A rapid and sensitive colorimetric assay method for Co2+ based on the modified Au nanoparticles (NPs): Understanding the involved interactions from experiments and simulations

We previously reported a colorimetric assay method for Co2+ based on the thioglycolic acid (TGA) functionalized hexadecyl trimethyl ammonium bromide (CTAB) modified Au NPs. However, the detection limit of 3 × 10−7 M was still higher than that of the sanitary standard for drinking water (6.8 × 10−8 M). In addition, the interactions between the modifier and Au NPs, and between the modifier-Au NPs and Co2+ remain to be clarified and confirmed. Thus, in the present study, the modified Au NPs solution was dialyzed and its detection limit was optimized to be 5 × 10−10 M. The interactions between the modifier and Au NPs, and between the modifier-Au NPs and Co2+ were investigated in both experimental characterizations and theoretical calculations, consistently confirming that the Au NPs were modified by the negatively charged anions of [SCH2CO2]2− through Au–S bonds and Co2+ was recognized by the modifier-Au NPs through CoO chelate bonds. The results of X-ray photoelectron spectroscopy (XPS) suggest that there were no chemical bonds formed between CTAB and Co2+. Moreover, the colorimetric assay of Co2+ using the modified Au NPs has been proved to be a rapid, very sensitive and highly selective method. The validation of the method was carried out by analysis of a certified reference material, GSBZ 50030-94.