A Hg2+-mediated label-free fluorescent sensing strategy based on G-quadruplex formation for selective detection of glutathione and cysteine

A novel label-free fluorescent strategy for the detection of glutathione (GSH) and cysteine (Cys) is presented. The system consists of two single stranded DNA (ssDNA) with thymine-thymine (T-T) mismatches and used Hg2+ as a mediator, and N-methyl mesoporphyrin IX (NMM) as the signal reporter. The assay is based on the competitive reaction of Hg2+ with GSH/Cys and T-T mismatched double stranded DNA (dsDNA). In the absence of the target, two ssDNA containing T-T mismatches react with Hg2+ to form a T-Hg2+-T dsDNA structure in the solution, which hampers the formation of a G-quadruplex structure. However, in the presence of the target, GSH/Cys reacts with Hg2+ to keep DNA probes in a free single state, resulting in the effective formation of a G-quadruplex structure of the DNA probe (GP). Subsequently, due to the strong interaction between the G-quadruplex structure and NMM, fluorescence was greatly enhanced. This fluorescence strategy does not require any chemical modification, making the assay convenient and cost-effective. This method exhibited a linear relationship between peak fluorescence intensity and concentration of GSH in the range of 10-400 nM with a limit of detection (LOD) of 9.6 nM. A linear range for Cys detection was obtained in the concentration range of 10-500 nM with an LOD of 10 nM. Moreover, the proposed method worked well for the analysis of complex biological samples.