Cryo-FIB specimen preparation for use in a cartridge-type cryo-TEM

Atomic-level imaging of beam-sensitive COFs and MOFs by low-dose electron microscopy

Electron microscopy, an important technique that allows for the precise determination of structural information with high spatiotemporal resolution, has become indispensable in unravelling the complex relationships between material structure and properties ranging from mesoscale morphology to atomic arrangement. However, beam-sensitive materials, particularly those comprising organic components such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), would suffer catastrophic damage from the high energy electrons, hindering the determination of atomic structures. A low-dose approach has arisen as a possible solution to this problem based on the integration of advancements in several aspects: electron optical system, detector, image processing, and specimen preservation. This article summarizes the transmission electron microscopy characterization of MOFs and COFs, including local structures, host-guest interactions, and interfaces at the atomic level. Revolutions in advanced direct electron detectors, algorithms in image acquisition and processing, and emerging methodology for high quality low-dose imaging are also reviewed. Finally, perspectives on the future development of electron microscopy methodology with the support of computer science are presented.

Cryo-FIB for TEM investigation of soft matter and beam sensitive energy materials

Primarily driven by structural biology, the rapid advances in cryogenic electron microscopy techniques are now being adopted and applied by materials scientists. Samples that inherently have electron transparency can be rapidly frozen (vitrified) in amorphous ice and imaged directly on a cryogenic transmission electron microscopy (cryo-TEM), however this is not the case for many important materials systems, which can consist of layered structures, embedded architectures, or be contained within a device. Cryogenic focused ion beam (cryo-FIB) lift-out procedures have recently been developed to extract intact regions and interfaces of interest, that can then be thinned to electron transparency and transferred to the cryo-TEM for characterization. Several detailed studies have been reported demonstrating the cryo-FIB lift-out procedure, however due to its relative infancy in materials science improvements are still required to ensure the technique becomes more accessible and routinely successful. Here, we review recent results on the preparation of cryo-TEM lamellae using cryo-FIB and show that the technique is broadly applicable to a range of soft matter and beam sensitive energy materials. We then present a tutorial that can guide the materials scientist through the cryo-FIB lift-out process, highlighting recent methodological advances that address the most common failure points of the technique, such as needle attachment, lift-out and transfer, and final thinning.

Molecular Dynamics Simulation and Cryo-Electron Microscopy Investigation of AOT Surfactant Structure at the Hydrated Mica Surface

Structural properties of the anionic surfactant dioctyl sodium sulfosuccinate (AOT or Aerosol-OT) adsorbed on the mica surface were investigated by molecular dynamics simulation, including the effect of surface loading in the presence of monovalent and divalent cations. The simulations confirmed recent neutron reflectivity experiments that revealed the binding of anionic surfactant to the negatively charged surface via adsorbed cations. At low loading, cylindrical micelles formed on the surface, with sulfate head groups bound to the surface by water molecules or adsorbed cations. Cation bridging was observed in the presence of weakly hydrating monovalent cations, while sulfate groups interacted with strongly hydrating divalent cations through water bridges. The adsorbed micelle structure was confirmed experimentally with cryogenic electronic microscopy, which revealed micelles approximately 2 nm in diameter at the basal surface. At higher AOT loading, the simulations reveal adsorbed bilayers with similar surface binding mechanisms. Adsorbed micelles were slightly thicker (2.2–3.0 nm) than the corresponding bilayers (2.0–2.4 nm). Upon heating the low loading systems from 300 K to 350 K, the adsorbed micelles transformed to a more planar configuration resembling bilayers. The driving force for this transition is an increase in the number of sulfate head groups interacting directly with adsorbed cations.

An introduction to cryo-FIB-SEM cross-sectioning of frozen, hydrated Life Science samples

The introduction of cryo‐techniques to the focused ion‐beam scanning electron microscope (FIB‐SEM) has brought new opportunities to study frozen, hydrated samples from the field of Life Sciences. Cryo‐techniques have long been employed in electron microscopy. Thin electron transparent sections are produced by cryo‐ultramicrotomy for observation in a cryo‐transmission electron microscope (TEM). Cryo‐TEM is presently reaching the imaging of macromolecular structures. In parallel, cryo‐fractured surfaces from bulk materials have been investigated by cryo‐SEM.

Both cryo‐TEM and cryo‐SEM have provided a wealth of information, despite being 2D techniques. Cryo‐TEM tomography does provide 3D information, but the thickness of the volume has a maximum of 200–300 nm, which limits the 3D information within the context of specific structures. FIB‐milling enables imaging additional planes by creating cross‐sections (e.g. cross‐sectioning or site‐specific X‐sectioning) perpendicular to the cryo‐fracture surface, thus adding a third imaging dimension to the cryo‐SEM. This paper discusses how to produce suitable cryo‐FIB‐SEM cross‐section results from frozen, hydrated Life Science samples with emphasis on ‘common knowledge’ and reoccurring observations.

Lay Description

Life Sciences studies life down to the smallest details. Visualising the smallest details requires electron microscopy, which utilises high‐vacuum chambers. One method to maintain the integrity of Life Sciences samples under vacuum conditions is freezing. Frozen samples can remain in a suspended state. As a result, research can be carried out without having to change the chemistry or internal physical structure of the samples.

Two types of electron microscopes equipped with cryo‐sample handling facilities are used to investigate samples: The scanning electron microscope (SEM) which investigates surfaces and the transmission electron microscope (TEM) which investigates thin electron transparent sections (called lamellae). A third method of investigation combines a SEM with a focused ion beam (FIB) to form a cryo‐FIB‐SEM, which is the basis of this paper. The electron beam images the cryo‐sample surface while the ion beam mills into the surface to expose the interior of the sample. The latter is called cross‐sectioning and the result provides a way of investigating the 3rd dimension of the sample. This paper looks at the making of cross‐sections in this manner originating from knowledge and experience gained with this technique over many years. This information is meant for newcomers, and experienced researchers in cryo‐microscopy alike.

Accessing crystal-crystal interaction forces with oriented nanocrystal atomic force microscopy probes

Biominerals serve as critical structures of living systems and play important roles in biochemical processes. Understanding their crystallization mechanisms is therefore central to many areas of biology, biogeoscience, and biochemistry. Some biominerals, such as bone and dentin, are hierarchical nanocomposite structures constructed by sequential addition of individual oriented nanocrystals. The driving forces that enable this oriented assembly are still poorly understood, with advances in understanding limited in part by the availability of techniques that can precisely measure the delicate interactions between nanocrystals as a function of their separation distance and mutual orientation. Here, we provide a comprehensive protocol for (i) fabricating oriented single-nanocrystal atomic force microscopy (AFM) probes using focused ion beam (FIB) milling and (ii) performing oriented nanocrystal interaction force measurements using dynamic force spectroscopy (DFS)-based AFM and environmental transmission electron microscopy (ETEM)-AFM techniques. We illustrate how to fabricate oriented nanocrystal force probes using commercial bulk crystals or nano/microcrystals of calcite, zinc oxide, and rutile. The typical protocol for fabricating one AFM crystal probe takes 2-3 h. In addition, we illustrate how to quantify the direction-specific interaction forces for a given pair of interacting oriented nanocrystal faces. The methods are fully transferrable to other minerals of interest, such as the apatites constituting bone minerals. This allows researchers across many fields to measure and understand particle-based crystallization processes.