Fig. 3. Typical UV-vis absorption spectra (A) and time kinetic curves through monitoring the maximum absorbance intensity at 652 nm (B) of TMB–H₂O₂ system in different reaction systems. Inset is the corresponding photographs of three samples. Conditions: 1 (black curve): TMB + H₂O₂, 2 (red curve): TMB + H₂O₂ + Cu NPs, and 3 (blue curve): TMB + H₂O₂ + Cu NPs + 10 μM DA in PBS buffer (pH 4.0) at room temperature.