Heating enhanced sensitive and selective electrochemical detection of Hg2+ based on T-Hg2+-T structure and exonuclease III-assisted target recycling amplification strategy at heated gold disk electrode

A sensitive and selective electrochemical Hg2+ sensor was developed based on T-Hg2+-T structure and exonuclease (Exo) III -assisted target recycling amplification at heated gold disk electrode (HAuDE). First, a DNA signal probe P1 was for the first time designed and labeled with ferrocene (Fc) near the attached SH-5′-end, so as to shorten the distance between Fc and the electrode and enhance the initial current of Fc compared with that labeled at the 3′-end far from the electrode. Then the signal amplification was achieved by Exo III-assisted Hg2+ recycling. Briefly, the P1 was complementary to the assistant DNA P2 except the T-T mismatches. In the presence of Hg2+, the P1 self-assembled on the HAuDE could hybridize with P2 and form DNA duplex with blunt end at the 3′- terminus, triggering Exo III to stepwise digest mononucleotides from the 3′-terminus of P1, ultimately liberating Hg2+ and P2, which could be “recycled”, resulting in the digestion of a large amount of P1 and significantly decrease the amount of Fc. The electrochemical signal difference before and after digestion was proportional to the Hg2+ concentration. Furthermore, during the digestion period, the Exo III activity could be significantly increased by elevating the electrode temperature, great improving the sensitivity and efficiency for Hg2+ detection. A detection limit of 6.2  pM (S/N = 3) could be obtained with an electrode temperature of 40 °C during 60 min digestion period, which was lower ca. two magnitudes than that at 0 °C and one magnitude than that at 25 °C.