A novel aptasensor based on single-molecule force spectroscopy for highly sensitive detection of mercury ions

We have developed a novel aptasensor based on single-molecule force spectroscopy (SMFS) capable of detecting mercury ions (Hg2+) with sub-nM sensitivity. The single-strand (ss) DNA aptamer used in this work is rich in thymine (T) and readily forms T-Hg2+-T complexes in the presence of Hg2+. The aptamer was conjugated to an atomic force microscope (AFM) probe, and the adhesion force between the probe and a flat graphite surface was measured by single-molecule force spectroscopy (SMFS). The presence of Hg2+ ions above a concentration threshold corresponding to the affinity constant of the ions for the aptamer (about 5 x 10(9) M-1) could be easily detected by a change of the measured adhesion force. With our chosen aptamer, we could reach an Hg2+ detection limit of 100 pM, which is well below the maximum allowable level of Hg2+ in drinking water. In addition, this aptasensor presents a very high selectivity for Hg2+ over other metal cations, such as K+, Ca2+, Zn2+, Fe2+, and Cd2+. Furthermore, the effects of the ionic strength and loading rate on the Hg2+ detection were evaluated. Its simplicity, reproducibility, high selectivity and sensitivity make our SMFS-based aptasensor advantageous with respect to other current Hg2+ sensing methods. It is expected that our strategy can be exploited for monitoring the pollution of water environments and the safety of potentially contaminated food.