Immunosensor with well-separated polyaniline nanowires for determination of isoelectric point based on electrochemical impedance spectroscopy

An immunosensor, with well-separated polyaniline (PANI) nanowires made by using an anodic aluminum oxide (AAO) template, is developed to detect the isoelectric point (pI) of a specific protein. The AAO template, with a pore diameter of 120 nm and thickness of 138 um, is made by the anodization process. The PANI nanowires are sequentially synthesized by potentiostatic electrochemical deposition within the AAO template. Consequently, the well-separated PANI nanowires are expected to provide a larger surface area for the immunoreaction, as well as the stable binding of antibodies through amino groups on PANI. In our experiments, we utilize a specific antibody (galectin-1) for the immunosensing of the bladder cancer cell lysate (T24). The measurement results by electromechanical impedance spectroscopy (EIS) show that the impedance variations increase after immunosensing, with the buffer pH values ranging from 3~5; however, in contrast, the impedance decreases after immunosensing in a buffer with pH 6~7. This result demonstrates that the net surface charge of a protein may change from positive to negative as the buffer pH varies between 5 ~ 6. The pI of galectin-1 protein, therefore, can be determined with a pH range of 5 ~ 6.