Observing the Growth of Metal–Organic Frameworks by in Situ Liquid Cell Transmission Electron Microscopy

In Situ Transmission Electron Microscopy of Electrocatalyst Materials: Proposed Workflows, Technical Advances, Challenges, and Lessons Learned

In situ electrochemical liquid phase transmission electron microscopy (LP-TEM) measurements utilize micro-chip three-electrode cells with electron transparent silicon nitride windows that confine the liquid electrolyte. By imaging electrocatalysts deposited on micro-patterned electrodes, LP-TEM provides insight into morphological, phase structure, and compositional changes within electrocatalyst materials under electrochemical reaction conditions, which have practical implications on activity, selectivity, and durability. Despite LP-TEM capabilities becoming more accessible, in situ measurements under electrochemical reaction conditions remain non-trivial, with challenges including electron beam interactions with the electrolyte and electrode, the lack of well-defined experimental workflows, and difficulty interpreting particle behavior within a liquid. Herein a summary of the current state of LP-TEM technique capabilities alongside a discussion of the relevant experimental challenges researchers typically face, with a focus on in situ studies of electrochemical CO2 conversion catalysts is provided. A methodological approach for in situ LP-TEM measurements on CO2R catalysts prepared by electro-deposition, sputtering, or drop-casting is presented and include case studies where challenges and proposed workflows for each are highlighted. By providing a summary of LP-TEM technique capabilities and guidance for the measurements, the goal is for this paper to reduce barriers for researchers who are interested in utilizing LP-TEM characterization to answer their scientific questions.

Multistep Growth Pathway of Covalent Organic Framework Onion Nanostructures

The growth of complex organic macromolecular materials in solution is a pervasive phenomenon in both natural and synthetic systems, yet the underlying growth mechanisms remain largely unresolved. Using liquid-phase transmission electron microscopy (TEM), we elucidate the real-time growth pathways of covalent organic framework (COF) onion nanostructures, which involve graphitic layer formation, subsequent layer attachment, onion ring closure, and structural relaxation. This process is marked by variations in orientation and curvature, driven by the dynamic formation of the COF structure, which further regulates order-disorder transition and defect generation within the framework. Our in situ TEM characterizations provide valuable insights into how molecular arrangement drives the formation of complex nanostructures. We anticipate that direct imaging of COF nanostructure growth in liquids will open new opportunities for controlling COF crystal morphology, composition, and hierarchical structure development.

Mechanism of Electron-Beam-Induced Structural Degradation in ZIF-8 and its Electron Dose Tolerance

Zeolitic-imidazolate frameworks (ZIFs) are crystalline microporous materials with promising potential for gas adsorption and catalysis application. Further research advances include studies on integrating ZIFs into nanodevice concepts. In detail for the application, e.g., electron-beam-assisted structural modifications or patterning, there is a need to understand potential structural degradation processes caused by such electron beams. Advanced transmission electron microscopy (TEM) has demonstrated its ability to study structures at the nanoscale. Here, we systematically investigated electron-beam-induced loss in crystallinity in ZIF-8 under various experimental conditions, using as measure the attenuation of the relative intensity and the relative displacement of electron diffraction Bragg planes with increasing cumulative electron dose. The {110} Bragg planes reflect the overall stability of the ZIF-8 unit-cell structure, while the {431} Bragg planes assess the stability of its micropore structure. We considered a relative loss of Bragg plane intensity of 37% as the threshold for determining the critical electron dose, which varied for different Bragg planes, with 35.6 ± 8.4 e-Å-2 for {110} and 11.4 ± 3.0 e-Å-2 for {431}. However, the critical dose per breakage of N-Zn bonds in a ZnN4 tetrahedra per different Bragg plane was found to be ∼3 e-Å-2, which indicates continuous, simultaneous breakage of N-Zn bonds throughout the crystal, confirming radiolysis as the dominant damage mechanism. In addition, we investigated the effects of TEM experiment parameters, including acceleration voltage, electron dose rate, cryogenic sample temperature, in situ sample drying, and change in conductivity of the sample substrate (e.g., graphene). Our results unravel the degradation mechanisms in ZIF-8 and provide threshold parameters for maximizing resolution in electron-beam-assisted experiments and processes.

Unveiling the Dynamic Pathways of Metal-Organic Framework Crystallization and Nanoparticle Incorporation for Li-S Batteries

Metal-organic frameworks (MOFs) present diverse building blocks for high-performance materials across industries, yet their crystallization mechanisms remain incompletely understood due to gaps in nucleation and growth knowledge. In this study, MOF structural evolution is probed using in situ liquid phase transmission electron microscopy (TEM) and cryo-TEM, unveiling a blend of classical and nonclassical pathways involving liquid-liquid phase separation, particle attachment-coalescence, and surface layer deposition. Additionally, ultrafast high-temperature sintering (UHS) is employed to dope ultrasmall Cobalt nanoparticles (Co NPs) uniformly within nitrogen-doped hard carbon nanocages confirmed by 3D electron tomography. Lithium-sulfur battery tests demonstrate the nanocage-Co NP structure's exceptional capacity and cycling stability, attributed to Co NP catalytic effects due to its small size, uniform dispersion, and nanocage confinement. The findings propose a holistic framework for MOF crystallization understanding and Co NP tunability through ultrafast sintering, promising advancements in materials science and informing future MOF synthesis strategies and applications.

Extent of Radiolytic Damage from Liquid Cell TEM Experiments on Metal-Organic Frameworks via Post-Mortem 4D-STEM

We report a systematic analysis of electron beam damage of the zeolitic imidazolate framework (ZIF-8) during liquid cell transmission electron microscopy (LCTEM). Our analysis reveals ZIF-8 morphology is strongly affected by solvent used (water vs dimethylformamide), electron flux applied, and imaging mode (i.e., TEM vs STEM), while ZIF-8 crystallinity is primarily affected by accumulated electron fluence. Our observations indicate that the stability of ZIF-8 morphology is higher in dimethylformamide (DMF) than in water. However, in situ electron diffraction indicates that ZIF-8 nanocrystals lose crystallinity at critical fluence of ∼80 e-Å-2 independent of the presence of solvent. Furthermore, 4D-STEM analysis as a post-mortem method reveals the extent of electron beam damage beyond the imaging area and indicates that radiolytic reactions are more pronounced in TEM mode than in STEM mode. These results illustrate the significance of radiolysis occurring while imaging ZIF-8 and present a workflow for assessing damage in LCTEM experiments.

Synthetic and analytical considerations for the preparation of amorphous metal-organic frameworks

Metal-organic frameworks (MOFs) are hybrid porous materials presenting several tuneable properties, allowing them to be utilised for a wide range of applications. To date, focus has been on the preparation of novel crystalline MOFs for specific applications. Recently, interest in amorphous MOFs (aMOFs), defined by their lack of correlated long-range order, is growing. This is due to their potential favourable properties compared to their crystalline equivalents, including increased defect concentration, improved processability and gas separation ability. Direct synthesis of these disordered materials presents an alternative method of preparation to post-synthetic amorphisation of a crystalline framework, potentially allowing for the preparation of aMOFs with varying compositions and structures, and very different properties to crystalline MOFs. This perspective summarises current literature on directly synthesised aMOFs, and proposes methods that could be utilised to modify existing syntheses for crystalline MOFs to form their amorphous counterparts. It outlines parameters that could discourage the ordering of crystalline MOFs, before examining the potential properties that could emerge. Methodologies of structural characterisation are discussed, in addition to the necessary analyses required to define a topologically amorphous structure.

Hierarchically Ordered Pore Engineering of Metal-Organic Framework-Based Materials for Electrocatalysis

Ordered pore engineering that embeds uniform pores with periodic alignment in electrocatalysts opens up a new avenue for achieving further performance promotion. Hierarchically ordered porous metal-organic frameworks (HOP-MOFs) possessing multilevel pores with ordered distribution are the promising precursors for the exploration of ordered porous electrocatalysts, while the scalable acquisition of HOP-MOFs with editable components and adjustable pore size regimes is critical. This review presents recent progress on hierarchically ordered pore engineering of MOF-based materials for enhanced electrocatalysis. The synthetic strategies of HOP-MOFs with different pore size regimes, including the self-assembly guided by reticular chemistry, surfactant, nanoemulsion, and nanocasting, are first introduced. Then the applications of HOP-MOFs as the precursors for exploring hierarchically ordered porous electrocatalysts are summarized, selecting representatives to highlight the boosted performance. Especially, the intensification of molecule and ion transport integrated with optimized electron transfer and site exposure over the hierarchically ordered porous derivatives are emphasized to clarify the directional transfer and integration effect endowed by ordered pore engineering. Finally, the remaining scientific challenges and an outlook of this field are proposed. It is hoped that this review will guide the hierarchically ordered pore engineering of nanocatalysts for boosting the catalytic performance and promoting the practical applications.

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.

Observing the Dynamics of an Electrochemically Driven Active Material with Liquid Electron Microscopy

Electrochemical liquid electron microscopy has revolutionized our understanding of nanomaterial dynamics by allowing for direct observation of their electrochemical production. This technique, primarily applied to inorganic materials, is now being used to explore the self-assembly dynamics of active molecular materials. Our study examines these dynamics across various scales, from the nanoscale behavior of individual fibers to the micrometer-scale hierarchical evolution of fiber clusters. To isolate the influences of the electron beam and electrical potential on material behavior, we conducted thorough beam-sample interaction analyses. Our findings reveal that the dynamics of these active materials at the nanoscale are shaped by their proximity to the electrode and the applied electrical current. By integrating electron microscopy observations with reaction-diffusion simulations, we uncover that local structures and their formation history play a crucial role in determining assembly rates. This suggests that the emergence of nonequilibrium structures can locally accelerate further structural development, offering insights into the behavior of active materials under electrochemical conditions.

Thermally activated structural phase transitions and processes in metal-organic frameworks

The structural knowledge of metal-organic frameworks is crucial to the understanding and development of new efficient materials for industrial implementation. This review classifies and discusses recent advanced literature reports on phase transitions that occur during thermal treatments on metal-organic frameworks and their characterisation. Thermally activated phase transitions and procceses are classified according to the temperaturatures at which they occur: high temperature (reversible and non-reversible) and low temperature. In addition, theoretical calculations and modelling approaches employed to better understand these structural phase transitions are also reviewed.

Zeta Potential and Size Analysis of Zeolitic Imidazolate Framework-8 Nanocrystals Prepared by Surfactant-Assisted Synthesis

The crystal nucleation and growth mechanism of monodispersed metal-organic framework nanoparticles were studied using time-resolved light dynamic, electrokinetic, and powder X-ray diffraction methods. We confirmed that zeolitic imidazolate framework-8 (ZIF-8) nanocrystals follow a nonclassical crystal growth pathway, where a fast nucleation occurs with dense liquid clusters or nanocrystals forming spontaneously when two precursors are mixed. We also explored the zeta potential and solvodynamic size changes of ZIF-8 prepared by a surfactant-assisted synthesis. Three modulators, including 1-methylimidazole (1-mIm), tris(hydroxymethyl)aminomethane (THAM), and (1-hexadecyl)trimethylammonium bromide (CTAB), were studied. We found that 1-mIm dramatically increases the rate of nucleation of ZIF-8. With an increasing amount of 1-mIm, which functions as a coordination modulator, the size increases, and the zeta potential of ZIF-8 decreases. Whereas THAM, as both a coordination and a deprotonation modulator, increases the size and zeta potential of ZIF-8 simultaneously, CTAB, as a long alkyl cationic surfactant, mainly adsorbs on the surface of ZIF-8, and the zeta potential of the formed ZIF-8 is controlled by the amount of CTAB in solution compared with its critical micelle concentration. Overall, we reveal that the modulator type and concentration can be used to control the size and zeta potential of the dispersed ZIF-8 nanocrystals in a colloid system. The experiments also enable identification of the nucleation and crystal growth processes of ZIF-8. The findings will be applicable to other nanocrystals in colloid systems, which are used for heterogeneous catalysis and guest molecular loadings.

The effect of nanochannel length on in situ loading times of diffusion-propelled nanoparticles in liquid cell electron microscopy

Liquid cell transmission electron microscopy is a powerful tool for visualizing nanoparticle (NP) assemblies in liquid environments with nanometer resolution. However, it remains a challenge to control the NP concentration in the high aspect ratio liquid enclosure where the diffusion of dispersed NPs is affected by the exposed surface of the liquid cell walls. Here, we introduce a semi-empirical model based on the 1D diffusion equation, to predict the NP loading time as they pass through the nanochannel into the imaging volume of the liquid cell. We show that loading of NPs into the imaging volume of the liquid cell may take several days if NPs are prone to attach to the surface of the mm-long nanochannel when using an industry-standard flat microchip. As a means to facilitate mass transport via diffusion, we tested a liquid cell incorporating a microchannel geometry resulting in a NP loading time in the order minutes that allowed us to observe the formation of a randomly oriented self-assembled monolayer in situ using scanning transmission electron microscopy.

In Situ Transmission Electron Microscopy Study of Bubble Behavior Near the Surface of Ice Crystals by Using a Liquid Cell With a Peltier Cooling Holder

Liquid cell transmission electron microscopy (LC-TEM) is a unique technique that permits in situ observations of various phenomena in liquids with high spatial and temporal resolutions. One difficulty with this technique is the control of the environmental conditions in the observation area. Control of the temperature ranging from room temperature to minus several tens of degrees Celsius, is desirable for controlling the supersaturation in various materials and for observing crystallization more easily. We have developed a cooling transmission electron microscopy specimen holder that uses Peltier devices, and we have combined it with a liquid cell to realize accurate temperature control in LC-TEM. We evaluated this system by using water as a specimen. Motionless bubbles, shown to be voids containing pressurized gas, formed in the specimen sometime after the temperature had reached -12°C. An electron diffraction pattern showed that the specimen turned into ice Ih after the formation of these bubbles, confirming that our system works properly and can induce crystallization. In addition, we analyzed the behavior of bubbles formed in the ice Ih, and we discussed the formation of these bubbles and their internal pressure.

Understanding and controlling the nucleation and growth of metal-organic frameworks

Metal-organic frameworks offer a diverse landscape of building blocks to design high performance materials for implications in almost every major industry. With this diversity stems complex crystallization mechanisms with various pathways and intermediates. Crystallization studies have been key to the advancement of countless biological and synthetic systems, with MOFs being no exception. This review provides an overview of the current theories and fundamental chemistry used to decipher MOF crystallization. We then discuss how intrinsic and extrinsic synthetic parameters can be used as tools to modulate the crystallization pathway to produce MOF crystals with finely tuned physical and chemical properties. Experimental and computational methods are provided to guide the probing of MOF crystal formation on the molecular and bulk scale. Lastly, we summarize the recent major advances in the field and our outlook on the exciting future of MOF crystallization.

Exploration of Organic Nanomaterials with Liquid-Phase Transmission Electron Microscopy

ConspectusOrganic, soft materials with solution-phase nanoscale structures, such as emulsions, hydrogels, and thermally responsive materials, are inherently difficult to directly image via dry state and cryogenic-transmission electron microscopy (TEM). Therefore, we lack a routine microscopy method with sufficient resolution that can, in tandem with scattering techniques, probe the morphology and dynamics of these and many related systems. These challenges motivate liquid cell (LC) TEM method development, aimed at making the technique generally available and routine. To date, the field has been and continues to be dominantly focused on analyzing solution-phase inorganic materials. These mostly metallic nanoparticles have been studied at electron fluxes that can allow for high-resolution imaging, in the range of hundreds to thousands of e- Å-2 s-1. Despite excellent contrast, in these cases, one often contends with knock-on damage, direct radiolysis, and sensitization of the solvent by virtue of enhanced secondary electron production by the impinging electron beam. With an interest in soft materials, we face both related and distinct challenges, especially in achieving a high-enough contrast within solvated liquid cells. Additionally, we must be aware of artifacts associated with high-flux imaging conditions in terms of direct radiolysis of the solvent and the sensitive materials themselves. Regardless, with care, it has become possible to gain real insight into both static and dynamic organic nanomaterials in solution. This is due, in large part, to key advances that have been made, including improved sample preparation protocols, image capture technologies, and image analysis, which have allowed LCTEM to have utility. To enable solvated soft matter characterization by LCTEM, a generalizable multimodal workflow was developed by leveraging both experimental and theoretical precedents from across the LCTEM field and adjacent works concerned with solution radiolysis and nanoparticle tracking analyses. This workflow consists of (1) modeling electron beam-solvent interactions, (2) studying electron beam-sample interactions via LCTEM coupled with post-mortem analysis, (3) the construction of "damage plots" displaying sample integrity under varied imaging and sample conditions, (4) optimized LCTEM imaging, (5) image processing, and (6) correlative analysis via X-ray or light scattering. In this Account, we present this outlook and the challenges we continue to overcome in the direct imaging of dynamic solvated nanoscale soft materials.

Understanding ZIF particle chemical etching dynamics and morphology manipulation: in situ liquid phase electron microscopy and 3D electron tomography application

In situ liquid phase transmission electron microscopy (TEM) and three-dimensional electron tomography are powerful tools for investigating the growth mechanism of MOFs and understanding the factors that influence their particle morphology. However, their combined application to the study of MOF etching dynamics is limited due to the challenges of the technique such as sample preparation, limited field of view, low electron density, and data analysis complexity. In this research, we present a study employing in situ liquid phase TEM to investigate the etching mechanism of colloidal zeolitic imidazolate framework (ZIF) nanoparticles. The etching process involves two distinct stages, resulting in the development of porous structures as well as partially and fully hollow morphologies. The etching process is induced by exposure to an acid solution, and both in situ and ex situ experiments demonstrate that the outer layer etches faster leading to overall volume shrinking (stage I) while the inner layer etches faster giving a hollow morphology (stage II), although both the outer layer and inner layer have been etched in the whole process. 3D electron tomography was used to quantify the properties of the hollow structures which show that the ZIF-67 crystal etching rate is larger than that of the ZIF-8 crystal at the same pH value. This study provides valuable insights into MOF particle morphology control and can lead to the development of novel MOF-based materials with tailored properties for various applications.

Research Progress on Metal-Organic Frameworks by Advanced Transmission Electron Microscopy

Metal-organic frameworks (MOFs), composed of metal nodes and inorganic linkers, are promising for a wide range of applications due to their unique periodic frameworks. Understanding structure-activity relationships can facilitate the development of new MOFs. Transmission electron microscopy (TEM) is a powerful technique to characterize the microstructures of MOFs at the atomic scale. In addition, it is possible to directly visualize the microstructural evolution of MOFs in real time under working conditions via in situ TEM setups. Although MOFs are sensitive to high-energy electron beams, much progress has been made due to the development of advanced TEM. In this review, we first introduce the main damage mechanisms for MOFs under electron-beam irradiation and two strategies to minimize these damages: low-dose TEM and cryo-TEM. Then we discuss three typical techniques to analyze the microstructure of MOFs, including three-dimensional electron diffraction, imaging using direct-detection electron-counting cameras, and iDPC-STEM. Groundbreaking milestones and research advances of MOFs structures obtained with these techniques are highlighted. In situ TEM studies are reviewed to provide insights into the dynamics of MOFs induced by various stimuli. Additionally, perspectives are analyzed for promising TEM techniques in the research of MOFs' structures.

Oriented attachment interfaces of zeolitic imidazolate framework nanocrystals

Understanding the growth and coarsening mechanisms of metal-organic framework (MOF) nanoparticles is crucially important for the design and fabrication of MOF materials with diverse functionalities and controllable stability. Oriented attachment (OA) growth is a common manner of MOF nanocrystal coarsening and agglomeration, but the underlying molecular mechanisms have not been well understood to date. Here we report the molecular-scale characterization of the OA interfaces of zeolitic imidazolate framework (ZIF) crystals by state-of-the-art low-dose aberration-corrected transmission electron microscopy. A series of OA interfaces with different molecular structures are captured, implying that multiple kinetic steps are involved in the OA growth of ZIF crystals from non-directional physical attractions between primary nanocrystals, lattice-aligned attachment of the ligand-capped nanocrystals, to coherent interfaces with perfect lattice alignment or stacking faults. It was found that the surface-capping organic ligands not only play an essential role in crystal lattice alignment by near-field directional interactions, but also dominate the interfacial reaction kinetics by interfacial diffusion-controlled elimination of excess surface-capping ligands. These observations provide molecular-scale insights into the OA growth mechanisms of ZIF crystals, which is important for engineering MOF crystal growth pathways by designing surface-capping ligands.

Sequential Sol-Gel Self-Assembly and Nonclassical Gel-Crystal Transformation of the Metal-Organic Framework Gel

Metal-organic framework (MOF) gel, an emerging subtype of MOF structure, is unique in formation and function; however, its evolutionary process remains elusive. Here, the evolution of a model gel-based MOF, UiO-66(Zr) gel, is explored by demonstrating its sequential sol-gel self-assembly and nonclassical gel-crystal transformation. The control of the sol-gel process enables the observation and characterization of structures in each assembly stage (phase-separation, polycondensation, and hindered-crystallization) and facilitates the preparation of hierarchical materials with giant mesopores. The gelation mechanism is tentatively attributed to the formation of zirconium oligomers. By further utilizing the pre-synthesized gel, the nonclassical gel-crystal transformation is achieved by the modulation in an unconventional manner, which sheds light on crystal intermediates and distinct crystallization motions ("growth and splitting" and "aggregation and fusion"). The overall sol-gel and gel-crystal evolutions of UiO-66(Zr) enrich self-assembly and crystallization domains, inspire the design of functional structures, and demand more in-depth research on the intermediates in the future.

Mechanistic Investigation of the Synthesis of Dianionic In-Derived Coordination Polymers

The mechanism of formation of crystalline coordination polymers is as complex as the architectures themselves. In this Communication, we detail a three-tiered approach using density functional theory (DFT) analysis, synthesis, and in situ Raman spectroscopy to study the formation of coordination polymers. Specifically, the previously reported coordination polymers YCM-22 and YCM-51 containing the [In(CO₂R)₂X₃]^2- (X = halogen) molecular building unit (MBU) were investigated. DFT revealed two potential pathways of formation, involving the initial formation of either [InCl₄]^- or [In(CO₂R)Cl₃]^-. A molecular dimeric In species (8a) containing two [In(CO₂R)Cl₄]^2- centers bridged by 2,5-thiophenedicarboxylic acid was isolated. When a suspension of 8a was treated with a solution of 2,5-thiophenedicarboxylic acid, an isomer of the coordination polymer YCM-22 (denoted as YCM-22') was formed. In situ Raman analysis of the formation of YCM-22 confirms that [InCl₄]^- forms at the onset of the reaction and that the [In(CO₂R)₂X₃]^2- MBU forms at its expense. The totality of the data presented support a mechanism of formation of one-dimensional In-derived coordination polymers and present a roadmap for future investigations into the formation of other crystalline coordination polymers.

Ouzo Effect Examined at the Nanoscale via Direct Observation of Droplet Nucleation and Morphology

Herein, we present the direct observation via liquid-phase transmission electron microscopy (LPTEM) of the nucleation and growth pathways of structures formed by the so-called "ouzo effect", which is a classic example of surfactant-free, spontaneous emulsification. Such liquid-liquid phase separation occurs in ternary systems with an appropriate cosolvent such that the addition of the third component extracts the cosolvent and makes the other component insoluble. Such droplets are homogeneously sized, stable, and require minimal energy to disperse compared to conventional emulsification methods. Thus, ouzo precipitation processes are an attractive, straightforward, and energy-efficient technique for preparing dispersions, especially those made on an industrial scale. While this process and the resulting emulsions have been studied by numerous indirect techniques (e.g., X-ray and light scattering), direct observation of such structures and their formation at the nanoscale has remained elusive. Here, we employed the nascent technique of LPTEM to simultaneously evaluate droplet growth and nanostructure. Observation of such emulsification and its rate dependence is a promising indication that similar LPTEM methodologies may be used to investigate emulsion formation and kinetics.

Unveiling growth and dynamics of liposomes by graphene liquid cell-transmission electron microscopy

Liposome is a model system for biotechnological and biomedical purposes spanning from targeted drug delivery to modern vaccine research. Yet, the growth mechanism of liposomes is largely unknown. In this work, the formation and evolution of phosphatidylcholine-based liposomes are studied in real-time by graphene liquid cell-transmission electron microscopy (GLC-TEM). We reveal important steps in the growth, fusion and denaturation of phosphatidylcholine (PC) liposomes. We show that initially complex lipid aggregates resembling micelles start to form. These aggregates randomly merge while capturing water and forming small proto-liposomes. The nanoscopic containers continue sucking water until their membrane becomes convex and free of redundant phospholipids, giving stabilized PC liposomes of different sizes. In the initial stage, proto-liposomes grow at a rate of 10-15 nm s^-1, which is followed by their growth rate of 2-5 nm s^-1, limited by the lipid availability in the solution. Molecular dynamics (MD) simulations are used to understand the structure of micellar clusters, their evolution, and merging. The liposomes are also found to fuse through lipid bilayers docking followed by the formation of a hemifusion diaphragm and fusion pore opening. The liposomes denaturation can be described by initial structural destabilization and deformation of the membrane followed by the leakage of the encapsulated liquid. This study offers new insights on the formation and growth of lipid-based molecular assemblies which is applicable to a wide range of amphiphilic molecules.

Direct synthesis of amorphous coordination polymers and metal–organic frameworks

Coordination polymers (CPs) and their subset, metal-organic frameworks (MOFs), can have porous structures and hybrid physicochemical properties that are useful for diverse applications. Although crystalline CPs and MOFs have received the most attention to date, their amorphous states are of growing interest as they can be directly synthesized under mild conditions. Directly synthesized amorphous CPs (aCPs) can be constructed from a wider range of metals and ligands than their crystalline and crystal-derived counterparts and demonstrate numerous unique material properties, such as higher mechanical robustness, increased stability and greater processability. This Review examines methods for the direct synthesis of aCPs and amorphous MOFs, as well as their properties and characterization routes, and offers a perspective on the opportunities for the widespread adoption of directly synthesized aCPs.

Caught in Action: Visualizing Dynamic Nanostructures Within Supramolecular Systems Chemistry

Supramolecular systems chemistry has been an area of active research to develop nanomaterials with life-like functions. Progress in systems chemistry relies on our ability to probe the nanostructure formation in solution. Often visualizing the dynamics of nanostructures which transform over time is a formidable challenge. This necessitates a paradigm shift from dry sample imaging towards solution-based techniques. We review the application of state-of-the-art techniques for real-time, in situ visualization of dynamic self-assembly processes. We present how solution-based techniques namely optical super-resolution microscopy, solution-state atomic force microscopy, liquid-phase transmission electron microscopy, molecular dynamics simulations and other emerging techniques are revolutionizing our understanding of active and adaptive nanomaterials with life-like functions. This Review provides the visualization toolbox and futuristic vision to tap the potential of dynamic nanomaterials.

In Situ Tracking of Crystal-Surface-Dependent Cu2O Nanoparticle Dissolution in an Aqueous Environment

Metal-oxide-based nanoparticles (MONPs) such as Cu₂O NPs have attracted growing attention, but the potential discharges of MONPs have raised considerable concern of their environmental fate including their dissolution behavior. The impacts of morphology on MONP dissolution are largely uncertain due to the lack of in situ tracking techniques. In this study, we combined a series of in situ technologies including liquid-cell transmission electron microscopy and fluorescence probes to reveal the in situ dissolution process of Cu₂O NPs in freshwater. Our results suggest that cubic Cu₂O NPs exhibit a higher dissolution quantity compared with spherical NPs of the same surface area. The difference was mainly related to the crystal surface, while other factors such as particle size or aggregation status showed minor effects. Importantly, we demonstrated the simultaneous growth of new small NPs and the dissolution of pristine Cu₂O NPs during the dissolution of Cu₂O NPs. Cubic Cu₂O NPs became much less soluble under O₂-limited conditions, suggesting that O₂ concentration largely affected the dependence of dissolution on the NP morphology. Our findings highlight the potential application of in situ techniques to track the environmental fates of MONPs, which would provide important information for assessing the ecological risks of engineered NPs.

Computer simulation of the early stages of self-assembly and thermal decomposition of ZIF-8

We employ all-atom well-tempered metadynamics simulations to study the mechanistic details of both the early stages of nucleation and crystal decomposition for the benchmark metal–organic framework (MOF) ZIF-8. To do so, we developed and validated a force field that reliably models the modes of coordination bonds via a Morse potential functional form and employs cationic and anionic dummy atoms to capture coordination symmetry. We also explored a set of physically relevant collective variables and carefully selected an appropriate subset for our problem at hand. After a rapid increase of the Zn–N connectivity, we observe the evaporation of small clusters in favor of a few large clusters, which leads to the formation of an amorphous highly connected aggregate. [Formula: see text] and [Formula: see text] complexes are observed with lifetimes in the order of a few picoseconds, while larger structures, such as four-, five-, and six-membered rings, have substantially longer lifetimes of a few nanoseconds. The free ligands act as “templating agents” for the formation of sodalite cages. ZIF-8 crystal decomposition results in the formation of a vitreous phase. Our findings contribute to a fundamental understanding of MOF’s synthesis that paves the way to controlling synthesis products. Furthermore, our developed force field and methodology can be applied to model solution processes that require coordination bond reactivity for other ZIFs besides ZIF-8.

Observation of formation and local structures of metal-organic layers via complementary electron microscopy techniques

Metal-organic layers (MOLs) are highly attractive for application in catalysis, separation, sensing and biomedicine, owing to their tunable framework structure. However, it is challenging to obtain comprehensive information about the formation and local structures of MOLs using standard electron microscopy methods due to serious damage under electron beam irradiation. Here, we investigate the growth processes and local structures of MOLs utilizing a combination of liquid-phase transmission electron microscopy, cryogenic electron microscopy and electron ptychography. Our results show a multistep formation process, where precursor clusters first form in solution, then they are complexed with ligands to form non-crystalline solids, followed by the arrangement of the cluster-ligand complex into crystalline sheets, with additional possible growth by the addition of clusters to surface edges. Moreover, high-resolution imaging allows us to identify missing clusters, dislocations, loop and flat surface terminations and ligand connectors in the MOLs. Our observations provide insights into controllable MOL crystal morphology, defect engineering, and surface modification, thus assisting novel MOL design and synthesis.

Strategy for optimizing experimental settings for studying low atomic number colloidal assemblies using liquid phase scanning transmission electron microscopy

Observing processes of nanoscale materials of low atomic number is possible using liquid phase electron microscopy (LP-EM). However, the achievable spatial resolution (d) is limited by radiation damage. Here, we examine a strategy for optimizing LP-EM experiments based on an analytical model and experimental measurements, and develop a method for quantifying image quality at ultra low electron dose D_e using scanning transmission electron microscopy (STEM). As experimental test case we study the formation of a colloidal binary system containing 30 nm diameter SiO₂ nanoparticles (SiONPs), and 100 nm diameter polystyrene microspheres (PMs). We show that annular dark field (DF) STEM is preferred over bright field (BF) STEM for practical reasons. Precise knowledge of the material's density is crucial for the calculations in order to match experimental data. To calculate the detectability of nano-objects in an image, the Rose criterion for single pixels is expanded to a model of the signal to noise ratio obtained for multiple pixels spanning the image of an object. Using optimized settings, it is possible to visualize the radiation-sensitive, hierarchical low-Z binary structures, and identify both components.

Direct Observation of Emulsion Morphology, Dynamics, and Demulsification

Herein, we present the direct observation and quantification of a water-in-oil (w/o) emulsion, its destabilization, and the effect of additives on such processes at the nanoscale. This is achieved via liquid phase transmission electron microscopy (LPTEM), wherein a small volume of emulsion is encapsulated against vacuum in its liquid state to allow observation of its initial morphology and its evolution over time at excellent spatial and temporal resolution. Emulsions of this class are useful for delivering payloads of materials insoluble in their delivery medium and are currently widely used across food science, pharmaceuticals, and environmental applications. However, their utility is inherently limited by their thermodynamic tendency to demulsify, eventually leading to bulk phase separation. This occurs via several degradation mechanisms, operating at times collectively, and which are difficult to differentiate via traditional ensemble methods (e.g., light scattering), obscuring mechanistic nuances. LPTEM as a characterization technique has the potential to augment our understanding of emulsion behavior and improve performance and formulations. In this work, we also emphasize the importance of the included videographic Supporting Information data in demonstrating the behavior of the studied materials.

Feasibility of Control of Particle Assembly by Dielectrophoresis in Liquid-Cell Transmission Electron Microscopy

Liquid-cell transmission electron microscopy (LC-TEM) is a useful technique for observing phenomena in liquid samples with spatial and temporal resolutions similar to those of conventional transmission electron microscopy (TEM). This method is therefore expected to permit the visualization of phenomena previously inaccessible to conventional optical microscopy. However, dynamic processes such as nucleation are difficult to observe by this method because of difficulties in controlling the condition of the sample liquid in the observation area. To approach this problem, we focused on dielectrophoresis, in which electrodes are used to assemble particles, and we investigated the phenomena that occurred when an alternating-current signal was applied to an electrode in an existing liquid cell by using a phase-contrast optical microscope (PCM) and TEM. In PCM, we observed that colloidal particles in a solution were attracted to the electrodes to form assemblies, that the particles aligned along the electric field to form pearl chains and that the pearl chains accumulated to form colloidal crystals. However, these phenomena were not observed in the TEM study because of differences in the design of the relevant holders. The results of our study imply that the particle assembly by using dielectrophoretic forces in LC-TEM should be possible, but further studies, including electric device development, will be required to realize this in practice.

A review of recent developments for the in situ/operando characterization of nanoporous materials

This is a review on up-to-date in situ/operando methods for a comprehensive characterization of nanoporous materials. The group of nanoporous materials is constantly growing, and with it, the variety of...

Observation of Liquid-Liquid-Phase Separation and Vesicle Spreading during Supported Bilayer Formation via Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables the real-time visualization of nanoscale dynamics in solution. This technique has been used to study the formation and transformation mechanisms of organic and inorganic nanomaterials. Here, we study the formation of block-copolymer-supported bilayers using LP-TEM. We observe two formation pathways that involve either liquid droplets or vesicles as intermediates toward supported bilayers. Quantitative image analysis methods are used to characterize vesicle spread rates and show the origin of defect formation in supported bilayers. Our results suggest that bilayer assembly methods that proceed via liquid droplet intermediates should be beneficial for forming pristine supported bilayers. Furthermore, supported bilayers inside the liquid cells may be used to image membrane interactions with proteins and nanoparticles in the future.

Prospects of Exploring the Metal-Organic Framework for Combating Antimicrobial Resistance

Infectious diseases are a major public health concern globally. Infections caused by pathogens with resistance against commonly used antimicrobial drugs or antibiotics (known as antimicrobial resistance, AMR) are becoming extremely difficult to control. AMR has thus been declared as one of the top 10 global public health threats, as it has very limited solutions. The drying pipeline of effective antibiotics has further worsened the situation. There is no absolute treatment, and the limitations of existing methods warrant further development in antimicrobials. Recent developments in the nanomaterial field present them as promising therapeutics and effective alternative to conventional antibiotics and synthetic drugs. The metal-organic framework (MOF) is a recent addition to the antimicrobial category with superior properties. The MOF exerts antimicrobial action on a wide range of species and is highly biocompatible. Additionally, their porous structures allow the incorporation of biomolecules and drugs for synergistic antimicrobial action. This review provides an inclusive summary of the molecular events responsible for resistance development and current trends in antimicrobials to combat antibiotic resistance and explores the potential role of the MOF in tackling the drug-resistant microbial species.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

Thermoresponsive polymer assemblies via variable temperature liquid-phase transmission electron microscopy and small angle X-ray scattering

Herein, phase transitions of a class of thermally-responsive polymers, namely a homopolymer, diblock, and triblock copolymer, were studied to gain mechanistic insight into nanoscale assembly dynamics via variable temperature liquid-cell transmission electron microscopy (VT-LCTEM) correlated with variable temperature small angle X-ray scattering (VT-SAXS). We study thermoresponsive poly(diethylene glycol methyl ether methacrylate) (PDEGMA)-based block copolymers and mitigate sample damage by screening electron flux and solvent conditions during LCTEM and by evaluating polymer survival via post-mortem matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). Our multimodal approach, utilizing VT-LCTEM with MS validation and VT-SAXS, is generalizable across polymeric systems and can be used to directly image solvated nanoscale structures and thermally-induced transitions. Our strategy of correlating VT-SAXS with VT-LCTEM provided direct insight into transient nanoscale intermediates formed during the thermally-triggered morphological transformation of a PDEGMA-based triblock. Notably, we observed the temperature-triggered formation and slow relaxation of core-shell particles with complex microphase separation in the core by both VT-SAXS and VT-LCTEM.

De Novo Approach to Encapsulating Biocatalysts into Synthetic Matrixes: From Enzymes to Microbial Electrocatalysts

Biocatalysts hold great promise in chemical and electrochemical reactions. However, biocatalysts are prone to inhospitable physiochemical conditions. Encapsulating biocatalysts into a synthetic host matrix can improve their stability and activity, and broaden their operational conditions. In this Review, we summarize the emerging de novo approaches to encapsulating biocatalysts into synthetic matrixes. Here, de novo means that embedding of biocatalysts and construction of matrixes take place simultaneously. We discuss the advantages and limitations of the de novo approach. On the basis of the nature of the biocatalysts and the synthetic frameworks, we specifically focus on two aspects: (1) encapsulation of enzymes (in vitro) in metal-organic frameworks and (2) encapsulation of microbial electrocatalysts (in vivo) on the electrode. For both cases, we discuss how the encapsulation improves biocatalysts' performance (stability, viability, activity, and etc.). We also highlight the benefit of encapsulation in facilitating the transport of charge carriers in microbial electrocatalysis.

Stable Zr-Based Metal-Organic Framework Nanoporous Membrane for Efficient Desalination of Hypersaline Water

Treatment of hypersaline waters is a critical environmental challenge. Pervaporation (PV) desalination is a promising technique to address this challenge, but current PV membranes still suffer from challenging issues such as low flux and insufficient stability. Herein, we propose in situ nanoseeding followed by a secondary growth strategy to fabricate a high-quality stable metal-organic framework (MOF) thin membrane (UiO-66) for high-performance pervaporation desalination of hypersaline waters. To address the issue of membrane quality, a TiO₂ nano-interlayer was introduced on coarse mullite substrates to favor the growth of a UiO-66 nanoseed layer, on which a well-intergrown UiO-66 selective membrane layer with thickness as low as 1 μm was finally produced via subsequent secondary growth. The PV separation performance for hypersaline waters was systematically investigated at different salt concentrations, feed temperatures, and long-term operation in different extreme chemical environments. Besides having nearly complete rejection (99.9%), the UiO-66 membrane exhibited high flux (37.4 L·m^-2·h^-1) for hypersaline waters, outperforming current existing zeolite and MOF membranes. The membrane also demonstrated superior long-term operational stability under various harsh environments (hypersaline, hot, and acidic/alkaline feed water) and mild fouling behavior. The rational design proposed in this study is not only applicable for the development of a high-quality UiO-66 membrane enabling harsh hypersaline water treatment but can also be potentially extended to other next-generation nanoporous MOF membranes for more environmental applications.

Dipeptide Nanostructure Assembly and Dynamics via in Situ Liquid-Phase Electron Microscopy

In this paper, we report the in situ growth of FF nanotubes examined via liquid-cell transmission electron microscopy (LCTEM). This direct, high spatial, and temporal resolution imaging approach allowed us to observe the growth of peptide-based nanofibrillar structures through directional elongation. Furthermore, the radial growth profile of FF nanotubes through the addition of monomers perpendicular to the tube axis has been observed in real-time with sufficient resolution to directly observe the increase in diameter. Our study demonstrates that the kinetics, dynamics, structure formation, and assembly mechanism of these supramolecular assemblies can be directly monitored using LCTEM. The performance of the peptides and the assemblies they form can be verified and evaluated using post-mortem techniques including time-of-flight secondary ion mass spectrometry (ToF-SIMS).

Revealing Reactions between the Electron Beam and Nanoparticle Capping Ligands with Correlative Fluorescence and Liquid-Phase Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables real-time imaging of nanoparticle self-assembly, formation, and etching with single nanometer resolution. Despite the importance of organic nanoparticle capping ligands in these processes, the effect of electron beam irradiation on surface-bound and soluble capping ligands during LP-TEM imaging has not been investigated. Here, we use correlative LP-TEM and fluorescence microscopy (FM) to demonstrate that polymeric nanoparticle ligands undergo competing crosslinking and chain scission reactions that nonmonotonically modify ligand coverage over time. Branched polyethylenimine (BPEI)-coated silver nanoparticles were imaged with dose-controlled LP-TEM followed by labeling their primary amine groups with fluorophores to visualize the local thickness of adsorbed capping ligands. FM images showed that free ligands crosslinked in the LP-TEM image area over imaging times of tens of seconds, enhancing local capping ligand coverage on nanoparticles and silicon nitride membranes. Nanoparticle surface ligands underwent chain scission over irradiation times of minutes to tens of minutes, which depleted surface ligands from the nanoparticle and silicon nitride surface. Conversely, solutions of only soluble capping ligand underwent successive crosslinking reactions with no chain scission, suggesting that nanoparticles enhanced the chain scission reactions by acting as radiolysis hotspots. The addition of a hydroxyl radical scavenger, tert-butanol, eliminated chain scission reactions and slowed the progression of crosslinking reactions. These experiments have important implications for performing controlled and reproducible LP-TEM nanoparticle imaging as they demonstrate that the electron beam can significantly alter ligand coverage on nanoparticles in a nonintuitive manner. They emphasize the need to understand and control the electron beam radiation chemistry of a given sample to avoid significant perturbations to the nanoparticle capping ligand chemistry, which are invisible in electron micrographs.

Sonocrystallisation of ZIF-8 in water with high excess of ligand: Effects of frequency, power and sonication time

A systematic study on the sonocrystallisation of ZIF-8 (zeolitic imidazolate framework-8) in a water-based system was investigated under different mixing speeds, ultrasound frequencies, calorimetric powers and sonication time. Regardless of the synthesis technique, pure crystals of ZIF-8 with high BET (Brunauer, Emmett and Teller) specific surface area (SSA) can be obtained in water after only 5 s. Furthermore, 5 s sonication produced even smaller crystals (~0.08 µm). The type of technique applied for producing the ZIF-8 crystals did not have any significant impact on crystallinity, purity and yield. Crystal morphology and size were affected by the use of ultrasound and mixing, obtaining nanoparticles with a more spherical shape than in silent condition (no ultrasound and mixing). However, no specific trends were observed with varying frequency, calorimetric power and mixing speed. Ultrasound and mixing may have an effect on the nucleation step, causing the fast production of nucleation centres. Furthermore, the BET SSA increased with increasing mixing speed. With ultrasound, the BET SSA is between the values obtained under silent condition and with mixing. A competition between micromixing and shockwaves has been proposed when sonication is used for ZIF-8 production. The former increases the BET SSA, while the latter could be responsible for porosity damage, causing a decrease of the surface area.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

Growth Dynamics of Vertical and Lateral Layered Double Hydroxide Nanosheets during Electrodeposition

Layered double hydroxides (LDHs) are a class of lamellar materials with a wide range of potential catalytic applications. LDHs form from positively charged 2D atomic layers separated by charge-balancing anions and solvent molecules. Typically, nanoscale LDH sheets can grow vertical or parallel to a substrate, exposing their different active facets. These two growth modes of LDH nanosheets have a significant impact on their electrocatalytic properties, yet the details of their growth remain unknown, hindering our ability to design and synthesize high-performance LDH-based electrocatalysts. Here, we investigate the growth pathways of LDH nanosheets using in situ electrochemical liquid-phase transmission electron microscopy (TEM) and show that the growth modes of LDH nanosheets can be controlled by tuning the precursor concentrations. Moreover, our observations reveal that LDH nanosheets grow via two pathways: (1) monomer addition, where the adatoms are heterogeneously deposited onto the LDH nanosheets, and (2) coalescence, where adjacent nanosheets merge together.