Highly sensitive detection of biomarkers is essential for disease prevention and early diagnosis. Herein, a highly sensitive strategy was proposed for microRNA-21 (miRNA-21) detection by the incorporation of programmable toehold-mediated strand displacement (TMSD) and dark-field microscopy imaging. Firstly, efficient and specific TSMD was carried out via hybridization between the substrate strand (Sub) and two short probe strands (P1, P2). Then, miRNA-21 could specifically hybridize with Sub due to the toehold that existed on its tail, which triggered the amplification with the help of the assist strands, and forming a large number of Sub-assist double-stranded DNA (dsDNA). This process realized the targeted highly specific recognition of miRNA-21 and the amplification of the trace target to high-output dsDNA. Additionally, as glucose oxidase (Gox) was modified on the end of the assist strands in advance, hydrogen peroxide was generated after adding glucose to the system, which further etched gold–silver core–shell nanocubes (Au@Ag NCs). As a result, the size of Au@Ag NCs decreased and the scattering intensity reduced simultaneously. The scattering intensity reduction value of Au@Ag NCs has a linear relationship with miRNA-21 concentration in the range of 1.0 to 100.0 fM with a limit of detection of 1.0 fM. Finally, the proposed method has been successfully demonstrated for the determination of miRNA-21 in lung cancer cell A549 lysate.
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