Detection of Ag⁺ ions and cysteine based on chelation actions between Ag⁺ ions and guanine bases.

A selective and sensitive fluorescence biosensor for Ag(+) ions and cysteine (Cys) was developed based on the chelation actions between Ag(+) ions and guanine bases of G-rich fluorogenic oligonucleotide (FAM-ssDNA) and the different electrostatic affinity between FAM-ssDNA and graphene oxide (GO). FAM-ssDNA adsorbed onto the surface of GO through π-π stacking interaction between the ring structure in the nucleobases and the hexagonal cells of GO, and the fluorescence of the dye was quenched. In the presence of Ag(+) ions, the random coil structure changed into a G-Ag(+) architecture. As a result, the binding released FAM-ssDNA signal probe from the surface of GO, which disrupted the energy transfer from FAM-ssDNA to GO, recovering the fluorescence emission of FAM-ssDNA. On the other hand, because Cys was a strong Ag(+) ions binder, it could deactivate the sensor fluorescence by rewrapping FAM-ssDNA around GO. In this way, these changes in fluorescence intensity allowed the selective detection of Ag(+) ions and Cys.