Nd³⁺-Sensitized multicolor upconversion luminescence from a sandwiched core/shell/shell nanostructure

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted interest for use in bioimaging, biosensing, and therapeutic applications. These motivations are empowered by multicolor upconversion luminescence (UCL) under single near infrared wavelength excitation at ∼980 nm. However, this wavelength overlaps with the absorption peak of water that is dominant in the biological environment, eliciting a serious biological heating problem. This study reports tailored multicolor UCL from a Nd³⁺-sensitized sandwich-structure of core/shell/shell UCNPs of NaYbF₄:0.5%Tm,1%Nd@ CaF₂:30%Nd@CaF₂:1%Nd,20%Yb,2%Er that can be excited at single wavelength of ∼800 nm without producing any local heating. Incorporation of substantial Nd³⁺ sensitizers in the middle shell region allows efficient harvesting of excitation light, with the excitation then migrating bidirectionally across the core/shell interfaces to simultaneously activate blue emission from Tm in the core as well as green and red emission from Er in the outermost shell layer. By precise control of the content of lanthanide ions in each domain, a palette of multicolor UCL can be produced, ranging from blue to white. The described Nd³⁺-sensitized multicolor UCNPs hold promises for a variety of multiplexed biological applications, without complications from heating effects.