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Cryo-Ultramicrotomy


Figure 2 : Schematics of “thin fi lm” (a and b) and “bulk” (c and d) approaches to prepare cryo-TEM specimens of soft-matter materials. Procedures (a) and (b) are used to plunge freeze lyotropic LC (a) and thermotropic LC (b) thin fi lms that are partially electron-transparent; (c) and (d) are applied to cryo-sectioning of high-pressure frozen lyotropic (c) and plunge frozen thermotropic (d) “bulk” LC samples using cryo-ultramicrotomy.


Materials and Methods Liquid crystals . T e LCs examined in this article include thermotropic and lyotropic LCs of diff erent viscosities and various compositions (single and multiple component molecules, chiral dopants, nanoparticles). Table 1 provides basic information about some common LCs presented in this article. T in fi lm specimens . For comparative studies, LC thin


fi lm specimens were prepared on holey carbon-coated TEM grids and rapidly quenched in liquid ethane or liquid nitrogen ( Figures 2 a and 2 b). For lyotropic LCs and low-viscosity thermotropic LCs (for example, 5CB), an automatic plunge freezer (FEI Mark IV Vitrobot) was employed to produce the thin fi lm specimens by blotting and rapid plunge freezing.


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Electron-transparent thin fi lms of thermotropic LCs can be prepared in several ways [ 5 ]. Here we used a simple procedure ( Figure 2b ) consisting of the following steps: dissolving LC in volatile solvent, thin fi lm formation through solvent evaporation, thin fi lm thermal treatment (heating to isotropic phase and then cooling to achieve the desired phase), and manual rapid plunge freezing [ 4 ]. Bulk specimens . Our preferred freezing method for bulk specimens employed a Leica EM PACT 2 high-pressure unit to rapidly freeze “bulk” lyotropic LCs ( Figure 2c ). A high-pressure (~2000 atm) was applied briefl y (~20 ms) to the specimen solution sealed in a copper capillary tube (~350 µm inner diameter) prior to the freezing process. T is prevents volume expansion, and thus the crystallization of water, so


www.microscopy-today.com • 2018 March


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