Confinement of a tris-aqua Gd(iii) complex in silica nanoparticles leads to high stability and high relaxivity and supresses anion binding.

The water soluble tris-aqua complex [Gd(dhqN-SO3)(H2O)3](3-) based on a hexadentate hydroxyquinoline ligand shows high thermodynamic stability and high relaxivity (12.54 mM(-1) s(-1) at 1.2 T). Its non-covalent confinement in 25 nm silica nanoparticles prevents transmetallation and endogenous anion binding and leads to higher relaxivity over a wide range of magnetic fields.