Fig.S4 displays the color change during Au-NPs synthesis in the absence and presence of melamine (1.6×10−7M) at different time. Compared Fig.S4 A to Fig.S4B,there is a significant color change of system ‘b’ (with1.6×10−7M melamine) than that of system ‘a’ (with out melamine),indicating the reaction in system ‘b’ was faster than that in system ‘a’.Compared Fig.S4C with Fig.S4D, the obvious color change of system ‘a’ than that of system ‘b’ suggested that the synthesis of Au-NPs in system ‘a’ is slower than that in system ‘b’,that is to say the presence of melamine accelerated the synthesis of Au-NPs. As depicted in Fig.1A, there is an intramolecular H bond formed by adjacent sulfurnate and phenolic -OH in PD in water solution, thus the synthesis of Au-NPs was relatively slow. On the other hand, some free PD molecules were attached to the surface of Au-NPs through sulfurnate. Hydrogen
bonding interactions between neighboring –OH (free PD) and Ogroups (oxidized PD) resulted in the aggregations of Au-NPs. In the presence of melamine,the itramolecular H bond of PD was interrupted by the O···HN formed between PD and melamine (Fig.1B).Therefore,the phenolic -OH was free from intra molecular H bond, which increased the reactivity of PD, and the formation of Au-NPs was accelerated. Meanwhile, Au-NPs were relatively dispersed, which might be ascribed to steric hindrance induced by binding of melamine to PD.