- According to Derjaguin− Landau− Verwey− Overbeek (DLVO) theory, anions stabilize solutions of metal (0) nanoparticles by binding to the metal surface. However, the structure of the metal− anion interface in these colloidal systems has eluded direct observation. To address this, we deployed a 1.2-nm sized heteropolytungstate cluster anion, α- AlW11O399−(1), which features 11 W atoms (Z= 183) for effective imaging by electron microscopy, and a high negative charge to enhance binding to a prototypical Ag (0) nanoparticle. Cryogenic methods were then used to “trap” the 1-protected Ag (0) nanoparticles at liquid-N2 temperatures within a water− glass matrix. The “solution-state” structures thus captured provide the first reported TEM images of self-assembled monolayers (SAMs) of anions on a colloidal metal (0) nanoparticle.