Dual spectral and temporal encoding in luminescent polymeric films using a EuIII complex and a persistent phosphor for anti-counterfeiting applications

Abstract

Anti-counterfeiting systems based on luminescent materials play a key role in protecting currency, artworks, and brands from forgery. Nevertheless, as existing systems become easier to replicate, developing harder-to-reproduce methods with higher expertise is increasingly important. In this study, we report the development of dual-mode luminescent polymeric films exhibiting both spectral and temporal encoding for advanced anti-counterfeiting applications. The films are based on the green-emitting persistent luminescent phosphor SrAl₂O₄:Eu^II, Dy^III and the red-emitting [Eu(nta)₃(phen)] complex (nta: 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butadione; phen: 1,10-phenanthroline), both dispersed in polymethyl methacrylate (PMMA). The [Eu(nta)₃(phen)] complex provides strong UV-excitable red emission, while SrAl₂O₄:Eu^II, Dy^III contributes with green persistent luminescence. The emission color gradually shifts from red to green as excitation changes from 380 to 440 nm. Moreover, under 380 nm excitation, the [Eu(nta)₃(phen)] red emission alters to green after the light source is ceased, arising from the SrAl₂O₄:Eu^II, Dy^III persistent luminescence. The films also display absolute emission quantum yield within the 40–60% range and remain thermally stable up to 200 °C, demonstrating their robustness. These results show that dispersing conventional and persistent luminescent materials in a polymer matrix enables simple yet sophisticated temporal and spectral dual-mode codification, offering a promising approach for advanced anti-counterfeiting technologies.