of the citrate-stabilized AuNPs incubated with various concentra-tions of OPD ranging from 1.6 to 3.4 M. As shown in Fig. 1(a), thecolor of AuNP dispersion changes more and more obviously fromwine-red to blue gray with the increase of OPD concentration from1.6 to 3.4 M. From the corresponding UV–vis spectra (Fig. 1(b)), itis exactly observed that, with increasing of OPD concentration, themaximum absorption peak at 520 nm decreases and meanwhilea new absorption peak emerges at around 680 nm, which origi-nated 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