Core–Shell–Shell NaYbF₄:Tm@CaF₂@NaDyF₄ Nanocomposites for Upconversion/T₂-Weighted MRI/Computed Tomography Lymphatic Imaging

To circumvent the defects of different bioimaging techniques, the development of multifunctional probes for multimodality bioimaging is required. In the present study, a lanthanide-based core-shell-shell nanocomposite NaYbF4:Tm@CaF2@NaDyF4 composed of an ∼9.5 nm NaYbF4:Tm nanocrystal as the core, ∼2 nm CaF2 as the middle layer, and 1-2 nm NaDyF4 as the outermost shell was designed and synthesized. Following surface modification with the ligand, citrate acid, this nanocomposite was hydrophilic, emitted intense upconversion luminescence (UCL), and displayed a high X-ray computed tomography (CT) value of ∼490 Hounsfield units (HU) and excellent r2 relaxivity of 41.1 mM(-1) s(-1). These results confirmed that the introduction of a middle CaF2 layer was necessary as a barrier to reduce cross-relaxation and the surface quenching effect, thus enhancing the upconversion emission of Tm(3+). This citrate-modified NaYbF4:Tm@CaF2@NaDyF4 nanocomposite was used as a multifunctional contrast agent for trimodal lymphatic bioimaging with T2-weighted magnetic resonance imaging (MRI), CT, and UCL imaging. The concept of fabricating a core-multishell nanostructure and the introduction of a Dy(3+)-based host as an outer layer is a useful strategy and can be used to develop a novel multifunctional nanoprobe for multimodality bioimaging.