Current magnetic resonance imaging (MRI) contrast agents often produce insufficient contrast for diagnosis of early disease stages, and do not sense their biochemical environments. Herein, we report a highly sensitive nanoparticle-based MRI probe with r1 relaxivity up to 51.7 ± 1.2 mM(-1) s(-1) (3T). Nanoparticles were co-assembled from Gd(3+) complexed to heparin-poly(dimethylsiloxane) copolymer, and a reduction-sensitive amphiphilic peptide serving to induce responsiveness to environmental changes. The release of the peptide components leads to a r1 relaxivity increase under reducing conditions and increases the MRI contrast. In addition, this MRI probe has several advantages, such as a low cellular uptake, no apparent cellular toxicity (tested up to 1 mM Gd(3+)), absence of an anticoagulation property, and a high shelf stability (no increase in free Gd(3+) over 7 months). Thus, this highly sensitive T1 MRI contrast nanoparticle system represents a promising probe for early diagnosis through possible accumulation and contrast enhancement within reductive extracellular tumour tissue.