Lignin-Inspired Surface Modification of Nanocellulose by Enzyme-Catalyzed Radical Coupling of Coniferyl Alcohol in Pickering Emulsion

A biomimetic approach to the surface modification of nanocellulose is proposed. This strategy was inspired by plant cell wall lignification, in which lignin, a hydrophobic biopolymer, tightly assembles cellulose microfibrils and matrix polysaccharides to confer the cell walls with essential mechanical properties. Enzymatic dehydrogenative polymerization of coniferyl alcohol in a nanocellulose-stabilized oil-in-water Pickering emulsion system efficiently yielded hydrophobic dehydrogenative polymers (DHPs) on hydrophilic nanocellulose without any phase separation. The DHPs were formed at the oil-water interface, where nanocellulose acted as a solid surfactant, covering the surface of the emulsion particles. DHP-integrated nanocellulose was casted to form thin films, with water contact angles on the as-prepared films increased by DHP incorporation, demonstrating the successful modulation of hydrophilicity of nanocellulose. In-depth structural analysis of DHPs by heteronuclear single quantum coherence (HSQC) NMR demonstrated that β-5 linkages were markedly increased compared with β-O-4 and β-β linkages in DHPs prepared using the nanocellulose-assisted Pickering emulsion system, while DHPs prepared using conventional nonemulsion polymerization systems were barely influenced by the presence of nanocellulose. These results suggested that nanocellulose acted as an efficient scaffold for monolignol radical coupling at the interface of the oil-in-water Pickering emulsion system, which provides new insight into the surface modification of nanocellulose in a biomimetic fashion.