of the citrate-stabilized AuNPs incubated with various concentrations of OPD ranging from 1.6 to 3.4 M. As shown in Fig. 1(a), the color of AuNP dispersion changes more and more obviously fromwinered to blue gray with the increase of OPD concentration from 1.6 to 3.4 M. From the corresponding UV–vis spectra (Fig. 1(b)), it is exactly observed that, with increasing of OPD concentration, the maximum absorption peak at 520 nm decreases and meanwhilea new absorption peak emerges at around 680 nm, which originated from the aggregation of AuNPs. The OPD concentration isfixed at 3.1 M for the subsequent experiments because the colorof AuNP dispersion has completely turned to be blue gray at thisOPD concentration