We report on a novel Cu2+-complex of nitrogen-rich polymer dots for magnetic resonance imaging (MRI). The N-rich polymer dots are prepared from N-vinyl imidazole (VIm) by a one-pot hydrothermal synthesis at 220 °C (24 h) and used later on to fabricate a Cu2+-PVIm dot complex via efficient incorporation of Cu2+ into aqueous medium. The obtained Cu2+-PVIm dot complexes display relaxivity (r1 = 1.05 mM-1 s-1) two times higher than Cu2+ in aqueous solution (r1 = 0.43 mM-1 s-1) and three times higher than Cu2+ in aqueous solution coordinated with VIm monomers (r1 = 0.32 mM-1 s-1), which show a remarkable contrast enhancement for T1-weighted MRI while efficiently labeling MCF-7 cells and other biomedical applications.