A ratiometric electrochemical biosensor for sensitive detection of Hg2+ based on thymine–Hg2+–thymine structure

• A ratiometric electrochemical biosensor based on thymine–Hg2+–thymine structure.
• Hg2+ was sensitively detected by the developed electrochemical biosensor.
• The ratiometric electrochemical biosensor shows excellent analytical performance.

In this paper, a simple, selective and reusable electrochemical biosensor for the sensitive detection of mercury ions (Hg2+) has been developed based on thymine (T)-rich stem–loop (hairpin) DNA probe and a dual-signaling electrochemical ratiometric strategy. The assay strategy includes both “signal-on” and “signal-off” elements. The thiolated methylene blue (MB)-modified T-rich hairpin DNA capture probe (MB-P) firstly self-assembled on the gold electrode surface via Au–S bond. In the presence of Hg2+, the ferrocene (Fc)-labeled T-rich DNA probe (Fc-P) hybridized with MB-P via the Hg2+-mediated coordination of T–Hg2+–T base pairs. As a result, the hairpin MB-P was opened, the MB tags were away from the gold electrode surface and the Fc tags closed to the gold electrode surface. These conformation changes led to the decrease of the oxidation peak current of MB (IMB), accompanied with the increase of that of Fc (IFc). The logarithmic value of IFc/IMB is linear with the logarithm of Hg2+ concentration in the range from 0.5 nM to 5000 nM, and the detection limit of 0.08 nM is much lower than 10 nM (the US Environmental Protection Agency (EPA) limit of Hg2+ in drinking water). What is more, the developed DNA-based electrochemical biosensor could be regenerated by adding cysteine and Mg2+. This strategy provides a simple and rapid approach for the detection of Hg2+, and has promising application in the detection of Hg2+ in real environmental samples.

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