Single-molecule force spectroscopy is a powerful technique for studying the detailed behaviour of biopolymers such as DNA and proteins: by applying pN-scale forces to individual molecules, properties such as conformations, folding pathways, and intermolecular interaction strengths can be determined. Traditionally these studies have been carried out under static tension. The dynamic response of polymers to a sudden change in force is exper- imentally more challenging as the polymer is often coupled to an external molecular handle, which suppresses
important physics at short (∼ms) timescales. Here we use a nanopore to electrically control the application of
force to the end of a double-stranded DNA molecule; the other end of the molecule is attached to a bead held in
an optical trap. By shutting off the voltage, the fast relaxation dynamics of the free polymer end can be studied. We observe for the first time an enhanced viscous friction which arises from the rapid internal contraction of the DNA, which is fully explained by theory. These studies pave the way for new dynamic force spectroscopy experiments, such as investigations of tension propagation along biomolecules, which has applications for both polymer theory as well as biological systems such as the cytoskeleton where dynamic tension can affect cellular response.
We present measurements on single and multiple DNA molecules inside a nanocapillary. Nanocapillaries are single
molecule sensors with similar properties as standard solid-state nanopores made from silicon nitride. For stalling of
DNA in the nanocapillaries, we apply optical fiber illumination in combination with video detection for real-time
tracking of optically trapped colloids on a microsecond time scale. Using a cross-correlation based algorithm after image
acquisition from a CMOS camera we are able to measure the position of a colloid at rates up to 40,000 frames per
second over hours.
A full understanding of electrokinetic transport of polyelectrolytes in strongly confined environments is still elusive. In
order to shed new light on this process we perform electrophoretic force and ionic-current measurements on single and
multiple DNA molecules inside nanocapillaries attached to an optically trapped colloid. The hydrodynamic interaction
between single DNA molecules was investigated by capturing multiple strands inside the tip of a capillary. We find that
the capture force depends linearly on the number of DNA molecules.