A colorimetric assay is described for sensitive determination of Hg2+ ions based on the MOS2-Au composites as peroxidase mimetics, which are synthesized by microwave-assisted solvothermal method. The addition of Hg2+ stimulates their peroxidase-like activity, along with lower Michaelis constant toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with H2O2, allowing the composites for direct determination of Hg2+. A broad linear response is obtained ranging from 20 nM to 20 mu M with a detection limit (LOD) of 5 nM. The superior peroxidase-like activity is attributed to the large surface area of MOS2 nanosheets and the synergistic catalytic effect of MOS2 and Au. The Hg2+-stimulation effect implies the strong interaction between Hg2+ and MOS2-Au, where the XPS results confirm the presence of metallic Hg-0, indicative of an Au-Hg amalgam. This colorimetric assay is successfully applied for the determination of Hg2+ in environmental water (tap water and Yellow River water) with excellent selectivity over interfering cations. (C) 2018 Elsevier Inc. All rights reserved.