Photochromic cellulose nanofibers-reinforced polyacrylic acid self-healable hydrogel toward dual-mode security authentication stamp

Abstract

Photochromic inks have displayed various disadvantages such as poor durability, high cost and limited efficiency. The self-healing inks have shown to be more durable and photostable. In order to create an authentication stamp, UV-induced photochromic hydrogel inks with self-healable capabilities were developed. Nanoparticles of rare-earth aluminate (NREA) were synthesized and dispersed in a combination of cellulose nanofibers (CNF) and polyacrylic acid (PAA) to create an organic-inorganic hybrid ink. NREA were produced and analyzed using transmission electron microscope (TEM), revealing diameters of 1–4 nm. CNF were synthesized to act as a nanofiller and dispersing agent to improve the mechanical strength of the polyacrylic acid bulk and to prevent the agglomerates of the NREA. In order to create transparent prints, NREA must be uniformly disseminated in the CNF/PAA hybrid hydrogel ink without clumping. A paper surface was effectively stamped with the photochromic ink, and then the ink was thermofixed. Transparency was achieved by direct stamping of a consistent photochromic layer on the sheet surface, and this film was shown to become green when exposed to ultraviolet light as verified by photoluminescence spectral and CIE Lab analysis. The luminous paper sheets had an emission wavelength at 519 nm when irradiated at 368 nm. Energy-dispersive X-ray (EDX), infrared spectroscopy (FT-IR), X-ray fluorescence and scanning electron microscope (SEM) were utilized to examine the morphology of the stamped sheets. The cellulose nanofibers displayed diameters of 9–17 nm. The printed hydrogel ink displayed highly reversible photochromic emission without fatigue. Mechanical testing of the printed sheets and examination of the rheology of the luminous hydrogel was explored. The current NREA@CNF/PAA hydrogel ink offers a competent method for anticounterfeiting that can be used in a variety of authentication industries.