The effect of nanobubbles on Langmuir-Blodgett films

Nanobubbles (NBs) are classified in two distinct categories: surface and bulk. Surface NBs are readily observed using atomic force microscopy (AFM), while the existence of bulk NBs has been a subject of debate, conflicting with the diffusion theory's predictions. Current methodologies for identifying bulk NBs yield inconclusive results. In this study, Langmuir Blodgett (LB) technique and AFM, are utilized to visualize NB imprints on anionic, cationic and zwitterionic lipid films deposited on glass-slide substrates. Our analysis of Langmuir monolayers compression isotherms reveals the impact of bulk NBs on lipid monolayer development. AFM scans of the deposited lipid films consistently show NB imprints. Notably, cationic and zwitterionic film depositions exhibit NB formations from the 1st layer, whereas in anionic films, these formations are observed only after the 3rd layer. These results suggest that the origin of these imprinted formations may be attributed to bulk NBs.

Impact of chlorogenic acid on surface and phase properties of cholesterol-enriched phosphatidylcholine membranes

This study analyses the insertion of Chlorogenic acid (CGA) in phosphatidylcholine (PC) membranes enriched with cholesterol (Chol). While cholesterol decreases the area per lipid and increases the dipole potential, CGA increases and decreases these values, respectively. When CGA is inserted into cholesterol-containing DMPC membranes, these effects cancel out, resulting in values that overlap with those of DMPC monolayers without Chol and CGA. The presence of CGA also compensates the increase of dipole potential produced by Chol which can be explain as a consequence of the orientation of CGA molecule at the interphase opposing the cholesterol dipole moieties and water dipoles. This compensatory effect is less effective when lipids lack carbonyl groups (CO). When monolayers are composed by unsaturated PCs the Chol compensation is found at higher concentrations of CGA due to the direct interaction between CGA and Chol. These results suggest that cholesterol modulates the interaction and distribution of CGA in the lipid membrane, which may have implications for its biological activity.

Advantages of the classical thermodynamic analysis of single-and multi-component Langmuir monolayers from molecules of biomedical importance-theory and applications

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

Probing the Mechanism of Targeted Delivery of Molecular Surfactants Loaded into Nanoparticles after Their Assembly at Oil-Water Interfaces

A targeted and controlled delivery of molecular surfactants at oil-water interfaces using the directed assembly of nanoparticles, NPs, is reported. The mechanism of NP assembly at the interface and the release of molecular surfactants is followed by laser scanning confocal microscopy and surface force spectroscopy. The assembly of positively charged polystyrene NPs at the oil-water interface was facilitated by the introduction of carboxylic acid groups in the oil phase (e.g., by adding 1 wt % stearic acid to hexadecane to produce a model oil). The presence of positively charged NPs consistently lowers the stiffness of the water-oil interface. The effect is lessened, when the NPs are present in a solution of NaCl or deionized water at pH 2, consistent with a less dense monolayer of NPs at the interface in the last two systems. In addition, the NPs reduce the interfacial adhesion (i.e., the "stickiness" of the interface or, put differently, the pull-off force experienced by the atomic force microscopy (AFM) tip during retraction). After the assembly, the NPs can release a previously loaded cargo of surfactant molecules, which then facilitate the formation of a much finer oil-water emulsion. As a proof of concept, we demonstrate the release of octadecyl amine, ODA, that has been incorporated into the NPs prior to the assembly. The release of ODA causes the NPs to detach from the interface altering the interfacial properties and leads to finer oil droplets. This approach can be exploited in applications in several fields ranging from pharmaceutical and cosmetics to hydrocarbon recovery and oil-spill remediation, where a targeted and controlled release of surfactants is wanted.

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

Developing better separation technologies for rare earth metals, an important aspect of a sustainable materials economy, is challenging due to their chemical similarities. Identifying molecular-scale interactions that amplify the subtle differences between the rare earths can be useful in developing new separation technologies. Here, we describe the ion-dependent monolayer to inverted bilayer transformation of extractant molecules at the air/aqueous interface. The inverted bilayers form with Lu³⁺ ions but not with Nd³⁺. By introducing Lu³⁺ ions to preformed monolayers, we extract kinetic parameters corresponding to the monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the character of the background anion, although anions are expected to be repelled from the interface. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods.

Fluid Films as Models for Understanding the Impact of Inhaled Particles in Lung Surfactant Layers

Pollution is currently a public health problem associated with different cardiovascular and respiratory diseases. These are commonly originated as a result of the pollutant transport to the alveolar cavity after their inhalation. Once pollutants enter the alveolar cavity, they are deposited on the lung surfactant (LS) film, altering their mechanical performance which increases the respiratory work and can induce a premature alveolar collapse. Furthermore, the interactions of pollutants with LS can induce the formation of an LS corona decorating the pollutant surface, favoring their penetration into the bloodstream and distribution along different organs. Therefore, it is necessary to understand the most fundamental aspects of the interaction of particulate pollutants with LS to mitigate their effects, and design therapeutic strategies. However, the use of animal models is often invasive, and requires a careful examination of different bioethics aspects. This makes it necessary to design in vitro models mimicking some physico-chemical aspects with relevance for LS performance, which can be done by exploiting the tools provided by the science and technology of interfaces to shed light on the most fundamental physico-chemical bases governing the interaction between LS and particulate matter. This review provides an updated perspective of the use of fluid films of LS models for shedding light on the potential impact of particulate matter in the performance of LS film. It should be noted that even though the used model systems cannot account for some physiological aspects, it is expected that the information contained in this review can contribute on the understanding of the potential toxicological effects of air pollution.

Role of Iron Phthalocyanine Coordination in Catecholamines Detection

Catecholamines are an important class of neurotransmitters responsible for regularizing, controlling, and treating neural diseases. Based on control and diseases treatment, the development of methodology and dives to sensing is a promissory technology area. This work evaluated the role of iron phthalocyanine coordination (FePc) with the specific groups from catecholamine molecules (L-dopa, dopamine, epinephrine, and the amino acid tyrosine) and the effect of this coordination on electrochemical behavior. The in situ coordination analysis was performed through isotherms π-A of FePc Langmuir films in the absence and presence of catecholamines. The π-A isotherm indicates a strong interaction between FePc monolayer and L-Dopa and DA, which present a catechol group and a side chain with a protonated amino group (-NH3+). These strong interactions with catechol and amine groups were confirmed by characterization at the molecular level using the surface-enhanced Raman spectroscopy (SERS) from a Langmuir–Schaefer monolayer deposited onto Ag surfaces. The electrochemical measurements present a similar tendency, with lower oxidation potential observed to DA>L-Dopa>Ep. The results corroborate that the coordination of the analyte on the electron mediator surface plays an essential role in an electrochemical sensing application. The FePc LS film was applied as a sensor in tablet drug samples, showing a uniformity of content of 96% for detecting active compounds present in the L-Dopa drug samples.

Characterizing and Tuning the Properties of Polydiacetylene Films for Sensing Applications

Self-assembled, polymerized diacetylene (DA) nanostructures and two-dimensional films have been studied over the past two decades for sensor applications because of their straightforward visual readout. DA monomers, when exposed to UV light, polymerize to produce a visibly blue polymer. Blue phase polydiacetylenes (PDAs) when exposed to an external stimuli, such as temperature or UV light, undergo a chromatic phase transition to a fluorescent, visibly red phase. The tunability of the monomer to blue to red chromatic phase transitions by choice of diacetylene monomer in the presence of metal cations is systematically and comprehensively investigated to determine their effects on the properties of PDA Langmuir films. The polymerization kinetics and domain morphology of the PDA films were characterized using polarized fluorescent microscopy, UV-vis-fluorescent spectroscopy, and Fourier transform infrared spectroscopy (FTIR). Increasing the monomer alkyl tail length was found to strongly increase the UV dose necessary to produce optimally blue films and fully red films. A decrease in the polymer domain size was also correlated with longer-tailed DA molecules. Metal cations have a diverse effect on the film behavior. Alkaline-earth metals such as Mg, Ca, and Ba have a negligible effect on the phase transition kinetics but can be used to tune PDA polymer domain sizes. The Ni and Fe cations increase the UV dose necessary to produce red phase PDA films and significantly decrease the polymer domain sizes. The Zn, Cd, and Cu ions exhibit strong directed interactions with the PDA carboxylic acid headgroups, resulting in quenched fluorescence and a unique film morphology. FTIR analysis provides insight into the metal-PDA binding mechanisms and demonstrates that the coordination between the PDA film headgroups and the metal cations can be correlated with changes in the film morphology and kinetics. The findings from these studies will have broad utility for tuning PDA-based sensors for different applications and sensitivity ranges.

Interactions of particulate matter and pulmonary surfactant: Implications for human health

Particulate matter (PM), which is the primary contributor to air pollution, has become a pervasive global health threat. When PM enters into a respiratory tract, the first body tissues to be directly exposed are the cells of respiratory tissues and pulmonary surfactant. Pulmonary surfactant is a pivotal component to modulate surface tension of alveoli during respiration. Many studies have proved that PM would interact with pulmonary surfactant to affect the alveolar activity, and meanwhile, pulmonary surfactant would be adsorbed to the surface of PM to change the toxic effect of PM. This review focuses on recent studies of the interactions between micro/nanoparticles (synthesized and environmental particles) and pulmonary surfactant (natural surfactant and its models), as well as the health effects caused by PM through a few significant aspects, such as surface properties of PM, including size, surface charge, hydrophobicity, shape, chemical nature, etc. Moreover, in vitro and in vivo studies have shown that PM leads to oxidative stress, inflammatory response, fibrosis, and cancerization in living bodies. By providing a comprehensive picture of PM-surfactant interaction, this review will benefit both researchers for further studies and policy-makers for setting up more appropriate regulations to reduce the adverse effects of PM on public health.

Multiscale Self-Assembly of Distinctive Weblike Structures from Evaporated Drops of Dilute American Whiskeys

When a sessile droplet of a complex mixture evaporates, its nonvolatile components may deposit into various patterns. One such phenomena, the coffee ring effect, has been a topic of interest for several decades. Here, we identify what we believe to be a fascinating phenomenon of droplet pattern deposition for another well-known beverage-what we have termed a "whiskey web". Nanoscale agglomerates were generated in diluted American whiskeys (20-25% alcohol by volume), which later stratified as microwebs on the liquid-air interface during evaporation. The web's strandlike features result from monolayer collapse, and the resulting pattern is a function of the intrinsic molecular constituents of the whiskey. Data suggest that, for our conditions (diluted 1.0 μL drops evaporated on cleaned glass substrates), whiskey webs were unique to diluted American whiskey; however, similar structures were generated with other whiskeys under different conditions. Further, each product forms their own distinct pattern, demonstrating that this phenomenon could be used for sample analysis and counterfeit identification.

Spontaneous collapse of palmitic acid films on an alkaline buffer containing calcium ions

Understanding the interaction of ions with fatty acids is important to identify their roles in various bioprocesses and to build novel biomimetic systems. In this study, the molecular organization of palmitic acid (PA) films on alkaline buffer solutions (pH 7.4) with and without divalent Ca²⁺ was measured at a constant surface area using Langmuir troughs coupled with microscopy and X-ray interfacial techniques. Without Ca²⁺, PA molecules remained a monolayer organization; however, with Ca²⁺, formation of the inverted bilayers of PA-Ca²⁺ superstructures caused a spontaneous 2D to 3D transformation under no compression due to the strong interaction between PA and the divalent cation. Self-assembly of this highly-organized inverted bilayer superstructure involved a two-step process of nucleation and nuclei growth. During nucleation, densely packed PA and Ca²⁺ monolayer firstly corrugated and some of PA and Ca²⁺ molecules ejected out from the monolayer; the ejected molecules then reorganized and formed the inverted bilayer nuclei. Nucleation was followed by nuclei growth, during which PA and Ca²⁺ in the monolayer kept integrating into the inverted bilayer structure through molecule migration and PA rotation around Ca²⁺.

Effect of Mineral-Oil Addition on Film Stability of Polar Lipid Constituents Derived from Meibum

We studied the effects of mineral oil (MO) on the properties and structure of a spread monolayer of polar lipid constituents in meibum, by performing cyclic lateral compression-expansion experiments using a Langmuir trough. A meibum sample without nonpolar lipids (meibumΔnonpolar-lipid) was prepared by removing the nonpolar lipids from biological meibum extruded from rabbit eyelids and spread on a water surface for measuring the cyclic surface pressure (π)-film area (A) isotherms with in situ observation of the film morphology using a Brewster angle microscope. The meibumΔnonpolar-lipid formed a homogeneous fluid monolayer and underwent collapse upon compression. The π-A isotherm shifted to a smaller area upon repeating the compression-expansion cycles. These observations contrasted those obtained for meibum previously, which may have resulted from the absence of nonpolar lipids. The recovery of the film stability against the lateral compression-expansion cycles was analyzed by adding MO as a nonpolar compound to the film system. A spread film of 1:1 mixture (by weight) could recover the high reversibility of the π-A isotherms during the repeated compression and expansion processes.

Unusual Behavior of the Bipolar Molecule 25-Hydroxycholesterol at the Air/Water Interface-Langmuir Monolayer Approach Complemented with Theoretical Calculations

In this study, 25-hydroxycholesterol (25-OH), a biamphiphilic compound with a wide range of biological activities, has been investigated at the air/water interface. We were interested in how two hydroxyl groups attached at distal positions of the 25-OH molecule (namely, at C(3) in the sterane system and at C(25) in the side chain) influence its surface behavior. Apart from traditional Langmuir monolayers, other complementary surface-sensitive techniques, such as electric surface potential measurements, Brewster angle microscopy (BAM, enabling texture visualization and film thickness measurements), and polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), were applied. Experimental data have been interpreted with the aid of theoretical study. Our results show that 25-OH molecules in the monomolecular layer are anchored to the water surface alternatively with C(3) or C(25) hydroxyl groups. Theoretical calculations revealed that the populations of these alternative orientations were not equal and molecules anchored with C(3) hydroxyl groups were found to be in excess. As a consequence of such an arrangement, surface films of 25-OH are of lower stability as compared to cholesterol (considered as a non-oxidized analogue of 25-OH). Moreover, it was found that, upon compression, the transition from mono- to bilayer occurred. The molecular mechanism and interactions stabilizing bilayer structure were proposed. The explanation of the observed unusual surface behavior of 25-OH may contribute to an understanding of differences in biological activity between chain- and ring-oxidized sterols.

Phase Behavior of GM1-Containing DMPC-Cholesterol Monolayer: Experimental and Theoretical Study

Organization and distribution of lipids in cellular membranes play an important role in a diverse range of biological processes, such as membrane trafficking and signaling. Here, we present the combined experimental and simulated results to elucidate the phase behavioral features of ganglioside monosialo 1 (GM1)-containing mixed monolayer of the lipids 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and cholesterol (CHOL). Two monolayers having compositions DMPC-CHOL and GM1-DMPC-CHOL are investigated at air-water and air-solid interfaces using Langmuir-Blodgett experiments and scanning electron microscopy (SEM), respectively, to ascertain the phase behavior change of the monolayers. Surface pressure isotherms and SEM imaging of domain formation indicate that addition of GM1 to the monolayer at low surface pressure causes a fluidization of the system but once the system attains the surface pressure corresponding to its liquid-condensed phase, the monolayer becomes more ordered than the system devoid of GM1 and interacts among each other more cooperatively. Besides, the condensing effect of cholesterol on the DMPC monolayer was also verified by our experiments. Apart from these, the effects induced by GM1 on the phase behavior of the binary mixture of DMPC-CHOL were studied with and without applying liquid-expanded (L_E)-liquid-condensed (L_C) equilibrium surface pressure using molecular dynamics (MD) simulation. Our molecular dynamics (MD) simulation results give an atomistic-level explanation of our experimental findings and furnish a similar conclusion.

Molecular rotors confined at an ordered 2D interface

Intramolecular rotation of molecules contained in a two-dimensional monolayer or a three-dimensional collapsed film at an air-water interface was investigated by in situ fluorescence spectroscopy of twisted intramolecular charge transfer (TICT) type 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) derivatives. The TICT type molecules, CCVJ-C12 and CCVJ-Chol, that contain a linear alkyl dodecyl chain or a cholesteryl group, respectively, as their hydrophobic group, were designed and synthesized to manipulate them at the air-water interface. These lipophilized molecular rotors showed the general properties of TICT molecules in solutions that the fluorescence intensity increases with increasing viscosity of the solvent, which is induced by inhibition of internal molecular rotations. The molecular rotors CCVJ-C12 and CCVJ-Chol formed monolayers at the air-water interface and in situ fluorescence spectroscopy was performed during the in-plane compression of the monolayers. It was revealed that the monomer emissions were suppressed and only after the collapse of monolayers, excimer emission from both layers consisting of CCVJ-C12 or CCVJ-Chol was observed. Suppressed monomer emission from monolayers suggests that intramolecular rotation is not inhibited in dense ordered monolayers. Furthermore, fluorescence spectroscopy of Langmuir-Blodgett (LB) films indicated that molecular rotations are not inhibited in the monolayer transferred on the solid substrates.

Molecular Explanation for the Abnormal Flux of Material into a Hot Spot in Ester Monolayers

Langmuir monolayers of certain surfactants show a negative derivative of the surface pressure with respect to temperature. In these monolayers, a local temperature gradient leads to local yielding of the solid phase to a kinetically flowing liquid, so that the material flows toward the hotter regions that act as sinks. The accumulation of material leads to the formation of nonequilibrium multilamellar bubbles of different sizes. Here we investigate the molecular factors leading to such a peculiar behavior. First, we identify the required structural molecular moieties, and second we vary the composition of the subphase in order to analyze its influence. We conclude that esters appear to be unique in two key aspects: they form monolayers whose compression isotherms shift to lower areas as the temperature increases, and thus collapse into a hot spot; and they bind weakly to the aqueous subphase, i.e., water does not attach to the monolayer at the molecular level, but only supports it. Molecular simulations for a selected system confirm and help explain the observed behavior: surfactant molecules form a weak hydrogen bonding network, which is disrupted upon heating, and also the molecular tilting changes with temperature, leading to changes in the film density.

Evaluating polymeric biomaterial-environment interfaces by Langmuir monolayer techniques

Polymeric biomaterials are of specific relevance in medical and pharmaceutical applications due to their wide range of tailorable properties and functionalities. The knowledge about interactions of biomaterials with their biological environment is of crucial importance for developing highly sophisticated medical devices. To achieve optimal in vivo performance, a description at the molecular level is required to gain better understanding about the surface of synthetic materials for tailoring their properties. This is still challenging and requires the comprehensive characterization of morphological structures, polymer chain arrangements and degradation behaviour. The review discusses selected aspects for evaluating polymeric biomaterial-environment interfaces by Langmuir monolayer methods as powerful techniques for studying interfacial properties, such as morphological and degradation processes. The combination of spectroscopic, microscopic and scattering methods with the Langmuir techniques adapted to polymers can substantially improve the understanding of their in vivo behaviour.

Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation

A Langmuir monolayer can be considered as a two-dimensional (2D) sheet at higher surface pressure which structurally deform with mechanical compression depending upon the elastic nature of the monolayer. The deformed structures formed after a certain elastic limit are called collapsed structures. To explore monolayer collapses at lower surface pressure and to see the effect of ions on such monolayer collapses, out-of-plane structures and in-plane morphologies of stearic acid Langmuir monolayers have been studied both at lower (≈6.8) and higher (≈9.5) subphase pH in the presence of Mg^{2+},Ca^{2+},Zn^{2+},Cd^{2+}, and Ba^{2+} ions. At lower subphase pH and in the presence of all cations, the stearic acid monolayer remains as a monolayer before collapse, which generally takes place at higher surface pressure (π_{c}>50mN/m). However, at higher subphase pH, structural changes of stearic acid monolayers occur at relatively lower surface pressure depending upon the specific dissolved ions. Among the same group elements of Mg^{2+},Ca^{2+}, and Ba^{2+}, only for Ba^{2+} ions does monolayer to multilayer transition take place from a much lower surface pressure of the monolayer, remaining, however, as a monolayer for Mg^{2+} and Ca^{2+} ions. For another same group elements of Zn^{2+} and Cd^{2+} ions, a less covered bilayer structure forms on top of the monolayer structure at lower surface pressure, which is evidenced from both x-ray reflectometry and atomic force microscopy. Fourier transform infrared spectroscopy confirms the presence of two coexisting conformations formed by the two different metal-headgroup coordinations and the monolayer to trilayer or multilayer transformation takes place when the coverage ratio of the two molecular conformations changes from the critical value (p_{c}) of ≈0.66. Such ion-specific monolayer collapses are correlated with the 2D lattice percolation model.