Direct observation of biological molecules in liquid by environmental phase-plate transmission electron microscopy

Nanoscale Chemical and Structural Analysis during In Situ Scanning/Transmission Electron Microscopy in Liquids

Liquid-cell scanning/transmission electron microscopy (S/TEM) has impacted our understanding of multiple areas of science, most notably nanostructure nucleation and growth and electrochemistry and corrosion. In the case of electrochemistry, the incorporation of electrodes requires the use of silicon nitride membranes to confine the liquid. The combined thickness of the liquid layer and the confining membranes prevents routine atomic-resolution characterization. Here, we show that by performing electrochemical water splitting in situ to generate a gas bubble, we can reduce the thickness of the liquid to a film approximately 30 nm thick that remains covering the sample. The reduced thickness of the liquid allows the acquisition of atomic-scale S/TEM images with chemical and valence analysis through electron energy loss spectroscopy (EELS) and structural analysis through selected area electron diffraction (SAED). This contrasts with a specimen cell entirely filled with liquid, where the broad plasmon peak from the liquid obscures the EELS signal from the sample and induces beam incoherence that impedes SAED analysis. The gas bubble generation is fully reversible, which allows alternating between a full cell and thin-film condition to obtain optimal experimental and analytical conditions, respectively. The methodology developed here can be applied to other scientific techniques, such as X-ray scattering, Raman spectroscopy, and X-ray photoelectron spectroscopy, allowing for a multi-modal, nanoscale understanding of solid-state samples in liquid media.

Mapping and Controlling Liquid Layer Thickness in Liquid-Phase (Scanning) Transmission Electron Microscopy

Liquid-Phase (Scanning) Transmission Electron Microscopy (LP-(S)TEM) has become an essential technique to monitor nanoscale materials processes in liquids in real-time. Due to the pressure difference between the liquid and the microscope vacuum, bending of the silicon nitride (SiN_x ) membrane windows generally occurs. This causes a spatially varying liquid layer thickness that makes interpretation of LP-(S)TEM results difficult due to a locally varying achievable resolution and diffusion limitations. To mediate these difficulties, it is shown: 1) how to quantitatively map liquid layer thickness for any liquid at less than 0.01 e^- Å^-2 total dose; 2) how to dynamically modulate the liquid thickness by tuning the internal pressure in the liquid cell, co-determined by the Laplace pressure and the external pressure. It is demonstrated that reproducible inward bulging of the window membranes can be realized, leading to an ultra-thin liquid layer in the central window area for high-resolution imaging. Furthermore, it is shown that the liquid thickness can be dynamically altered in a programmed way, thereby potentially overcoming the diffusion limitations towards achieving bulk solution conditions. The presented approaches provide essential ways to measure and dynamically adjust liquid thickness in LP-(S)TEM experiments, enabling new experiment designs and better control of solution chemistry.

Environmental Liquid Cell Technique for Improved Electron Microscopic Imaging of Soft Matter in Solution

Liquid-phase transmission electron microscopy is a technique for simultaneous imaging of the structure and dynamics of specimens in a liquid environment. The conventional sample geometry consists of a liquid layer tightly sandwiched between two Si3N4 windows with a nominal spacing on the order of 0.5 μm. We describe a variation of the conventional approach, wherein the Si3N4 windows are separated by a 10-μm-thick spacer, thus providing room for gas flow inside the liquid specimen enclosure. Adjusting the pressure and flow speed of humid air inside this environmental liquid cell (ELC) creates a stable liquid layer of controllable thickness on the bottom window, thus facilitating high-resolution observations of low mass-thickness contrast objects at low electron doses. We demonstrate controllable liquid thicknesses in the range 160 ± 34 to 340 ± 71 nm resulting in corresponding edge resolutions of 0.8 ± 0.06 to 1.7 ± 0.8 nm as measured for immersed gold nanoparticles. Liquid layer thickness 40 ± 8 nm allowed imaging of low-contrast polystyrene particles. Hydration effects in the ELC have been studied using poly-N-isopropylacrylamide nanogels with a silica core. Therefore, ELC can be a suitable tool for in situ investigations of liquid specimens.

Phase plates in the transmission electron microscope: operating principles and applications

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

Liquid-Phase Electron Microscopy for Soft Matter Science and Biology

Innovations in liquid-phase electron microscopy (LP-EM) have made it possible to perform experiments at the optimized conditions needed to examine soft matter. The main obstacle is conducting experiments in such a way that electron beam radiation can be used to obtain answers for scientific questions without changing the structure and (bio)chemical processes in the sample due to the influence of the radiation. By overcoming these experimental difficulties at least partially, LP-EM has evolved into a new microscopy method with nanometer spatial resolution and sub-second temporal resolution for analysis of soft matter in materials science and biology. Both experimental design and applications of LP-EM for soft matter materials science and biological research are reviewed, and a perspective of possible future directions is given.

Investigation of hydrated smectite microstructure through wet environmental transmission electron microscopy

Water is an essential constituent of all biological materials as well as many non-biological materials. Not only the removal of water may result in undesirable morphological and structure change, the inability to sustain the hydrated conditions in the microscope also prevents the study of reactions which take place in aqueous environment. In order to overcome these problems we used wet environmental-cell transmission electron microscopy TEM (WETEM). Conventional TEM of dry smectite showed well-defined particle outlines (but without a specific shape) and typical smectite aggregates. Selected area electron diffraction (SAD) of dry particles showed stacking of smectite particles (i.e., aggregate) in very clear dot and ring patterns. In contrast, WETEM depicted well-dispersed clay particles showing a variety of different particle shapes. Analysis of SAD patterns obtained from dry and hydrated states illustrated a lattice change in different environments. The small lattice expansion in (h k 0) resulted from the expansion of the (0 0 l) plane resulting from the addition of water molecules in the crystal along the c-axis.

Liquid-Phase Electron Microscopy with Controllable Liquid Thickness

Liquid-phase electron microscopy (LPEM) is capable of imaging nanostructures and processes in a liquid environment. The spatial resolution achieved with LPEM critically depends on the thickness of the liquid layer surrounding the object of interest. An excessively thick liquid results in broadening of the electron beam and a high background signal that decreases the resolution and contrast of the object in an image. The liquid thickness in a standard liquid cell, consisting of two liquid enclosing membranes separated by spacers, is mainly defined by the deformation of the SiN membrane windows toward the vacuum side, and the effective thickness may differ from the spacer height. Here, we present a method involving a pressure controller setup to balance the pressure difference over the membrane windows, thus manipulating the shape profiles of the used silicon nitride membrane windows. Electron energy loss spectroscopy (EELS) measurements to determine the liquid thickness showed that it is possible to control the thickness precisely during an LPEM experiment by regulating the interior pressure of the liquid cell. We demonstrated atomic resolution on gold nanoparticles and the phase contrast using silica nanoparticles in liquid with controlled thickness.

Contrast enhancement of nanomaterials using phase plate STEM

Visualizing materials composed of light elements is difficult, and the development of an imaging method that enhances the phase contrast of such materials has been of much interest. In this study, we demonstrate phase-plate scanning transmission electron microscopy (P-STEM), which we developed recently, and its application to nanomaterials. An amorphous carbon film with a small hole in its center was used to control the phase of incident electron waves, and the phase-contrast transfer function (PCTF) was modified from sine-type to cosine-type. The modification of the PCTF enhances image contrast with a spatial frequency below 1 nm^-1. The PCTF for P-STEM with a spatial frequency below 1 nm^-1 is about three times stronger than that of bright field STEM. The ratio obtained using power spectra is consistent with the result obtained from images of quantum dots. The image contrast of biological materials was also enhanced by P-STEM.

Operando chemistry of catalyst surfaces during catalysis

The chemistry of a catalyst surface during catalysis is crucial for a fundamental understanding of the mechanisms of a catalytic reaction performed on the catalyst in the gas or liquid phase.

Generation and application of bessel beams in electron microscopy

We report a systematic treatment of the holographic generation of electron Bessel beams, with a view to applications in electron microscopy. We describe in detail the theory underlying hologram patterning, as well as the actual electron-optical configuration used experimentally. We show that by optimizing our nanofabrication recipe, electron Bessel beams can be generated with relative efficiencies reaching 37±3%. We also demonstrate by tuning various hologram parameters that electron Bessel beams can be produced with many visible rings, making them ideal for interferometric applications, or in more highly localized forms with fewer rings, more suitable for imaging. We describe the settings required to tune beam localization in this way, and explore beam and hologram configurations that allow the convergences and topological charges of electron Bessel beams to be controlled. We also characterize the phase structure of the Bessel beams generated with our technique, using a simulation procedure that accounts for imperfections in the hologram manufacturing process.

Recent developments of the in situ wet cell technology for transmission electron microscopies

In situ wet cells for transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) allow studying structures and processes in a liquid environment with high temporal and spatial resolutions, and have been attracting increasing research interests in many fields. In this review, we highlight the structural and functional developments of the wet cells for TEM and STEM. One of the key features of the wet cells is the sealing technique used to isolate the liquid sample from the TEM/STEM vacuum environments, thus the existing in situ wet cells are grouped by different sealing methods. In this study, the advantages and shortcomings of each type of in situ wet cells are discussed, the functional developments of different wet cells are presented, and the future trends of the wet cell technology are addressed. It is suggested that in the future the in situ wet cell TEM/STEM technology will have an increasing impact on frontier nanoscale research.

Nanoscale analysis of unstained biological specimens in water without radiation damage using high-resolution frequency transmission electric-field system based on FE-SEM

Scanning electron microscopy (SEM) has been widely used to examine biological specimens of bacteria, viruses and proteins. Until now, atmospheric and/or wet biological specimens have been examined using various atmospheric holders or special equipment involving SEM. Unfortunately, they undergo heavy radiation damage by the direct electron beam. In addition, images of unstained biological samples in water yield poor contrast. We recently developed a new analytical technology involving a frequency transmission electric-field (FTE) method based on thermionic SEM. This method is suitable for high-contrast imaging of unstained biological specimens. Our aim was to optimise the method. Here we describe a high-resolution FTE system based on field-emission SEM; it allows for imaging and nanoscale examination of various biological specimens in water without radiation damage. The spatial resolution is 8 nm, which is higher than 41 nm of the existing FTE system. Our new method can be easily utilised for examination of unstained biological specimens including bacteria, viruses and protein complexes. Furthermore, our high-resolution FTE system can be used for diverse liquid samples across a broad range of scientific fields, e.g. nanoparticles, nanotubes and organic and catalytic materials.

Direct observation of unstained biological specimens in water by the frequency transmission electric-field method using SEM

Scanning electron microscopy (SEM) is a powerful tool for the direct visualization of biological specimens at nanometre-scale resolution. However, images of unstained specimens in water using an atmospheric holder exhibit very poor contrast and heavy radiation damage. Here, we present a new form of microscopy, the frequency transmission electric-field (FTE) method using SEM, that offers low radiation damage and high-contrast observation of unstained biological samples in water. The wet biological specimens are enclosed in two silicon nitride (SiN) films. The metal-coated SiN film is irradiated using a focused modulation electron beam (EB) at a low-accelerating voltage. A measurement terminal under the sample holder detects the electric-field frequency signal, which contains structural information relating to the biological specimens. Our results in very little radiation damage to the sample, and the observation image is similar to the transmission image, depending on the sample volume. Our developed method can easily be utilized for the observation of various biological specimens in water.

Atomic force microscopy: High resolution dynamic imaging of cellular and molecular structure in health and disease

The atomic force microscope (AFM), invented in 1986, and a member of the scanning probe family of microscopes, offers the unprecedented ability to image biological samples unfixed and in a hydrated environment at high resolution. This opens the possibility to investigate biological mechanisms temporally in a heretofore unattainable resolution. We have used AFM to investigate: (1) fundamental issues in cell biology (secretion) and, (2) the pathological basis of a human thrombotic disease, the antiphospholipid syndrome (APS). These studies have incorporated the imaging of live cells at nanometer resolution, leading to discovery of the "porosome," the universal secretory portal in cells, and a molecular understanding of membrane fusion from imaging the interaction and assembly of proteins between opposing lipid membranes. Similarly, the development of an in vitro simulacrum for investigating the molecular interactions between proteins and lipids has helped define an etiological explanation for APS. The prime importance of AFM in the success of these investigations will be presented in this manuscript, as well as a discussion of the limitations of this technique for the study of biomedical samples.

A carbon sandwich environmental cell for wet specimens

A carbon sandwich environmental cell for wet specimens was developed for environmental transmission electron microscopy (E-TEM). A plastic film with many holes was used to form carbon capsules. The carbon sandwich environmental cells were composed of the spacer and two carbon films and enclosing the samples and experimental solution. The thickness of the water layer can be controlled by changing the conditions used to prepare the plastic spacers. The quality of the images was improved over our previous E-TEM, which could circulate gas around samples. A resolution of 2 nm was obtained by using loosely condensed DNAs.