Core-shell nanoparticles - structure and self-assembly at liquid-liquid interfaces

Polymer capped core-shell nanoparticles (NP) are of immense scientific and technological interest due their potential responsiveness to external stimuli and their usability as building blocks for the self-assembly of new functional materials. Vast efforts have been made to synthesize and characterize such NP systems. Promising applications such as ultrafiltration, drug encapsulation and delivery are already targeted. Nevertheless, due to the large variety of synthesis conditions and structural parameters, a detailed understanding of the structure- property relationships is still lacking. Our aim is a fundamentally improved basis to understand and predict the thermoresponsive behavior and self-assembly kinetics of superparamagnetic solid core polymer brush shell NP as related to their core-shell structure. An improved understanding will allow tailored synthesis and strict control of active nano- Pickering emulsions (self-assembled from these particles at droplet interfaces) through the shape deformation of the shell. In order to obtain fundamental relationships, we base our studies on our recently developed unique platform of monodisperse NP with iron-oxide cores and densely grafted polymer shells that allows detailed control of structural properties. We will create a toolbox of core-shell NP with independently varied core radius, shell thickness and grafting density and systematically investigate the structure and thermo-responsiveness in bulk and the self-assembly and resulting structure at liquid interfaces in-situ. Small-angle x- ray and neutron scattering methods (SAXS and SANS) are ideally suited for this purpose due to their nanometer resolution, volume penetration, non-destructive nature and good time- resolution. We will therefore focus on SAXS and SANS to in-situ characterize the core nucleation during synthesis and study the core-shell structure. Preliminary results have proven the suitability of SAXS for characterization of very dense polymer shells. SAXS at the synchrotron shall provide ultra-high time resolution to study fast thermoresponsive shell switching. Grazing incidence SAXS (GISAXS) and X-ray reflectivity (XR) will be used to follow the structure of NP assemblies by SALI. In this way we will be able to directly link structural properties of core-shell NP with thermoresponsive behavior and self-assembly kinetics. For his project, two groups of BOKU Vienna have teamed up, providing scientific excellence in NP synthesis and assembly (E. Reimhult) and in scattering techniques (H. Lichtenegger). The project is further linked to the Partnership for Soft Condensed Matter (PSCM) initiated by the European Synchrotron Radiation Facility and Institute Laue Langevin in Grenoble. This will ensure unique support for synchrotron and neutron experiments. The results of this project are expected to lay the fundamental basis for understanding and tailored fabrication of a new class of materials: ultra-thin mechanically robust membranes built from core-shell NP, the structure of which can be controlled and actuated on demand through magnetic fields.

We have developed new generations of magnetic nanoparticles ultimately destined for biomedical applications. Using a core of biocompatible iron oxide nanocrystal that behaves as a magnet only when subject to a magnetic field, these nanoparticles can be controlled, e.g. heated up inside the body or extract out of solution, by magnetic fields that are harmless for the body. This is valuable, e.g., to locate, move or release drugs inside the body. However, for this the nanocrystals must be protected by a shell that responds to an increase in temperature induced by the target tissue, or even better, by magnetic heating of the nanoparticle core. We have developed such shells that respond to heat to turn the particle from invisible in a biological system to recognised and taken up by cells. Important discoveries in our project relate the structure of how the polymer is bound to the particle core to how it responds to heat and thereby enable a control of the nanoparticles in biofluids that previously was not possible. The superb control that can be exerted over these smart nanoparticles also makes it possible to cluster them into a desired size with new, size- dependent properties. This is used to control their motion or removal by magnetic fields. Further, we have shown that these particles can be used to create and shield tiny droplets of vegetable oils, which automatically form to a uniform size smaller than cells. They are therefore useful for transporting poorly water-soluble compounds, such as many drugs, and release them using the magnetically and thermally responsive nanoparticles. Previously, creating such tiny oil droplets of precise size in water could only be achieved by using detergents and mechanical processes.