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Nanoparticles Encapsulated in Immobilized Dendrimers: Use of Supercritical Carbon Dioxide as the Processing Medium

Sreenivasa Reddy Puniredd and M.P. Srinivasan. Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Block E5, Singapore, Singapore

Nanostructured thin films on solid supports containing sequestered nanoparticles have numerous applications in areas ranging from biomedical devices to molecular electronics. Covalent molecular assembly and use of dendrimeric species have been, respectively, among the preferred methods for building nanostructured films and for sequestering nanoparticles. In this work, we have used supercritical carbon dioxide (SCCO2) as the vehicle for forming multilayered ultrathin films by layer-wise deposition of second generation polyamidodiamine (PAMAM) and pyromellitic dianhydride (PMDA) alternately on derivatised silicon dioxide surfaces, while ensuring covalent bonding between successive layers, thus contributing to the robustness of the structure.

The functional property of the PAMAM-containing film as a sequestering agent for metallic nanoparticles was exploited by the introduction of copper acetylacetonate (Cu(acac)2) as a precursor for copper nanoparticles. Introduction of the nanoparticle precursors into the matrix was performed in SCCO2 as well as in a conventional solvent medium (tetra hydro furan-THF). The precursors were subsequently reduced to form the nanoparticles. Formation of Cu nanoparticles within the dendrimer films is confirmed by X-ray photoelectron spectroscopy (XPS), UV-visible absorption spectroscopy, Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Atomic force microscopy (AFM), and ellipsometry. Processing in the supercritical phase resulted in a higher yield and a denser and more stable distribution of nanoparticles than when THF was employed, possibly due to rapid solvent separation and accelerated reaction rates available in SCF media. TEM images revealed that the embedded nanoparticles are fairly tightly distributed in terms of size with an average diameter of 7 nm. The SCF processed film exhibited good stability towards polar solvent attack when compared to the THF-processed film

We have also demonstrated the formation of monometallic (Pd, and Pt) and bimetallic Pd-Pt nanoparticles with various metal compositions within the dendrimer films in SCCO2. These nanoparticles are prepared by sequestering acetylacetonate complexes (Pd(acac)2 and Pt(acac)2) within the dendrimer film followed by thermal reduction of the complexes to yield the corresponding nanoparticles. XPS and UV-visible absorption spectroscopy confirm that the metal complexes are completely reduced to zerovalent nanoparticles. TEM, FESEM, and AFM study show the Pd, Pt, and Pd-Pt nanoparticles are well separated and are of average sizes 1.3 nm, 1.6 nm and 8 nm, respectively. FESEM and TEM-EDX analyses of individual nanoparticles confirm that the films contain both Pd and Pt atoms and have approximately the same atomic compositions as the ratio of the acetylacetonates loaded into the SCCO2 stream. The spatial distribution of the bimetallic nanoparticles depends on the proportion of the metal precursors loaded into the dendrimer matrix. This study also suggests that the other metal nanoparticles with a narrow size distribution can be synthesized in SCCO2 using corresponding organometallic compounds.