Polydopamine-coated silicon substrates were immersed in 10 mM acetate/phosphate/Tris co-buffers (pH 5.5, 6.4, 7.4, 8.4, and 9.5) with 0.1 mM of His-EG3-Lys for 5 h. Subsequently, 10 mM NHS-Biotin (PIERCE, Rockford, IL) was reacted with the surface-immobilized His-EG3-Lys in 10 mM sodium phosphate buffer (pH 7.8) for 4 h. The substrates were transferred into 10 mM sodium phosphate buffer (pH 7.0), 100 mM NaCl with 1 μg mL-1 streptavidin-horseradish peroxidase for biotin-streptavidin linkage (1 h). Surface-immobilized horseradish peroxidase activity was monitored at absorbance 420 nm using a UV-vis spectrophotometer (HP-8453, Agilent). The substrate was placed inside a UV-vis cuvette, and then solution was added: 2 mL of sodium phosphate buffer (100 mM, pH 6.0), 0.3 mL of pyrogallol solution (10 mg mL-1 dissolved in the same phosphate buffer), and 0.2 mL of 0.4% (v/v) hydrogen peroxide.