The pH plays a critical role on the stability and sensing ability of CSDTC-Au NPs. Therefore, the influence of pH was investigated on the sensing behavior of CSDTC-Au NPs towards Cd2+ ion using PBS pH ranging from 2.0 to 12. Fig. 5a demonstrates the UV–visible spectra of CSDTC-Au NPs in the presence of only PBS pH from 2.0 to 12 (without addition of Cd2+ ion). It can be noticed that the absorption spectra of CSDTC-Au NPs were red-shifted to 668 nm in the pH range from 2.0 to 4.0 which is due to the surface charge neutralization via cleavage of CSDTC ligands on the surfaces of Au NPs, yielding the color change from red to purple. However, the SPR peak and color of CSDTC-Au NPs remain unchanged at pH 5.0–12, which confirms that the CSDTC-Au NPs are stable and well dispersed. Furthermore, we investigated the effect of PBS on the aggregation of CSDTC-Au NPs induced by Cd2+ ion at different PBS pH ranging from 2.0 to 12 ( Fig. 5b). As shown in Fig. 5b, the SPR peak of CSDTC-Au NPs was red-shifted towards longer wavelength by the addition of Cd2+ ion at pH 2.0–12. Since, the red-shift in SPR peak (710 nm) at pH 2.0–4.0 may be attributed to the surface charge neutralization of Au NPs in acidic medium leading to the self-aggregation of CSDTC-Au NPs. Furthermore, the SPR peak of CSDTC-Au NPs was drastically decreased at 523 nm and a new SPR peak was generated at 685 nm by the addition of Cd2+ ion at PBS (pH 5.0–12), which confirms the aggregation of CSDTC-Au NPs induced by Cd2+ ion. These results demonstrate that the interparticle distance of CSDTC-Au NPs was greatly reduced by the formation of coordination covalent bonds between Cd2+ ion and CSDTC-Au NPs in the PBS pH range of 5.0–12.0. However, the maximum absorption ratio (A685nm/A523nm) was observed at pH 8.0 than the other pH. Therefore, PBS pH 8.0 was chosen as the best pH for the sensitive colorimetric detection of Cd2+ ion using CSDTC-Au NPs as a probe