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Cryo-Ultramicrotomy


Figure 3 : Cryo-TEM results using plunge-frozen LC thin fi lms. (a) Side-view and top-view (inset) of a Ag-containing nematic lyotropic chromonic LC. The dark dots in the top-view are corresponding to the elongated aggregates. (b) A select-area electron diffraction pattern of a bent-core nematic LC; (c) smectic structure in a bent-core thermotropic LC. The dotted line marks curved smectic layers that are parallel to the electron beam. (d) A double-twisted helix in a cholesteric blue phase LC.


nematic LC, the molecules and their arrangement usually cannot be resolved by routine cryo-TEM imaging, but they can be detected by local electron diffraction ( Figure 3b ). The inner arcs match the long dimension of the bent-core LC molecules and are along the preferred molecular orientation (director). The weak intensity and the absence of higher-order diffraction indicate the lack of a well-defined long-range 1D lattice. The diffuse outer arcs correspond to the short- range order perpendicular to the bent-core molecules. Figures 3 c and 3 d are cryo-TEM images of a smectic phase [ 4 ] and a self-assembled double-twisted helical structure in a blue phase [ 7 ], respectively. Compared to the traditional freeze fracture replica TEM technique used in LC studies, the cryo-TEM observations in Figure 3 demonstrate certain advantages, such as higher image resolution, examination through the actual material, and the availability of additional TEM techniques (diffraction, spectroscopy, tomography, etc.). Surface effects with thin film


approach . The thin film approach ( Figures 2 a and 2 b) is a relatively easy way to prepare cryo-TEM specimens of LCs. In this case, the LC molecular orientation is known to be largely controlled by surface/interface properties. The so-called surface anchoring effect has significant applications in the LCD screen industry, but it imposes challenges on the preparation of thin film specimens representing the LC bulk structure [ 4 ]. Figure 4a is a cryo-TEM image of a thin film lyotropic chromonic LC called DSCG (see Table 1 ), known also as an anti-asthmatic drug. The orientation of the columnar aggregates is strongly influenced by the hole shape and is roughly along the long direction of the hole. Figure 4c shows layered smectic


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nanoclusters in a bent-core thermotropic nematic LC [ 4 ]. The layer number and cluster width vary strongly as a function of the LC film thickness. CEMOVIS of lyotropic chromonic LCs . Figure 5 shows results for a model lyotropic chromonic LC, SSY ( Table 1 ) also known as sunset yellow FCF (a food color dye), using the bulk approach of combining high-pressure freezing, cryo-ultramicrotomy, and cryo-TEM, that is, CEMOVIS. In a typical lyotropic chromic LC solution, disk-like molecules stack face-to-face and form columnar aggregates that are the assembly units of the LC structure ( Figure 5a ). Lyotropic chromonic LCs often feature close distances between aggregates and require high-resolution imaging techniques to resolve the aggregates. Figure 5b is a cryo-TEM image of a ~30 wt% SSY solution prepared by thin film plunge freezing, and Figure 5c shows the same structure prepared


Figure 4 : Examples showing the surface effect on cryo-TEM results in LC thin fi lms. (a) Suspended lyotropic chromonic disodium cromoglycate (DSCG) thin fi lm. The elongated aggregates are roughly aligned along the long direction of the holes in the supporting carbon fi lm. (b–d) Morphology of smectic nanoclusters in a bent-core nematic LC varies with fi lm thickness: <100 nm (b), ~150 nm (c), >200 nm (d), respectively.


www.microscopy-today.com • 2018 March


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