pH-dependent aggregation of citrate-capped Au nanoparticles induced by Cu2+ ions: The competition effect of hydroxyl groups with the carboxyl groups

The citrate-capped Au nanoparticles were found to undergo pH-dependent aggregation induced by Cu2+ ions. At low pH value (8.9), the Au nanoparticles aggregated into fractal-like structure induced by the coordination interactions between the carboxyl groups of citrate ligands and Cu2+ ions. At high pH value (10.1), the Au nanoparticles aggregated into close-packed structure attributed to the increased concentration of hydroxyl groups. The coordination interactions between the metal ions and the ligands were suppressed as a result of the competition effect of the hydroxyl groups with the carboxyl groups. The aggregation process was identified to be dominated by the cluster-cluster mechanism at the low pH value and the particle-cluster mechanism at the high pH value.