PubMed Health⌕ Search

Biomedical subjects

Alexander Böker

Publications and source records attributed to Alexander Böker.

9 recordsLinked to original sources
PubMed2006

Janus particles at liquid-liquid interfaces.

Following recent theoretical predictions, we report on the first experiments on the interfacial activity of so-called Janus nanoparticles (i.e., bifacial particles consisting of a gold and an iron oxide moiety). Using pendant drop tensiometry, we show that the amphiphilicity derived from the Janus character of the particles leads to a significantly higher interfacial activity compared to that of the respective homogeneous particles of the same size. The self-assembly of Janus particles at the hexane-water interface results in a significant decrease in the interfacial tension. Furthermore, we demonstrate control over the interfacial activity by tuning the particles' amphiphilicity via ligand-exchange reactions.

Journal Article↗
PubMed2005

Influence of initial order on the microscopic mechanism of electric field induced alignment of block copolymer microdomains.

We investigate the mechanism of microdomain orientation in concentrated block copolymer solutions exposed to a dc electric field by in situ synchrotron small-angle X-ray scattering (SAXS). As a model system, we use concentrated solutions of a lamellar polystyrene-b-polyisoprene block copolymer in toluene. We find that both the microscopic mechanism of reorientation and the kinetics of the process strongly depend on the initial degree of order in the system. In a highly ordered lamellar system with the lamellae being aligned perpendicular to the electric field vector, only nucleation and growth of domains is possible as a pathway to reorientation and the process proceeds rather slowly. In less ordered samples, grain rotation becomes possible as an alternative pathway, and the process proceeds considerably faster. The interpretation of our finding is strongly corroborated by dynamic self-consistent field simulations.

Electricity↗
PubMed2005

Self-directed self-assembly of nanoparticle/copolymer mixtures.

The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoscopic elements as a strategy for tailoring the electrical, magnetic or photonic properties of the material. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.

Journal Article↗
PubMed2005

Nanoparticle assembly at fluid interfaces: structure and dynamics.

The self-assembly of nanoparticles at fluid interfaces, driven by the reduction in interfacial energy, was investigated. With spherical, tri-n-octyl-phosphine-oxide covered cadmium selenide (CdSe) nanoparticles (1-8 nm), thermal fluctuations compete with the interfacial segregation giving rise to a size-dependent self-assembly of the particles. The structure of the nanoparticle assembly was studied using electron microscopy, atomic force microscopy, and X-ray scattering in situ, which indicate that the particles form a densely packed monolayer. The energetics of the adsorption of nanoparticles onto the interface was revealed by time-dependent fluorescence studies on a mixture of two different sized nanoparticles at the interface. The dynamics of the nanoparticles at the fluid interface, probed using fluorescence photobleaching methods, suggests a liquid-like behavior. The results have implications in the design of hierarchical self-assemblies of nanoparticles for the one-step fabrication of devices on multiple length scales.

Journal Article↗
PubMed2004

Hierarchical nanoparticle assemblies formed by decorating breath figures.

The combination of two self-assembly processes on different length scales leads to the formation of hierarchically structured nanoparticle arrays. Here, the formation of spherical cavities, or 'breath figures'-made by the condensation of micrometre-sized water droplets on the surface of a polymer solution-that self-assemble into a well-ordered hexagonal array, is combined with the self-assembly of CdSe nanoparticles at the polymer solution-water droplet interface. Complete evaporation of the solvent and water confines the particle assembly to an array of spherical cavities and allows for ex situ investigation. Fluorescence confocal, transmission electron and scanning electron microscope images show the preferential segregation of the CdSe nanoparticles to the polymer solution-water interface where they form a 5-7-nm-thick layer, thus functionalizing the walls of the holes. This process opens a new route to fabricating highly functionalized ordered microarrays of nanoparticles, potentially useful in sensory, separation membrane or catalytic applications.

Cadmium Compounds↗
PubMed2003

Self-assembly of functional nanostructures from ABC triblock copolymers.

The spontaneous formation of nanostructured materials by molecular self-assembly of block copolymers is an active area of research, driven both by its inherent beauty and by a wealth of potential technological applications. Thin films of block copolymers have attracted increasing interest, particularly in view of possible applications in nanotechnology. Although much of the work has concentrated on block copolymers consisting of two components, the insertion of a third block greatly enlarges the structural diversity and allows incorporation of additional chemical functionality. Here we describe a highly ordered hexagonally perforated lamella structure based on an ABC triblock copolymer thin film. By suitable choice of the three blocks a versatile structure is formed. The perforated lamella can serve as a lithographic mask, it can be chemically converted into an amphiphilic structure without losing its order, and after selective removal of one of its constituents it could be used as a responsive membrane. Intriguingly, the particular choice of the blocks ensures that the structure is formed irrespective of the chemical nature of the solid substrate. The experimental results are supported by mesoscale computer simulations.

Crystallization↗
PubMed2003

Ultrathin cross-linked nanoparticle membranes.

The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Here, the self-assembly of functionalized, photoluminescent nanoparticles at liquid interfaces, followed by cross-linking of the associated ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic yet permeable and have potential applications involving controlled permeability and diffusion. Cadmium selenide (CdSe) nanoparticles are used, since their inherent photoluminescence offers a direct way to probe the spatial organization of the particles. Functionalized ligands attached to the nanoparticles provide an effective means to stabilize the interfacial assembly by cross-linking. The concepts shown are adaptable to other type of nanoparticles, ligands, and solvent combinations.

Colloids↗
PubMed2003

Amphiphilic Janus micelles with polystyrene and poly(methacrylic acid) hemispheres.

We describe the synthesis and the solution properties of Janus micelles containing a polybutadiene (PB) core and a compartmentalized corona consisting of a poly(methacrylic acid) (PMAA) and a polystyrene (PS) hemisphere. The Janus micelles were obtained via cross-linking the middle block of a microphase-separated PS-block-PB-block-PMMA triblock copolymer in the bulk state, followed by alkaline hydrolysis of the poly(methyl methacrylate) (PMMA) ester groups. Results of fluorescence correlation spectroscopy, field flow fractionation, light scattering, cryogenic transmission electron microscopy, scanning electron microscopy, and scanning force microscopy indicate that above a critical aggregation concentration of about 0.03 g/L spherical supermicelles are formed from about 30 PS-PMAA micelles in aqueous solution in the presence of NaCl. These supermicelles have radii of 40-60 nm, significantly increasing on ionization (pH >6). In addition, very large spherical objects are observed with radii of 100-250 nm.

Journal Article↗