Cation Exchange: A Facile Method To Make NaYF4:Yb,Tm-NaGdF4 Core–Shell Nanoparticles with a Thin, Tunable, and Uniform Shell

Cation exchange was performed on up-conversion NaYF4:Yb,Tm nanoparticles, resulting in NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles as indicated by electron energy-loss spectroscopy 2D mapping. Results show that core–shell nanoparticles with a thin, tunable, and uniform shell of subnanometer thickness can be made via this cation exchange process. The resulting NaYF4:Yb,Tm-NaGdF4 core–shell nanoparticles have an enhanced up-conversion intensity relative to the initial core nanoparticles. As potential magnetic resonance imaging (MRI) contrast agents, they were tested for their proton relaxivities. The r1 relaxivity per Gd3+ ion of the nanoparticles with a thin NaGdF4 shell (ca. 0.6 nm thick) measured at 9.4 T was found to be 2.33 mM–1·s–1. This r1 relaxivity is among the highest in all the reported NaYF4–NaGdF4 core–shell nanoparticles. The r1 relaxivity per nanoparticle is 1.56 × 104 mM–1·s–1, which is over 4000 times higher than commercial Gd3+-complexes. The very high proton relaxivity per nanoparticle is critical for targeted MRI as such nanoparticles provide strong contrast even in low concentrations. The presented cation exchange method is a promising way to manufacture core–shell nanoparticles with up-conversion NaYF4:Yb,Tm core and paramagnetic NaGdF4 shell for bimodal imaging, i.e. MR and optical imaging.