Ultrasensitive Electrochemiluminescence Biosensor Using Sulfur Quantum Dots as an Emitter and an Efficient DNA Walking Machine with Triple-Stranded DNA as a Signal Amplifier

In this study, sulfur quantum dots (SQDs) with superior near-infrared electrochemiluminescence (ECL) performance were synthesized by the H2O2-assisted top-down approach. Through H2O2 etching, the size and dispersity of SQDs were adjusted, reducing the aggregation-caused quenching effect and obviously promoting the ECL performance. Using the obtained SQDs as an emitter, a super-sensitive ECL biosensor of microRNA-21 (miRNA-21) detection was constructed, which was based on an efficient DNA walking machine with triple-stranded DNA (tsDNA) nanostructures as tracks. Compared with the common single-stranded DNA or double-stranded DNA, the tsDNA nanostructures on the electrode interface could avert probe entanglement and decrease local overcrowding effects. The walking efficiency of the DNA walking machine was also improved and the signal-amplification efficacy was greatly enhanced, which was benefited from the fact that tsDNA nanostructures were highly rigid scaffolds and provided orderly tracks for the DNA walking machine to walk. Thus, the designed ECL biosensor demonstrated outstanding performance for miRNA-21 detection in the concentration range of 20 aM to 1 nM with a low detection limit of 6.67 aM. Remarkably, this work enriched the application of pure element quantum dots in the ECL field and offered a new avenue for ultra-sensitive detection in clinical and biochemical analysis.