Furthermore, the absorbance spectra and TEM photographs support this observation. Addition of single component, AChE or ATCh, to the LA capped AuNPs solutions do not lead to any change in the color or dispersing status. While upon TCh adding, the LA capped AuNPs change from a well-dispersed to an obvious aggregation one, and the solution color switches from red to steel-blue, which reflects in the absorbance spectra with a decrease in absorption at 523 nm and generation of a new absorption peak at 625 nm (see Fig. S4 in Supplementary data). The status of LA capped AuNPs in the presence of ATCh, AChE or TCh shown in TEM images (Fig. 2) are quite fit the phenomenon observed by naked eyes and UV–vis spectra. This result is considered to be a consequence of the suppressive effect of cationic TCh on the surface of AuNPs against the coulombic repulsive interaction between the anionic LA stabilized AuNPs. The alteration effect of TCh on the LA capped AuNPs absorbance forms the basis for colorimetric measurement of OPs.