A520/A680(ratio of the absorbance value at 520 nm to that at680 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 OPDincubation 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