Contamination with heavy metal ions may have severe effects on human health and the environment. Mercury contamination is widespread and arises from a variety of natural sources. ¹ In the reported small-molecule sensors, only a few compounds can selectively detect Hg²⁺ ion in aqueous solutions. Herein we propose the use of hyperpolarized ¹²⁹Xe magnetic resonance image (MRI) for the sensitive detection of Hg²⁺ ions in aqueous solution. To achieve this goal, we develop a biosensor whose molecular structure is like a clamp. The biosensor is developed using dipyrrolylquinoxaline as the basic molecular clamp scaffold, pyrrole and the hydrazone as the recognition site, and cryptophane as the ¹²⁹Xe NMR signal reported moiety. ^{2, 3} When interact with Hg²⁺ in aqueous solution, the molecular structure of the biosensor could be changed as a clamp from "open" to "closed". This molecular structure change causes the distance between the two cryptophane cages of the biosensor become closer, and the electron cloud of them overlapped. As a result, comparing with normal downfield chemical shifts of the reported xenon biosensors formetallic ions, the Xe caged in the cryptophane moiety shows a upfield chemical shift change from 66.5 ppm to 66.1 ppm (Δ＝38.6 Hz). Images were obtained using a CSI method preciously used for clinical MRI. (Fig. 1).