Magnetic resonance imaging (MRI) is one of the most important medical imaging techniques for clinical diagnosis. Contrast agents (CAs) are commonly necessarily used to enhance the imaging quality of MRI and differentiate diseased tissues from normal ones. Herein, we introduced a macromolecular carrier, spherical polyelectrolyte brushes (SPBs), which consists of a solid polystyrene (PS) core and polyacrylic acid (PAA) chains as a brush layer to host Gd(iii) complexes. The cationic Gd(iii) complex Gd-DTPA-NO-C4 was synthesized through a 6-step approach and then formed electrostatic self-assemblies with SPBs to afford magnetic assemblies. The regular appearance of the core-shell type structure of the assemblies was confirmed by TEM and SEM. Besides, a remarkable enhancement in relaxivity up to 62 mM-1 s-1 of these assemblies was determined, much higher than that of clinically used small molecule CAs (4-5 mM-1 s-1). It is noteworthy that the assemblies exhibit non-cytotoxicity even at the concentration of Gd(iii) up to 150 μM, showing great potential for clinical MRI applications.

A novel platform for the development of highly efficient magnetic resonance imaging (MRI) contrast agents has been demonstrated. New contrast agents are designed and produced through electrostatic self-assembly of cationic gadolinium(III) complexes onto anionic spherical polyelectrolyte brushes (SPB). The structurally well-defined SPB are composed of polystyrene core and polyacrylic acid brush layer, where numerous binding sites and confined microenvironments are available for the embedment of the gadolinium(III) contrast agents. Both in vitro and in vivo experiments show excellent biocompatibility and relaxometric performance of these SPB-based gadolinium hybrid materials. The enhanced relaxivity value is up to 86.2 mM s per Gd, a remarkably high record value at 1.5 T magnetic field. In vivo imaging displays a prolonged blood circulation time and massive accumulation of the contrast agents at the tumor region due to the enhanced permeability and retention effect. The SPB-based gadolinium hybrid materials not only broaden the horizons of new MRI contrast agents, but also have a great potential for tumor diagnosis.