When the AuNPs are well dispersed in solution, the colloidexhibit ruby-red color with a strong absorption peak at 520 nm and an extremely high extinction coefficients [41]. However, the color changes from red to blue and a red shift of surface plasmon absorption would occur after aggregation of the AuNPs [42]. Therefore,the visualization of the AuNP aggregation could be observed by the naked eyes, and the quantification could be monitored by UV–visspectra. Fig. 1 shows the photographic image and UV–vis spectra