PEG-capped, lanthanide doped GdF3nanoparticles: luminescent and T2 contrast agents for optical and MRI multimodal imaging

Abstract

A facile method for the synthesis of water dispersible Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped upconverting GdF₃ nanoparticles is reported. Strong upconversion emissions are observed in the red (for Er/Yb doped) and near-infrared (for Tm/Yb doped) regions upon laser excitation at 980 nm. The PEG coating ensures a good dispersion of the system in water and reduces the radiationless de-excitation of the excited states of the Er³⁺ and Tm³⁺ ions by water molecules. The r₂ relaxivity values are quite high with respect to the common T₂-relaxing agents (22.6 ± 3.4 mM⁻¹ s⁻¹ and 15.8 ± 3.4 mM⁻¹ s⁻¹ for the Tm/Yb and Er/Yb doped samples, respectively), suggesting that the present NPs can be interesting as T₂ weighted contrast agents for proton MRI purpose. Preliminary experiments conducted in vitro, in stem cell cultures, and in vivo, after subcutaneous injection of the lanthanide-doped GdF₃ NPs, indicate scarce toxic effects. After an intravenous injection in mice, the GdF₃ NPs localize mainly in the liver. The present results indicate that the present Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped GdF₃ NPs are suitable candidates to be efficiently used as bimodal probes for both in vitro and in vivo optical and magnetic resonance imaging.