Ten years ago I wrote a review about the important field of core-shell nanoparticles, focussing mainly on our own work about tracer systems, and briefly addressing polymer-coated nanoparticles as fillers for homogeneous polymer-colloid composites. Since then, the potential use of core-shell nanoparticles as multifunctional sensors or potential smart drug-delivery vehicles in biology and medicine has gained more and more importance, affording special types of multi-functionalized and bio-compatible nanoparticles. In this new review article, I try to address the most important developments during the last ten years. This overview is mainly based on frequently cited and more specialized recent review articles from leaders in their respective field. We will consider a variety of nanoscopic core-shell architectures from highly fluorescent nanoparticles (NPs), protected magnetic NPs, multifunctional NPs, thermoresponsive NPs and biocompatible systems to, finally, smart drug-delivery systems.

The cover picture shows mesoscopic strands of highly fluorenscent perylene bisimide ± melamine assemblies as visualized by confocal fluorenscence microscopy. This technique does not only provide topological information such as related AFM or STM images, but truly shows the functionality of fluorescent optical networks of the present supramolecular system. The synthesis and structural investigations of the fluorescent mesoscopic superstructures is presented by F. Würthner et al. on p. 3871 ff. The author wishes to thank the BASF AG for kindly covering the costs for the cover picture

Core–shell microgels are crosslinked nanosized spherical particles with a chemical composition that is different on the surface compared to the core region. By employing a core with special optical properties, e.g., a core labeled either with organic dye molecules or noble metal clusters (see Figure), these particles are perfectly suited as optical tracers in diffusion measurements. Here, the shell may be important for several reasons: (i) as a protective coating to suppress any influence of the labels on particle mobility, (ii) to optically separate individual particles even at high concentrations, and (iii) to compatibilize the particles with e.g., polymeric chains. Recent developments in the field of crosslinked core–shell particles are reviewed here, with a focus on such optical tracer systems.

We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional “brute force’’ ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above φc we find indications that additional dynamic processes modify the β relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an increase of the particle compressibility and the surface friction on decreasing the cross-link density from its hard sphere limit.

We describe the preparation of hairy nanospheres by grafting of polystyrene macromonomer chains onto polyorganosiloxane microgels. Our strategy was to obtain special surface-modified colloidal particles, which can be blended with linear polymer chains without depletion demixing found for standard colloid-polymer mixtures. For this purpose, the molecular weight of the polymer hairs and of the linear chains has been varied between 4000 and 19 000 g/mol. In all cases, the number of hairs per single particle with core radius about 10 nm exceeded 150. Studying the internal structure of mixtures of these hairy spheres with linear polymer chains by transmission electron microscopy and small-angle X-ray scattering, we identified homogeneous systems, i.e., suppression of depletion demixing, in case the molecular weight of the polymer hairs on the particle surface is at least as large as the molecular weight of the single polymer chains. This result is interpreted in terms of a new depletion model, taking into account the surface roughness of colloidal spheres with a surface of polymer hairs, and the probability of single polymer chains to partially penetrate into this hairy particle surface.