Liquid-Phase Electron Microscopy for Soft Matter Science and Biology

Nanoscopic Imaging of Self-Propelled Ultrasmall Catalytic Nanomotors

Ultrasmall nanomotors (<100 nm) are highly desirable nanomachines for their size-specific advantages over their larger counterparts in applications spanning nanomedicine, directed assembly, active sensing, and environmental remediation. While there are extensive studies on motors larger than 100 nm, the design and understanding of ultrasmall nanomotors have been scant due to the lack of high-resolution imaging of their propelled motions with orientation and shape details resolved. Here, we report the imaging of the propelled motions of catalytically powered ultrasmall nanomotors─hundreds of them─at the nanometer resolution using liquid-phase transmission electron microscopy. These nanomotors are Pt nanoparticles of asymmetric shapes ("tadpoles" and "boomerangs"), which are colloidally synthesized and observed to be fueled by the catalyzed decomposition of NaBH4 in solution. Statistical analysis of the orientation and position trajectories of fueled and unfueled motors, coupled with finite element simulation, reveals that the shape asymmetry alone is sufficient to induce local chemical concentration gradient and self-diffusiophoresis to act against random Brownian motion. Our work elucidates the colloidal design and fundamental forces involved in the motions of ultrasmall nanomotors, which hold promise as active nanomachines to perform tasks in confined environments such as drug delivery and chemical sensing.

High-Resolution Electron Diffraction of Hydrated Protein Crystals at Room Temperature

Structural characterization is crucial to understanding protein function. Compared with X-ray diffraction methods, electron crystallography can be performed on nanometer-sized crystals and can provide additional information from the resulting Coulomb potential map. Whereas electron crystallography has successfully resolved three-dimensional structures of vitrified protein crystals, its widespread use as a structural biology tool has been limited. One main reason is the fragility of such crystals. Protein crystals can be easily damaged by mechanical stress, change in temperature, or buffer conditions as well as by electron irradiation. This work demonstrates a methodology to preserve these nanocrystals in their natural environment at room temperature for electron diffraction experiments as an alternative to existing cryogenic techniques. Lysozyme crystals in their crystallization solution are hermetically sealed via graphene-coated grids, and their radiation damage is minimized by employing a low-dose data collection strategy in combination with a hybrid-pixel direct electron detector. Diffraction patterns with reflections of up to 3 Å are obtained and successfully indexed using a template-matching algorithm. These results demonstrate the feasibility of in situ protein electron diffraction. The method described will also be applicable to structural studies of hydrated nanocrystals important in many research and technological developments.

Liquid Transmission Electron Microscopy for Probing Collagen Biomineralization

Collagen biomineralization is fundamental to hard tissue assembly. While studied extensively, collagen mineralization processes are not fully understood, with the majority of theories derived from electron microscopy (EM) under static, dehydrated, or frozen conditions, unlike the liquid phase environment where mineralization occurs. Herein, novel liquid transmission EM (TEM) strategies are presented, in which collagen mineralization was explored in liquid for the first time via TEM. Custom thin-film enclosures were employed to visualize the mineralization of reconstituted collagen fibrils in a calcium phosphate and polyaspartic acid solution to promote intrafibrillar mineralization. TEM highlighted that at early time points precursor mineral particles attached to collagen and progressed to crystalline mineral platelets aligned with fibrils at later time points. This aligns with observations from other techniques and validates the liquid TEM approach. This work provides a new liquid imaging approach for exploring collagen biomineralization, advancing toward understanding disease pathogenesis and remineralization strategies for hard tissues.

Electric Fields in Liquid Water Irradiated with Protons at Ultrahigh Dose Rates

We study the effects of irradiating water with 3 MeV protons at high doses by observing the motion of charged polystyrene beads outside the proton beam. By single-particle tracking, we measure a radial velocity of the order of microns per second. Combining electrokinetic theory with simulations of the beam-generated reaction products and their outward diffusion, we find that the bead motion is due to electrophoresis in the electric field induced by the mobility contrast of cations and anions. This work sheds light on the perturbation of biological systems by high-dose radiations and paves the way for the manipulation of colloid or macromolecular dispersions by radiation-induced diffusiophoresis.

An Unusual Microdomain Factor Controls Interaction of Organic Halides with the Palladium Phase and Influences Catalytic Activity in the Mizoroki-Heck Reaction

In this work, using a combination of scanning and transmission electron microscopy (SEM and TEM), the transformations of palladium-containing species in imidazolium ionic liquids in reaction mixtures of the Mizoroki-Heck reaction and in related organic media are studied to understand a challenging question of the relative reactivity of organic halides as key substrates in modern catalytic technologies. The microscopy technique detects the formation of a stable nanosized palladium phase under the action of an aryl (Ar) halide capable of forming microcompartments in an ionic liquid. For the first time, the correlation between the reactivity of the aryl halide and the microdomain structure is observed: Ar-I (well-developed microdomains) > Ar-Br (microphase present) > Ar-Cl (minor amount of microphase). Previously, it is assumed that molecular level factors, namely, carbon-halogen bond strength and the ease of bond breakage, are the sole factors determining the reactivity of aryl halides in catalytic transformations. The present work reports a new factor connected with the nature of the organic substrates used and their ability to form a microdomain structure and concentrate metallic species, highlighting the importance of considering both the molecular and microscale properties of the reaction mixtures.

In Situ and Emerging Transmission Electron Microscopy for Catalysis Research

Catalysts are the primary facilitator in many dynamic processes. Therefore, a thorough understanding of these processes has vast implications for a myriad of energy systems. The scanning/transmission electron microscope (S/TEM) is a powerful tool not only for atomic-scale characterization but also in situ catalytic experimentation. Techniques such as liquid and gas phase electron microscopy allow the observation of catalysts in an environment conducive to catalytic reactions. Correlated algorithms can greatly improve microscopy data processing and expand multidimensional data handling. Furthermore, new techniques including 4D-STEM, atomic electron tomography, cryogenic electron microscopy, and monochromated electron energy loss spectroscopy (EELS) push the boundaries of our comprehension of catalyst behavior. In this review, we discuss the existing and emergent techniques for observing catalysts using S/TEM. Challenges and opportunities highlighted aim to inspire and accelerate the use of electron microscopy to further investigate the complex interplay of catalytic systems.

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.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Direct Imaging of the Kinetic Crystallization Pathway: Simulation and Liquid-Phase Transmission Electron Microscopy Observations

The crystallization of materials from a suspension determines the structure and function of the final product, and numerous pieces of evidence have pointed out that the classical crystallization pathway may not capture the whole picture of the crystallization pathways. However, visualizing the initial nucleation and further growth of a crystal at the nanoscale has been challenging due to the difficulties of imaging individual atoms or nanoparticles during the crystallization process in solution. Recent progress in nanoscale microscopy had tackled this problem by monitoring the dynamic structural evolution of crystallization in a liquid environment. In this review, we summarized several crystallization pathways captured by the liquid-phase transmission electron microscopy technique and compared the observations with computer simulation. Apart from the classical nucleation pathway, we highlight three nonclassical pathways that are both observed in experiments and computer simulations: formation of an amorphous cluster below the critical nucleus size, nucleation of the crystalline phase from an amorphous intermediate, and transition between multiple crystalline structures before achieving the final product. Among these pathways, we also highlight the similarities and differences between the experimental results of the crystallization of single nanocrystals from atoms and the assembly of a colloidal superlattice from a large number of colloidal nanoparticles. By comparing the experimental results with computer simulations, we point out the importance of theory and simulation in developing a mechanistic approach to facilitate the understanding of the crystallization pathway in experimental systems. We also discuss the challenges and future perspectives for investigating the crystallization pathways at the nanoscale with the development of in situ nanoscale imaging techniques and potential applications to the understanding of biomineralization and protein self-assembly.

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.

Spatioselective surface chemistry for the production of functional and chemically anisotropic nanocellulose colloids

Maximizing the benefits of nanomaterials from biomass requires unique considerations associated with their native chemical and physical structure. Both cellulose nanofibrils and nanocrystals are extracted from cellulose fibers via a top-down approach and have significantly advanced materials chemistry and set new benchmarks in the last decade. One major challenge has been to prepare defined and selectively modified nanocelluloses, which would, e.g., allow optimal particle interactions and thereby further improve the properties of processed materials. At the molecular and crystallite level, the surface of nanocelluloses offers an alternating chemical structure and functional groups of different reactivity, enabling straightforward avenues towards chemically anisotropic and molecularly patterned nanoparticles via spatioselective chemical modification. In this review, we will explain the influence and role of the multiscale hierarchy of cellulose fibers in chemical modifications, and critically discuss recent advances in selective surface chemistry of nanocelluloses. Finally, we will demonstrate the potential of those chemically anisotropic nanocelluloses in materials science and discuss challenges and opportunities in this field.

Time-Resolved Thickness and Shape-Change Quantification using a Dual-Band Nanoplasmonic Ruler with Sub-Nanometer Resolution

Time-resolved measurements of changes in the size and shape of nanobiological objects and layers are crucial to understand their properties and optimize their performance. Optical sensing is particularly attractive with high throughput and sensitivity, and label-free operation. However, most state-of-the-art solutions require intricate modeling or multiparameter measurements to disentangle conformational or thickness changes of biomolecular layers from complex interfacial refractive index variations. Here, we present a dual-band nanoplasmonic ruler comprising mixed arrays of plasmonic nanoparticles with spectrally separated resonance peaks. As electrodynamic simulations and model experiments show, the ruler enables real-time simultaneous measurements of thickness and refractive index variations in uniform and heterogeneous layers with sub-nanometer resolution. Additionally, nanostructure shape changes can be tracked, as demonstrated by quantifying the degree of lipid vesicle deformation at the critical coverage prior to rupture and supported lipid bilayer formation. In a broader context, the presented nanofabrication approach constitutes a generic route for multimodal nanoplasmonic optical sensing.

Radiolysis-Driven Evolution of Gold Nanostructures - Model Verification by Scale Bridging In Situ Liquid-Phase Transmission Electron Microscopy and X-Ray Diffraction

Utilizing ionizing radiation for in situ studies in liquid media enables unique insights into nanostructure formation dynamics. As radiolysis interferes with observations, kinetic simulations are employed to understand and exploit beam-liquid interactions. By introducing an intuitive tool to simulate arbitrary kinetic models for radiation chemistry, it is demonstrated that these models provide a holistic understanding of reaction mechanisms. This is shown for irradiated HAuCl₄ solutions allowing for quantitative prediction and tailoring of redox processes in liquid-phase transmission electron microscopy (LP-TEM). Moreover, it is demonstrated that kinetic modeling of radiation chemistry is applicable to investigations utilizing X-rays such as X-ray diffraction (XRD). This emphasizes that beam-sample interactions must be considered during XRD in liquid media and shows that reaction kinetics do not provide a threshold dose rate for gold nucleation relevant to LP-TEM and XRD. Furthermore, it is unveiled that oxidative etching of gold nanoparticles depends on both, precursor concentration, and dose rate. This dependency is exploited to probe the electron beam-induced shift in Gibbs free energy landscape by analyzing critical radii of gold nanoparticles.

Multistep Crystallization of Pharmaceutical Amorphous Nanoparticles via a Cognate Pathway of Oriented Attachment: Direct Evidence of Nonclassical Crystallization for Organic Molecules

Crystallization of organic molecules is important in a wide range of scientific disciplines. However, in contrast to maturely studied crystallization of inorganic materials, the crystallization mechanisms of organic molecules involving nucleation and crystal growth are still poorly understood. Here, we used time-resolved cryogenic transmission electron microscopy to directly map the morphological evolution of amorphous cyclosporin A (CyA) nanoparticles during CyA crystallization. We successfully observed its initial nucleation and found that the amorphous CyA nanoparticles crystallized via a pathway cognate with oriented attachment, which is the nonclassical crystallization mechanism usually reported for inorganic compounds. Crystalline mesostructured intermediates (mesocrystals) were formed during crystallization. This study revealed clear and direct evidence of mesocrystal formation and oriented attachment in organic pharmaceuticals, providing new insights into the crystallization of organic molecules and theories of nonclassical crystallization.

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.

Nanoscale Faceting and Ligand Shell Structure Dominate the Self-Assembly of Nonpolar Nanoparticles into Superlattices

Self-assembly of nanoscale structures at liquid-solid interfaces occurs in a broad range of industrial processes and is found in various phenomena in nature. Conventional theory assumes spherical particles and homogeneous surfaces, but that model is oversimplified, and nanoscale in situ observations are needed for a more complete understanding. Liquid-phase scanning transmission electron microscopy (LP-STEM) is used to examine the interactions that direct the self-assembly of superlattices formed by gold nanoparticles (AuNPs) in nonpolar liquids. Varying the molecular coating of the substrate modulates short-range attraction and leads to switching between a range of different geometric structures, including hexagonal close-packed (hcp), simple hexagonal (sh), dodecahedral quasi-crystal (dqc), and body-centered cubic (bcc) lattices, as well as random distributions. Langevin dynamics simulations explain the experimental results in terms of the interplay between nanoparticle faceting, ligand shell structure, and substrate-NP interactions.

Inactivation of Fluorescent Lipid Bilayers by Irradiation With 300 keV Electrons Using Liquid Cell Transmission Electron Microscopy

Liquid cell transmission electron microscopy allows for imaging of samples in a fully hydrated state at high resolution and has the potential for visualizing static or dynamic biological structures. However, the ionizing nature of the electron beam makes it difficult to discern real physiological dynamics from radiation induced artifacts within liquid cell samples. Electron flux thresholds for achieving high resolution structures from biological samples frozen in ice have been described extensively by the cryo-electron microscopy field, while electron flux thresholds which do not result in a functional change for biological samples within the hydrated environment of a transmission electron microscope liquid cell is less clear. Establishing these functional thresholds for biologically relevant samples is important for accurate interpretation of results from liquid cell experiments. Here we demonstrate the electron damage threshold of fluorescently tagged lipid bilayers by quantifying the change in fluorescence before and after electron exposure. We observe the reduction of fluorescent signal in bilayers by 25% after only 0.0005 e−/Å2 and a reduction of over 90% after 0.01 e−/Å2. These results indicate that the loss of function occurs at irradiation thresholds far below a typical single high resolution (scanning) transmission electron microscopy image and orders of magnitude below fluxes used for preserving structural features with cryo-electron microscopy.

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.

Coordination Cage-Based Emulsifiers: Templated Formation of Metal Oxide Microcapsules Monitored by In Situ LC-TEM

Metallo‐supramolecular self‐assembly has yielded a plethora of discrete nanosystems, many of which show competence in capturing guests and catalyzing chemical reactions. However, the potential of low‐molecular bottom‐up self‐assemblies in the development of structured inorganic materials has rarely been methodically explored so far. Herein, we present a new type of metallo‐supramolecular surfactant with the ability to stabilize non‐aqueous emulsions for a significant period. The molecular design of the surfactant is based on a heteroleptic coordination cage (CGA‐3; CGA=Cage‐based Gemini Amphiphile), assembled from two pairs of organic building blocks, grouped around two Pd(II) cations. Shape‐complementarity between the differently functionalized components generates discrete amphiphiles with a tailor‐made polarity profile, able to stabilize non‐aqueous emulsions, such as hexadecane‐in‐DMSO. These emulsions were used as a medium for the synthesis of spherical metal oxide microcapsules (titanium oxide, zirconium oxide, and niobium oxide) from soluble, water‐sensitive alkoxide precursors by allowing a controlled dosage of water to the liquid‐liquid phase boundary. Synthesized materials were analyzed by a combination of electron microscopic techniques. In situ liquid cell transmission electron microscopy (LC‐TEM) was utilized for the first time to visualize the dynamics of the emulsion‐templated formation of hollow inorganic titanium oxide and zirconium oxide microspheres.

Some living eukaryotes during and after scanning electron microscopy

Electron microscopy (EM) is an essential imaging method in biological sciences. Since biological specimens are exposed to radiation and vacuum conditions during EM observations, they die due to chemical bond breakage and desiccation. However, some organisms belonging to the taxa of bacteria, fungi, plants, and animals (including beetles, ticks, and tardigrades) have been reported to survive hostile scanning EM (SEM) conditions since the onset of EM. The surviving organisms were observed (i) without chemical fixation, (ii) after mounting to a precooled cold stage, (iii) using cryo-SEM, or (iv) after coating with a thin polymer layer, respectively. Combined use of these techniques may provide a better condition for preservation and live imaging of multicellular organisms for a long time beyond live-cell EM.

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).

Understanding Symmetry Breaking at the Single-Particle Level via the Growth of Tetrahedron-Shaped Nanocrystals from Higher-Symmetry Precursors

The vast majority of single crystalline metal nanoparticles adopt shapes in the O_h point group as a consequence of the symmetry of the underlying face-centered cubic (FCC) crystal lattice. Tetrahedra are a notable exception to this rule, and although they have been observed in several syntheses, their growth mechanism, and the symmetry-reduction process that necessarily characterizes it, is poorly understood. Here, a symmetry breaking mechanism is revealed by in situ liquid flow cell transmission electron microscopy (TEM) observation of seeded growth in which tetrahedra nanoparticles are formed from higher symmetry seeds. Real-time observation of the growth demonstrates a kinetically driven pathway during which rhombic dodecahedra nanoparticles transition to tetrahedra through tristetrahedra intermediates, with an accompanying surface facet evolution from {110} to {111} via {hhl} (where h > l), respectively. On the basis of these data, we propose a mechanism that relies on a rapid loss of inversion symmetry in the initial stages of the reaction, followed by differential reactivity of tips vs faces under conditions of relatively high supersaturation and moderate ligand concentration. The application of these insights to ex situ synthesis conditions allowed for an improved yield of tetrahedra nanoparticles. This work sheds an important mechanistic light on the crystallographic underpinnings of nanoparticle shape and symmetry transformations and highlights the importance of single-particle characterization tools for monitoring nanoscale phenomena.

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.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

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.

In Situ TEM Imaging of Solution-Phase Chemical Reactions Using 2D-Heterostructure Mixing Cells

Liquid-phase transmission electron microscopy is used to study a wide range of chemical processes, where its unique combination of spatial and temporal resolution provides countless insights into nanoscale reaction dynamics. However, achieving sub-nanometer resolution has proved difficult due to limitations in the current liquid cell designs. Here, a novel experimental platform for in situ mixing using a specially developed 2D heterostructure-based liquid cell is presented. The technique facilitates in situ atomic resolution imaging and elemental analysis, with mixing achieved within the immediate viewing area via controllable nanofracture of an atomically thin separation membrane. This novel technique is used to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment. The observations provide the first direct visual confirmation of the recently developed liquid-liquid phase separation theory, while the technological advancements open an avenue for many other studies of early stage solution-phase reactions of great interest for both the exploration of fundamental science and developing applications.

Studying Reaction Mechanisms in Solution Using a Distributed Electron Microscopy Method

Electron microscopy (EM) of materials undergoing chemical reactions provides knowledge of the underlying mechanisms. However, the mechanisms are often complex and cannot be fully resolved using a single method. Here, we present a distributed electron microscopy method for studying complex reactions. The method combines information from multiple stages of the reaction and from multiple EM methods, including liquid phase EM (LP-EM), cryogenic EM (cryo-EM), and cryo-electron tomography (cryo-ET). We demonstrate this method by studying the desilication mechanism of zeolite crystals. Collectively, our data reveal that the reaction proceeds via a two-step anisotropic etching process and that the defects in curved surfaces and between the subunits in the crystal control the desilication kinetics by directing mass transport.

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

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

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.

Anomalous nanoparticle surface diffusion in LCTEM is revealed by deep learning-assisted analysis

The motion of nanoparticles near surfaces is of fundamental importance in physics, biology, and chemistry. Liquid cell transmission electron microscopy (LCTEM) is a promising technique for studying motion of nanoparticles with high spatial resolution. Yet, the lack of understanding of how the electron beam of the microscope affects the particle motion has held back advancement in using LCTEM for in situ single nanoparticle and macromolecule tracking at interfaces. Here, we experimentally studied the motion of a model system of gold nanoparticles dispersed in water and moving adjacent to the silicon nitride membrane of a commercial LC in a broad range of electron beam dose rates. We find that the nanoparticles exhibit anomalous diffusive behavior modulated by the electron beam dose rate. We characterized the anomalous diffusion of nanoparticles in LCTEM using a convolutional deep neural-network model and canonical statistical tests. The results demonstrate that the nanoparticle motion is governed by fractional Brownian motion at low dose rates, resembling diffusion in a viscoelastic medium, and continuous-time random walk at high dose rates, resembling diffusion on an energy landscape with pinning sites. Both behaviors can be explained by the presence of silanol molecular species on the surface of the silicon nitride membrane and the ionic species in solution formed by radiolysis of water in presence of the electron beam.

Ionic liquids: prospects for nucleic acid handling and delivery

Operations with nucleic acids are among the main means of studying the mechanisms of gene function and developing novel methods of molecular medicine and gene therapy. These endeavours usually imply the necessity of nucleic acid storage and delivery into eukaryotic cells. In spite of diversity of the existing dedicated techniques, all of them have their limitations. Thus, a recent notion of using ionic liquids in manipulations of nucleic acids has been attracting significant attention lately. Due to their unique physicochemical properties, in particular, their micro-structuring impact and tunability, ionic liquids are currently applied as solvents and stabilizing media in chemical synthesis, electrochemistry, biotechnology, and other areas. Here, we review the current knowledge on interactions between nucleic acids and ionic liquids and discuss potential advantages of applying the latter in delivery of the former into eukaryotic cells.

Enhancing and Mitigating Radiolytic Damage to Soft Matter in Aqueous Phase Liquid-Cell Transmission Electron Microscopy in the Presence of Gold Nanoparticle Sensitizers or Isopropanol Scavengers

In this work, we describe the radiolytic environment experienced by a polymer in water during liquid-cell transmission electron microscopy (LCTEM). We examined the radiolytic environment of aqueous solutions of poly(ethylene glycol) (PEG, 2400 g/mol) in the presence of sensitizing gold nanoparticles (GNPs, 100 nm) or radical scavenging isopropanol (IPA). To quantify polymer damage, we employed post-mortem analysis via matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). This approach confirms IPA (1-10% w/v) can significantly mitigate radiolysis-induced damage to polymers in water, while GNPs significantly enhance damage. We couple LCTEM experiments with simulations to provide a generalizable strategy for assessing radiolysis mitigation or enhancement. This study highlights the caution required for LCTEM experiments on inorganic nanoparticles where solution phase properties of surrounding organic materials or the solvent itself are under investigation. Furthermore, we anticipate an increased use of scavengers for LCTEM studies of all kinds.

100th Anniversary of Macromolecular Science Viewpoint: Polymeric Materials by In Situ Liquid-Phase Transmission Electron Microscopy

A century ago, Hermann Staudinger proposed the macromolecular theory of polymers, and now, as we enter the second century of polymer science, we face a different set of opportunities and challenges for the development of functional soft matter. Indeed, many fundamental questions remain open, relating to physical structures and mechanisms of phase transformations at the molecular and nanoscale. In this Viewpoint, we describe efforts to develop a dynamic, in situ microscopy tool suited to the study of polymeric materials at the nanoscale that allows for direct observation of discrete structures and processes in solution, as a complement to light, neutron, and X-ray scattering methods. Liquid-phase transmission electron microscopy (LPTEM) is a nascent in situ imaging technique for characterizing and examining solvated nanomaterials in real time. Though still under development, LPTEM has been shown to be capable of several modes of imaging: (1) imaging static solvated materials analogous to cryo-TEM, (2) videography of nanomaterials in motion, (3) observing solutions or nanomaterials undergoing physical and chemical transformations, including synthesis, assembly, and phase transitions, and (4) observing electron beam-induced chemical-materials processes. Herein, we describe opportunities and limitations of LPTEM for polymer science. We review the basic experimental platform of LPTEM and describe the origin of electron beam effects that go hand in hand with the imaging process. These electron beam effects cause perturbation and damage to the sample and solvent that can manifest as artefacts in images and videos. We describe sample-specific experimental guidelines and outline approaches to mitigate, characterize, and quantify beam damaging effects. Altogether, we seek to provide an overview of this nascent field in the context of its potential to contribute to the advancement of polymer science.

In Situ Quantification of Interactions between Charged Nanorods in a Predefined Potential Energy Landscape

Quantitative understanding of nanoscale interactions is a prerequisite for harnessing the remarkable collective properties of nanoparticle systems. Here, we report the combined use of liquid-phase transmission electron microscopy and electron beam lithography to elucidate the interactions between charged nanorods in a predefined potential energy landscape. In situ site-selective lift-off of surface-functionalized lithographed gold nanorods is achieved by patterning them with adhesion layer materials that undergo etching at different rates. Analysis of the subsequent nanorod motion, which is two-dimensionally confined as a result of the particle-substrate attraction, allows quantification of interparticle interactions in a lithographically engineered environment. For lithographed nanorods patterned with the same adhesion layer material, their self-assembly behavior following lift-off is tuned by changing their starting spatial arrangement. Our approach facilitates investigation of interparticle interactions in designed nanoparticle systems and affords fundamental insights into the role of the potential energy landscape in determining the kinetic pathway for nanoparticle self-assembly.

Investigating states of gas in water encapsulated between graphene layers

Conventionally, only two states are assumed to exist in water: well-dispersed gas monomers and gas bubbles. Rarely is this paradigm explored experimentally. To close this gap, here we used transmission electron microscopy (TEM) to study degassed water, deionized water, and gas-supersaturated water encapsulated in graphene liquid cells. While neither degassed water nor deionized water yielded specific features, two major microscopic structures were evident in gas-supersaturated water: (1) polycrystalline nanoparticles formed of gas molecules and (2) a high density of tiny cells. Dark-field TEM imaging revealed that water molecules surrounding each cell form crystalline structures-a surprising discovery of a clathrate state in gas-supersaturated water that may help resolve several long-standing puzzles. Overall, this study suggests that water may form a matrix that actively interacts with gas molecules in complex and subtle ways.

100th Anniversary of Macromolecular Science Viewpoint: Single-Molecule Studies of Synthetic Polymers

Single polymer studies have revealed unexpected and heterogeneous dynamics among identical or seemingly similar macromolecules. In recent years, direct observation of single polymers has uncovered broad distributions in molecular behavior that play a key role in determining bulk properties. Early single polymer experiments focused primarily on biological macromolecules such as DNA, but recent advances in synthesis, imaging, and force spectroscopy have enabled broad exploration of chemically diverse polymer systems. In this Viewpoint, we discuss the recent study of synthetic polymers using single-molecule methods. In terms of polymer synthesis, direct observation of single chain polymerization has revealed heterogeneity in monomer insertion events at catalytic centers and decoupling of local and global growth kinetics. In terms of single polymer visualization, recent advances in super-resolution imaging, atomic force microscopy (AFM), and liquid-cell transmission electron microscopy (LC-TEM) can resolve structure and dynamics in single synthetic chains. Moreover, single synthetic polymers can be probed in the context of bulk material environments, including hydrogels, nanostructured polymers, and crystalline polymers. In each area, we highlight key challenges and exciting opportunities in using single polymer techniques to enhance our understanding of polymer science. Overall, the expanding versatility of single polymer methods will enable the molecular-scale design and fundamental understanding of a broad range of chemically diverse and functional polymeric materials.

Liquid-phase electron microscopy imaging of cellular and biomolecular systems

Liquid-phase electron microscopy, a new method for real-time nanoscopic imaging in liquid, makes it possible to study cells or biomolecules with a singular combination of spatial and temporal resolution. We review the state of the art in biological research in this growing and promising field.