Structural influences on lattice distortion in luminescence finetuning of core-multi-shell upconversion nanoparticles...

Core-multi-shell upconversion nanoparticles (UCNPs) highly doped with sensitizers and activators were synthesized by co-precipitation methods. With Zn²⁺ separately doped with Yb³⁺ or Ho³⁺ ions in different core-multi-shell structures, the nanoparticle morphologies, diffraction peak shifts, variations of upconversion luminescence (UCL) intensity and lifetime, underlying energy mechanisms, as well as multifunction potentials, were characterized and analyzed in detail. Lattice distortion promotes Yb³⁺ and Ho³⁺ 4f-4f transitions, but co-doping Gd³⁺ and Zn²⁺ ions in the NaHoF₄ matrix may induce offset of lattice distortion and lengthen the overall Yb³⁺-Ho³⁺ energy transfer distance. Zn²⁺-induced F^- vacancies can also quench energy as inner defects. And even in layers with identical content, the optimal Zn²⁺ doping can change with multi-shell structures. Besides, UCL intensity and lifetime may be non-positively correlated, especially in highly doped UCNPs with Zn²⁺ doping and multi-shell structures. With core-doped Zn²⁺ hetero-ions and controlled Yb³⁺ concentration, NaGdF₄:Yb³⁺@NaHoF₄@NaGdF₄:Yb³⁺ structures can achieve lifetimes as high as 635.82 μs; while with core- or shell-doped Zn²⁺ hetero-ions, NaHoF₄@NaHoF₄@NaGdF₄:Yb³⁺@NaGdF₄:Yb³⁺ structures can achieve comparable UCL intensities with varied lifetimes ranging from 111.58 to 245.29 μs. Multifunction characterizations manifested the high application potential of designed UCNPs in photo-induced bio-imaging and as contrast agents for both computed tomography and T₁/T₂-weighed magnetic resonance imaging. Our work can deepen the theory study of UCL finetuning with lattice distortion, and widen the multifunction application of NaHoF₄-based nanomaterials.