Local symmetry distortion-induced enhancement of upconversion luminescence in Gd₂O₃:Ho³⁺/Yb³⁺/Zn²⁺ nanoparticles for solid-state lighting and bioimaging

The Ho³⁺/Yb³⁺/Zn²⁺-tridoped Gd₂O₃ nanoparticles were prepared by a simple urea-based homogeneous precipitation method. Under near-infrared (NIR) light excitation, all the synthesized nanoparticles exhibit bright green and red upconversion (UC) emissions corresponding to the intra-4f transitions of Ho³⁺ ions and the UC mechanism is found to be a two-photon process. With the introduction of Zn²⁺ ions, not only the local symmetry surrounding the dopants is decreased, but also the UC emission intensity is also enhanced, which is further verified by the Judd-Ofelt theory. The temperature-dependent UC emission spectra were recorded to examine the thermal stability of the final products. From theoretical calculations, the activation energy is found to be about 0.18 eV. A novel green light-emitting diode device, which consists of the resultant nanoparticles and a NIR chip, was fabricated to examine their suitability for solid-state lighting. Meanwhile, the synthesized nanoparticles exhibit low cytotoxicity in various cell lines, suggesting their potential applications in in vivo UC luminescence imaging. Additionally, the applicability of the Ho³⁺/Yb³⁺/Zn²⁺-tridoped Gd₂O₃ nanoparticles for in vivo bioimaging applications was also analyzed.