The invention provides a
highly sensitive immunoassay for detection of a
biological species. The
immunoassay comprises exposing an
electrode to an
analyte liquid putatively containing the
biological species so as to couple the
biological species, if present in the
analyte liquid, to a binding
antibody on the
electrode. The
electrode comprises a binding
antibody and an anchor group, each being coupled to an
electrically conductive substrate, said binding
antibody being capable of binding to the biological species and said anchor group being capable of binding to a
redox polymer. The electrode is then exposed to an antibody-
enzyme liquid comprising an antibody-
enzyme species, said antibody-
enzyme species comprising a detection antibody capable of binding to the biological species, said detection antibody being coupled to a
redox enzyme, whereby, if the
analyte liquid comprises the biological species, the
redox enzyme couples to the electrode by means of the
coupling of both the detection antibody and the binding antibody to the biological species. The electrode is then exposed to a
polymer solution comprising the redox
polymer and to an enzyme substrate, whereby if the redox enzyme is coupled to the anchor group on the electrode the redox polymer is reduced and couples to the anchor group on the electrode. A
voltage is then applied between the electrode and a
reference electrode and the electrode is exposed to an oxidisable species, whereby a magnitude of an
electric current between said electrode and a
reference electrode is indicative of the presence or absence of the biological species.