Optimizing bead size reduces errors in force measurements in optical traps

Rebecca K. Montange, Matthew S. Bull, Elisabeth R. Shanblatt, and Thomas T. Perkins
Optical traps are used to measure force (F) over a wide range (0.01 to 1,000 pN). Variations in bead radius (r) hinder force precision since trap stiffness (ktrap) varies as r3 when r is small. Prior work has shown ktrap is maximized when r is approximately equal to the beam waist (w0), which on our instrument was ~400 nm when trapping with a 1064-nm laser. In this work, we show that by choosing r ≈w0, we improved the force precision by 2.8-fold as compared to a smaller bead (250 nm). This improvement in force precision was verified by pulling on a canonical DNA hairpin. Thus, by using an optimum bead size, one can simultaneously maximize ktrap while minimizing errors in F.
DOI