808 nm excited energy migration upconversion nanoparticles driven by a Nd3+–Trinity system with color-tunability and superior luminescence properties

Abstract

We have developed energy migration upconversion (EMU) nanoparticles (UCNPs) with optimal Nd³⁺-sensitization under excitation of an 808 nm laser to avoid over-heating effects caused by a 980 nm laser while maximizing the excitation efficiency. To realize efficient 808 nm sensitization, a “Nd³⁺–Trinity system” was implemented in the energy migration upconversion (EMU) cores (NaGdF₄:Yb,Tm@NaGdF₄:Yb,X, X = Eu/Tb), resulting in a core–multishell structure of EMU cores (accumulation layer@activation layer)@transition layer@harvest layer@activation layer. The spatially separated dopants and optimized Yb³⁺/Nd³⁺ content effectively prevented severe quenching events in the UCNPs and their Nd³⁺-sensitized EMU-based photoluminescence mechanism was studied under 808 nm excitation. These Nd³⁺–Trinity EMU system UCNPs presented enhanced upconversion luminescence and prolonged lifetime compared to the 980 nm excited UCNPs of the EMU system. It is proposed that 975 nm and 1056 nm NIR photons induced from the Nd³⁺ → Yb³⁺ energy transfer facilitate the Tm³⁺ accumulation process due to the matched energy gaps, which contributes to the extended lifetimes. More importantly, the synthesized UCNPs had a small average size of sub-15 nm and they not only exhibited color-tunability via Eu³⁺/Tb³⁺ activators, but also released a larger portion of Tm³⁺ red emission at 647 nm and had better penetration ability in water under 808 nm excitation, which are favorable for bioimaging applications.