Optical response of photonic cellulose nanocrystal film for a novel humidity indicator

Herein, an optical colorimetric sensor based on photonic cellulose nanocrystals (CNCs) was successfully developed to offer a convenient approach for measuring the qualitative index of high humidity (>85% RH). The addition of monovalent electrolyte (0.11 wt% NaCl) to CNC suspension before gelation enabled CNC film to bear consistent pinkish color, which is far better to readily reflect the visible color appeared as the pitch increases by water uptake. Further study of virtual reality simulation revealed that the increase of ambient humidity alters the color of photonic CNC film to be sufficiently observable either by optical instruments or human eyes. Under polarized optical microscope, CNC films appeared initially green and gradually turned to red color that is in agreement with red shift in UV-Vis absorption spectrum. Whereas, the reflected color change from pinkish to bluish was well-recognized by human eyes indicating photonic CNC is capable of being an effective colorimetric indicator. In a variable circumstance of low and high humidity, consecutive cycle test showed that the as-prepared indicator fully recovered to the original color in 30 min with 99.8% colorimetric response value. This film can be potentially used for an accurate and reversible humidity optical indicator in intelligent packaging for products that undergo deterioration along with high moisture, e.g., pharmaceutical products, cereals and grain seeds, and electronic parts, to monitor the quality during export or storage.