Internal Structure of Silver−Poly(amidoamine) Dendrimer Complexes and Nanocomposites

Abstract

Poly(amidoamine) PAMAM dendrimers of various generations (generation 2 to generation 6) with different surfaces (amine, Tris, and pivalate) were used as organic templates to form complexes and nanocomposites of silver. Starting from silver salts and PAMAM dendrimers, first a silver−dendrimer complex and then a nanocomposite were synthesized. To avoid chemical contamination, transformation of silver from Ag+ to Ag0 was achieved by irradiating the solution of silver complex with an excessive dose of X-rays. Intensity of the plasmon resonance at 400−420 nm was found to be the function of silver content, nanoparticle size, and architecture. Internal structural changes during this transformation from complexes into nanocomposites have been studied in the solution state by the electron paramagnetic resonance (EPR) technique. Copper(II) ions were used as a probe. Measurements were performed at various silver/dendrimer ratios and at various temperatures. We determined the structure of the copper(II) ions interacting with the [(Ag+)n−PAMAM] silver(I)−poly(amidoamine) dendrimer complexes and compared those to the structure of the respective Cu(II) complexes formed with {(Ag0)n−PAMAM} silver−dendrimer nanocomposites. In the presence of silver ions, the interior of the dendrimer was found to be partially positively charged and it repulsed copper ions, which were then confined to sites not occupied by silver ions. Transformation of Ag+ into Ag0 completely modified the Cu(II) interactions with the interior of the dendrimer in the direction of favoring the internalization of copper(II) ions in the dendrimer structure. In nanocomposites, Ag(0) atoms occupy the smaller outer cavities of the dendrimer interior forcing a portion of copper ions into the larger cavities. The conclusion above is also supported by high resolution energy-filtered TEM images of silver nanocomposites. The significance of various surface variations is also quantitatively described and discussed.