Synthesis of upconversion zirconia nanoparticles for bioimaging

In this work, the upconversion Er³⁺/Yb³⁺ co-doped ZrO₂ nanoparticles were synthesized and characterized. The nanoparticles were obtained via sol-gel method. To achieve upconversion activity ZrO₂ nanoparticles were doped with lanthanide ion pair of the Yb³⁺ and Er³⁺ and further calcined at 800°C. According to X-ray powder and high-resolution transmission electron microscopy the well-crystallized monoclinic nanoparticles were formed, which size ranges from 20 to 30 nm for undoped ZrO₂ and from 30 to 40 nm for doped ZrO₂. The calcined Er³⁺/Yb³⁺ co-doped ZrO₂ nanoparticles demonstrated an upconversion luminescence visible by the bare eye. Namely, when excited with NIR-laser, the nanoparticles emit visible green light. Moreover, the Er³⁺/Yb³⁺ co-doped ZrO₂ nanoparticles showed red (650-673 nm) and green (520–570 nm) upconversion luminescence due to the ⁴F_9/2 → ⁴I_15/2 of and (²H_11/2 and ⁴S_3/2) → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The obtained nanoparticles exhibited intense luminescence alongside with low toxicity according to colorimetric assay for cellular growth and survival. It was concluded that Er³⁺/Yb³⁺ co-doped ZrO₂ nanoparticles are perfect prospective bioimaging agents for using as extremely sensitive detectors of biomarkers or excessive contrast deep tissue indicators.