In-situ transmission electron microscopy of liposomes in an aqueous environment

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.

Messenger RNA in lipid nanoparticles rescues HEK 293 cells from lipid-induced mitochondrial dysfunction as studied by real time pulse chase NMR, RTPC-NMR, spectroscopy

Analytical tools to study cell physiology are critical for optimizing drug-host interactions. Real time pulse chase NMR spectroscopy, RTPC-NMR, was introduced to monitor the kinetics of metabolite production in HEK 293T cells treated with COVID-19 vaccine-like lipid nanoparticles, LNPs, with and without mRNA. Kinetic flux parameters were resolved for the incorporation of isotopic label into metabolites and clearance of labeled metabolites from the cells. Changes in the characteristic times for alanine production implicated mitochondrial dysfunction as a consequence of treating the cells with lipid nanoparticles, LNPs. Mitochondrial dysfunction was largely abated by inclusion of mRNA in the LNPs, the presence of which increased the size and uniformity of the LNPs. The methodology is applicable to all cultured cells.

Dynamics of Polymer Nanocapsule Buckling and Collapse Revealed by In Situ Liquid-Phase TEM

Nanocapsules are hollow nanoscale shells that have applications in drug delivery, batteries, self-healing materials, and as model systems for naturally occurring shell geometries. In many applications, nanocapsules are designed to release their cargo as they buckle and collapse, but the details of this transient buckling process have not been directly observed. Here, we use in situ liquid-phase transmission electron microscopy to record the electron-irradiation-induced buckling in spherical 60-187 nm polymer capsules with ∼3.5 nm walls. We observe in real time the release of aqueous cargo from these nanocapsules and their buckling into morphologies with single or multiple indentations. The in situ buckling of nanoscale capsules is compared to ex situ measurements of collapsed and micrometer-sized capsules and to Monte Carlo (MC) simulations. The shape and dynamics of the collapsing nanocapsules are consistent with MC simulations, which reveal that the excessive wrinkling of nanocapsules with ultrathin walls results from their large Föppl-von Kármán numbers around 10⁵. Our experiments suggest design rules for nanocapsules with the desired buckling response based on parameters such as capsule radius, wall thickness, and collapse rate.

Lipid in Chips: A Brief Review of Liposomes Formation by Microfluidics

Liposomes are ubiquitous tools in biomedical applications, such as drug delivery, membrane science and artificial cell. Micro- and nanofabrication techniques have revolutionized the preparation of liposomes on the microscale. State-of-the-art liposomal formation on microfluidic chips and its associated applications are introduced in this review. We attempt to provide a reference for liposomal researchers by comparing various microfluidic techniques for liposomes formation.

In situ Electron Microscopy of Complex Biological and Nanoscale Systems: Challenges and Opportunities

In situ imaging for direct visualization is important for physical and biological sciences. Research endeavors into elucidating dynamic biological and nanoscale phenomena frequently necessitate in situ and time-resolved imaging. In situ liquid cell electron microscopy (LC-EM) can overcome certain limitations of conventional electron microscopies and offer great promise. This review aims to examine the status-quo and practical challenges of in situ LC-EM and its applications, and to offer insights into a novel correlative technique termed microfluidic liquid cell electron microscopy. We conclude by suggesting a few research ideas adopting microfluidic LC-EM for in situ imaging of biological and nanoscale systems.

Hybrid nanocapsules for in situ TEM imaging of gas evolution reactions in confined liquids

Liquid cell transmission electron microscopy (TEM) enables the direct observation of dynamic physical and chemical processes in liquids at the nanoscale. Quantitative investigations into reactions with fast kinetics and/or multiple reagents will benefit from further advances in liquid cell design that facilitate rapid in situ mixing and precise control over reagent volumes and concentrations. This work reports the development of inorganic-organic nanocapsules for high-resolution TEM imaging of nanoscale reactions in liquids with well-defined zeptoliter volumes. These hybrid nanocapsules, with 48 nm average diameter, consist of a thin layer of gold coating a lipid vesicle. As a model reaction, the nucleation, growth, and diffusion of nanobubbles generated by the radiolysis of water is investigated inside the nanocapsules. When the nanobubbles are sufficiently small (10-25 nm diameter), they are mobile in the nanocapsules, but their movement deviates from Brownian motion, which may result from geometric confinement by the nanocapsules. Gases and fluids can be transported between two nanocapsules when they fuse, demonstrating in situ mixing without using complex microfluidic schemes. The ability to synthesize nanocapsules with controlled sizes and to monitor dynamics simultaneously inside multiple nanocapsules provides opportunities to investigate nanoscale processes such as single nanoparticle synthesis in confined volumes and biological processes such as biomineralization and membrane dynamics.

In Situ Visualization of Ferritin Biomineralization via Graphene Liquid Cell-Transmission Electron Microscopy

Ferritin biomineralization is essential to regulate the toxic Fe²⁺ iron ions in the human body. Unravelling the mechanism of biomineralization in ferritin facilitates our understanding of the causes underlying many iron disorder-related diseases. Until now, no report of in situ visualization of ferritin biomineralization events at nanoscale exists due to the requirement for high-resolution imaging of nanometer-sized ferritin proteins in their hydrated states. Herein, for the first time, we show that the biomineralization processes within individual ferritin proteins can be visualized by means of graphene liquid cell-transmission electron microscopy (GLC-TEM). The increase in the ratio of Fe³⁺/Fe²⁺ ions over time monitored via electron energy loss spectroscopy (EELS) reveals the change in oxidation state of iron oxide phases with time. This study lays a foundation for future investigations on iron regulation mechanisms in healthy and dysfunctional ferritins.

On the structure and chemistry of iron oxide cores in human heart and human spleen ferritins using graphene liquid cell electron microscopy

Ferritin is a protein that regulates the iron ions in humans by storing them in the form of iron oxides. Despite extensive efforts to understand the ferritin iron oxide structures, it is still not clear how ferritin proteins with a distinct light (L) and heavy (H) chain subunit ratio impact the biomineralization process. In situ graphene liquid cell-transmission electron microscopy (GLC-TEM) provides an indispensable platform to study the atomic structure of ferritin mineral cores in their native liquid environment. In this study, we report differences in the iron oxide formation in human spleen ferritins (HSFs) and human heart ferritins (HHFs) using in situ GLC-TEM. Scanning transmission electron microscopy (STEM) along with selected area electron diffraction (SAED) of the mineral core and electron energy loss spectroscopy (EELS) analyses enabled the visualization of morphologies, crystal structures and the chemistry of iron oxide cores in HSFs and HHFs. Our study revealed the presence of metastable ferrihydrite (5Fe2O3·9H2O) as a dominant phase in hydrated HSFs and HHFs, while a stable hematite (α-Fe2O3) phase predominated in non-hydrated HSFs and HHFs. In addition, a higher Fe3+/Fe2+ ratio was found in HHFs in comparison with HSFs. This study provides new understanding on iron-oxide phases that exist in hydrated ferritin proteins from different human organs. Such new insights are needed to map ferritin biomineralization pathways and possible correlations with various iron-related disorders in humans.

The Why, Where, Who, How, and What of the vesicular delivery systems

Though vesicular delivery systems have been widely explored and reviewed, no comprehensive review exists that covers their development from the inception of the concept to its culmination in the form of regulated marketed formulations. With the advancement of scientific research in the field of nanomedicine, certain category of vesicular delivery systems have successfully reached the global market. Despite extensive research and highly encouraging results in a plethora of pathological conditions in the preclinical studies, translation of these nanomedicines from laboratory to market has been very limited. Aim of this review is to describe comprehensively the various colloidal delivery systems, focusing mainly on their conventional and advanced methods of preparation, different characterization techniques and main success stories of their journey from bench to bedside of the patient. The review also touches the finer nuances of the use of modern formulation approach of DoE (Design of Experiments) in their formulation and the status of regulatory guidelines for the approval of these nanomedicines.

Synthesis of complex rare earth nanostructures using in situ liquid cell transmission electron microscopy

Energy and cost efficient synthesis pathways are important for the production, processing, and recycling of rare earth metals necessary for a range of advanced energy and environmental applications. In this work, we present results of successful in situ liquid cell transmission electron microscopy production and imaging of rare earth element nanostructure synthesis, from aqueous salt solutions, via radiolysis due to exposure to a 200 keV electron beam. Nucleation, growth, and crystallization processes for nanostructures formed in yttrium(iii) nitrate hydrate (Y(NO3)3·4H2O), europium(iii) chloride hydrate (EuCl3·6H2O), and lanthanum(iii) chloride hydrate (LaCl3·7H2O) solutions are discussed. In situ electron diffraction analysis in a closed microfluidic configuration indicated that rare earth metal, salt, and metal oxide structures were synthesized. Real-time imaging of nanostructure formation was compared in closed cell and flow cell configurations. Notably, this work also includes the first known collection of automated crystal orientation mapping data through liquid using a microfluidic transmission electron microscope stage, which permits the deconvolution of amorphous and crystalline features (orientation and interfaces) inside the resulting nanostructures.

Imaging of soft materials using in situ liquid-cell transmission electron microscopy

This review summarizes the breakthroughs in the field of soft material characterization by in situ liquid-cell transmission electron microscopy (TEM). The focus of this review is mostly on soft biological species such as cells, bacteria, viruses, proteins and polymers. The comparison between the two main liquid-cell systems (silicon nitride membranes liquid cell and graphene liquid cell) is also discussed in terms of their spatial resolution and imaging/analytical capabilities. We have showcased how liquid-cell TEM can reveal the structural details of whole cells, enable the chemical probing of proteins, detect the structural conformation of viruses, and monitor the dynamics of polymerization. In addition, the challenges faced by decoupling electron beam effect on beam-sensitive soft materials are discussed. At the end, future perspectives of in situ liquid-cell TEM studies of soft materials are outlined.

Rational Design of an Amphiphilic Coordination Cage-Based Emulsifier

Self-assembled, porous coordination cages with a functional interior find application in controlled guest inclusion/release, drug delivery, separation processes, and catalysis. However, only few studies exist that describe their utilization for the development of self-assembled materials based on their 3-dimensional shape and external functionalization. Here, dodecyl chain-containing, acridone-based ligands (L^A) and shape-complementary phenanthrene-derived ligands (L^B) are shown to self-assemble to heteroleptic coordination cages cis-[Pd₂(L^A)₂(L^B)₂]⁴⁺ acting as a gemini amphiphile (CGA-1; Cage-based Gemini Amphiphile-1). Owing to their anisotropic decoration with short polar and long nonpolar side chains, the cationic cages were found to assemble into vesicles with diameters larger than 100 nm in suitable polar solvents, visualized by cryo-TEM and Liquid-Cell Transmission Electron Microscopy (LC-TEM). LC-TEM reveals that these vesicles aggregate into chains and necklaces via long-range interactions. In addition, the cages show a rarely described ability to stabilize oil-in-oil emulsions.

Monitoring the dynamics of cell-derived extracellular vesicles at the nanoscale by liquid-cell transmission electron microscopy

Cell-derived extracellular vesicles (EVs) circulating in body fluids hold promises as bioactive therapeutic agents and as biomarkers to diagnose a wide range of diseases. However nano-imaging methods are needed to characterize these complex and heterogeneous soft materials in their native wet environment. Herein, we exploit liquid-cell transmission electron microscopy (LCTEM) to characterize the morphology and dynamic behavior of EVs in physiological media with nanometer resolution. The beam-induced controlled growth of Au nanoparticles on bilayer membranes is used as an original in situ staining method to improve the contrast of EVs and artificial liposomes. LCTEM provides information about the size distribution and concentration of EVs that are consistent with Cryo-TEM and nanoparticle tracking analysis measurements. Moreover, LCTEM gives a unique insight into the dynamics of EVs depending on their liquid environment. The size-dependent morphology of EVs is sensitive to osmotic stress which tends to transform their spherical shape to ellipsoidal, stomatocyte or discocyte morphologies. In the liquid-cell, EVs exhibit a sub-diffusive motion due to strong interactions between the Au nanoparticles and the liquid-cell windows. Finally, the high-resolution monitoring of EV aggregation and fusion illustrate that LCTEM opens up a new way to study cell-membrane dynamics.

Rapid and Controlled In Situ Growth of Noble Metal Nanostructures within Halloysite Clay Nanotubes

A rapid (≤2 min) and high-yield low-temperature synthesis has been developed for the in situ growth of gold nanoparticles (NPs) with controlled sizes in the interior of halloysite nanotubes (HNTs). A combination of HAuCl₄ in ethanol/toluene, oleic acid, and oleylamine surfactants and ascorbic acid reducing agent with mild heating (55 °C) readily lead to the growth of targeted nanostructures. The sizes of Au NPs are tuned mainly by adjusting nucleation and growth rates. Further modification of the process, through an increase in ascorbic acid, allows for the formation of nanorods (NRs)/nanowires within the HNTs. This approach is not limited to gold-a modified version of this synthetic strategy can also be applied to the formation of Ag NPs and NRs within the clay nanotubes. The ability to readily grow such core-shell nanosystems is important to their further development as nanoreactors and active catalysts. NPs within the tube interior can further be manipulated by the electron beam. Growth of Au and Ag could be achieved under a converged electron beam suggesting that both Au@HNT and Ag@HNT systems can be used for the fundamental studies of NP growth/attachment.

Liquid-Cell Electron Microscopy of Adsorbed Polymers

Individual macromolecules of polystyrene sulfonate and poly(ethylene oxide) are visualized with nanometer resolution using transmission electron microscopy (TEM) imaging of aqueous solutions with and without added salt, trapped in liquid pockets between creased graphene sheets. Successful imaging with 0.3 s per frame is enabled by the sluggish mobility of the adsorbed molecules. This study finds, validating others, that an advantage of this graphene liquid-cell approach is apparently to retard sample degradation from incident electrons, in addition to minimizing background scattering because graphene windows are atomically thin. Its new application here to polymers devoid of metal-ion labeling allows the projected sizes and conformational fluctuations of adsorbed molecules and adsorption-desorption events to be analyzed. Confirming the identification of the observed objects, this study reports statistical analysis of datasets of hundreds of images for times up to 100 s, with variation of the chemical makeup of the polymer, the molecular weight of the polymer, and the salt concentration. This observation of discrete polymer molecules in solution environment may be useful generally, as the findings are obtained using an ordinary TEM microscope, whose kind is available to many researchers routinely.

Cryo-EM Visualization of Nanobubbles in Aqueous Solutions

The detection of nanobubbles on surfaces is well established (e.g., AFM and optical microscopy methods), but currently no methods exist for the direct detection of bulk nanobubbles. Here, cryo-electron microscopy (cryo-EM) has been employed to observe bubbles in aqueous solutions for the first time. Nitrogen bubbles generated by a chemical reaction were observed in amorphous ice trapped between two carbon films. The cryo-EM images of bubbles showed the same features as predicted by theory. The fact that no bubbles were observed near an air-water interface suggests that bubbles may diffuse to the nearby air-water interface and escape. The estimate of the bubble diffusion coefficient is about 30-250 μm²/s.

Opportunities and challenges in liquid cell electron microscopy

Transmission electron microscopy offers structural and compositional information with atomic resolution, but its use is restricted to thin, solid samples. Liquid samples, particularly those involving water, have been challenging because of the need to form a thin liquid layer that is stable within the microscope vacuum. Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to imaging and analysis of liquid specimens. We describe its impact in materials science and biology. We discuss how its applications have expanded via improvements in equipment and experimental techniques, enabling new capabilities and stimuli for samples in liquids, and offering the potential to solve grand challenge problems.

Electron Microscopy and Image Processing: Essential Tools for Structural Analysis of Macromolecules

Macromolecular electron microscopy typically depicts the structures of macromolecular complexes ranging from ∼200 kDa to hundreds of MDa. The amount of specimen required, a few micrograms, is typically 100 to 1000 times less than needed for X-ray crystallography or nuclear magnetic resonance spectroscopy. Micrographs of frozen-hydrated (cryogenic) specimens portray native structures, but the original images are noisy. Computational averaging reduces noise, and three-dimensional reconstructions are calculated by combining different views of free-standing particles ("single-particle analysis"). Electron crystallography is used to characterize two-dimensional arrays of membrane proteins and very small three-dimensional crystals. Under favorable circumstances, near-atomic resolutions are achieved. For structures at somewhat lower resolution, pseudo-atomic models are obtained by fitting high-resolution components into the density. Time-resolved experiments describe dynamic processes. Electron tomography allows reconstruction of pleiomorphic complexes and subcellular structures and modeling of macromolecules in their cellular context. Significant information is also obtained from metal-coated and dehydrated specimens.

Real-time imaging of lead nanoparticles in solution – determination of the growth mechanism

In situ electron microscopy is a tool which offers great promise for studying the mechanisms responsible for nanoparticle growth.

Improved microchip design and application for in situ transmission electron microscopy of macromolecules

Understanding the fundamental properties of macromolecules has enhanced the development of emerging technologies used to improve biomedical research. Currently, there is a critical need for innovative platforms that can illuminate the function of biomedical reagents in a native environment. To address this need, we have developed an in situ approach to visualize the dynamic behavior of biomedically relevant macromolecules at the nanoscale. Newly designed silicon nitride devices containing integrated "microwells" were used to enclose active macromolecular specimens in liquid for transmission electron microscopy imaging purposes.We were able to successfully examine novel magnetic resonance imaging contrast reagents, micelle suspensions, liposome carrier vehicles, and transcribing viral assemblies. With each specimen tested, the integrated microwells adequately maintained macromolecules in discrete local environments while enabling thin liquid layers to be produced.

Dynamics of soft nanomaterials captured by transmission electron microscopy in liquid water

In this paper we present in situ transmission electron microscopy of synthetic polymeric nanoparticles with emphasis on capturing motion in a solvated, aqueous state. The nanoparticles studied were obtained from the direct polymerization of a Pt(II)-containing monomer. The resulting structures provided sufficient contrast for facile imaging in situ. We contend that this technique will quickly become essential in the characterization of analogous systems, especially where dynamics are of interest in the solvated state. We describe the preparation of the synthetic micellar nanoparticles together with their characterization and motion in liquid water with comparison to conventional electron microscopy analyses.

Studies of the dynamics of biological macromolecules using Au nanoparticle–DNA artificial molecules

The recent development of graphene liquid cells, a nanoscale version of liquid bubble wrap, is a breakthrough for in situ liquid phase electron microscopy (EM). Using ultrathin graphene sheets as the liquid sample container, graphene liquid cells have allowed the unprecedented atomic resolution observation of solution phase growth and dynamics of nanocrystals. Here we explore the potential of this technique to probe nanoscale structure and dynamics of biomolecules in situ, using artificial Au nanoparticle–DNA artificial molecules as model systems. The interactions of electrons with both the artificial molecules and the liquid environment have been demonstrated and discussed, revealing both the opportunities and challenges of using graphene liquid cell EM as a new method of bio-imaging.