Fig. 2 shows the differential pulse voltammograms of 0.08 μM BPR at CPE in the presence of SDS. The anodic peak current of BPR achieves the highest value at the pH value of 3.0, therefore this pH value was chosen for further study. Also, peak potential shifted to negative value with increasing of pH. The inset B of Fig. 2 displayed the relationships between the oxidation peak potential of BPR and pH. A linear relationship with equation of E(V) = −0.0457 pH + 0.7466 (R2 = 0.9948) suggests that equal electron and proton are involved in the oxidation of BPR at the CPE.