A fuel-initiated DNA molecular machine for microRNA detection in serum via poly-adenine-mediated spherical nucleic acids

MicroRNAs (miRNAs) have been identified as promising disease diagnostic biomarkers. However, it is challenging to sensitively detect miRNAs, especially in complex biological environments, due to their low abundance and small size. Herein, we have developed a DNA-fueled molecular machine for sensitive detection of miRNA-22 (miR-22) in undiluted serum by combining poly-adenine-mediated spherical nucleic acids (polyA-SNAs) with a toehold mediated strand displacement reaction (TMSDR). The polyA-SNAs are constructed by the assembly of diblock DNA probes on a AuNP surface through the high binding affinity of polyA to AuNPs. The surface density of the diblock DNA probe can be controlled by tuning the length of the polyA block, and the orientation of the diblock DNA probe can adopt an upright conformation, which is beneficial to target hybridization and TMSDRs. TMSDR is an enzyme-free target recycling amplification approach. Taking advantage of polyA-mediated SNAs and TMSDR, the operation of the molecular machine based on two successive TMSDRs on polyA20-SNAs is rapid and efficient, which can significantly amplify the fluorescence response for detection of miR-22 in an undiluted complex matrix. The developed sensor can detect as low as 10 pM of target miRNA/DNA in undiluted fetal bovine serum within 30 min. The synergetic effect of polyA-mediated SNAs and TMSDR presents a potential alternative tool for the detection of biomarkers in real biological samples.