Total Bilayer Thickness Research Articles

Lossy mode resonance-based highly sensitive fiber optic refractive index sensor using the bilayer of FTO/HfO2 for operation in the visible region

A lossy mode resonance (LMR) based fiber optic refractive index (RI) sensor utilizing a layer of fluorine-doped tin oxide (FTO) over the unclad core of the fiber is theoretically analyzed. To enhance its sensitivity, the thickness of the FTO layer is optimized. For further sensitivity enhancement, an over-layer of two different materials, T i O 2 and H f O 2 , is used. The sensitivity of F T O / T i O 2 and F T O / H f O 2 bilayer based LMR sensors increases with the increase in the thickness of the over-layer for a fixed thickness of the bilayer. Further, the sensitivity of the sensor utilizing the F T O / H f O 2 bilayer is found to be more than the sensitivity of the F T O / T i O 2 bilayer sensor. To keep the operating spectral range of the sensor in the visible region, the total thickness of the bilayer is adjusted. It is found that a total bilayer thickness of 30 nm with 90% thickness of H f O 2 gives the maximum sensitivity of 4400 nm/RIU for 1.33 RI of the sensing medium, which is more than three times the sensitivity of the FTO coated LMR sensor. The sensitivity of the proposed LMR sensor utilizing the F T O / H f O 2 bilayer is compared with the other LMR sensors reported in the literature, and it is found that the proposed sensor possesses the highest sensitivity. The other advantage of the proposed sensor is that it works in the visible region, which reduces the cost of the sensor.

Molecular Structure of Sphingomyelin in Fluid Phase Bilayers Determined by the Joint Analysis of Small-Angle Neutron and X-ray Scattering Data.

We have determined the fluid bilayer structure of palmitoyl sphingomyelin (PSM) and stearoyl sphingomyelin (SSM) by simultaneously analyzing small-angle neutron and X-ray scattering data. Using a newly developed scattering density profile (SDP) model for sphingomyelin lipids, we report structural parameters including the area per lipid, total bilayer thickness, and hydrocarbon thickness, in addition to lipid volumes determined by densitometry. Unconstrained all-atom simulations of PSM bilayers at 55 °C using the C36 CHARMM force field produced a lipid area of 56 Å2, a value that is 10% lower than the one determined experimentally by SDP analysis (61.9 Å2). Furthermore, scattering form factors calculated from the unconstrained simulations were in poor agreement with experimental form factors, even though segmental order parameter (SCD) profiles calculated from the simulations were in relatively good agreement with SCD profiles obtained from NMR experiments. Conversely, constrained area simulations at 61.9 Å2 resulted in good agreement between the simulation and experimental scattering form factors, but not with SCD profiles from NMR. We discuss possible reasons for the discrepancies between these two types of data that are frequently used as validation metrics for molecular dynamics force fields.

Towards biomimics of cell membranes: Structural effect of phosphatidylinositol triphosphate (PIP3) on a lipid bilayer.

Phosphoinositide (PIP) lipids are anionic phospholipids playing a fundamental role for the activity of several transmembrane and soluble proteins. Among all, phosphoinositol-3',4',5'-trisphosphate (PIP3) is a secondary signaling messenger that regulates the function of proteins involved in cell growth and gene transcription. The present study aims to reveal the structure of PIP-containing lipid membranes, which so far has been little explored. For this purpose, supported lipid bilayers (SLBs) containing 1,2-dioleoyl-sn-glycero-3-phospho-(1'-myo-inositol-3',4',5'-trisphosphate (DOPIP3) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were used as mimics of biomembranes. Surface sensitive techniques, i.e. Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), Atomic Force Microscopy (AFM) and Neutron Reflectometry (NR), provided detailed information on the formation of the SLB and the location of DOPIP3 in the lipid membrane. Specifically, QCM-D and AFM were used to identify the best condition for lipid deposition and to estimate the total bilayer thickness. On the other hand, NR was used to collect experimental structural data on the DOPIP3 location and orientation within the lipid membrane. The two bilayer leaflets showed the same DOPIP3 concentration, thus suggesting the formation of a symmetric bilayer. The headgroup layer thicknesses of the pure POPC and the mixed POPC/DOPIP3 bilayer suggest that the DOPIP3-headgroups have a preferred orientation, which is not perpendicular to the membrane surface, but instead it is close to the surrounding lipid headgroups. These results support the proposed PIP3 tendency to interact with the other lipid headgroups as PC, so far exclusively suggested by MD simulations.

The Molecular Structure of Sphingomyelin in Fluid Phase Bilayers Determined by the Joint Analysis of Neutron and X-Ray Scattering Data

We have determined the fluid bilayer structure of palmitoyl sphingomyelin (PSM) and stearoyl sphingomyelin (SSM) by simultaneously analyzing small-angle neutron and X-ray scattering data. We report structural parameters obtained using the newly developed scattering density profile (SDP) model for sphingomyelin lipids, including the area per lipid, total bilayer thickness and hydrocarbon thickness, in addition to lipid volumes determined by densitometry. Unconstrained all-atom simulations of PSM bilayers at 55°C using the C36 CHARMM force field produced a lipid area of 56 A2, a value that was 10% lower than the one determined experimentally (61.9 A2). Furthermore, scattering form factors calculated from the unconstrained simulations were in poor agreement with experimental form factors, despite the fact that segmental order parameter (SCD) profiles calculated from the simulations were in relatively good agreement with previously published SCD profiles obtained from NMR experiments. Conversely, constrained area simulations at 61.9 A2 resulted in good agreement between the simulation and experimental scattering form factors, but not with SCD profiles. We discuss possible reasons for the discrepancies between these two important types of data that are frequently used as validation metrics for molecular dynamics force fields.

Enhancement of thermoelectric effect in diffusive superconducting bilayers with magnetic interfaces

We demonstrate that thermoelectric currents in superconducting bilayers with a spin-active interface are controlled by the two competing processes. On one hand, spin-sensitive quasiparticle scattering at such an interface generates an electron-hole imbalance and yields an orders-of-magnitude enhancement of the thermoelectric effect in the system. On the other hand, this electron-hole imbalance gets suppressed in the superconductor bulk due to electron scattering on nonmagnetic impurities. As a result, large thermoelectric currents can only flow in the vicinity of the spin-active interface and decay away from this interface at a distance exceeding the electron elastic mean free path $\ensuremath{\ell}$. The magnitude of the thermoelectric effect reaches its maximum provided $\ensuremath{\ell}$ becomes of order of the total bilayer thickness.

Theoretical modeling of lossy mode resonance based refractive index sensors with ITO/TiO₂ bilayers.

A lossy mode resonance (LMR) based fiber optic refractive index (RI) sensor utilizing indium tin oxide (ITO)/titanium dioxide (TiO₂) bilayers has been theoretically investigated. Specifically, the effect of different thickness ratios of bilayers on the sensitivity to surrounding medium RI variations was studied. It was observed that for a specific thickness ratio, a two fold increase in sensitivity can be achieved in comparison to conventional LMR sensors using a single absorbing thin film. The sensitivity can be enhanced by tuning the total bilayer thickness and bilayer thickness ratio for the desired application, which is not possible in conventional LMR sensors.

Reactive Ni/Ti nanolaminates

Nickel/titanium nanolaminates fabricated by sputter deposition exhibited rapid, high-temperature synthesis. When heated locally, self-sustained reactions were produced in freestanding Ni/Ti multilayer foils characterized by average propagation speeds between ∼0.1 and 1.4 m/s. The speed of a propagating reaction front was affected by total foil thickness and bilayer thickness (layer periodicity). In contrast to previous work with compacted Ni–Ti powders, no preheating of Ni/Ti foils was required to maintain self-propagating reactions. High-temperature synthesis was also stimulated by rapid global heating demonstrating low ignition temperatures (Tig)∼300–400 °C for nanolaminates. Ignition temperature was influenced by bilayer thickness with more coarse laminate designs exhibiting increased Tig. Foils reacted in a vacuum apparatus developed either as single-phase B2 cubic NiTi (austenite) or as a mixed-phase structure that was composed of monoclinic B19′ NiTi (martensite), hexagonal NiTi2, and B2 NiTi. Single-phase, cubic B2 NiTi generally formed when the initial bilayer thickness was made small.

Contact Instability of Elastic Bilayers: Miniaturization of Instability Patterns

AbstractWe show, both experimentally and theoretically, that the free surface of an elastic bilayer becomes spontaneously rough when brought in contact with another rigid surface. The lateral length scales of these self‐organized structures were found to scale as: λ = RF h, where h is the total bilayer thickness. The scale factor, RF could be modulated by the ratios of the individual film thicknesses and shear moduli. This is unlike the case of a single elastic film where the scale factor is independent of all material properties, RF ∼ 3. A linear stability analysis shows that the instability patterns in the bilayer can be tuned from the short waves (∼ 0.5 h) to long waves (∼ 8 h). Experiments show good agreement with the theoretical predictions regarding the existence of short wave deformations. Further, a rather catastrophic change in the wavelength from its minimum value, ∼h/2, to a limiting value of ∼ 3 h occurs by very small changes in the film thicknesses. This effect is explained by a switching of states in the bilayer energy curve which displays two minima at different wave numbers. Thus, the contact instability of elastic bilayers suggests novel strategies for the control of adhesion and engineering of feature sizes in a wide range. In particular, these findings have implications for further pattern miniaturization in the elastic contact lithography using pre‐patterned stamps.

Interface effects in ferroelectric bilayers and heterostructures

We study the role of interlayer interfaces on the polarization response of ferroelectric-paraelectric bilayers using a nonlinear thermodynamic model. We carry out a numerical analysis for prototypical BaTiO3–SrTiO3 bilayers ranging from 40to800nm total thickness as a function of SrTiO3 fraction. There exists a critical fraction of SrTiO3 at which the polarization is suppressed due to the depolarization field arising from the interlayer coupling and a large dielectric response is predicted. It is shown that this critical fraction decreases with decreasing total bilayer thickness, indicating that the interfacial effects are more pronounced in thinner bilayers.

Aggregation and Phase Behavior of a Double-Chain Surfactant, N-Dodecyl-N-octyl-N-methylamine Oxide, as a Function of the Protonation Degree

The phase sequence of the N-dodecyl-N-octyl-N-methylamine oxide (C12C8MAO)/HCl/water system with increasing apparent degree of protonation, X, defined as [HCl]/[C12C8MAO], has been studied. For a 100 mM concentration of C12C8MAO the following sequence of phases has been observed: L1/L2, L1/Lalpha/L2, L1/Lalpha, Lalpha, Lalpha/L2. The single-phase Lalpha region begins at X = 0.007 and ends at X = 0.35. The upper phase boundary, X*, depends strongly on the acid that is used for the protonation of the surfactant. It is shifted for increasing hydrophilicity of the acid to higher X values. For formic acid X* = 0.95, and for HBr X* = 0.05. A weakly protonated 1% solution of the surfactant is an iridescent Lalpha phase. Both unilamellar vesicles and multilamellar vesicles are observed in cryo transmission electron microscopy and freeze fracture transmission electron microscopy images in the Lalpha phase. The phase sequence with protonation differs from that of single-chain amine oxide surfactants. The synergism between the protonated and the nonprotonated species is very weak in the range X < X*, while the transition from the Lalpha phase to the Lalpha/L2 two-phase region is considered to be due to synergism. Little or no synergism is observed regarding the surface tension, but synergism does appear in the interfacial tension between decane and the aqueous solution. The viscoelastic properties of the vesicle/Lalpha phase resemble those of densely packed hard spheres. The effects of electric charge on the elastic property of the vesicles could be understood in terms of the osmotic pressure of the solutions. The interlamellar spacing evaluated by small-angle X-ray scattering showed a minimum around X approximately 0.1, which is interpreted as a result of two opposing contributions. One contribution is the suppression of undulation of bilayer membranes by introduction of electric charges, and the other comes from the increasing total bilayer thickness due to the increasing hydrogen bond formation with increasing X.

Systematic control of high-speed damping in doped/undoped Ni81Fe19 (50 nm) bilayer thin films

We show that the relaxation of GHz precessional motion in soft ferromagnetic thin films can be controlled through the inhomogeneous placement of dopants. Bilayers of doped and undoped permalloy (Ni81Fe19), Ni81Fe19/Ni81Fe19:Tb (Tb 5%), varied in net relaxation rate λ according to the thickness fraction of the doped layer. The behavior is roughly independent of total bilayer thickness and deposition order in the range studied (50–100 nm). The technique provides a means to control the high-speed response of magnetoelectronic devices with limited effect on signal, as magnetically active dopants can be separated from the interfaces relevant for spin transport.

Structure, Composition, and Peptide Binding Properties of Detergent Soluble Bilayers and Detergent Resistant Rafts

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70°C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 Å, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 Å for DRMs, which was ∼5 Å greater than that of 1:1:1 PC:SM:cholesterol and 10 Å greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.

Rolled-up In(Ga)As/GaAs Nanotubes Diameter as a Function of Structural Properties

AbstractInherently strained bilayers of In(Ga)As/GaAs, Epitaxially grown on top of an AlAs sacrificial buffer layer roll-up into free-standing nanotubes if they are released from their substrate by selective underetching, Here, we preseant a systematic study of the tube diameter as a function of total bilayer thickness and the ratio between the two layer thinknesses. We show that the diameters of the tubes are well described by a continous mechanical model.

Effect of pH and fatty acid chain length on the interaction of myelin basic protein with phosphatidylglycerol.

The basic protein of myelin binds electrostatically to acidic lipids but has several hydrophobic segments which may penetrate into the lipid bilayer. Calorimetric and spin-label evidence suggests that below the phase transition temperature, Tc, several phase states occur in the complex of phosphatidylglycerol with basic protein, possibly due to differences in the degree of penetration of the protein and/or interdigitation of the lipid acyl chains. One of these states is a metastable state which starts to melt 10 degrees C below the Tc of the pure lipid and then refreezes, with release of heat, into a stable state. The stable state melts near the Tc of the pure lipid but restricts the motion of fatty acid spin-labeled near the terminal methyl much more than does the pure lipid. The relationship between the rate of conversion to the stable state and the degree of penetration of the protein at varying pH, in the range 4--8, and the lipid acyl chain length, in the range 14 to 18 carbons, was investigated. Altering the pH in this range affects protonation of the histidines of the protein but has no effect on the lipid at pH 4 and above. The rate of conversion of the sample to both the metastable state and the stable state decreased with increase in pH for phosphatidylglycerol with all lipid chain lengths. It also decreased with decreasing chain length at constant pH. This suggested that the lipid could refreeze into the stable state more readily if a smaller proportion of the total bilayer thickness was occupied by the hydrophobic segments of the protein. The consistency of these results with the concept of penetration of portions of the protein partway into the bilayer lends support to this hypothesis.

The intrinsic structural asymmetry of highly curved phospholipid bilayer membranes

Phosphorus-31 NMR studies of solutions of small l-α-dipalmitoyl phosphatidylcholine bilayer vesicles containing sodium dimethyl phosphate uniformly distributed between the continuous external and the intravesicular aqueous spaces, with the paramagnetic shift reagent Pr 3+ present only in the external space, are reported. These studies give the distribution both of dipalmitoyl phosphatidylcholine in the vesicle inner and outer monolayers and of dimethyl phosphate in the aqueous spaes. With the third necessary parameter obtained from the vesicle sedimentation coefficient, the very different packing parameters of dipalmitoyl phosphatidylcholine in inner and outer monolayers can be determined. The vesicle outer radius is 109 Å. Although the total bilayer thickness is virtually identical to that of planar bilayers, the outer monolayer is thicker (20 Å) and the inner monolayer thinner (15 Å). The area per head group at the inner surface, 68 Å 2, is like the planar value, but the tails are much more folded, so as to decrease the radial lengths and increase the tangential spread (to 94 Å 2). The reverse is true in the outer layer: the surface per head group is 76 Å 2, tapering to 51 Å 2 in the tail region, so that outer layer tails are relatively extended. The difference is equivalent to a shift of about two 2g1 kinks from outer to inner layers; the uneven packing certainly affects fluidity, and may have important biological consequences.