Solid-state Thermal Transformation of Methylammonium Monomolybdate to Polyoxomolybdates and their Applications as Staining Reagents for Transmission Electron Microscopy Observations of Viruses

The solid-state heating of methylammonium monomolybdate, (CH3NH3)2[MoO4] (prepared by dissolving MoO3 in an aqueous methylamine solution), in air released methylamine and water to produce (CH3NH3)6[Mo7O24] and followed by (CH3NH3)8[Mo10O34] before transformation to MoO3. We report the first single-crystal X-ray structural analysis of (CH3NH3)2[MoO4] and (CH3NH3)6[Mo7O24] and demonstrate that the decamolybdate structure is different from that obtained from ammonium molybdates. Furthermore, (CH3NH3)6[Mo7O24] is a good negative staining reagent for transmission electron microscopy observations of viruses, such as SARS-CoV-2 and influenza virus.