It can be clearly seen that the color of AuNP dispersions (incubated with 3.1 M OPD for 20 min) in the presence or absence of Hg2+ stepwise changes from red to blue with decreasing of the pH value from 9.0 to 4.6 (Fig. S3). Because the color difference in thepresence or absence of Hg2+is most obvious at pH 6.5, the pH value is fixed at 6.5 for the subsequent experiments