Credit: Nature Physics

Solid-state nanopores can be designed to mimic the porous walls of biological membranes, providing an artificial ‘cell’ in which to study and manipulate molecular dynamics. When the end of a DNA molecule, for example, is brought close to an electrically biased nanopore, the electrostatic force pulls the DNA – which carries an electric charge in solution – through the pore.

Recently, attempts have been made to use nanopores to analyze the structure of DNA and RNA, but it has proved difficult to manipulate the molecules. However, using a combination of solid-state nanopores and optical 'tweezers', Ulrich Keyser and colleagues1 at the Delft University of Technology in the Netherlands have demonstrated that they can control the rate of passage of a single DNA molecule through pores that are 6–15 nm in diameter.

The DNA molecules are tethered to a micro-bead, which is held and moved by optical tweezers near the pore. A well-defined drop in ionic current through the pore indicates the capture of one of the strands of DNA on the bead. The set-up permits the first direct measurement of the charge density of DNA in solution. By balancing the restoring force on the bead against the electrostatic force of the nanopore on the DNA, the authors determine a charge of 0.50 electrons per DNA base pair. Keyser and colleagues suggest that by controlling the passage of DNA through pores just 2 nm across, it should be possible to sequence the DNA.