Highly sensitive label-free fluorescent detection of Hg2+ ions by DNA molecular machine-based Ag nanoclusters

We present here a highly selective and sensitive label-free method to detect Hg2+ ions in aqueous solution by using DNA molecular machine-based fluorescent Ag nanoclusters (AgNCs). This mechanism is based on the Hg2+ ions triggering machine-like operations of DNA and the product of the machine being used to stabilize fluorescent AgNCs. In this method, a tailored DNA, containing a sequence for Hg2+ ions recognition, a sequence-specific nicking site for Nb BbvC I and a sequence complementary to the DNA as a template for the synthesis of fluorescent AgNCs, was firstly designed. In the presence of Hg2+ ions, the machine's function operations were triggered. A series of machine-like operations, including replication, scission, and displacement then occurred with the addition of polymerase/dNTPs/Nb BbvC I, which manufactured lots of product DNA. The product DNA could act as a template for the preparation of fluorescent AgNCs. Thus the fluorescence of the AgNCs could be used as a signal transduction of this DNA machine, which was related to the concentration of the Hg2+ ions. The repeated synthesis of the product and its template effect for AgNCs synthesis led to signal amplification in the assay of Hg2+ ions. A linear response to the concentration of Hg2+ ions was observed in the range from 0.08 nM to 20 nM and a detection limit of 0.08 nM was obtained. By contrast, the operation of the machine could not be executed in an Hg2+ ion-free system. Moreover, the detection was not only label-free but also specific for Hg2+ ions without being affected by other metal ions.