Abstract
Infrared photo-induced force microscopy (IR PiFM) was applied for imaging ultrathin sections of Norway spruce (Picea abies) at 800–1885 cm−1 with varying scanning steps from 0.6 to 30 nm. Cell wall sublayers were visualized in the low-resolution mode based on differences in their chemical composition. The spectra from the individual sublayers demonstrated differences in the orientation of cellulose elementary fibrils (EFs) and in the content and structure of lignin. The high-resolution images revealed 5–20 nm wide lignin-free areas in the S1 layer. Full spectra collected from a non-lignified spot and at a short distance apart from it verified an abrupt change in the lignin content and the presence of tangentially oriented EFs. Line scans across the lignin-free areas corresponded to a spatial resolution of ≤ 5 nm. The ability of IR PiFM to resolve structures based on their chemical composition differentiates it from transmission electron microscopy that can reach a similar spatial resolution in imaging ultrathin wood sections. In comparison with Raman imaging, IR PiFM can acquire chemical images with ≥ 50 times higher spatial resolution. IR PiFM is also a surface-sensitive technique that is important for reaching the high spatial resolution in anisotropic samples like the cell wall. All these features make IR PiFM a highly promising technique for analyzing the recalcitrant nature of lignocellulosic biomass for its conversion into various materials and chemicals.Graphic abstract
Highlights
Replacing fossil carbon sources with biomass as a source of materials and chemicals sets increasing needs for understanding the structure of biomass on molecular to nanostructural scales
The hyperspectral PiFM IR (hyPIR) images at 1505, 1471 and 1220 cm-1 had similar high intensity profiles in the cell corner middle lamella (CCML) and narrow compound middle lamella (CML) regions (Fig. 3). All these absorption bands are characteristic for lignin and indicative of high lignin content in CCML and CML (Bock et al 2020; Larsen and Barsberg 2010)
A similar, weaker, intensity profile was observed in the hyPIR image at 1145 cm-1, indicating that this band originated from lignin (Bock et al 2020)
Summary
Replacing fossil carbon sources with biomass as a source of materials and chemicals sets increasing needs for understanding the structure of biomass on molecular to nanostructural scales This knowledge is required in developing more efficient ways of fractionating and processing plant fibers and their constituents (Charrier et al 2018; Sorieul et al 2016). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), especially have provided detailed information on the organization of cellulose elementary fibrils (EFs) in the sublayers of wood cell walls (CW). The thick S2 layer of normal wood fibers consists of close to axially oriented EFs that may aggregate to form (micro) fibrils or lamellae (Abe et al 1991; Donaldson 2008). Many researchers have reported the occurrence of EFs in S1, S2, and S3 layers, where S1, S3 has transversely oriented and S2 has axially oriented EFs (Abe et al 1992; Abe and Funada 2005; Brandstrom et al 2003; Donaldson 2008; Donaldson and Xu 2005; Maaß et al 2020; Huang et al 2003)
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