Polytyrosine as an electroactive label for signal amplification in electrochemical immunosensors

Small electroactive chemicals (e.g. ferrocene and metals) have been used as labels in electrochemical biosensors, including immunosensors. However, the fact that each of these molecules generates only one electron during oxidation suggests they may not be the most sensitive electroactive labels. An electroactive polymer, which can generate a large number of electrons during electrochemical oxidation, could serve to amplify the electrochemical signal and therefore enhance the detection sensitivity. In this study, polytyrosine was chosen to test this hypothesis. We first characterized the oxidation of polytyrosine by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) on bare and multiwalled carbon nanotubes (MWCNTs)-modified glassy carbon electrodes at different pH and varying amount of MWCNTs. The use of MWCNTs on the electrodes enhanced the electrochemical signal by 84 times both in CV and DPV when compared with bare electrodes. This enabled the detection of as low as 10 ng (or 0.4 pmol) of polytyrosine. Polytyrosine was then conjugated to a prostate-specific antigen (PSA) peptide and used as an electroactive label in competitive electrochemical immunoassays in which a PSA antibody was immobilized on MWCNTs. Addition of the free peptide to the assays decreased the current in a concentration dependent manner. The sensitivity of the detection was calculated to be 12.8% inhibition/nM peptide and the limit of detection was approx. 1 nM. These results therefore demonstrated that polytyrosine could potentially serve as a label for signal amplification in electrochemical biosensors.