Tutorials - Optical Tweezers

Force-sensing optical tweezers

Quantitative optical tweezers - also known as photonic force microscopy (PFM) - combines the technique of optical trapping with high-resolution detection of forces or tracking of positions.

The basic physical principle underlying optical tweezers (OT) is the radiation pressure exerted by light when colliding with matter. The effect was postulated and first demonstrated by Arthur Ashkin in the late 1960s. It has become known as optical trapping, in which dielectric particles can be stably trapped in three dimensions in a tightly focused laser beam. The work by Steven Chu on cooling and trapping atoms culminated in the award of a Nobel Prize in 1997. Yet again it was the group around Ashkin that extended the application of OT to the field of biology by trapping cells and viruses in the 1980s.

The radiation pressure exerted on the trapped particle by tightly focused laser light pulles it into the center of the focus and thus enables manipulation by steering of the laser beam. Brownian motion, viscous drag or any other force exerted on the particle moves the probe against the force gradient out of the center of the trap.

As for a normal elastic spring, the stiffness of the trap defines the relationship between this deflection of the particle from the center and the restoring force acting on it. The combination of optical trapping with a position detection system finally paved the way for force-sensing optical tweezers. The most prominent solution is the detection of the interference pattern of the scattered and unscattered light on a quadrant photo diode (QPD). With this detection scheme, the picoNewton forces exerted on the particle can be measured down to a fraction of a picoNewton, as well as the position of the particle with nanometer precision.

In order to measure the forces associated with single molecule interactions and kinetics, the biomolecule of interest should be attached to a probe. The optical tweezers probe - a bead with the diameter of several tens of nanometers to several micrometers - can be used to perform force spectroscopy, tracking or imaging experiments. Thanks to optical tweezers, a lot could be learned about the mechanics of molecular motors.

Stepping of kinesin and dynein on microtubule filaments was as well detected as base-pair stepping of RNA polymerase and codon stepping of ribosomes. Many reports also appeared on stretching and the elasticity of DNA. Furthermore, the technique has already been used by researchers in the field of microrheology, and the detection of diffusion in cell membranes represents the groundbreaking experiments of force-sensing optical tweezers on cells.