Hydrogelation from Scaled-Down Chitin Nanofibers by Reductive Amination of Monosaccharide Residues

Polysaccharides are widely distributed in nature and are regarded as structural materials and reservoirs of water and energy [1]. Among the natural polysaccharides, chitin, comprising main chains of β(1 →4)-linked N -acetyl-D-glucosamine (GlcNAc) residues, is an important biomass resource because it is one of the most abundant polysaccharides present in nature, mainly prevalent in the exoskeletons of crustaceans [2‒4]. However, chitin remains largely underutilized because of its poor solubility and processability, owing to its intractable bulk structure composed of numerous intraand inter-molecular hydrogen bonds. The fabrication of nanoscale polymeric assemblies (e.g., nanofibers and nanowhiskers) is a useful method for the functionalization of chitin [5‒7] because of the remarkable properties of bio-based nanomaterials, such as low weight, high tensile strength, and biocompatibility [8‒14]. Efficient procedures for the preparation of chitin nanofibers (ChNFs) and nanowhiskers have been developed using a top-down approach that breaks down the starting bulk materials from native chitin sources [5, 6, 15‒18]. Based on another approach‒the bottom-up technique‒we previously developed a facile method to fabricate ChNFs with a width of approximately 20‒60 nm and length of several hundred nanometers. Selfassembling regeneration at the nanoscale from an ion gel of a chitin/ionic liquid, namely, 1-allyl-3methylimidazolium bromide (AMIMBr), was achieved using methanol [19, 20]. This was based on our previous findings that AMIMBr efficiently dissolves and swells chitin [21]. Isolation of the resulting ChNFs from the methanol dispersion via filtration produced a ChNF film possessing a heavily entangled nanofiber morphology. Furthermore, the self-assembled ChNFs had a bundle-like structure consisting of an assembly of thinner fibrils [22]. The treatment of the ChNF film with an aqueous NaOH induced the partial generation of amino groups on the chitin chains (i.e., partially deacetylated chitin nanofibers (PDA-ChNFs)). This led to the successful disentanglement of the bundles by cationization and electrostatic repulsion in 1.0 mol/L aqueous acetic acid with ultrasonication, yielding 【Transaction】