Photoluminescent Cellulose Nanofibers-Reinforced Alginate Hydrogel with Color-Tunable and Self-Healing Properties for Authentication Applications

Photochromic inks have showed many drawbacks such as low efficiency, high price and low durability. Self-healable inks have proved to be highly photostable and durable. Ultraviolet-stimulated photochromic hydrogels with self-healing characteristics were produced and used to create an authentication stamp. An organic-inorganic hybrid hydrogel ink was developed by the synthesis of nanoparticles of lanthanide-doped aluminate (NLdA), and the dispersion of those nanoparticles in a mixture of cellulose nanofibers (CNF) and sodium alginate (SA). Transmission electron microscopy analysis of NLdA revealed sizes of 8-12 nm, whereas the diameters of cellulose nanofibers ranged from 33 to 63 nm. The mechanical strength of the sodium alginate bulk was enhanced by synthesizing CNF and using it as nanofiller and reinforcement agent in the CNF/SA hybrid hydrogel. Transparent prints need NLdA to be dispersed evenly throughout the cellulose nanofibers/sodium alginate hydrogel without aggregation. The thermofixation process efficiently imprinted the photochromic ink into the paper surface. By directly stamping a homogeneous photochromic film onto paper surface, we were able to achieve transparency. Both photoluminescent spectral and CIE (Commission internationale de l'eclairage) Lab measurements confirmed that the colorless film turns green when illuminated with UV. Upon excitation at 370 nm, the emission wavelength of the glowing paper sheets was monitored at 518 nm. The morphology of the stamped sheets was analyzed using scanning electron microscope, infrared spectroscopy and X-ray fluorescence, energy-dispersive X-ray. The hydrogel ink prints showed highly reversible fluorescent-based photochromism. The mechanical properties of prints were tested, and the rheology of the glowing hydrogel was also studied. The currently available NLdA@CNF/SA hydrogel ink provides a reliable anticounterfeiting technology applicable to many types of products.