Photophysical behavior and biological safety of <scp> YVO ₄ </scp> :Er,Yb upconversion nanocomposites decorated with <scp> Fe ₃ O ₄ </scp> for optical‐magnetic bioapplications

Abstract This work reports the synthesis, physicochemical characterization, and toxicity assessment of bifunctional YVO 4 :Er,Yb@Fe 3 O 4 nanocomposites designed for optical–magnetic bioapplications. Upconversion nanoparticles (UCNPs) of YVO 4 :Er,Yb were synthesized and subsequently decorated with Fe 3 O 4 nanoparticles. Nanocomposites exhibit superparamagnetic behavior and efficiently upconvert 980 nm near‐infrared radiation (NIR) into visible green emission at 545 nm, making them attractive candidates for multimodal bioimaging. Structural, optical, and magnetic properties were analyzed by X‐ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). A slight redshift of the characteristic vanadate Raman modes was observed, suggesting interfacial interactions between Fe 2+ /Fe 3+ sites and Y‐O or V‐O surface groups. Upconversion emission intensity decreased with increasing magnetite content. The lifetime of the green 4 S 3/2 → 4 I 15/2 transition decreased from 5.11 to 4.62 microseconds upon magnetite decoration, indicating enhanced nonradiative relaxation. Toxicity assays using Artemia salina indicated that the nanocomposites are non‐toxic, yielding LC 50 values of 204 ppm for YVO 4 :Er,Yb, 698 ppm for Fe 3 O 4 , and 345 ppm for the nanocomposite containing 30 wt% Fe 3 O 4 . These results demonstrate that Fe 3 O 4 decoration not only imparts magnetic functionality but also reduces the toxicity of UCNPs. The combined optical–magnetic properties and low toxicity support their potential use in photoactive nanomaterial systems.