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

Abstract

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 PtCl 4 2− 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 PtCl 4 2− 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–(Pt 2+) 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.