Structural color film with humidity response based on cellulose nanocrystals from waste cotton

Utilizing sulfuric acid acidolysis to extract cellulose nanocrystals (CNCs) from waste cotton offers a sustainable resource for humidity-responsive materials. Prolonging the acidolysis duration from 30 to 90 min systematically reduces the CNCs' aspect ratio from approximately 30 to 17. Notably, CNC suspensions with an aspect ratio of ~30 self-assemble into red structural color films, whereas those with an aspect ratio of ~17 give rise to blue films. By tuning the aspect ratio, the self-assembly behavior of CNCs can be precisely controlled, resulting in humidity-responsive films manifesting a spectrum of vivid structural colors. These films exhibit fully reversible color variation under fluctuating humidity levels, effectively translating changes in ambient moisture into distinctive visual signals. In particular, when the relative humidity increases from 11 % to 97 %, the color evolves from blue to red, accompanied by a diffraction peak shift from 475 nm to 693 nm. Beyond demonstrating a facile route to humidity-responsive photonic materials, this work underscores the high-value reuse of cotton waste, paving the way for broader waste-to-resource approaches and contributing to primary resource conservation.