Cr3+-Doped γ- and β-Gallium Oxide Nanoprobes for Bioimaging: Synthesis, Persistent Luminescence, and Biocompatibility

Persistent luminescent (PersL) nanophosphors that emit in the near infrared (NIR) region are promising nanoprobes for in vivo bioimaging. Although Cr³⁺-doped zinc gallate nanoparticles (NPs) have been widely studied as in vivo bioimaging nanoprobes due to their NIR PersL emission at 695 nm, the simpler Cr³⁺-doped gallium oxide system has been less explored despite its deeper NIR emission (760 nm), which favors tissue penetration. This is likely due the lack of synthesis methods that render Ga₂O₃-based NPs suitable for in vivo applications. In this paper, a novel method for the synthesis of uniform and hydrophilic γ-Ga₂O₃:Cr³⁺ NPs is reported, whose photoluminescence (PL) and PersL are optimized by adjusting their Cr³⁺ content. Such properties are further greatly improved through an annealing process at high temperature, which result in the transformation of its crystal structure into the β-phase. The obtained β-Ga₂O₃:Cr³⁺ NPs are colloidally stable in a physiological pH simulator medium and are nontoxic for cells. Finally, this work studies, for the first time in literature, the in vivo biocompatibility of such NPs using a Caenorhabditis elegans (C. elegans) animal model, finding that their morbidity and reproductive toxicity are negligible. In summary, the reported NPs are excellent candidates for their use as a NIR PersL probes for in vivo bioimaging.