Power-Dependent Optimal Concentrations of Tm³⁺ and Yb³⁺ in Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) have enabled a broad range of emerging nanophotonics and biophotonics applications. Here, we provide a quantitative guide to the optimum concentrations of Yb³⁺ sensitizer and Tm³⁺ emitter ions, highly dependent on the excitation power densities. To achieve this, we fabricate the inert-core@active-shell@inert-shell architecture to sandwich the same volume of the optically active section. Our results show that highly doped UCNPs enable an approximately 18-fold enhancement in brightness over that of conventional ones. Increasing the Tm³⁺ concentration improves the brightness by 6 times and increases the NIR/blue ratio by 11 times, while the increase of Yb³⁺ concentration enhances the brightness by 3 times and only slightly affects the NIR/blue ratio. Moreover, the optimal doping concentration of Tm³⁺ varies from 2% to 16%, which is highly dependent on the excitation power density ranging from 10² to 10⁷ W/cm². This work provides a guideline for designing bright UCNPs under different excitation conditions.