Fluorescence off-on nanosensor based on MoS₂ nanosheets and oligonucleotides for the alternative detection of mercury(II) ions or silver(I) ions

As traditional methods for detection of heavy metal pollution in water involve complex procedures and require expensive equipment, there is a great deal of interest in the development of rapid and simple methods for determining heavy metal ions in water. Here, a nanobiosensor based on molybdenum disulphide (MoS₂) nanosheets and fluorophore (FAM) labeled oligonucleotides was proposed, and fluorescence spectroscopy was adopted for detection of Hg²⁺ or Ag⁺ ions in aqueous solution. The principle underlying detection by the sensor involves the formation of T-Hg²⁺-T or C-Ag⁺-C mismatches by single-stranded DNA (ssDNA) rich in thymine (T) or cytosine (C), thereby forming stable double-stranded DNA (dsDNA) structures. By exploiting the different adsorption capacity of MoS₂ nanosheets for ssDNA and dsDNA, when oligonucleotides were in a single chain state, MoS₂ nanosheets possessed a strong adsorption capacity for ssDNA, resulting in fluorescence quenching of FAM. After the addition of Hg²⁺ or Ag⁺, ssDNA formed double chains structure, the fluorescence recovered due to the weak adsorption capacity of MoS₂ nanosheets for dsDNA. Along this line, an "off-on" mode fluorescence nanobiosensor was designed to alternatively detect these two heavy metal ions in water. The sensor showed high sensitivity and excellent selectivity for both Hg²⁺ and Ag⁺ ions, with minimum detection limits of 6.8 nM and 8.9 nM, respectively.