A label-free GR-5DNAzyme sensor for lead ions detection based on nanoporous gold and anionic intercalator

A label-free electrochemical sensor, based on a classic lead ions (Pb²⁺)-dependent GR-5DNAzyme as the catalytic unit, disodium-anthraquinone-2,6-disulfonate (AQDS) as DNA intercalator, and nanoporous gold (NPG) for signal amplification, was designed for sensitive and selective detection of Pb²⁺. Firstly, NPG modified electrode surface were employed as a platform for the immobilization of thiolated probe DNA, and then, hybridized with DNAzyme catalytic beacons. The Pb²⁺-induced catalytic reaction makes the substrate strand break at the cleavage sitGe irreversibly, which disturbs the formation of DNA strands. AQDS served as an indicator that intercalated into the base-pairing regions of DNAzyme, resulting in a strong electrochemical signal. In the presence of Pb²⁺, the complementary regions were reduced, due to the fracture of the DNA strand, resulting in the release of AQDS. And a decreased current was obtained, corresponding to Pb²⁺ concentration. Taking advantage of the amplification effect of NPG electrode for increasing the reaction sites of thiol modified capture probe, the proposed electrochemical biosensor could detect Pb²⁺ quantitatively, in the range of 1-120nM, with a limit of detection as low as 0.02nM, which is much lower than the maximum contamination level for Pb²⁺ in drinking water defined by the U.S. Environmental Protection Agency. The electrochemical sensor was also used to detect Pb²⁺ from real water samples, and the results showed excellent agreement with the values determined by inductively coupled plasma mass spectroscopy. This biosensor showed a promising potential for on-site detecting Pb²⁺ in aqueous environment.