Enhanced magnetic relaxation switching immunoassay for chlorpyrifos based on tyramine signal amplification

Chlorpyrifos, an organophosphorus pesticide, is widely used in agriculture to protect crops from insects. However, the presence of its residues in food has caused widespread concern due to the serious risks to human health. Traditional detection methods suffer from limitations, such as low sensitivity, long detection time, and complicated operations. Herein, based on the tyramine signal amplification (TSA) strategy, we developed a sensitive and rapid magnetic relaxation switching (MRS) immunosensor for the detection of chlorpyrifos. Wherein, magnetic Fe₃O₄ nanoparticles modified with tyramine (MNP₁₅₀-tyramine) acted as magnetic probes for magnetic relaxation signal output. In the presence of hydrogen peroxide (H₂O₂) and horseradish peroxidase (HRP), tyramine can be converted to the highly reactive intermediate that covalently binds with the nearby proteins, such as HRP and antibody, thus assembling MNPs to magnetic clusters and showing changes in transverse relaxation time (T₂) signals. Based on the “antibody–antigen” immunoreaction, chlorpyrifos could make a connection of HRP/antibody-modified MNPs and MNP₁₅₀-tyramine with a result of MNPs aggregation and strong T₂ signals. In this study, the TSA-MRS method showed sensitive detection of chlorpyrifos with a limit of detection of 0.54 ng/mL, a 4.5-fold enhancement in the sensitivity compared with the ELISA method, providing an alternative method for the detection of harmful substances in food samples.