Porous Silica Nanoparticle Entrapped Small Gd(III) and Mn(II) Complexes as MRI Contrast Agents

Among the in vivo imaging techniques, magnetic resonance imaging (MRI) provides soft-tissue images with high spatial resolution without using any harmful ionizing radiation. Prior to in vivo imaging, the administration of a bolus injection of paramagnetic species, coined as contrast agents (CAs), has become almost routine to facilitate conspicuous imaging in a relatively short measurement period. The contrast agents are mainly small Gd(III)-complexes of macrocyclic and acyclic organic ligands with polar pendant arms. Nonetheless, reports on some adverse effects due to the accumulation of bare Gd(III) ions in the human body from the used gadolinium-based contrast agents necessitate extensive investigations on Mn(II)-complexes to engender potential alternatives. While thermodynamically stable and kinetically inert Mn-complexes with inner-sphere water molecule(s) have been developed and tested as CAs, the enhancement in the relaxivity value beyond 3.5 mM^-1 s^-1 has been intriguing. This review discloses the recent strategies for incorporating paramagnetic small Gd(III) and Mn(II) complexes within the porous nanosystems, the physicochemical properties, and stability and contrast efficiency improvement after confinement. The generation of "smart" and environmentally responsive contrasting probes by incorporating appropriate functional groups onto the surface of the robust nanosystems is also presented herein.