Recent advances in biological-environment-responsive smart MRI contrast agents

With the rapid advancement in biology and medicine, there is an urgent need to study metabolisms in lives and mechanisms of diseases at molecular levels. Magnetic resonance imaging (MRI), which is capable of non-invasive and three-dimensional deep imaging, is a powerful tool to meet this demand. Environment-responsive contrast agents, whose contrast abilities change once stimulated, are necessary for this purpose. With the use of these responsive contrast agents, related biological information, such as pH, concentration of ions, activity of enzymes, could be easily extracted and analyzed from MR images. Considerable progress has been made in the field of biological-environment-responsive smart MRI contrast agents over the two decades. In this review, we describe achievements in recent years, discuss several representative works in detail, summarize current difficulties that have been met and possible solutions that have been formulated, and finally look into the promising future of biological-environment-responsive smart MRI contrast agents.