Au Cluster-Cored Dendrimers Fabricated by Direct Synthesis and Post-functionalization Routes

The use of dendrimers and dendrons as stabilizing agents for metal nanoparticles and nanoclusters has captured interest in both the biomedicine and catalysis fields. Herein, we describe the synthesis of Au cluster-cored dendrimers by either direct synthesis or multi-step functionalization pathways. Direct synthesis of Au cluster-cored dendrimers was performed by the Brust-Schiffrin method using cystamine core poly(amidoamine) (PAMAM) dendrons as capping agents. Alternatively, a divergent approach to make nanoclusters with dendritic branching groups by functionalizing glycine-cystamine Au clusters was also carried out. This synthesis involved sequential Michael addition reactions of methyl acrylate followed by a subsequent amide coupling reaction with ethylenediamine on amine-terminated Au nanoclusters to form dendritic architectures around the Au core. The chemical structure of the ligands was confirmed by proton nuclear magnetic resonance after each functionalization reaction, and the cluster size was characterized by transmission electron microscopy. Au cluster-cored dendrimers with amine or ester terminal groups on the surface were produced. The resulting amine- and ester-terminated Au cluster-cored dendrimers synthesized by the divergent method are stable in solution and in the presence of excess reducing agent. In contrast, amine-terminated Au cluster-cored dendrimers synthesized by direct synthesis undergo aggregation in solution over time as a result of the high reactivity of the surface, while ester-terminated Au cluster-cored dendrimers formed by direct synthesis have much larger core sizes than seen using the divergent approach. Finally, the catalytic activities of these clusters for 4-nitrophenol reductions have been investigated. Cluster-cored dendrimers formed by direct synthesis had larger core sizes and higher catalytic activities than those formed by the divergent approach, which is likely due to the poor passivation of the Au surface for the directly synthesized clusters. Furthermore, Au cluster-cored dendrimers with less sterically bulky dendrons showed higher catalytic activities.