Electrochemical sandwich immunoassay for vitellogenin by sequential injection analysis using antibody immobilized magnetic microbeads

A rapid and sensitive sandwich immunoassay based on a sequential injection analysis (SIA) for the determination of carp vitellogenin (Vg) is described. The SIA system was constructed from a syringe pump, a multiposition valve, a flow-through type immunoreaction cell equipped with a magnet and an amperometric detector. Magnetic microbeads immobilized with an anti-Vg monoclonal antibody (primary antibody) were used as a solid support. The primary antibody was immobilized on magnetic microbeads by coupling the primary antibody with a polylactic acid-layer, which was coated on the surface of the magnetic beads, after activated with N-hydroxysuccinimide. The introduction, trapping and flushing out of the magnetic microbeads in the immunoreaction cell were controlled by the magnet and the flow of the carrier solution. After the primary antibody-immobilized magnetic beads were introduced and trapped in the immunoreaction cell, a Vg sample solution, an alkaline phosphatase (AP)-labeled anti-Vg polyclonal antibody (secondary antibody) solution and a p-aminophenyl phosphate (PAPP) solution were sequentially introduced into the immunoreaction cell based on an SIA programmed sequence. Vg was determined by the electrochemical detection of p-aminophenol (PAP), an enzymatic product of PAPP via the action by AP labeled on the secondary antibody. A solution containing PAP, which was generated in the immunoreaction cell and transiently held in a holding coil, was transported to the amperometric detector and the oxidation current of PAP on a working electrode applied at +0.20 V was measured. A sigmoidal calibration curve was obtained in the concentration range from 1 ppb to 500 ppb in a plot of oxidation current against the logarithm of the Vg concentration. The lower detection limit of the immunoassay was about 2-3 ppb. The time required for an analysis was ca. 15 min/sample.