▎ 摘　　要

Cryogenic electron microscopy (cryo-EM) is a powerful tool for imaging liquid and semiliquid systems. While cryogenic transmission electron microscopy (cryo-TEM) is a standard technique in many fields, cryogenic scanning electron microscopy (cryo-SEM) is still not that widely used and is far less developed. The vast majority of systems under investigation by cryo-EM involve either water or organic components. In this paper, we introduce the use of novel cryo-TEM and cryo-SEM specimen preparation and imaging methodologies, suitable for highly acidic and very reactive systems. Both preserve the native nanostructure in the system, while not harming the expensive equipment or the user. We present examples of direct imaging of single-walled, multiwalled carbon nanotubes and graphene, dissolved in chlorosulfonic acid and oleum. Moreover, we demonstrate the ability of these new cryo-TEM and cryo-SEM methodologies to follow phase transitions in carbon nanotube (CNT)/superacid systems, starting from dilute solutions up to the concentrated nematic liquid-crystalline CNT phases, used as the dope' for all-carbon-fibre spinning. Originally developed for direct imaging of CNTs and graphene dissolution and self-assembly in superacids, these methodologies can be implemented for a variety of highly acidic systems, paving a way for a new field of nonaqueous cryogenic electron microscopy. Lay description It has been shown that ultra-long carbon nanotubes (CNTs) spontaneously dissolve in superacids (particularly in chlorosulfonic acid, CSA), and, at high concentrations form a nematic liquid crystalline phase, with individual CNTs arranged in long domains. This discovery lays the foundation for fiber spinning from solutions of high quality ultra-long carbon nanotubes. The transition between the isotropic and liquid crystalline phases depends strongly on the CNT type, concentration, and solvent strength. The need to image those systems, which are difficult to handle, and are potentially dangerous to the microscope and the microscopist, has emerged when we realized that superacids are the only effective solvent for carbon nanotubes and graphene sheets without the presence in solution of stabilizers or surfactants. In this work we describe a unique cryogenic electron microscopy (cryo-EM) specimen preparation and imaging methodology, suitable with highly acidic systems that we have developed for direct imaging of CNT/CSA solutions. Controlled cryo-specimen preparation and optimal imaging conditions allow imaging vitrified specimens without harming the equipment and personnel. The cryogenic transmission electron microscope (cryo-TEM) methodology is useful for imaging of non-viscous and dilute solutions, while the methodology we have developed for cryogenic scanning electron microscope (cryo-SEM) allows direct imaging of viscous CNT/CSA systems, including liquid-crystalline phases and their development, from dilute solutions to concentrated liquid-crystalline CNT phases, used as the "dope" for fiber spinning. The new cryo-EM methodology allows the study of basic scientific phenomena at nanometric level in CNT/superacid systems, such as the CNT dissolution mechanism, direct observation of CNT filling with a solvent, correlation between CNT quality and self-assembling into ordered phase, as well as measurement of nanotube length. While originally developed for direct imaging of CNTs and graphene dissolution and for the study of self-assembly in superacids, these methodologies may be implemented in the study of a variety of highly acidic systems.