Ultrasonic fabrication of all-cellulose-based films with vibrant tunable structural colors for visual soil moisture detection.

The development of nanomaterial-based intelligent stimuli-responsive devices with structural colors has drawn considerable attention. However, the simultaneous achievement of high color saturation and green degradability in these materials still remains major challenge. Herein, cellulose, the most abundant carbohydrate on Earth, was chosen as the raw material, and an ultrasound energy-disturbance strategy was employed to modulate the uniform evaporation-induced self-assembly (EISA) of cellulose nanocrystals (CNCs). This approach successfully produced photonic crystal films with chiral structure and tunable vibrant colors. The precise control over the reflective wavelength and helical pitch of the CNC film can be achieved by simply adjusting the input ultrasonic energy. In addition, the dimensional homogeneity of CNCs endows the film with bright color presentation. Due to the rapid water-responsive properties, favorable biocompatibility and complete soil degradation properties of CNCs, this nature-derived vision sensor has great potential in environmental humidity sensing and greenhouse soil moisture detection.