Tutorials - Optical Tweezers

Relation between optical tweezers and AFM

The equivalent to the force-sensing mechanical probe in AFM is an optically trapped particle in force-sensing optical tweezers. The substitution of the AFM cantilever with a mechanically uncoupled spherical probe opens new fields of application: while AFM is a surface-based technique, optical tweezers allow for measurements in solution, away from surfaces.

AFM is the technique of choice for probing the topography and the physical and chemical properties of a surface. The sharp tip on the cantilever of an AFM allows performing high resolution imaging of a biological sample. Furthermore, attractive and repulsive forces between the cantilever tip and the sample can be investigated. Force spectroscopy experiments can finally unfold and stretch single molecules.

The main difference between AFM and optical tweezers is the applicable range of forces: with an AFM this is limited to a few tens of pN on the bottom side due to a limit in reducing the stiffness of the cantilever. Here, AFM is complemented by force sensing optical tweezers, which allow to probe the numerous biomolecular interactions and processes that involve forces well below 100 pN. The stalling force of motor proteins, unfolding of single molecules and ligand-receptor interaction, to name a few, are all in the range of several or some tens of pNs. For this kind of experiments, force-sensing optical tweezers is in the optimal range. On top of this, thermal fluctuations can be investigated due to the high resolution of optical tweezers, which can be as low as 100fN.

Another advantage of optical tweezers is its mechanical independence. The particle probe is not attached to any lever, but steered around by a laser beam, allowing it to perform floating experiments and even penetrate cell membranes and perform measurements inside closed compartments.

On top of the possibility to investigate biomolecules by binding it to handle beads, optical tweezers also allow to directly probe viruses or vesicles on cell membranes or assembled surfaces.

Another particular feature of the force-sensing optical tweezers is the utilization of the particle's thermal motion to scan its local environment. By strongly decreasing the stiffness of the trap, the particle probe can move around almost undisturbed and probe its environment. Binding kinetics and mobility studies can thus be easily performed with a high bandwidth.