High Stability Hydrogel Magnetic Relaxation Switch Sensor Driven by pH for the Sensitive Detection of Cd²⁺

The traditional magnetic relaxation switching (MRS) sensors have excellent sensitivity, but their stability is poor because the magnetic relaxation signal is easily affected by the external magnetic field or environmental oxidation. In this study, a highly stable hydrogel bead-based MRS (Gel-MRS) sensor was established for the accurate and sensitive detection of Cd²⁺ in rice. A pH-responsive hydrogel bead was applied as a core element for the target stimulus and transverse relaxation signal transduction. The stability experiments showed that the transverse relaxation time (<i>T</i>₂) change of the Gel-MRS sensor was one-seventh that of traditional magnetic nanoparticles under an external magnetic field and less than a fifth that of Fe²⁺/Fe³⁺ conversion in air. The excellent stability was due to the fact that <i>T</i>₂ of the Gel-MRS sensor came from the swelling system mediated by pH rather than the traditional aggregation/dispersion or Fe²⁺/Fe³⁺ conversion of magnetic nanoparticles. In addition, the target Cd²⁺ could exclusively trigger a pH response of the hydrogel beads, altering the <i>T</i>₂, thus resulting in excellent relaxation properties (R₂ = 56.89) and pH responsiveness of the Gel-MRS sensor. The swelling process of the hydrogel beads followed quasi-second-order dynamics. The Gel-MRS sensor demonstrated a remarkable limit of detection as low as 0.009 ng/mL for Cd²⁺, with a linear range of 0.01-5 ng/mL. The excellent stability and sensitivity made the Gel-MRS sensor have great application potential in food and environmental detection.