Pseudo homogeneous amperometric immunosensor for the detection of 2,4-D based on a displacement format

A pseudo-homogeneous amperometric displacement immunosensor for the detecti on of the herbicide 2,4-dichlorophenoxyacetic acid is described. The displa cement assay is performed on disposable screen-printed carbon electrodes wi thout any washing steps and takes advantage of the cross-reactivity of a mo noclonal anti-2,4-D antibody toward immobilised 2-methyl-4-chlorophenoxyace tic acid (MCPA). Due to the low affinity of the antibody toward MCPA (cross -reactivity of approximately 30%), the addition of 2,4-D results in a displ acement of previously bound antibody. Subsequently the peroxidase label of the remaining anti-2,4-D antibodies is detected at the carbon electrode by adding hydrogen peroxide and KI as redox mediator, which shuttles the elect ron transfer between the enzyme and the surface of the electrode at a poten tial of 0 V vs Ag/AgCl. This pseudohomogeneous assay is compared with a het erogeneous assay format, which includes a washing step after the displaceme nt reaction. To avoid the addition of hydrogen peroxide the co-immobilisati on of glucose oxidase was also investigated. With the pseudo-homogeneous as say format a detection limit of approximately 0.1 mug/l 2,4-D was obtained, which complies with the European Union Drinking Water Directives. Data abo ut 2,4-D recovery in spiked water samples are presented.