An electrochemical impedance biosensor for Hg2+ detection based on DNA hydrogel by coupling with DNAzyme-assisted target recycling and hybridization chain reaction

In this work, an electrochemical impedance biosensor for high sensitive detection of Hg2+ was presented by coupling with Hg2+-induced activation of Mg2+-specific DNAzyme (Mg2+-DNAzyme) for target cycling and hybridization chain reaction (HCR) assembled DNA hydrogel for signal amplification. Firstly, we synthesized two different copolymer chains P1 and P2 by modifying hairpin DNA H3 and H4 with acrylamide polymer, respectively. Subsequently, Hg2+ was served as trigger to activate the Mg2+-DNAzyme for selectively cleavage ribonucleobase-modified substrate in the presence of Mg2+. The partial substrate strand could dissociate from DNAzyme structure, and hybridize with capture probe H1 to expose its concealed sequence for further hybridization. With the help of the exposed sequence, the HCR between hairpin DNA H3 and H4 in P1 and P2 was initiated, and assembled a layer of DNA cross-linked hydrogel on the electrode surface. The formed non-conductive DNA hydrogel film could greatly hinder the interfacial electronic transfer which provided a possibility for us to construct a high sensitive impedance biosensor for Hg2+ detection. Under the optimal conditions, the impedance biosensor showed an excellent sensitivity and selectivity toward Hg2+ in a concentration range of 0.1 pM - 10 nM with a detection limit of 0.042 pM Moreover, the real sample analysis reveal that the proposed biosensor is capable of discriminating Hg2+ ions in reliable and quantitative manners, indicating this method has a promising potential for preliminary application in routine tests.