Ethynyl and π-stacked thymine–Hg2+–thymine base pairs enhanced fluorescence quenching via photoinduced electron transfer and simple and sensitive mercury ion sensing

Ethynyl triggered enhanced fluorescence quenching effect was first reported based on photoinduced electron transfer (PET) between fluorophore and ethynyl. Ethynyl and 6-carboxyl-fluorescein (FAM) were labeled at the 5′- and 3′-termini of a poly d(T) sequence containing three thymine units, respectively. In the presence of Hg 2+ , Hg 2+ binds to two thymine residues through T–Hg 2+ –T interactions and the formed T–Hg 2+ –T base pairs bring FAM and ethynyl from two matched sequences into close proximity . On the one hand, π-stacked T–Hg 2+ –T mediates and accepts electron transfer of the excited state of FAM and quenches the fluorescence through PET. On the other hand, π-stacked interactions between ethynyl and FAM also provide a de-excitation process for the excited state of FAM via PET and trigger an enhanced fluorescence quenching. On the basis of fluorescence quenching, a simple, rapid, and sensitive Hg 2+ sensor was constructed; under optimum conditions, a good linear relationship was obtained over the concentration range of 5–150   nM for Hg 2+ and a very low detection limit (0.42   nM) was reached. The developed system was further designed as a cysteine (Cys) sensor based on fluorescence recovery resulted from the strong interaction between Cys and Hg 2+ ; the proposed Cys sensor has a linear range of 10–550   nM and a detection limit of 2.5   nM. The two sensors were used for analysis of real samples and the satisfactory results were achieved.