Abstract:

Thallium (Tl) is a highly toxic heavy metal situated between mercury and lead in the periodic table. While its neighbors have been thoroughly studied for DNA-based sensing, little is known about thallium detection. In this work, in vitro selection of RNA-cleaving DNAzymes is carried out using Tl3+ as the target metal cofactor. Both normal DNA and phosphorothioate (PS)-modified DNA are tested for this purpose. While no Tl3+-dependent DNAzymes are obtained, a DNA oligonucleotide containing a single PS-modified RNA nucleotide is found to cleave by ∼7% by Tl3+ at the RNA position. The remaining 93% are desulfurized. By hybridization of this PS-modified oligonucleotide with the Tm7 DNAzyme, the cleavage yield increases to ∼40% in the presence of Tl3+ and Er3+. Tm7 is an Er3+-dependent RNA-cleaving DNAzyme. It cleaves only the normal substrate but is completely inactive using the PS-modified substrate. Tl3+ desulfurizes the PS substrate to the normal substrate to be cleaved by Tm7 and Er3+. This system is engineered into a catalytic beacon for Tl3+ with a detection limit of 1.5 nM, which is below its maximal contamination limit defined by the U.S. Environmental Protection Agency (10 nM).