A label-free DNAzyme sensor for lead(II) detection by quantitative polymerase chain reaction.

A label-free sensor was developed for sensitive detection of lead(II), combining high selectivity of a Pb(2+)-dependent DNAzyme with enormous signal amplification of quantitative polymerase chain reaction (QPCR). Specifically, a substrate strand was designed to have two primer-hybridization sequences at either terminus. The presence of lead ion (Pb(2+)) catalyzed cleavage of the substrate strands. This resulted in a concentration decrease of the substrate strand that could be detected by QPCR. Compared with existing DNAzyme-based protocols for Pb(2+) assay, this strategy circumvented the use of various optical or electrical labels that might be difficult to be synthesized. Also, the incorporation of QPCR furnished our approach with high sensitivity and superb reproducibility. In addition, QPCR allowed an immediate quantification of the cleavage efficiency that could be useful for evaluation of the DNAzyme activity. The results obtained revealed that our approach exhibited a dynamic response toward Pb(2+) within a three-decade concentration range from 10 nM to 5 microM with a detection limit of 1 nM. This approach also demonstrated good selectivity against other metal ions that commonly coexisted with Pb(2+).