Direct force measurements on DNA in a solid-state nanopore

Among the variety of roles for nanopores in biology, an important one is enabling polymer transport, for example in gene transfer between bacteria(1) and transport of RNA through the nuclear membrane(2). Recently, this has inspired the use of protein(3-5) and solid-state(6-10) nanopores as single-molecule sensors for the detection and structural analysis of DNA and RNA by voltage-driven translocation. The magnitude of the force involved is of fundamental importance in understanding and exploiting this translocation mechanism, yet so far it has remained unknown. Here, we demonstrate the first measurements of the force on a single DNA molecule in a solid-state nanopore by combining optical tweezers(11) with ionic-current detection. The opposing force exerted by the optical tweezers can be used to slow down and even arrest the translocation of the DNA molecules. We obtain a value of 0.24 +/- 0.02 pN mV(-1) for the force on a single DNA molecule, independent of salt concentration from 0.02 to 1MKCl. This force corresponds to an effective charge of 0.50 +/- 0.05 electrons per base pair equivalent to a 75% reduction of the bare DNA charge.