Ink formulations using Eu³⁺ doped strontium aluminates for security printing

Counterfeiting is a widespread menace that affects multiple sectors of society, which is increasing due to technological advancements and growing globalization of trade. It undermines economic growth, endangers public safety, damages brand integrity, and facilitates criminal activities. The present study focuses on the synthesis of thermally stable europium (Eu³⁺) doped orthorhombic Sr₄Al_l4O₂₅ (SAO) system by solid state method for use as a colorant in the formulation of viz based and polyvinyl alcohol (PVA) based screen inks. The steady state spectrum of Eu³⁺ doped SAO depicts the charge transfer taking place between host (O^2-) and half-filled f-orbital of Eu³⁺ ion. Mono-exponential decay with lifetime value recorded in milliseconds indicates substitution of single Sr host lattice. The screen prints obtained on UV dull paper using PVA based ink with Eu³⁺ doped SAO as pigment displayed better photostability and abrasion resistance. Both Viz and PVA based formulations could serve as invisible inks for security printing and information storage applications. The prints produced using the viz-based and PVA-based formulations appeared fluorescent blue under UV illumination, while they exhibited red and green fluorescence, respectively, when viewed through long-pass filters. A user familiar with these features can easily authenticate the prints, whereas replicating them is challenging for counterfeiters.