Platinum Ion Uptake by Dendrimers:  An NMR and AFM Study

The reaction of generation 2 and generation 4 poly(amidoamine) (PAMAM) dendrimers with K₂PtCl₄ was studied by several NMR methods. The time dependency of the Pt(II) complexation was followed with ¹⁹⁵Pt NMR for both dendrimers and the equilibrium product was further characterized with ¹H NMR, and indirectly detected ¹³C NMR, in the case of the generation 2 dendrimer. After 2 days, a black precipitate of Pt(0) was observed, half the original ¹⁹⁵Pt signal was lost, and approximately 20% of the initial Pt(II) was coordinated to the tertiary and secondary nitrogens of the generation 2 dendrimer. The uptake of Pt(II) by the generation 4 dendrimer was much slower, consistent with the steric crowding of the surface groups on the generation 4 dendrimer compared to the more open generation 2. After 10 days, 80% of the Pt(II) was deep within the generation 4 dendrimer; the remaining 20% was unreacted or bound near the surface nitrogens of a single dendrimer. The location and time course of the platinum ion uptake by the dendrimers provides valuable insight into the formation of Pt⁰ nanoparticles made in the presence of dendrimers as stabilizers, visualized by atomic force microscopy.