An ultrasensitive electrochemical biosensor for uranyl detection based on DNAzyme and target-catalyzed hairpin assembly

The authors describe a highly sensitive platform for the determination of uranyl ion (UO2 2+) that is based on the integration of an UO2 2+-specific DNAzyme and a target-catalyzed hairpin assembly. Each DNAzyme consists of an enzyme strand (E-DNA) and a substrate strand (S-DNA). The S-DNA is selectively cleaved in the presence of UO2 2+. The released fragment of S-DNA hybridizes with hairpin probe 1 (H1) immobilized on a gold electrode. This is accompanied by unfolding of H1 which induces the hybridization with hairpin 2 (H2). The DNA fragment spontaneously dissociates from the surface and then initiates the next hybridization cycle. After multiple cycling, the electrochemical probe Methylene Blue (MB) is added to intercalate into the minor grooves of the dsDNA. The combination of signal amplification and the use of a highly specific DNAzyme results in a highly sensitive sensor for UO2 2+. At a typical working voltage of −0.22 V (vs Ag/AgCl), a linear response is found range for the 10 pM to 1 nM concentration range, and the detection limit is as low as 2 pM. This strategy provides a powerful and generally applicable tool that may be extended to other metal ions for which specific DNAzymes and signal probes are available.