Investigation of the factors influencing the formation of dendrimer/polyanion multilayer films

. Poly(amidoamine) dendrimer (PAMAM)/poly(styrenesulfonate) (PSS) multilayer films were prepared by an iterative electrostatic self-assembly process, viz., the layer-by-layer deposition of PAMAM and PSS onto planar supports. Multilayer film growth was monitored by UV-vis spectrophotometry, by use of a quartz crystal microbalance (QCM), and by ellipsometry and atomic force microscopy (AFM). The UV-vis data revealed an adsorption-desorption phenomenon that was sensitive to the pH and ionic strength of the PSS and dendrimer solutions, the dendrimer generation (2G, 3G, or 4G) and concentration, and the PSS molecular weight and concentration. This behavior was due to the different nature of interaction and complexation between the hierarchical (PAMAM) and linear (PSS) polyelectrolytes, both at the film-solution interface and in bulk solution. QCM experiments provided evidence for multilayer film growth, but the adsorption-desorption behavior precluded quantitative evaluation of the amount of polyelectrolyte deposited. The use of "fresh" polyelectrolyte solutions for deposition of each layer was found to facilitate the regular formation of PSS/PAMAM multilayers. For PSS/4G PAMAM films prepared in this way, the bilayer thickness was determined to be 3.9 +/- 0.2 nm from ellipsometry, while AFM showed that 10-layer films were rather smooth, with a root-mean-square roughness of ca. 0.6 +/- 0.1 nm. Investigation of the formation of thin films from two different classes of polyelectrolytes (hierarchical and linear) provides insights into their interactions and opens the possibility of constructing complex films based on dendrimers that can be exploited as nanoreactors and nanoreservoirs.