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An on-site ultrasensitive sandwich immunoassay was developed for the detection of ractopamine (RAC) using magnetic calixarene as capture probe and Antibody (Ab)-Envision polymer-invertase on Au NPs (EV-Ab/Au-INT) as signal labels.Fe3O4@SiO2-calixarene nanocomposites were used to capture one of phenolic hydroxyl of ractopamine.The antibody bound to the probe can combine with the residue groups in RAC due to the specific antigen-antibody reaction.The quantification is based on invertase conjugate,thereby transforming the concentration of ractopamine into glucose through invertase-catalyzed hydrolysis of sucrose.The concentration of RAC can be calculated based on the linear relation between RAC and the glucose.The developed method in this work shows good stability and reproducibility.This new method could be applied for detecting any other small molecular toxins with the corresponding antibody.