We previously demonstrated that metal nanoclusters ranging in size from 1 to 2 nm can be prepared within dendrimer templates. In this two-step synthesis, metal ions (for example, Cu2+, Pd2+, and Pt2+) first partition into the interior of, for example, a hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimer, and then the resulting nanocomposite is reduced with BH4- to yield a dendrimer-encapsulated, zerovalent metal nanocluster. The critical step in this procedure is partitioning of a particular number of metal ions into the dendrimer interior. This process is normally driven by strong association of metal ions with intradendrimer tertiary amine groups. However, for metal ions that do not form either covalent bonds or strong complexes with the interior amine groups (for example, Au3+ and Ag+), an alternative procedure is required. Here we report that dendrimer-encapsulated metal nanoclusters can undergo multiple, in-situ displacement reactions. For example, a 55-Cu-atom-containing sixth-generation PAMAM dendrimer (G6-OH(Cu-55)) can be prepared by direct BH4- reduction of the corresponding Cu2+-containing dendrimer. When G6-OH(Cu-55) is exposed to a solution containing ions more noble than Cu, the Cu is displaced and the more noble ions are reduced. Here we show that Ag, Au, Pd, and Pt dendrimer-encapsulated metal particles can be prepared by this sort of primary displacement reaction. Such reactions are fast and go to completion, and the resulting particles are stable (no agglomeration or precipitation) and small (1-3 nm in diameter) and can be relatively monodisperse. Moreover, depending on the pH at which the displacement is carried out, the displaced Cu2+ ions may be retained within the dendrimer interior. Au, Pt, and Pd nanoparticles can be also prepared by a secondary displacement reaction between dendrimer-encapsulated Ag nanoclusters (prepared from a primary displacement reaction) and Au3+, Pt2+, or Pd2+ ions, respectively. Pd and Pt dendrimer-encapsulated nanoparticles prepared by direct reduction, as well as by primary or secondary displacement reactions, are catalytically active for electrochemical reduction of O-2. The materials resulting from this study are characterized by UV-vis spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and electrochemical methods.
