Fluorescent Polymer Composites Based on Core-Shell NaYF4:Yb/Er@NaGdF4:Ce/Tb Structures for Temperature Monitoring and Anti-Counterfeiting Protection

Synthesis and detailed morphological, structural, and photophysical analysis were conducted on core@shell NaYF4:Yb/Er@NaGdF4:Ce/Tb nanoparticles (C/S NPs). It was found that the NaGdF4:Ce/Tb shell significantly reduces defects in the NaYF4:Yb/Er core, enhancing the emission resolution of Er3+ ions. These C/S NPs exhibit dual luminescence functionality: upconversion with a quantum yield of 0.19 %, driven by energy transfer between Yb3+ and Er3+ ions, and downshifting luminescence with a remarkable quantum yield of 96 %, attributed to the energy transfer between Ce3+ and Tb3+ ions. Polymer nanocomposites, polystyrene-C/S (PS–C/S), and polyvinyl acetate-C/S (PVA-C/S) were synthesized by co-dissolving the polymers and nanoparticles in chloroform, followed by the removal of chloroform. Thin films based on the PS-C/S composite exhibited upconversion luminescence, enabling spatial temperature measurements with a lateral resolution of 10 μm and an accuracy of 1.6 K on a non-uniformly heated sample. The PVA-C/S composite exhibited dual-mode luminescence (upconversion and downshifting), which was employed to create fluorescent images activated by either NIR or UV irradiation. Additionally, these images demonstrated high stability against alcohols, acetone, and alkaline aqueous solutions, offering new anti-counterfeiting measures.