Organic-Inorganic Hybrid Particles by Dendrimer Nanotemplating
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Organic-Inorganic Hybrid Particles by Dendrimer Nanotemplating Franziska Gröhn, Barry J. Bauer, Eric J. Amis Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ABSTRACT Poly(amidoamine) (PAMAM) dendrimers are used to create organic-inorganic hybrid colloids in aqueous solution. The formation of gold colloids upon reduction of a gold salt precursor serves as a model reaction to study the influence of reaction conditions and dendrimer generation on the resulting nanostructures. The hybrid particles were characterized by transmission electron microscopy (TEM), small angle neutron scattering (SANS), and small angle x-ray scattering (SAXS). A transition is found from “colloid stabilization” by low molecular mass molecules to “polymer nanotemplating” with increasing dendrimer generation, i.e, increasing molecular mass but retaining the chemical nature of the stabilizing species. I.
INTRODUCTION Organic-inorganic hybrid nanostructures have generated significant interest due to a variety of potential applications as electrical, optical, medical and information storage materials. Biological nanocomposites like bone or sea shells produced in nature have excellent properties. The method of polymer nanotemplating, i.e., using a polymeric matrix to control the growth of an inorganic crystal, has been shown to be an effective synthetic route for producing polymer/inorganic composites[1]. However, the success of this approach requires understanding formation mechanisms and properties. Thus there is a need for model systems of polymer nanotemplating. Since dendrimers are well defined organic molecules in the size range of (1 to 15) nm and are known to act as hosts for guest molecules [2,3], they are promising candidates as templates for the formation of inorganic nanoclusters. Organic-inorganic hybrid materials based on dendrimers have been recently reported by several authors [4-9]. However, our motivation for using dendrimers comes from a desire to understand the mechanisms of polymer nanotemplating in a model system. Dendrimers can serve a model system since they are monodisperse and well characterized molecules [10]. The size of different generation dendrimers spans the characteristic sizes of low molecular mass molecules, polymers and colloids, thereby offering a unique mesoscopic system. In this study, we use charged polyamidoamine dendrimers in aqueous solution as potential nanotemplates. Our approach that is shown schematically in Figure 1 relies on the attraction between charged dendrimers and oppositely charged metal ions. Performing chemical reactions on the metal ions is expected to produce colloid structures that are controlled by the dendrimer. We use the reduction of gold salt to colloidal gold as a classical model reaction. The concept of using charged, solvent-penetrable templates in aqueous solution was first applied to polyelectrolyte microgels [11]. In this study, we investigate dendrimers from generation 2 to 10 (G2 to G10) for the formation of inorganic-organic hybr
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