Intradendrimer exchange of metal nanoparticles

We previously demonstrated that metal nanoclusters ranging in size from 1 t o 2 nm can be prepared within dendrimer templates. In this two-step synthes is, metal ions (for example, Cu2+, Pd2+, and Pt2+) first partition into the interior of, for example, a hydroxyl-terminated poly(amidoamine) (PAMAM) d endrimer, and then the resulting nanocomposite is reduced with BH4- to yiel d a dendrimer-encapsulated, zerovalent metal nanocluster. The critical step in this procedure is partitioning of a particular number of metal ions int o the dendrimer interior. This process is normally driven by strong associa tion 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 proce dure is required. Here we report that dendrimer-encapsulated metal nanoclus ters can undergo multiple, in-situ displacement reactions. For example, a 5 5-Cu-atom-containing sixth-generation PAMAM dendrimer (G6-OH(Cu-55)) can be prepared by direct BH4- reduction of the corresponding Cu2+-containing den drimer. When G6-OH(Cu-55) is exposed to a solution containing ions more nob le than Cu, the Cu is displaced and the more noble ions are reduced. Here w e show that Ag, Au, Pd, and Pt dendrimer-encapsulated metal particles can b e prepared by this sort of primary displacement reaction. Such reactions ar e fast and go to completion, and the resulting particles are stable (no agg lomeration or precipitation) and small (1-3 nm in diameter) and can be rela tively monodisperse. Moreover, depending on the pH at which the displacemen t is carried out, the displaced Cu2+ ions may be retained within the dendri mer interior. Au, Pt, and Pd nanoparticles can be also prepared by a second ary displacement reaction between dendrimer-encapsulated Ag nanoclusters (p repared from a primary displacement reaction) and Au3+, Pt2+, or Pd2+ ions, respectively. Pd and Pt dendrimer-encapsulated nanoparticles prepared by d irect reduction, as well as by primary or secondary displacement reactions, are catalytically active for electrochemical reduction of O-2. The materia ls resulting from this study are characterized by UV-vis spectroscopy, X-ra y photoelectron spectroscopy, transmission electron microscopy, and electro chemical methods.