Synthesis, characterization, and stability of dendrimer-encapsulated palladium nanoparticles

Here we report on the synthesis, physical and chemical properties, and stability of I'd nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers. Specifically, amine- and hydroxyl-terminated PAMAM dendrimers ranging in generation from 4 to 8 were studied. Under appropriate conditions, addition of K2PdCl4 results in covalent attachment of the PdCl3- hydrolysis product of this complex to tertiary amines within the dendrimers. Reduction with NaBH4 results in conversion of dendrimer-encapsulated PdCl3- to nearly size monodisperse, encapsulated, zerovalent Pd nanoparticles. Details regarding the Pd species present in solution and within the dendrimer prior to reduction are reported, as is the maximum Pd2+ loading of the dendrimers. Dendrimer-encapsulated Pd nanoparticles undergo oxidation in air, but this process is slowed significantly when coordinating ions are removed from solution. In the absence of O-2, dendrimer-encapsulated Pd nanoparticles are stable indefinitely. The oxidation product is not PdO, but rather Pd ions coordinated to the dendrimer interior. Dendrimer generation does not affect the rate of Pd oxidation. The dendrimer itself undergoes irreversible oxidation in the presence of O-2. Finally, the oxidation of dendrimer-encapsulated Pd nanoparticles is reversible. Specifically, H-2 gas can be used to re-reduce partially oxidized Pd nanoparticles without changing their average size.