The effect of scan rate on the oxidation peak current of 0.08 μM BPR was investigated in the 0.01–0.11 V s−1 range of scan rate (Fig. 3B). As shown in the inset of Fig. 3B, the value of peak current reaches itsmaximum at 0.09 V s−1, and then the peak current remains constant. Thus, this value was selected as an optimum scan rate.