A sensitive,label-free electrochemical immunosensor based on the combination of polythionine/methylene blue(PTH/MB)and gold nanoparticles(GNPs)amplification was proposed for the determination of okadaic acid(OA).A gold electrode was firstly modified with an electropolymerized film of PTH/MB,which not only provided a stable sensing matrix with abundant amino-groups,but also improved the conductivity of the interface.Subsequently,gold nanoparticle monolayer was assembled onto the resulting surface.Making use of the unique properties of GNPs,antibodies can be self-assembled onto the surface-confined GNPs via amine-Au affinity with a high loading amount and reserve high immunological activity.Due to its ionizable carboxylic and phenolic moieties,OA could offer additional negative charge upon binding to anti-OA.After the introduction of OA,the anion forms of antibody-antigen complex formed on the sensing interface can efficiently block the electron transfer between the redox species and the electrode surface,resulting in a decrease of redox peak current.The current reduction is proportional to amount of analyte.The electrochemical characteristics of the immunosensor were studied by cyclic voltammetry(CV)，electrochemical impedance spectroscopy(EIS) and differential pulse voltammetry(DPV).The factors influencing the performance of the immunosensor were investigated.Under the optimized experimental conditions,the proposed sensing strategy provides a linear dynamic range from 0.2 μg/L to 100 μg/L and the regression equation was ΔI=1.721 7+1.083 6lgρ with a correlation coefficient of 0.992 0.The detection limit was estimated to be 0.1 μg/L.With excellent selectivity and reproducibility,the proposed immunosensor was applied in the determination of OA in real shellfish samples with spiked recoveries of 85.3%-112%.