Synthesis of Germanium Nanospheres as High-Precision Optical Tweezers Probes.

Force spectroscopy on single molecular machines generating piconewton forces is often performed using optical tweezers. Since trapping forces scale with the particle volume, piconewton-force measurements so far required micron-sized probes practically limiting the spatiotemporal resolution. Here, we have overcome this limit by developing high-refractive index germanium nanospheres as ultraresolution trapping probes. With a refractive index of 4.4, their trapping efficiency and maximum force per power is more than 10-fold higher compared to silica spheres of equal size. Therefore, the use of germanium allows piconewton-force measurements with nanometer sized probes. Using 70-nm-diameter germanium nanospheres as trappable optical probes (GeNTOPs), we could show that kinesin-1 walks with 4-nm-center-of-mass steps. In the long-term, the application of these novel high-precision GeNTOPs will provide new insight into the working mechanism of molecular machines and are promising candidates for other applications in microscopy, optoelectronics, and nanophotonics.