Monitoring the surface aging of wood through its pits using atomic force microscopy with functionalized tips

Wood aging is a topic of great interest due to its impact on wood performance and durability as a construction material. Therefore, it is important to develop accurate methods to monitor aging and understand its mechanisms. Atomic force microscopy (AFM) is a powerful tool to study the changes in materials properties in the nanoscale. We aim in this paper at developing an accurate AFM-based method to understand the wood aging phenomenon by studying the surface changes of wood pits as they are the most susceptible wood structures to aging. For this objective, surface adhesion measurements were performed to detect any chemical changes on the surface of wood structures by estimating the relative adhesion force (RAF). The results showed that unfunctionalized AFM tips should not be used for monitoring aging as they fail to simply differentiate between the different wood structures (cell wall, lumen, pit torus and pit margo) as the RAF values were around 10 for the four structures. With a focus on pit structures (torus and margo), AFM tips functionalized with hydroxyl groups showed significantly different RAF values for torus and margo suggesting the ability of these tips in detecting possible chemical changes. The RAF values of torus and margo were around 216 and 530 for unaged wood, respectively. This indicates that, for the first time, the use of functionalized tips does not only improve the interaction between the tips and wood in general but also improves the sensitivity to its different structures. This also allowed monitoring wood aging as the RAF values for torus and margo dropped to 152 and 301 upon aging, respectively. This work suggests that using AFM tips with different functional groups, such as carboxyl, acetyl, methoxyl, and hydroxyl groups can bring a more thorough understanding of wood aging.