Super-stable, solvent-resistant and uniform lignin nanorods and nanospheres with a high yield in a mild and facile process

Lignin-based nanomaterials have attracted much attention in value-added functional material fields due to their green/sustainable nature; however, it is a challenge to control the morphology of lignin nanoparticles, and furthermore, to make them stable in organic solvent systems. Herein, for the first time we developed a green and simple approach for producing lignin nanorods, in addition to lignin nanospheres, both of which are stable in various organic solvents. The preparation process involves two steps: (1) lignosulfonate (LS) is fractionated into three fractions, namely, LS90, LS70, and LS40 using 90% ethanol, 70% ethanol and 40% ethanol sequentially; (2) lignin nanorods are obtained from the LS40 fraction, while lignin nanospheres are obtained from LS70 by an anti-solvent method. During the lignin self-assembly process, for the LS40 fraction, the lignin structures are in a flat oblate ellipsoid conformation, and the J-aggregation of their aromatic structures occurs on the flat sides of the oblate ellipsoid, leading to the formation of lignin nanorods. In contrast, for the LS70 fraction, lignin aggregation occurs randomly because of its near-spherical small ellipsoid conformation in solution, leading to the formation of lignin nanospheres. The entire preparation process of lignin nanorods and lignin nanospheres is simple, without the use of templates, complex chemical reactions and rigorous conditions. The lignin nanorods and lignin nanospheres exhibit excellent uniformity and dispersibility, and long-term stability in various organic solvents. This study not only presents a green, facile and economical approach for preparing lignin nanorods, as well as lignin nanospheres, but also provides a promising new value-added utilization pathway for lignosulfonate.