Cyclic voltammetry studies of Cd2+ and Zn2+ complexation with hydroxyl-terminated polyamidoamine generation 2 dendrimer at a mercury microelectrode

Cyclic voltammetry was used to determine the stability constants for the complexation of cadmium and zinc ions with hydroxyl-terminated polyamidoamine generation 2 dendrimers with an ethylenediamine core (PAMAM G2-OH) at a mercury ultramicroelectrode. Cadmium and zinc ions were found to form poly-metal complexes through the interactions with interior amino groups rather than hydroxyl groups and the maximum number of metal ions per dendrimer or stoichiometry was 4. Complexation is accompanied by a significant decrease in the limiting current and by measuring the changes in the magnitude of cathodic current with the analytical dendrimer:metal ion ratio and fitting the resulting curves, the four cumulative consecutive complexation constants were determined: 1.3 × 105 M−1, 1.6 × 109 M−2, 3.3 × 1012 M−3 and 4.6 × 1014 M−4 for Cd2+ and 1.0 × 106 M−1, 1.6 × 109 M−2, 4.6 × 1014 M−3 and 2.6 × 1017 M−4 for Zn2+. The stronger interaction of zinc ions with PAMAM G2-OH compared to cadmium was accompanied with a sharper decrease of the cathodic current with the addition of dendrimer and a larger half-wave potential shift than cadmium. This was rationalized by evaluating the changes of the diffusion coefficients for these two different systems by electrochemical means.