Brewster Angle Research Articles

Unsaturated lipids modulating the interaction of the antileishmanial isolinderanolide E with models of cellular membranes

The present work evaluated the antiprotozoal activity of isolinderanolide E, isolated from the Brazilian plant Nectandra oppositifolia, against promastigote forms of Leishmania (Leishmania) amazonensis. The compound exhibited an EC50 value of 20.3 μM, similar to the positive control miltefosine (IC50 of 19.4 μM), and reduced toxicity to macrophages (CC50 > 200 μM). Based on these results, Langmuir monolayers of two unsaturated lipids: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), were employed as a model of mammalian and parasite membranes, respectively, to study the interaction of isolinderanolide E at a molecular level. The films were characterized with tensiometry (surface pressure-area isotherms and surface pressure-time curves), infrared spectroscopy, and Brewster angle microscopy (BAM). This compound changed the profile of the isotherms leading to fluid DOPC and DOPE monolayers, which were not able to attain rigid states even with compression. Infrared spectroscopy showed that the bioactive compound decreases the trans/gauche ratio conformers related to the molecular conformational disorder. BAM showed the formation of specific aggregates upon drug incorporation. In conclusion, isolinderanolide E changes the thermodynamic, mechanical, structural, and morphological characteristics of the monolayer of these unsaturated lipids, which may be essential to understand the action at the molecular level bioactives in biointerfaces.

Multifunctional hybrid films from sudan III and multiwalled carbon nanotubes: electrical conduction and photoinduced molecular orientation

Multifunctional materials (MFMs) can be an important system because of their capacity to combine different properties useful for electronic devices. Multiwalled carbon nanotubes (MWCNTs) and azobenzene derivate Sudan III (SIII) are compounds with great potential to form this type of system. These compounds exhibit properties of electrical conduction and photoinduced molecular orientation, respectively, which can be combined in a single film-shaped system. In this study, we report on the fabrication and characterization of multifunctional hybrid films created from SIII mixed with MWCNTs via both Langmuir and Langmuir–Blodgett (LB) techniques. For SIII+MWCNTs monolayers obtained using the Langmuir technique, the surface pressure isotherms revealed that the aggregation process of pure SIII drives the formation of monolayers. Brewster angle microscopy of the monolayers indicated that MWCNTs form loop-shaped structures resembling rings. Such structures were not present in films of pure MWCNTs or pure SIII. The current–voltage characteristic curve showed that the films have an ohmic behavior associated to free electrons in MWCNTs, exhibiting conductivity on the order of that of semiconductors. The SIII+MWCNTs LB films exhibited a photoinduced molecular orientation determined via SIII photoisomerization. These processes, occurring in a single film, are the conceptual proof that these systems are MFMs with potential applications in developing hybrid devices.

Polarization Measurement Method Based on Liquid Crystal Variable Retarder (LCVR) for Atomic Thin-Film Thickness

Atomic thickness thin films are critical functional materials and structures in atomic and close-to-atomic scale manufacturing. However, fast, facile, and highly sensitive precision measurement of atomic film thickness remains challenging. The reflected light has a dramatic phase change and extreme reflectivity considering the Brewster angle, indicating the high sensitivity of the optical signal to film thickness near this angle. Hence, the precision polarization measurement method focusing on Brewster angle is vital for the ultrahigh precision characterization of thin films. A precision polarization measurement method based on a liquid crystal variable retarder (LCVR) is proposed in this paper, and a measurement system with a high angular resolution is established. A comprehensive measurement system calibration scheme is also introduced to accommodate ultrahigh precision film thickness measurement. Repeatable measurement accuracy to the subnanometer level is achieved. Standard silicon oxide film samples of different thicknesses were measured around Brewster angle using the self-developed system and compared with a commercial ellipsometer to verify the measurement accuracy. The consistency of the thickness measurement results demonstrates the feasibility and robustness of the measurement method and calibration scheme. This study also demonstrates the remarkable potential of the LCVR-based polarization method for atomic film thickness measurement in ultraprecision manufacturing.

Optical resonances in graded index spheres: A resonant-state-expansion study and analytic approximations

Recent improvements in the resonant-state expansion (RSE), focusing on the static mode contribution, have made it possible to treat transverse-magnetic (TM) modes of a spherically symmetric system with the same efficiency as their transverse-electric (TE) counterparts. We demonstrate here that the efficient inclusion of static modes in the RSE results in its quick convergence to the exact solution regardless of the static mode set used. We then apply the RSE to spherically symmetric systems with continuous radial variations of the permittivity. We show that in TM polarization, the spectral transition from whispering gallery to Fabry-Perot modes is characterized by a peak in the mode losses and an additional mode as compared to TE polarization. Both features are explained quantitatively by the Brewster angle of the surface reflection which occurs in this frequency range. Eliminating the discontinuity at the sphere surface by using linear or quadratic profiles of the permittivity modifies this peak and increases the Fabry-Perot mode losses, in qualitative agreement with a reduced surface reflectivity. These profiles also provide a nearly parabolic confinement for the whispering gallery modes, for which an analytical approximation using the Morse potential is presented. Both profiles result in a reduced TE-TM splitting, which is shown to be further suppressed by choosing a profile radially extending the mode fields. Based on the concepts of ray optics, phase analysis of the secular equation, and effective quantum-mechanical potential for a wave equation, we have further developed a number of useful approximations which shed light on the physical phenomena observed in the spectra of graded-index systems.

Tunable giant two-dimensional optical beam shift from a tilted linear polarizer

We demonstrate an intriguing giant optical beam shift in a tilted polarizer system analogous to the Imbert-Fedorov shift in partial reflection around the Brewster's angle of incidence. We explain this giant shift using a generalized theoretical treatment for beam shift in a tilted uniaxial anisotropic material incorporating the three-dimensional orientation of its optic axis in the laboratory frame. A regulated control and tunability of both the magnitude and direction of the shifts are demonstrated further by wisely tuning the input polarization and the corresponding orientation angles. These findings open up the possibility of precise micron-level beam steering using a simple linear polarizer.

Quantization of Goos–Hänchen shift in monolayer graphene under partial and total internal reflection conditions

We theoretically investigate the Goos-Hänchen (GH) shifts of a reflected light beam from the dielectric interface containing a monolayer graphene sheet in the presence of an external perpendicular magnetic field. Using Kubo formalism we derive the expressions for the magneto-optical (MO) conductivities. Based on the angular spectrum analysis, we calculate and demonstrate that quantized GH shifts on the surface of graphene monolayer can be tuned by varying the intensity of the applied magnetic field and the beam incidence angle. We show that the GH shifts are quantized due to the Landau level (LL) quantization of the magneto-optical conductivities. In the vicinity of Brewster's angle the GH shift exhibit extreme positive or negative peaks around the magneto-excitation photonic energies in the terahertz regime. We discuss the dependence of the GH shifts on the strength of the magnetic field, the incidence angle, chemical potential, and the impinging frequency of the Gaussian beam. We also discuss the GH shifts for partial reflection (PR) and total internal reflection (TIR) conditions. We find that in the total internal reflection geometry, we have giant angular and spatial GH shifts in the vicinity of the Brewster angle as well as near the critical angle. The MO-modulated GH shift in graphene–substrate system provides a new mechanism to realize photonic devices in the terahertz region.

Surface mechanical behavior of water-spread poly(styrene)–poly(ethylene glycol) (PS–PEG) micelles at the air–water interface: Effect of micelle size and polymer end/linking group chemistry

Hypothesis:The surface mechanical properties of poly(styrene)-poly(ethylene glycol) (PS–PEG) micelles are influenced by the PEG corona structure. Changes in micelle aggregation number as well as changes in the PEG end group and linking group chemistry of the PS–PEG block copolymer are expected to alter PEG corona characteristics and therefore affect surface mechanical properties of the resulting micelle film. Experiments:Different sized micelles comprised of PS–PEG block copolymer chains were formulated by equilibrating micelles in different ratios of acetone/water mixtures and subsequently removing acetone using dialysis. Additionally, micelles of a similar size and PS–PEG molecular weight but slightly different chemistry were formulated. The micelles were characterized using dynamic light scattering (DLS), transmission electron microscopy (TEM), 1H NMR, surface pressure–area isotherms and Brewster angle microscopy (BAM). Findings:The reduction in micelle aggregation number results in the subsequent monolayer having higher compressibility moduli and bending stiffnesses and collapsing at lower surface pressures. Micelle hydrophobicity was shown to improve readsorption of micelles to interface after collapse. Analysis of Brewster angle microscopy images of out-of-plane wrinkle structures which formed upon monolayer collapse indicates the presence of continuous 1 nm thick PEG layer which allows micelle monolayers to bend under high compression.

Penicillin-binding proteins (PBPs) determine antibiotic action in Langmuir monolayers as nanoarchitectonics mimetic membranes of methicillin-resistant Staphylococcus aureus

The membrane of methicillin-resistant Staphylococcus aureus (MRSA) contains penicillin-binding proteins (PBPs) in the phospholipidic bilayer, with the protein PBP2a being linked with the resistance mechanism. In this work we confirm the role of PBP2a with molecular-level information obtained with Langmuir monolayers as cell membrane models. The MRSA cell membrane was mimicked with a mixed monolayer of dipalmitoyl phosphatidyl glycerol (DPPG) and cardiolipin (CL), also incorporating PBP2a. The surface pressure-area isotherms and the Brewster angle microscopy (BAM) images for these mixed monolayers were significantly affected by the antibiotic meropenem, which is PBP2a inhibitor. The meropenem effects were associated with the presence of PBP2a, as they were absent in the Langmuir monolayers without PBP2a. The relevance of PBP2a was confirmed with results where the antibiotic methicillin, known to be unsuitable to kill MRSA, had the same effects on mixed DPPG/CL and DPPG/CL-PBP2a monolayers since it prevented PBP2a from incorporating in the monolayer. The biological implication of the findings presented here is that a successful antibiotic against MRSA should be able to interact with PBP2a, but in the membrane.

Oxysterols can act antiviral through modification of lipid membrane properties – The Langmuir monolayer study

In this paper we tested how oxysterols influence on fusion process between viral lipid envelope and host cells membranes. For this purpose, the Zika virus was selected, while dendritic cell (DC) and neural cell (NC) membranes were chosen as target membranes. The investigated systems were modeled as multicomponent Langmuir monolayers and characterized using surface manometry and imaging in micro- (Brewster angle microscopy, BAM) and nanoscale (Atomic Force Microscopy, AFM) to monitor local heterogeneity. The fusion process was conducted by mixing viral and host cell membranes devoid and in the presence of oxysterols: 25-hydroxycholesterol (25-OH) and 7β-hydroxycholesterol (7β-OH) as representatives of chain- and ring-oxidized oxysterols, respectively. Our results show that oxysterols hinder the fusion with host cell membranes by modifying their biophysical properties. Moreover, oxysterols applied to an already infected membrane reverse the changes caused by the infection. It could therefore be concluded that oxysterols may display antiviral activity in two ways: they prevent the healthy membrane from viral infection by blocking the fusion process; and protect already infected membrane from pathological changes induced by the virus.

Synthesis and characterisation of carboxy amide-bonded pyridinium Gemini surfactants: influence of the nature of the spacer group and counterions on the aggregation behaviour

Abstract A series of novel dimeric pyridinium surfactants has been synthesised and the effects of a semi-flexible p-xylyl spacer and flexible, polyethylene glycol spacers have been studied. The nature of the spacer determines solubility and aggregation behaviour in two- and three-dimensional systems. Some of these insoluble compounds form two-dimensional, rigid-condensed structures at the air–water interface, while others form liquid-analogue monolayers. Whereas the latter compounds become soluble after exchange of the counterions, the former remain insoluble. The aggregation behaviour of Langmuir layers was, inter alia, investigated by Brewster angle microscopy. The micellisation behaviour of diluted aqueous solutions of soluble surfactants was primarily investigated by conductometric measurements and thermodynamic parameters of aggregation have been deduced with respect to the spacer length.

The interactions of trace amounts of ionic surfactants with mixed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/Cholesterol membranes

The influence of dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylamonium bromide (CTAB) and sodium dodecylsulfate (SDS) on the properties of mixed DPPC/cholesterol layers with 30% of cholesterol was investigated. The studies proved that even a tiny amount of surfactant (below 10 µM) alters the zeta potential of the cholesterol containing liposomes significantly, to the greater extent for the anionic surfactant. Moreover, the surfactant effect increases with the increasing concentration. In the case of the cationic surfactants the effect increases with the carbon chain length. In order to confirm the observed findings the experiment was extended to the study of the corresponding Langmuir monolayers. The discussion based on the π-A isotherms, penetration tests and Brewster angle microscopy provided additional evidence of the surfactants interactions with the membrane lipids. It was also found that the penetration of DTAB and SDS surfactants was favoured by the presence of cholesterol molecules in the DPPC layers.

Vertically Heterogeneous 2D Semi-Interpenetrating Networks Based on Cellulose Acetate and Cross-Linked Polybutadiene.

This work reports the feasibility of polybutadiene (PB) cross-linking under UV irradiation in the presence of a linear polymer, cellulose acetate (CA), to form semi-interpenetrating polymer networks at the air-water interface. The thermodynamic properties and the morphology of two-dimensional (2D) CA/PB blends are investigated after UV irradiation and for a wide range of CA volume fractions. A contraction of the mixed Langmuir films is observed independent of the composition, in agreement with that recorded for the individual PB monolayer after cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic CA/PB mixed film. From Brewster angle microscopy observations, the PB network synthesis does not induce any morphology change at the mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect of PB cross-linking on the vertical structure of CA/PB mixed films. For all studied compositions, significant thickening of the films is evidenced, consistent with their contraction ratio. This thickening is accompanied by a partial expulsion of the PB toward the film-air interface, which is attributed to the hydrophobic character of the PB. This phenomenon is stronger for films rich in PB. In particular, the structure of the PB-rich film undergoes a transition from vertically homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating polymer networks can thus be synthesized at the air-water interface with a morphology that is strongly influenced by the polymer-polymer and polymer-environment interactions.

Mechanisms of hypericin incorporation to explain the photooxidation outcomes in phospholipid biomembrane models

Cell membranes are the first barriers for drug binding and key for the action of photosensitizers (PS). Herein, we report on the incorporation of the PS hypericin into Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) to represent eukaryotic cell membranes, and 1,2-dioleoyl-sn-glycero-3-phospho(1’-rac-glycerol) (DOPG) to mimic bacterial membranes. Surface pressure (π) vs mean molecular area (Å) isotherms showed a high degree of interaction (binding, penetration and relative solubilization) of hypericin into DPPC and DOPC monolayers. On the other hand, electrostatic repulsions govern the interactions with DOPG and DOPS, favoring hypericin self-aggregation, as visualized by Brewster angle microscopy (BAM). Indeed, the larger domains in BAM were consistent with the greater expansion of DOPG monolayers with incorporated hypericin, owing to stronger electrostatic repulsions. In contrast to DPPC, light-irradiation of DOPC monolayers containing hypericin induced loss of material due to hydrocarbon chain cleavage triggered by contact-dependent reactions between triplet excited state of hypericin and chain unsaturations. The mild effects noted for both irradiated DOPS and DOPG monolayers are attributed to hypericin self-aggregation, which may have decreased the singlet oxygen quantum yield (Φ1O2) via self-quenching, despite the increased instability induced in the monolayers.

Determination of tilt degree and Weyl-node separation by the spatial Imbert-Fedorov shift near the Brewster angle

The Imbert-Fedorov (IF) shift at the optical interface of Weyl semimetal (WSM) is extremely significant to recognize the optical characteristics of WSM. In this paper, we have studied the spatial IF shift on the surface of a bulk WSM by theoretically deriving a 4 \ifmmode\times\else\texttimes\fi{} 4 magneto-optic matrix. It is found that the phenomenon of spatial IF shift changing from positive to negative occurs at an incident angle near the Brewster angle. Remarkably, this special phenomenon can be regulated by changing the frequency of incident light, tilt degree, and Weyl node separation. Specially, for a fixed Weyl node separation, the phenomena of spatial IF shift changing from positive to negative can be realized at different tilt degrees by simply changing the frequency, and there exists one-to-one correspondence between the tilt degree and frequency at these situations. Similarly, for a fixed tilt degree, these phenomena can be achieved at different Weyl node separations by changing the frequency, and there also exists one-to-one correspondence between the Weyl node separation and frequency. Therefore, these tunable phenomena of spatial IF shift changing from positive to negative enable the feasibility of accurate determination of tilt degree and Weyl node separation. Our study can also provide a reference for the research of IF shift in such similar structures.

Graphene Oxide Modulating the Bioelectronic Properties of Penicillinase Immobilized in Lipid Langmuir–Blodgett Films

In this paper, graphene oxide was incorporated in penicillinase-lipid Langmuir monolayers and transferred to solid supports as Langmuir-Blodgett (LB) films so that the enzyme catalytic properties could be evaluated. Adsorption of penicillinase and graphene oxide on dimyristoylphosphatidic acid (DMPA) monolayers at the air-water interface was investigated by tensiometry, vibrational spectroscopy, and Brewster angle microscopy. The LB films were characterized by quartz crystal microbalance, infrared spectroscopy, luminescence spectroscopy, and atomic force microscopy. Enzyme activity was studied with UV-vis spectroscopy, and the feasibility of the supramolecular device nanostructured as ultrathin films was essayed as an optical sensor device. The presence of graphene oxide in the enzyme-lipid LB film not only tuned the catalytic activity of penicillinase but also helped conserve its enzyme activity after weeks. These results may be related not only to the molecular architecture provided by the film but also to the synergism between the compounds on the active layer, leading to a molecular architecture that allowed a fast analyte diffusion owing to a suitable molecular accommodation which also preserved the penicillinase activity. This work then demonstrates the feasibility of employing LB films composed of lipids, graphene oxide, and enzymes as optical devices for biosensing applications as a proof-of-concept experiment.

Topological Imbert–Fedorov shifts in silicene

In this paper, we study the topological Imbert–Fedorov (IF) shifts of a light beam reflected from a silicene–substrate system in the presence of an externally applied electric field and circularly polarized beam. Silicene is a topological material and due to the strong spin–orbit interaction has a rich phase diagram. Using the famous Kubo formalism, we derive analytical expressions for the longitudinal and transverse Hall conductivities of silicene. By employing angular spectrum analysis, the analytical expression of the spatial IF shift is obtained. We show that electric field/circularly polarized light beam modulated positive and negative spatial beam shifts can be obtained by tuning the angle of incidence near the Brewster angle and the incident photonic energy in the terahertz regime. Furthermore, we also investigate the IF shifts in different topological phases of the silicene by driving the system through distinct topological quantum phase transitions (TQPTs), i.e., from topologically nontrivial to a semi-metallic state and further to a band insulating state by manipulating the interplay between the electric field and circularly polarized light beam. We believe these results are helpful for developing novel optoelectronic, spintronic, and valleytronic devices and may provide a possible technique for probing Berry curvature, topological Chern numbers, and TQPTs by direct optical measurement.

Morphological Behavior of DPPC-DMPC Two-dimensional Mixed Monolayer on Water Surface by Dropping Method

We have investigated the morphology of dipalmitoyl phosphatidyl choline (DPPC)-dimyristoyl PC (DMPC) mixed monolayer formed on the water surface by the dropping method using surface tension measurement (STm), Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IERS). The limiting molecular area (A0) obtained by STm showed that the negative deviation at < mole fraction (xDMPC)= 0.4 whereas the positive deviation at > xDMPC =0.6 when DMPC was added to the DPPC monolayer. BAM images also showed that the condensed rigid monolayer at xDMPC < 0.4 changed to the expandable monolayer at xDMPC > 0.5. Moreover, IER spectra showed that the alkyl chains of both DPPC and DMPC molecules are in trans conformation in mixed monolayer at xDMPC =0.2 whereas gauche conformation existed at xDMPC = 0.8. Above findings have justified the transition of flexible gauche conformation alkyl chains of DMPC molecule into rigid trans conformation by the addition of DPPC molecule at xDMPC < 0.4 whereas alkyl chains retained flexible gauche conformation in both DPPC and DMPC molecules at xDMPC > 0.6.

Self-Suppression of the Giant Coherent Anti-Stokes Raman Scattering Background for Detection of Buried Interfaces with Submonolayer Sensitivity.

Despite its success in many fields, the implementation of coherent anti-Stokes Raman spectroscopy (CARS) in tackling the problems at interfaces was hindered by the enormous resonant and nonresonant background from the bulk. In this work, we have developed a novel CARS scheme that can probe a buried interface via ≥105-fold suppression of the nonresonant and resonant bulk contribution. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we can resolve the vibrational spectrum of submonolayer interfacial polar/apolar species immersed in the surrounding medium with huge CARS responses. We expect that our approach opens up the opportunity to interrogate the interfaces involving apolar molecules and benefits other nonlinear optical spectroscopic techniques, e.g., sum-frequency spectroscopy and four-wave mixing spectroscopy in general, to promote the signal-to-background noise ratio.

Phase behavior and miscibility in lipid monolayers containing glycolipids

HypothesisGlycolipids in biological membranes are ubiquitous and believed to be involved in the formation of ordered functional domains. However, our current knowledge about such glycolipid-enriched domains is limited because they are inherently difficult to characterize. ExperimentsWe use grazing-incidence X-ray diffraction, isotherm measurements, and Brewster angle microscopy to investigate the phase behavior and miscibility in Langmuir lipid monolayers containing glycolipids. FindingsGlycolipid-enriched domains give rise to distinct diffraction patterns that allow for a systematic structural investigation and reveal a rich phenomenology, ranging from near-complete demixing to the formation of mixed domains with unique features. The phase behavior is governed by the headgroup chemistry and by the length and saturation of the tails.

Purified myelin lipids display a critical mixing point at low surface pressure

Lipids extracted from Purified Myelin Membranes (LPMM) were spread as monomolecular films at the air/aqueous interface. The films were visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (π) and ionic environments. Coexistence of Liquid-Expanded (LE) and cholesterol-enriched (CE) rounded domains persisted up to π ≈ 5 mN/m but the monolayers became homogeneous at higher surface pressures. Before mixing, the domains distorted to non-rounded domains. We experimentally measured the line tension (λ) for the lipid monolayers at the domain borders by a shape relaxation technique using non-homogeneous electric fields. Regardless of the subphase conditions, the obtained line tensions are of the order of pN and tended to decrease as lateral pressure increased toward the mixing point. From the mean square displacement of nested trapped domains, we also calculated the dipole density difference between phases (μ). A non-linear drop was detected in this parameter as the mixing point is approached. Here we quantitively evaluated the π-dependance of both parameters with proper power laws in the vicinity of the critical mixing surface pressure, and the exponents showed to be consistent with a critical phenomenon in the two-dimensional Ising universality class. This idea of bidimensionality was found to be compatible only for simplified lipidic systems, while for whole myelin monolayers, that means including proteins, no critical mixing point was detected.Finally, the line tension values were related with the thickness differences between phases (Δt) near the critical point.