Phase transition beyond the monolayer collapse – The case of stearic acid spread at the air/water interface

Combined impact of pesticides on mono- and bilayer lipid membranes

The increased use of agrochemicals in crop production raises concerns about the risk of combined pesticide exposure through water and food, potentially leading to a 'cocktail effect' with synergistic impacts on human health. To investigate such effects, we used the pesticides acephate and diuron interacting with a mimetic system of the cell membrane, composed of lipid monolayers. These mimetic systems were composed by a mixture of POPC, cholesterol and sphingomyelin (70/20/10 mol%), respectively, close to the composition found in mammalian membranes. Results from Langmuir monolayers, including surface pressure-area isotherms, polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), and Brewster angle microscopy (BAM), showed that the pesticides interact predominantly with the polar head region of the lipids, a finding supported by density functional theory (DFT) calculations and molecular dynamics simulations. The cocktail had a similar effect in π-A isotherms; however, PM-IRRAS data suggests a stronger effect of the cocktail on the ternary monolayer at the molecular level, once the pesticide mixture changed the conformation and orientation of the headgroup and disturbed the hydrocarbon chain. These results evidence the impact of the 'cocktail effect' on lipid membranes, highlighting potential health risks associated with pesticide mixtures.

Using Carbon Nanotubes to Improve Enzyme Activity and Electroactivity of Fatty Acid Langmuir-Blodgett Film-Incorporated Galactose Oxidase for Sensing and Energy Storage Applications

Incorporating enzymes into nanostructured supercapacitor devices represents a groundbreaking advancement in energy storage. Enzyme catalysis using nanomaterials enhances performance, efficiency, and stability by facilitating precise charge transfer, while the nanostructure provides a high surface area and improved conductivity. This synergy yields eco-friendly, high-performance energy storage solutions crucial for diverse applications, from portable electronics to renewable energy systems. In this study, we harnessed the versatility of Langmuir-Blodgett films to create meticulously organized thin films with specific enzyme properties, coupled with carbon nanotubes, to develop biosupercapacitors. Langmuir monolayers were constructed with stearic acid, carbon nanotubes, and galactose oxidase. Following comprehensive characterization using tensiometric, rheological, morphological, and spectroscopic techniques, the monolayers were transferred to solid supports, yielding Langmuir-Blodgett films. These films exhibited superior performance, with persisting enzyme activity. However, increasing film thickness did not enhance enzymatic activity values, indicating a surface-driven process. Subsequently, we explored the electrochemical properties of the films, revealing stability compatible with supercapacitor applications. The introduction of carbon nanotubes demonstrated a higher capacitance, indicating the potential viability of the films for energy storage applications.

Filling the gaps: Introducing plasticizers into π-conjugated OPE-NH2 Langmuir layers for defect-free anisotropic interfaces and membranes towards unidirectional mass, charge, or energy transfer

The construction of ultrathin membranes from linearly aligned π-electron systems is advantageous for targeted energy, charge, or mass transfer. The Langmuir-Blodgett (LB) technique enables the creation of such membranes, especially with amphiphilic π-electron systems. However, these systems often aggregate, forming rigid Langmuir monolayers with defects or holes. In this study we introduce plasticizers to effectively address this issue. To create anisotropic membranes, we used an oligo(phenylene ethynylene) derivative (OPE-NH2) as an linear amphiphile and bisphenol A di-tert-butyl ester (BPAE) as a plasticizer. We analyzed surface pressure (mean molecular area) (Π(mma)) isotherms and characterized Langmuir monolayers with Brewster Angle Microscopy (BAM), to determine the optimal miscibility of OPE-NH2 with BPAE. Detailed analysis of hole areas filled was performed through image binarization. We identified an optimal BPAE concentration of 4 mol-% in the OPE-NH2 Langmuir monolayer. Our BAM image evaluation via binarization determined the difference between the mean molecular areas of close-packed Langmuir domains and those quantified via the Π(mma) isotherm. This study presents an automated method for BAM image analysis and a new approach for fabricating defect-free anisotropic molecular monolayers of π-conjugated amphiphiles.

Lage E, Casas M, Concheiro A, Alvarez-Lorenzo C. Langmuir monolayer studies of non-ionic surfactants and DOTMA for the design of ophthalmic niosomes

The worldwide increase in diabetes entails a rise in associated diseases, with diabetic retinopathy on the forefront of the ocular complications. To overcome the challenges posed by ocular barriers, self-assembled nanocarriers have gathered increasing attention in recent years, with niosomes revealing themselves to be suitable for the delivery of a variety of drugs. This study investigated the mechanical properties of Langmuir monolayers comprising cholesterol, Tween 60, and 1,2-di-O-octadecenyl-3-trimethylammonium propane (DOTMA), both individually and in binary and ternary systems. The cholesterol monolayer was characterized by an L-shaped isotherm, reflecting two surface aggregation states. Tween 60 exhibited expanded conformation and progressive aggregation, transitioning through a phase change. The addition of cholesterol to Tween 60 resulted in a subtle reduction in surface compressional modulus. The compression isotherms highlighted the stabilizing effect of cholesterol on the monolayer, affecting the film's resistance to compression. The introduction of DOTMA in Tween 60 monolayers revealed concentration-dependent effects, where the compression resistance of the film was proportional to DOTMA concentration. Ternary systems of cholesterol, DOTMA and Tween 60 exhibited unique behavior, with DOTMA enhancing film stability and cholesterol modulating this effect. Temperature and subphase ionic strength variations further exacerbated the effects of DOTMA concentration. Brewster Angle Microscopy confirmed the absence of microdomains in the compressed monolayer, supporting the hypothesis of a monolayer collapse. Overall, the research provided valuable insights into the intricate interactions and mechanical behavior of these surfactant systems and the feasibility of obtaining cationic niosome-based drug delivery.

Four-parameter model of thin surface layer contribution to reflectance-absorbance spectroscopy and ellipsometry

The contribution of the surface layer to the reflection coefficients is shown to be determined by four surface integral values, which can be interpreted as real and imaginary parts of two complex permittivity excesses. The reflectance-absorbance spectra are determined by the spectra of these parameters. The spectra of the surface excess integrals cannot be found with the angular measurements of reflection-absorption spectra, which are determined by only three angular dependent terms. To determine these four surface excess integrals, it is necessary to involve the experimental data of spectroscopic ellipsometry or polarization-modulation infrared reflection absorption spectroscopy providing equivalent information about surface. In the case of weakly absorbing bulk medium, the real parts of the excesses can be neglected, permitting calculation of their imaginary parts using the angular dependence of the absorbance. The calculation of these parameters allows to check consistency of the data obtained. Measurements of the angular dependence of the absorbance of p-polarized radiation reflected from the DPPC monolayer upon distilled water were performed. The data obtained turned to be in good agreement with the proposed theoretical analysis.

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.

Site of the Hydroxyl Group Determines the Surface Behavior of Bipolar Chain-Oxidized Cholesterol Derivatives─Langmuir Monolayer Studies Supplemented with Theoretical Calculations

Cholesterol oxidation products (called oxysterols) are involved in many biological processes, showing both negative (e.g., neurodegenerative) and positive (e.g., antiviral and antimicrobial) effects. The physiological activity of oxysterols is undoubtedly closely related to their structure (i.e., the type and location of the additional polar group in the cholesterol skeleton). In this paper, we focus on determining how a seemingly minor structural change (introduction of a hydroxyl moiety at C(24), C(25), or C(27) in the isooctyl chain of cholesterol) affects the organization of the resulting molecules at the phase boundary. In our research, we supplemented the classic Langmuir monolayer technique, based on the surface pressure and electric surface potential isotherms, with microscopic (BAM) and spectroscopic (PM-IRRAS) techniques, as well as theoretical calculations (DFT and MD). This allowed us to show that 24-OH behaves more like cholesterol and forms stable, rigid monolayers. On the other hand, 27-OH, similar to 25-OH, undergoes the phase transition from monolayer to bilayer structures. Theoretical calculations enabled us to conclude that the formation of bilayers from 27-OH or 25-OH is possible due to the hydrogen bonding between adjacent oxysterol molecules. This observation may help to understand the factors responsible for the unique biological activity (including antiviral and antimicrobial) of 27-OH and 25-OH compared to other oxysterols.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.