A Pr³⁺ doping strategy for simultaneously optimizing the size and near infrared persistent luminescence of ZGGO:Cr³⁺ nanoparticles for potential bio-imaging

Spinel-phase Zn₂Ga_2.98−xGe_0.75O₈:Cr_0.020,Pr_x (ZGGO:Cr³⁺,Pr³⁺) near infrared (NIR) persistent luminescence nanoparticles (PLNPs) with different amounts of Pr³⁺ dopant were prepared by a hydrothermal method in combination with a subsequent annealing in a vacuum. For these nanoparticles, the averaged particle size decreases from 64 to 37 nm with increasing Pr³⁺ doping concentration from 0 to 0.025 and Cr³⁺ and Pr³⁺ ions are uniformly doped into the interior and surface of a single nanoparticle. It can be found that Pr³⁺ doping leads to the appearance of more anti-site pairs () around distorted octahedral Cr³⁺ ions and enhanced NIR emissions around 697 nm, which originate from the ²E(²G) → ⁴A₂(⁴F) and ⁴T₂(⁴F) → ⁴A₂(⁴F) transitions of the interior and surface Cr³⁺ ions in the nanoparticles. In particular, for the interior Cr³⁺ ions in the Pr³⁺ doped nanoparticles, the enhanced NIR luminescence can be attributed to the suppressed energy transfer of the excited electrons from the ⁴T₂(⁴F) level to the trap level related to anti-site pairs () around the distorted octahedral Cr³⁺ ions. Our results suggest that Pr³⁺ doped ZGGO:Cr³⁺ PLNPs have potential applications for bio-imaging.