Synthesis and electrochemical characterization of highly monodisperse dendrimer-templated monolayer protected clusters
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We described the synthesis of multilayer organic thin films prepared by sequential vapor-phase coupling of monomers. The reactions were carried out at room temperature and atmospheric pressure. Films prepared using up to six sequential coupling reactions are reported. Homobifunctionalized monomers, such as hexamethylenediamine, react primarily via a single endgroup rather than cross coupling to the reactive surface via both reactive groups. We synthesized bifunctionalized polyamidoamine (PAMAM) dendrimers having both quaternary ammonium groups and primary amines on their periphery were prepared. The high positive charge on the surface of these dendrimers prevents agglomeration, and the unquanternized amine groups provide a reactive handle for immobilizing the dendrimer-encapsulated nanoparticles onto surfaces. We prepared highly monodisperse, 1-2 nm diameter Au nanoparticles using bifunctionalized PAMAM dendrimers as templates. The synthesis is carried out in water, takes less than 30 min, and requires no subsequent purification. The high monodispersity is a function of the template synthesis, which avoids size variations arising from random nucleation and growth phenomena, and the use of magic number equivalent ratios of AuCl4-/dendrimer. We investigated the electrochemical properties of Au, Pd and PdAu monolayer-protected clusters (MPCs), prepared by dendrimer-templating and subsequent extraction, are described. Purification of the extracted Au, Pd and PdAu nanoparticles was not required to obtain well-defined differential pulse voltammetry peaks arising from quantized double-layer charging. The calculated sizes of the nanoparticles were essentially identical to those determined from the electrochemical data. The capacitance of the particles was independent of the composition of core metal.