Key factors of scanning a plant virus with AFM in air and aqueous solution

For tobacco mosaic virus (TMV) as a model virus, this article shows typical issues of scanning soft biological matter by atomic force microscopy (AFM). TMV adsorbed on chemically different flat surfaces, gold, mica, and APDMES-functionalized silicon, is studied in air and aqueous environment. In air, the TMV particles arrangement shows some variety, depending on the substrate. The height of TMV is reduced to 13.7, 15.8, and 15.6 nm, for gold, APDMES, and mica, respectively while the width is about ∼30 nm due to the influence of the tip radius. In aqueous solution, the surface charges of the virus and the solid support play an important role in the virus adsorption process. While deposition on negatively charged mica is favored only at low pH values, it is shown that positively charged APDMES functionalized silicon can be a suitable substrate to work with at neutral pHs. The effects of cantilever oscillation's free amplitude (A₀ ) and the amplitude set-point (A) are also assessed here. While high A₀ prompt reversible deformation of TMV in measurements performed in air, irreversible damage of the virus in liquid conditions (water) is observed using stiff cantilevers (0.35 N m^-1 ) and high A₀ (81 nm), leading to a 6 nm reduction in the height of TMV after the first scan. Finally, low values of the amplitude set-point (A/A₀  = 0.3), which means applying higher forces to the sample, also brings the damage of TMV virus assemblies, reducing its monolayer roughness to 0.3 nm. Microsc. Res. Tech. 80:18-29, 2017. © 2016 Wiley Periodicals, Inc.