Calibration of Holographic Optical Tweezers for Force Measurements on Biomaterials

Abstract

Optical tweezers have been widely applied in the field of single-molecule biophysics, as the picoNewton forces that can be exerted and measured with this noninvasive technique lie in the force range of many biomolecular properties and events. By using trapped micrometer-sized particles as handles, the force-extension relations of macromolecules such as DNA and proteins have been probed. When probing more complex systems, however, such as cells or protein networks, the 3D character of these materials requires more flexibility in manipulating particles. With holographic optical tweezers, multiple optical traps can be manipulated independently in three dimensions in real time, adding this necessary flexibility to the interactive control over multiple particles. Thus far, however, holographic tweezers have not been an accepted tool in the biophysics community, in large part due to lack of evidence as to how exerted forces vary as the positions of holographic traps are changed. To perform quantitative force measurements, parameters such as trap stiffness and its position dependence, range of trap steering, and minimum step size are of key importance. Here, we systematically characterize the stiffness of traps within our holographic tweezers setup, in which high-speed (>kHz) camera imaging is used for particle position detection. We create multiple traps and steer one or more over small and large distances, and find that over a range of ∼25 μm the trap stiffness does not change significantly. Also, we determine the efficiency with which the laser power is directed towards intended traps. In addition, we control and detect trap displacements to ∼1 nm, comparable to the position detection limit of our system. Our results suggest that after full characterization, holographic optical tweezers can be successfully employed in quantitative experiments on biomaterials, e.g., probing elastomeric properties of structural protein networks.