Photon upconversion of Nd3+–Yb3+–Tb3+ doped core–shell–shell–shell nanoparticles†

808 nm excited Nd³⁺ doped upconversion nanoparticles (UCNPs) have become a new research hotspot due to their reduced thermal effect when applied in a water-rich system. Different from Tm³⁺-mediated energy migration upconversion of Tb³⁺, in this work, NaYF₄:Yb/Tb@NaYF₄:Yb@NaNdF₄:Yb@NaYF₄ core–shell–shell–shell nanoparticles were designed based on the Yb³⁺ sensitized cooperative energy-transfer process to realize photon upconversion. NaYF₄:Yb served as the migration layer, separating the activator Tb³⁺ from the sensitizer Nd³⁺ and minimizing the deleterious cross-relaxation between them. At 40% Yb³⁺ doping concentration in the NaYF₄:Yb layer, bright green luminescence was achieved under 808 nm excitation. Meanwhile, the presence of Yb³⁺ in the sensitization layer (NaNdF₄:Yb) reduced Nd³⁺ cross-relaxation, leading to improved energy migration efficiency of Nd³⁺ → Yb³⁺. Finally, the incorporation of an inert NaYF₄ shell resulted in a significant luminescence boost. These nanoparticles can be dispersed in water after surface modification and could be effectively excited after laser penetration of 3 cm of water, indicating the enormous potential of the designed UCNPs in the biological system.