A signal-on ratiometric fluorometric heparin assay based on the direct interaction between amino-modified carbon dots and DNA

Amino-modified carbon dots (C-dots) with positively charged surface were prepared. They display strong blue fluorescence and are shown to act as quenchers of the green fluorescence of FAM-labeled ssDNA such as the F-probe used in this work that was immobilized on the C-dots. On the addition of highly negatively charged heparin (Hep), it will interact with the C-dots and displace the F-probe from C-dots. Once the F-probe is displaced by Hep, its green fluorescence is restored. The intrinsic blue fluorescence of the C-dots remains stable after addition of Hep. Thus, a signal-on ratiometric fluorometric assay was developed for the ultra-sensitive detection of Hep. The underlying mechanisms of quenching and recovery are discussed. Under optimized conditions, the recovery of the ratiometric fluorescence of the system composed of C-dots and quenched F-probe is proportional to the Hep concentration in the range of 0.01-2.0 mu g.mL(-1) (=0.00125-0.25 U.mL(-1)). The method was successfully applied to the determination of Hep in spiked serum samples.