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

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Polyphenol-Derived Carbonaceous Frameworks with Multiscale Porosity for High-Power Electrochemical Applications

With the escalating global demand for electric vehicles and sustainable energy solutions, increasing focus is placed on developing electrochemical systems that offer fast charging and high-power output, primarily governed by mass transport. Accordingly, porous carbons have emerged as highly promising electrochemically active or supporting materials due to expansive surface areas, tunable pore structures, and superior electrical conductivity, accelerating surface reaction. Yet, while substantial research has been devoted to crafting various porous carbons to increase specific surface areas, the optimal utilization of the surfaces remains underexplored. This review emphasizes the critical role of the fluid dynamics within multiscale porous carbonaceous electrodes, leading to substantially enhanced pore utilization in electrochemical systems. It elaborates on strategies of using sacrificial templates for incorporating meso/macropores into microporous carbon matrix, while exploiting the unique properties of polyphenol moieties such as sustainable carbons derived from biomass, inherent adhesive/cohesive interactions with template materials, and facile complexation capabilities with diverse materials, thereby enabling adaptive structural modulations. Furthermore, it explores how multiscale pore configurations influence pore-utilization efficiency, demonstrating advantages of incorporating multiscale pores. Finally, synergistic impact on the high-power electrochemical systems is examined, attributed to improved fluid-dynamic behavior within the carbonaceous frameworks, providing insights for advancing next-generation high-power electrochemical applications.

Operando Electrodeposition of Nonprecious Metal Copper Nanocatalysts on Low-Dimensional Support Materials for Nitrate Reduction Reactions

Supported nonprecious metal catalysts such as copper (Cu) are promising replacements for Pt-based catalysts for a wide range of energy-related electrochemical reactions. Direct electrochemical deposition is one of the most straightforward and versatile methods to synthesize supported nonprecious metal catalysts. However, further advancement in the design of supported nonprecious metal catalysts requires a detailed mechanistic understanding of the interplay between kinetics and thermodynamics of the deposition phenomena under realistic reaction conditions. Here, we study the electrodeposition of Cu on carbon nanotubes and graphene derivatives under electrochemical conditions using in situ liquid cell transmission electron microscopy (TEM). By combining real-time imaging, electrochemical measurements, X-ray photoelectron spectroscopy (XPS), and finite-element analysis (FEA), we show that low-dimensional support materials, especially carbon nanotubes, are excellent for generating uniform and finely dispersed platinum group metal-(PGM)-free catalysts under mild electrochemical conditions. The electrodeposited Cu on graphene and carbon nanotubes is also observed to show good electrochemical activity toward nitrate reduction reactions (NO3RRs), further supported by density functional theory (DFT) calculations. Nitrogen doping plays an important role in guiding nonprecious metal deposition, but its low electrical conductivity may give rise to lower NO3RR activity compared to its nondoped analogue. The development of supported nonprecious metals through interfacial and surface engineering for the design of supported catalysts will substantially reduce the demand for precious metals and generate robust catalysts with better durability, thereby presenting opportunities for solving the critical problems in energy storage and electrocatalysis.

Theoretical Investigation of the Adsorbate and Potential-Induced Stability of Cu Facets During Electrochemical CO2 and CO Reduction

The activity and product selectivity of electrocatalysts for reactions like the carbon dioxide reduction reaction (CO2RR) are intimately dependent on the catalyst's structure and composition. While engineering catalytic surfaces can improve performance, discovering the key sets of rational design principles remains challenging due to limitations in modeling catalyst stability under operating conditions. Herein, we perform first-principles density functional calculations adopting implicit solvation methods with potential control to study the influence of adsorbates and applied potential on the stability of different facets of model Cu electrocatalysts. Using coverage dependencies extracted from microkinetic models, we describe an approach for calculating potential and adsorbate-dependent contributions to surface energies under reaction conditions, where Wulff constructions are used to understand the morphological evolution of Cu electrocatalysts under CO2RR conditions. We identify that CO*, a key reaction intermediate, exhibits higher kinetically and thermodynamically accessible coverages on (100) relative to (111) facets, which can translate into an increased relative stabilization of the (100) facet during CO2RR. Our results support the known tendency for increased (111) faceting of Cu nanoparticles under more reducing conditions and that the relative increase in (100) faceting observed under CO2RR conditions is likely attributed to differences in CO* coverage between these facets.

Photoilluminated Redox-Processed Rh2P Nanoparticles on Photocathodes for Stable Hydrogen Production in Acidic Environments

While photoelectrochemical (PEC) cells show promise for solar-driven green hydrogen production, exploration of various light-absorbing multilayer coatings has yet to significantly enhance their hydrogen generation efficiency. Acidic conditions can enhance the hydrogen evolution reaction (HER) kinetics and reduce overpotential losses. However, prolonged acidic exposure deactivates noble metal electrocatalysts, hindering their long-term stability. Progress requires addressing catalyst degradation to enable stable, efficient, and acidic PEC cells. Here, we proposed a process design based on the photoilluminated redox deposition (PRoD) approach. We use this to grow crystalline Rh2P nanoparticles (NPs) with a size of 5-10 on 30 nm-thick TiO2, without annealing. Atomically precise reaction control was performed by using several cyclic voltammetry cycles coincident with light irradiation to create a system with optimal catalytic activity. The optimized photocathode, composed of Rh2P/TiO2/Al-ZnO/Cu2O/Sb-Cu2O/ITO, achieved an excellent photocurrent density of 8.2 mA cm-2 at 0 VRHE and a durable water-splitting reaction in a strong acidic solution. Specifically, the Rh2P-loaded photocathode exhibited a 5.3-fold enhancement in mass activity compared to that utilizing just a Rh catalyst. Furthermore, in situ scanning transmission electron microscopy (STEM) was performed to observe the real-time growth process of Rh2P NPs in a liquid cell.

Modes of Nanodroplet Formation and Growth on an Ultrathin Water Film

Using nanobubbles as geometrical confinements, we create a thin water film (∼10 nm) in a graphene liquid cell and investigate the evolution of its instability at the nanoscale under transmission electron microscopy. The breakdown of the water films, resulting in the subsequent formation and growth of nanodroplets, is visualized and generalized into different modes. We identified distinct droplet formation and growth modes by analyzing the dynamic processes involving 61 droplets and 110 liquid bridges within 31 Graphene Liquid Cells (GLCs). Droplet formation is influenced by their positions in GLCs, taking on a semicircular shape at the edge and a circular shape in the middle. Growth modes include liquid mass transfer driven by Plateau-Rayleigh instability and merging processes in and out-of-plane of the graphene interface. Droplet growth can lead to the formation of liquid bridges for which we obtain multiview projections. Data analysis reveals the general dynamics of liquid bridges, including drawing liquids from neighboring residual water films, merging with surrounding droplets, and merging with other liquid bridges. Our experimental observations provide insights into fluid transport at the nanoscale.

Graphene Electrode for Studying CO2 Electroreduction Nanocatalysts under Realistic Conditions in Microcells

The ability to resolve the dynamic evolution of electrocatalytically induced processes with electrochemical liquid-phase electron microscopy (EM) is limited by the microcell configuration. Herein, a free-standing tri-layer graphene is integrated as a membrane and electrode material into the electrochemical chip and its suitability as a substrate electrode at the high cathodic potentials required for CO2 electroreduction (CO2ER) is evaluated. The three-layer stacked graphene is transferred onto an in-house fabricated single-working electrode chip for use with bulk-like reference and counter electrodes to facilitate evaluation of its effectiveness. Electrochemical measurements show that the graphene working electrode exhibits a wider inert cathodic potential range than the conventional glassy carbon electrode while achieving good charge transfer properties for nanocatalytic redox reactions. Operando scanning electron microscopy studies clearly demonstrate the improvement in spatial resolution but reveal a synergistic effect of the electron beam and the applied potential that limits the stability time window of the graphene-based electrochemical chip. By optimizing the operating conditions, in situ monitoring of Cu nanocube degradation is achieved at the CO2ER potential of -1.1 V versus RHE. Thus, this improved microcell configuration allows EM observation of catalytic processes at potentials relevant to real systems.

Microfabricated double-tilt apparatus for transmission electron microscope imaging of atomic force microscope probe

Atomic force microscopy (AFM) uses a scanning stylus to directly measure the surface characteristics of a sample. Since AFM relies on nanoscale interaction between the probe and the sample, the resolution of AFM-based measurement is critically dependent on the geometry of the scanning probe tip. This geometry, therefore, can limit the development of related applications. However, AFM itself cannot be effectively used to characterize AFM probe geometry, leading researchers to rely on indirect estimates based on force measurement results. Previous reports have described sample jigs that enable the observation of AFM probe tips using Transmission Electron Microscopy (TEM). However, such setups are too tall to allow sample tilting within more modern high-resolution TEM systems, which can only tilt samples less than a few millimeters in thickness. This makes it impossible to observe atomic-scale crystallographic lattice fringes by aligning the imaging angle perfectly or to view a flat probe tip profile exactly from the side. We have developed an apparatus that can hold an AFM tip for TEM observation while remaining thin enough for tilting, thereby enabling atomic-scale tip characterization. Using this technique, we demonstrated consistent observation of AFM tip crystal structures using tilting in TEM and found that the radii of curvature of nominally identical probes taken from a single box varied widely from 1.4 nm for the sharpest to 50 nm for the most blunt.

Impact of palladium/palladium hydride conversion on electrochemical CO2 reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

High-resolution and analytical electron microscopy in a liquid flow cell via gas purging

Liquid-phase transmission electron microscopy (LPTEM) technique has been used to perform a wide range of in situ and operando studies. While most studies are based on the sample contrast change in the liquid, acquiring high qualitative results in the native liquid environment still poses a challenge. Herein, we present a novel and facile method to perform high-resolution and analytical electron microscopy studies in a liquid flow cell. This technique is based on removing the liquid from the observation area by a flow of gas. It is expected that the proposed approach can find broad applications in LPTEM studies.

A multiple Kirkendall strategy for converting nanosized zero-valent iron to highly active Fenton-like catalyst for organics degradation

Nanosized zero-valent iron (nZVI) is a promising persulfate (PS) activator, however, its structurally dense oxide shell seriously inhibited electrons transfer for O-O bond cleavage of PS. Herein, we introduced sulfidation and phosphorus-doped biochar for breaking the pristine oxide shell with formation of FeS and FePO4-containing mixed shell. In this case, the faster diffusion rate of iron atoms compared to shell components triggered multiple Kirkendall effects, causing inward fluxion of vacancies with further coalescing into radial nanocracks. Exemplified by trichloroethylene (TCE) removal, such a unique "lemon-slice-like" nanocrack structure favored fast outward transfer of electrons and ferrous ions across the mixed shell to PS activation for high-efficient generation and utilization of reactive species, as evidenced by effective dechlorination (90.6%) and mineralization (85.4%) of TCE. [Formula: see text] contributed most to TCE decomposition, moreover, the SnZVI@PBC gradually became electron-deficient and thus extracted electrons from TCE with achieving nonradical-based degradation. Compared to nZVI/PS process, the SnZVI@PBC/PS system could significantly reduce catalyst dosage (87.5%) and PS amount (68.8%) to achieve nearly complete TCE degradation, and was anti-interference, stable, and pH-universal. This study advanced mechanistic understandings of multiple Kirkendall effects-triggered nanocrack formation on nZVI with corresponding rational design of Fenton-like catalysts for organics degradation.

Comparative effects of curcumin versus nano-curcumin on histological, immunohistochemical expression, histomorphometric, and biochemical changes to pancreatic beta cells and lipid profile of streptozocin induced diabetes in male Sprague-Dawley rats

Diabetes mellitus is a worldwide problem characterized by hyperglycemia as well as the damage of the microscopic structure of the beta cells of Langerhans pancreatic islets. In the present study, the histological, immunohistochemical, morphometric, and biochemical alterations to pancreatic beta cells in streptozocin (STZ)-induced diabetes were assessed in rats treated with curcumin (CU) (100 mg/kg/day) or nano-curcumin (nCU) (100 mg/kg/day) for 1 month. Twenty-four adult male Wistar albino rats were distributed into four groups: the nondiabetic control group, the diabetic untreated group, and two diabetic groups treated with CU or nCUR, respectively. Blood glucose, serum insulin levels, and lipid profile were measured. The pancreatic tissues were collected and processed into paraffin sections for histological and immunohistochemical examination, oxidative stress markers, and real-time PCR expression for pancreatic and duodenal homeobox 1 (PDX1). The insulin expression in beta cells was assessed using immunohistochemistry. Morphometrically, the percentage area of anti-insulin antibody reaction and the percentage area of islet cells were determined. STZ-induced deteriorating alteration in beta cells led to declines in the number of functioning beta cells and insulin immunoreactivity. In STZ-treated rats, CU and nCUR significantly reduced blood glucose concentration while increasing blood insulin level. It also caused a significant increase in the number of immunoreactive beta cells to the insulin expression and significant reduction of the immunoreactive beta cells to the caspase-3 expression. In conclusion, CU and nCUR could have a therapeutic role in the biochemical and microscopic changes in pancreatic beta cells in diabetes-induced rats through STZ administration with more bio-efficacy of nCUR.

Operando studies reveal active Cu nanograins for CO2 electroreduction

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals¹. Although Cu enables CO₂-to-multicarbon product (C₂₊) conversion, the nature of the active sites under operating conditions remains elusive². Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques^3-5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu₂O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO₂ reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. Quantitative structure-activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C₂₊ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C₂₊ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

Liquid cell electrochemical TEM: Unveiling the real-time interfacial reactions of advanced Li-metal batteries

Li metal batteries (LMBs) reveal great application prospect in next-generation energy storage, because of their high energy density and low electrochemical potential, especially when paired with elemental sulfur and oxygen cathodes. Complex interfacial reactions have long been a big concern because of the elusive formation/dissolution of Li metal at the solid-electrolyte interface (SEI) layer, which leads to battery degradation under practical operating conditions. To precisely track the reactions at the electrode/electrolyte interfaces, in the past ten years, high spatio-temporal resolution, in situ electrochemical transmission electron microscopy (EC-TEM) has been developed. A preliminary understanding of the structural and chemical variation of Li metal during nucleation/growth and SEI layer formation has been obtained. In this perspective, we give a brief introduction of liquid cell development. Then, we comparably discuss the different configurations of EC-TEM based on open-cell and liquid-cell, and focus on the recent advances of liquid-cell EC-TEM and its investigation in the electrodes, electrolytes, and SEI. Finally, we present a perspective of liquid-cell EC-TEM for future LMB research.

Observation of H2 Evolution and Electrolyte Diffusion on MoS2 Monolayer by In Situ Liquid-Phase Transmission Electron Microscopy

Unit-cell-thick MoS₂ is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its tunable catalytic activity, which is determined based on the energetics and molecular interactions of different types of HER active sites. Kinetic responses of MoS₂ active sites, including the reaction onset, diffusion of the electrolyte and H₂ bubbles, and continuation of these processes, are important factors affecting the catalytic activity of MoS₂ . Investigating these factors requires a direct real-time analysis of the HER occurring on spatially independent active sites. Herein, the H₂ evolution and electrolyte diffusion on the surface of MoS₂ are observed in real time by in situ electrochemical liquid-phase transmission electron microscopy (LPTEM). Time-dependent LPTEM observations reveal that different types of active sites are sequentially activated under the same conditions. Furthermore, the electrolyte flow to these sites is influenced by the reduction potential and site geometry, which affects the bubble detachment and overall HER activity of MoS₂ .

Visualizing Dynamic Environmental Processes in Liquid at Nanoscale via Liquid-Phase Electron Microscopy

Visualizing the structure and processes in liquids at the nanoscale is essential for understanding the fundamental mechanisms and underlying processes of environmental research. Cutting-edge progress of in situ liquid-phase (scanning) transmission electron microscopy (LP-S/TEM) and inferred possible applications are highlighted as a more and more indispensable tool for visualization of dynamic environmental processes in this Perspective. Advancements in nanofabrication technology, high-speed imaging, comprehensive detectors, and spectroscopy analysis have made it increasingly convenient to use LP S/TEM, thus providing an approach for visualization of direct and insightful scientific information with the exciting possibility of solving an increasing number of tricky environmental problems. This includes evaluating the transformation fate and path of contamination, assessing toxicology of nanomaterials, simulating solid surface corrosion processes in the environment, and observing water pollution control processes. Distinct nanoscale or even atomic understanding of the reaction would provide dependable and precise identification and quantification of contaminants in dynamic processes, thus facilitating trouble-tracing of environmental problems with amplifying complexity.

Elucidating Cathodic Corrosion Mechanisms with Operando Electrochemical Transmission Electron Microscopy

Cathodic corrosion represents an enigmatic electrochemical process in which metallic electrodes corrode under sufficiently reducing potentials. Although discovered by Fritz Haber in the 19th century, only recently has progress been made in beginning to understand the atomistic mechanisms of corroding bulk electrodes. The creation of nanoparticles as the end-product of the corrosion process suggests an additional length scale of complexity. Here, we studied the dynamic evolution of morphology, composition, and crystallographic structural information of nanocrystal corrosion products by analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM). Our operando/in situ electron microscopy revealed, in real-time, at the nanometer scale, that cathodic corrosion yields significantly higher levels of structural degradation for heterogeneous nanocrystals than bulk electrodes. In particular, the cathodic corrosion of Au nanocubes on bulk Pt electrodes led to the unexpected formation of thermodynamically immiscible Au-Pt alloy nanoparticles. The highly kinetically driven corrosion process is evidenced by the successive anisotropic transition from stable Pt(111) bulk single-crystal surfaces evolving to energetically less-stable (100) and (110) steps. The motifs identified in this microscopy study of cathodic corrosion of nanocrystals are likely to underlie the structural evolution of nanoscale electrocatalysts during many electrochemical reactions under highly reducing potentials, such as CO₂ and N₂ reduction.

Liquid-EM goes viral - visualizing structure and dynamics

Liquid-electron microscopy (EM), the room temperature correlate to cryo-EM, is an exciting new technique delivering real-time data of dynamic reactions in solution. Here, we explain how liquid-EM gained popularity in recent years by examining key experiments conducted on viral assemblies and host-pathogen interactions. We describe developing workflows for specimen preparation, data collection, and computing processes that led to the first high-resolution virus structures in a liquid environment. Equally important, we review why liquid-electron tomography may become the next big thing in biomedical research due to its ability to monitor live viruses entering cells within seconds. Taken together, we pose the idea that liquid-EM can serve as a dynamic complement to current cryo-EM methods, inspiring the "real-time revolution" in nanoscale imaging.

Unusual Mesoporous Titanium Niobium Oxides Realizing Sodium-Ion Batteries Operated at -40 °C

Sodium-ion batteries (SIBs) are a promising candidate for grid-scale energy storage, however, the sluggish ion-diffusion kinetics brought by the large radius of Na⁺ seriously limits the performance of SIBs, let alone at low temperatures. Herein, a confined acid-base pair self-assembly strategy to synthesize unusual Ti_0.88 Nb_0.88 O_{4- x} @C for high-performance SIBs operating at room and low temperatures is proposed. The confinement self-assembly of the acid-base pair around the micelles and confined crystallization by the carbon layer realize the formation of ordered and stoichiometric mesoporous frameworks with opened ion channels. Thus, the mesoporous Ti_0.88 Nb_0.88 O_{4- x} @C exhibits rapid Na⁺ diffusion kinetics at 25 and -40 °C, which are one order higher than that of the nonporous one. A high reversible capacity of 233 mAh g^-1 , excellent rate (a specific capacity of 103 mAh g^-1 at 50 C), and cycling performances (<0.03% fading per cycle) can be observed at 25 °C. More importantly, even at -40 °C, the mesoporous Ti_0.88 Nb_0.88 O_{4- x} @C can still deliver the 161 mAh g^-1 capacity, a high initial Coulombic efficiency of 60% and outstanding cycling stability (99 mAh g^-1 at 0.5 C after 500 cycles). It is believed this strategy opens a new avenue for constructing novel mesoporous electrode materials for low-temperature energy storage.

Automated Tools to Advance High-Resolution Imaging in Liquid

Liquid-electron microscopy (EM), the room-temperature correlate to cryo-EM, is a rapidly growing field providing high-resolution insights of macromolecules in solution. Here, we describe how liquid-EM experiments can incorporate automated tools to propel the field to new heights. We demonstrate fresh workflows for specimen preparation, data collection, and computing processes to assess biological structures in liquid. Adeno-associated virus (AAV) and the SARS-CoV-2 nucleocapsid (N) were used as model systems to highlight the technical advances. These complexes were selected based on their major differences in size and natural symmetry. AAV is a highly symmetric, icosahedral assembly with a particle diameter of ~25 nm. At the other end of the spectrum, N protein is an asymmetric monomer or dimer with dimensions of approximately 5–7 nm, depending upon its oligomerization state. Equally important, both AAV and N protein are popular subjects in biomedical research due to their high value in vaccine development and therapeutic efforts against COVID-19. Overall, we demonstrate how automated practices in liquid-EM can be used to decode molecules of interest for human health and disease.

Hydroxyapatite as a scavenger of reactive radiolysis species in graphene liquid cells forin situelectron microscopy

Liquid cell electron microscopy is an imaging technique allowing for the investigation of the interaction of liquids and solids at nanoscopic length scales. Suchin situobservations are increasingly in-demand in an array of fields, from biological sciences to medicine to batteries. Graphene liquid cells (GLCs), in particular, have generated a great interest as a low-scattering window material with the potential for increasing the quality of both imaging and spectroscopy. However, preserving the stability of the liquid and of the sample in the GLC remains a considerable challenge. In the present work we encapsulate water and hydroxyapatite (HAP), a pH-sensitive biological material, in GLCs to observe the interactions between the graphene, HAP, and the electron beam. HAP was chosen for several reasons. One is its ubiquity in biological specimens such as bones and teeth, and the second is the presence of phosphate ions in common buffer solutions. Finally, there is its sensitivity to changes in pH, which result from beam-induced hydrolysis in liquid cells. A dynamic process of dissolution and recrystallization of HAP was observed, which correlated with the production of H⁺ions by the beam during imaging. In addition, a large increase in the stability of the GLC under irradiation was noted. Specifically, no stable hydrogen bubbles were detected under the electron fluxes routinely exceeding 170 e^-Å^-2s^-1. With the measured threshold dose for the bubble formation in pure water equaling 9 e^-Å^-2s^-1, it was concluded that the presence of HAP increases the resistance of water against radiolysis in the GLC by more than an order of magnitude. These results confirm the possibility of using biological materials, such as HAP, as stabilizers in liquid cell electron microscopy. They outline a potential route for stabilization of specimens in liquid cells through the addition of a scavenger of reactive species generated by the beam-induced hydrolysis of water. These improvements are essential for enhancing both the resolution of imaging and the available imaging time, as well as avoiding the beam-induced artifacts.