Direct imaging of the ligand monolayer on an anion-protected metal nanoparticle through cryogenic trapping of its solution-state structure

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-AlW₁₁O₃₉^9- (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.