Multifunctional CNF-Based Composite with Photochromic and Upconversion Luminescence Properties for Advanced Anticounterfeiting Applications

The development of advanced security materials with multimodal optical responses is of increasing relevance for anticounterfeiting technologies. In this study, we report the successful synthesis and comprehensive characterization of a multifunctional composite based on cellulose nanofibrils (CNFs) covalently functionalized with photochromic PSPA oligomers and cross-linked with upconversion nanoparticles (UCNPs). Covalent grafting of PSPA onto the CNF backbone was done via Cu0-mediated radical polymerization (SET-LRP) and confirmed by spectroscopic analyses, while microscopic and structural investigations verified the homogeneous incorporation of UCNPs within the nanofibril-based hybrid gel. The resulting material combines reversible photochromic behavior under ultraviolet irradiation with near-infrared (NIR)-to-visible upconversion luminescence, enabling dual-mode optical signaling. The nanocomposite exhibited distinct and reproducible color changes upon UV exposure and characteristic luminescent emission under NIR excitation, demonstrating its suitability for advanced anticounterfeiting applications. Furthermore, the formulation as a stable, printable gel underscores its practical processability and compatibility with scalable anticounterfeiting technologies. The integration of photochromic and upconversion optical functionalities within a renewable CNF-based platform provides a robust, versatile, and sustainable approach for secure labeling and product authentication.