Super-stable, highly monodisperse plasmonic faradaurate-500 nanocrystals with 500 Gold atoms: Au_∼500(SR)_∼120

Determining the composition of plasmonic nanoparticles is challenging due to a lack of tools to accurately quantify the number of atoms within the particle. Mass spectrometry plays a significant role in determining the nanoparticle composition at the atomic level. Significant progress has been made in understanding ultrasmall gold nanoparticles such as Au₂₅(SR) ₁₈ and Au₃₈(SR)₂₄, which have Au core diameters of 0.97 and 1.3 nm, respectively. However, progress in 2-5 nm-diameter small plasmonic nanoparticles is currently impeded, partially because of the challenges in synthesizing monodisperse nanoparticles. Here, we report a plasmonic nanocrystal that is highly monodisperse, with unprecedentedly small size variability. The composition of the superstable plasmonic nanocrystals at 115 kDa was determined as Au_500±10SR_120±3. The Au_∼500 system, named Faradaurate-500, is the largest system to be characterized using high resolution electrospray (ESI) mass spectrometry. Atomic pair distribution function (PDF) data indicate that the local atomic structure is consistent with a face-centered cubic (fcc) or Marks decahedral arrangement. High resolution scanning transmission electron microscopy (STEM) images show that the diameter is 2.4 ± 0.1 nm. The size and the shape of the molecular envelope measured by small-angle X-ray scattering (SAXS) confirms the STEM and PDF analysis.