  Metallic nanoparticles have been used as catalysts for various reactions, and
the huge literature on the subject is hard to overlook. In many applications,
the nanoparticles must be affixed to a colloidal carrier for easy handling
during catalysis. These "passive carriers" (e.g., dendrimers) serve for a
controlled synthesis of the nanoparticles and prevent coagulation during
catalysis. Recently, hybrids from nanoparticles and polymers have been
developed that allow us to change the catalytic activity of the nanoparticles
by external triggers. In particular, single nanoparticles embedded in a
thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have
become the most-studied examples of such hybrids: Immersed in cold water, the
PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic
substrates can diffuse easily through the network, and react at the surface of
the nanoparticles. Above the volume transition located at 32{\deg}C, the
network becomes hydrophobic and shrinks. Now hydrophobic substrates will
preferably diffuse through the network and react with other substrates in the
reaction catalyzed by the enclosed nanoparticle. Such "active carriers", may
thus be viewed as true nanoreactors that open new ways for the use of
nanoparticles in catalysis.
