Introducing Hydrogen Bond Networks in the Self-Assembly of Chitin Nanocrystals: Strong and Flexible Bioactive Films Containing Natural Polyphenols

Free-standing, highly transparent and flexible films are obtained from solvent casting of aqueous colloidal dispersions of surface-deacetylated chitin nanocrystals. The Young's modulus and the water absorption of the films is further modulated by the addition of three natural polyphenols, i.e., epigallocatechingallate, tannic acid and one lignosulfonate, which differ one another in terms of molecular weight, and overall amount of hydroxy, phenolic and catecholic functionalities. The polyphenolic molecules create an extensive network of hydrogen bonds with the nanocrystals, thus controlling interfacial interactions. Therefore, they act as crosslinkers exerting a reinforcing and structuring action and hampering water absorption. The films do not show dissolution in water upon 7 days of incubation at room temperature, and the release profiles of the polyphenols in aqueous media evidence hindered Fickian diffusion kinetics confirming the presence of interactions with the nanostructured matrix. Lastly, the developed films possess bioactive properties, as they show both radical scavenging and antimicrobial activity. These characteristics are enhanced by the phenolic and, most importantly, catecholic moieties present in tannins (and to a lesser extent in lignins), allowing to reach bactericidal effects as high as 99.99% against both Gram-positive and Gram-negative strains.