Metal-ion-induced DNAzyme on magnetic beads for detection of lead(II) by using rolling circle amplification, glucose oxidase, and readout of pH changes

This work reports on a method for determination of lead(II) ion in environmental water samples. A Pb2+-specific DNAzyme immobilized on magnetic beads was coupled to rolling circle amplification (RCA) and pH meter-based readout. On addition of Pb2+ ion, it induces partial cleavage of the DNAzyme on the magnetic beads. The single-stranded DNA remaining on the magnetic beads is used as the primer to trigger the RCA reaction with the assistance of a circular DNA template, polymerase and dNTPs. This results in the formation of numerous oligonucleotide repeats on the magnetic bead. Subsequently, these repeats hybridize with glucose oxidase-labeled single-stranded DNA (GOx-ssDNA) to form a long concatamer containing tens to hundreds of GOx-ssDNA tandem repeats. The concatenated GOx molecules oxidize glucose, and this is accompanied by a drop in the local pH value. The pH values (vs. background signal) drop linearly when Pb2+ concentrations increase from 1.0-100nM, and the detection limit is 0.91nM. The method displays good reproducibility, high specificity and acceptable accuracy. It was applied to the analysis of spiked water samples, and results compared favorably with those obtained by ICP-MS.