Meniscus-Mediated Imaging of Soft Biological Membranes Using 200 kN/m Ultra-Stiff 15 MHz MEMS AFM Probes.

Abstract

Coronaviruses, among other membrane-enveloped viruses, can remain infectious on surfaces in ambient air for days. Many bacteria and fungi also expose their plasma membranes to air environments. Key survival strategies in air include hydrophobic coatings to prevent water loss and waxy or silica-based resistant cell walls. Yet, many aspects remain largely unexplored due to the lack of suitable nanoscale imaging. While Atomic Force Microscopy (AFM) imaging in the liquid native environment of the biological samples gives access to many details of the membranes, operation in air damages these soft samples, being unable to resolve these fragile structures. Here, we demonstrate that ultra-stiff microelectromechanical (MEMS)-based AFM probes, with an ultra-high stiffness of 200 kN m^-1 and a resonance frequency of 15 MHz, can image soft cell membranes of Halobacterium Sallinarium in ambient air without structural disruption. Although meniscus-mediated tip stabilization has been previously observed for MEMS and optomechanical probes, this study provides the first demonstration of its application to biological imaging. By operating in a capillary-coupled regime with sub-ångström oscillation amplitude, the probe stabilizes at approximately 40% of the meniscus rupture distance, dissipating only ~35% of the maximum interaction energy. This enables noncontact imaging at tip-sample separations of ~5 nm and faithful recovery of the membrane topography (~5 nm thickness). The results establish a pathway for ultra-stiff AFM probe to perform noninvasive imaging of soft biological structures in air.