Ensemble Sensing Using Single-Molecule DNA Copolymers.

While single-molecule sensing has offered ultimate mass sensitivity at the precision of individual molecules, it requires a longer time to detect analytes at lower concentrations when analyte binding to single-molecule probes becomes diffusion-limited. Here, we solved this accuracy problem in the concentration sensitivity determination by using single-molecule DNA homopolymers, in which up to 473 identical sensing elements (DNA hairpins) were introduced by rolling circle amplification. Surprisingly, the DNA homopolymers containing as few as 10 tandem hairpins displayed ensemble unfolding/refolding transitions, which were exploited to recognize microRNAs (miRNAs) that populated unfolded hairpins. Within 20 min, the femtomolar detection limit for miRNAs was observed, 6 orders of magnitude better than standalone hairpins. By incorporating different hairpin probes in an alternating DNA copolymer, multiplex recognition of different miRNAs was demonstrated. These DNA co-polymers represent new materials for innovative sensing strategies that combine the single-molecule precision with the accuracy of ensemble assays to determine concentration sensitivities.