Fig. 6(a) shows the UV–vis absorption spectra of the AuNPs-based detection systems (containing 3.1 M of OPD) incubated with various concentrations of Hg2+. From Fig. 6(a), it is found that,with the increase of Hg2+concentration, the absorption intensity at 680 nm decreases and that at 520 nm increases. That’s because Hg2+inhibits the aggregation of the AuNPs induced by OPD. TheLOD of the Hg2+detection system is observed to be 5 nM by UV–vis spectroscopy, which is lower than the mercury toxic level (10 nM)defined by the US Environmental Protection Agency and reachesthe sanitary standard of drinking water of China [26]. Fig. 6(b)shows the plot of A520/A680(ratio of the absorbance value at 520 nmto that at 680 nm in the UV–vis spectra) as a function of Hg2+concentration. We can see that the A520/A680 value increases fast with Hg2+concentration lower than 2.00 M, but cease to increase with Hg2+concentration higher than 2.00 M. The inset in Fig. 6(b)shows a plot of A520/A680versus Hg2+concentration in the range of 0.01–2.00 M. We can find that it’s a good linear relationship(R2= 0.9989) between A520/A680and Hg2+concentration in the range from 0.01 to 2.00 M, which indicates that our detection system is applicable for the quantitative assay of Hg2+. Furthermore,this method is also highlighted by its selectivity and sensitivity compared to other various typical techniques as shown in TableS1.