Gum kondagogu reduced/stabilized silver nanoparticles as direct colorimetric sensor for the sensitive detection of Hg2+ in aqueous system

A highly sensitive and selective method is reported for the colorimetric detection of Hg2+ in aqueous system by using label free silver nanoparticles (Ag NPs). Ag NPs used in this method were synthesized by gum kondagogu (GK) which acted as both reducing and stabilizing agent. The average size of the GK–Ag NPs was found to be 5.0±2.8 nm as revealed by transmission electron microscope (TEM) analysis and the nanoparticles were stable at various pH conditions (pH 4–11) and salt concentrations (5–100 mM). The GK reduced/stabilized Ag NPs (GK–Ag NPs) were directly used for the selective colorimetric reaction with Hg2+ without any further modification. The bright yellow colour of Ag NPs was found to fade in a concentration dependent manner with the added Hg+ ions. The fading response was directly correlated with increasing concentration of Hg2+. More importantly, this response was found to be highly selective for Hg2+ as the absorption spectra were found to be unaffected by the presence of other ions like; Na+, K+, Mg2+, Ca2+, Cu2+, Ni2+, Co2+, As3+, Fe2+, Cd2+, etc. The metal sensing mechanism is explained based on the turbidometric and X-ray diffraction (XRD) analysis of GK–Ag NPs with Hg2+. The proposed method was successfully applied for the determination of Hg2+ in various ground water samples. The reported method can be effectively used for the quantification of total Hg2+ in samples, wherein the organic mercury is first oxidized to inorganic form by ultraviolet (UV) irradiation. The limit of quantification for Hg2+ using the proposed method was as low as 4.9×10−8 mol L−1 (50 nM). The proposed method has potential application for on-field qualitative detection of Hg2+ in aqueous environmental samples.