Developing novel simple and ultrasensitive strategies for detecting microRNAs (miRNAs) is highly desirable because of their association with early cancer diagnostic and prognostic processes. Here a new chronocoulometric sensor, based on semiconducting 2H MoS2 nanosheets (MoS2 NSs) decorated with a controlled density of monodispersed small gold nanoparticles (AuNPs@MoS2), was fabricated via electrodeposition, for the highly sensitive detection of miRNA-21. The size and interparticle spacing of AuNPs were optimized by controlling nucleation and growth rates through the tuning of deposition potential and Au precursor concentration and by getting simultaneous feedback from morphological and electrochemical activity studies. The sensing strategy, involved the selective immobilization of the thiolated capture probe DNA (CP) at AuNPs and hybridization of CP to a part of the miRNA target, whereas the remaining part of the target was complementary to a signaling nonlabeled DNA sequence that served to amplify the target upon hybridization. Chronocoulometry provided precise quantification of nucleic acids at each step of the sensor assay by interrogating [Ru(NH3)6]3+ electrostatically bound to phosphate backbones of oligonucleotides. A detailed and systematic optimization study demonstrated that the thinnest and smallest MoS2 NSs improved the sensitivity of the AuNP@MoS2 sensor, achieving an impressive detection limit of ≈100 aM, which is 2 orders of magnitude lower than that of a bare Au electrode and also enhanced the DNA-miRNA hybridization efficiency by 25%. Such an improved performance can be attributed to the controlled packing density of CPs achieved by their self-assembly on AuNPs, large interparticle density, small size, and intimate coupling between AuNPs and MoS2. Alongside the outstanding sensitivity, the sensor exhibited an excellent selectivity down to femtomolar concentrations, for discriminating a complementary miRNA-21 target in a complex system composed of different foreign targets including mismatched and noncomplementary miRNA-155. These advantages make our sensor a promising contender in the point of care miRNA sensor family for medical diagnostics.
Keywords: chronocoulometric detection; electrochemical deposition; electrochemical microRNA sensor; gold-decorated MoS2 hybrid nanosheets; ionic liquid-assisted grinding exfoliation; microRNA-21 (miRNA-21); molybdenum disulfide (MoS2) nanosheets; screen-printed gold electrodes.