Interactions of hydrophilic quantum dots with defect-free and defect containing supported lipid membranes.

Quantum dots (QDs) are semiconductor nanoparticles with unique optical and electronic properties, whose interest as potential nano-theranostic platforms for imaging and sensing is increasing. The design and use of QDs requires the understanding of cell-nanoparticle interactions at a microscopic and nanoscale level. Model systems such as supported lipid bilayers (SLBs) are useful, less complex platforms mimicking physico-chemical properties of cell membranes. In this work, we investigated the effect of topographical homogeneity of SLBs bearing different surface charge in the adsorption of hydrophilic QDs. Using quartz-crystal microbalance, a label-free surface sensitive technique, we show significant differences in the interactions of QDs onto homogeneous and inhomogeneous SLBs formed following different strategies. Within short time scales, QDs adsorb onto topographically homogeneous, defect-free SLBs is driven by electrostatic interactions, leading to no layer disruption. After prolonged QD exposure, the nanomechanical stability of the SLB decreases suggesting nanoparticle insertion. In the case of inhomogeneous, defect containing layers, QDs target preferentially membrane defects, driven by a subtle interplay of electrostatic and entropic effects, inducing local vesicle rupture and QD insertion at membrane edges.

Two-Step Nanoscale Approach for Well-Defined Complex Alkanethiol Films on Au Surfaces.

Controlling the molecular organization of organic self-assembled monolayers (SAM) is of utmost importance in nanotechnology, molecular electronics, and surface science. Here we propose two well-differentiated approaches, double printing based on microcontact printing (µ-cp) and molecular backfilling adsorption, to produce complex alkanethiol films. The resulting films on model Au surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Double printing alkanethiols results in clear coexisting regions where no molecular displacement is observed, highlighting the slow diffusion rates of long alkanethiols and large attractive interaction between long alkyl chains. Exposing a single-print µ-cp Au substrate to an additional alkanethiol solution yields the formation of differently ordered domain boundaries with different thickness and micrometer lateral size. The high order is a result of enhanced molecular mobility and restructuring during solution backfilling. The formed molecular assemblies constitute an excellent testing ground for nanoscale phenomena that strongly depend on the nanoscale geometrical and chemical features of the surface such as designed functionality or corrosion initiation and inhibition.

Effect of cholesterol on the phase behavior of solid-supported lipid vesicle layers.

The interest in solid-supported biomimetic membranes stems from their utility in nanotechnology and biosensing. In particular, supported lipid vesicles (SLVs) have become popular in both fundamental biophysical studies and pharmaceutical screening applications. It is thus essential to gain information on the structural properties and phase behavior of SLVs. Here we report on a study on the influence of cholesterol on the phase behavior of SLVs of saturated phospholipids by using quartz crystal microbalance with dissipation monitoring, a label-free and nonintrusive surface-sensitive technique. Two complementary approaches have been used, a Voigt-based viscoelastic model yielding shear viscosity temperature profiles and the first-order derivative of the frequency (mass-sensitive) shifts. Anomalies in the shear viscosity and extrema in the first-order derivative frequency curves stand as a token of the main phase transition and provide information on its gradual suppression upon addition of cholesterol. This method proves convenient for its small sample volume needed, its short temperature equilibration time and the non-necessity of external labels. This work can be regarded as a starting point for further studies on more rare lipid systems and different geometries, such as tethered SLVs or biologically relevant vesicles produced by living cells.

Melittin disruption of raft and non-raft-forming biomimetic membranes: a study by quartz crystal microbalance with dissipation monitoring.

In this work we examine the role of lateral phase separation in cholesterol-containing biomimetic membranes on the disrupting action of melittin using a label-free surface-sensitive technique, quartz crystal microbalance with dissipation monitoring (QCM-D). Melittin disruption mechanisms depend strongly on the geometry of the lipid layer; however, despite the interplay between layer geometry/thickness and melittin activity, results indicate that the presence of lipid heterogeneity and lateral phase separation greatly influences the disrupting efficiency of melittin. In homogeneous non-raft forming membranes with high cholesterol content, melittin spontaneous activity is strongly delayed compared to heterogeneous raft-forming systems with the same amount of cholesterol. These results confirm the importance of lateral phase separation as a determinant factor in peptide activity. The information provided can be used for the design of more efficient antimicrobial peptides and the possibility of using a label-free approach for tailored-membranes and interactions with other types of peptides, such as amyloid peptides.

High-resolution birefringence investigation of octylcyanobiphenyl (8CB): an upper bound on the discontinuity at the smectic-A to nematic phase transition.

We report optical birefringence data by two different methods with high temperature resolution for octylcyanobiphenyl (8CB) near the smectic-A to nematic (Sm-A-N) phase transition temperature T(AN). Within the resolution of our experiments, we find that the Sm-A-N phase transition is continuous. For a possible discontinuity in the orientational order parameter S(T) at T(AN), we arrive at an upper limit of 0.0002, which is substantially smaller than other estimates in literature, but consistent with the value of 0.00008 derived from the upper limit of the latent heat from high-resolution adiabatic scanning calorimetry (ASC), which is itself consistent with the Halperin-Lubensky-Ma theory. The temperature derivative of the order parameter exhibits a power law divergence with a critical exponent that is consistent with the value α = 0.31 ± 0.03 for the specific heat capacity obtained by ASC.

Evidence from adiabatic scanning calorimetry for the Halperin-Lubensky-Ma effect at the N-SmA phase transitions in mixtures of 7OCB+heptane with an injected SmA phase.

The high-resolution adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A transition (N-SmA in binary mixtures of the non-smectogenic liquid crystal heptyloxycyanobiphenyl (7OCB) and heptane, exhibiting a so-called injected smectic A phase. With the exception of a mixture with the lowest heptane mole fraction for which only an upper limit of 0.2 ± 0.2 J kg(-1) for a possible latent heat could be obtained, for all other mixtures finite latent heats were obtained. The mole fraction dependence of the latent heat could be well fitted with a crossover function consistent with a mean-field free energy expression with a non-zero cubic term arising from the Halperin-Lubensky-Ma (HLM) coupling between the SmA order parameter and the orientational director fluctuations. The mole fraction dependence of the heat capacity effective critical exponents is similar to that observed in mixtures of the two liquid crystals octyloxycyanobiphenyl (8OCB) and nonylcyanobiphenyl (9OCB). The thermal behavior observed along the N-SmA phase transition line yields further strong evidence for the HLM coupling effect.

Asymmetric criticality in weakly compressible liquid mixtures.

The thermodynamics of asymmetric liquid-liquid criticality is updated by incorporating pressure effects into the complete-scaling formulation earlier developed for incompressible liquid mixtures [C. A. Cerdeirina et al., Chem. Phys. Lett. 424, 414 (2006); J. T. Wang et al., Phys. Rev. E 77, 031127 (2008)]. Specifically, we show that pressure mixing enters into weakly compressible liquid mixtures as a consequence of the pressure dependence of the critical parameters. The theory is used to analyze experimental coexistence-curve data in the mole fraction-temperature, density-temperature, and partial density-temperature planes for a large number of binary liquid mixtures. It is shown how the asymmetry coefficients in the scaling fields are related to the difference in molecular volumes of the two liquid components. The work resolves the question of the so-called "best order parameter" discussed in the literature during the past decades.