We report the formation of Pd, Pt and Pd/Pt nanoparticles in dendrimer-laden assemblies immobilized within a silica gel matrix and constructed by covalent layer-by-layer (LbL) assembly using supercritical carbon dioxide (SCCO2) as the processing medium. We show that the dendrimer plays a critical role in retaining the nanoparticle precursor within the silica gel. Smaller particle size, higher yield, a denser distribution of nanoparticles and greater stability were observed when the structures were assembled in SCCO2. (C) 2008 Elsevier Inc. All rights reserved.