Near-infrared-emitting upconverting BiVO₄ nanoprobes for in vivo fluorescent imaging

Near-infrared (NIR) emitting BiVO₄:Yb³⁺,Tm³⁺ nanoparticles are synthesized by a new solvothermal strategy using solvents of oleic acid and methanol. The obtained BiVO₄:Yb³⁺,Tm³⁺ samples show an average particle size of ≈164 nm and exhibit an asymmetry monoclinic crystal structure of BiVO₄. At NIR excitation of 980 nm, the BiVO₄:Yb³⁺,Tm³⁺ sample exhibits a nearly single NIR emission at ≈796 nm with extremely weak blue emissions from Tm³⁺ ions. These high-energy visible emissions are absorbed by the semiconducting host of BiVO₄ that possesses a bandgap of ≈2.2 eV. Therefore, the NIR excitation to a single intense NIR emission fluorescent BiVO₄ materials could be a potential ideal probe for deep-tissue high-resolution bioimaging. To validate the ability of BiVO₄ materials for bio-applications, we conduct the cytotoxicity experiments. The results show that the cytotoxicity of HeLa cells is negligible at a concentration of 0.2 mg/ml of BiVO₄:Yb³⁺,Tm³⁺ , and the cell viability approaches 90% at a high dosage of 0.5 mg/ml. The Daphnia magna and Zebrafish treated with nanoparticles (0.5 mg/ml) display bright NIR emission without any background, indicating the excellent in vivo fluorescent imaging capacity of BiVO₄:Yb³⁺,Tm³⁺ nanoparticles. Our findings offer an environment-friendly strategy to synthesize BiVO₄ UCL nanophosphors and provide a promising new class of fluorescent probes for biological applications.