Highly Water-Dispersible Upconverting GdPO ₄ :Er ³⁺ /Yb ³⁺ Nanophosphors for Optical, Magnetic, Magnetic Hyperthermia, Anticounterfeit, and In Vitro and Ex-Vivo Multimodal Applications

Highly water-dispersible Er 3+ /Yb 3+ -activated GdPO 4 nanophosphors were synthesized via a polyol-mediated coprecipitation route. In situ X-ray diffraction reveals a thermally induced phase transformation from hexagonal to monoclinic symmetry upon annealing between 50 and 900 °C, accompanied by a morphology evolution from nanorods to dumbbell-like structures as confirmed by HRTEM (high resolution transmission electron microscopy). The phase transition is attributed to pressure buildup within the confined hexagonal nanorods due to trapped water molecules. Under 980 nm excitation, the nanophosphors exhibit strong green and red upconversion emissions at 523, 545, and 655 nm arising from the characteristic of the Er 3+ ion having 4f–4f transitions. Detailed magnetic studies indicate paramagnetic behavior, highlighting their potential for magnetic resonance imaging. Furthermore, PEG-functionalized GdPO 4:Er 3+ /Yb 3+ –Fe 3 O 4 hybrid nanostructures (PEG = polyethylene glycol) demonstrate efficient magnetic hyperthermia. The nanophosphors show good biocompatibility toward MCF-7, A549, and WI-26 cell lines and enable effective ex-vivo bioimaging of muscle and bone tissues. Real-time anticounterfeiting is successfully demonstrated under continuous near-infrared excitation, establishing prepared nanophosphors as multifunctional materials for optical, magnetic, and biomedical applications.