Adsorption of poly(amidoamine) dendrimers on gold

Adsorption of poly(amidoamine) (PAMAM) dendrimers from dilute solution onto cleaned gold has been studied in-situ by a quartz crystal microbalance technique. All experiments were carried out at 21 degreesC, with either anhydrous ethanol or deionized water as solvent. In ethanol, the equilibrium frequency shifts, proportional to the equilibrium surface coverages, Gamma [M/L-2], correspond to about a monolayer and increase weakly (roughly Linearly) with generation, G. The adsorption is likely driven by the weak favorable interaction between the primary amine end groups on PAMAM and gold. In aqueous media, the results are completely different. Under the conditions employed (low ionic strength and pH 6.5-7) the PAMAM amine end groups tend to protonate in aqueous solution, so that the macromolecules bear positive charge. The equilibrium frequency shift grows exponentially with generation G up to G = 6. For G = 7 a drastic drop is observed, but the increasing trend resumes again for G = 7 through G = 9. Estimates of the adsorbed layer thickness indicate that multilayers form on the clean gold surface in the aqueous system with the number of layers increasing exponentially up to G = 6. The formation of multilayers can be explained by a favorable electrostatic image-charge interaction between the positively charged dendrimers in aqueous solution and the gold substrate. By accounting for charge renormalization and screening, this picture quantitatively accounts for the dependence of Gamma on G, for G less than or equal to 6. The drop in frequency shift from G = 6 to G = 7 may be the result of surface crowding discussed in connection with the dense-shell transition of the dendrimer external surface.