High Stability\nHydrogel Magnetic Relaxation Switch\nSensor Driven by pH for the Sensitive Detection of Cd<sup>2+</sup>

The traditional magnetic relaxation switching (MRS) sensors\nhave\nexcellent sensitivity, but their stability is poor because the magnetic\nrelaxation signal is easily affected by the external magnetic field\nor environmental oxidation. In this study, a highly stable hydrogel\nbead-based MRS (Gel-MRS) sensor was established for the accurate and\nsensitive detection of Cd<sup>2+</sup> in rice. A pH-responsive hydrogel\nbead was applied as a core element for the target stimulus and transverse\nrelaxation signal transduction. The stability experiments showed that\nthe transverse relaxation time (<i>T</i><sub>2</sub>) change\nof the Gel-MRS sensor was one-seventh that of traditional magnetic\nnanoparticles under an external magnetic field and less than a fifth\nthat of Fe<sup>2+</sup>/Fe<sup>3+</sup> conversion in air. The excellent\nstability was due to the fact that <i>T</i><sub>2</sub> of\nthe Gel-MRS sensor came from the swelling system mediated by pH rather\nthan the traditional aggregation/dispersion or Fe<sup>2+</sup>/Fe<sup>3+</sup> conversion of magnetic nanoparticles. In addition, the target\nCd<sup>2+</sup> could exclusively trigger a pH response of the hydrogel\nbeads, altering the <i>T</i><sub>2</sub>, thus resulting\nin excellent relaxation properties (R<sub>2</sub> = 56.89) and pH\nresponsiveness of the Gel-MRS sensor. The swelling process of the\nhydrogel beads followed quasi-second-order dynamics. The Gel-MRS sensor\ndemonstrated a remarkable limit of detection as low as 0.009 ng/mL\nfor Cd<sup>2+</sup>, with a linear range of 0.01–5 ng/mL. The\nexcellent stability and sensitivity made the Gel-MRS sensor have great\napplication potential in food and environmental detection.