Lanthanide-doped Sandwich Structure Nanocrystals Achieving Highly Efficient Energy Migration Upconversion Emissions

Lanthanide-doped upconversion nanocrystals (NCs) are promising candidates for various bioapplications, and Tb³⁺-doped energy migration upconversion (EMU) NCs are the most prominent ones.

Aiming at acquiring strong EMU emissions, in the traditional design scheme of Tb³⁺-doped EMU NCs, the sensitizer Yb³⁺ and accumulator Tm³⁺ ions are co-doped in the inner core, and the activator Tb³⁺ ion is spatially confined in the shell of NC. However, in vivo bioapplications based on such design scheme are confronted with the dilemma of background biofluorescence induced by the ultraviolet upconversion emissions of Tm³⁺ which are always concomitant with the EMU emissions of Tb³⁺.

In a study published in CCS Chemistry, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences (CAS) reported a novel sandwich structured core@shell@shell scheme for the design of EMU NCs wherein Yb³⁺, Tm³⁺, and Tb³⁺ were separately incorporated into the inner core, middle shell, and outer shell of NC.

In comparison with the traditional non-sandwich structure NaLuF₄:Yb/Gd/Tm@NaGdF₄@NaLuF₄:Tb NCs, the sandwich structure NaLuF₄:Yb/Gd@NaGdF₄:Tm@NaLuF₄:Tb NCs showed a great superiority in acquiring strong upconversion emissions of Tb³⁺, while nearly completely suppressing the Tm³⁺ ultraviolet upconversion emissions.

Specifically, under 980 nm excitation at a moderate powder density (~ 200 W/cm²), the intensity ratio between the strongest emission band of Tb³⁺ and the maximum ultraviolet emission of Tm³⁺ reached a value of ~ 58.9 for the sandwich structure NCs, largely exceeding the small value of less than 1.2 for the non-sandwich structure NCs counterparts.

Based on the steady-state and transient photoluminescence spectroscopy analyses, the researchers unveiled that the depressed random energy hopping between Gd³⁺ ions, in combination with the effective Gd³⁺→Tb³⁺ inter-shell energy transfer that largely enhanced the Yb³⁺-Tm³⁺ five-photon upconversion and subsequent Tm³⁺ to Gd³⁺ energy transfer processes, were responsible for the more remarkable EMU emissions of Tb³⁺ in the sandwich structure NCs.

This study presents a prototypical sandwich structure NaLuF₄:Yb/Gd@NaGdF₄:Tm@NaLuF₄:Tb NC to circumvent the issue of background biofluorescence induced by the ultraviolet upconversion emissions of Tm³⁺ for the in vivo bioapplications based on Tb³⁺-doped EMU NCs. The findings may attract researchers interested in the fundamental upconversion mechanism involving energy management on the nanometer-length scale in lanthanide-doped NCs.