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We report on an on-site ultrasensitive sandwich immunoassay for ractopamine (RAC) that is based on the use of a personal glucose meter. RAC is preconcentrated with the help of magnetic nanoparticles coated with β-cyclodextrin (FeO@SiO-CD), and a novel polymer tag labeled with invertase is employed to catalyze the hydrolysis of sucrose to form glucose which is detected with the glucose meter. Invertase-modified gold nanoparticles were prepared and immobilized, along with antibodies against RAC, on an Envision reagent (a polymer that contains many anti-IgG antibodies). Because of its high loading with the enzyme, the resulting copolymers strongly amplify the signal. The concentration of RAC is calculated on the basis of the linear relationship between the concentration of RAC and the amount of glucose formed. The method exhibits a detection limit of 0.34 μg kg as determined for a sample of pork. This on-site testing method may also be applied to the detection of other low-molecular toxins by simply changing the antibody. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

We report on a novel enzyme-enhanced label for the electrochemical determination of diethylstilbestrol (DES). The label was obtained by orientation-controlled immobilization of a multiplex horseradisch peroxidase (HRP) conjugated polymer on gold nanoparticles (AuNPs) using the Envision reagent (EV) which is an enzyme-polymer complex that contains HRP and anti-IgG antibody in a polydextrin amine skeleton. The AuNPs were modified with Concanavalin A (Con A) and served as a carrier for immobilization of the EV−DES antibody composite. This resulted in a bioconjugate of the type AuNP−Con A−EV−DES Ab which was employed as the label. On exposure to samples containing DES, a sandwich immunocomplex is formed between antibody against DES (which was immobilized on a glassy carbon electrode and is acting as a capture probe), DES (the analyte), and the above label as the signal tracer. Hemin was used as an electronic mediator in the reaction of HRP. The HRP on the label catalyzes the oxidative formation of hydrogen peroxide at pH 7.0, and this induces an increased reductive current in the presence of hemin as an electron mediator. Under optimal conditions, the current increases linearly with increasing concentrations of DES in the range from 5 to 500 pg · mL, with a detection limit as low as 2 pg · mL (at an S/N of 3). The method exhibits high selectivity and good stability. It works without incubation so that the time for an assay is shortened to 5 min. The assays was successfully applied to the determination of DES in milk samples. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]