Dual Stimuli-Responsive Inks Based on Orthogonal Upconversion Three-Primary-Color Luminescence for Advanced Anticounterfeiting Applications

Stimuli-responsive fluorescent materials have recently been developed and employed as fluorescent security inks for applications in the fields of high-level security protection and multi-anticounterfeiting because they can provide extra security features by switching luminescence outputs in response to external stimuli. However, conventional stimuli-responsive fluorescent inks are generally limited to unicolor or dual-color fluorescent response behaviors in monotonous stimuli mode, resulting in being comparatively less effective for advanced anticounterfeiting applications. Herein, we report a new strategy for constructing a dual stimuli-responsive multicolor security ink, which is based on the integration of the near-infrared (NIR) light-responsive orthogonal three-primary-color luminescence properties of upconversion nanoparticles and pH-sensitive color-changing behavior of thymolphthalein dye. Unlike most other invisible security inks, this new dual stimuli-responsive security ink can realize visualization of hidden information. The information recorded by this ink is initially visible under natural light and then spontaneously becomes invisible within several tens of seconds. The feature of disappearance in seconds allows customers to check and correct concealed information before hiding it. In addition, upon pH stimulation, the concealed information can be reversibly interconverted between visible and invisible states. Meanwhile, upon ternary NIR quadrature excitation (808/980/1560 nm), the information can emit intense green, blue, and red fluorescence, respectively. More importantly, the information printed with the security ink exhibits robust stability against various chemical treatments, which demonstrates great potential for practical applications in advanced anticounterfeiting.