Square wave voltammetry versus electrochemical impedance spectroscopy as a rapid detection technique at electrochemical immunosensors

Square wave voltammetry (SWV) was compared to electrochemical impedance spectroscopy (EIS) in developing a label-free electrochemical immunosensor for the hormone estradiol. The immunosensor consists of a Au electrode anchored with a Au nanoparticle|thiolated Protein G-scaffold to facilitate immobilisation of an enhanced quantity of an almost uprightly aligned anti-estradiol capture antibody. Competitive immunoassays between an estradiol-bovine saline albumin complex and free estradiol in a sample were then promoted at the immunosensor. Next, SWV and EIS of [Fe(CN)₆]^3-/4- were sequentially conducted at the immunosensor. SWV yielded familiar peak-shaped voltammograms with the peak currents readily employable in establishing calibration. A dynamic range up to ∼1200 pg mL^-1 and a detection limit of 18 pg mL^-1 estradiol were achieved. In EIS, an electron transfer resistance estimated from the Nyquist plots was used in the calibration experiments. A comparable dynamic range up to ∼1000 pg mL^-1 and a detection limit of 26 pg mL^-1 estradiol were obtained. However, a significantly 10 times longer analysis time and substantial effort were required to complete the EIS determinations relative to SWV. Moreover, a large amount of EIS data involving phase angle was collected but ignored because they would not contribute any useful information to quantitative determination. Overall, SWV was determined to be a more rapid, efficient, effective and low cost detection technique than EIS at label-free electrochemical immunosensors.