Trapping of Highly Birefringent Rutile Nanocylinders in the Optical Torque Wrench

The optical torque wrench (OTW) is a powerful technique to measure the torsional properties of different biomolecules, including DNA, DNA-processing protein complexes and rotary motors. To date, quartz has proven to be a convenient birefringent material out of which to synthesize the micron-sized particles essential for this technique. However, the relatively low birefringence of quartz, which limits the maximal torque that can be applied in OTW, hampers the study of certain biological systems. A more attractive material is rutile, which has a thirty-fold higher birefringence. To date, however, the application of rutile in the trapping has been restricted due to its high refractive index, which results in low trapping efficiency. Here, we have employed finite element method calculations to determine the optimal dimensions of sub-micron-sized rutile cylinders for tight stable optical trapping. Using these calculations as a guideline, we have designed and developed a nanofabrication protocol that allows us to produce rutile cylinders with the desired sizes at high yield. We have characterized the fabricated cylinders in the OTW setup and quantified both their linear and angular trapping properties. In addition, we demonstrate full translational and rotational control of these functionalized cylinders tethered to individual DNA molecules for use in single-molecule applications.