Thymine-mediated electrochemical aptasensor for sensitive and simultaneous detection of Hg2+and CH3Hg+in fish samples

In this study, a thymine-mediated electrochemical aptasensor was constructed for instantaneous detection of Hg2+ and CH3Hg+ in fish muscle using ferrocene (Fc) and methylene blue (MB) as electrochemical tags. In the presence of Hg2+, a HP-Fc (ferrocene and thiol-labeled hairpin DNA) was opened and hybridized with a com-plementary S1 (single-stranded DNA) to form a stable thymine-Hg2+-thymine (T-Hg2+-T) structure. Then, the Fc tag got far away from the gold electrode surface, inducing a distinct decrease of its electrochemical signal. Meanwhile, SA_MB (methylene blue labeled single-stranded DNA) was designed to complement with S2 (thiol-labeled single-stranded DNA). When CH3Hg+ was present, the double-stranded DNA was dissociated, and a more stable CH3Hg+-T structure was generated by binding of screened S2 and CH3Hg+. Thus, the SA_MB was away from electrode surface and the significantly reduced current of MB was observed. As a result, Hg2+ and CH3Hg+ were differentiated through the thymine-mediating, realizing the simultaneous detection of Hg2+ and CH3Hg+ without extra separation procedure. Such a sensing strategy allowed sensitive and selective detection of Hg2+ and CH3Hg+ down to the picomolar level, with limits of detection (LOD, S/N = 3) of 0.06 pM and 0.13 pM, respectively. The sensor was finally successfully applied for the detection of Hg2+ and CH3Hg+ in fish samples, and the results were in good agreement with those obtained by liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS). This technology provides a sensitive and simple method for speciation analysis and monitoring of Hg2+ as well as CH3Hg+ in biological samples.