Label-Free Electrochemical Biosensor of Mercury Ions Based on DNA Strand Displacement by Thymine-Hg(II)-Thymine Complex

A novel electrochemical biosensor for the determination of mercury ions in aqueous solution has been reported. The sensing substrate with a layer of gold nanoparticles on the gold electrode surface was formed through 1,6-hexanedithiol connection, which could enhance the surface loading of the capture DNA probe and improve the electron transfer performance. Another DNA probe used as mercury ion specific binding (MSB) probe hybridized with the capture probe. In the presence of mercury ions, the easy formation of thymine-Hg2+-thymine structure destabilized the hybrid complex and caused MSB probe released from the interface. As a result, the amount of electroactive indicator Methylene blue (MB) adsorbed by the remained DNA on the electrode surface decreased, which weakened the current signal. Therefore, the change of the redox current could reflect the concentration of the analyte. A linear relationship between the current signal and the logarithm of the target concentration up to 500 nM was obtained, with a detection limit of 0.32 nM. The fabricated sensor is shown to exhibit high sensitivity, desirable selectivity and excellent application in real sample analysis.