Abstract: With deeper understanding of resource crisis and environmental pollution,the development of low-priced, renewable and biodegradable natural polymers is drawing great attention. Lignin is the second-largest amount natural high molecular material, only next to cellulose, and is the only non-fossil natural resource, which can provide renewable aryl compounds. The research on the molecular structure of lignin is therefore arousing more concern. The main constitutive monomeric units of lignin are syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H). Lignin can not only strengthen cell walls but also waterproof and protect them against microorganisms. Lignin and hemicelluloses are covalently linked forming lignin-carbohydrate, which fills the voids between microfibrils, and further creats a highly complex 3-D hierarchy structure as a natural biomass recalcitrance against degradation. In biomass refinery, the distribution of lignin directly affects the biomass conversion efficiency. Therefore, in situ study on the molecular structure, micro-distribution and solution pattern of lignin at cellular level is of great significance. In traditional wet-chemistry, milled wood lignin and Klason lignin are commonly used for quantitative and qualitative investigation of the lignin molecular structure. However, the physical or chemical pretreatments required will inevitably disrupted the native-state molecular structure of lignin samples. Although the general optical and electronic microscopy can provide the information on lignin micro-distribution in plant cell walls, complicated staining and embedding are needed for sample preparation. By comparison, Raman spectroscopy with its features of non-destruction, rapidity, high resolution and high sensitivity shows unique advantage in the studies on the macromolecular structure and topochemistry. In the present review, Raman band assignments of G, S, H lignin units and biomass materials are summarized, and the factors influencing lignin Raman spectra are briefly introduced. Progress of the application of Raman spectroscopy to lignin micro-distribution and cellular level dissolution is discussed in detail. The prospect of Raman spectroscopy in the research on lignin is discussed with new ideas and methods for the plant physiology and biomass refinery, especially the design of highly-efficient biomass pretreatment.

Key words: lignin, Raman spectroscopy, Raman imaging, band assignment, topochemistry