Stable Magnetic Relaxation Switch Sensor Based on Fe₃O₄@Gel for Ultrafast Detection of Cd²⁺

To overcome the dual challenges of signal instability and prolonged detection in conventional magnetic relaxation switching (MRS) systems, a novel Fe₃O₄-encapsulated alginate hydrogel nanocomposite (Fe₃O₄@Gel) sensor was designed for rapid screening of the cadmium ion. Compared with the traditional Fe₃O₄-based sensors, the Fe₃O₄ was embedded in the gel network framework to avoid magnetic field-induced aggregation, which helped to improve the stability of MRS. On the other hand, compared with MRS based on gel, the Fe₃O₄ accelerated the relaxation process of water molecules inside the gel, obtaining a fast detection time of the sensor within 38 s, which is one-fifth of the detection time of the traditional magnetic relaxation switch sensor with pure hydrogel of 191 s. Mechanistically, target-induced immunocomplex formation modulates alkaline phosphatase activity, triggering cascade enzymatic reactions that precisely regulate hydrogel swelling dynamics. This stimuli-responsive behavior translates quantitative Cd²⁺ concentrations into reproducible transverse relaxation time (<i>T</i>₂) signal shifts (<i>R</i>² = 0.987), achieving sub-ppt sensitivity (6 pg/mL) across linearity (0.01-10 ng/mL). Practical validation in complex matrices demonstrated 96.62%-109.97% spike recoveries. This multifunctional nanoplatform establishes a new paradigm for high-fidelity, field-deployable hazard screening in complex systems.