A520/A680(ratio of the absorbance value at 520 nm to that at 680 nm in the UV–vis spectra) of the AuNP dispersions (with pH6.5) incubated with OPD (3.1 M) is plotted as a function of OPD incubation time (Fig. S2). Because the A520/A680value decreasessignificantly in 20 min and then keeps almost constant after 20 min,the OPD incubation time is fixed at 20 min as an optimized valuefor the subsequent experiments