A multichannel electrochemical detector coupled with an ELISA microtiter plate for the immunoassay of 2,4-dichlorophenoxyacetic acid

A multichannel electrochemical detector (MED) consisting of eight sets of electrodes held on an ELISA microtiter plate for immunoassay of a herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) was described. The eight-channel electrodes were characterized in terms of repeatibility, homogeneity and stability. The direct competitive immunoassay format was employed for the determination of 2,4-D. Monoclonal antibody (mAb) against 2,4-D, alkaline phosphatase-2,4-D (ALP-2,4-D) and p-aminophenyl phosphate (p-APP) were used as coating protein, enzyme labeled conjugate and substrate, respectively. The immuno-reaction and enzymatic reaction were carried out in an ELISA microtiter plate. The p-aminophenol (p-AP) produced was amperometrically detected by the pre-equilibrated eight-channel electrodes in another plate. The current responses measured were inversely proportional to the 2,4-D concentration. Under optimal conditions, the standard calibration curve was constructed in the dynamic range of 0.1–330 ng/ml and the detection limit of 0.072 ng/ml was achieved. The recoveries of 2,4-D in tap water samples spiked at the concentration of 0.1, 1.0, 10 and 100 ng/ml were found within 89.0–116.0%, and the relative standard derivation (R.S.D.) values of the intra-assay and inter-assay were less than 20%. The advantages of the presented multichannel electrochemical detector coupled with an ELISA plate for immunoassay were those with high stability and throughput, low detection limit, the ability of being repeatedly used and no need for regeneration.