Thioglycolic acid-functionalized gold nanoparticles: Capping agent-affected color perception stability towards nitrate sensing purpose

Abstract

In comparison with colorimetric detection of metal ions, inorganic anions sensing on gold nanoparticles (AuNPs) seems to be more challenging and less explored due to their weak affinity towards the surface of such metal nanoparticles. Early efforts to detect nitrate in water mainly focused on amino thiol-functionalized AuNPs, thanks to the gold-sulfur interactions, sometimes adjusting the pH was required. In the present work, thioglycolic acid was preliminarily introduced for functionalizing purpose, in combination with the stability of colloidal AuNPs suspensions capped by either sodium citrate or ascorbic acid, which have not been previously reported. In contrast to the precipitation observed on ascorbic acid-capped AuNPs, the stability of color perception of freshly citrate-capped AuNPs and corresponding functionalized samples during 10 days extended their realistic nitrate sensing applications. A straightforward protocol involving three successive stages (synthesis of AuNPs, functionalizing and colorimetric sensing) exposed the color variations from red to purple and then blue, with the detection limit of nitrate of 50 ppm, qualifying the nitrate limit of drinking water authorized by the World Health Organization (WHO). The color signals were strictly relevant to surface plasmon resonance and particle size, evidenced by ultraviolet – visible spectroscopy (UV–vis) and transmission electron microscopy (TEM), respectively.