Quantitative analysis of Pt–PAMAM ligand exchange reactions: Time and concentration effects

The goal of this work is to quantitatively evaluate the extent of Pt–PAMAM complexation for conditions used in practice to prepare PAMAM-stabilized Pt nanoparticles for catalysis applications. UV–visible spectrophotometry (UV–vis), atomic absorption (AA) spectroscopy, and ion chromatography (IC) are used to monitor the progress of ligand exchange reactions between PtCl42− and G4OH PAMAM dendrimers. A specific objective was to quantify the fraction of Pt that covalently binds with PAMAM as a function of time, nominal Pt:PAMAM ratio, and PAMAM concentration. UV–vis and IC results show that ligand exchange reactions between PtCl42− and G4OH continue to produce new Pt–PAMAM bonds and release chloride ions over periods as long as 3 months. The overall kinetics of ligand exchange follows a log(time) dependence. Dialysis of G4OH–(Pt2+)n solutions followed by AA analysis shows that fractional covalent binding is greater than 90% for n < 30. Fractional binding decreases significantly for n > 40, showing that the actual number of Pt atoms that are irreversibly, covalently bound by G4OH is less than the nominal Pt:G4OH ratio (n). Incomplete binding leaves free, aquated Pt complexes in solution that may impact subsequent reduction and nanoparticle formation. We show that the 250 nm peak absorbance due to ligand-to-metal charge transfer (LMCT) increases non-linearly with the bound Pt:G4OH ratio. We speculate that the change in the extinction coefficient may be due to a graduate transition from multidentate to monodentate complexation as the bound Pt:G4OH ratio increases.