Monolayer Technique Research Articles

Use of tyrosinase nanobiosensor for detection of atrazine and simazine and its application in real water samples

ABSTRACT This study presents the development and characterization of cantilever nanobiosensors functionalized with the tyrosinase enzyme (0.05–0.001%) for the detection of atrazine and simazine in water. The optimization of tyrosinase activity through precise immobilization and sensitive layer adjustments was evaluated. Topographical analysis revealed the successful immobilization of the tyrosinase enzyme using the self-assembled monolayer technique, enhancing sensor sensitivity. Upon comparing the enzyme concentrations, it was found that the 0.006% exhibited remarkable results, including a limit of detection (LOD = 0.0080 ppb) and a limit of quantification (LOQ = 0.0267 ppb), along with enhanced storage stability for 15 days for atrazine detection. For simazine, the nanobiosensor exhibited high sensitivity (8.1532 nm/ppb) and LOD (0.0117 ppb). They demonstrated excellent reversibility (99%) over six cycles and selectivity against interfering pesticides commonly found in agricultural environments, indicating their suitability for complex sample matrices. Real water samples met regulatory standards for atrazine levels, further validating the efficacy of the nanobiosensors in detecting pesticide contamination. The enzymatic nanobiosensor achieved recoveries ranging from 97 to 101%, indicating high reproducibility. These findings demonstrate the potential of the developed nanobiosensors for sensitive and reliable pesticide detection in environmental monitoring, with implications for future applications.

Nanobiosensors based on tyrosinase with alkanethiols for detection commercial formulation of atrazine in artesian water samples applied in agriculture crop

This study aims to develop cantilever nanobiosensors functionalized with commercial tyrosinase enzymes and alkanethiols for the detection of a commercial formulation of atrazine in artesian water samples used in agricultural crops. Functionalization was carried out using the self-assembled monolayer (SAM) technique with different alkanethiols, namely, 16-mercaptohexadecanoic acid (16-MHDA) and 11-mercaptoundecanoic acid (11-MUA), in conjunction with commercial tyrosinase enzymes. Surface characterization techniques confirm the successful deposition of the sensing layer. The nanobiosensors exhibited the capability to detect various concentrations of atrazine in water, achieving high sensitivity with detection and quantification limits in the µg/L range, while maintaining 100 % reversibility. The enzyme retained 87 % and 80 % of its activity when immobilized on 16-MHDA and 11-MUA linkages, respectively. The response of cantilever nanobiosensors was stable after 13 min, with deflection values showing a direct proportional relationship with atrazine concentrations ranging from 0.001 to 100 µg/L. Both cantilever nanobiosensors demonstrated promising responses, with 11-MUA differing statistically (P<0.05) from 16-MHDA. The rapid detection of commercial atrazine was successfully conducted in real water samples collected directly during its application in crop fields. Consequently, the developed devices hold significant potential for practical use in the detection of atrazine in water, contributing to assessments of environmental quality.

In situ via Contact to hBN-Encapsulated Air-Sensitive Atomically Thin Semiconductors.

Establishing reliable electrical contacts to atomically thin materials is a prerequisite for both fundamental studies and applications yet remains a challenge. In particular, the development of contact techniques for air-sensitive monolayers has lagged behind, despite their unique properties and significant potential for applications. Here, we present a robust method to create contacts to device layers encapsulated within hexagonal boron nitride (hBN). This method uses plasma etching and metal deposition to create 'vias' in the hBN with graphene forming an atomically thin etch-stop. The resulting partially fluorinated graphene (PFG) protects the underlying device layer from air-induced degradation and damage during metal deposition. PFG is resistive in-plane but maintains high out-of-plane conductivity. The work function of the PFG/metal contact is tunable through the degree of fluorination, offering opportunities for contact engineering. Using the in situ via technique, we achieve ambipolar contact to air-sensitive monolayer 2H-molybdenum ditelluride (MoTe2) with more than 1 order of magnitude improvement in on-current density compared to previous literature. The complete encapsulation provides high reproducibility and long-term stability. The technique can be extended to other air-sensitive materials as well as air-stable materials, offering highly competitive device performance.

Effect of Edge Activator Combinations in Transethosomal Formulations for Skin Delivery of Thymoquinone via Langmuir Technique

Thymoquinone (TQ), a bioactive compound found in Nigella sativa seeds, possesses diverse therapeutic properties for skin conditions. However, formulating TQ presents challenges due to its hydrophobic nature and chemical instability, which hinder its skin penetration. Transethosomes, as a formulation, offer an environment conducive to enhancing TQ’s solubility, stability, and skin permeation. To optimize TQ transethosomal formulations, we introduced a combination of ionic and nonionic surfactants, namely Tween 20 and sodium lauryl sulfate (SLS) or sodium lauroyl glutamate (SLG). Surfactants play a crucial role in stabilizing the formulation, reducing aggregation, improving biocompatibility, and minimizing potential toxicity. We fine-tuned the formulation composition and gained insights into its interfacial behavior using the Langmuir monolayer technique. This method elucidated the interfacial properties and behavior of phospholipids in ethosome and transethosome formulations. Our findings suggest that monolayer studies can serve as the initial step in selecting surfactants for nanocarrier formulations based on their interfacial dilational rheology studies. It was found that the addition of surfactant to the formulation increased the elasticity considering the capability of transethosomes to significantly decrease their radius when permeating the skin barrier. The results of the dilational rheology experiments were most relevant to drug permeation through the skin for the largest amplitude of deformation. The combination of Tween 20 and SLS efficiently modified the rheological behavior of lipids, increasing their elasticity. This conclusion was supported by in vitro studies, where formulation F2 composed of Tween 20 and SLS demonstrated the highest permeation after 24 h (300.23 µg/cm2). Furthermore, the F2 formulation showed the highest encapsulation efficiency (EE) of 94%, surpassing those of the control and ethosomal formulations. Additionally, this transethosomal formulation exhibited antimicrobial activity against S. aureus, with a zone of inhibition of 26.4 ± 0.3 mm. Importantly, we assessed the cytotoxicity of both ethosomes and transethosomes at concentrations ranging from 3.5 µM to 50 µM on HaCaT cell lines and found no cytotoxic effects compared to TQ hydroethanolic solution. These results suggest the potential safety and efficacy of TQ transethosomal formulations.

Susceptibility of Legionella gormanii Membrane-Derived Phospholipids to the Peptide Action of Antimicrobial LL-37-Langmuir Monolayer Studies.

LL-37 is the only member of the cathelicidin-type host defense peptide family in humans. It exhibits broad-spectrum bactericidal activity, which represents a distinctive advantage for future therapeutic targets. The presence of choline in the growth medium for bacteria changes the composition and physicochemical properties of their membranes, which affects LL-37's activity as an antimicrobial agent. In this study, the effect of the LL-37 peptide on the phospholipid monolayers at the liquid-air interface imitating the membranes of Legionella gormanii bacteria was determined. The Langmuir monolayer technique was employed to prepare model membranes composed of individual classes of phospholipids-phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), cardiolipin (CL)-isolated from L. gormanii bacteria supplemented or non-supplemented with exogenous choline. Compression isotherms were obtained for the monolayers with or without the addition of the peptide to the subphase. Then, penetration tests were carried out for the phospholipid monolayers compressed to a surface pressure of 30 mN/m, followed by the insertion of the peptide into the subphase. Changes in the mean molecular area were observed over time. Our findings demonstrate the diversified effect of LL-37 on the phospholipid monolayers, depending on the bacteria growth conditions. The substantial changes in membrane properties due to its interactions with LL-37 enable us to propose a feasible mechanism of peptide action at a molecular level. This can be associated with the stable incorporation of the peptide inside the monolayer or with the disruption of the membrane leading to the removal (desorption) of molecules into the subphase. Understanding the role of antimicrobial peptides is crucial for the design and development of new strategies and routes for combating resistance to conventional antibiotics.

Another Brick in the Wall of Tear Film Insights Added Through the Total Synthesis and Biophysical Profiling of anteiso-Branched Wax and Cholesteryl Esters.

The tear film lipid layer (TFLL) plays a vital part in maintenance of ocular health and represents a unique biological barrier comprising unusual and specialized lipid classes and species. The wax and cholesteryl esters (WEs and CEs) constitute roughly 80-90% of the TFLL. The majority of species in these lipid classes are branched and it is therefore surprising that the synthesis and properties of the second largest category of species, i.e., the anteiso-branched species, remain poorly characterized. In this study, we have developed a total synthesis route and completed a detailed NMR spectroscopic characterization of two common anteiso-branched species, namely: (22S)-22-methyltetracosanyl oleate and cholesteryl (22'S)-22'-methyltetracosanoate. In addition, we have studied their structural properties in the bulk state by wide-angle and small-angle X-ray scattering and their behavior at the aqueous interface using Langmuir monolayer techniques. A comparison to the properties displayed by iso-branched and straight-chain analogues indicate that branching patterns lead to distinct properties in the CE and WE lipid classes. Overall, this study complements the previous work in the field and adds another important brick in the tear film insights wall.

Insight into the Molecular Mechanism of Surface Interactions of Phosphatidylcholines─Langmuir Monolayer Study Complemented with Molecular Dynamics Simulations.

Mutual interactions between components of biological membranes are pivotal for maintaining their proper biophysical properties, such as stability, fluidity, or permeability. The main building blocks of biomembranes are lipids, among which the most important are phospholipids (mainly phosphatidylcholines (PCs)) and sterols (mainly cholesterol). Although there is a plethora of reports on interactions between PCs, as well as between PCs and cholesterol, their molecular mechanism has not yet been fully explained. Therefore, to resolve this issue, we carried out systematic investigations based on the classical Langmuir monolayer technique complemented with molecular dynamics simulations. The studies involved systems containing 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) analogues possessing in the structure one or two polar functional groups similar to those of DPPC. The interactions and rheological properties of binary mixtures of DPPC analogues with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol were compared with reference systems (DPPC/POPC and DPPC/cholesterol). This pointed to the importance of the ternary amine group in PC/cholesterol interactions, while in PC mixtures, the phosphate group played a key role. In both cases, the esterified glycerol group had an effect on the magnitude of interactions. The obtained results are crucial for establishing structure-property relationships as well as for designing substitutes for natural lipids.

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.

Aβ Amyloid Fibers Drastically Alter the Topography and Mechanical Properties of Lipid Membranes.

Alzheimer's disease (AD) is related to the fibrillation of the Aβ peptides at neuronal membranes, a process that depends on the lipid composition and may impart different physical states to the membrane. In the present work, we study the properties of the Aβ peptide when mixed with a zwitterionic lipid (DMPC), using the Langmuir monolayer technique as an approach to control membrane physical conditions. First, we build on previous characterizations of pure Aβ monolayers and observe that, in addition to high shear, these films present a pronounced compressional hysteresis. When Aβ is assembled with DMPC in a binary film, the resulting membranes become heterogeneous, with a peptide-enriched phase distributed in a network-like pattern, and they exhibit a lateral transition that depends on the Aβ content. At lower peptide proportions, the films segregate into two well-defined phases: one consisting of lipids and another enriched with peptides. The reflectivity of these phases differs from that obtained for pure Aβ films. Thus, the formed fibers effectively cover most of the interface area and remain stable at higher pressures (from 20 to 30 mN m-1 depending on Aβ content) compared to pure peptide films (17 mN m-1). Furthermore, such structures induce a compressional hysteresis in the film, similar to that of pure peptide films (which is nonexistent in the pure lipid monolayer), even at low peptide proportions. We claim that the mechanical properties at the interface are governed by the size of the fibril-like structures. Based on the low molar fractions and surface packing at which these phenomena were observed, we postulate that as a consequence of peptide intermolecular interactions, Aβ may have drastic effects on the molecular arrangement and mechanical properties of a lipid membrane.

Biochemical and Functional Characterization by Site-Directed Mutagenesis of a Phospholipase A2 from Scorpio maurus Venom

The study of amino acid interactions in the active site of scorpion venom phospholipases A2 could help to gain insights into the structure–function relationship and the biological activities of the enzyme. In the secreted phospholipase A2 of Scorpio maurus venom glands, Glutamate 63 and Tyrosine 122 amino acids play critical roles in the catalytic mechanism through interactions with residues around the calcium-binding loop. We constructed mutants at these positions by overexpression in Escherichia coli cells. After refolding and purification of recombinant enzymes, we studied their kinetic properties using pH-stat and monolayer techniques. The mutant Glutamate 63–Aspartate (E63D) exhibited a reduced activity, while the second mutant Tyrosine 122–Arginine (Y122R) retained some activity with a 14-fold reduction in catalytic efficiency. However, both mutants remained stable in pH values ranging from 2 to 12 whereas the double mutant D63–R122 was catalytically inactive. Comparative analysis of wild-type and mutant 3-D models showed various modifications of the hydrogen-binding network linking residues Glutamate 63 and Tyrosine 122. These modifications of interactions could explain the reduction in enzymatic activity. The kinetic behavior on phosphatidylcholine and phosphatidylethanolamine monolayers of three mutants was evaluated using a baro-stat system to assess the potential association between the hydrolysis of erythrocyte membrane phospholipids and the enzyme’s capability to penetrate phospholipid monolayers at high surface pressure. Mutants’ kinetic behaviors were similar to the wild-type form with slightly modified specific activities at high surface pressure. All mutants were more active on phosphatidylethanolamine than phosphatidylcholine films at high surface pressure. This study provided new information to further elucidate structure–function relationships of scorpion venom-secreted phospholipases A2 and the design of novel potent drug molecules.

Preparation of Thin Films Containing Modified Hydroxyapatite Particles and Phospholipids (DPPC) for Improved Properties of Biomaterials

The main aims of thin biofilm synthesis are to either achieve a new form to promote the transport of drugs in oral delivery systems or as a coating to improve the biocompatibility of the implant’s surface. In this study, the Langmuir monolayer technique was employed to obtain films containing Mg-doped hydroxyapatite with 0.5%, 1.0%, and 1.5% Mg(II). The obtained modified HA particles were analysed via the FT-IR, XRD, DLS, and SEM methods. It was shown that the modified hydroxyapatite particles were able to form thin films at the air/water interface. BAM microscopy was employed to characterized the morphology of these films. In the next step, the mixed films were prepared using phospholipid (DPPC) molecules and modified hydroxyapatite particles (HA-Mg(II)). We expected that the presence of phospholipids (DPPC) in thin films improved the biocompatibility of the preparing films, while adding HA-Mg(II) particles will promote antibacterial properties and enhance osteogenesis processes. The films were prepared in two ways: (1) by mixing DPPC and HA-Mg (II) and spreading this solution onto the subphase, or (2) by forming DPPC films, dropping the HA-Mg (II) dispersion onto the phospholipid monolayer. Based on the obtained π–A isotherms, the surface parameters of the achieved thin films were estimated. It was observed that the HA-Mg(II) films can be stabilized with phospholipid molecules, and a more stable structure was obtained from films synthesied via method (2).

Effects of subphase pH and temperature on the interfacial behavior of double hydrophilic diblock copolymer PDEGMA‐b‐PDIPAEMA

AbstractThe aggregation behavior of two pH‐ and temperature‐responsive diblock copolymers of poly[di‐(ethylene glycol) methyl ether methacrylate]‐block‐poly[2‐(diisopropylamino) ethyl methacrylate] (PDEGMA‐b‐PDIPAEMA) at the air/water interface and the structures of their Langmuir–Blodgett (LB) films were studied by the Langmuir monolayer technique and atomic force microscopy, respectively. At the air/water interface, PDEGMA‐b‐PDIPAEMA tends to form the core‐shell‐corona micellar structure composed of a PDIPAEMA main chain core, an amino ethyl ester shell, and a PDEGMA corona. Under acidic, neutral, and alkaline conditions, PDIPAEMA blocks are completely protonated, partially protonated, and completely non‐protonated, respectively, and the protonated amino ethyl ester groups are immersed in water before monolayer compression, whereas PDEGMA coronas are adsorbed at the interface. At pH 3, 7, and 10, the limiting areas (A0) for PDEGMA42%‐PDIPAEMA58% (weight percents) and PDEGMA55%‐PDIPAEMA45% are 8.2/10.2/14.0 and 6.7/8.3/8.4 nm2, respectively. The A0 values of the former copolymer are larger than those of the latter. This is because the shells in the former copolymer are denser due to the higher polymerization degree of PDIPAEMA blocks, providing greater steric hindrance for PDEGMA coronas and making the latter more extended at the interface. In contrast to other copolymer systems, the effect of temperature on the isotherms of PDEGMA‐b‐PDIPAEMA is less obvious.

Effect of lipid-polymer hybrid nanoparticles on the biophysical function and lateral structure of pulmonary surfactant: Mechanistic in vitro studies

The interaction between inhaled drug-loaded nanoparticles and pulmonary surfactant (PS) is critical for the efficacy and safety of inhaled nanomedicines. Here, we investigated the effect of small interfering RNA (siRNA)-loaded lipid-polymer hybrid nanoparticles (LPNs), which are designed for treatment of lung inflammation, on the physiological function of PS. By using biophysical in vitro methods we show that siRNA-loaded LPNs affect the biophysical function and lateral structure of PS. We used the Langmuir monolayer technique to demonstrate that LPNs display intrinsic surface activity by forming interfacial films that collapse at 49 mN/m, and they competitively inhibit the adsorption and spreading of PS components at the air–liquid interface. However, LPNs are excluded from the interface into the aqueous subphase at surface pressures above 49 mN/m, and hence they overcome the PS monolayer film barrier. Epifluorescence microscopy data revealed that LPNs influence the lateral structure of PS by: (i) affecting the nucleation, shape, and growth of compression-driven segregated condensed PS domains, and (ii) facilitating intermixing of liquid-expanded and tilted-condensed domains. However, the total surface area occupied by a highly condensed phase, presumably enriched in the highly surface tension-reducing dipalmitoylphosphatidylcholine, remained constant upon exposure to LPNs. These results suggest that surface-active LPNs influence the lateral structure of PS during translocation from the interface into the subphase, but LPNs do apparently not affect the biophysical function of PS under physiologically relevant conditions.

The effect of trans-resveratrol on the physicochemical properties of lipid membranes with different cholesterol content

Resveratrol is one of the most popular phytoalexins, which naturally occurs in grapes and red wine. This compound not only has beneficial effects on the human body, especially on the cardiovascular system, but also has antiviral, antibacterial and antifungal properties. In addition, resveratrol may have therapeutic effects against various types of cancer. The mechanism of action of resveratrol is not fully understood, but it is suspected that one of the most important steps is its interaction with the cell membrane and changing its molecular organization. Therefore, in the present study, we investigated the effects of resveratrol at different concentrations (0–75 μM) on model membranes composed of POPC, SM and cholesterol, in systems with different cholesterol contents and a constant POPC/SM molar ratio (1:1). Our tests included systems containing 5, 15 and 33.3 mol% cholesterol. Tests were carried out for monolayers using the Langmuir monolayer technique supported by Brewster angle microscopy and penetration experiments. Bilayer (liposome) experiments included calcein release, steady-state DPH fluorescence anisotropy and partition coefficients. The results showed that resveratrol interacts with model cell membranes (lipid monolayers and lipid bilayers), and its incorporation into membranes is accompanied by changes in their physicochemical parameters, such as lipid packing, fluidity and permeability. Furthermore, we showed that the cholesterol content of the membrane significantly affects the degree of incorporation of resveratrol into the model membrane, which may indicate that the molecular mechanism of action of this compound is closely related to its interactions with lipid rafts, domains responsible for regulating various cellular functions.

The Effect of the Interface Layer on the Electronic Parameters of the Light Emitting Electrochemical Cell (LEC) Device Based on Iridium (III) Complex that Calculated by Different Methods

We report a calculation of the electronic parameters and some optical measurement results of a light-emitting electrochemical cell (LEC), one of the most basic forms of electroluminescent films by modifying 4-(4-methoxy-N-naphthalen-1-ylanilino) benzoic acid (MA) as hole injection layer doped with an emissive layer that is ionic transition metal complex (Ir[dF(CF3)ppy]2(dtbpy))PF6-(Ir-iTMC). The electronic parameters of LEC device were computed using three different methods; Cheung and Cheung’s, Conventional Schottky and Norde, to evaluate the ideality factor (n), the barrier height () and the series resistance (Rs) of the LEC device. The formation on indium tin oxide (ITO) substrate was analyzed by Atomic Force Microscopy (AFM). The HOMO-LUMO energy level and electronic charge distribution of the MA molecule were computed theoretically using Chemissian software. The MA’s HOMO-LUMO energy level is determined to be EHOMO = −5.27 eV and ELUMO = −1.40 eV. The ITO surface has been modified with the Self-Assembled Monolayers (SAM) technique as an interlayer to improve the surface working function by simplifying hole injection from the ITO anode to the Hole Transport Layer (HTL) in the LEC. The Ir- iTMC complexes containing Π-conjugated bridging ligand shown better external quantum efficiency (EQE), reaching to 1.49%. The max luminance values of the bare-LEC and LEC-MA are measured as 145 cd m−2 and 1240 cd m−2, respectively.

Specific Ion Effect on the “Water Bridge” Interaction at an Interface: A New Understanding into the Extraction and Separation Selectivity Based upon Competitive Diffusion and Adsorption

Understanding the essence of a specific ion effect on the intermolecular interaction at the near-interface boundary layer during solvent extraction is crucial for developing new approaches to achieve enhanced extraction and separation of various valuable metals from complex aqueous solutions. The present work aims to detect the microscopic effect from competitive diffusion and adsorption of various salt cations in the near-interface boundary layer on their interaction with amphiphilic model molecules, 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (P507), at the air–water interface by using the Langmuir–Blodgett (LB) monolayer technique. It was found that the interaction between cations and P507 molecules through a long-range hydrogen bond “water bridge” confined in the near-interface boundary layer plays a crucial role in determining the difference in the diffusion mass transfer rate of ions with different hydration abilities. Conventional understanding about competitive extraction of various metal cations controlled mainly by their thermodynamic difference in the interaction intensity with the P–O and P═O groups in P507 molecules cannot explain such a kind of specific ion effect on the mass transfer kinetics via ion hydration. Molecular dynamics simulation revealed that the hydration ability of ions is the key factor determining the mass transfer rate and interaction intensity. In the low salt concentration region, the interaction intensity of salt cations with the P–O and P═O groups in P507 molecules is one of the determinants of the competitive adsorption behavior at the interface. However, in a higher salt concentration region, the specific ion salting-out effect via ion hydration becomes significant, causing a decrease in hydration of the target salt cation; therefore, it dominated the diffusion mass transfer rate of ions. This work provides a new insight to understand the kinetic role of competitive diffusion and adsorption of ions in the near-interface boundary layer and their effect on extraction and separation selectivity. It lays the foundation for achieving controllable separation of target metal ions by adjusting the coexisting ion species and their concentrations.

Novel Glycolipid Chips with a Double Layer of Au Nanoparticles for Biological Toxin Detection.

Glycolipid chips having a double layer of Au nanoparticles are proposed for detection of biological toxins. The sugar-modified chips constitute an under and an upper layer of Au nanoparticles of 20-80 nm diameter on glass plates, and Au nanoparticles of each layer are linked with 1,8-octanedithiol by a self-assembled monolayer (SAM) technique. A tris-sialo glycosphingolipid, ganglioside GT1b, having lipoic amide at the sphingosine part was immobilized on the Au outside surface of the upper layer, and botulinum toxin (type A heavy chain) was detected by localized surface plasmon resonance (LSPR). The GT1b-Cer-coated chip having a double layer of Au nanoparticles enhanced the toxin detection by LSPR more than those with single monolayers. The LSPR response changed according to the sizes of Au nanoparticles in each under and upper layer. The combination of 60 and 40 nm Au nanoparticles in the under and upper layer, respectively, gave the best result, which enabled the toxin detection at concentrations below 5 ng/mL with the portable LSPR device.

Physicochemical Characteristics of Model Membranes Composed of Legionella gormanii Lipids

Legionella gormanii is one of the species belonging to the genus Legionella, which causes atypical community-acquired pneumonia. The most important virulence factors that enable the bacteria to colonize the host organism are associated with the cell surface. Lipids building the cell envelope are crucial not only for the membrane integrity of L. gormanii but also by virtue of being a dynamic site of interactions between the pathogen and the metabolites supplied by its host. The utilization of exogenous choline by the Legionella species results in changes in the lipids' composition, which influences the physicochemical properties of the cell surface. The aim of this study was to characterize the interfacial properties of the phospholipids extracted from L. gormanii cultured with (PL+choline) and without exogenous choline (PL-choline). The Langmuir monolayer technique coupled with the surface potential (SPOT) sensor and the Brewster angle microscope (BAM) made it possible to prepare the lipid monomolecular films (model membranes) and study their properties at the liquid/air interface at 20 °C and 37 °C. The results indicate the effect of the choline addition to the bacterial medium on the properties of the L. gormanii phospholipid membranes. The differences were revealed in the organization of monolayers, their molecular packing and ordering, degree of condensation and changes in the components' miscibility. These findings are the basis for further research on the mechanisms of adaptation of this pathogen, which by changing the native composition and properties of lipids, bypasses the action of antimicrobial compounds and avoids the host immune attack.

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.

Study of Interactions between Saponin Biosurfactant and Model Biological Membranes: Phospholipid Monolayers and Liposomes

The aim of this study was to determine the effect of saponins-rich plant extract on two model biological membranes: phospholipid monolayers and liposomes. The Langmuir monolayer technique was used to study the interactions of model phospholipid membranes with saponins. The π-A isotherms were determined for DPPE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine) monolayer with the addition of various concentrations of licorice saponins extracts and subjected to qualitative as well as quantitative analysis. Additionally, relaxation studies of the obtained monolayers were carried out and morphological changes were examined using Brewster angle microscopy. Moreover, changes in the structure of phospholipid vesicles treated with solutions of saponins-rich plant extracts were assessed using the FTIR technique. The size and zeta potential of the liposomes were estimated based on DLS methods. The obtained results indicated that the saponins interact with the phospholipid membrane formed by DPPE molecules and that the stability of the mixed DPPE/saponins monolayer strongly depends on the presence of impurities in saponins. Furthermore, it was found that the plant extract rich in saponins biosurfactant interacts mainly with the hydrophilic part of liposomes.