Dmitry Ershov

Capillarity-induced ordering of spherical colloids on an interface with anisotropic curvature

Objects floating at a liquid interface, such as breakfast cereals floating in a bowl of milk or bubbles at the surface of a soft drink, clump together as a result of capillary attraction. This attraction arises from deformation of the liquid interface due to gravitational forces; these deformations cause excess surface area that can be reduced if the particles move closer together. For micrometer-sized colloids, however, the gravitational force is too small to produce significant interfacial deformations, so capillary forces between spherical colloids at a flat interface are negligible. Here, we show that this is different when the confining liquid interface has a finite curvature that is also anisotropic. In that case, the condition of constant contact angle along the three-phase contact line can only be satisfied when the interface is deformed. We present experiments and numerical calculations that demonstrate how this leads to quadrupolar capillary interactions between the particles, giving rise to organization into regular square lattices. We demonstrate that the strength of the governing anisotropic interactions can be rescaled with the deviatoric curvature alone, irrespective of the exact shape of the liquid interface. Our results suggest that anisotropic interactions can easily be induced between isotropic colloids through tailoring of the interfacial curvature.

Cytotoxicity and cellular uptake of tri-block copolymer nanoparticles with different size and surface characteristics

BackgroundPolymer nanoparticles (PNP) are becoming increasingly important in nanomedicine and food-based applications. Size and surface characteristics are often considered to be important factors in the cellular interactions of these PNP, although systematic investigations on the role of surface properties on cellular interactions and toxicity of PNP are scarce.ResultsFluorescent, monodisperse tri-block copolymer nanoparticles with different sizes (45 and 90 nm) and surface charges (positive and negative) were synthesized, characterized and studied for uptake and cytotoxicity in NR8383 and Caco-2 cells. All types of PNP were taken up by the cells. The positive smaller PNP45 (45 nm) showed a higher cytotoxicity compared to the positive bigger PNP90 (90 nm) particles including reduction in mitochondrial membrane potential (ΔΨm), induction of reactive oxygen species (ROS) production, ATP depletion and TNF-α release. The negative PNP did not show any cytotoxic effect. Reduction in mitochondrial membrane potential (ΔΨm), uncoupling of the electron transfer chain in mitochondria and the resulting ATP depletion, induction of ROS and oxidative stress may all play a role in the possible mode of action for the cytotoxicity of these PNP. The role of receptor-mediated endocytosis in the intracellular uptake of different PNP was studied by confocal laser scanning microscopy (CLSM). Involvement of size and charge in the cellular uptake of PNP by clathrin (for positive PNP), caveolin (for negative PNP) and mannose receptors (for hydroxylated PNP) were found with smaller PNP45 showing stronger interactions with the receptors than bigger PNP90.ConclusionsThe size and surface characteristics of polymer nanoparticles (PNP; 45 and 90 nm with different surface charges) play a crucial role in cellular uptake. Specific interactions with cell membrane-bound receptors (clathrin, caveolin and mannose) leading to cellular internalization were observed to depend on size and surface properties of the different PNP. These properties of the nanoparticles also dominate their cytotoxicity, which was analyzed for many factors. The effective reduction in the mitochondrial membrane potential (ΔΨm), uncoupling of the electron transfer chain in mitochondria and resulting ATP depletion, induction of ROS and oxidative stress likely all play a role in the mechanisms behind the cytotoxicity of these PNP.

Surface charge-specific cytotoxicity and cellular uptake of tri-block copolymer nanoparticles

Abstract A series of monodisperse (45 ± 5 nm) fluorescent nanoparticles from tri-block copolymers (polymeric nanoparticles (PNPs)) bearing different surface charges were synthesised and investigated for cytotoxicity in NR8383 and Caco-2 cells. The positive PNPs were more cytotoxic and induced a higher intracellular reactive oxygen species production than the neutral and negative ones. The cytotoxicity of positive PNPs with quaternary ammonium groups decreased with increasing steric bulk. The intracellular uptake and cellular interactions of these different PNPs were also tested in NR8383 cells by confocal laser scanning microscopy, which revealed higher uptake for positive than for negative PNPs. Also positive PNPs were found to interact much more with cell membranes, whereas the negative PNPs were internalised mainly by lysosomal endocytosis. Uptake of positive PNPs decreased with increasing steric bulk around the positive charge. A surface charge-specific interaction of clathrin for positive PNPs and caveolin receptors for negative PNPs was observed. These findings confirm that surface charge is important for the cytotoxicity of these PNPs, while they additionally point to considerable additional effects of the steric shielding around positive charges on PNP cytotoxicity.

In vitro nanoparticle toxicity to rat alveolar cells and coelomocytes from the earthworm Lumbricus rubellus

Abstract Sensitivity of immune cells (coelomocytes) of Lumbricus rubellus earthworms was investigated for exposure to selected nanoparticles, in order to obtain further insight in mechanisms of effects observed after in vivo C60 exposure. In the in vivo study, tissue damage appeared to occur without accompanying increased immune responses. Coelomocytes exposed in vitro to C60 showed no decrease of their cellular viability, but demonstrated a decrease in gene expression of the cytokine-like protein CCF-1, indicating immunosuppression. Experiments with NR8383 rat macrophage cells and tri-block copolymer nanoparticles were used to compare sensitivity and to demonstrate the usefulness of coelomocytes as a test system for nano-immunotoxicity, respectively. Overall, the results imply that sensitivity towards nanoparticles differs between cell types and nanoparticles. Moreover, this study indicates that injuries in absence of an immune response, observed after in vivo C60 exposure in our earlier work, are caused by immunosuppression rather than coelomocyte mortality.

Coalescence and compression in centrifuged emulsions studied with in situ optical microscopy

We report an experimental method to investigate droplet dynamics in centrifuged emulsions and its application to study droplet compression and coalescence. The experimental setup permits in situ monitoring of an ensemble of droplets in a centrifuged monolayer of monodisperse emulsion droplets using optical microscopy. We studied a hexadecane-in-water emulsion stabilized by the ionic surfactant sodium-n-dodecyl sulfate as a model system. With a microfluidic T-junction emulsion droplets of 97 μm diameter are produced which are subsequently inserted into a rectangular glass chamber of 100 μm height. Using an emulsion which is stable against coalescence, we measured the steady-state oil volume fraction in the compressed layer as a function of the compressive force induced by centrifugation, and quantified the deformation of droplets upon compression. To induce coalescence, we decreased the SDS bulk concentration by dilution of the continuous phase with water before the start of centrifugation. The growth rate of the separated oil phase, which forms on top of the emulsion, and the extent of drop–drop coalescence in the droplet layer upon centrifugation were evaluated as a function of the radial acceleration. The potential of the method for studies in emulsion science and possible improvements of the experimental setup are discussed.

Interfacial tension measured at high expansion rates and within milliseconds using microfluidics

To understand droplet formation and stabilisation, technologies are needed to measure interfacial tension at micrometer range and millisecond scale. In this paper, microtechnology is used, and that allows us to access these ranges and derive a model for surfactant free systems. The predicting power of the model was tested, and we found that it can be used to accurately (validated with >60 experiments) describe droplet size for a wide range of flow rates, interfacial tensions, and continuous phase viscosities. The model was used next to determine interfacial tensions in a system with hexadecane and sodium dodecylsulfate (SDS) solutions, and it was found that the model can be used for droplet formation times ranging from 0.4 to 9.4ms while using a wide range of process conditions. The method described here differs greatly from standard dynamic interfacial tension methods that use quiescent, mostly diffusion-limited situations. The effects that we measured are much faster due to enhanced mass transfer; this allows us to assess the typical time scales used in industrial emulsification devices.

Role of membrane disturbance and oxidative stress in the mode of action underlying the toxicity of differently charged polystyrene nanoparticles

Surface charge is often hypothesized to influence toxicity of nanoparticles (NPs) including polymeric nanoparticles (PNPs) while oxidative stress is considered to be an important mode of action (MOA) for such toxicity. In order to investigate the role of membrane disturbance and oxidative stress in the MOA of PNPs, the cytotoxicity and a range of related cellular endpoints induced by monodisperse, fluorescent, cationic and anionic polystyrene nanoparticles (PSNPs) of 50 and 100 nm sizes were investigated in vitro in macrophage NR8383 cells. Only amine-terminated cationic PSNPs exhibited cytotoxicity which was accompanied by induction of intracellular reactive oxygen species (ROS), increased levels of cytoplasmic free calcium, a reduced phagocytic index, a reduced mitochondrial membrane potential (ΔΨm) and a decreased intracellular ATP content with the effects being more pronounced for 50 nm than 100 nm PSNPs. Both cationic and anionic PSNPs were found to increase the roughness of the cell membrane with the effect being more profound for cationic PSNPs. The pattern of protection by cellular antioxidants against the effects induced by positive PSNPs was similar to the pattern of protection against effects induced by the mitochondrial electron transport disrupting agent 2,4-dinitrophenol (DNP) and dissimilar to that for protection against the model compound for oxidative stress, i.e. hydrogen peroxide (H2O2). Surface charge influences the cellular interaction for NPs. The results collectively indicated that membrane interaction, and disturbance of the mitochondrial electronic transport chain (ETC) may represent a principal mechanism of toxicity for cationic PSNPs resulting in ROS production and oxidative stress as secondary effects.

Two modes of phase inversion in a drying emulsion

We report two different modes of phase inversion in surfactant-stabilized oil-in-water emulsions subjected to a unidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. This drying-induced coalescence results from the development of a pressure gradient from the drying front into the bulk of the sample, established by limited water transport through the Plateau borders. Depending on the steepness of this pressure profile, coalescence occurs throughout the sample or only at the drying front. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure at which coalescence occurs. Very stable emulsions, characterized by a high critical pressure, tend to develop steep pressure profiles, which favours front-dominated coalescence, while unstable emulsions with low critical pressures develop shallow pressure gradients, favouring nucleation-and-growth dominated coalescence. These results offer new insights into the microscopic mechanisms governing the complex and poorly understood macroscopic phenomena during phase inversion of drying emulsions.

Mechanical properties of reconstituted actin networks at an oil–water interface determined by microrheology

There have been various attempts to investigate the mechanical properties of the actin cortex in cells, but the factors that control them remain poorly understood. To make progress, we develop a reconstituted model of the actin cortex that mimics its structure. We attach actin filaments to lipids lining the surface of an oil droplet using biotin–streptavidin bonds. In this way we can form a thin actin network that can be visualized and studied by confocal microscopy. Our approach allows incorporation of different actin-binding and motor proteins into this 2D network and characterization of their effect on its mechanical properties in a quantitative way. To study the viscoelasticity of the network, we use passive particle tracking microrheology, which allows storage and loss moduli to be extracted from the mean square displacement of tracer particles. We show that adding cross-linkers to the cortex increases its elasticity by several orders of magnitude and addition of myosin in the presence of ATP results in a strong and rapid stiffening of the network. This approach opens up a variety of possibilities to study viscoelastic properties of the actin cortex in vitro, allowing incorporation of any protein of interest into the system.