Adjustable hydro-thermochromic green nanofoams and films obtained from shapable hybrids of cellulose nanofibrils and ionic liquids for smart packaging

Engendering stimuli-responsive and green shapable materials is a critical aspect of intelligent, responsive packaging technologies. Thermochromism, i.e., the optical response of materials to thermal stimuli, merges the visual appearance of the packaging with the temperature of the surroundings and cargo. Herein, sustainable, functional two-dimensional (2D) and three-dimensional (3D) hybrids of natural nanoribbons, i.e., cellulose nanofibrils (CNFs) and ionic liquids (ILs), were introduced as highly porous hydrothermochromic nanofoams, spheres, and flexible films. Hygroscopic ILs of nickel (II) or chromium (III) salts and imidazolium derivatives (1-ethyl-3-methylimidazolium chloride) were incorporated into the nanofibrils network, driving reversible color-switching via moisture adsorption controlled by temperature (hydrothermochromism, HTC). Multicolored HTC hybrids with a vast and adjustable color range from pale green to blue and from green to red with a color transition at 20 °C–80 °C were obtained by tailoring the composition (nickel and chromium chloride chemistry) and shape of the CNF–IL samples. These humidity- and temperature-responsive hybrids derived from biobased nanomaterials can pave the way toward future green smart packaging, which meets the requirements of sustainable development. The hybrids also provide advanced performance by monitoring and responding to the conditions of items and the surroundings.