A Pr 3+ doping strategy for simultaneously optimizing the size and near infrared persistent luminescence of ZGGO:Cr 3+ nanoparticles for potential bio-imaging.

Spinel-phase Zn2 Ga2.98-x Ge0.75 O8 :Cr0.020 ,Prx (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 the2 E(2 G) →4 A2 (4 F) and4 T2 (4 F) →4 A2 (4 F) 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 the4 T2 (4 F) 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.