Au_329-xAg_x(SR)₈₄ Nanomolecules: Plasmonic Alloy Faradaurate-329

Though significant progress has been made to improve the monodispersity of larger (>10 nm) alloy metal nanoparticles, there still exists a significant variation in nanoparticle composition, ranging from ±1000s of atoms. Here, for the first time, we report the synthesis of atomically precise (±0 metal atom variation) Au_329-xAg_x(SCH₂CH₂Ph)₈₄ alloy nanomolecules. The composition was determined using high resolution electrospray ionization mass spectrometry. In contrast to larger (>10 nm) Au-Ag nanoparticles, the surface plasmon resonance (SPR) peak does not show a major shift, but a minor ∼10 nm red-shift, upon increasing silver content. The intensity of the SPR peak also varies in an intriguing manner, where a dampening is observed with medium silver incorporation, and a significant sharpening is observed upon higher Ag content. The report outlines (a) an unprecedented advance in nanoparticle mass spectrometry of high mass at atomic precision; and (b) the unexpected optical behavior of Au-Ag alloys in the region where nascent SPR emerges; specifically, in this work, the SPR-like peak does not show a major ∼100 nm blue-shift with Ag alloying of Au₃₂₉ nanomolecules, as shown to be common in larger nanoparticles.