Doping and luminescence mechanisms of broadband NIR‐II emitting Zn2(1−x)Ni2xGa3Ge0.75O8 nanoparticles

Abstract In this study, Zn 2(1− x ) Ni 2 x Ga 3 Ge 0.75 O 8 ( x = 0.0002, 0.001, 0.002, 0.010, 0.020, and 0.030) nanoparticles with broadband NIR‐II emissions were synthesized by a hydrothermal synthesis combined with a vacuum annealing. For the Ni 2+ ‐doped ZGGO samples ( x = 0–0.03), with increasing concentration, the particle shape gradually becomes spherical and the average particle size decreases from 124.4 to 74.2 nm. Meanwhile, for the ZGGO:Ni 2+ 0.01 nanoparticles, the asymmetrically broad emission peak around 1290 nm, which is the superposition of the two peaks locating at 1280 and 1450 nm, can be observed and the afterglow time exceeds 30 min. Based on the spectral data, luminescence decay curves, first‐principles calculations, and Tanabe–Sugano theory, it is found that Ni 2+ ions can occupy not only tetrahedral but also octahedral Zn 2+ sites (locating in anti‐site defects pair) in the spinel ZGGO host, and they have the contributions to the 1450 and 1280 nm emission peaks, respectively. Furthermore, the surface‐modified ZGGO:Ni 2+ nanoparticles exhibited good stability in the H 2 O and HSA (5% human serum albumin, pH = 7.4) solutions and the occurred agglomeration sinking in the SLS (simulate lysosomal solution, pH = 4.7) solution. Compared to the narrow‐band NIR‐II emitting persistent luminescence nanoparticles (ZGGO:Cr 3+ ,Er 3+ and ZGGO:Cr 3+ ,Nd 3+ ), broadband NIR‐II emitting persistent luminescence nanoparticles (ZGGO:Ni 2+ NIR‐II) possess stronger persistent luminescence intensity and can effectively avoid the water absorption of biological tissues. Our results suggest that ZGGO:Ni 2+ persistent luminescence nanoparticles have a potential to become optical probes for deep‐tissue autofluorescence‐free bioimaging in the biomedical field.