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) nanopartictes 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, alpha-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-N-2 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.