“…4B, the peak current increased with the pH value of the supporting electrolyte from 5.0 to 7.0, and then decreased steeply when the pH value further increased to 9.0. The reason could be attributed to the protonation states of the trans ligands of the heme iron and amino acids around the heme, or the protonation of the water molecule coordinated to the central iron [40,41], which can increase the barrier for the electron transfer [42,43]. Thus, the phosphate buffer solution with the pH value of 7.0 was selected as the supporting electrolyte in the following experiments.…”