Monolayer Phase Research Articles

Effects of γ-zein peptides on lipid membrane organization: Quartz crystal microbalance with dissipation and Langmuir monolayer studies

γ-zein peptides (VHLPPP)n present great potential as efficient molecule carriers across the cell membrane. To further understand the binding behavior of peptides with cell membrane, simplified models consisting of peptides and mono/bi-layer membranes (Langmuir monolayer and liposome) were used to studied the resulting response in the structure of peptide and cell membrane in the present work. The results showed that (VHLPPP)n=1,3 bound to dipalmitoylphosphatidylcholine (DPPC) liposomes and was induced to self-assemble into polyproline II (PP II) structure. The incorporation of (VHLPPP)n=1,3 in the DPPC Langmuir monolayers resulted in an increase in the surface pressure and a decrease in the collapse pressure, suggesting the increase of lipid monolayer fluidity. Fluorescence images further confirmed that the presence of (VHLPPP)n=1,3 promoted the formation of liquid-expanded (LE, dis-ordered) phases in lipid monolayers. Atomic force microscopy images found that (VHLPPP)n=1,3 preferred to penetrate in LE phase domains. In addition, (VHLPPP)3 possesses higher binding affinity with DPPC membrane models compared to (VHLPPP)1.

Temperature-Responsive Hydrophobic Silica Nanoparticle Ultrasound Contrast Agents Directed by Phospholipid Phase Behavior.

In this paper, we report ultrasonically active nanoscale contrast agents that behave as thermometric sensors through phase change in their stabilizing phospholipid monolayer. Phospholipid-stabilized, hydrophobic mesoporous silica nanoparticles (P@hMSNs) are known to interact with high-intensity focused ultrasound (HIFU) to promote cavitation at their surfaces, which can be used for both imaging and therapy. We show that the lateral lipid phase behavior of the phosphocholine lipid dictates the acoustic contrast of the P@hMSNs. When the lipids are in the gel phase below their melting temperature, the P@hMSNs generate detectable microbubbles when exposed to HIFU. However, if the lipids exhibit a liquid expanded phase, the P@hMSNs cease to generate bubbles in response to HIFU insonation. We verify that the heating and subsequent transition of lipid coating the hMSN are associated with the loss of acoustic response by doping laurdan dye into the lipid monolayer and imaging lipid phase through red shifts in emission spectra. Similarly, cessation of cavitation was also induced by adding a fluidizing surfactant such as Triton X, which could be reversed upon washing away the excess surfactant. Finally, by controlling for the partial fluidization caused by the adsorption of protein, P@hMSNs may be used as thermometric sensors of the bulk fluid temperature. These findings not only impact the utilization of nanoscale agents as stimulus-responsive ultrasound contrast agents but also have broader implications for how cavitation may be initiated at surfaces coated by a surfactant.

Correction to "Molecular Dynamics and Energy Landscape of Decanethiolates in Self-Assembled Monolayers on Au(111) Studied by Scanning Tunneling Microscopy".

The energetics and dynamics of the various phases of decanethiolate self-assembled monolayers on Au(111) surfaces were studied with scanning tunneling microscopy. We have observed five different phases of the decanethiolate monolayer on Au(111): four ordered phases (β, δ, χ*, and ) and one disordered phase (e). We have determined the boundary free energies between the disordered and order phases by analyzing the thermally induced meandering of the domain boundaries. On the basis of these results, we are able to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system. The order–disorder phase transition of the χ* phase occurs at 295 K, followed by the order–disorder phase transition of the β phase at 325 K. Above temperatures of 325 K, only the densely packed and disordered e phases remain. Our findings are in good agreement with the phase diagram of the decanethiolate/Au(111) system that was put forward by Poirier et al

H-CaS and h-CaSe nanosheets in CaX (X = O, S, Se and Te) series: promising thermoelectric materials under DFT investigation

The present work reports systematic study of a series of calcium chalcogenides CaX (X = O, S, Se and Te) in the architecture of rock-salt and hexagonal monolayer phases. Using first principle investigation within density functional theory (DFT) framework, we have computed the equilibrium structure and phonon dispersion curves for the dynamic stability, which follows the calculation of electronic properties like electronic band structure and projected density of state for the considered chalcogenide series. Furthermore, the thermoelectric properties such as thermal and electrical conductivities, Seebeck coefficient (S) and figure of merit (ZT) of the considered compounds are computed using the semi-classical Boltzmann transport equations (BTE). The present work reports the monolayer calcium chalcogenides as potential candidate for thermoelectric applications.

Prediction of MoO2 as high capacity electrode material for (Na, K, Ca)-ion batteries

Na, K and Ca-ion battery electrode materials with appropriate electrochemical properties are desirable candidates for replacing lithium-ion batteries (LIBs) because of their natural richness and low cost. Recently, MoO2 has been reported as the anode material in LIBs, but so far not received attention in Na and other ion batteries. In this paper, the behaviors of Na, K, and Ca on MoO2 are investigated by first-principles calculations. These metal atoms strongly absorb on the hexagonal center of MoO2 and the adsorption results in semiconducting-metallic transition. The low diffusion barrier, 0.13, 0.08, 0.22 eV for Na, K, Ca, respectively, leads to an ultrahigh diffusivity. Importantly, the maximum metal-storage phases of MoO2 monolayer correspond to Na4MoO2, K2.5MoO2 and Ca3MoO2, with considerable theoretical specific capacities of 837, 523 and 1256 mAh g−1. The electrode materials exhibit moderate average voltage of 0.30, 0.75, 0.35 V, respectively. Our findings suggest that MoO2 monolayer can be utilized as a promising anode material with high capacities and high rate performance for next generation ion batteries.

Single-Metal Atom Anchored on Boron Monolayer (β12) as an Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions: A First-Principles Study

On the basis of the first-principles calculation, single transition-metal (TM) atoms of first and second transition series are anchored on the β12 phase of the boron monolayer (BM) electrocatalyst ...

Structural phase transitions in VSe2: energetics, electronic structure and magnetism.

First principles calculations of the magnetic and electronic properties of VSe2 describing the transition between two structural phases (H,T) were performed. The results of the calculations evidence a rather low energy barrier (0.60 eV for the monolayer) for the transition between the phases. The energy required for the deviation of a Se atom or whole layer of selenium atoms by a small angle of up to 10° from their initial positions is also rather low, 0.32 and 0.19 eV/Se, respectively. The changes in the band structure of VSe2 caused by these motions of Se atoms should be taken into account for analysis of the experimental data. Simulations of the strain effects suggest that the experimentally observed T phase of the VSe2 monolayer is the ground state due to substrate-induced strain. Calculations of the difference in the total energies of the ferromagnetic and antiferromagnetic configurations evidence that the ferromagnetic configuration is the ground state of the system for all stable and intermediate atomic structures. Calculated phonon dispersions suggest a visible influence of the magnetic configurations on the vibrational properties.

Molecular interactions of phospholipid monolayers with a model phospholipase.

The intrinsic overexpression of secretory phospholipase A2 (sPLA2) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA2 (from bee venom) with the DPPC monolayer exhibited Ca2+ dependence. DPPC molecules at the interface without Ca2+ retained a monolayer organization; upon adsorption of sPLA2 to the monolayer the packing became tighter. In contrast, sPLA2-catalyzed degradation of DPPC occurred in the presence of Ca2+, leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA2. A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

Exploring the electronic and magnetic properties of new metal halides from bulk to two-dimensional monolayer: RuX3 (X = Br, I)

Theoretical and experimental studies present that metal halogens in MX$_3$ forms can show very interesting electronic and magnetic properties in their bulk and monolayer phases. Many MX$_3$ materials have layered structures in their bulk phases, while RuBr$_3$ and RuI$_3$ have one-dimensional chains in plane. In this paper, we show that these metal halogens can also form two-dimensional layered structures in the bulk phase similar to other metal halogens, and cleavage energy values confirm that the monolayers of RuX$_3$ can be possible to be synthesised. We also find that monolayers of RuX$_3$ prefer ferromagnetic spin orientation in the plane for Ru atoms. Their ferromagnetic ground state, however, changes to antiferromagnetic zigzag state after U is included. Calculations using PBE+U with SOC predict indirect band gap of 0.70 eV and 0.32 eV for the optimized structure of RuBr$_3$ and RuI$_3$, respectively. Calculation based on the Monte Carlo simulations reveal interesting magnetic properties of RuBr$_3$, such as large Curie temperature against RuI$_3$, both in bulk and monolayer cases. Moreover, as a result of varying exchange couplings between neighboring magnetic moments, magnetic properties of RuBr$_3$ and RuI$_3$ can undergo drastic changes from bulk to monolayer. We hope our findings can be useful to attempt to fabricate the bulk and monolayer of RuBr$_3$ and RuI$_3$.

Kidney organoids—a new tool for kidney therapeutic development

Kidney organoids, derived from human pluripotent stem cells, have the potential to greatly facilitate drug development. Boreström etal. have used CRISPR/Cas9 to create kidney fluorescent lineage markers for SIX2 and NPHS1 to monitor the differentiation process to tubular and glomerular structures and optimize maturity. The convergence of "personalized" kidney organoids with genome editing and single-cell sequencing technology hold great promise to result in better insight to disease, better human cell disease models, more predictive toxicology, and potentially "clinical trials in a dish."

Solid–Liquid Phase Transition in the Octadecanoic Acid Film Adsorbed on the Toluene–Water Interface

The structure of the soluble protonated (pH=2) octadecanoic acid film adsorbed on the saturated hydrocarbon (n-hexane) - water and aromatic hydrocarbon (toluene)-water interfaces is studied by X-ray reflectometry using synchrotron radiation. The experimental data demonstrate that a solid phase of a Gibbs monolayer (26 +/- 1) Angstrom thick, in which aliphatic tails are perpendicular to the surface and the area per molecule is A=(18 +/- 2) Angstrom^2, forms in the film at the n-hexane - water interface. The solid monolayer on the toluene - water interface in the adsorbed film melts when temperature increases, and this transition is caused by disordering the hydrocarbon tails of the acid. During the solid - liquid transition, the Gibbs monolayer thickness remains almost the same, (22 +/- 1) Angstrom. In the solid phase, we have A=(20+/- 2) Angstrom^2 and the angle of deviation of the molecular tails from the normal to the surface is about ~ 30 deg. The density of the liquid monolayer phase with A=(24 +/- 2) Angstrom^2 corresponds to liquid n-octadecane.

Material Genome Explorations and New Phases of Two-Dimensional MoS2, WS2, and ReS2 Monolayers

Two-dimensional transition metal dichalcogenides have attracted intense interests in recent years. Existing studies have fully explored the properties of their ground-state structures, but their global energy landscapes are still not well understood. The global energy landscape is important for understanding the experimental synthesis and thermal dynamic properties as well as for discovering new phases. This work uses material genome techniques (huge global search and data mining) to explore the global energy landscapes and new phases of two-dimensional MoS2, WS2, and ReS2 monolayers at ab initio level. Our results show that their energy landscapes have two or three major funnels, each of which consists of a few local minima with hexagonal, octahedral, quadrilateral, nanoribbon structures as well as their distorted and hybrid structures. The global-minimum structures are confined in deep funnels while the higher-energy minima stay in flat funnels and can transform to other minima at high temperatures. A few new phases with high geometry symmetry are found, e.g., quadrilateral phase, nanoribbon phase, distorted hexagonal phase, 4-2-coordination phase, and Pmm2 space group phase. These new phases exhibit novel phonon and electron properties (e.g., direct Γ point band gap and distorted Dirac cone), and could be the new candidates for devices applications. We further demonstrate that these phases could have better stability with charge doping and thus could be fabricated in experiments.

Electronic structure and nearly room-temperature ferromagnetism in V-doped monolayer and bilayer MoS2

The electronic structure, magnetic properties and ferromagnetic transition temperature (Tc) of Vandium (V) doped monolayer (ML) and bilayer (BL) MoS2 are investigated using density function theory (DFT) plus on-site Hubbard potential correction (U). The results show that substitution of V dopant atom at the Mo sites are energetically favorable and magnetic interaction between two dopants in ML and BL MoS2 oscillates from ferromagnetic (FM) to antiferromagnetic (AF) depending on atomic distance between dopants. Our result also shows that a pair of V dopants in different layers of BL MoS2 interacts antiferromagnetically. Moreover, it is obtained that interlayer interaction in BL MoS2 affects the magnetic interaction in V-doped BL MoS2. The calculated ferromagnetic transition temperatures (Tc) value for impurity concentration of 12.5% and 22.22% are 242 and 285 K, respectively, for ML phase. However, for BL phase T[Formula: see text] values are 187 and 256 K for concentration of 6.25% and 11.11%, respectively, these values are closer to room-temperature. Our calculations indicate that, V-doped ML and BL MoS2 are promising candidates for 2D dilute magnetic semiconductors for spintronics applications.

Ab initio study of the (2 × 2) phase of barium on graphene

We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 × 2) spots induced by barium adatoms with BaC8 stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC6 monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC8 phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene π states and new barium-derived hole Fermi surface at the M-point of the (2 × 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron–phonon coupling of this novel (2 × 2) obtaining λ = 0.26, which excludes the stabilization of a superconducting phase.

Over-Stoichiometry in Heavy Metal Oxides: The Case of Iono-Covalent Tantalum Trioxides.

Oxides of tantalum (common examples including TaO, TaO2, and Ta2O5) are key oxide materials for modern electronic devices, such as dynamic random-access memory and field effect transistors. Of late, new forms of stable tantalum oxides have been proposed as two-dimensional nanosheet structures with a nonconventional stoichiometry of TaO3 via soft-chemical delamination of RbTaO3. However, not much is known about the elusive nanosheet-structured TaO3, unlike other closely related common trioxides of W and Mo. In this work, using first-principles density functional theory calculations, we have studied various TaO3 structures as inspired from previous theoretical and experimental studies and discuss their properties with respect to the more conventional oxide of tantalum, Ta2O5. We have calculated their thermodynamics and lattice properties and have found a new stable-layered β-TaO3 and its exfoliated monolayer phase (β'). By further analyzing their electronic structures, we discuss the mixed iono-covalent bonding characteristics in the TaO3 phases, challenging the conventional formal oxidation state model for metal oxides. Finally, we propose how these new TaO3 oxide materials may be potentially useful in photodevice applications.

Liquid-crystal Assembly of Semiflexible-coil/Homopolymer Blends: a Dissipative Particle Dynamics Study

The liquid-crystal assembly of semiflexible-coil diblock copolymers with coil or semiflexible homopolymers is studied by dissipative particle dynamics simulation. Phase diagrams of the blends and orientation ordering parameters among semiflexible blocks are constructed as a function of chain stiffness and homopolymer volume fraction. For semiflexible-coil/coil blends with varying stiffness of semiflexible blocks, we display the rich phase behaviors of the system transited from coil-coil/coil to rod-coil/coil blends. The disorder-lamellae or lamellae-liquid crystalline transition and “dry brush” phenomenon induced by coil homopolymers are observed. For semiflexible-coil/semiflexible blends, adding semiflexible homopolymers also leads to a disorder-order transition and even a transition between monolayer and bilayer smectic-A phase. The results demonstrate that blending homopolymers into semiflexible copolymers can induce liquid-crystal assembly and even improve the orientation ordering of semiflexible blocks effectively.

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

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

Pb arachidate Langmuir-Blodgett coatings of silicon wafers: relation between Pb particle density and subphase composition

The formation of lead arachidate (Pb(AA)2) coatings produced by Langmuir tensiometry (LT) was studied in dependence of lead nitrate (Pb(NO3)2) concentration in the aqueous subphase at pH 5.5. The stable Pb(AA)2 monolayers were transferred to Si wafers and TEM grids at 35 mN/m in the liquid condensed phase of the AA monolayer applying the Langmuir-Blodgett (LB) (ten monolayers) or the Langmuir-Schaefer (LS) technique, respectively. The most homogeneous monolayer was obtained at a Pb(NO3)2 concentration of 0.1 mM by using the LS method. Most homogeneous films (five bilayers) were obtained for a Pb(NO3)2 concentration of 0.01 mM by using the LB method. The formed films are investigated by transmission electron microscopy (monolayer produced by LS), X-ray reflectivity (XRR), and atomic force microscopy (films produced by LB). These methods revealed the formation of homogeneously distributed lead inclusions in the formed monolayers and films. Pb aggregates increase in number whereas their average size stays constant at ≈ 50 nm2 with increasing Pb(NO3)2 concentration. Thickness parameters were determined by XRR. With increasing Pb(NO3)2 concentration, the bilayer thickness was found to increase. Furthermore, the patterns (111) and (200) for a cubic lead structure were found. The experimental results are compared with simulations obtained from geometry optimization on a semi-empirical quantum level to discuss the lead-ion binding to the AA molecules at pH 5.5.

Single layer of carbon phosphide as an efficient material for optoelectronic devices

Recently, a single layer of carbon phosphide allotropes were theoretically investigated and show finite energy band gap and high carrier mobility to attract rapidly growing interests. Here, we study the structural, electronic and optical properties of single-layer carbon phosphide (CP) allotropes (α-, β- and γ-phases) based on density functional theory. The thermoelectric properties like electrical conductivity, thermal conductivity, thermoelectric power, figure of merit (ZT) and compatibility factor as a function of temperature are calculated by using BoltzTrap code. The electronic band structures reveal the direct band gap of α- and β-CP monolayer (i.e., semiconducting nature), while γ-CP monolayer is semimetallic with Dirac fermions. The significant absorption is observed in α-, β- and γ-CP monolayer, which can be used as an ultraviolet–optical–nanodevice, and all three phases of monolayer of CP are directionally transparent. The transmission spectrum of monolayer of β- and γ-phase in the visible region is much higher; therefore, it is used in an anti-reflecting layer in solar cell also. The α-phase of CP monolayer in the ZT increases linearly up to 1500 K, and beyond it reached maximum values as compared to other phases. The results show that below 550 K, CP allotropes (both n- and p-types) are hitherto the best-promising thermoelectric materials. These theoretical investigations suggest that the different phases of semiconducting materials of CP are better candidate for potential application in micro-/nanoscale device, photovoltaic and optoelectronics.

Evidence of lying molecules in the structure of the most condensed phase of semi-fluorinated alkane monolayers.

Semifluorinated alkanes are known to form peculiar nano-structured monolayers on the surface of water. A comprehensive analysis of in situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) proves that the structure of their condensed phase consists of domains of upright molecules surrounded by lying down molecules. Such a model explains the non-coalescence of the domains and is in agreement with the high resolution AFM images of monolayers at the surfaces of solid substrates. The interaction of the lying molecule dipoles with the dipole of the water surface is proposed to explain the observed structuration.