Infrared photo-induced force microscopy unveils nanoscale features of Norway spruce fibre wall

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

Super-resolution mid-infrared spectro-microscopy of biological applications through tapping mode and peak force tapping mode atomic force microscope

“…Adapted from ref. [111] with permission. (b) AFM Topography and PiFM images at 1658 cm -1 (protein absorption) and 1700 cm -1 (off-resonance) of bacterial cell walls of Enterococcus faecium.…”

“…As an emerging technique with the non-intrusive tapping mode operation, IR PiFM has also been implemented in analyzing biological specimens with nanoscale resolution. For instance, Kesari et al utilized IR PiFM to investigate nanoscale heterogeneities of Norway spruce fiber wall [111]. On the ultrathin sample cross-section of the spruce, different cell wall layers were differentiated based on lignin and cellulose contents, and locations of small cellulose elementary fibrils (EFs) with widths of only 5 -6 nm were identified in PiFM images (Figure 5a).…”

“…The 1051 cm -1 map shows cellulose enriched elementary fibrils (EFs). Adapted from ref [111]. with permission.…”

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…54,55 In a slightly different area of research, Kesari et al's study of ultrathin microtome sections of Norway Spruce demonstrates the potential of PiFM for visualising the chemical composition of cell walls. 56 Using vibrational bands at 1505, 1471, 1220 and 1145 cm À1 , the distribution of lignin and cellulose was mapped across cell walls, Fig. 10.…”

Microspheres and Nanorods Produced in the Dissolution of Mildly Carboxylated Cellulose Fibers in Alkaline Solutions

“…Despite the fact that the primary wall layer is thin and the cellulose amount is very low, because the micro brils in this wall are interwoven randomly, it cannot swell considerably and wraps around the SW unless it is broken apart either by a su cient internal swelling force or by external mechanical action. S1, with 5-20 nm lignin-free voids (Kesari et al 2021), is also very thin, however it has enough cellulose in the form of helical micro brils (50-80° in Norway spruce) to render it highly exible with an enormous expansion capability. Brändström et al (2003) indicated that the S1 layer of Norway spruce is a homogeneous layer oriented approximately perpendicular to the tracheid axis without a cross brillar structure in alternate S (clockwise) and Z (anticlockwise) helices.…”