Preparation of cellulose nanocrystal-reinforced alginate hydrogel to optimize fluorescence and persistent emission of self-healable adhesive to combat counterfeiting

Abstract

Fluorescent adhesives have shown various drawbacks, including expensive price, low efficiency, and poor durability. Recently, various studies have shown that self-healing adhesives outperform their conventional counterparts in terms of durability and stability. Herein, hydrogel adhesives were developed to provide self-healing authentication stamps. Those adhesive stamps showed photoresponsive capability, displaying a color change to green emission under ultraviolet irradiation according to photoluminescence spectra and CIE Lab coordinates. A composite of nanoparticles of lanthanide strontium aluminate (NSA), sodium alginate (ALG), and cellulose nanocrystals (CNCs), was mixed to form a self-healable hydrogel adhesive. In order to avoid agglomeration of NSA and improve the hydrogel mechanical strength, CNCs were used as nanofiller and a dispersant agent. In order to develop colorless stamps, it is important that NSA must not agglomerate when mixed with CNCs/ALG hydrogel. Afterglow and fluorescence emission peak was determined at 519 nm when the paper sheets were illuminated at 370 nm. Persistently greenish-yellow emission was noticeable in darkness after illumination with smart phone flashlight for a very few seconds. In order to analyze the morphologies of prints, different microscopic and spectroscopic methods were used. NSA exhibited diameters of 6–14 nm according to transmission electron microscope (TEM) images, whereas cellulose nanocrystals exhibited diameters of 8–13 nm and lengths of 95–135 nm according to scanning electron microscope (SEM) images. The present hydrogel provides a trustworthy photoresponsive anti-counterfeiting for a variety of commercial products.