Positive carbon dots with dual roles of nanoquencher and reference signal for the ratiometric fluorescence sensing of DNA

Specific modification of carbon dots (CDs) exhibit excellent properties and are suitable for numerous applications. The exploitation of multifunctional CDs in the area of fluorescent assay is still urgent and attractive. Herein, using a simple two-step heating procedure, positive charged carbon dots (P-CDs) were prepared with the co-doping of the elements of N and S. The morphology, optical properties and stability of P-CDs was investigated. Furthermore, P-CDs show the dual functions of unprecedented quenching performance toward fluorophore-modified DNA coupling the retaining fluorescence of P-CDs self. The quenching effect and mechanism of P-CDs to fluorophore-modified DNA was investigated. The fluorescence and quenching properties of P-CDs were used to design a ratiometric fluorescence platform for DNA based on the complex of P-CDs and FAM-labeled ssDNA. The ratiometric method allows the detection of target DNA because of the different affinities of P-CDs to ssDNA and dsDNA with acceptable sensitivity and improved reproducibility, when using sequence fragment of the gene of Gardnerella vaginalis as model. This method provides a new approach to widen the future of the biological applications based on some special CDs.