Morphology of amphiphilic gold/dendrimer nanocomposite monolayers

We have synthesized amphiphilic gold/poly(amidoamine) (PAMAM) dendrimer nanocomposites (DNCs) using hydrophobically modified ethylenediamine (EDA) core generation-2 and -4 dendrimers as templates. Monolayers of gold DNCs were spread at the air/water interface of a Langmuir trough, and X-ray reflectivity was performed both in situ and on transferred LB layers. From the scattering length density and the specular reflectivity profiles, we find that the dendrimer layer is hydrated and the Au is uniformly distributed within the dendrimer body. Diffuse scattering shows no distinct features in agreement with TEM images, which show that the Au is disordered. The second-generation dendrimer was spherical on the water surface, whereas the fourth-generation became oblate at high pressures. The isotherms were also measured as a function of the pH of the water subphase. The results show that, at high pH, the dendrimers become "soft" and the films collapse at smaller areas. For lower pH values, a distinct plateau is observed that is postulated to arise from crystallization of the carbon side chains. The presence of Au in the dendrimer appears to amplify the effects of pH on the isotherms.