Grids designed for tomography: Stereovision transmission electron microscopy makes it easy to determine the winding handedness of helical nanocoils.

Determining the handedness of helical nanocoils using transmission electron microscopy (TEM) has traditionally been challenging due to the deep depth of field and transmission nature of TEM, complementary techniques are considered necessary and have been practiced such as low angle rotary shadowing, scanning electron microscopy (SEM), or atomic force microscopy (AFM). These methods require customized sample preparation, making direct comparison difficult. Inspired by the need to identify the helical winding direction from TEM images alone, we developed a specialized tomography grid to capture stereo-pair images, enabling stereopsis. By leveraging previous research on nano-coiled structures using identical materials and tomography grids, we successfully identified the handedness of helical coils. Our model sample consisted of graphitic nanotubes with bilayer ribbons of π-stacked hexa-peri-hexabenzocoronene (HBC) units, forming right- and left-handed helical coils from (S)- and (R)-enantiomers of the amphiphile [Jin W. et al. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10801-10806]. Using stereo-pair TEM images, we evaluated the accuracy of our approach in visually determining the handedness of helical coils. The technique provides a valuable tool for sample inspection, screening, and assessing relative positions, including the determination of helical handedness.