Hydration number: crucial role in nuclear magnetic relaxivity of Gd(III) chelate-based nanoparticles

Today, nanostructure-based contrast agents (CA) are emerging in the field of magnetic resonance imaging (MRI). Their sensitivity is reported as greatly improved in comparison to commercially used chelate-based ones. The present work is aimed at revealing the factors governing the efficiency of longitudinal magnetic relaxivity (r(1)) in aqueous colloids of core-shell Gd(III)-based nanoparticles. We report for the first time on hydration number (q) of gadolinium(III) as a substantial factor in controlling r(1) values of polyelectrolyte-stabilized nanoparticles built from water insoluble complexes of Gd(III). The use of specific complex structure enables to reveal the impact of the inner-sphere hydration number on both r(1) values for the Gd(III)-based nanoparticles and the photophysical properties of their luminescent Tb(III) and Eu(III) counterparts. The low hydration of TTA-based Gd(III) complexes (q approximate to 1) agrees well with the poor relaxivity values (r(1) = 2.82 mM(-1)s(-1) and r(2) = 3.95 mM(-1)s(-1)), while these values tend to increase substantially (r(1) = 12.41 mM(-1)s(-1), r(2) = 14.36 mM(-1)s(-1)) for aqueous Gd(III)-based colloids, when macrocyclic 1,3-diketonate is applied as the ligand (q approximate to 3). The regularities obtained in this work are fundamental in understanding the efficiency of MRI probes in the fast growing field of nanoparticulate contrast agents.