Colloidal Deacetylation of Chitin Nanocrystals Results in Amorphous and Patchy Chitosan Chains

Controlling the surface chemistry of biobased nanomaterials is crucial for unlocking their full potential in advanced applications. However, the impact of such chemical modifications on nanoscale morphology remains poorly understood. In this work, we investigate the sequential deacetylation of chitin nanocrystals (ChNCs) into chitosan nanocrystals (ChsNCs), a transformation that significantly alters their ultrastructural properties through the introduction of amine functionalities. By combining bulk and nanoscale characterization techniques─including electron diffraction, cryo-transmission electron microscopy (cryoTEM), and scattering-type scanning near-field optical microscopy (s-SNOM)─we can track the chemical and structural evolution during the deacetylation process. Our findings demonstrate that partially deacetylated ChNCs (20-60% degree of deacetylation) exhibit chitosan-rich surface patches, revealing nanoscale heterogeneity in surface modification. Furthermore, we observed that such a patchy distribution is accompanied by a decrease in nanocrystal bundling, suggesting changes in interparticle interactions. Finally, at higher degrees of deacetylation, ChsNCs exhibit mobile chitosan chains surrounding cores composed of chitosan-rich or residual chitin regions. We believe that our results provide critical insights into the nanostructural identity of ChsNCs, with implications for understanding and tuning their structure-property-function relationships, which are critical for the fabrication of chitin-derived biomaterials.