PEG-capped, lanthanide doped GdF3 nanoparticles: luminescent and T-2 contrast agents for optical and MRI multimodal imaging

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 r(2) relaxivity values are quite high with respect to the common T-2-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 T-2 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.