MicroRNAs (miRNAs) play vital roles in physiologic and pathologic processes and are significant biomarkers for disease diagnostics and therapeutics. However, rapid, low-cost, sensitive, and selective detection of miRNAs remains a challenge because of their short length, sequence homology, and low abundance. Herein, we report for the first time that WS2 nanosheet can exhibit differential affinity toward short oligonucleotide fragment versus ssDNA probe and act as an efficient quencher for adsorbed fluorescent probes. This finding is utilized to develop a new strategy for simple, sensitive, and selective detection of miRNA by combining WS2 nanosheet based fluorescence quenching with duplex-specific nuclease signal amplification (DSNSA). This assay exhibits highly sensitive and selective with a detection limit of 300 fM and even discriminate single-base difference between the miRNA family members. The result indicates that this simple and cost-effective strategy holds great potential application in biomedical research and clinical diagnostics.

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Aptamer-based rolling circle amplification (aptamer-RCA) was developed as a novel versatile electrochemical platform for ultrasensitive detection of protein. This method utilized antibodies immobilized on the electrode surface to capture the protein target, and the surface-captured protein was then sandwiched by an aptamer-primer complex. The aptamer-primer sequence mediated an in situ RCA reaction that generated hundreds of copies of a circular DNA template. Detection of the amplified copies via enzymatic silver deposition then allowed enormous sensitivity enhancement in the assay of target protein. This novel aptamer-primer design circumvented time-consuming preparation of the antibody-DNA conjugate for the common immuno-RCA assay. Moreover, the detection strategy based on enzymatic silver deposition enabled a highly efficient readout of the RCA product as compared to a redox-labeled probe based procedure that might exhibit low detection efficiency due to RCA product distance from the electrode. With the platelet-derived growth factor B-chain (PDGF-BB) as a model target, it was demonstrated that the presented method was highly sensitive and specific with a wide detection range of 4 orders of magnitude and a detection limit as low as 10 fM. Because of the wide availability of aptamers for numerous proteins, this platform holds great promise in ultrasensitive immunoassay.

DNAzymes, screened through in vitro selection, have shown great promise as molecular tools in the design of biosensors and nanodevices. The catalytic activities of DNAzymes depend specifically on cofactors and show multiple enzymatic turnover properties, which make DNAzymes both versatile recognition elements and outstanding signal amplifiers. Combining nanomaterials with unique optical, magnetic and electronic properties, DNAzymes may yield novel fluorescent, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical and chemiluminescent biosensors. Moreover, some DNAzymes have been utilized as functional components to perform arithmetic operations or as "walkers" to move along DNA tracks. DNAzymes can also function as promising therapeutics, when designed to complement target mRNAs or viral RNAs, and consequently lead to down-regulation of protein expression. This feature article focuses on the most significant achievements in using DNAzymes as recognition elements and signal amplifiers for biosensors, and highlights the applications of DNAzymes in logic gates, DNA walkers and nanotherapeutics.