A label-free and enzyme-free fluorescent assay for mercury ions based on T-Hg(II)-T nanoladders and DNA-templated silver nanoclusters/graphene oxide nanocomposites

A label-free and enzyme-free signal-on biosensor for Hg(II) ions was constructed based on T-Hg(II)-T nanoladders and fluorescent DNA-templated silver nanoclusters/graphene oxide nanocomposites (DNA-AgNCs/GO). The T-rich sequences (P1 and P2) were used to capture Hg(II) and form the T-Hg(II)-T nanoladders. C 6 G 5 C 6 was extended at the 5′ end of P1, used as a template for synthesizing fluorescent DNA-AgNCs. In the presence of mercury ions , the T-Hg(II)-T nanoladders significantly enhanced the emission of AgNCs. GO can attract free P1-C 6 G 5 C 6 and P2 that were not involved in the formation of T-Hg(II)-T nanoladders, thus employed as the energy acceptor to quench the background fluorescence of AgNCs. The fluorescence enhancement induced by mercury ions can be used for quantitative detection of Hg(II) over the range 0.1–30 nM with a detection limit as low as 7.35 pM. This method was used to detect Hg(II) in the spiked samples of Songhua River water and grass carp, with the recoveries of 97.61%− 103.79% and 96.52%− 105.58% respectively, consistent with the results from atomic fluorescence spectrometry. Obviously, in this assay, enzyme-free signal amplification and signal identification are integrated in the ingenious design of nucleic acid sequences, thus achieving the label-free fluorescence detection of mercury ions.