Application of X-ray Diffraction and Electron Crystallography for Solving Complex Structure Problems

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g-1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Single-Crystal Three-Dimensional Covalent Organic Framework Constructed from 6-Connected Triangular Prism Node

The limited structural diversity of three-dimensional covalent organic frameworks (3D COFs) greatly restricts their application exploration. Therefore, there is an urgent need to expand their library of molecular building blocks, such as the development of highly connected (>4 reaction sites) polyhedral nodes. Herein, by precisely controlling the precursor conformation, we rationally designed a new 6-connected triangular prism node derived from the triphenylbenzene molecule and further used it to construct a novel 3D COF (3D-TMTAPB-COF) via imine condensation reaction. Surprisingly, without the addition of competing reagents, 3D-TMTAPB-COF crystallized directly into single crystals of ∼15 μm in size and was determined to adopt a rare 6-fold interpenetrated (Class IIIa interpenetration) acs topology. In addition, 3D-TMTAPB-COF showed a high SF6 adsorption capacity (60.9 cm3 g-1) and good SF6/N2 selectivity (335) at 298 K and 1 bar, superior to those of most crystalline porous materials. This work not only confirms the possibility of growing large-size single-crystal 3D COFs formed with strong covalent bonds by a solvothermal method in the absence of modulators, but also reports a novel triangular prism node for future construction of 3D COFs with interesting applications.

Type-II Red Phosphorus: Wavy Packing of Twisted Pentagonal Tubes

Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 Å3 , which is the largest unit cell for elemental phosphorus crystals up to now and contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems.

Non-Covalent Reactions Supporting Antiviral Development

Viruses are the current big enemy of the world's healthcare systems. As the small infector causes various deadly diseases, from influenza and HIV to COVID-19, the virus continues to evolve from one type to its mutants. Therefore, the development of antivirals demands tremendous attention and resources for drug researchers around the world. Active pharmaceutical ingredients (API) development includes discovering new drug compounds and developing existing ones. However, to innovate a new antiviral takes a very long time to test its safety and effectiveness, from structure modeling to synthesis, and then requires various stages of clinical trials. Meanwhile, developing the existing API can be more efficient because it reduces many development stages. One approach in this effort is to modify the solid structures to improve their physicochemical properties and enhance their activity. This review discusses antiviral multicomponent systems under the research phase and has been marketed. The discussion includes the types of antivirals, their counterpart compound, screening, manufacturing methods, multicomponent systems yielded, characterization methods, physicochemical properties, and their effects on their pharmacological activities. It is hoped that the opportunities and challenges of solid antiviral drug modifications can be drawn in this review as important information for further antiviral development.

Digitization of imaging plates from Guinier powder X-ray diffraction cameras

A Guinier camera equipped with an imaging plate is used to investigate and eliminate the sources of instrumental errors affecting the quality of the obtained scanned Guinier data. A program with a graphical user interface is presented which converts the data of the scanned images into different standard file formats for powder X-ray patterns containing intensities, their standard deviations and the diffraction angles. The program also allows for manual and automatic correction of the 2θ scale against a known reference material. It is shown using LaB6 that the exported X-ray diffraction patterns provide a 2θ scale reproducible enough to allow for averaging diffractograms obtained from different exposures of the imaging plate for the same sample. As shown on a mixture of NaCl and sodalite, the quality of the produced data is sufficient for Rietveld refinement. The software including source code is made available under a free software license.

Atomic-resolution structures from polycrystalline covalent organic frameworks with enhanced cryo-cRED

The pursuit of atomic precision structure of porous covalent organic frameworks (COFs) is the key to understanding the relationship between structures and properties, and further developing new materials with superior performance. Yet, a challenge of how to determine their atomic structures has always existed since the first COFs reported seventeen years ago. Here, we present a universal method for ab initio structure determination of polycrystalline three-dimensional (3D) COFs at atomic level using enhanced cryo-continuous rotation electron diffraction (cryo-cRED), which combines hierarchical cluster analysis with cryo-EM technique. The high-quality datasets possess not only up to 0.79-angstrom resolution but more than 90% completeness, leading to unambiguous solution and precise refinement with anisotropic temperature factors. With such a powerful method, the dynamic structures with flexible linkers, degree of interpenetration, position of functional groups, and arrangement of ordered guest molecules are successfully revealed with atomic precision in five 3D COFs, which are almost impossible to be obtained without atomic resolution structure solution. This study demonstrates a practicable strategy for determining the structures of polycrystalline COFs and other beam-sensitive materials and to help in the future discovery of novel materials on the other.

Charge-Discharge Mechanism of High-Entropy Co-Free Spinel Oxide Toward Li+ Storage Examined Using Operando Quick-Scanning X-Ray Absorption Spectroscopy

Transition metal high-entropy oxides (HEOs) are an attractive class of anode materials for high-performance lithium-ion batteries (LIBs). However, owing to the multiple electroactive centers of HEOs, the Li+ storage mechanism is complex and debated in the literature. In this work, operando quick-scanning X-ray absorption spectroscopy (XAS) is used to study the lithiation/delithiation mechanism of the Cobalt-free spinel (CrMnFeNiCu)3 O4 HEO. A monochromator oscillation frequency of 2 Hz is used and 240 spectra are integrated to achieve a 2 min time resolution. High-photon-flux synchrotron radiation is employed to increase the XAS sensitivity. The results indicate that the Cu2+ and Ni2+ cations are reduced to their metallic states during lithiation but their oxidation reactions are less favorable compared to the other elements upon delithiation. The Mn2+/3+ and Fe2+/3+ cations undergo two-step conversion reactions to form metallic phases, with MnO and FeO as the intermediate species, respectively. During delithiation, the oxidation of Mn occurs prior to that of Fe. The Cr3+ cations are reduced to CrO and then Cr0 during lithiation. A relatively large overpotential is required to activate the Cr reoxidation reaction. The Cr3+ cations are found after delithiation. These results can guide the material design of HEOs for improving LIB performance.

Resolving alternative structure determinations of indapamide using 13C solid-state NMR

The conflict between alternative crystal structures in the Cambridge Structural Database for the diuretic drug indapamide hemihydrate (IND) has been resolved with the aid of ¹³C solid-state NMR. IND is seen to contain multiple distinct molecules in the asymmetric unit (Z' = 4) rather than exhibiting disorder in the orientation of sulfonamide groups. The NMR crystallographic approach is a more effective tool for distinguishing between alternative structures than naïve judgements of quality based on crystallographic refinement agreement factors.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

A kiloelectron-volt ultrafast electron micro-diffraction apparatus using low emittance semiconductor photocathodes

We report the design and performance of a time-resolved electron diffraction apparatus capable of producing intense bunches with simultaneously single digit micrometer probe size, long coherence length, and 200 fs rms time resolution. We measure the 5d (peak) beam brightness at the sample location in micro-diffraction mode to be 7 × 10 13   A / m 2   rad 2 . To generate high brightness electron bunches, the system employs high efficiency, low emittance semiconductor photocathodes driven with a wavelength near the photoemission threshold at a repetition rate up to 250 kHz. We characterize spatial, temporal, and reciprocal space resolution of the apparatus. We perform proof-of-principle measurements of ultrafast heating in single crystal Au samples and compare experimental results with simulations that account for the effects of multiple scattering.

Tailoring the Pore Surface of 3D Covalent Organic Frameworks via Post-Synthetic Click Chemistry

Three-dimensional covalent organic frameworks (3D COFs) have gained increasing attention for their attractive features. However, the development of 3D COFs is strongly restricted, mainly due to their synthetic difficulty and complicated structure determination. Post-synthetic modification, which can avoid these problems by incorporating functional moieties into a predetermined framework, provides an alternative way to construct 3D COFs with specific functions. Herein, we report the designed synthesis and characterization of a series of highly crystalline 3D COFs with different loadings of ethynyl groups. Notably, these alkyne-tagged 3D COFs provide a platform for targeted anchoring various specific groups onto the pore walls via click reactions. Moreover, the pore surface engineering can accordingly change their properties, for example, the obtained click products exhibited higher CO₂ /N₂ selectivity. We describe a simple but powerful strategy to build functional 3D COFs, which will certainly advance them for a ranging of interesting applications in the future.

Monolayer Support Control and Precise Colloidal Nanocrystals Demonstrate Metal-Support Interactions in Heterogeneous Catalysts

Electronic and geometric interactions between active and support phases are critical in determining the activity of heterogeneous catalysts, but metal-support interactions are challenging to study. Here, it is demonstrated how the combination of the monolayer-controlled formation using atomic layer deposition (ALD) and colloidal nanocrystal synthesis methods leads to catalysts with sub-nanometer precision of active and support phases, thus allowing for the study of the metal-support interactions in detail. The use of this approach in developing a fundamental understanding of support effects in Pd-catalyzed methane combustion is demonstrated. Uniform Pd nanocrystals are deposited onto Al₂ O₃ /SiO₂ spherical supports prepared with control over morphology and Al₂ O₃ layer thicknesses ranging from sub-monolayer to a ≈4 nm thick uniform coating. Dramatic changes in catalytic activity depending on the coverage and structure of Al₂ O₃ situated at the Pd/Al₂ O₃ interface are observed, with even a single monolayer of alumina contributing an order of magnitude increase in reaction rate. By building the Pd/Al₂ O₃ interface up layer-by-layer and using uniform Pd nanocrystals, this work demonstrates the importance of controlled and tunable materials in determining metal-support interactions and catalyst activity.

Cryo-EM as a powerful tool for drug discovery: recent structural based studies of SARS-CoV-2

The novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has arisen as a global pandemic affecting the respiratory system showing acute respiratory distress syndrome (ARDS). However, there is no targeted therapeutic agent yet and due to the growing cases of infections and the rising death tolls, discovery of the possible drug is the need of the hour. In general, the study for discovering therapeutic agent for SARS-CoV-2 is largely focused on large-scale screening with fragment-based drug discovery (FBDD). With the recent advancement in cryo-electron microscopy (Cryo-EM), it has become one of the widely used tools in structural biology. It is effective in investigating the structure of numerous proteins in high-resolution and also had an intense influence on drug discovery, determining the binding reaction and regulation of known drugs as well as leading the design and development of new drug candidates. Here, we review the application of cryo-EM in a structure-based drug design (SBDD) and in silico screening of the recently acquired FBDD in SARS-CoV-2. Such insights will help deliver better understanding in the procurement of the effective remedial solution for this pandemic.

[Cu36H10(PET)24(PPh3)6Cl2] Reveals Surface Vacancy Defects in Ligand-Stabilized Metal Nanoclusters

Precise identification and in-depth understanding of defects in nanomaterials can aid in rationally modulating defect-induced functionalities. However, few studies have explored vacancy defects in ligand-stabilized metal nanoclusters with well-defined structures, owing to the substantial challenge of synthesizing and isolating such defective metal nanoclusters. Herein, a novel defective copper hydride nanocluster, [Cu₃₆H₁₀(PET)₂₄(PPh₃)₆Cl₂] (Cu36; PET: phenylethanethiolate; PPh₃: triphenylphosphine), is successfully synthesized at the gram scale via a simple one-pot reduction method. Structural analysis reveals that Cu36 is a distorted half cubic nanocluster, evolved from the perfect Nichol's half cube. The two surface copper vacancies in Cu36 are found to be the principal imperfections, which result in some structural adjustments, including copper atom reconstruction near the vacancies as well as ligand modifications (e.g., substitution, migration, and exfoliation). Density functional theory calculations imply that the above-mentioned defects have a considerable influence on the electronic structure and properties. The modeling suggests that the formation of defective Cu36 rather than the perfect half cube is driven by the enlargement of the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the nanocluster. The structural evolution induced by the surface copper atom vacancies provides atomically precise insights into the defect-induced readjustment of the local structure and introduces new avenues for understanding the chemistry of defects in nanomaterials.

Reticular design and crystal structure determination of covalent organic frameworks

Reticular chemistry of covalent organic frameworks (COFs) deals with the linking of discrete organic molecular building units into extended structures adopting various topologies by strong covalent bonds. The past decade has witnessed a rapid development of COF chemistry in terms of both structural diversity and applications. From the structural perspective, irrespective of our subject of concern with regard to COFs, it is inevitable to take into account the structural aspects of COFs in all dimensions from 1D ribbons to 3D frameworks, for which understanding the concepts of reticular chemistry, based mainly on 'reticular design', will seemingly lead to unlimited ways of exploring the exquisiteness of this advanced class of porous, extended, and crystalline materials. A comprehensive discussion and understanding of reticular design, therefore, is of paramount importance so that everyone willing to research on COFs can interpret well and chemically correlate the geometrical structures of this subset of reticular materials and their practical applications. This article lies at the heart of using the conceptual basis of reticular chemistry for designing, modeling, and determination of novel infinite and crystalline structures. Especially, the structure determinations are described by means of chronological advances of discoveries and development of COFs whereby their crystal structures are elucidated by modeling through the topological approach, 3D electron diffraction, single-crystal X-ray diffraction, and powder X-ray diffraction techniques.

Celecoxib Crystallized from Hydrophilic Polymeric Solutions Showed Modified Crystalline Behavior with an Improved Dissolution Profile

The crystallization technique has been established as a cost-effective and simple approach to improve the dissolution rate and oral bioavailability of poorly soluble drugs. This study was carried out to study the effect of some selected hydrophilic polymers such as methyl cellulose, hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol, and carboxymethyl cellulose on the crystal behavior and dissolution properties of celecoxib (CLX), a common nonsteroidal anti-inflammatory drug. Structural and spectral characteristics of crystallized CLX have been studied by Fourier transform infrared (FTIR) spectroscopy, diffraction scanning calorimetry (DSC), and X-ray diffraction (XRD) analysis. From FTIR and DSC analysis, no significant shifting of peaks or appearance of any new peaks (for polymers) were observed, which indicated the absence of any major interaction between drug and polymers as well as the absence of polymers in the final crystallized product of CLX. The XRD analysis showed a change in crystalline morphology to some extent. The dissolution rate of crystallized CLX in the presence of polymers (particularly with HPMC) was significantly improved compared with plain CLX. The improved dissolution profile of the experimental CLX crystal products could be an indication of improved bioavailability of CLX for better clinical outcome.

Tuning the Topology of Three-Dimensional Covalent Organic Frameworks via Steric Control: From to Unprecedented

Whether or not the topology of three-dimensional covalent organic frameworks (3D COFs) can be tuned via steric control remains a big question and has never been reported. Herein, we describe the designed synthesis of two highly crystalline 3D COFs (3D-TPB-COF-OMe and 3D-TPB-COF-Ph), through the polycondensation of tetra(p-aminophenyl)methane and methoxy- or phenyl- substituted 1,2,4,5-tetrakis(4-formylphenyl)benzene on the 3- and 6-positions. Amazingly, by using the continuous rotation electron diffraction technique, 3D-TPB-COF-OMe is determined to have a 5-fold interpenetrated structure with a reported pts net, while 3D-TPB-COF-Ph adopts an unprecedented self-penetrated ljh topology (ljh = Luojia Hill) that does not exist in the database of ToposPro. Therefore, by altering the substituents from methoxy to phenyl groups, the topology of designed 3D COFs changes accordingly, and a rare net is now available. This result clearly demonstrates that such COF structures need to be carefully determined due to its complexity, and moreover, it is promising to design 3D COFs with new topology for interesting application by increasing the steric hindrance of molecular building blocks.

Quantifying the Likelihood of Structural Models through a Dynamically Enhanced Powder X-Ray Diffraction Protocol

Structurally characterizing new materials is tremendously challenging, especially when single crystal structures are hardly available which is often the case for covalent organic frameworks. Yet, knowledge of the atomic structure is key to establish structure-function relations and enable functional material design. Herein, a new protocol is proposed to unambiguously predict the structure of poorly crystalline materials through a likelihood ordering based on the X-ray diffraction (XRD) pattern. Key of the procedure is the broad set of structures generated from a limited number of building blocks and topologies, which is submitted to operando structural characterization. The dynamic averaging in the latter accounts for the operando conditions and inherent temporal character of experimental measurements, yielding unparalleled agreement with experimental powder XRD patterns. The proposed concept can hence unquestionably identify the structure of experimentally synthesized materials, a crucial step to design next generation functional materials.

Chromosome inner structure investigation by electron tomography and electron diffraction in a transmission electron microscope

Our understanding of the inner structure of metaphase chromosomes remains inconclusive despite intensive studies using multiple imaging techniques. Transmission electron microscopy has been extensively used to visualize chromosome ultrastructure. This review summarizes recent results obtained using two transmission electron microscopy-based techniques: electron tomography and electron diffraction. Electron tomography allows advanced three-dimensional imaging of chromosomes, while electron diffraction detects the presence of periodic structures within chromosomes. The combination of these two techniques provides results contributing to the understanding of local structural organization of chromatin fibers within chromosomes.

A Crystalline Three-Dimensional Covalent Organic Framework with Flexible Building Blocks

The construction of three-dimensional covalent organic frameworks (3D COFs) has proven to be very challenging, as their synthetic driving force mainly comes from the formation of covalent bonds. To facilitate the synthesis, rigid building blocks are always the first choice for designing 3D COFs. In principle, it should be very appealing to construct 3D COFs from flexible building blocks, but there are some obstacles blocking the development of such systems, especially for the designed synthesis and structure determination. Herein, we reported a novel highly crystalline 3D COF (FCOF-5) with flexible C-O single bonds in the building block backbone. By merging 17 continuous rotation electron diffraction data sets, we successfully determined the crystal structure of FCOF-5 to be a 6-fold interpenetrated pts topology. Interestingly, FCOF-5 is flexible and can undergo reversible expansion/contraction upon vapor adsorption/desorption, indicating a breathing motion. Moreover, a smart soft polymer composite film with FCOF-5 was fabricated, which can show a reversible vapor-triggered shape transformation. Therefore, 3D COFs constructed from flexible building blocks can exhibit interesting breathing behavior, and finally, a totally new type of soft porous crystals made of pure organic framework was announced.

Structure–direction towards the new large pore zeolite NUD-3

A subtle structure–direction allows the crystallization of the ordered and fully connected zeolite NUD-3 instead of disordered or interrupted versions.

Redox-triggered switching in three-dimensional covalent organic frameworks

The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years. Herein, we report a switchable three-dimensional covalent organic framework (3D COF), which can undergo a reversible transformation through a hydroquinone/quinone redox reaction while retaining the crystallinity and porosity. Our results clearly show that the switching process gradually happened through the COF framework, with an almost quantitative conversion yield. In addition, the redox-triggered transformation will form different functional groups on the pore surface and modify the shape of pore channel, which can result in tunable gas separation property. This study strongly demonstrates 3D COFs can provide robust platforms for efficient tuning of molecular switches in solid state. More importantly, switching of these moieties in 3D COFs can remarkably modify the internal pore environment, which will thus enable the resulting materials with interesting stimuli-responsive properties.

Tuning of molecular switches in solid state toward stimuli-responsive materials attracted attention in recent years but has not yet been realized in three-dimensional (3D) covalent organic frameworks (COFs). Herein, the authors demonstrate a stable and switchable 3D COF which undergoes reversible transformation through a hydroquinone/quinone redox reaction.

Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ

CeNbO_4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO₄, CeNbO_4.08, CeNbO_4.25, CeNbO_4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO_4.08 and CeNbO_4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO_4.08, and the supercell structure of CeNbO_4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO_4.08 and CeNbO_4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb₂O₉ units. However, the excess oxygen in CeNbO_4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO_4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

Electron diffraction of CS2 nanoclusters embedded in superfluid helium droplets

We report experimental results from electron diffraction of CS2 nanoclusters embedded in superfluid helium droplets. From detailed measurements of the sizes of doped droplets, we can model the doping statistics under different experimental conditions, thereby obtaining the range of cluster sizes of CS2. Using a least squares fitting procedure, we can then determine the structures and contributions of dimers, trimers, and tetramers embedded in small droplets. While dimers prefer a stable gas phase structure, trimers and tetramers seem to forgo the highly symmetric gas phase structures and prefer compact cuts from the crystalline structure of CS2. In larger droplets containing more than 12 CS2 monomers, the diffraction profile is consistent with a three-dimensional nanostructure of bulk CS2. This work demonstrates the feasibility of electron diffraction for in situ monitoring of nanocluster formation in superfluid helium droplets.

Isostructural Three-Dimensional Covalent Organic Frameworks

Herein, we reported the designed synthesis of three isostructural three-dimensional covalent organic frameworks (3D COFs) with -H, -Me, or -F substituents, which have similar crystallinity and topology. Their crystal structures were determined by continuous rotation electron diffraction (cRED), and all three 3D COFs were found to adopt a fivefold interpenetrated pts topology. More importantly, the resolution of these cRED datasets reached up to 0.9-1.0 Å, enabling the localization of all non-hydrogen atomic positions in a COF framework directly by 3D ED techniques for the first time. In addition, the precise control of the pore environments through the use of different functional groups led to different selectivities for CO₂ over N₂ . We have thus confirmed that polycrystalline COFs can be definitely studied to the atomic level as other materials, and this study should also inspire the design and synthesis of 3D COFs with tailored pore environments for interesting applications.

Analytical techniques for characterization of biological molecules - proteins and aptamers/oligonucleotides

With the advent of the high-throughput technologies and exciting times for biology, the discipline of analytical methodology is experiencing a surge in the growth and the scope. Over the years, multitude of analytical techniques have evolved from a work-intensive, low sensitivity and high volume of reagent and sample consumption endeavor to automated, better selectivity, lower limit of quantification and cost-effective techniques for biological research. In this review, we give an overview of the currently available wide range of cell-based and noncell based and structural based analytical techniques, their principle and biological applications. The analytical techniques discussed in this paper includes surface plasmon resonance, electrophoresis, enzyme linked immunosorbent assay, Western blotting, flow cytometry, fluorescence activated cell sorting, mass spectrometry, nuclear magnetic resonance and x-ray crystallography.

Characterization of Atomic Structures of Nanosized Intermetallic Compounds Using Electron Diffraction Methods

In metallurgy, many intermetallic compounds crystallize as nanosized particles in metallic matrices. These particles influence dramatically the physical properties of engineering materials such as alloys and steels. Since properties and crystal structure are intimately linked, characterization of the atomic model of these intermetallides is crucial for the development of new alloys. However, this structural information usually cannot be attained using traditional X-ray diffraction methods, limited by the small volume and size of the precipitates. In these cases, electron diffraction (ED) is the most suitable method. In the last few decades, ED has experienced a tremendous leap forward. Many structures, including intermetallides, are solved using these methods. The class of intermetallides should be discussed independently since these phases do not comprise regular polyhedrals; moreover, the interatomic distances and angles vary drastically even in the same compositional system. These facts point to difficulties that have to be overcome during the solution path. Furthermore, intermetallic compounds can be of high complexity-possessing hundreds of atoms in the unit cell. Here, this topic is expanded with an emphasis on novel developments in the field.

Nanospace within metal-organic frameworks for gas storage and separation

Porous metal-organic frameworks (MOFs), also known as porous coordination polymers, represent a new class of porous materials, and one of their striking features lies in their tunable, designable, and functionalizable nanospace. This nanospace within MOFs provides virtually plenty of room for imagination, allowing designed incorporation of different size, shape, and functionalities for targeted gas storage and separation applications. Furthermore, the features of high porosities, tunable framework structures and pore sizes, and immobilized functional sites enable MOF materials to fully make use of their nanopore space for gas storage, to optimize their sieving effects, and to differentiate their interactions with gas molecules for gas separation. In this review article, we highlight some recent significant advances in developing microporous MOFs for some of the most important gas storage and separation applications.