In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1-2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.

In this article, we provide a detailed description of the synthesis and properties of Pt dendrimer-encapsulated nanoparticles (DENs) prepared using sixth-generation, hydroxyl-terminated, poly(amidoamine) (PAMAM) dendrimers (G6−OH) and three different PtCl4 2−/G6−OH ratios: 55, 147, and 240. Results obtained from UV−vis spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, X-ray absorption spectroscopy, and high-energy X-ray diffraction show that only a fraction of the Pt2+/dendrimer precursors are reduced by BH4 − and that the reduction process is highly heterogeneous. That is, after reduction each Pt2+/dendrimer precursor complex is either fully reduced, to yield a DEN having a size and structure consistent with the original PtCl4 2−/dendrimer ratio used for the synthesis, or the precursor is not reduced at all. This result is consistent with an autocatalytic process that entails slow formation of a nascent catalytic Pt seed within the dendrimer, followed by rapid, catalytic reduction of nearby Pt2+ ions. Details concerning the formation of the Pt2+/dendrimer precursor are also discussed.

High-resolution solution (1)H NMR spectroscopy has been used to characterize the size of Pd dendrimer-encapsulated nanoparticles (DENs). The Pd nanoparticles measured by this technique contain 55, 147, 200, or 250 atoms, and they are encapsulated within sixth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G6-OH). Detailed analysis of the NMR data shows that signals arising from the innermost protons of G6-OH(Pd(n)) decrease significantly as the size of the encapsulated nanoparticles increase. A mathematical correlation between this decrease in the integral value and the theoretical number of Pd atoms in the nanoparticle is extracted. It enables the elucidation of the size of Pd DENs by (1)H NMR spectroscopy. NMR pulse-field gradient spin-echo experiments demonstrate that G6-OH with and without DENs have identical hydrodynamic radii, which excludes the presence of dendrimer/nanoparticle aggregates.