Structure Transformation of Methylammonium Polyoxomolybdates via In-Solution Acidification and Solid-State Heating from Methylammonium Monomolybdate and Application as Negative Staining Reagents for Coronavirus Observation.

We prepared polyoxomolybdates with methylammonium countercations from methylammonium monomolybdate, (CHNH)[MoO], through two dehydrative condensation methods, acidifying in the aqueous solution and solid-state heating. Discrete (CHNH)[MoO(OH)(HO)], polymeric ((CHNH)[MoO(HO)]), and polymeric ((CHNH)[γ-MoO]) were selectively isolated via pH control of the aqueous (CHNH)[MoO] solution. The HSO-acidified solution of pH < 1 produced "sulfonated α-MoO", polymeric ((CHNH)[(MoO)(SO)]). The solid-state heating of (CHNH)[MoO] in air released methylamine and water to produce several methylammonium polyoxomolybdates in the sequence of discrete (CHNH)[MoO-MoO], discrete (CHNH)[MoO], discrete (CHNH)[MoO], and polymeric ((CHNH)[γ-MoO]), before their transformation into molybdenum oxides such as hexagonal-MoO and α-MoO. Notably, some of their polyoxomolybdate structures were different from polyoxomolybdates produced from ammonium molybdates, such as (NH)[MoO] or (NH)[MoO], indicating that countercation affected the polyoxomolybdate structure. Moreover, among the tested polyoxomolybdates, (CHNH)[MoO] was the best negative staining reagent for the observation of the SARS-CoV-2 virus using transmission electron microscopy.