Huge enhancement of upconversion luminescence by dye/Nd³⁺ sensitization of quenching-shield sandwich structured upconversion nanocrystals under 808 nm excitation

Lanthanide-doped upconversion nanoparticles (UCNPs) have shown potential applications in diverse fields. However, their upconversion luminescence (UCL) intensity and excitation wavelength range are limited by the weak and narrowband absorption of lanthanide ions. Herein, we introduce and validate a strategy to largely increase the absorptivity and upconversion luminescence intensity under 808 nm excitation by broadband dye-sensitized quenching-shield sandwich structured upconversion nanocrystals NaLuF₄:Gd,Yb,Tm@NaLuF₄:Gd,Yb@NaNdF₄:Yb. The dye molecules anchored on the surface of the UCNPs serve as an antenna which can broadly and strongly harvest NIR light. The Nd³⁺ facilitates the energy transfer and photon upconversion of the lanthanide activator at a biocompatible excitation wavelength (around 800 nm) with a significant increase in penetration ability and minimizes the overheating problem associated with conventional 980 nm excitation. The quenching-shield sandwich structure can greatly eliminate the deleterious cross-relaxation pathway between the activator and sensitizer. This approach combines the merits of the use of Nd³⁺ as a sensitizer, a quenching-shield sandwich structure and the "antenna" effect, leading to a tremendous enhancement of UCL under excitation at 808 nm. These well-designed UCNPs excited at 808 nm with improved optical performances will outperform conventional UCNPs excited at 980 nm and play an important role in the development of luminescent probes for future biological and medical applications. © The Royal Society of Chemistry 2017.