Synthesis and characterization of Au core-Au-Ag shell nanoparticles from gold seeds: Impacts of glycine concentration and pH

This paper describes the preparation of Au core-Au-Ag shell nanoparticles (NPs) in different morphologies by controlling both the pH and the glycine concentration. Using a seed-growth method, we prepared high-quality Au core-Au-Ag shell NPs from a glycine solution under alkaline conditions (pH > 8.5). By controlling both the pH and the glycine concentration, we prepared dumbbell-shaped and peanut-shaped Au core-Au-Ag shell NPs readily by depositing gold and silver, reduced by ascorbate, onto the gold nanorods. We have found that the glycine concentration that is optimal for preparing high-quality Au core-Au-Ag shell NPs differs at the various values of pH. At pH < 8.5, the glycine concentration is not important, but, when preparing dumbbell- and peanut-shaped Au core-Au-Ag shell NPs, it should be greater than 50 mM and greater than 20 mM at pH 9.5 and 10.5, respectively. Glycine plays a number of roles during the synthesis of the Au core-Au-Ag shell NPs by controlling the solution pH, altering the reduction potentials of gold and silver ions through forming complexes with metal ions (Au⁺ and Ag⁺), minimizing the formation of Ag₂O, AgCl, and AgBr precipitates, and stabilizing the thus-prepared NPs. At pH 9.7, we observed the changes in the morphologies of the Au core-Au-Ag shell NPs-from regular (rectangular) to peanut- and dumbbell-shaped, and finally to jewel-, diamond-, and/or sphere-shaped-that occurred during the course of a 60-min reaction. In addition, we were able to affect the shapes and sizes of the Au core-Au-Ag shell NPs by controlling the reaction time.