Early diagnosis of cancer is extremely crucial for radical treatment of cancer because cancer patients diagnosed in the early stage and subjected to surgery show far better survival rates than those not subjected to surgery [1].The sensitivity of magnetic resonance imaging (MRI) for detecting early tumor can be increased with the assistance of a contrast agent [2].However,the traditional positive MRI contrast agents,such as Gd-chelates and Gd-based inorganic nanoparticles,are often limited by the cytotoxicity and low specificity [3].