Chiral Nematic Cellulose Nanocrystal/Shikonin Hetero-paper towards Multi-modal Optical Responses and Intelligent Monitoring

• The hetero-paper is fabricated via one-step impregnation of cellulose nanocrystals and shikonin. • The paper synergizes structural, fluorescent, and pigment colors for bright, non-interfering hues. • The hetero-paper has excellent pH, solvent and Fe³⁺ ions discrimination ability. • The paper serves as smart labels for real-time visual detection of food freshness. Intelligent materials that transform environmental stimuli into distinct optical signals hold great potential for multidimensional sensing and real-time monitoring. In this study, we developed a hetero-paper based on cellulose nanocrystals and shikonin (CNC/S) through a simple one-step impregnation method on semi-transparent base paper. This hetero-paper integrates the structural color originating from the chiral nematic phase of cellulose nanocrystals, the pigment color of shikonin, and its intrinsic fluorescence into a single platform, enabling multimodal optical outputs under natural light, polarized light, and UV light. The hetero-paper showed enhanced mechanical properties, with a tensile strength increase from 34 MPa to 42 MPa. It was also highly responsive to various environmental stimuli. For example, its color red-shifts with decreasing solvent polarity, changes markedly across pH 1 to pH 13 driven by shikonin’s protonation equilibrium, and enables selective detection of Fe³⁺ ions through coordination. The hetero-paper was further successfully applied for the real-time visual monitoring of shrimp freshness, where the changes in color and fluorescence highly correlated with the total volatile basic nitrogen (TVBN) content, accurately indicating the progression from freshness to spoilage. This bio-based, environmentally friendly, and multi-stimulus-responsive hetero-paper could open new applications in intelligent packaging and sensing, and environmental monitoring.