A carbon dot and molecular beacon based fluorometric sensor for the cancer marker microRNA-21

A carbon quantum dot (CQD) labeled molecular beacon was synthesized and applied to the detection of microRNA-21. The CQDs possess low cytotoxicity, excellent water solubility, and photostability. The CQDs were characterized by transmission electron microscopy, dynamic light scattering, Fourier-transform infrared spectroscopy, and fluorescence spectroscopy. The molecular beacon (MB) was labeled with the CQDs at the 5 end, and with Black Hole Quencher 1 (BHQ1) at the 3 end. The two labels act as the donor and acceptor parts of a FRET system, respectively. Only weak fluorescence is observed in the absence of microRNA-21, and in the presence of scrambled or mismatched sequences. However, in the presence of microRNA-21, fluorescence intensity of the CQDs at 460nm (excitation at 360nm) recovers. The hybridization of the hairpin structure of the MB with microRNA-21 opens the loop of MB. Consequently, the distance between the BHQ1 quencher and the CQDs is increasedand fluorescence changes. The probe has high sensitivity (with a 0.3nM limit of detection) and specificity. It can distinguish between microRNA-21 and its single mismatch mutant and hence represents a valuable tool for the early cancer diagnosis.