G-Quadruplex DNA for Fluorescent and Colorimetric Detection of Thallium(I)

This is the authors' abstract. We don't add key points for this paper.

Thallium is a highly toxic heavy metal, but its sensing is underexplored compared to its neighboring elements in the periodic table: lead and mercury. Thallium has two oxidation states. A DNAzyme-based biosensor for Tl 3+ was reported recently, representing the first work in this area. However, the most environmentally abundant thallium is monovalent Tl +, which is the focus of this work. Since Tl + is similar to K + in terms of size and charge, G-quadruplex DNAs are herein tested for Tl + detection. First, nine dual fluorophore labeled DNA probes are screened. Among them, a DNA designated PS2.M has the largest increase in fluorescence resonance energy transfer (FRET) efficiency upon Tl + addition. This FRET-based assay is directly used as a biosensor yielding a detection limit of 59 μM Tl + . In comparison, K + had a much lower response and the other tested monovalent metals do not produce a significant signal increase. In addition, a colorimetric sensor was developed based on DNA protected gold nanoparticles. When folded by Tl +, the nonlabeled PS2.M DNA cannot effectively adsorb onto gold nanoparticles. This leads to a color change from red to blue upon salt addition. The detection limit is 4.6 μM Tl +, and Tl + spiked in a lake water sample can also be detected. CD spectroscopy is used to further understand Tl + binding to PS2.M. This study demonstrates that DNA can also be used for detecting Tl +, and this work gives rise to a highly effective probe for this purpose.