Nano-mechanical characterization of the wood cell wall by AFM studies: comparison between AC- and QI™ mode

Background: Understanding the arrangement and mechanical properties of wood polymers within the plant cell wall is the basis for unravelling its underlying structure-property relationships. As state of the art Atomic Force Microscopy (AFM) has been used to visualize cell wall layers in contact resonance- and amplitude controlled mode (AC) on embedded samples. Most of the studies have focused on the structural arrangement of the S-2 layer and its lamellar structure. Results: In this work, a protocol for AFM is proposed to characterize the entire cell wall mechanically by quantitative imaging (QI (TM)) at the nanometer level, without embedding the samples. It is shown that the applied protocol allows for distinguishing between the cell wall layers of the compound middle lamella, S-1, and S-2 of spruce wood based on their Young's Moduli. In the transition zone, S-12, a stiffness gradient is measured. Conclusions: The QI (TM) mode pushes the limit of resolution for mechanical characterization of the plant cell wall to the nanometer range. Comparing QI (TM) - against AC images reveals that the mode of operation strongly influences the visualization of the cell wall.