Hydrothermal synthesis of core–shell structured TbPO4:Ce3+@TbPO4:Gd3+ nanocomposites for magnetic resonance and optical imaging

Abstract

Multi-modal imaging based on multifunctional nanoparticles provides deep, non-invasive and highly sensitive imaging and is a promising alternative approach that can improve the sensitivity of early cancer diagnosis. In this study, two nanoparticles, TbPO₄:Ce³⁺ and TbPO₄:Ce³⁺@TbPO₄:Gd³⁺, were synthesized via the citric-acid-mediated hydrothermal route, and then systematically characterized by means of microstructure, photoluminescence, magnetic resonance imaging (MRI), biocompatibility, and bioimaging. The results of energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) line scans indicated that TbPO₄:Gd³⁺ nanoshells about 5 nm in thickness were successfully coated on the TbPO₄:Ce³⁺ nanocores. X-ray diffraction (XRD) and Fourier transforms of high-resolution transmission electron microscopy (TEM) images indicated that the core–shell nanocomposites had a single crystal structure. The photoluminescence of the TbPO₄:Ce³⁺@TbPO₄:Gd³⁺ and TbPO₄:Ce³⁺ nanoparticles was greatly intensified by 200 times and 100 times, respectively, compared with pure TbPO₄ nanoparticles. In vitro cytotoxicity tests based on the methyl thiazolyl tetrazolium (MTT) assay demonstrated that the monodispersed nanoparticles of TbPO₄:Ce³⁺@TbPO₄:Gd³⁺ had low toxicity. The intracellular luminescence of the nanoparticles after being internalized by HeLa cells was also observed using confocal fluorescence microscopes. MRI showed that the nanoshells of Gd-doped TbPO₄ possessed a longitudinal relaxivity of 4.067 s⁻¹ mM⁻¹, which is comparable to that of the commercial MRI contrast Gd-TDPA. As a result, the core–shell structured TbPO₄:Ce³⁺@TbPO₄:Gd³⁺ nanoparticles can potentially serve as multifunctional nanoprobes for both optical biolabels and MRI contrast agents.