Controlled Encapsulation of Gold Nanoparticles into Zr-Metal-Organic Frameworks with Improved Detection Limitation of Volatile Organic Compounds via Surface-Enhanced Raman Scattering.

Volatile organic compounds (VOCs) have harmful effects on human health and the environment but detecting low levels of VOCs is challenging due to a lack of reliable biomarkers. However, incorporating gold nanoparticles (Au NPs) into metal-organic frameworks (MOFs) shows promise for VOC detection. In this study, we developed nanoscale Au@UiO-66 that exhibited surface-enhanced Raman scattering (SERS) activity even at very low levels of toluene vapors (down to 1.0 ppm) due to the thickness of the shell and strong π-π interactions between benzenyl-type linkers and toluene. The UiO-66 shell also increased the thermal stability of the Au NPs, preventing aggregation up to 550 °C. This development may be useful for sensitive detection of VOCs for environmental protection purposes.

An archetype of the electron-unobstructed core-shell composite with inherent selectivity: conductive metal-organic frameworks encapsulated with metal nanoparticles.

The acquisition of monodisperse metal nanoparticles covered by conductive metal-organic frameworks (cMOFs) is an archetype of an electron-unobstructed core-shell composite, valued for its potential electrocatalytic ability and selectivity enhancement. In this work, Pt@cMOF composites with direct interfaces showed better performance in the oxygen reduction reaction than composites with indirect interfaces or with lower electroconductivity shells. This composite was proved to exhibit the ability to expedite electron transfer with different thicknesses of electrode materials. The detailed mechanism was studied by exploring the conductivity of shell materials, interfaces between cores and shells, and the surface electronic structure of the nanoparticles. We also report reaction selectivity from the inherent porous shells in the selective reduction of cinnamyl alcohol.

Hierarchically encapsulating enzymes with multi-shelled metal-organic frameworks for tandem biocatalytic reactions.

Biocatalytic transformations in living organisms, such as multi-enzyme catalytic cascades, proceed in different cellular membrane-compartmentalized organelles with high efficiency. Nevertheless, it remains challenging to mimicking biocatalytic cascade processes in natural systems. Herein, we demonstrate that multi-shelled metal-organic frameworks (MOFs) can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency. Encapsulating multi-enzymes with multi-shelled MOFs by epitaxial shell-by-shell overgrowth leads to 5.8~13.5-fold enhancements in catalytic efficiencies compared with free enzymes in solution. Importantly, multi-shelled MOFs can act as a multi-spatial-compartmental nanoreactor that allows physically compartmentalize multiple enzymes in a single MOF nanoparticle for operating incompatible tandem biocatalytic reaction in one pot. Additionally, we use nanoscale Fourier transform infrared (nano-FTIR) spectroscopy to resolve nanoscale heterogeneity of vibrational activity associated to enzymes encapsulated in multi-shelled MOFs. Furthermore, multi-shelled MOFs enable facile control of multi-enzyme positions according to specific tandem reaction routes, in which close positioning of enzyme-1-loaded and enzyme-2-loaded shells along the inner-to-outer shells could effectively facilitate mass transportation to promote efficient tandem biocatalytic reaction. This work is anticipated to shed new light on designing efficient multi-enzyme catalytic cascades to encourage applications in many chemical and pharmaceutical industrial processes.

Molecular-Level Insights into Selective Transport of Mg2+ in Metal-Organic Frameworks.

Metal-organic frameworks (MOF) are promising media for achieving solid-state Mg2+ conduction and developing a magnesium-based battery. To this end, the chemical behavior and transport properties of an Mg(TFSI)2/DME electrolyte system inside Mg-MOF-74 were studied by density functional theory (DFT). We found that inside the MOF chemical environment, solvent and anion molecules occupy the coordinatively unsaturated open metal sites of Mg-MOF-74, while Mg2+ ions adsorb directly onto the carboxylate group of the MOF organic linker. These predicted binding geometries were further corroborated by IR spectroscopy. We computed the free energies of desolvation of Mg2+ ions inside MOF to investigate the capacity of Mg-MOF-74 thin film to act as a separator for selective Mg2+ transport. We showed that Mg-MOF-74 could facilitate partial, but not full, desolvation of Mg2+. We found that the dominant minimum-energy pathway (MEP) for Mg2+ conduction inside Mg-MOF-74 corresponds to a "solvent hopping" mechanism, with an energy barrier of 4.4 kcal/mol. The molar conductivity of Mg2+ associated with the idealized solvent hopping mechanism along the MOF one-dimensional channel was predicted to be 2.4 × 10-3 S cm-1 M-1, which is one to two orders of magnitude greater than the experimentally measured value of 1.2 × 10-4 S cm-1 M-1 (with an estimated Mg2+ concentration). We have discussed several possible factors contributing to this apparent discrepancy. The current work demonstrates the validity of the computational strategies applied and the structural models constructed for the understanding of fast and selective Mg2+ transport in Mg-MOF-74, which serves as a cornerstone for studying transport of multivalent ions in MOFs. Furthermore, it provides detailed molecular-level insights that are not yet accessible experimentally.

Tailoring Heterogeneous Catalysts at the Atomic Level: In Memoriam, Prof. Chia-Kuang (Frank) Tsung.

Professor Chia-Kuang (Frank) Tsung made his scientific impact primarily through the atomic-level design of nanoscale materials for application in heterogeneous catalysis. He approached this challenge from two directions: above and below the material surface. Below the surface, Prof. Tsung synthesized finely controlled nanoparticles, primarily of noble metals and metal oxides, tailoring their composition and surface structure for efficient catalysis. Above the surface, he was among the first to leverage the tunability and stability of metal-organic frameworks (MOFs) to improve heterogeneous, molecular, and biocatalysts. This article, written by his former students, seeks first to commemorate Prof. Tsung's scientific accomplishments in three parts: (1) rationally designing nanocrystal surfaces to promote catalytic activity; (2) encapsulating nanocrystals in MOFs to improve catalyst selectivity; and (3) tuning the host-guest interaction between MOFs and guest molecules to inhibit catalyst degradation. The subsequent discussion focuses on building on the foundation laid by Prof. Tsung and on his considerable influence on his former group members and collaborators, both inside and outside of the lab.

A direct solvent-free conversion approach to prepare mixed-metal metal-organic frameworks from doped metal oxides.

We propose a novel strategy to introduce platinum into the metal nodes of ZIF-8 by preloading Pt as a dopant in ZnO (Pt-ZnO) and then convert it to Pt doped ZIF-8 (Pt-ZIF-8) through a chemical vapor deposition (CVD) approach. The solvent-free conversion of Pt-ZnO to Pt-ZIF-8 allows the Pt dopant in ZnO to coordinate with organic linkers directly without the formation of Pt nanoparticles, which is a general issue of many methods. This general synthesis strategy may facilitate the discovery of MMOFs that have not been reported previously.

Creating an Aligned Interface between Nanoparticles and MOFs by Concurrent Replacement of Capping Agents.

Applying metal-organic frameworks (MOFs) on the surface of other materials to form multifunctional materials has recently attracted great attention; however, directing the MOF overgrowth is challenging due to the orders of magnitude differences in structural dimensions. In this work, we developed a universal strategy to mediate MOF growth on the surface of metal nanoparticles (NPs), by taking advantage of the dynamic nature of weakly adsorbed capping agents. During this colloidal process, the capping agents gradually dissociate from the metal surface, replaced in situ by the MOF. The MOF grows to generate a well-defined NP-MOF interface without a trapped capping agent, resulting in a uniform core-shell structure of one NP encapsulated in one single-crystalline MOF nanocrystal with specific facet alignment. The concept was demonstrated by coating ZIF-8 and UiO-66-type MOFs on shaped metal NPs capped by cetyltrimethylammonium surfactants, and the formation of the well-defined NP-MOF interface was monitored by spectroscopies. The defined interface outperforms ill-defined ones generated via conventional methods, displaying a high selectivity to unsaturated alcohols for the hydrogenation of an α,ß-unsaturated aldehyde. This strategy opens a new route to create aligned interfaces between materials with vastly different structural dimensions.

Electrolyte-Resistant Dual Materials for the Synergistic Safety Enhancement of Lithium-Ion Batteries.

Safety issues associated with lithium-ion batteries are of major concern, especially with the ever-growing demand for higher-energy-density storage devices. Although flame retardants (FRs) added to electrolytes can reduce fire hazards, large amounts of FRs are required and they severely deteriorate battery performance. Here, we report a feasible method to balance flame retardancy and electrochemical performance by coating an electrolyte-insoluble FR on commercial battery separators. By integrating dual materials via a two-pronged mechanism, the quantity of FR required could be limited to an ultrathin coating layer (4 µm) that rarely influences electrochemical performance. The developed composite separator has a four-times better flame retardancy than conventional polyolefin separators in full pouch cells. Additionally, this separator can be fabricated easily on a large scale for industrial applications. High-energy-density batteries (2 Ah) were assembled to demonstrate the scaling of the composite separator and to confirm its enhanced safety through nail penetration tests.

Metal-Organic Framework-Based Enzyme Biocomposites.

Because of their efficiency, selectivity, and environmental sustainability, there are significant opportunities for enzymes in chemical synthesis and biotechnology. However, as the three-dimensional active structure of enzymes is predominantly maintained by weaker noncovalent interactions, thermal, pH, and chemical stressors can modify or eliminate activity. Metal-organic frameworks (MOFs), which are extended porous network materials assembled by a bottom-up building block approach from metal-based nodes and organic linkers, can be used to afford protection to enzymes. The self-assembled structures of MOFs can be used to encase an enzyme in a process called encapsulation when the MOF is synthesized in the presence of the biomolecule. Alternatively, enzymes can be infiltrated into mesoporous MOF structures or surface bound via covalent or noncovalent processes. Integration of MOF materials and enzymes in this way affords protection and allows the enzyme to maintain activity in challenge conditions (e.g., denaturing agents, elevated temperature, non-native pH, and organic solvents). In addition to forming simple enzyme/MOF biocomposites, other materials can be introduced to the composites to improve recovery or facilitate advanced applications in sensing and fuel cell technology. This review canvasses enzyme protection via encapsulation, pore infiltration, and surface adsorption and summarizes strategies to form multicomponent composites. Also, given that enzyme/MOF biocomposites straddle materials chemistry and enzymology, this review provides an assessment of the characterization methodologies used for MOF-immobilized enzymes and identifies some key parameters to facilitate development of the field.

Engineering Second Sphere Interactions in a Host-Guest Multicomponent Catalyst System for the Hydrogenation of Carbon Dioxide to Methanol.

Many enzymes utilize interactions extending beyond the primary coordination sphere to enhance catalyst activity and/or selectivity. Such interactions could improve the efficacy of synthetic catalyst systems, but the supramolecular assemblies employed by biology to incorporate second sphere interactions are challenging to replicate in synthetic catalysts. Herein, a strategy is reported for efficiently manipulating outer-sphere influence on catalyst reactivity by modulating host-guest interactions between a noncovalently encapsulated transition-metal-based catalyst guest and a metal-organic framework (MOF) host. This composite consists of a ruthenium PNP pincer complex encapsulated in the MOF UiO-66 that is used in tandem with the zirconium oxide nodes of UiO-66 and a ruthenium PNN pincer complex to hydrogenate carbon dioxide to methanol. Due to the method used to incorporate the complexes in UiO-66, structure-activity relationships could be efficiently determined using a variety of functionalized UiO-66-X hosts. These investigations uncovered the beneficial effects of the ammonium functional group (i.e., UiO-66-NH3+). Mechanistic experiments revealed that the ammonium functionality improved efficiency in the hydrogenation of carbon dioxide to formic acid, the first step in the cascade. Isotope effects and structure-activity relationships suggested that the primary role of the ammonium functionality is to serve as a general Brønsted acid. Importantly, the cooperative influence from the host was effective only with the functional group in close proximity to the encapsulated catalyst. Reactions carried out in the presence of molecular sieves to remove water highlighted the beneficial effects of the ammonium functional group in UiO-66-NH3+ and resulted in a 4-fold increase in activity. As a result of the modular nature of the catalyst system, the highest reported turnover number (TON) (19 000) and turnover frequency (TOF) (9100 h-1) for the hydrogenation of carbon dioxide to methanol are obtained. Moreover, the reaction was readily recyclable, leading to a cumulative TON of 100 000 after 10 reaction cycles.

Structure of a seeded palladium nanoparticle and its dynamics during the hydride phase transformation.

Palladium absorbs large volumetric quantities of hydrogen at room temperature and ambient pressure, making the palladium hydride system a promising candidate for hydrogen storage. Here, we use Bragg coherent diffraction imaging to map the strain associated with defects in three dimensions before and during the hydride phase transformation of an individual octahedral palladium nanoparticle, synthesized using a seed-mediated approach. The displacement distribution imaging unveils the location of the seed nanoparticle in the final nanocrystal. By comparing our experimental results with a finite-element model, we verify that the seed nanoparticle causes a characteristic displacement distribution of the larger nanocrystal. During the hydrogen exposure, the hydride phase is predominantly formed on one tip of the octahedra, where there is a high number of lower coordinated Pd atoms. Our experimental and theoretical results provide an unambiguous method for future structure optimization of seed-mediated nanoparticle growth and in the design of palladium-based hydrogen storage systems.

The impact of synthetic method on the catalytic application of intermetallic nanoparticles.

Intermetallic alloy nanocrystals have emerged as a promising next generation of nanocatalyst, largely due to their promise of surface tunability. Atomic control of the geometric and electronic structure of the nanoparticle surface offers a precise command of the catalytic surface, with the potential for creating homogeneous active sites that extend over the entire nanoparticle. Realizing this promise, however, has been limited by synthetic difficulties, imparted by differences in parent metal crystal structure, reduction potential, and atomic size. Further, little attention has been paid to the impact of synthetic method on catalytic application. In this review, we seek to connect the two, organizing the current synthesis methods and catalytic scope of intermetallic nanoparticles and suggesting areas where more work is needed. Such analysis should help to guide future intermetallic nanoparticle development, with the ultimate goal of generating precisely controlled nanocatalysts tailored to catalysis.

Probing Interactions between Metal-Organic Frameworks and Freestanding Enzymes in a Hollow Structure.

It has been reported that the biological functions of enzymes could be altered when they are encapsulated in metal-organic frameworks (MOFs) due to the interactions between them. Herein, we probed the interactions of catalase in solid and hollow ZIF-8 microcrystals. The solid sample with confined catalase is prepared through a reported method, and the hollow sample is generated by hollowing the MOF crystals, sealing freestanding enzymes in the central cavities of hollow ZIF-8. During the hollowing process, the samples were monitored by small-angle X-ray scattering (SAXS) spectroscopy, electron microscopy, powder X-ray diffraction (PXRD), and nitrogen sorption. The interfacial interactions of the two samples were studied by infrared (IR) and fluorescence spectroscopy. IR study shows that freestanding catalase has less chemical interaction with ZIF-8 than confined catalase, and a fluorescence study indicates that the freestanding catalase has lower structural confinement. We have then carried out the hydrogen peroxide degradation activities of catalase at different stages and revealed that the freestanding catalase in hollow ZIF-8 has higher activity.

Reshaping of Truncated Pd Nanocubes: Energetic and Kinetic Analysis Integrating Transmission Electron Microscopy with Atomistic-Level and Coarse-Grained Modeling.

Stability against reshaping of metallic fcc nanocrystals synthesized with tailored far-from-equilibrium shapes is key to maintaining optimal properties for applications such as catalysis. Yet Arrhenius analysis of experimental reshaping kinetics, and appropriate theory and simulation, is lacking. Thus, we use TEM to monitor the reshaping of Pd nanocubes of ∼25 nm side length between 410 °C (over ∼4.5 h) and 440 °C (over ∼0.25 h), extracting a high effective energy barrier of Eeff ≈ 4.6 eV. We also provide an analytic determination of the energy variation along the optimal pathway for reshaping that involves transfer of atoms across the nanocube surface from edges or corners to form new layers on side {100} facets. The effective barrier from this analysis is shown to increase strongly with the degree of truncation of edges and corners in the synthesized nanocube. Theory matches experiment for the appropriate degree of truncation. In addition, we perform simulations of a stochastic atomistic-level model incorporating a realistic description of diffusive hopping for undercoordinated surface atoms, thereby providing a visualization of the initial reshaping process.

Applying a Nanoparticle@MOF Interface To Activate an Unconventional Regioselectivity of an Inert Reaction at Ambient Conditions.

Here we design an interface between a metal nanoparticle (NP) and a metal-organic framework (MOF) to activate an inert CO2 carboxylation reaction and in situ monitor its unconventional regioselectivity at the molecular level. Using a Kolbe-Schmitt reaction as model, our strategy exploits the NP@MOF interface to create a pseudo high-pressure CO2 microenvironment over the phenolic substrate to drive its direct C-H carboxylation at ambient conditions. Conversely, Kolbe-Schmitt reactions usually demand high reaction temperature (>125 °C) and pressure (>80 atm). Notably, we observe an unprecedented CO2 meta-carboxylation of an arene that was previously deemed impossible in traditional Kolbe-Schmitt reactions. While the phenolic substrate in this study is fixed at the NP@MOF interface to facilitate spectroscopic investigations, free reactants could be activated the same way by the local pressurized CO2 microenvironment. These valuable insights create enormous opportunities in diverse applications including synthetic chemistry, gas valorization, and greenhouse gas remediation.

Enhancing Four-Carbon Olefin Production from Acetylene over Copper Nanoparticles in Metal-Organic Frameworks.

Four-carbon olefins, such as 1-butene and 1,3-butadiene, are important chemical feedstocks for the production of adhesives and synthetic rubber. These compounds are found in the C4 fraction of "green oil" products that can arise during the hydrogenation of acetylene. Here, we demonstrate that control of the catalyst structure increases the yield and productivity of these important olefins with a family of catalyst materials comprising Cu nanoparticles (CuNPs) bound within the pores of Zr-based metal-organic frameworks. Using carbon monoxide as a probe molecule, we characterize the surfaces of these catalytic CuNPs with diffuse reflectance infrared Fourier transform spectroscopy, revealing that the electronic structure of the CuNP surfaces is size-dependent. Furthermore, we find that as the CuNP diameter decreases, the selectivity for C4 products increases and that lowering the stoichiometric ratio of H2/acetylene improves the selectivity and productivity of the catalyst.

Stabilizing DNAzymes through Encapsulation in a Metal-Organic Framework.

DNAzymes are a promising class of bioinspired catalyst; however, their structural instability limits their potential. Herein, a method to stabilize DNAzymes by encapsulating them in a metal-organic framework (MOF) host is reported. This biomimetic mineralization process makes DNAzymes active under a wider range of conditions. The concept is demonstrated by encapsulating hemin-G-quadruplex (Hemin-G4) into zeolitic imidazolate framework-90 (ZIF-90), which indeed increases the DNAzyme's structural stability. The stabilized DNAzymes show activities in the presence of Exonuclease I, organic solvents, or high temperature. Owing to its elevated stability and heterogeneous nature, it is possible to perform catalysis under continuous-flow conditions, and the DNAzyme can be reactivated in situ by introducing K+ . Moreover, it is found that the encapsulated DNAzyme maintains its high enantiomer selectivity, demonstrated by the sulfoxidation of thioanisole to (S)-methyl phenyl sulfoxide. This concept of stabilizing DNAzymes expands their potential application in chemical industry.

Investigating lattice strain impact on the alloyed surface of small Au@PdPt core-shell nanoparticles.

We investigated lattice strain on alloyed surfaces using ∼10 nm core-shell nanoparticles with controlled size, shape, and composition. We developed a wet-chemistry method for synthesizing small octahedral PdPt alloy nanoparticles and Au@PdPt core-shell nanoparticles with Pd-Pt alloy shells and Au cores. Upon introduction of the Au core, the size and shape of the overall nanostructure and the composition of the alloyed PdPt were maintained, enabling the use of the electrooxidation of formic acid as a probe to compare the surface structures with different lattice strain. We have found that the structure of the alloyed surface is indeed impacted by the lattice strain generated by the Au core. To further reveal the impact of lattice strain, we fine-tuned the shell thickness. Then, we used synchrotron-based X-ray diffraction to investigate the degree of lattice strain and compared the observations with the results of the formic acid electrooxidation, suggesting that there is an optimal intermediate shell thickness for high catalytic activity.

Strain-Enhanced Metallic Intermixing in Shape-Controlled Multilayered Core-Shell Nanostructures: Toward Shaped Intermetallics.

Controlling the surface composition of shaped bimetallic nanoparticles could offer precise tunability of geometric and electronic surface structure for new nanocatalysts. To achieve this goal, a platform for studying the intermixing process in a shaped nanoparticle was designed, using multilayered Pd-Ni-Pt core-shell nanocubes as precursors. Under mild conditions, the intermixing between Ni and Pt could be tuned by changing layer thickness and number, triggering intermixing while preserving nanoparticle shape. Intermixing of the two metals is monitored using transmission electron microscopy. The surface structure evolution is characterized using electrochemical methanol oxidation. DFT calculations suggest that the low-temperature mixing is enhanced by shorter diffusion lengths and strain introduced by the layered structure. The platform and insights presented are an advance toward the realization of shape-controlled multimetallic nanoparticles tailored to each potential application.

Surface softening in palladium nanoparticles: effects of a capping agent on vibrational properties.

The presence of a capping agent (CTAB) on Pd nanoparticles produces a strong static disorder in the surface region. This results in a surface softening, which contributes to an overall increase in the Debye-Waller coefficient measured by X-ray powder diffraction. Molecular dynamics and density functional theory simulations show that the adsorption-induced surface disorder is strong enough to overcome the effects of nanoparticle size and shape.