New insights into beech wood structure through photothermal AFM-IR

Understanding the polymeric architecture and distribution in wood cell walls is the key to developing efficient treatment and modification methods for both downstream conversion and advanced material development. This paper reports the spatial chemical mapping of aromatic structures of lignin and β-glycosidic linkages of polysaccharides in the middle lamella of beech wood cell walls using high resolution AFM-IR methodology. The study confirms that lignification in the cell walls is highly heterogeneous, with higher lignin concentration in the central middle lamella than the secondary cell walls. Contrary to previous findings, the S1/S2 transition layer may not be a region of depleted lignin content but it is possible that it is a region of complex and differing cellulose chain organisation, as indicated by the lack of difference in C-O-C vibration when IR polarisation was changed from 90° to 0°. Based on spatial spectral differences between the aromatic C[double bond, length as m-dash]C vibration and conjugated structures in the various cell walls, we suggest that lignin in the secondary cell walls may be structurally different from lignin in the middle layer, the latter being dominated by the conjugated structures. The study also confirms the presence of a steep lignin gradient between the central middle lamella and the S1 layer.