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

We previously reported that a scaled-down chitin nanofiber (SD-ChNF) dispersion can be obtained from partially deacetylated chitin nanofiber films by ultrasonic treatment in 1.0 mol/L aqueous acetic acid. In this study, we demonstrated that hydrogelation from SD-ChNFs occurs via the reductive amination of monosaccharide residues with the amino groups of chitin. The reductive amination of D-xylose (Xyl) was carried out in the presence of NaBH3CN as a reducing agent in the SD-ChNF/aqueous acetic acid dispersion to produce the Xyl-modified ChNFs. Under selected conditions, a hydrogel was formed in the reaction mixture. The scanning electron microscopy (SEM) image of the lyophilized hydrogel product displayed a network structure at the nanoscale. Hydrogels were also prepared by the reductive amination of D-glucose and N-acetyl-D-glucosamine on SD-ChNFs under the same conditions. Furthermore, the lyophilized products were re-swollen in water, and their weak gel behaviors were characterized by dynamic viscoelastic measurements. Based on the results of reductive amination, precisely performed under various conditions, a mechanism for hydrogelation via the formation of a network structure during reductive amination is proposed.