Fluorometric determination of microRNA by using an entropy-driven three-dimensional DNA walking machine based on a catalytic hairpin assembly reaction on polystyrene microspheres

An entropy-driven 3-D DNA walking machine is presented which involves catalytic hairpin assembly (CHA) for detection of microRNA. A 3-D DNA walking machine was designed that uses streptavidin-coated polystyrene microspheres as track carriers to obtain reproducibility. The method was applied to microRNA 21 as a model analyte. Continuous walking on the DNA tracks is achieved via entropy increase. This results in a disassembly of ternary DNA substrates on polystyrene microspheres and leads to cycling of microRNA 21. The release of massive auxiliary strands from ternary DNA substrates induces the CHA. This is accompanied by in increase in fluorescence, best measured at excitation/emission wavelengths of 480/520 nm. On account of entropy-driven reaction, the assay is remarkably selective. It can differentiate microRNA 21 from homologous microRNAs in giving a signal that is less than 5% of the signal for microRNA 21 except for microRNA-200b. The assay works in the 50 pM to 20 nM concentration range and has a 41 pM detection limit. The method displays good reproducibility (between 1.1 and 4.2%) and recovery (from 99.8 to 104.0%).