Water-Soluble GdF3 and GdF3/LaF3 NanoparticlesPhysical Characterization and NMR Relaxation Properties

Nanoparticles consisting of either a solid core of GdF3 or an 80/20 mixture of GdF3 and LaF3 have been prepared for use as NMR and MRI relaxation agents. To obtain high aqueous solubilities, the particles were coated with either citrate (cit) groups (in the case of GdF3 nanoparticles), giving the nanoparticle a negatively charged surface, or 2-aminoethyl phosphate (AEP) groups (in the case of GdF3/LaF3 = 80/20), giving the nanoparticle a positively charged surface at physiological pH. In the presence of the 80/20 GdF3/LaF3:AEP, the paramagnetic contribution to the water spin−lattice relaxation rate was observed to be 7.5 s-1 at a nanoparticle concentration of 9.0 nM (0.78 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Similarly, paramagnetic rates of 10.5 s-1 were observed for water using the GdF3:cit nanoparticles at a nanoparticle concentration of 0.55 nM (0.77 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Relaxivity measurements confirmed the potential of the particles for applications as contrast agents at MRI imaging field strengths. T 1 and T 2 experiments of the GdF3:cit revealed mass relaxivities of 7.4 ± 0.2 and 8.4 ± 0.2 s-1 (mg/mL)-1, respectively, at 1.5 T, whereas identical measurements at 3.0 T revealed respective relaxivities of 8.8 ± 0.2 and 9.4 ± 0.2 s-1 (mg/mL)-1. The relatively high mass relaxivities exhibited by the nanoparticles may also find uses in NMR studies in which spin−lattice relaxation times are prohibitively long, as in the case of highly deuterated proteins. Direct interaction with the protein can be minimized by the use of surface charges opposite to the net charge of the molecule, whereas the nanoparticles are easily removed by ultracentrifugation.