Fig. 3. (a) The detection range of the immunosensor is from 2.5 ng mL⁻¹ to 2.5 × 10⁴ pg mL⁻¹. (b) Quantitative measurements of the peak currents as a function of the concentration of AFP. (c) Specificity investigation of the electrochemical immunosensor for AFP (25 ng mL⁻¹), CEA (1 μg mL⁻¹) and PSA (1 μg mL⁻¹); (d) the reproducibility study of the immunosensor; (e) the time stability study of the immunosensor.