Electro-osmotic trapping and compression of single DNA molecules while passing through a nanopore

Complicated DNA molecular behaviors exist during translocation into a nanopore because their large and coiled structure needs to unwind. In this work, we investigated DNA translocation dynamics through a 200 nm pore using a fast photon counting system (FPCS). We found that the dwell time of the DNA molecules depends on the inverse of voltage (τ∝V^-1.02) with a large constant term (∼1 ms). In other words, spherical fluorescence bead translocation involves electrophoresis as well as other additional factors. Our theoretical calculation suggested that one additional factor is electro-osmotic trapping associated with the instantaneous Brownian motion before and after translocation. Furthermore, compressed DNA molecular conformation was seen as a result of the increase of peak photon counts and the decrease of electrophoretic mobility with voltage. Our experiments showed that the polymers at the vicinity of a nanopore can be trapped and compressed, which is necessary to understand how to control the polymer translocation into a nanopore.