Nanofibrillation of lignocellulosic biomass into lignin-containing nanocellulose for 3D printing and antimicrobial applications

Abstract

Lignin-containing nanocellulose (LNC) has become an emerging subcategory of nanocellulose. Compared toedtraditional pure nanocellulose, LNC contains lignin and hemicellulose that are not fully removed from original lignocellulosic biomass and thus is regarded a novel nanocomposite material. With lignin and hemicellulose, the physicochemical properties of LNC could be enriched and its potential for high-end applications could be extended. On the other hand, the complex composition of LNC requires in-depth understanding of the properties of each component, their physical/chemical changes in LNC production, and how those properties affect the application performances of LNC. To achieve this goal, this study aims at understanding the roles of lignin and hemicellulose in nanofibrillation of lignocellulose and exploring 3D printing and antimicrobial applications of LNC. In the production section (Chapter 3 and 4), optimizations of TEMPO-mediated oxidation and enzymatic nanofibrillation have been explored. By TEMPO-mediated oxidation, several LNC products having high lignin contents (as high as 27.21 wt percent) were successfully produced, which was among the first few studies in the field. TEMPO-mediated oxidation also grafted LNC with richcarboxyl groups that introduced reactivity with polyethylenimine (PEI). The as-synthesized PEI-LNC showed attractive antioxidant and antibacterial properties, and the lignin residues played negatively against those functions but provided a beneficial anti-aggregation property. With enzymatic nanofibrillation, a series of variables, including enzyme loading, enzyme composition, pulp composition, enzyme inhibitor, and enzyme permeation, were tested to enhance LNC productivity while avoiding extensivehydrolysis of polysaccharides. LNC with good dispersibility and size distribution was successfully produced at a yield of above 70 wt percent. Lignin modification by diol in shorter chain tended to block enzyme more and led to the higher LNC yield. Meanwhile, low CTec2 loading, enzyme inhibitors, or short hydrolysis time led to lower hydrolysis level, which could potentially serve as key factors for optimized enzymatic nanofibrillation. In the application section (Chapter 5 and 6), two major applications based on LNC, antibacterial and 3D printing, are introduced. The antibacterial agent was prepared by AgAu-based nanoparticles synthesized by LNC in an in situ chemical-free route. With optimized Au/Ag ratio, a Ag-Au-AgCl nanohybrid-LNC samples showed extraordinary antibacterial activity against both Gram-positive and Gram-negative species. In 3D printing, a series of all LNC-based inks were prepared with specifically controlled lignin and hemicellulose contents. Rheological properties of the inks were studied systematically to gain insights into the roles of lignin and hemicellulose. In the end, 3D printing was carried out with the LNC-based inks and their performances were qualitatively and quantitively evaluated. Both lignin and hemicellulose enhanced shear-thinning properties of LNC-based inks, and lignin affected LNC alignment in different manners depending on its content.