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.
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