Constructing a small core–multishell nanostructure for Ho-based red upconversion emission

Abstract

The development of Ho-based red-emitting upconversion (UC) nanoparticles with small size is of great significance in scientific research and practical application. Besides, a time-saving and environment-friendly strategy for the synthesis of core–multishell UC nanoparticles is urgently needed. In this work, small NaHoF₄@NaYF₄:Tb@NaGdF₄:Yb,Tm@NaYF₄ core/shell/shell/shell nanoparticles were successfully prepared by a modified co-precipitation method, in which sequential growth of shells could be realized. By means of Tb³⁺-mediated interfacial energy transfer (IET) and energy migration (EM), a novel mechanism for Ho-based red UC emission is presented under the premise of a small core size and thin shell. In the system, Ho-based red dominant emission mainly comes from the cross-relaxation process among Ho³⁺, which is induced by IET from Tb³⁺ to Ho³⁺. The EM process among Tb³⁺ can facilitate efficient energy transport. Furthermore, a mouse experiment demonstrates the deep penetration of 980 nm excitation and Ho-based red dominant emission. This new type of Ho-based red-emitting core–multishell UC nanoparticles not only provides an in-depth understanding of the IET-mediated energy-managing strategy but also displays special advantages in biomedical applications.