Effect of PAMAM Dendrimer Size and pH on the Electrostatic Binding of Metal Complexes Using Cyclic Voltammetry

Abstract

Cyclic voltammetry was employed to investigate the extent of electrostatic binding for metal complexes such as ferrocyanide, [Fe(CN)6]4-, to full generations of polyamidoamine (PAMAM) dendrimers (G1.0−G5.0) as a function of size and pH. Addition of dendrimer solutions to metal complexes caused a large net positive shift in E1/2 and reduced peak currents that varied with the size of the dendrimer. Binding ratios, K4-/K3-, calculated from the potential shifts changed from a ratio of five with small dendrimers (G1.0−G3.0) to 10 for G4.0 and G5.0, consistent with known structural changes in PAMAM dendrimers. Conditional binding constants for [Fe(CN)6]4-, calculated from the diffusion-limited oxidation currents, also increased from G3.0 (8.8 × 104 M-1) to G4.0 (31 × 104 M-1). At pH 5, the binding ratios increased dramatically at all generations, suggesting that the interior amine sites in the dendrimer are easily accessible to metal complexes and that dendrimers show some charge selectivity.