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

Abstract

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. DHPintegrated 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.

Recent developments in catalysis with Pickering Emulsions

“…with respect to resources used, energy consumption and process intensification. 93,[119][120][121] Provided that the reactions that are catalyzed are in and of themselves sound (i.e. the 'right thing' is to target a chemical conversion based on renewables and with benign reagents), PE catalysis can ensure that 'the right thing is done right", i.e.…”

“…Naturally, materials made from abundant resources are preferred, as are scalable and efficient synthesis routes. 120,128 Currently, those routes may often still be cumbersome and wasteful, in particular when the more advanced solid emulsifiers are concerned. For example, the use of Janus particles as advanced emulsifiers offers many opportunities for PE catalysis, having both hydrophilic and hydrophobic faces, similar to molecular surfactants.…”

“…24,68,129 But synthesizing such asymmetric particles and functionalizing the individual patches selectively is often an elaborate (and wasteful) synthetic exercise 130 and it is therefore also essential to explore how such engineered, colloidal particles can be made efficiently in a scalable manner, from green, sustainable materials. 120,[128][129][130] Furthermore, the functionalities of a given emulsifier need to be stable, so that the corresponding properties (wetting, stimulus-response, catalysis, etc.) are maintained.…”

Enzymatic Preparation and Characterization of Spherical Microparticles Composed of Artificial Lignin and TEMPO-Oxidized Cellulose Nanofiber

“…In the last two decades, a new type of cellulose nanomaterial, cellulose nanofiber (CNF), has attracted much attention owing to its extraordinary nanoarchitectures and inherent crystalline fiber structure [ 14 , 23 , 24 , 26 ]. This novel nanomaterial is expected to have applications as a natural filler for reinforced plastics [ 27 , 28 , 29 ], food and cosmetic additive [ 30 , 31 ], flexible electronic devices [ 32 , 33 ], and in optoelectronics [ 34 , 35 ] owing to its fascinating physicochemical properties [ 14 , 23 ]. Recently, various MPs prepared by CNF-stabilized Pickering emulsion templating methods, in which thin CNF shells cover polymer cores, have received much interest regarding practical applications [ 36 , 37 , 38 , 39 ].…”

“…However, nature-derived lignin has crude and complex structures, which strongly affect its biodegradability and biocompatibility. On the other hand, to determine the biosynthesis mechanism of lignin in vivo, the in vitro synthesis of dehydrogenative polymers (DHPs) of coniferyl alcohol has long been investigated for use as an artificial lignin, enabling rough control of molecular structures [ 29 , 42 , 43 ]. Mićić et al found that synthetic DHP molecules self-assemble to form fine particles [ 44 , 45 ].…”