Preparation And Application Of Microcapsules Using Pickering Emulsion As Template

Pickering emulsion is stabilized by solid particles,which has many advantages such as high safety,simple preparation,environmentally friendlygood,excellent biocompatibility,stimulation response and low toxicity.The water phase inside the emulsion can provide a suitable microenvironment for enzymes to maximize the activity of enzymes.The enzyme can be encapsulated in Pickering emulsion only through one step emulsification process,which catalytic reactions can be realized.At present,the continuous flow catalytic reaction for up to 4000 hours has been realized by encapsulating lipase in Pickering droplets.However,chemical reactions often require a certain temperature,pressure and polar solvents,which Pickering emulsion can not meet these conditions,and often appears deformation,convergence and even damage.The preparation of microcapsules by crosslinking the interface of Pickering emulsion is to reinforce the surface of Pickering emulsion,so as to enhance the Pickering emulsion stability,mechanical strength,resistance to polar solvents and high substrate concentration.By coating the active substance in the shell,it can prevent the adverse or toxic side effects of the external environment on the enzyme and cofactor activities,which has a wider application prospect.In this paper,Pickering emulsion microcapsules were prepared by chemical crosslinking method,which the W/O Pickering emulsion as template.Four kinds of crosslinker including polyethoxysiloxane（PEOS）,tetraethyl orthosilicate（TEOS）,tetramethoxysilane（TMOS）and trimethoxymethylsilane（MTMS）were added to the emulsion,and crosslinking was performed at 30 ^oC for 24 h.It was observed that PEOS and TEOS could not form shell in a short time.Under neutral conditions,the crosslinking process is slow,which is not conducive to the encapsulation of acid-base sensitive active substances.TMOS is a great crosslinker which hydrolyzes easily to form shells under mild conditions,but the hydrolyzed TMOS is more inclined to enter the water phase to form solid spheres.Using MTMS as the crosslinker,the Pickering emulsion microcapsules formed were slightly crumpled and maintained the hollow structure.After drying and in ethanol,the capsules could maintain the complete structure and no rupture occurred.Therefore,MTMS was selected as the crosslinker in this study.The structure and properties of Pickering emulsion microcapsules prepared by MTMS were adjusted and analyzed,such as the size,shell thickness,permeability,pressure resistance,polar solvent resistance and mechanical strength of microcapsules.The experimental results showed that the prepared microcapsules had good solvent tolerance,adjustable permeability,size and shell thickness.It is a good carrier of biocatalyst enzyme.In order to explore the catalytic application of the prepared microcapsules,we encapsulated enzymes and cofactors in Pickering emulsion to prepare microcapsules containing KRED-NADP⁺.Pickering emulsion microcapsules can confine the water-soluble cofactors inside and make the enzymes strongly adsorbed on the surface.This new model of co-immobilization allows cofactors to access the enzyme freely and regenerate completely within the droplet.They were loaded into a fixed-bed reactor for continuous flow reaction to prepare chiral alcohols.The gap between droplets allowed the oil phase containing the substrate to diffuse to the capsule interface and catalyze the reaction at the interface.After continuous operation for 450 h,the conversion remained above 90%,the enantioselectivity of chiral alcohols was>99%,the space time yield was up to 420 g L^-1 d^-1,and the total conversion number of cofactors was up to 350,000 mol mol^-1.Then,the effects of enzyme concentration,capsule size,shell thickness and other factors on the catalytic performance were explored.In addition,the microcapsules were applied to high substrate concentration and polar solvent to verify the advantages of crosslinking.Finally,the system was applied to different reactants.The results showed that the reaction system has good universality and high catalytic efficiency,indicating that KRED-NADP⁺microcapsules have broad prospects in industrial application.