Fig. 2 shows TEM images of the citrate-stabilized AuNPs at dif-ferent conditions demonstrating the influence of OPD and Hg2+ on the stability of the AuNPs. The AuNPs without OPD and Hg2+is well-dispersed (Fig. 2(a)) with a narrow size distribution (Fig. S1), which is used as a control. The AuNPs incubated with 3.1 M of OPD aggre-gate severely (Fig. 2(b)) with a very broad size distribution (Fig. S1)compared with the control. However, further introducing of Hg2+may inhibit the aggregation to some extent and the inhibition effect is positively correlative with the Hg2+concentration (Fig. 2(c) and(d), Fig. S1). These results verify the above-mentioned mechanism for Hg2+detection, i.e. OPD can easily induce the aggregation of AuNPs and the aggregation could be prevented by Hg2+.