Dendrimer-cyclodextrin assemblies as stabilizers for gold and platinum nanoparticles

Aqueous assemblies of adamantyl-derivatized poly(propylene imine) (PPI) dendrimers and B-cyclodextrin (B-CD) have been used as nanoreactors in the preparation of gold and platinum nanoparticles in water. These particles have been formed by the reduction of aurate or platinate anions in the presence of the generation 4 (4·(B-CD)32) and 5 ( 5·(B-CD)40) assemblies. Lower generation assemblies did not provide stable nanoparticles. A kinetic model is proposed in which the particles form inside the dendrimer assemblies owing to preferred nucleation as a result of the electrostatic attraction between the polycationic core and the metallate anions. The persistent shape of the adamantyl-derivatized dendrimers and the dense shell of adamantyl-B-CD complexes provide a kinetic barrier for nanoparticle escape thus prolonging their lifetime. Exchange of the dendrimers for a cationic disulfide provided stable, water-soluble metal nanoparticles without change of their size distribution.