Lignin supported bio-based multifunctional cellulose nanocrystal photonic films for sensing applications

Cellulose nanocrystal (CNC) derived photonic materials hold transformative potential for sustainable photonics, particularly in applications such as multifunctional sensors, intelligent detection, and information storage devices. However, their practical implementation is limited by mechanical fragility, strong dependence on substrate background for color visibility, and compromise between mechanical and optical properties associated with conventional plasticizers. Herein, a fully bio-based lignin-CNC-glycerol bilayer architecture is developed, where a CNC photonic layer is assembled on glycerol-doped lignin substrates to enhance flexibility, hygroscopicity, adhesion, and tune optical properties. Lignin provides intrinsic light-absorbing properties, serving as a built-in dark background for background-independent structural coloration. The resulting bilayer films exhibit pronounced angle-dependent structural color and markedly improved thermal stability, with maximum decomposition temperatures delayed to 390–393 °C due to the high thermal resistance of lignin and strong CNC-lignin interfacial interactions. Nanoindentation analysis reveals glycerol-induced surface softening of the photonic layer, which regulates swelling behavior and enables reversible optical responses under humidity and solvent stimuli. Furthermore humidity-driven bending arises from swelling-induced strain mismatch between layers, producing reversible actuation across 64–94% relative humidity. Overall, this work offers a multifunctional, sustainable platform for responsive photonic materials, providing a versatile approach for environmentally friendly sensors, smart coatings, and information storage systems.