Ultrasensitive electrochemical detection of Pb²⁺ based on rolling circle amplification and quantum dots tagging.

A novel highly sensitive and selective electrochemical sensing system based on DNAzyme and rolling circle amplification (RCA) for the determination of Pb²⁺ was developed in the present study. Firstly, the DNAzyme catalytic strands were immobilized onto magnetic beads surface and then hybridized with substrate strands. In the presence of Pb²⁺, the DNAzyme could be activated to cleave the substrate strand into two DNA fragments. After RCA reaction, a long ssDNA product with repeating sequence was obtained. Subsequently, CdS QDs modified ssDNA (CdS QD-ssDNA) were used as signaling probes to hybridize with the long ssDNA product. Due to the dramatic signal amplification by the numerous QDs and the low background signal by magnetic separation, ultra-low level (7.8 pM) of Pb²⁺ could be detected. Furthermore, with the application of Pb²⁺ dependent DNAzyme, the proposed sensing system exhibited high selectivity. The proposed sensing system showed a promising potential for on-site testing and would gain wide applications in real sample analysis.