Stable water-based luminescent suspensions of Eu3+:YPO4 nanophosphors in sodium alginate medium: Sustainable inks for potential anticounterfeiting applications

The development of sustainable luminescent inks is attracting increasing attention for applications in security printing, bio-imaging, and visualization technologies. Rare-earth-doped YPO4nanophosphors are particularly promising due to their high efficiency, chemical stability, and well-defined emission features. In this work, we present a simple and eco-friendly strategy to formulate fully water-based luminescent inks by combining Eu3+:YPO4(5 mol% Eu3+) nanophosphors, synthesized via a microwave-assisted method, with sodium alginate (SA) as a dispersant and rheology modifier. Different grades of SA were investigated to identify the conditions that optimize both dispersion stability and optical performance. Suspensions containing 2 wt% SA and a Eu3+:YPO4/SA ratio of 1:10 (w/w) exhibited excellent colloidal stability, shear-thinning behavior, and anti-thixotropic properties. ζ-potential and photoluminescence analyses confirmed strong interactions between SA chains and Eu3+:YPO4nanoparticles, enabling long-term stability while preserving the characteristic emission of the phosphor. Notably, the SA matrix contributed to enhanced luminescence in the 350–400 nm range, resulting in a dual-emission response from both polymer and nanophosphor. These findings provide a clear proof-of-concept for cost-effective, solvent-free luminescent inks based on environmentally friendly components, highlighting their potential for future integration in anti-counterfeiting and security printing applications.