MicroRNAs (miRNAs) are found in extremely low concentrations in cells, so highly sensitive quantitation is a great challenge. Herein, a simple dual-amplification strategy involving target-activated catalytic hairpin assembly (CHA) coupled with multiple fluorophores concentrated on one X-shaped DNA is reported. In this strategy, four hairpin probes (H1, H2, H3, and H4) are modified with FAM and BHQ1 at both sticky ends, while a circulating hairpin probe (H0) is used to activate CHA circuits once it binds to complementary sequences in the target miR-21 (T). The powerful dual-amplification cascades in Förster resonance energy transfer (FRET)-based nonenzymatic nucleic acid circuits are triggered by T–H0-activated formation of the X-shaped DNA nanostructure, freeing T–H0 for the next CHA reaction cycle. CHA circuits increase the fluorescence due to the wide distance between FAM and BHQ1 in the formed X-shaped DNA nanostructure, resulting in signal amplification and highly sensitive detection of miR-21, with a limit of detection (LOD, 3σ) of 0.025 nM, which is 25.6 or 57.6 times lower than that obtained through a single-amplification strategy without multiple fluorophores on one X-shaped DNA or CHA circuit. Furthermore, this cascade reaction was completed in 45 min, effectively avoiding target degradation. This new enzyme-free signal amplification strategy holds promising potential for sensitively detecting different DNA or RNA sequences by simply adapting the fragment of the H0 sequence complementary to the target.

Nonalcoholic fatty liver disease (NAFLD) can progress gradually to liver failure, early warning of which is critical for improving the cure rate of NAFLD. In situ imaging and monitoring of overexpressed miR-21 is an advanced strategy for NAFLD diagnosis. However, this strategy usually suffers from the high background imaging in living cells owing to the complexity of the biological system. To overcome this problem, herein, we have developed a one-donor–two-acceptor nanoprobe by assembling gold nanoparticles (AuNPs) coupled with BHQ2 (AuBHQ) and quantum dots (QDs) through DNA hybridization for imaging of miR-21 in living cells. The fluorescence of QDs was quenched up to 82.8% simultaneously by the AuNPs and the BHQ2 via nanometal surface energy transfer and fluorescence resonance energy transfer, reducing the background signals for target imaging. This low background fluorescent nanoprobe was successfully applied for imaging the target miR-21 in nonalcoholic fatty liver cells by catalyzing the disassembly of QDs with the AuBHQ and the fluorescence recovery of QDs. In addition, the sensitivity of this nanoprobe has also been enhanced toward detecting miR-21 in the range of 2.0–15.0 nM with the detection limit (LOD, 3σ) of 0.22 nM, which was 13.5 times lower than that without BHQ2. The proposed approach provides a new way for early warning, treatments, and prognosis of NAFLD.

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