Unraveling single-stranded DNA in a solid-state nanopore.

Solid-state nanopores are an emerging class of single-molecule sensors. Whereas most studies so far focused on double-stranded DNA (dsDNA) molecules, exploration of single-stranded DNA (ssDNA) is of great interest as well, for example to employ such a nanopore device to read out the sequence. Here, we study the translocation of long random-sequence ssDNA through nanopores. Using atomic force microscopy, we observe the ssDNA to hybridize into a random coil, forming blobs of around 100 nm in diameter for 7 kb ssDNA. These large entangled structures have to unravel, when they arrive at the pore entrance. Indeed, we observe strong blockade events with a translocation time that is exponentially dependent on voltage, tau approximately e(-V/V(0)). Interestingly, this is very different than for dsDNA, for which tau approximately 1/V. We report translocations of ssDNA but also of ssDNA-dsDNA constructs where we compare the conductance-blockade levels for ssDNA versus dsDNA as a function of voltage.