Reliable characterization of cellulose nanomaterials is critical for their utilization in various applications.

Deconstruction of lignocellulosic plant cell walls to fermentable sugars by thermochemical and/or biological means is impeded by several poorly understood ultrastructural and chemical barriers. A promising thermochemical pretreatment called ammonia fiber expansion (AFEX) overcomes the native recalcitrance of cell walls through subtle morphological and physicochemical changes that enhance cellulase accessibility without extracting lignin and hemicelluloses into separate liquid streams. Multi-scale visualization and characterization of Zea mays (i.e., corn stover) cell walls were carried out by laser scanning confocal fluorescence microscopy (LSCM), Raman spectroscopy, atomic force microscopy (AFM), electron microscopy (SEM, TEM), nuclear magnetic resonance (NMR), and electron spectroscopy for chemical analysis (ESCA) to elucidate the mechanism of AFEX pretreatment. AFEX first dissolves, then extracts and, as the ammonia evaporates, redeposits cell wall decomposition products (e.g., amides, arabinoxylan oligomers, lignin-based phenolics) on outer cell wall surfaces. As a result, nanoporous tunnel-like networks, as visualized by 3D-electron tomography, are formed within the cell walls. We propose that this highly porous structure greatly enhances enzyme accessibility to embedded cellulosic microfibrils. The shape, size (10 to 1000 nm), and spatial distribution of the pores depended on their location within the cell wall and the pretreatment conditions used. Exposed pore surface area per unit AFEX pretreated cell wall volume, estimated via TEM-tomogram image analysis, ranged between 0.005 and 0.05 nm² per nm³. AFEX results in ultrastructural and physicochemical modifications within the cell wall that enhance enzymatic hydrolysis yield by 4–5 fold over that of untreated cell walls.

This review is a summary of the Raman spectroscopy applications made over the last 10 years in the field of cellulose and lignocellulose materials. This paper functions as a status report on the kinds of information that can be generated by applying Raman spectroscopy. The information in the review is taken from the published papers and author’s own research—most of which is in print. Although, at the molecular level, focus of the investigations has been on cellulose and lignin, hemicelluloses have also received some attention. The progress over the last decade in applying Raman spectroscopy is a direct consequence of the technical advances in the field of Raman spectroscopy, in particular, the application of new Raman techniques (e.g., Raman imaging and coherent anti-Stokes Raman or CARS), novel ways of spectral analysis, and quantum chemical calculations. On the basis of this analysis, it is clear that Raman spectroscopy continues to play an important role in the field of cellulose and lignocellulose research across a wide range of areas and applications, and thereby provides useful information at the molecular level.