surface of the as-formed AgNPs and change the surface pro-perties of AgNPs when AgNPs possessed weak catalysis. The decreased catalytic ability of AgNPs resulted in extremely weak fluorescence intensity and light yellow color as shown in Fig. 1B(d). Furthermore, the introduction of Hg2+ ions in the absence of Ag+ had almost no adverse impact (Fig. 1B(c)). From the phenomena mentioned above, we speculated that a fluorescence method for the detection of Hg2+ can be proposed.
In order to confirm the effect of Hg2+ on the Ag autocata-lysis, some control experiments were performed. Firstly, 16 metal ions were introduced alone into the OPD solution, respectively. Except for Ag+, there was no obvious fluorescence and color observed (ESI, Fig. S3†). Secondly, the inhibitory effects of various metal ions on the OPD–Ag+ autocatalysis system were also verified. The time-dependent fluorescence variations of Ag autocatalysis in the presence of 3 µM of certain metal ions are shown in Fig. 2. Unlike other metal ions, Pb2+ was capable of enhancing the Ag(I)-mediated oxi-dation of OPD in the initial stage. As the reaction progressed, the fluorescence intensity reached almost the same level as the blank sample. However, the addition of Cu2+, Fe2+, Co2+, Ni2+ and Hg2+ led to more or less inhibition. It is believed that the selective enhancement or inhibition effect is mainly attributed to the interaction between the metal ions and the as-formed AgNPs. Among these five inhibitory ions, Hg2+ ions exhibited a much lower IC50 value (concentration causing 50% inhibition) of 1.133 µM than that of Fe2+ ions (6.223 µM), Cu2+ ions (9.418 µM), Co2+ (31.19 µM) and Ni2+ (7.294 µM) (ESI, Fig. S4A–E†), thus revealing good selectivity toward Hg2+. Thirdly, Hg2+ che-lating agents, such as the ethylenediamine tetraacetic acid disodium salt (EDTA·Na2·H2O), were introduced to further illustrate the inhibiting effect of Hg2+. With the addition of EDTA, Hg2+ ions were shielded and OPD could be oxidized to OPDox with the appearance of a strong signal output. The fluor-escence intensity could be effectively restored and returned to 103% of that in the absence of Hg2+ (ESI, Fig. S5†).