Microwave–assisted synthesis of water–disperse and biocompatible NaGdF₄: Yb,Ln@NaGdF₄ nanocrystals for UCL/CT/MR multimodal imaging

In this paper, we have developed a series of rare earth fluoride nanocrystals (NCs) that can be used for multimodal imaging applications. Three highly monodisperse and water dispersible NaGdF₄:Yb,Ln@NaGdF₄ (Ln = Er, Tm, Ho) upconversion luminescence (UCL) NCs with an average size of 75 nm were prepared using a facile and efficient microwave–assisted solvothermal reaction in ethylene glycol (EG). The morphology, crystal structures, size distributions and zeta potentials of the resulting NCs were fully characterized by transmission electron microscopy (TEM), X–ray diffraction (XRD) and Nano–ZS90 Zetasizer. The obtained NaGdF₄:Yb,Ln@NaGdF₄ (Ln = Er, Tm, Ho) NCs could emit bright blue, green and red luminescence in water upon laser excitation at 980 nm. Furthermore, these core–shell NCs exhibited low cytotoxicity for HeLa and C9H2 cells, separately. Moreover, these NCs also displayed excellent Hounsfield units (HU) values in X–ray computed tomography (CT) imaging and high relaxivity parameters (r₁) in magnetic resonance (MR) imaging in vitro. Thus, these small, water–dispersible and biocompatible NCs show excellent potential as contrast agents for UCL/CT/MR multimodal imaging and other biomedical applications.