Dendrimer-mediated synthesis and shape evolution of gold–silver alloy nanoparticles

We report the dendrimer-mediated synthesis and shape evolution of gold–silver alloy nanoparticles (Au–Ag alloy NPs) with different metal compositions. In this study, amine-terminated generation 5 poly(amidoamine) dendrimers were used as stabilizers to prepare Au–Ag alloy NPs with different gold atom/silver atom/dendrimer molar ratios without the assistance of additional reducing agents. Following a one-step acetylation reaction to transform the dendrimer terminal amines to acetyl groups, a series of Au–Ag alloy NPs with surface acetyl groups were formed. The alloy NPs before and after acetylation reaction were characterized using 1H NMR, UV–vis spectrometry, transmission electron microscopy, inductively coupled plasma-optical emission spectroscopy, and X-ray absorption coefficient measurements. We showed that the optical property, the size, and the morphology of the bimetallic NPs were greatly affected by the metal composition and their surface modification. At the constant total metal atom/dendrimer molar ratio, the alloy NPs experienced a shape evolution from spherical particles and polyhedrons to curved nanowires with the Au content. Acetylation of the dendrimer terminal amines seemed to thicken the nanowires. The formed Au–Ag alloy NPs were stable at different pH (pH 5–8) and temperature (4–50 °C) conditions. X-ray absorption coefficient measurements showed that the attenuation intensity of the alloy NPs was dependent on both the metal composition and surface functional groups. At a given metal composition, the X-ray attenuation intensity of the binary NPs was enhanced after acetylation. The formed metal alloy NPs with tunable size and shape prepared using the dendrimer stabilizers may be used in a range of applications in catalysis, sensing, and biomedical sciences.