Highly sensitive and efficient fluorescent sensing for Hg2+ detection based on triple-helix molecular switch and exonuclease III-assisted amplification

Abstract

Here, a novel fluorescent sensing for simple, highly sensitive and efficient detection of Hg²⁺ was developed as joint result of triple-helix molecular switch (THMS) and exonuclease III (Exo III)-assisted signal amplification. In this study, the special structure of THMS was used to realize efficient fluorescence quenching and excellent signal unit transformation to complete the output of signal FAM. In the absence of Hg²⁺, hairpin probe (HP) containing thymine-rich (T-rich) ssDNA strand can induce the dissociation of the THMS, causing FAM far away from BHQ1 and increasing fluorescence intensity. Nevertheless, Hg²⁺ could bind to the thymine (T) base to form the dsDNA with T-Hg²⁺-T structure that stimulates Exo III to digest it from the blunt 3'-terminus to 5'-terminus, causing Hg²⁺ to be released from the dsDNA. The released Hg²⁺ could initiate the next cycling, allowing a large number of hairpin probes to be cleaved by Exo III to form ssDNA. These ssDNA could inhibit the switch dissociation of THMS, causing a dramatic decrease in the fluorescence signal. This allowed for the highly sensitive detection of Hg²⁺ at concentrations as low as 1.04 pM. In addition, the sensing showed a linear detection range of 0.01-50 nM and was used for the assay of Hg²⁺ in real samples of Xiangjiang river water and tap water. These results showed that the provided fluorescent sensing has a good application prospect in environmental and food monitoring.

Keywords: Exonuclease III; Fluorescence; Mercury (Hg(2+)); Thymine-mercury-thymine; Triple-helix molecular switch.