If long polyelectrolyte chains are attached densely to colloidal latex particles, a spherical polyelectrolyte brush results. These spherical polyelectrolytes are dispersed in water and carry a high charge. We demonstrate that these systems can be used to immobilize ions of heavy metals, such as gold, as counter-ions. Reduction of these ions leads to metallic nanoparticles. In this way the brush layer attached to the surface of the particles becomes a "nanoreactor" that may be used for chemical conversions of the metal ions. We show that the reduction of AuCl4- ions within these nanoreactors leads to well-defined and rather monodisperse gold nanoparticles that are attached to the surface of the core. A stable dispersion of polymeric core particles with attached nanoparticles results. All results reported here suggest that chemical reactions of ions immobilized in spherical polyelectrolyte brushes provide a new route to I composite particles of inorganic and organic materials.