Stability evaluation of dual-excitation luminescent inks for anti-counterfeiting printing technology

Counterfeiting affects nearly all sectors of the global economy, increasing the demand for advanced anti-counterfeiting technologies. Here, we investigate the stability of multifunctional luminescent inks composed of Gd_0.95Eu_0.05AlO₃ (downshifting, DS) and Gd_0.94Er_0.01Yb_0.05AlO₃ (upconversion, UC) phosphors dispersed in poly(vinyl alcohol) (PVA) and glycerol (GLY) matrices. Both phosphors, synthesized by the urea combustion method, crystallized in the orthorhombic perovskite structure (Pbnm), confirmed by PXRD and Rietveld refinement. Zeta potential measurements indicated moderate colloidal stability and sufficient dispersion for pattern formation. Thermogravimetric analyses revealed thermal stability above 180 °C for GLY-based inks and 400 °C for PVA-based inks. Rheological measurements showed greater structural rigidity for GLY formulations, whereas PVA-based inks exhibited improved spreadability. Both inks displayed intense dual emission, producing red luminescence under UV excitation (Eu³⁺) and green emission under NIR excitation (Er³⁺). Successful printing on paper substrates and banknotes demonstrates the potential of these stable dual-emission inks for advanced anti-counterfeiting applications.