Adsorption of cationic dendrimers on negatively charged solid substrates

substrates Michal Borkovec Email. [email protected] July 31, 2012 Direct link www.colloid.ch/dendrimers Since their synthesis by Vögtle and Tomalia in the eighties, dendrimers have developed into an very interesting class of novel polymeric materials. Potential applications include surface patterning, synthesis of nanoparticles, or gene delivery. Our research group was much involved in the study of adsorption of cationic dendrimers to negatively charged solid substrates, mainly silica, mica, or sulfate latex particles. This essay presents a summary of these results. Polyamidoamine (PAMAM) dendrimers Dendrimers to be discussed are the commercially available polyamidoamine (PAMAM) dendrimers. Their chemical structure and architecture are illustrated in the figure below. They are synthesized in different generations, and each generation adds a corona of branching side groups. PAMAM dendrimers are available up to generation 10 (G10). The G10 dendrimer has a molecular mass of 935 kg/mol and radius in solution of about 7 nm. The dependence of their radius a can be described by the simple relation 4πa3/3= vM/φ where v is the molecular volume of the monomer unit, M the molecular mass, and φ the volume fraction of the polymer. This relation assumes that the dendrimer can be approximated by a porous sphere containing water. In the experimental data of the hydrodynamic radii are shown in the figure below and the solid line is the calculation based on the above relation with φ= 0.6. This model describes the data well, and one concludes that a dissolved PAMAM dendrimer consists of about half polymer and half water. PAMAM dendrimers in solution may accumulate a substantial charge [2]. Since their charge originates from amine groups, they are almost neutral in basic conditions, and they become positively charged in acidic conditions. Their charging behavior is characterized by the fact that ionizable amine groups are rather far apart and thus ionize almost independently. Since the primary amine groups at the exterior are more basic than the tertiary amine in the core, the primary amines protonate already around pH 10, while the tertiary amines around pH 7. For pH values below 5, the dendrimers are fully protonated. The bare charge of G10 dendrimer is about 8000 charges, but even at pH 10 there will be still about 2000.