NMR Characterization of Fourth-Generation PAMAM Dendrimers in the Presence and Absence of Palladium Dendrimer-Encapsulated Nanoparticles

High-resolution solution NMR spectroscopy has been used to characterize the structure of Pd dendrimer-encapsulated nanoparticles (DENS), consisting of approximately 55-atom nanoparticles encapsulated within fourth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G4-OH). Detailed analysis of 1D and 2D NMR spectra of dendrimers with (G4-OH(Pd-55)) and without (G4-OH) nanoparticles unambiguously demonstrate that single nanoparticles are encapsulated within individual dendrimers. This conclusion is based on the following results. First, the NMR data show that signals arising from the innermost methylenes of G4-OH(Pd-55) are more highly influenced by the presence of the Pd nanoparticles than are the terminal functional groups. This means that DENS are encapsulated within dendrimers rather than being adsorbed to their surface, as would be the case for aggregates consisting of multiple dendrimers and nanoparticles. Second, extraction of DENS from within their dendrimer hosts results in an increase in the NMR intensity associated with the interior methylenes, which corroborates the previous point. Third, NMR pulse-field gradient spin-echo experiments demonstrate that G4-OH and G4-OH(Pd-55) have identical hydrodynamic radii, and this finding excludes the presence of dendrimer/nanoparticle aggregates.