Spectrophotometric determination of mercury(II) ions based on their stimulation effect on the peroxidase-like activity of molybdenum disulfide nanosheets

The authors describe a colorimetric method for the determination of Hg(II) ions by exploiting the peroxidase-lile activity of few-layered MoS2 nanosheets (MoS2-NSs). These were prepared by sonication-induced exfoliation of bulk MoS2 crystals in aqueous surfactant solution. The MoS2-NSs were found to acts as a peroxidase mimic that is capable of oxidizing the substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2) to give a blue product with an absorption maximum at 652 nm. The addition of Hg(II) strongly accelerates the kinetics of this reaction. It is shown that the enzyme mimic possesses a high affinity for TMB and a lower pseudo-Michaelis-Menten constant. The stimulating effect of Hg(II) is seriously influenced by the change of surface charge. The use of nanosheets covered with (negatively charged) polystyrene sulfonate results in a decrease in the formation of blue dye, while those covered with (cationic) poly(diallyldimethyl ammonium) ions cause a small increase. Under optimal conditions, the peroxidase-like activity of MoS2-NSs is affected by Hg(II) in the 2.0 to 200 mu M concentration range. The method has a detection limit (LOD) of 0.5 mu M which is much below the allowed level in cosmetics (1 ppm; ca. 5 mu M). The method display excellent sensitivity, selectivity and stability. It was applied to the determination of total mercury in cosmetic samples, and results compared well with results obtained by ICP-AES.