Gd-Based Mesoporous Silica Nanoparticles as MRI Probes

Over the last decade, a number of GdIII chelates and paramagnetic ions were conjugated or included into mesoporous silica nanoparticles (MSNs) and tested as potential Magnetic Resonance Imaging (MRI) and theranostic probes. These systems are able to concentrate a large payload of paramagnetic sites in the region where they localize, thus increasing the sensitivity of the technique. In addition, when attached to the silica surface the GdIII complexes reduce their rotational tumbling motion leading to an improvement of their performance as relaxation agents. In this review, we describe different strategies to confine paramagnetic species into porous silica. The relationship between the structural and dynamic parameters of conjugated Gd-complexes on the surface of the MSNs and their effectiveness as relaxation agents is discussed, examining how the magnetic properties of the probes are influenced by the characteristics of the nanoparticles. Special attention is addressed to clarify the effect of silica support, in terms of porosity and surface reactivity, on the relaxometric properties of the final solids. A better understanding of the interactions between the silica nanoparticles and the conjugated GdIII chelate is crucial to design improved nanoprobes for MRI applications.