Inspired by natural molecular machines, scientists are devoted to designing nanomachines that can navigate in aqueous solutions, sense their microenvironment, actuate, and respond. Among different strategies, magnetically driven nanoactuators can easily be operated remotely in liquids and thus are valuable in biosensing. Here we report a magnetic nanoactuator swarm with rotating-magnetic-field-controlled conformational changes for reaction acceleration and target quantification. By grafting nucleic acid amplification primers, magnetic nanoparticle (MNP) actuators can assemble and be fixed with a flexible DNA scaffold generated by surface-localized hyperbranched rolling circle amplification in response to the presence of a target microRNA, osa-miR156. Net magnetic anisotropy changes of the system induced by the MNP assembly can be measured by ferromagnetic resonance spectroscopy as shifts in the resonance field. With a total assay time of ca. 120 min, the proposed biosensor offers a limit of detection of 6 fM with a dynamic detection range spanning 5 orders of magnitude. The specificity of the system is validated by testing different microRNAs and salmon sperm DNA. Endogenous microRNAs extracted from Oryza sativa leaves are tested with both quantitative reverse transcription-PCR and our approach, showing comparable performances with a Pearson correlation coefficient >0.9 (n = 20).
Keywords: ferromagnetic resonance spectroscopy; magnetic nanoparticles; microRNA detection; rolling circle amplification; rotating magnetic field.