Covalent Molecular Assembly in a Supercritical Medium:  Formation of Nanoparticles Encapsulated in Immobilized Dendrimers

This work demonstrates the feasibility of forming metallic nanoparticles within the confines of dendrimer-laden ultrathin films that are immobilized on a solid surface. The functional property of the poly amido amine dendrimer (PAMAM) as a sequestering agent for Cu nanoparticles was exploited by the introduction of copper acetylacetonate (Cu(acac) 2 ) as a precursor. The nanoparticle precursors were introduced into the matrix using supercritical carbon dioxide and liquid tetrahydrofuran (THF) as processing media. The precursors were subsequently reduced to form the nanoparticles. The resulting film assemblies were characterized via X-ray photoelectron and UV−visible absorption spectroscopies, atomic force, scanning electron, transmission electron microscopies, and ellipsometry. Higher yield, a denser distribution of nanoparticles, and greater stability toward polar solvent attack were observed when the structures were assembled in supercritical CO 2 than when THF was employed. TEM images revealed that the embedded nanoparticles are fairly tightly distributed in terms of size with an average diameter of 7 nm.