Ultrasensitive colorimetric miRNA detection based on magnetic 3D DNA walker and unmodified AuNPs

[Display omitted] •A magenetic 3D DNA walker/AuNPs-based colorimetric platform for detecting miRNA-155 is described.•The magenetic 3D DNA walker plays a signal-amplification role.•Detection limit has been achieved to a value as low as 16.7 fM.•Detection of miRNA-155 in human serum has been achieved without complicated sample pretreatment. Gold nanoparticles (AuNPs) based colorimetric assay is a promising platform for simple and low-cost point-of-care (POC) testing, but tends to have low sensitivity, which limits their clinical applications for trace nucleic acid assays. Herein, a simple colorimetric assay was developed by using unmodified AuNPs and magnetic 3D DNA walker, which could detect as few as 16.7 fM of miRNA-155, thus easily solves the main technical challenge of AuNPs-based colorimetric nucleic acid assays. In this platform, hundreds of substrate sing-stranded DNA (DNA tracks) and dozens of blocked walking strands were attached on an Au-Fe3O4 nanocomposite, applied as a magnetic 3D DNA walker. The target could activate this DNA walker powered by DNAzyme cleavage mechanism, generating a great deal of single-stranded product (displaced ssDNA). After magnetic separation, the generated displaced ssDNA (supernatant) was incubated with AuNPs to protect the unmodified nanoparticles from aggregation in high-salt solutions. Based on this, the concentration of miRNA-155 could be visualized through a color change from grayish black to purple and quantitatively determined by UV–vis spectroscopy. Furthermore, we demonstrated that our approach could be used to detect target miRNA-155 in real human serum samples without complicated sample pretreatment. Thus, the magnetic 3D DNA walker-AuNPs colorimetric method is a facile, sensitive and specific detection platform that shows great potential for development of next-generation POC nucleic acids diagnostics.