Intrinsic peroxidase-like activity of 4-amino hippuric acid reduced/stabilized gold nanoparticles and its application in the selective determination of mercury and iron in ground water.

Herein, we report a method for the synthesis of 4-aminohippuric acid (4-AHA) reduced/stabilized gold nanoparticles and their peroxidase mimicking properties for the colorimetric detection of Fe and Hg. The synthesis of nanoparticles was evidenced by appearance of bright red color and an absorption peak at 518 nm. Transmission electron microscopic (TEM) characterization revealed the nanoparticles to be spherical with average size of about 5.9 ± 1.7 nm. X-ray diffraction (XRD) analysis established highly crystalline nature of the nanoparticles. The synthesized nanoparticles have shown very good peroxidase mimicking property; exhibiting the catalytic oxidation of the chromogen 3,3',5,5'-tetramethyl benzidine (TMB) to a blue color product, in the presence of hydrogen peroxide. The peroxidase mimicking activity of the nanoparticles was found to be selectivity enhanced in the presence of Fe and Hg while there was no change in the activity in the presence of other concomitant ions. The mechanism studies revealed that the synthesized gold nanoparticles assisted in electron transfer during the catalytic process however the stimulation of peroxidase-like activity in the presence of Fe and Hg is owed to both generation of hydroxyl radical and accelerated electron transfer. The assay was made selective for iron by the addition of cysteine in acetate buffer; whereas the selective detection of mercury was achieved by carrying out the assay in citrate buffer. The linear ranges for the determination of Fe and Hg in deionized water were found to be: 5-50 ppb and 5-200 ppb respectively. The limits of quantification (LOQ) for Fe and Hg were 4.0 and 2.5 ppb respectively. The assay was applied for the determination of Fe and Hg in drinking and ground water samples. The method holds potential for the on-field screening of these metal ions in real environmental samples.