Understanding the effect of Mn2+ on Yb3+/Er3+ upconversion and obtaining a maximum upconversion fluorescence enhancement in inert-core/active-shell/inert-shell structures

Abstract

In this study, the energy transfer (ET) mechanism among Yb³⁺/Er³⁺/Mn²⁺ has been revealed by download conversion (DC) and upconversion (UC) transients and steady-state fluorescence spectra. Moreover, the novel structure, inert-core/active-shell/inert-shell cubic NaYF₄@NaYF₄:Er³⁺/Yb³⁺/Mn²⁺@NaYF₄, has been synthesized via a convenient hydrothermal method. With the excitation of a 980 nm laser diode, bright single-red upconversion (UC) fluorescence is observed, which shows the longest UC lifetime and highest efficiency compared with the other structures of NaYF₄:Er³⁺/Yb³⁺/Mn²⁺, NaYF₄:Er³⁺/Yb³⁺/Mn²⁺@NaYF₄, and NaYF₄@NaYF₄:Er³⁺/Yb³⁺/Mn²⁺. The inert-core/active-shell/inert-shell nanocubes are homogeneous with an ultra-small (sub-20 nm) size, which is suitable for distribution and elimination in biological tissue. Our results indicate that the Mn²⁺ doping of δ-doped structures is an effective method to significantly improve UC single-red light, which shows great potential application in vivo bioimaging.