Engineering Micrometer‐Sized DNA Tracks for High‐Speed DNA Synthesis and Biosensing

φ29 DNA polymerase (Polφ29) is capable of synthesizing long-chain single-stranded (ss) DNA molecules by copying the sequence of a small ss circular DNA template (ssCDT) in a process known as rolling circle amplification (RCA). The use of a ssCDT in RCA, however, comes with a key drawback: the rate of DNA synthesis is significantly reduced. We hypothesize that this issue can be overcome using a very long linear ssDNA template with a repeating sequence. To test this idea, we engineered a DNA assembly, which we denote "micrometer-sized DNA track" (μDT). This μDT, with an average length of ≈13.5 μm, is made of a long chain DNA with a primer-binding domain at its 3' end and ≈1000 repeating sequence units at its 5' end, each carrying a DNA anchor. We find that Polφ29 copies μDT at a speed ≈5-time faster than it does a related ssCDT. We use this to design a simple all-in-one printed paper device for rapid and sensitive detection of microRNA let-7. This paper sensor is capable of detecting 1 pM let-7a in 10 minutes.