Self-assembly of iron oxide-poly(ethylene glycol) core-shell nanoparticles at liquid-liquid interfaces

Nanoparticles (NPs) play an increasingly important role in the fabrication of functional advanced materials. Two major steps need to be carried out in order to achieve control of the material properties. First of all, the properties of the single NPs have to be under control, especially in relation to colloidal stability; aggregation and corrosion negate all the benefits associated to the nanoscopic dimensions. Secondly, the assembly process has to be controlled to achieve a material with the desired properties. We propose here to use stabilized ceramic NPs consisting of a magnetite core, coated by a poly(ethylene glycol) (PEG) shell and study their assembly at polar/ non-polar liquid interfaces, en route to fabricating functional NP membranes. These NPs show extraordinary stability in aqueous solutions achieved by anchoring linear PEG chains through an end-terminating nitroDOPA group to their surface. Furthermore, the core and shell sizes of these NPs can be independently varied with ease. We first describe the details of the NP synthesis and stabilization in bulk solutions, discussing the PEG molecular weight needed to achieve bulk stability. Subsequently, we demonstrate self-assembly of these particles at liquid-liquid interfaces (SALI) into monolayers of stable properties. SALI has been chosen as path for the assembly given its suitability for fabricating two-dimensional materials. We report here results from pendant drop tensiometry which illustrate the kinetics of NP adsorption at the liquid-liquid interface and highlight the role played by the molecular weight of the PEG shell in the interfacial assembly. In particular we show that the requisites to ensure particle stability at a liquid interface are more stringent compared to the bulk case.     

Microfluidic Fabrication of Capsule Sensor Platform with Double‐Shell Structure

Metal nanoparticles are frequently employed for the colorimetric detection of specific target molecules using an aggregation‐induced shift of the localized surface plasmon resonance. However, metal nanoparticles dispersed in bulk solutions are prone to be contaminated by adhesive molecules and the dispersions tend to be diluted by sample fluids, restricting direct application to unpurified pristine samples. Here, a versatile capsule sensor platform is proposed that can encompass a variety of different types of nanoparticle‐based sensors. The capsule sensors are microfluidically prepared to obtain close control over their dimensions and composition. Their aqueous cores that are loaded with sensing materials are surrounded by an ultrathin inner oil shell and an outer hydrogel shell. The hydrogel shell prevents the diffusion of large adhesive molecules into the core, thereby preventing contamination of the sensing materials. The oil shell is selectively permeable such that it further improves the sensor selectivity. Importantly, these shells confine the sensing materials and prevent them from being diluted, securing a consistent optical property. Moreover, the capsule‐based sensors display a higher sensitivity than bulk dispersions because a smaller amount of sensing materials is used. The power of nanoparticle‐loaded capsule sensors is demonstrated using lysine‐coated gold nanoparticles to detect mercury ions.     

Selectively Permeable Double Emulsions

Natural organisms are made of different types of microcompartments, many of which are enclosed by cell membranes. For these organisms to display a proper function, the microcompartments must be selectively permeable. For example, cell membranes are typically permeable toward small, uncharged molecules such as water, selected nutrients, and cell signaling molecules, but impermeable toward many larger biomolecules. Here, it is reported for the first time dynamic compartments, namely surfactant‐stabilized double emulsions, that display selective and tunable permeability. Selective permeability is imparted to double emulsions by stabilizing them with catechol‐functionalized surfactants that transport molecules across the oil shell of double emulsions only if they electrostatically or hydrophobically attract encapsulants. These double emulsions are employed as semipermeable picoliter‐sized vessels to controllably perform complexation reactions inside picoliter‐sized aqueous cores. This thus far unmet level of control over the transport of reagents across oil phases opens up new possibilities to use double emulsion drops as dynamic and selectively permeable microcompartments to initiate and maintain chemical and biochemical reactions in picoliter‐sized cell‐mimetic compartments.     

A meta-analysis of work–family conflict and various outcomes with a special emphasis on cross-domain versus matching-domain relations.

A literature review of studies analyzing work–family conflict and its consequences was conducted, and 427 effect sizes were analyzed meta-analytically. Work–family conflict was analyzed bidirectionally in terms of work interference with family (WIF) and family interference with work (FIW). We assessed 3 categories of potential outcomes: work-related outcomes, family-related outcomes, and domain-unspecific outcomes. Results show that WIF and FIW are consistently related to all 3 types of outcomes. Both types of interrole conflict showed stronger relationships to same-domain outcomes than to cross-domain outcomes. Thus, WIF was more strongly associated with work-related than with family-related outcomes, and FIW was more strongly associated with family-related than with work-related outcomes. In moderator analyses, parenthood could not explain variability in effect sizes. However, time spent at work did moderate the relationships between WIF and family-related outcomes, as well as FIW and domain-unspecific outcomes. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Stabilization and functionalization of iron oxide nanoparticles for biomedical applications

Superparamagnetic iron oxide nanoparticles (NPs) are used in a rapidly expanding number of research and practical applications in the biomedical field, including magnetic cell labeling separation and tracking, for therapeutic purposes in hyperthermia and drug delivery, and for diagnostic purposes, e.g., as contrast agents for magnetic resonance imaging. These applications require good NP stability at physiological conditions, close control over NP size and controlled surface presentation of functionalities. This review is focused on different aspects of the stability of superparamagnetic iron oxide NPs, from its practical definition to its implementation by molecular design of the dispersant shell around the iron oxide core and further on to its influence on the magnetic properties of the superparamagnetic iron oxide NPs. Special attention is given to the selection of molecular anchors for the dispersant shell, because of their importance to ensure colloidal and functional stability of sterically stabilized superparamagnetic iron oxide NPs. We further detail how dispersants have been optimized to gain close control over iron oxide NP stability, size and functionalities by independently considering the influences of anchors and the attached sterically repulsive polymer brushes. A critical evaluation of different strategies to stabilize and functionalize core-shell superparamagnetic iron oxide NPs as well as a brief introduction to characterization methods to compare those strategies is given.     

Creation of Faceted Polyhedral Microgels from Compressed Emulsions

Compressed monodisperse emulsions in confined space exhibit highly ordered structures. The influence of the volume fraction and the confinement geometry on the organized structures is investigated and the mechanism by which structural transition occurs is studied. Based on the understanding of ordering behavior of compressed emulsions, a simple and high‐throughput method to fabricate monodisperse polyhedral microgels using the emulsions as the template is developed. By controlling the geometry of the confined spaces, a variety of shapes such as hexagonal prism, Fejes Toth honeycomb prism, truncated octahedron, pyritohedron, and truncated hexagonal trapezohedron are implemented. Moreover, the edge sharpness of each shape is controllable by adjusting the drop volume fraction. This design principle can be readily extended to other shapes and materials, and therefore provides a useful means to create polyhedral microparticles for both fundamental study and practical applications.     

Clogging in parallelized tapered microfluidic channels

Nearly all tubes and pores used to transport solids in fluids, such as arteries and filters, are subject to clogging. The length scales and geometries of these tubes are well defined. In spite of this knowledge, the collective clogging behavior of multiple tubes has not yet been connected to their shapes and sizes. We investigate the clogging behavior of ten parallel tubes, which we model with ten parallel tapered microchannels using poly(styrene) beads to induce clogging events. The clogging behavior depends on the channel geometry as well as the shear stress particles are subjected to. Although our microchannels model filters, our results can be applied to the clogging behavior of a broad range of applications such as the clogging in arteries, inkjets, or xylem in trees.     

Reducing the shell thickness of double emulsions using microfluidics

Double emulsion drops are well-suited templates to produce capsules whose dimensions can be conveniently tuned by adjusting those of the drops. To closely control the release kinetics of encapsulants, the composition and thickness of the capsule shell must be precisely tuned; this is greatly facilitated if the shell is homogeneous in its composition and thickness. However, the densities of the two drops that form the double emulsion are often different, resulting in an offset of the two drop centers and therefore in an inhomogeneous shell thickness. This difficulty can be overcome if the shell is made very thin. Unfortunately, a controlled fabrication of double emulsions with thin shells is difficult. In this paper, we present a microfluidic squeezing device that removes up to 93 vol% of the oil from the shell of water–oil–water double emulsions. This is achieved by strongly deforming drops; this deformation increases their interfacial energy to sufficiently high values to make splitting of double emulsions into double emulsions with a much thinner shell and a single emulsion oil drop energetically favorable. Therefore, we can reduce the shell thickness of the double emulsion down to 330 nm. Because this method does not rely on solvent evaporation, any type of oil can be removed. Therefore, it constitutes a new way to produce double emulsions with very thin shells that can be converted into thin-shell capsules made of a broad range of materials.     

Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.