kinetics of adsorption on phosphatidylcholine (PC) monomolecular films of the same three lipases, YLLIP2 being the most tensioactive lipase. YLLIP2 adsorption onto a PC monolayer spread at the air/water interface was influenced by pH-dependent changes in the enzyme/lipid interfacial association constant (KAds) which was optimum at pH 6.0 on long-chain egg PC monolayer, and at pH 5.0 on medium chain dilauroylphosphatidylcholine film. Using substrate monolayers (1,2-dicaprin, trioctanoin), YLLIP2 displayed the highest lipolytic activities on both substrates in the 25-35 mN m(-1) surface pressure range. YLLIP2 was active in a large pH range and displayed a pH-dependent activity profile combining DGL and HPL features at pH values found in the stomach (pH 3-5) and in the intestine (pH 6-7), respectively. The apparent maximum activity of YLLIP2 was observed at acidic pH 4-6 and was therefore well correlated with an efficient interfacial binding at these pH levels, whatever the type of interfaces (Intralipid emulsions, substrate or PC monolayers). All these findings support the use of YLLIP2 in enzyme replacement therapy for the treatment of pancreatic exocrine insufficiency, a pathological situation in which an acidification of intestinal contents occurs.