Magnetic relaxation switching immunoassay for chlorpyrifos using enzyme-mediated Fe2+/Fe3+ conversion and magnetic separation.

Since pesticide residues in food have attracted increasing concern worldwide, it is crucial to develop rapid and sensitive analytical methods to detect pesticide residues and ensure food safety. In this work, via the biotin-streptavidin (SA) recognition system, we constructed a magnetic relaxation switching (MRS) immunosensor for sensitive detection of chlorpyrifos (CPF). With a competitive immunoassay mode, H2O2 and Fe2+ were firstly optimized as the reaction substrates. Wherein, horseradish peroxidase (HRP) acted as enzyme label to catalyze H2O2 and mediated the conversion of Fe2+/Fe3+. By introducing Fe2+/Fe3+ interconversion as MRS signal output, CPF was detected by the immunoreaction induced variations of the transverse relaxation time (ΔT2). The proposed MRS immunosensor exhibited a detection linear range from 0.01 to 1000 ng mL-1 (R2 = 0.9916), with the limit of detection (LOD) of 6 pg mL-1 (3S/M, n = 3). As a proof-of-concept, CPF was easily detected among five different pesticides at a low concentration level (0.1 ng mL-1), as well as in real samples (apple, tea, and lettuce) with recoveries of 80.70-115.30%. Besides, the sensor can realize one step of "separation and detection" towards CPF with the aid of magnetic nanoparticles, which demonstrate its promising potential for pesticide residue detection in food samples and environment.