Acid-base interactions between amino-silica and carboxyl-gold protected nanoparticles, functionalized or not with free stable radicals or fluorescent moieties, were studied by ESR, fluorescence, and TEM. While those nanoparticles are stable in solution, mixing them induces aggregation, due to acid-base interactions; addition of a base or an acid redissolves them. By ESR, strong spin-spin interactions were noticed; by fluorescence, only small differences may be observed, while by TEM, it was noticed that gold nanoparticles are distributed evenly on the surface of the silica nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.