Can low energy (1–20 eV) electron microscopy produce damage-free images of biological samples?

Electron microscopy constitutes an efficient and well-established method to visualize biological material on the nanoscale. The image is usually produced by a high energy electron beam, which can damage the biological sample. To reduce image degradation, Neu et al. [Ultramicroscopy 222 (2021) 113,199] recently suggested the possibility of damage-free imaging of such samples at nm resolution using as a probe low energy electron (LEEs). The aims of the present article are to 1) present a simple and short description of LEE inelastic scattering and attachment in molecular solids in the 0–20 eV range, 2) show that principally due to the formation of transient anions (TAs) in biological material, by temporary LEE attachment to molecular sites, damage-free electron microscopy may be difficult to achieve and 3) suggest specimen conditions that reduce the damage produced by TAs to inflict minimum damage to biological samples in LEE microscopy. We provide examples of lesions induced by electrons of energies below 3 eV in short DNA strands composed of 16 base-pair oligonucleotides and on the 1–20 eV dependence of effective damage yields from LEE-bombarded plasmid DNA. The damaged samples were produced from 5-monolayer films lyophilized on tantalum substrates and transferred to ultra-high vacuum to be bombarded with LEEs. The products were identified and quantified ex-vacuo by LC-MS-MS and electrophoresis, respectively. Such effective yields, and the corresponding absolute cross sections derived from model analysis, should allow estimating beam damage and image quality in the visualization of thin biological films by LEE microscopy.