Enhanced Photobiocatalytic Cascades at Pickering Droplet Interfaces.

Developing new methods to engineer photobiocatalytic reactions is of utmost significance for artificial photosynthesis, but it remains a grand challenge due to the intrinsic incompatibility of biocatalysts with photocatalysts. In this work, photocatalysts and enzymes were spatially colocalized at Pickering droplet interfaces, where the reaction microenvironment and the spatial distance between two distinct catalysts were exquisitely regulated to achieve unprecedented photobiocatalytic cascade reactions. As proof of the concept, ultrathin graphitic carbon nitride nanosheets loaded with Au nanoparticles were precisely positioned in the outer interfacial layer of Pickering oil droplets to produce H2O2 under light irradiation, while enzymes were exactly placed in the inner interfacial layer to catalyze the subsequent biocatalytic oxidation reactions using in situ formed H2O2 as an oxidant. In the alkene epoxidation and thioether oxidation, our interfacial photobiocatalytic cascades showed a 2.0-5.8-fold higher overall reaction efficiency than the photobiocatalytic cascades in the bulk water phase. It was demonstrated that spatial localization of the photocatalyst and the enzyme at Pickering oil droplet interfaces not only provided their respective preferable reaction environments and intimate proximity for rapid H2O2 transport but also protected the enzyme from oxidative inactivation caused by the photogenerated species. These remarkable interfacial effects contributed to the significantly enhanced photobiocatalytic cascading efficiency. Our work presents an innovative photobiocatalytic reaction system with manifold benefits, providing a cutting-edge platform for solar-driven chemical transformations via photobiocatalysis.

Amphiphilic Covalent Organic Framework Nanoparticles for Pickering Emulsion Catalysis with Size Selectivity.

Exploiting advanced amphiphilic solid catalysts is crucial to the development of Pickering emulsion catalysis. Herein, covalent organic framework (COF) nanoparticles constructed with highly hydrophobic monomers as linkers were found to show superior amphiphilicity and they were then developed as a new class of solid emulsifiers for Pickering emulsion catalysis. Employing amphiphilic COFs as solid emulsifiers, Pickering emulsions with controllable emulsion type and droplet sizes were obtained. COF materials have also been demonstrated to serve as porous surface coatings to replace traditional surface modifications for stabilizing Pickering emulsions. After implanting Pd nanoparticles into amphiphilic COFs, the obtained catalyst displayed a 3.9 times higher catalytic efficiency than traditional amphiphilic solid catalysts with surface modifications in the biphasic oxidation reaction of alcohols. Such an enhanced activity was resulted from the high surface area and regular porous structure of COFs. More importantly, because of their tunable pore diameters, Pickering emulsion catalysis with remarkable size selectivity was achieved. This work is the first example that COFs were applied in Pickering emulsion catalysis, providing a platform for exploring new frontiers of Pickering emulsion catalysis.

Bottom-Up Synthesis of Multicompartmentalized Microreactors for Continuous Flow Catalysis.

The bottom-up assembly of biomimetic multicompartmentalized microreactors for use in continuous flow catalysis remains a grand challenge because of the structural instability or the absence of liquid microenvironments to host biocatalysts in the existing systems. Here, we address this challenge using a strategy that combines stepwise Pickering emulsification with interface-confined cross-linking. Our strategy allows for the fabrication of robust multicompartmentalized liquid-containing microreactors (MLMs), whose interior architectures can be exquisitely tuned in a bottom-up fashion. With this strategy, enzymes and metal catalysts can be separately confined in distinct subcompartments of MLMs for processing biocatalysis or chemo-enzymatic cascade reactions. As exemplified by the enzyme-catalyzed kinetic resolution of racemic alcohols, our systems exhibit a durability of 2000 h with 99% enantioselectivity. Another Pd-enzyme-cocatalyzed dynamic kinetic resolution of amines further demonstrates the versatility and long-term operational stability of our MLMs in continuous flow cascade catalysis. This study opens up a new way to design efficient biomimetic multicompartmental microreactors for practical applications.

Phase-Controlled Cobalt Catalyst Boosting Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran.

The search for non-noble metal catalysts for chemical transformations is of paramount importance. In this study, an efficient non-noble metal catalyst for hydrogenation, hexagonal close-packed cobalt (HCP-Co), was synthesized through a simple one-step reduction of ß-Co(OH)2 nanosheets via a temperature-induced phase transition. The obtained HCP-Co exhibited several-times-higher catalytic efficiency than its face-centered cubic cobalt (FCC-Co) counterpart in the hydrogenation of the C=C/C=O group, especially for the 5-hydroxymethylfurfural (HMF) hydrogenation (8.5-fold enhancement). Density functional theory calculations demonstrated that HMF molecules were adsorbed more firmly on the (112_0) facet of HCP-Co than that on the (111) facet of FCC-Co, favoring the activation of the C=O group in the HMF molecule. The stronger adsorption on the (112_0) facet of HCP-Co also led to lower activation energy than that on the (111) facet of FCC-Co, thereby resulting in high activity and selectivity. Moreover, HCP-Co exhibited outstanding catalytic stability during the hydrogenation of HMF. These results highlight the possibility of fabricating hydrogenation catalysts with satisfactory catalytic properties by precisely tuning their active crystal phase.

Multi-compartmental MOF microreactors derived from Pickering double emulsions for chemo-enzymatic cascade catalysis.

Bioinspired multi-compartment architectures are desired in synthetic biology and metabolic engineering, as credited by their cell-like structures and intrinsic ability of assembling catalytic species for spatiotemporal control over cascade reactions like in living systems. Herein, we describe a general Pickering double emulsion-directed interfacial synthesis method for the fabrication of multicompartmental MOF microreactors. This approach employs multiple liquid-liquid interfaces as a controllable platform for the self-completing growth of dense MOF layers, enabling the microreactor with tailor-made inner architectures and selective permeability. Importantly, simultaneous encapsulation of incompatible functionalities, including hydrophilic enzyme and hydrophobic molecular catalyst, can be realized in a single MOF microreactor for operating chemo-enzymatic cascade reactions. As exemplified by the Grubb' catalyst/CALB lipase driven olefin metathesis/ transesterification cascade reaction and glucose oxidase (GOx)/Fe-porphyrin catalyzed oxidation reaction, the multicompartmental microreactor exhibits 2.24-5.81 folds enhancement in cascade reaction efficiency in comparison to the homogeneous counterparts or physical mixture of individual analogues, due to the restrained mutual inactivation and substrate channelling effects. Our study prompts further design of multicompartment systems and the development of artificial cells capable of complex cellular transformations.

Aggregation-Induced Structural Symmetry Breaking Promotes Charge Separation for Efficient Photocatalytic Hydrogen Production.

Recently, organic semiconductors have received much attention in the field of photocatalysis due to their tunable physicochemical properties. However, organic semiconductor photocatalysts typically suffer from severe charge recombination due to high exciton binding energy. Herein, we found that aggregation of pyrene results in a red-shift of the light absorption from UV to visible light region. Importantly, the aggregation can induce dipole polarization by spontaneous structural symmetry breaking, thus significantly accelerating the separation and transfer of charge carriers. As a result, the pyrene aggregates display enhanced hydrogen photosynthesis activity. Furthermore, the noncovalent interactions allow rational design of physicochemical and electronic properties of pyrene aggregates, further strengthening the charge separation and photocatalytic activity of aggregates. The quantum yield of pyrene aggregates for hydrogen production highly reaches 20.77 % at 400 nm. Moreover, we have also observed pyrene analogues (1-hydroxypyrene, 1-nitropyrene and perylene) after aggregation all display large dipole moments induced by structural symmetry breaking and therefore accelerate the separation of charge carriers, confirming its general principle. This work highlights the achievement of using aggregation-induced structural symmetry breaking to enable the separation and transfer of charge carriers.

Pickering Emulsion Droplet-Derived Multicompartmentalized Microspheres for Innovative Applications.

Multicompartmentalized microspheres with multilevel and complex interior structures have great potential in practical applications due to their cell-like structures and microscale dimension. The Pickering emulsion droplet-confined synthesis route has been demonstrated to be a promising strategy for fabricating multicompartmentalized microspheres. Since Pickering emulsion-templated formation of hollow microspheres is an interface-directed process in which the growth of shells occurs at the oil/water interface and the confined space of Pickering emulsion droplet accommodates a variety of behaviors, such as surfactant-guided assembly growth, confined pyrolysis transformation, tritemplated growth, and bottom-up assembly, the independent and free regulation of the interface and internal structure of microspheres is allowed. In this Perspective, we highlight the recent progress in the synthesis of microparticles with tunable interior structures via the Pickering emulsion droplet-based approach. And we delve into the innovative applications of these multilevel-structured microparticles benefiting from their biomimetic multicompartments. Finally, some fundamental challenges and opportunities are identified for regulating the interior structure within microspheres and promoting practical applications by virtue of the Pickering emulsion droplet-confined synthesis pathway.

Spatial Localization of Two Enzymes at Pickering Emulsion Droplet Interfaces for Cascade Reactions.

Developing biocatalytic cascades in abiological conditions is of utmost significance, but such processes often suffer from low reaction efficiency because of incompatible reaction environments and suppressed intermediate transportation. Herein we report a new type of biocatalytic cascade by localizing two different enzymes separately in the outer and inner interfacial layers of Pickering emulsion droplets. This versatile approach enables the localization of two enzymes in their preferred reaction microenvironments and simultaneously in nanoscale proximity of each other. The thus-designed interfacial biocatalytic cascades show outstanding catalytic efficiency in alkene epoxidation and thioether oxidation with in situ generation of hydrogen peroxide under mild conditions, 6.9-13.6 times higher than the catalytic efficiency of the free enzymes in solution and their multi-enzymatic counterparts. The remarkable interfacial effect of Pickering droplets was found to be responsible for the significantly enhanced cascading efficiency.

Boosting Catalytic Selectivity through a Precise Spatial Control of Catalysts at Pickering Droplet Interfaces.

Exploration of new methodologies to tune catalytic selectivity is a long-sought goal in catalytic community. In this work, oil-water interfaces of Pickering emulsions are developed to effectively regulate catalytic selectivity of hydrogenation reactions, which was achieved via a precise control of the spatial distribution of metal nanoparticles at the droplet interfaces. It was found that Pd nanoparticles located in the inner interfacial layer of Pickering droplets exhibited a significantly enhanced selectivity for p-chloroaniline (up to 99.6%) in the hydrogenation of p-chloronitrobenzene in comparison to those in the outer interfacial layer (63.6%) in pure water (68.5%) or in pure organic solvents (46.8%). Experimental and theoretical investigations indicated that such a remarkable interfacial microregion-dependent catalytic selectivity was attributed to the microenvironments of the coexistence of water and organic solvent at the droplet interfaces, which could provide unique interfacial hydrogen-bonding interactions and solvation effects so as to alter the adsorption patterns of p-chloronitrobenzene and p-chloroaniline on the Pd nanoparticles, thereby avoiding the unwanted contact of C-Cl bonds with the metal surfaces. Our strategy of precise spatial control of catalysts at liquid-liquid interfaces and the unprecedented interfacial effect reported here not only provide new insights into the liquid-liquid interfacial reactions but also open an avenue to boost catalytic selectivity.

Programmatically Dynamic Microcompartmentation in Coacervate-in-Pickering Emulsion Protocell.

The desire for exploration of cellular functional mechanisms has substantially increased the rapid development of artificial cells. However, the construction of synthetic cells with high organizational complexity remains challenging due to the lack of facile approaches ensuring dynamic multi-compartments of cytoplasm and stability of membranes in protocells. Herein, a stable coacervate-in-Pickering emulsion protocell model comprising a membraneless coacervate phase formed by poly-l-lysine (PLys) and adenosine triphosphate (ATP) encapsulated in Pickering emulsion is put forward only through simple one-step emulsification. The dynamic distribution of intracellular components (coacervates in this protocell model) can be manipulated by changes in temperature or pH. This coacervate-in-Pickering emulsion protocell system exhibits repeatable cycle stability in response to external stimuli (at least 24 cycles for temperature and 3 cycles for pH). By encapsulating antagonistic enzymes into coacervates, glucose oxidase (GOx) and urease as an example, the control of local enzyme concentration is achieved by introducing glucose and urea to adjust the pH value in Pickering emulsion droplets. This hybrid protocell model with programmatically dynamic microcompartmentation and sufficient stability is expected to be further studied and applied in cellular biology, facilitating the development of lifelike systems with potential in practical applications.

A supraparticle-based biomimetic cascade catalyst for continuous flow reaction.

Robust millimeter-sized spherical particles with controlled compositions and microstructures hold promises of important practical applications especially in relation to continuous flow cascade catalysis. However, the efficient fabrication methods for producing such particles remain scare. Here, we demonstrate a liquid marble approach to fabricate robust mm-sized porous supraparticles (SPs) through the bottom-up assembly of silica nanoparticles in the presence of strength additive or surface interactions, without the need for the specific liquid-repellent surfaces used by the existing methods. As the proof of the concept, our method was exemplified by fabricating biomimetic cascade catalysts through assembly of two types of well-defined catalytically active nanoparticles. The obtained SP-based cascade catalysts work well in industrially preferred fixed-bed reactors, exhibiting excellent catalysis efficiency, controlled reaction kinetics, high enantioselectivity (99% ee) and outstanding stability (200~500 h) in the cascades of ketone hydrogenation-kinetic resolution and amine racemization-kinetic resolution. The excellent catalytic performances are attributed to the structural features, reconciling close proximity of different catalytic sites and their sufficient spatial isolation.

Co-compartmentalization of Enzymes and Cofactors within Pickering Emulsion Droplets for Continuous-Flow Catalysis.

Co-immobilization of enzymes and cofactors in a manner suitable for use in continuous flow catalysis remains a great challenge because of the difficulty in ensuring the free accessibility of immobilized enzymes and cofactors. Herein, we present a continuous flow catalysis system based on co-compartmentalization of enzymes and cofactors within Pickering emulsion droplets, enabling regeneration of cofactors within the droplets. As exemplified by enzyme-catalyzed ketone enantioselective reduction and enantioselective transamination, our systems exhibit long-term stability (300-400 h), outstanding total turnover number (TTN, 59204 mol mol-1 ) and several-fold enhancement in the enzyme catalytic efficiency (CEe ) in comparison to conventional biphasic reactions. As well as giving insight into the co-compartmentalization effects, our system will provide the opportunity to significantly advance continuous-flow biocatalysis towards the level of practical applications.

Tri-templating Synthesis of Multilevel Mesoporous Silica Microspheres with a Complex Interior Structure for Efficient CO2 Capture and Catalysis.

Multilevel porous architectures with microscopic shape control and tailor-made complex structures offer great potential for various innovative applications, but their elaborate design and synthesis have remained a scientific and technological challenge. Herein, we report a simple and effective tri-templating method, in which microscale Pickering droplets, nanoscale polystyrene colloids (PS), and molecular cetyltrimethylammonium chloride micelles are synchronously employed, for the fabrication of such micro-nanohierarchical mesoporous silica microspheres. In this protocol, Pickering droplet-directed interfacial sol-gel growth and its spatially confined surfactant assembly-directed sol-gel coating on PS suspensions are coupled together, enabling the successful formation of structured mesoporous silica that consists of numerous nanocompartments enclosed by a permeable shell. By varying the quantity of PS colloidal templates, rational regulation of the complex interior structure is achieved. Also, ascribed to the multilevel arrangement, this peculiar architecture not only shows desirable fast mass transport of external molecules but also possesses easy handling ability. After loading with tetraethylenepentamine or enzyme species, the yielded microspherical CO2 sorbents or immobilized biocatalysts, respectively, exhibit enhanced CO2 capture capacity and enzymatic catalysis efficiency. Notably, taking advantage of their microscopic characteristics, the immobilized biocatalysts could be ideally packed in a fixed-bed reactor for long-term continuous-flow enzymatic reactions. This tri-templating strategy provides a new synthetic route to access other multilevel microscopic materials with fascinating complex structures and paves a way to promote their practical applications.

Light-Induced Synthesis of Oxygen-Vacancy-Functionalized Ni(OH)2 Nanosheets for Highly Selective CO2 Reduction.

Solar-driven CO2 reduction into fuels and chemicals has gained increasing attention in recent years. In this study, oxygen-vacancies-functionalized Ni(OH)2 (OVs-Ni(OH)2 ) nanosheets are synthesized by a photochemical method to serve as a catalyst for CO2 reduction. Characterization reveals that COOH* is the key intermediate for CO2 -to-CO photoreduction. Experimental results and theoretical calculations confirm that OVs modification can greatly modulate the interaction strength between the OVs-Ni(OH)2 and CO2 , while lowering the energy barrier for COOH* formation, thereby preferentially facilitating CO2 reduction. As a result, the OVs-Ni(OH)2 catalyst exhibits outstanding activity and selectivity for CO2 -to-CO photoreduction with visible light. A CO evolution rate of 31.58 µmol h-1 (0.35 mg catalyst, 90228 µmol h-1 g-1 ) with a selectivity of 98 % over OVs-Ni(OH)2 was achieved, outperforming most analogous reported catalysts. Moreover, even under a low CO2 concentration of 0.04 % (representative of the CO2 concentration in air) and low reaction temperature (273 K, 0 °C), this catalyst can still trigger CO2 reduction. This work provides a new method to synthesize OVs-Ni(OH)2 catalysts for efficient CO2 reduction and establishes a relationship between the OVs and the catalytic activity, which may guide the design of highly selective CO2 reduction catalysts.

High Thermoelectric Performance Achieved in Sb-Doped GeTe by Manipulating Carrier Concentration and Nanoscale Twin Grains.

Lead-free and eco-friendly GeTe shows promising mid-temperature thermoelectric applications. However, a low Seebeck coefficient due to its intrinsically high hole concentration induced by Ge vacancies, and a relatively high thermal conductivity result in inferior thermoelectric performance in pristine GeTe. Extrinsic dopants such as Sb, Bi, and Y could play a crucial role in regulating the hole concentration of GeTe because of their different valence states as cations and high solubility in GeTe. Here we investigate the thermoelectric performance of GeTe upon Sb doping, and demonstrate a high maximum zT value up to 1.88 in Ge0.90Sb0.10Te as a result of the significant suppression in thermal conductivity while maintaining a high power factor. The maintained high power factor is due to the markable enhancement in the Seebeck coefficient, which could be attributed to the significant suppression of hole concentration and the valence band convergence upon Sb doping, while the low thermal conductivity stems from the suppression of electronic thermal conductivity due to the increase in electrical resistivity and the lowering of lattice thermal conductivity through strengthening the phonon scattering by lattice distortion, dislocations, and twin boundaries. The excellent thermoelectric performance of Ge0.90Sb0.10Te shows good reproducibility and thermal stability. This work confirms that Ge0.90Sb0.10Te is a superior thermoelectric material for practical application.

Pickering emulsion droplet-based biomimetic microreactors for continuous flow cascade reactions.

A continuous flow cascade of multi-step catalytic reactions is a cutting-edge concept to revolutionize stepwise catalytic synthesis yet is still challenging in practical applications. Herein, a method for practical one-pot cascade catalysis is developed by combining Pickering emulsions with continuous flow. Our method involves co-localization of different catalytically active sub-compartments within droplets of a Pickering emulsion yielding cell-like microreactors, which can be packed in a column reactor for continuous flow cascade catalysis. As exemplified by two chemo-enzymatic cascade reactions for the synthesis of chiral cyanohydrins and chiral ester, 5 - 420 fold enhancement in the catalysis efficiency and as high as 99% enantioselectivity were obtained even over a period of 80 - 240 h. The compartmentalization effect and enriching-reactant properties arising from the biomimetic microreactor are theoretically and experimentally identified as the key factors for boosting the catalysis efficiency and for regulating the kinetics of cascade catalysis.

Pickering-Droplet-Derived MOF Microreactors for Continuous-Flow Biocatalysis with Size Selectivity.

Enzymatic microarchitectures with spatially controlled reactivity, engineered molecular sieving ability, favorable interior environment, and industrial productivity show great potential in synthetic protocellular systems and practical biotechnology, but their construction remains a significant challenge. Here, we proposed a Pickering emulsion interface-directed synthesis method to fabricate such a microreactor, in which a robust and defect-free MOF layer was grown around silica emulsifier stabilized droplet surfaces. The compartmentalized interior droplets can provide a biomimetic microenvironment to host free enzymes, while the outer MOF layer secludes active species from the surroundings and endows the microreactor with size-selective permeability. Impressively, the thus-designed enzymatic microreactor exhibited excellent size selectivity and long-term stability, as demonstrated by a 1000 h continuous-flow reaction, while affording completely equal enantioselectivities to the free enzyme counterpart. Moreover, the catalytic efficiency of such enzymatic microreactors was conveniently regulated through engineering of the type or thickness of the outer MOF layer or interior environments for the enzymes, highlighting their superior customized specialties. This study provides new opportunities in designing MOF-based artificial cellular microreactors for practical applications.

Strategy of Extra Zr Doping on the Enhancement of Thermoelectric Performance for TiZrxNiSn Synthesized by a Modified Solid-State Reaction.

Half-Heusler alloys, which possess the advantages of high thermal stability, a large power factor, and good mechanical property, have been attracting increasing interest in mid-temperature thermoelectric applications. In this work, extra Zr-doped TiZrxNiSn samples were successfully prepared by a modified solid-state reaction followed by spark plasma sintering. It demonstrates that extra Zr doping could not only improve the power factor on account of an increase in the Seebeck coefficient but also suppress the lattice thermal conductivity originated from the strengthened phonon scattering by the superlattice nanodomains and the secondary nanoparticles. As a consequence, an increased power factor of 3.29 mW m-1 K-2 and a decreased lattice thermal conductivity of 1.74 W m-1 K-1 are achieved in TiZr0.015NiSn, leading to a peak ZT as high as 0.88 at 773 K and an average ZT value up to 0.62 in the temperature range of 373-773 K. This work gives guidance for optimizing the thermoelectric performance of TiNiSn-based alloys by modulating the microstructures on the secondary nanophases and superlattice nanodomains.

Metal-Nanoparticles-Loaded Ultrathin g-C3N4 Nanosheets at Liquid-Liquid Interfaces for Enhanced Biphasic Catalysis.

Exploiting new interface-active solid catalysts is crucial to construct efficient Pickering emulsion systems for biphasic catalysis. In this work, ultrathin g-C3N4 nanosheets (g-C3N4-NSs) were developed as a new solid emulsifier to directly position catalytic sites at oil-water interfaces for improving the reaction efficiency of a biphasic reaction. Exemplified by a metal-involved biphasic reaction of nitroarenes reduction, the developed Pd/g-C3N4-NSs catalyst with Pd nanoparticles loaded on the surface of g-C3N4-NSs exhibited excellent activity with a catalytic efficiency of 1220 h-1. Such activity was 4.2 and 17.9 times higher than those of Pd/g-C3N4-bulk and the ordinary Pd/C8-SiO2 catalyst, respectively. Also, in the biphasic oxidation reaction of alcohols, Pd/g-C3N4-NSs achieved a 2.3-fold activity enhancement. It was found by analyzing the solidified emulsion droplets that the Pd/g-C3N4-NSs catalyst was parallelly assembled at the oil-water interfaces. Because of the ultrathin thickness of g-C3N4-NSs, such a unique interfacial assembly behavior allowed precise positioning of Pd nanoparticles at the oil-water interfaces. As a result, the oil-soluble reactant could directly react with the water-soluble reactant at the oil-water interface hosting the Pd nanoparticles. Our elaborately designed reaction interface was believed to substantially avoid the diffusion barrier between oil-soluble and water-soluble reactants and then to significantly enhance the reactivity of biphasic reactions. This work highlights the importance of the interfacial location of catalytic sites in biphasic catalysis.

Dual metal nanoparticles within multicompartmentalized mesoporous organosilicas for efficient sequential hydrogenation.

Controlling localization of multiple metal nanoparticles on a single support is at the cutting edge of designing cascade catalysts, but is still a scientific and technological challenge because of the lack of nanostructured materials that can not only host metal nanoparticles in different sub-compartments but also enable efficient molecular transport between different metals. Herein we report a multicompartmentalized mesoporous organosilica with spatially separated sub-compartments that are connected by short nanochannels. Such a unique structure allows co-localization of Ru and Pd nanoparticles in a nanoscale proximal fashion. The so designed cascade catalyst exhibits an order of magnitude activity enhancement in the sequential hydrogenation of nitroarenes to cyclohexylamines compared with its mono/bi-metallic counterparts. Crucially, an interesting phenomenon of neighboring metal-assisted hydrogenation via hydrogen spillover is observed, contributing to the significant enhancement in catalytic efficiency. The multicompartmentalized architectures along with the revealed mechanism of accelerated hydrogenation provide vast opportunity for designing efficient cascade catalysts.