Fig. 6. (a) Differential pulse voltammograms with various DA concentrations (0.1, 1, 3, 5, 7 10, 20, 25, 30, 35, 40, 50, 60 μM) at Pt–Ag/Gr/GCE in 0.1 mol L⁻¹ PBS at pH 6.5. (b) Relationship between peak current and DA concentration. (c) The calibration curve in low concentration range.