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 (T2) signal shifts (R2 = 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.