Sensitive impedimetric detection of microRNAs using a hairpin probe based on DNAzyme-functionalized gold nanoparticle tag-initiated deposition of an insulating film on gold electrode

•The reDNA (the reporter DNA and the DNAzyme)-functionalized AuNP was prepared and acted as a tag of the impedimetric sensor.•The signal amplification was achieved through the DNAzymatic oxidation of 4-chloro-1-naphthol to produce the precipitation on sensor surface.•The detection limit to subfemtomolar levels was achieved based on the enzymatic reactions and cumulative nature of the protocol.

An ultrasensitive impedimetric miRNA assay based on DNAzyme tag-initiated deposition of an insulating film on gold electrode was developed. Two strategies were proposed to introduce the DNAzyme tag onto the electrode surface. One is single DNAzyme strategy, which was based on structure switch of the immobilized hairpin probe DNA (P1) to the DNAzyme triggered by miRNA. Another is multiple-DNAzyme strategy, the DNAzyme-modified AuNP tags were linked to the electrode through a sandwich reaction after the target miRNA open hairpin structure of the immobilized DNA probe (P2). The DNAzyme was used to the biocatalytic oxidation of 4-chloro-1-naphthol (CN) to the insoluble product, which was then deposited onto the electrode surface. The insulating layer formed by insoluble product introduces a barrier for the electron transfer between the redox probe in the electrolyte solution and the electrode. Therefore, the electron-transfer resistance (Ret) of the sensor increased with the increase in the amount of the insoluble product. Electrochemical impedance spectroscopy and microgravimetric quartz crystal microbalance were used to monitor the precipitation of CN that had accumulated on the electrode surface. The Ret of the sensor directly correlated with the concentration of the target miRNA in the solution. Sensitivity of multiple-DNAzyme strategy is higher than that obtained using the single DNAzyme strategy. Under optimal conditions, a detection limit of 1.5 × 10−17 M for a miRNA (miR-26a) was obtained using multiple-DNAzyme strategy. The sensor also showed excellent differentiation ability for single-base mismatch.

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