An immunomagnetic electrochemical sensor based an a perfluorosulfonate-coated screen-printed electrode for the determination of 2,4-dichlorophenoxyacetic acid

A disposable immunomagnetic electrochemical sensor involving a magnetic particle-based solid phase and a Nafion film-coated screen-printed electrode (Nafion-SPE) stuck at the bottom of a polystyrene cylinder (microwell of 300 mu L) was developed and evaluated in a competitive immunoassay of the widely used herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The competitive binding of 2,4-D and 2,4-D labeled with alkaline phosphatase (AP) for a limited amount of polyclonal anti-2,4-D antibody-coated magnetic beads was monitored electrochemically by measuring the AP label activity bound to the beads. The phosphoric acid ester of [[(4-hydroxyphenyl)amino]carbonyl]cobaltocenium hexafluorophosphate was used as the AP substrate. This anionic substrate (S-) is enzymatically transformed at pH 9.0 into a cationic phenol derivative (pf) which can be easily accumulated in the polyanionic Nafion coating and determined by cyclic voltammetry. During the enzyme reaction, the AP-associated beads were localized on the surface of the Nafion-SPE with the aid of a magnet, thus effectively increasing the concentration of P+ in the Nafion-modified electrode vicinity. The enzyme generation of pf close to the electrode surface, and thereby to the Nafion film, resulted in a high amplification of the response. A detection limit of 0.01 mu g L-1 2,4-D was thus achieved. The performance of the sensor was successfully evaluated on river water samples spiked with 2,4-D, indicating that this convenient and sensitive technique offers great promise for decentralized environmental applications.