Thermosensitive core–shell microgels: From colloidal model systems to nanoreactors

Abstract We review recent work done on thermosensitive core–shell particles that consist of a solid core onto which a shell of crosslinked polymer as, e.g., poly(N-isopropylacrylamide) (PNIPA) has been grafted. Immersed in cold water the shell of these particles having dimensions of typically 150 nm in diameter will swell. At ca. 32 °C a volume transition takes place in the PNIPA-network and the shell shrinks considerably. In this review we present a survey of recent work done with these particles. The synthesis by a two-step route as well as by photo-emulsion polymerization will be discussed. Recent progress in the characterization of the systems by cryogenic transmission electron microscopy is presented and compared to work employing scattering methods. The phase diagram of the thermosensitive network and its description in terms of the Flory-Rehner-theory is reviewed. Moreover, the internal dynamics of the network could be studied by depolarized dynamic light scattering (DDLS). Since the degree of swelling can be reversibly adjusted by temperature, the effective volume fraction in aqueous solution can be adjusted within a wide range. Thus, the particles act as inflatable spheres and have been used for an in-depth study of the rheology of concentrated suspensions. As a final point, we present an overview of the use of these particles as nanoreactors. Here metal nanoparticles and enzymes are immobilized in the shell of the particles and used for catalysis. All results obtained so far demonstrate that thermosensitive core–shell particles present a class of model colloids with excellent stability and great versatility.