Background Signal-Free Magnetic Bioassay for Food-Borne Pathogen and Residue of Veterinary Drug via Mn(VII)/Mn(II) Interconversion

Paramagnetic ion-mediated sensors can greatly simplify current magnetic sensors for biochemical assays, but it remains challenging because of the limited sensitivity. Herein, we report a magnetic immunosensor relying on Mn(VII)/Mn(II) interconversion and the corresponding change in the low-field nuclear magnetic resonance (LF-NMR) of the transverse relaxation rate (R-2). The fact that the NMR R-2 of the water protons detected in Mn(II) aqueous solution is much stronger than Mn(VII) aqueous solution enables the modulation of the LF-NMR signal intensity of R-2. By employing immunomagnetic separation and enzyme-catalyzed reaction, this Mn(VII)/Mn(II) interconversion allows the development of a background signal-free magnetic immunosensor with a high signal-to-background ratio that enables detection of ractopamine and Salmonella with high sensitivity (the limits of detection for ractopamine and Salmonella are 8.1 pg/mL and 20 cfu/mL, respectively). This Mn-mediated magnetic immunosensor not only retains the good stability but also greatly improves the sensitivity of conventional paramagnetic ion-mediated magnetic sensors, offering a promising platform for sensitive, stable, and convenient bioanalysis.