Magnetic relaxation switching biosensor via polydopamine nanoparticle mediated click chemistry for detection of chlorpyrifos.

Traditional magnetic relaxation switching (MRS) biosensors suffer from poor sensitivity and unsatisfactory stability. In this study, a polydopamine (PDA) nanoparticles (NPs)-Cu chelate complex mediated signal conversion system and a Cu-catalyzed click chemistry triggered magnetic signal amplification system were evaluated and dynamically integrated into an MRS biosensor. Owing to abundant functional groups and a large surface area, PDA NPs enabled the absorption of a large amount of Cu ions by chelation. The residual Cu ions can be reduced with sodium ascorbate to Cu, which could initiate the click reaction between azide-functionalized magnetic NPs (MNPs) and alkyne-functionalized MNPs that resulted in the production of aggregated nanoclusters. The transverse relaxation time (T) depends on the degree of aggregation of MNPs; T is expressed as the magnetic signal readout. In addition, PDA NPs can be easily conjugated with antibodies by mixing, thus providing a straightforward bridge that integrates the immunoassay and magnetic signal readout. Combined with the high capacity of PDA NPs for chelating Cu and high efficiency of click reaction for changing the T signals, the PDA-MRS biosensor enables the detection of chlorpyrifos with a limit of detection of 0.084 ng/mL, providing 22-fold enhancement than traditional enzyme-linked immunosorbent assay (1.86 ng/mL). This demonstrates its great potential for the detection of hazardous chemical molecules in a complex sample matrix.