Cu(II)-induced magnetic resonance tuning and enhanced magnetic relaxation switching immunosensor for sensitive detection of chlorpyrifos and Salmonella.

Magnetic relaxation switching (MRS) biosensors have been recognized as useful analytical tools for a range of targets; however, traditional MRS biosensors are limited by the "prozone effect", resulting in a narrow linear range and low sensitivity. Herein, we proposed a paramagnetic Cu-induced magnetic resonance tuning (MRET) strategy, based on which Cu ions and magnetic nanoparticles (MNPs) were adopted to construct a Cu-MNP-mediated MRS (Cu-M-MRS) immunosensor with Cu ions acting as a quencher and MNPs as an enhancer. An FeO@polydopamine-secondary antibody conjugate was prepared and used to correlate the amount of Cu ions to the target concentration through an immunoassay. Based on the immunoreaction, the Cu-M-MRS immunosensor enabled the sensitive detection of chlorpyrifos (0.05 ng/mL, a 77-fold enhancement in sensitivity compared with the traditional MRS immunosensor) and Salmonella (50 CFU/mL). The proposed MRET strategy effectively improved the sensitivity and accuracy of the MRS immunosensor, offering a promising and versatile platform for food safety detection.