Colorful bioactive hydroxypropyl cellulose photonic materials synergistically reinforced by natural deep eutectic solvents and sodium copper chlorophyllin.

Abstract

Cellulose photonic materials are highly promising for food packaging and decoration. However, their applications are limited by challenge of simultaneously enhancing structural color and bioactive properties. Inspired by the coloration of blueberries, innovative, colorful, and multifunctional hydroxypropyl cellulose (HPC) photonic materials synergistically reinforced by natural deep eutectic solvents (NADES) and sodium copper chlorophyllin (SCC) were designed and characterized in terms of the color regulation mechanism, microstructure, color characteristics, mechanical properties, and antibacterial and antioxidant activities. The NADES differentially induced shifts in the structural color of HPC photonic gels through hydrogen-bonding and electrostatic interactions. Specifically, a blue shift was induced by choline- and acid-based NADES, as well as by NADES containing neither choline nor acids, whereas a red shift was caused only by acid-based NADES without choline. The NADES also differentially affected the responses to temperature, pressure, and tension deformation of the HPC photonic gels, with choline-containing NADES inhibiting their color response. Replacing conventional black absorbers with SCC produced spectral interplay between the characteristic absorption of SCC and the structural reflection of HPC, producing greenish tones and unique colors. The synergistic incorporation of representative NADES and SCC endowed the HPC photonic films with excellent antioxidant and antibacterial properties, while the NADES significantly enhanced the mechanical properties of the films, especially for films containing fructose- and citric acid-based NADES. These multifunctional HPC photonic materials with unique color retain multi-responsiveness and integrate antioxidant and antibacterial activities, color stability, and biodegradability, providing a highly promising platform for active and smart packaging or food coloring applications.