Property control and functional expression of partially deacetylated nanochitin with various organic acid counter ions: antibacterial activity and hydrogel formation

Cationic partially deacetylated nanochitin exhibits counter-ion-dependent physicochemical properties and nanofibrillation behavior. In this study, the effects of organic acid counter ions on the dispersion characteristics, nanofibrillation efficiency, and functional performance of partially deacetylated nanochitin were systematically investigated to establish rational guidelines for acid selection. Thirteen carboxylic acid derivatives, including aliphatic and aromatic acids, were employed as counter ions. The zeta potential of aqueous nanochitin dispersions strongly depended on the acid species and showed a clear positive correlation with optical transmittance and nanofibrillation yield. Aliphatic carboxylic acids provided higher dispersion stability, whereas aromatic acids tended to suppress nanofibrillation due to intermolecular interactions, which was reflected in nanofiber morphology. Nanochitin prepared using acetic acid exhibited a high nanofibrillation yield with an average fiber width of ∼3 nm and length of ∼430 nm. In addition, nanochitin acetate and benzoate showed enhanced antibacterial activity against Escherichia coli and Staphylococcus aureus. In contrast, nanochitin with acrylic acid underwent gelation through radical polymerization, forming pH-responsive hydrogels via ionic crosslinking with poly(acrylic acid). These findings demonstrate that organic acid counter ions play a crucial role in tailoring the structure–property relationships of partially deacetylated nanochitin.