Zn₃Ga₂Ge₂O₁₀:Cr³⁺ Uniform Microspheres: Template-Free Synthesis, Tunable Bandgap/Trap Depth, and In Vivo Rechargeable Near-Infrared-Persistent Luminescence

Near-infrared (NIR) emitting persistent phosphors of Cr³⁺-doped zinc gallogermanate have emerged for in vivo bioimaging with the advantage of no need for in situ excitation. However, it is challenging to synthesize well-dispersed and uniform spherical particles with high brightness, high resolution, and distinguished NIR long afterglow. In this work, Zn₃Ga₂Ge₂O₁₀:Cr³⁺ (ZGGC) monospheres were directly synthesized by a facile hydrothermal method with the assistance of citric anions (Cit^3-), which emit an NIR emission at ∼696 nm and exhibit excellent NIR-persistent luminescence with rechargeability. Controlled experiments indicated that the shape evolution of the ZGGC product is significantly affected by Cit^3-, solution pH, and the duration and temperature of hydrothermal reaction. Furthermore, compositional influence on the crystal structure, bandgap, trap depth, and luminescence characteristics of Zn_yGa₂Ge₂O_10-δ:Cr³⁺ (y = 2.8, 3.0, 3.2) were investigated in detail, which allows us to construct an energy level diagram of the ZGGC host, Cr³⁺ ions, and electron traps. It was found that the bandgap and conduction-band minimum (CBM) are significantly affected by the Zn content, while the valence-band maximum (VBM) is not. The y = 3.0 sample exhibited the best persistent luminescence, owing to its deepest defects. The ZGGC-NH₂ prepared through surface functionalization of ZGGC spheres showed distinguished NIR long afterglow, low toxicity, and great potential for in vitro cell imaging and in vivo bioimaging in the absence of excitation. Moreover, the persistent luminescence signal from the ZGGC-NH₂ can be repeated in vivo through in situ recharge with external excitation of a red LED lamp, indicating that the ZGGC-NH₂ is suitable for applications in long-term in vivo imaging.