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Cryo-Ultramicrotomy


the Au nanoparticles ( Figure 7a ). As a result, the distribution and concentration of Au nanoparticles in the suspended 5CB films can be modified during the blotting process. Cholesteric liquid crystals . Cholesteric (that is, chiral nematic) LC molecules form 2-dimensional nematic-like thin layers. Molecules in each layer orient along a common preferred direction (director). The directors of different layers twist to form a helical pattern ( Figure 8a ). Figures 8 b and 8 c show typical cryo-TEM images of the “fingerprint” texture of a cholesteric LC, namely a chirally doped E7 (a LC mixture of several cyanobi- phenyls with long aliphatic tails) [ 9 ], prepared by thin film plunge freezing ( Figure 2b ) and bulk cryo-ultramicrotomy ( Figure 2d ), respectively. The distinct alternating contrast in Figure 8b corresponds to the helical twist. Figure 8b also indicates that the surface anchoring effect tends to have the helix axis lying on the supporting carbon film. This simple geometry makes it convenient to determine the pitch of the helical twist (roughly 2 times the periodicity of the distinct alternating pattern). In contrast, the sectioning direction is normally at an angle to the helical axis in the bulk approach without pre-alignment ( Figure 8c ), which often results in larger contrast periodicity.


Twist-bend nematic phase of liquid crystals . Figure 9 shows another interesting example: twist-bend nematic phase (N tb ) [ 10 ]. This “new” nematic structure has been long predicated in addition to the earlier normal nematic ( Figure 1 ) and chiral nematic ( Figure 8a ) LCs, but was only confirmed recently thanks to combined freeze fracture TEM and cryo-TEM [ 10 ]. A major difference between chiral nematic and twist-bend nematic phases is that the former has a 90° angle between twist axis and the directors, whereas the latter has a smaller angle ( Figure 9a ). In addition, there is no modulated density but only a 1D modulation of orientation order [ 4 , 10 ]. Liquid crystal molecules, typically rod-like or long bent molecules, tend to fracture along their long directions, so the orientational ordering can be revealed by the surface-sensitive freeze-fracture TEM as a clear 1D


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modulation of ~8 nm in periodicity in 1”,7”-bis(4-cyanobi- phenyl-4’-yl)heptane (CB7CB) ( Figure 9b ). In contrast, cryo-TEM of cryo-sectioned specimens shows homoge- neous contrast ( Figure 9c ). Note similar procedures (plunge freezing, sample dimensions, etc.) were used in the freezing process before both the fracture and the cryo-sectioning. The comparative result ( Figures 9 b and 9 c) indicates that CB7CB has 1D orientational order, but no modulated density, which provides one of the strongest evidences of the existence of the N tb phase so far [ 4 , 10 ].


Figure 6 : CEMOVIS of a nematic disodium cromoglycate (DSCG) solution with added dextran. (a) and (b) Side- and top-views of the columnar aggregates (dark lines and dots, respectively); (c) variations of aggregate orientation. The white dashed line indicates a domain boundary. The short yellow dotted lines track local aggregate orientations (directors). Red dots are placed in the areas where the aggregates are parallel to the electron beam. The FFT patterns on the right side correspond to the areas enclosed by squares in the same outline colors. The black arrow indicates a knife mark (bright line).


www.microscopy-today.com • 2018 March


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