Germanium nanospheres as high precision optical tweezers probes (Conference Presentation)

Force spectroscopy on single molecular machines is often performed using optical tweezers. However, the use of common microspheres composed of silica or polystyrene may have limitations in the maximum force, measurement precision, or the degrees of freedom that can be measured. For example, the ultimate precision of the experiment is limited by the drag coefficient, i.e. the size of the microsphere. Thus, ideally, microspheres should be as small as possible. However, if microspheres are too small, maximum trapping forces are smaller than biological motor-generated forces creating a lower practical size limit of about 200 nm for polystyrene. Here, we have developed germanium nanospheres with diameters ranging from 30--200 nm. With a high refractive index of 4.4, their trapping efficiency is more than 10-fold improved compared to silica. Using 70-nm-diameter germanium nanospheres, we measured the stepping behavior of the molecular motor kinesin-1. With an improved precision, we could measure intermediate steps. In the long-term, the development of novel probes enables novel applications.