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.

Graphical abstractThe preparation of Au core–Au–Ag shell nanoparticles in different morphologies in 0.1 M glycine solution at pH 9.7 by controlling the reaction time.Figure optionsDownload full-size imageDownload as PowerPoint slide