A QCM immunosensor to rapidly detect ractopamine using bio-polymer conjugate and magnetic β-cyclodextrins

An enzyme-amplified magnetic immunosensor was developed for the sensitive and selective detection of ractopamine (RAC) in fodder using a quartz crystal microbalance (QCM). Magnetic capture probes were prepared by immobilizing β-cyclodextrins (β-CD) on Fe3O4 nanoparticles in order to enrich RAC. PowerVision (PV), a kind of enzyme–polymer complex with a high enzyme-to-antibody ratio, served as a signal label. Horseradish peroxidase (HRP) on the immnunocomplex, which arises from sandwich-type immunoreactions, causes 3,3′,5,5′-tetramethylbenzidine (TMB) to produce large amounts of sedimentation with a mass proportional to RAC. The large amounts of HRP on the PV greatly amplify the QCM sensor's signal. Under optimum conditions, the differences of the QCM frequency (Δf) are proportional to the concentration of RAC over the range from 0.03 to 25 ng mL−1. The limit of detection was 0.01 ng mL−1. More importantly, this assay's results in swine fodder samples were consistent with those of ELISA, with recoveries between 92.3% and 110%. Due to its high sensitivity, portability and selectivity, the immunosensor achieved reliable quantification of RAC in real fodder samples and has the potential to become an on-site screening tool to enhance food safety.