Gold electrodes modified with short-chain ss-DNA oligonucleotides were used for trace-level voltammetric determination of mercury cation. External redox marker, methylene blue, was employed for these measurements. Due to the competition between Hg2+ and methylene blue, current signal decreases proportionally to the analyte concentration. It was shown that 5'-SH-(CH2)(6)-(T)(15)-3' oligonucleotide, when employed as a receptor layer, induces the strongest electrochemical signal, among all tested DNA sequences. Electrochemical impedance spectroscopy measurements confirmed a specific interaction between the oligonucleotide receptor layer and Hg2+ ions. Proposed biosensor exhibited significant selectivity toward mercury ion, with strong discrimination of other common metal cations. The remarkably low detection limit of 4.63 nmol.L-1 for Hg2+ ions was calculated for proposed biosensor. The usefulness of developed biosensor was verified by its application for analysis of Hg2+ in aqueous samples. (C) 2013 The Electrochemical Society. All rights reserved.