Structural and Mechanical Response of Two-Component Photoswitchable Lipid Bilayer Vesicles.

Optical control of phospholipids is an attractive option for the rapid, reversible, and tunable manipulation of membrane structure and dynamics. Azo-PC, a lipid with an azobenzene group within one acyl chain, undergoes a light-induced trans-to-cis isomerization and thus arises as a powerful tool for manipulating lipid order and dynamics. Here, we report on vesicle-scale micropipette measurements and atomistic simulations to probe the elastic stretching modulus, water permeability, toughness, thickness, and membrane area upon isomerization. We investigated both dynamics and steady-state properties. In pure azo-PC membranes, we found that the molecular area in trans was 16% smaller than that in cis, the membrane's stretching modulus kA was 2.5 ± 0.3 times greater, and the water permeability PW was 3.5 ± 0.5 times smaller. We also studied mixtures of azo-PC with the miscible, unsaturated lipid DOPC. Atomistic molecular dynamics simulations show how the membrane thickness, chain order, and correlations across membrane leaflets explain the experimental data. Together, these data show how one rotating bond changes the molecular- and membrane-scale properties. These results will be useful for photopharmacology and for developing new materials whose permeability, elasticity, and toughness may be switched on demand.

Adsorption of Hydrophilic Silica Nanoparticles at Oil-Water Interfaces with Reversible Emulsion Stabilization by Ion Partitioning.

Adsorption of particles at oil-water interfaces is the basis of Pickering emulsions, which are common in nature and industry. For hydrophilic anionic particles, electrostatic repulsion and the absence of wetting inhibit spontaneous adsorption and limit the scope of materials that can be used in emulsion-based applications. Here, we explore how adding ions that selectively partition in the two fluid phases changes the interfacial electric potential and drives particle adsorption. We add oil-soluble tetrabutyl ammonium perchlorate (TBAP) to the nonpolar phase and Ludox silica nanoparticles or silica microparticles to the aqueous phase. We find a well-defined threshold TBAP concentration, above which emulsions are stable for months. This threshold increases with the particle concentration and with the oil's dielectric constant. Adding NaClO4 salt to water increases the threshold and causes spontaneous particle desorption and droplet coalescence even without agitation. The results are explained by a model based on the Poisson-Boltzmann theory, which predicts that the perchlorate anions (ClO4-) migrate into the water phase and leave behind a net positive charge in the oil. Our results show how a large class of inorganic hydrophilic, anionic nanoparticles can be used to stabilize emulsions in a reversible and stimulus-responsive way, without surface modifications.

Vesicle-Based Gel via Polyelectrolyte-Induced Adhesion: Structure, Rheology, and Response.

We describe an experimental study of soft solids composed of micron-scale lipid bilayer vesicles that adhere to one another through electrostatic attraction to an oppositely charged polymer (PDADMAC). As the polymer concentration was increased, we found a fluid phase, a solid gel phase, and a gel composed of internally reorganized vesicles. Optical microscopy images showed a nearly close-packed structure of adhered vesicles that retained their closed-cell morphology. Shear rheology measurements showed that the gel phase is a solid with a modulus at the Pa scale and with linear response up to 70% strain. We found that the modulus depends on the energy per area of membrane-membrane adhesion but does not depend on the vesicle size. We further found that the gels survived osmotic stress or dilution of the adhering polymer but could be rapidly disrupted in response to the addition of strongly binding silica nanoparticles. These results demonstrate the potential for cell-sized lipid vesicles to form a solid platform that maintains the responsive properties of the membranes. Such materials may find applications as triggerable, protective coatings of delicate surfaces.

Membrane Remodeling by DNA Origami Nanorods: Experiments Exploring the Parameter Space for Vesicle Remodeling.

Inspired by the ability of cell membranes to alter their shape in response to bound particles, we report an experimental study of long, slender nanorods binding to lipid bilayer vesicles and altering the membrane shape. Our work illuminates the role of particle concentration, adhesion strength, and membrane tension in determining the membrane morphology. We combined giant unilamellar vesicles with oppositely charged nanorods, carefully tuning the adhesion strength, membrane tension, and particle concentration. With increasing adhesion strength, the primary behaviors observed were membrane deformation, vesicle-vesicle adhesion, and vesicle rupture. These behaviors were observed in well-defined regions in the parameter space with sharp transitions between them. We observed the deformation of the membrane resulting in tubulation, textured surfaces, and small and large lipid-particle aggregates. These responses are robust and repeatable and provide a new physical understanding of the dependence on the shape, binding affinity, and particle concentration in membrane remodeling. The design principles derived from these experiments may lead to new bioinspired membrane-based materials.

Triggered interactions between nanoparticles and lipid membranes: design principles for gel formation or disruption-and-release.

Lipid bilayer vesicles offer exciting possibilities for stimulated response, taking advantage of the membrane's flexibility and impermeability. We show how synergistic interactions between vesicles and polymer-based nanoparticles can be triggered at the nanoscale using UV light. This interaction leads either to adhesion and a membrane-based gel, or to nanoscale wrapping of the particles by the membrane and then vesicle destruction. To map the response, we varied the particle-membrane interactions via their surface charge densities. We found a crossover from adhesion to destruction at a well-defined region in parameter space. We modeled these results by accounting for the electrostatic attraction and the energy of membrane bending. We then synthesized amphiphilic polymers containing a UV-responsive nitrobenzyl moiety that switches its charge, and showed how a trigger predictably led to either a vesicle gel or disruption and release. The results pave the way to a new triggering mechanism and new response modes in soft materials.

Response of a raft of particles to a local indentation.

Interfaces that are coated with a layer of adsorbed particles (particle "rafts") are common in natural and industrial settings. Particle-coated interfaces may be useful in part because the particulate structure can endow the fluid interface with physical properties distinct from molecular surfactants. We study the mechanics of particulate assemblies by measuring the raft's response to indentation in the vertical direction by a flat, circular disc. We measured force (f) vs. indentation depth (δ) and found two linear regions with different slopes. The first linear region started at δ = 0 and persisted over a range of δ much less than the capillary length. In the second linear region, the raft had the same stiffness (df/dδ) as a liquid interface with no particles. Further, we show that, as long as the indenter was larger than a single particle, the azimuthal compression imposed by the interface deformation relaxed through in-plane rearrangement of particles rather than by the radial wrinkles that are characteristic of thin elastic sheets at fluid interfaces. We show how the force-displacement curves and stiffnesses depended on fluid mass densities, interfacial tensions, and indenter radius. For all cases studied, the particle-raft coated interfaces had a stiffness equal to or smaller than that of a bare fluid interface. Although the interfacial particle raft behaved like a pure fluid interface under a wide range of displacements, we show that the raft could nonetheless withstand substantially greater applied force (up to 2×) and greater indentation depth (up to 2.6×), so that the range of reversible behavior was greatly extended. These results improve our understanding of the mechanics of particulate assemblies at interfaces.

Wax Spreading in Paper under Controlled Pressure and Temperature.

This work describes a novel rapid method to fabricate high-resolution paper-based microfluidic devices using wax-ink-based printing. This study demonstrates that both temperature and pressure are important knobs in controlling the device resolution. High-resolution lines and patterns were obtained by heating the paper asymmetrically from one side up to 110 °C while applying pressure up to 49 kPa. Starting with wax lines with an initial width of 130 µm, we achieve a thorough penetration through a 190 µm-thick paper with lateral spreading on the front as narrow as 90 µm. The role of temperature and pressure are systematically studied and compared with the prediction of the Lucas-Washburn equation. We found that the temperature dependence of spreading can be explained by the viscosity change of the wax, according to the Lucas-Washburn equation. The pressure dependence deviates from Lucas-Washburn behavior because of compression of the paper. An optimal condition for achieving full depth penetration of the wax yet minimizing lateral spreading is suggested after exploring various parameters including temperature, pressure, and paper type. These findings could lead to a rapid roll-to-roll fabrication of high-resolution paper-based diagnostic devices.

Wetting, meniscus structure, and capillary interactions of microspheres bound to a cylindrical liquid interface.

Wetting, meniscus structure, and capillary interactions for polystyrene microspheres deposited on constant curvature cylindrical liquid interfaces, constructed from nonvolatile ionic or oligomeric liquids, were studied by optical interferometry and optical microscopy. The liquid interface curvature resulted from the preferential wetting of finite width lines patterned onto planar silicon substrates. Key variables included sphere diameter, nominal (or average) contact angle, and deviatoric interfacial curvature. Menisci adopted the quadrupolar symmetry anticipated by theory, with interfacial deformation closely following predicted dependences on sphere diameter and nominal contact angle. Unexpectedly, the contact angle was not constant locally around the contact line, the nominal contact angle varied among seemingly identical spheres, and the maximum interface deviation did not follow the predicted dependence on deviatoric interfacial curvature. Instead, this deviation was up to an order-of-magnitude larger than predicted. Trajectories of neighboring microspheres visually manifested quadrupole-quadrupole interactions, eventually producing square sphere packings that foreshadow interfacial assembly as a potential route to hierarchical 2D particle structures.

Confined Assemblies of Colloidal Particles with Soft Repulsive Interactions.

We investigate the microconfinement of charged silica nanoparticles dispersed in refractive index matching monomers in poly(dimethylsiloxane) (PDMS) porous membrane. Here, the silica colloidal particles interact with each other and the pore wall via electrostatic double layer forces. Different from the hard sphere systems where the assembled morphologies are prescribed by the diameter ratio between the cylindrical confinement and the nanoparticles, here we observe a much richer variety of assemblies that are highly sensitive to both bulk and local nanoparticle concentration with fixed particle size and channel size. The experimentally observed assembly morphologies are consistent with theoretical predictions from the literature, based on Yukawa potential in the low packing density regime. Also, most of the configurations found in the experiment are well described by computer simulations using pairwise additive long-range repulsive interactions, demonstrating the ability to control the system to obtain a desired structure.

Deformation of the contact line around spherical particles bound at anisotropic fluid interfaces.

When a particle adsorbs at a liquid interface, the 3-phase contact line geometry depends on the shape of the particle and of the liquid interface. The shape of the contact line is the key to controlling capillary forces among particles, and is therefore a useful means to direct assembly of interfacial particles. We measured the shape of the contact line around millimeter-sized PDMS-coated glass spheres at water/air interfaces with anisotropic shapes. We studied the advancing and receding conditions separately. We focused on interfaces with a cylindrical shape, where the predominant deformation of the meniscus and the contact line both have quadrupolar cos(2ϕ) symmetry. We related the measured magnitude of the quadrupolar deformation to the applied vertical force on the sphere and the interface's deviatoric curvature, D0. For modest curvature (D0 < 0.1 × sphere radius), our results agree with the theoretical prediction for free particles. At higher curvature, the measurements exceed the theory. The theory appears to apply even when there is contact-angle hysteresis, as long as the measured contact angle is used rather than the equilibrium (Young-Dupré) angle. The magnitude of the quadrupolar deformation depends on the applied force. Together, these results show the range of validity of the theory.

Osmotically-induced tension and the binding of N-BAR protein to lipid vesicles.

The binding affinity of a curvature-sensing protein domain (N-BAR) is measured as a function of applied osmotic stress while the membrane curvature is nearly constant. Varying the osmotic stress allows us to control membrane tension, which provides a probe of the mechanism of binding. We study the N-BAR domain of the Drosophila amphiphysin and monitor its binding on 50 nm-radius vesicles composed of 90 mol% DOPC and 10 mol% PIP. We find that the bound fraction of N-BAR is enhanced by a factor of approximately 6.5 when the tension increases from zero to 2.6 mN m(-1). This tension-induced response can be explained by the hydrophobic insertion mechanism. From the data we extract a hydrophobic domain area that is consistent with known structure. These results indicate that membrane stress and strain could play a major role in the previously reported curvature-affinity of N-BAR.

Solid particles adsorbed on capillary-bridge-shaped fluid polystyrene surfaces.

Particles adsorbed on microscopic polystyrene (PS) capillary bridge surfaces were observed to investigate their motion under capillary forces arising from a nonuniform shape. Capillary bridges were created by placing thin PS films, heated above the glass transition temperature (Tg), between two electrodes with an air gap between the surface of the PS and the upper electrode. Silica particles, 100 nm in diameter, were placed on the surface of the PS capillary bridges, and the sample was heated above the Tg of PS to enable particle motion. Samples were cooled to below Tg, and the locations of the particles were observed using scanning electron microscopy. The particles did not preferentially locate around the center of the capillary bridge, as predicted by others, but instead segregated to the edges. These results indicate that the forces driving particles to the three-phase contact line (air/PS/electrode surface) are greater than those locating particles around the center.

Effect of interface shape on advancing and receding fluid-contact angles around spherical particles.

The angle of contact between a solid surface and a fluid interface plays a key role in wetting and is therefore a focus in studies of a wide range of natural phenomena and fluidic technologies. The contact angle ranges between two values, a maximum (advancing) angle and a minimum (receding) angle. These limiting angles are thought to be properties of the fluids and of the chemistry or topography of the solid. By contrast, we find that the value of the receding angle can be significantly reduced by altering the interface shape. Using millimeter-sized spheres coated with polydimethylsiloxane and pulled through an air-water interface, we observe that the receding angle decreases from 101 ± 1° at a planar interface to as low as 80 ± 1° at saddle- or cylinder-shaped interfaces. The angle decreases smoothly with the deviatoric curvature of the interface (a measure of the shape anisotropy) and is linked to a non-circular contact line.

Change of line tension in phase-separated vesicles upon protein binding.

We measured the effect of a model membrane-binding protein on line tension and morphology of phase-separated lipid-bilayer vesicles. We studied giant unilamellar vesicles composed of a cholesterol/dioleoylphosphatidylcholine/palmitoylsphingomyelin mixture and a controlled mole fraction of a Ni-chelating lipid. These vesicles exhibited two coexisting fluid-phase domains at room temperature. Owing to the line tension, σ, between the two phases, the boundary between them was pulled like a purse string so that the smaller domain formed a bud. While observing the vesicles in a microscope, histidine-tagged green fluorescent protein was added, which bound to the Ni-chelating lipid. As protein bound, the vesicle shape changed and the length of the phase boundary increased. The change in morphology was attributed to a reduction of σ between the two phases because of preferential accumulation of histidine-tagged green fluorescent protein-Ni-chelating lipid clusters at the domain boundary. Greater reductions of σ were found in samples with higher concentrations of Ni-chelating lipid; this trend provided an estimate of the binding energy at the boundary, approximately k(B)T. The results show how domain boundaries can lead to an accumulation of membrane-binding proteins at their boundaries and, in turn, how proteins can alter line tension and vesicle morphology.

Adsorption energy of nano- and microparticles at liquid-liquid interfaces.

We study experimentally the energy of adsorption, DeltaE, of nanoparticles and microparticles at the oil-water interface by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we find DeltaE = -5.1 k(B)T for particle radius R = 2.5 nm and DeltaE proportional, variant R(2) for larger sizes. Gold nanoparticles with (1-mercaptoundec-11-yl)tetra(ethylene glycol) ligand have a much larger binding energy (DeltaE = -60.4 k(B)T) and an energy barrier against adsorption. For polystyrene spheres with R = 1.05 microm, we find DeltaE = -0.9 x 10(6) k(B)T. We also find that the binding energy depends on the composition of the oil phase and can be tuned by the salt concentration of the nanoparticle suspension. These results will be useful for controlling the assembly of nanoparticles at liquid interfaces, and the method reported here should be broadly useful for quantitative measurements of binding energy.

Nanoparticles binding to lipid membranes: from vesicle-based gels to vesicle tubulation and destruction.

While cells offer numerous inspiring examples in which membrane morphology and function are controlled by interactions with viruses or proteins, we still lack design principles for controlling membrane morphology in synthetic systems. With experiments and simulations, we show that spherical nanoparticles binding to lipid-bilayer membrane vesicles results in a remarkably rich set of collective morphologies that are controllable via the particle binding energy. We separately study cationic and anionic particles, where the adhesion is tuned by addition of oppositely charged lipids to the vesicles. When the binding energy is weak relative to a characteristic membrane-bending energy, vesicles adhere to one another and form a soft solid gel, a novel and useful platform for controlled release. With larger binding energy, a transition from partial to complete wrapping of the nanoparticles causes a remarkable vesicle destruction process culminating in rupture, nanoparticle-membrane tubules, and an apparent inversion of the vesicles. These findings help unify the diverse phenomena observed previously. They also open the door to a new class of vesicle-based, closed-cell gels that are more than 99% water and can encapsulate and release on demand, and show how to drive intentional membrane remodeling for shape-responsive systems.

Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures.

Combined Kelvin probe force microscopy and wavelength-resolved photoluminescence measurements on individual pre- and post-cross-linked poly(3-hexylthiophene)-b-poly(3-methyl alcohol thiophene) (P3HT-b-P3MT) nanofibers have revealed striking differences in their optical and electronic properties driven by structural perturbation of the crystalline aggregate nanofiber structures after cross-linking. Chemical cross-linking from diblock copolymer P3HT-b-P3MT using a hexamethylene diisocyanate cross-linker produces a variety of morphologies including very small nanowires, nanofiber bundles, nanoribbons, and sheets, whose relative abundance can be controlled by reaction time and cross-linker concentration. While the different cross-linked morphologies have almost identical photophysical characteristics, KPFM measurements show that the surface potential contrast, related to the work function of the sample, depends sensitively on nanostructure morphology related to chain-packing disorder.

Cluster formation in polyelectrolyte-micelle complex coacervation.

The temperature-induced liquid-liquid phase transition (complex coacervation) of a polycation-anionic/nonionic mixed micelle system was examined over a range of macroion concentrations and polycation molecular weights (MW) using turbidimetry and dynamic light scattering (DLS). DLS revealed a progressive increase in complex/aggregate size with temperature up to the phase transition at T(φ), followed by splitting of these clusters into respectively smaller and larger particles. We present two explanations: (1) large (200-400 nm) clusters (soluble aggregates) are necessary and sufficient coacervation precursors, and (2) the process of coacervation itself is accompanied by the expulsion of smaller aggregates to form submicrometer droplets. Although a reduction in T(φ) for higher MW appears to be correlated with larger clusters, in support of model 1, the opposite correlation between cluster size and T(φ) is seen upon isoionic dilution. We conclude that enhanced coacervation and increased cluster size at high polymer MW arise independently from increased intercomplex attractive forces. Dilution, on the other hand, leads to diminished cluster size, whereas the decrease in T(φ) on dilution is a reflection of coacervate self-suppression, previously observed for this system. The splitting of clusters into large and small species near T(φ) is explained by macroion disproportionation, as proposed by Shkolvskii et al for DNA condensation. We demonstrate and explain a similar phenomenon: broadening of the phase transition by an increase in cluster polydispersity, resulting from an increase in surfactant polydispersity.

Self-assembled shells composed of colloidal particles: fabrication and characterization.

We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles. These techniques reproducibly yield shells with controllable properties such as elastic moduli and breaking forces. To enable diffusive exchange through the particle shells, we transfer them into solvents that are miscible with the encapsulant. We characterize the mechanical properties of the shells by measuring the response to deformation by calibrated microcantilevers.