Biomimetic Microreactors Derived From Pickering Droplet For Biocatalysis Applications

Inspired by cells,biomimetic microreactors with specific structures and functions are constructed by simulating cells,which simplify complex cellular structures and functions,so as to achieve the purpose of regulating material transport,enhancing molecular mass transfer and improving catalytic reaction efficiency.It has been a hot topic in the field of catalysis and materials.Metal-organic frameworks（MOFs）have abundant microporous structures,which can adjust pore size at the molecular level,and have a good application prospect in the regulation of permeability.Ascribing to their unique properties,including a rigid and adjustable framework structure,the facile and biocompatible assembly process,such a platform exhibits powerful capability to better mimic the cellular systems.Therefore,the preparation of MOF biomicroreactor have been received extensive attention of scientists.However,the traditional adsorption or encapsulation synthesis methods often suffer from low loading efficiency,incompact confinement or mismatching of pore size,leading to the decrease of catalytic activity.Moreover,these reported microreactors lack the regulation of the internal microenvironment,and the internal structure is limited to single compartment structures,thus resulting in lack of control over the spatial organization of different catalytic actives.More urgently,the present MOF-microreactors are far less implemented for biocatalytic continuous flow processes that are crucial for practical applications.Therefore,the exploration and design of MOF microreactors with selective permeability together with tunable microenvironments,and the simultaneous integration of multiple catalytic active centers,especially incompatible functional components such as chemical catalysts and enzymes,into an integrated system in an efficient and intelligent manner,is a dream goal and a major challenge for synthetic biologists.To solve these problems,we present a simple and robust Pickering emulsion-based interfacial growth method to directly fabricate MOF biomimetic microreactor.In this proposed method,Si O₂-stabilized microscale Pickering droplets are utilized as an‘architecture directing agent’for the interfacial deposition of a MOF layer.During this process,the metal ions and organic linkers are available in a form soluble respectively in the dispersed and continuous phases.As these two precursors encounter each other from opposite sides of the oil-water interface,they react based on metal-coordination interactions,forming a shaped MOF exoskeleton.At the same time,the construction of biomimetic MOF-microreactor can be realized by in situ loading a spot of catalytic species during synthesis.The applications of MOF biomicroreactor in continuous flow biocatalysis and chemo-enzymatic cascade reactions were investigated.The main results are as follows:（1）The fabrication of enzymatic MOF-microreactor was based on water-in-oil Pickering emulsion stabilized by hydrophobic Si O₂ nanoparticles.The compartmentalized interior droplets can provide biomimetic microenvironment to host free enzymes,while the outer MOF layer secludes active species from surroundings and endows the microreactor with size-selective permeability.We then studied the impacts of the concentration of MOF precursors on the thickness of formed MOF layer by tuning initial metal salt dosage while keeping organic linker always excessive.Impressively,the thus-designed enzymatic microreactor exhibited excellent size-selectivity in enzymatic hydrolysis kinetic resolution reactions.Moreover,the catalytic efficiency of such enzymatic microreactors was conveniently regulated through rational engineering the type or thickness of outer MOF layer,highlighting its superior customized specialities.Furthermore,such an interfacial growth method is also feasible to other types of MOF microreactors,indicating the precise control and feasibility of our interfacial growth approach.This study provides new opportunities in designing MOF-based artificial cellular microreactors for practical applications.（2）We also tried to improve the activity by modulating interior environment of microreactors through embedding biocompatible polyethylene glycol（PEG）within the confined space of the developed biomimetic MOF microreactor.Encouragingly,compared with pure MOF-microreactor and free enzyme,impressive enhancement effects for transesterification reaction were observed after PEG modulation.The microreactors were packed in the fixed-bed reactor directly.The results show s full conversion after flow,and these high ee values could be well maintained over a period of 1000 h,suggesting the long-term stability.Moreover,the MOF microreactor showed excellent catalytic activity and stability in the continuous flow reaction for the synthesis of thioester,further highlighting the extensive applicability.Impressively,the MOF microreactors can not only create the alternative interior microenvironment to improve the catalytic activity but also afford excellent durability of the catalysis efficiency and enantioselectivity over a long period of1000 h in continuous flow catalysis,which provides a new way for the efficient synthesis of chiral compounds and other fine chemicals.（3）To move closer to a truly applicable cell-like microsystem in terms of structure and function,we developed a Pickering double emulsion-directed interfacial synthesis method for efficiently constructing multi-compartmental crystalline MOF microreactors,which can be utilized as a powerful platform for integrating orthogonal catalytic species for multistep chemo-enzymatic cascade reactions.A series of multi-compartmental MOF microreactors with unprecedented well-tailored inner architectures have been obtained by tuning the inner droplet size and concentration in multiple emulsions.Benefiting from the complex structure of Pickering double emulsion,we integrated hydrophilic enzymes and hydrophobic molecular catalysts into the tailored synthetic compartments in a bottom-up manner,forming biomimetic functional systems for operating concurrent chemo-enzymatic reactions in a one-pot fashion.The power of our designed multi-compartmental microreactor was definitely demonstrated by the significantly enhanced catalytic efficiency.The restrained mutual inactivation of enzyme and molecular catalyst within the compartments,together with the promoted substrate channelling effects,might be attributed to the enhanced cascade efficiency.We also extended this approach to the synthesis of a series of structurally diverse MOFs,in which individual microspheres with multi-compartmental interior structures are achieved.We believe that this rational synthesis approach offers new opportunities for the design and development of complex multi-compartmental cellular models with spatially organized functions,taking a step forward the artificial cells that is closer to living systems.