Energy transfer in Tm, Gd, Eu triple-doped LiYbF ₄ nanoparticles enables simplified synthesis design

Because the emission of lanthanide-doped upconversion (UC) nanoparticles (NPs) can be tuned both spectrally and temporally, these materials have become indispensable tools for bioimaging and optical encoding. Most colour-tunable designs rely on multilayer core–shell structures that physically separate Yb, Tm, and Eu (or Tb) activators into separate shells, but placing all three activators in a single core has seldom been explored. Here, we examine how confining Tm, Gd, and Eu activators within the LiYbF 4 core of LiYbF4: Tm, Gd, Eu@LiYbF4@LiYF4 core-shell-shell nanoparticles affects their upconversion emission colour. Spectroscopic analysis reveals a unidirectional energy-transfer Tm → Gd → Eu that enables rapid, efficient colour switching across the visible spectrum. Following a nanosecond excitation pulse at 959 nm, the temporal UC emission shifts completely from Tm-dominated blue (450 nm) to Eu-dominated orange (600 nm) within 3 ms after the laser pulse. This single-core approach delivers temporal colour tuning in a more compact architecture for imaging and encoding applications.