In this work, the colorimetric detection assay of mercury has been proposed based on silver nanoparticles (AgNPs) aggregation. In basic medium with 1.75 mmol/L NaOH, the stable and monodisperse silver nanoparticles, which coated by polydopamine, was prepared by a one-pot reduction process using dopamine as a reducing agent. The polydopamine adsorption with Hg²⁺ can induce the aggregation of silver nanoparticles resulting in the decrease of absorption signal of AgNPs at 405 nm and the color change of the solutions. It was found that the decreased absorption signal intensity was proportional to the mercury content in a range of 0.5～5.0 μmol/L. According to this, the ultraviolet-visible absorption spectrum method and colorimetric method for determination of Hg²⁺ was developed. The limit of detection is 50 nmol/L. In this work, UV-vis absorption spectrum, scanning electron microscope (SEM) and infrared (IR) spectrum were investigated. The mechanism of this reaction was discussed. The IR spectrum of special functional groups approved that polydopamine was formed on the surface of AgNPs. The UV-Vis absorption peak at 405 nm and SEM photos indicated that the stable and monodisperse AgNPs was prepared and the aggregation of AgNPs was occurred after the addition of Hg²⁺. The tolerance of coexisting foreign substances in system was also studied and the experimental results indicated that the interferences of other common metal ions were small. The experimental condition optimization results show that when the NaOH concentration is 1.75 mmol/L and dopamine concentration is 40 μmol/L, the system has a good response for Hg²⁺. The proposed method was successfully used to detection of Hg²⁺ in tap water with a recovery range of 93.0% to 105.5% and RSD≤4.4%. This method show some advantages including low-cost, rapidity, simplicity and selectivity. Moreover, it can achieve a visual sensing of Hg²⁺.

Key words: dopamine, nanoparticles, mercury, colorimetric method, UV/Vis spectroscopy