Near-infrared DNA-AgNCs enzyme-free fluorescence biosensing for microRNA imaging in living cells based on self-replicating catalytic hairpin self-assembly

In this work, a fast signal amplification system mediated by self-replicating catalytic hairpin self-assembly (SCHA) was established for microRNA-155 using near-infrared DNA-Ag Nanoclusters (DNA-AgNCs) as fluorescence signal output. Among them, two fission target-like DNA sequences are merged into two hairpin DNA H1 and H2, and the AgNCs template sequence is designed at the sticky end of H1 and H2. The target can be recycled in the system to form a double-stranded DNA structure (H1-H2), which will detach the H1/H2-AgNCs from the surface of the polypyrrole nanoparticles (PPy NPs) and cause the near-infrared fluorescence signal of DNA-AgNCs to be restored. At this point, the two-split target-like DNA sequences will be reassembled to initiator DNA. The acquired replicas can also be recycled as a brand-new activation unit to initiate the SCHA response, resulting in rapid replication of the target/triggered DNA, accompanied by the generation of higher fluorescence signals. This autocatalytic signal amplification approach has been successfully applicable to fast signal amplification, enzyme-free and label-free for microRNA-155 assay in biological samples, and the detection limit (LOD) is 240 fM (S/N = 3). At the same time, this SCHA system can realize intracellular microRNA fluorescence imaging, which presents a promising approach to developing advanced molecular diagnostic tools.