Dendrimer-encapsulated metals and semiconductors: Synthesis, characterization, and applications

This chapter describes composite materials composed of dendrimers and metal s or semiconductors. Three types of dendrimer/metal-ion composites are disc ussed: dendrimers containing structural metal ions, nonstructural exterior metal ions, and nonstructural interior metal ions. Nonstructural interior m etal ions can be reduced to yield dendrimer-encapsulated metal and semicond uctor nanoparticles. These materials are the principal focus of this chapte r. Poly(wamidoamine) (PAMAM) and poly(propylene imine) dendrimers, which ar e the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent number of interior tertiary amines. These are able to complex a range of metal ion s including Cu2+, Pd2+, and Pt2+. The maximum number of metal ions that can be sorbed within the dendrimer interior depends on the metal ion, the dend rimer type, and the dendrimer generation. For example, a generation six PAM AM dendrimer can contain up to 64 Cu2+ ions. Nonstructural interior ions ca n be chemically reduced to yield dendrimer-encapsulated metal nanoparticles . Because each dendrimer contains a specific number of ions, the resulting metal nanoparticles are in many cases of nearly monodisperse size. Nanopart icles within dendrimers are stabilized by the dendrimer framework; that is, the dendrimer first acts as a molecular template to prepare the metal nano particles and then as a stabilizer to prevent agglomeration. These composit es are useful for a range of catalytic applications including hydrogenation s and Heck chemistry. The unique properties of the interior dendrimer micro environment can result in formation of products not observed in the absence of the dendrimer. Moreover the exterior dendrimer branches act as a select ive gate that controls access to the interior nanoparticle, which results i n selective catalysis. In addition to single-metal nanoparticles, it is als o possible to prepare bimetallic nanoclusters and dendrimer-encapsulated se miconductor nanoparticles, such as CdS, using this same general approach.