Nanoscopic lignin mapping on cellulose nanofibers via scanning transmission electron microscopy and atomic force microscopy

Cellulose has been developed as an alternative to petrochemical materials. By comparison with refined nanofibers (RCNFs), lignocellulose nanofibers (LCNFs) show particular promise because it is produced from biomass using only mild pretreatment. The mechanical properties of LCNFs depend on the contained lignin. However, the microscopic location of the lignin contained in LCNFs has not been determined. Thus, we developed two methods to detect and visualize lignin. One uses a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray spectroscopy detector. The other method uses an atomic force microscope (AFM) equipped with a cantilever coated with an aromatic molecule. Both methods revealed that the lignin in LCNFs covers a thin cellulose fiber and is precipitated in a grained structure. In particular, the AFM system was able to determine the nanoscopic location of lignin-rich areas. The present study establishes a strong tool for analyzing the characteristics of lignin-containing materials.