A ratiometric electrochemical DNA biosensor for detection of exosomal MicroRNA

The detection of exosomal microRNAs (miRNAs) derived from cancer cells with sensitive and selective methods has stimulated increasing interest due to its potential utility in the application of tumor diagnosis. Here, we developed a ratiometric electrochemical DNA biosensor based on a locked nucleic acid (LNA)-modified "Y" shape-like structure for the detection of exosomal miRNA-21 (miR-21). When miR-21 is present, the LNA-assisted strand displacement reaction on the "Y" shape-like structure is activated, leading to a structure change and augmentation of the signal ratio, which reflects the different distances between the electrode surface and two electroactive molecules labeled on the "Y" shape-like structure. With this dual signal ratiometric method, the biosensor shows high accuracy and sensitivity with a limit of detection as low as 2.3 fM. Moreover, because of the logarithm of the signal ratio displays a linear relationship with the logarithm of the miR-21 concentration, the biosensor is stable enough to be used in the detection of miR-21 in MCF-7 cell-derived exosomes. In addition, the biosensor shows good selectivity even in the detection of even a single base-mismatched target due to the LNA-assisted strand displacement reaction. Notably, the sensor is both regenerative and robust. In brief, the high sensitivity and selectivity, combined with the low cost of the glassy carbon electrode, make this biosensor a promising tool for the development of point-of-care testing in cancer.