A label-free electrochemical biosensor based on magnetic biocomposites with DNAzyme and hybridization chain reaction dual signal amplification for the determination of Pb2+

A highly sensitive and selective electrochemical biosensor for Pb2+ with a dual-amplification strategy is proposed. The first amplification step was realized by the cycle of Pb2+ and 8–17 DNAzyme (S2), and the hybridization chain reaction (HCR) triggered by S1 further amplified the electrochemical signal. Fe3O4@Au NPs, as a multifunctional magnetic carrier, is not only manifested in the construction of a magnetically controlled electrochemical response interface, but also has significant contribution in the purifying system, reducing interference, increasing the specific surface area, and the DNA loading. The magnetic nanocomposites were characterized by TEM as spheres with particle size of around 39 nm. When there was no Pb2+, long double-strand DNA (dsDNA) is formed on the surface of Fe3O4@Au NPs by the S1-triggered HCR; in the presence of Pb2+, S2 is activated and S1 on the surface of magnetic biocomposites (Fe3O4@Au NPs-S1) is continuously cleaved with the cycle of Pb2+ and S2, leading to a significant decrease of methylene blue (MB) absorbed on dsDNA. Such reverse dual-signal amplification strategy effectively increased the current difference and improved the sensitivity of the proposed sensor. The electrochemical signal of MB was obtained by differential pulse voltammetry (DPV) with preconcentration, showing a linear response toward Pb2+ ranging from 50 pM to 1 μM with a detection limit of 15 pM. The proposed method has feasible applications in detecting other heavy metal ions based on other metal-dependent DNAzyme.