Highly selective and sensitive method for cysteine detection based on fluorescence resonance energy transfer between FAM-tagged ssDNA and graphene oxide.

In this work, a new platform for effective sensing cysteine (Cys) was developed based on fluorescence resonance energy transfer (FRET) between FAM-tagged single-stranded DNA (FAM-ssDNA) and graphene oxide (GO). Due to the noncovalent assembly between FAM-ssDNA and GO, fluorescence quenching of the FAM took place because of FRET. This method relied on the competitive ligation of Ag(+) by Cys and "cytosine-cytosine" (C-C) mismatches in a FAM-labeled DNA strand of the self-hybridizing strand. At first, enough amount of Ag(+) was introduced to bind "C-C" mismatches and form double-stranded DNA (dsDNA), which had weak affinity to GO and kept FAM away from GO surface. However, the presence of Cys removed Ag(+) away from "cytosine-Ag(+)-cytosine" (C-Ag(+)-C) base pairs, leading to the formation of ssDNA again and FRET, and then fluorescence of the FAM-ssDNA was efficiently quenched. The fluorescence intensity decrease was found to be proportional to the increase of concentration of Cys in both aqueous buffer (2-200 nM) and human serum (5-200 nM), and the sensitivity of the proposed method towards Cys was much higher than that of other reported assays for Cys.