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Nanostructured gold electrodes have been widely used for electrochemical trace analysis of Hg2+ because Hg2+ has superior specificity for the formation of gold amalgam. However, it is still very challenging to achieve both high sensitivity and reproducibility, especially for real environmental water samples due to surface poisoning by organic residues. In this work, we developed two strategies for addressing these challenges. First, we added NaBr (with a final concentration of 0.01 M) to the analytes; this enhanced the sensitivity by two orders of magnitude and improved the reproducibility to be better than RSD <15% because of the synergetic interaction of Br− with Hg2+ and gold at the interface. Second, we developed a pre-oxidation method using a glassy carbon electrode to remove organic residue from the analyte before Hg2+ analysis. The combination of these two approaches results in an efficient, reliable, and fast electrochemical detection technique for on-site trace analysis of Hg2+ that is in concentrations lower than that required by WHO for drinking water. Image 1 • The batch preparation of a novel gold micro-nanostructured electrode. • The significantly improved sensitivity and reproducibility with the addition of small amount of Br−. • The effective removal of organic residues in ambient water by simple electrochemical oxidation pretreatment. [ABSTRACT FROM AUTHOR]