Preparation of solid,hollow,hole-shell and asymmetric silica microspheres by microfluidic-assisted solvent extractionprocessMinhua Ju,Xiaobo Ji,Chongqing Wang,Ruwei Shen,Lixiong Zhang ⇑State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemistry and Chemical Engineering,Nanjing University of Technology,Nanjing 210009,PR Chinastructures are prepared microfluidic device.a r t i c l e i n f o Article history:Received 2January 2014Received in revised form 25March 2014Accepted 2April 2014Available online 13April 2014Keywords:MicrofluidicSilica microspheres Solvent extraction Hollow spheres Interfaciala b s t r a c tPresent work demonstrated the facile preparation of silica microspheres with various structures (solid,hollow,hollow with a hole and filbert-like solid).These were prepared by first forming monodisperse silica sol droplets in a simple microfluidic device,followed by extracting the solvent from the droplets in an extractant or at the interface between the extractant and liquid paraffin at different conditions.The effect of different extractants and extracting temperature was investigated.The products were characterized by optical microscope and scanning electron microscope.Extraction in fatty acid methyl ester (FAME)at room temperature led to formation of solid silica microspheres,while extraction at the interface between FAME and liquid paraffin at 60°C resulted in formation of hollow silica e of mixture of castor oil (CO)and dimethyl carbonate (DMC)as extractant resulted in formation of hollow silica microspheres with a hole on the surface,whereas increase in the DMC content in extracting medium led to formation of filbert-like silica solid microspheres.Change in size of cavity and hole was studied by changing the extracting temperature.The formation process and mechanism of these silica microspheres are proposed based on the diffusion rate.The relationship between the size of the microspheres and the state of the droplet at the interface is correlated.Ó2014Elsevier B.V.All rights reserved.1.IntroductionInorganic microspheres have attracted much attention due to their wide applicationsin many areas,such as catalyst supports,adsorbents,sensors,drug carriers,and photoelectric materials [1–5].According to their internal structure,they can be cataloged as solid and hollow ones.The inorganic solid microspheres can be synthesized by the Stöber method [6],spray drying [7],hydrothermal synthesis [8],emulsion polymerization [9]and so on.The resulting products always exhibit good sphericity./10.1016/j.cej.2014.04.0081385-8947/Ó2014Elsevier B.V.All rights reserved.⇑Corresponding author.Tel.:+862583172265;fax:+862583172261.E-mail address:lixiongzhang@ (L.Zhang).However,asymmetricalfilbert-like inorganic solid microspheres have not been reported yet.On the other hand,inorganic hollow microspheres have recently been paid much more attentions than their solid counter-parts because of their lower density,larger specific surface area and higher capacity.They are most commonly prepared by the template methods[10–12],although other template-free methods, such as Ostwald ripening process[13],self-assembly[14],emul-sion polymerization[15],and chemical selective etching[16],are also reported.These methods are either dependent on the uniform sacrificial templates,or careful controlling of synthesis conditions, or use of multisteps.Recently,the microfluidic technology has shown remarkable capabilities for preparing monodisperse parti-cles[17–21],inorganic TiO2,carbon,SiO2hollow microspheres based on interfacial polymerization[22–25],chitosan and chito-san/silica hybrid microspheres[26,27],Fe3O4@ZIF-8magnetic core–shell microspheres[28],however,it is still a challenge to eas-ily tune the internal structure of the microspheres between hollow and solid.Another type of the inorganic hollow microspheres,i.e.,the one with a hole in the surface of the microsphere,is recently developed.This type of microspheres exhibits‘‘lock–key’’effect and are more useful for loading objects such as cells and confining a microreaction[29].Up to now,SiO2and TiO2are the mainly inorganic components which exhibit this structure,although it has been seen on some organic components,including poly(o-methoxyaniline)[30],polystyrene(PS)[31–33],poly (methyl methacrylate)(PMMA)[32],poly-(L-lactide)(PLLA)[32], poly(acrylamide-ethylene glycol dimethacrylate)[34],polymethylsilsesquioxane(PMSQ)[35],[36],PS/polydivinylbenzene(PDVB)[37],thylolpropane triacrylate)(PETPTA)[29],andrylamide)[38].Mainly,particle orare used to prepare these organic hollowin the shell[37].Similarly,the SiO2hollowhole in the shell can be also prepared by themethods[39–42].On the other hand,the TiO2with a hole are prepared by hydrothermal synthesisthese methods are difficult to control the hole sizesize distribution of the microspheres is quitepreparation of inorganic microspheres withspherical morphology is still a challenge.Herein,we report a versatile method tospheres with solid,hollow,hollow with a hole andsolid structures in a simple microfluidic device.process is based on solvent extraction in a silicathat is suspended on the interface between thethe non-extractant.Although the dropletmethod has been applied to prepare solidTiO2[44],SiO2[45],ordered mesoporous silica[46–48]and hollow colloidal crystals microspheresducted by immersing the droplet containing thein the extractant,leading to equal solventtion.To the best of our knowledge,this is thefirstaration of materials based on interfacialbetween extractant and non-extractant.Therates of the two parts of the microdroplets resultof hollow structure,which can be further tunedtures with a hole in the shell.When theenough,filbert-like microspheres can be obtained.ing the operating mode and the composition ofsilica microspheres with several kinds of internalmorphology can be produced.Thesepotentially used as catalyst supports,for cells[29].2.Materials and methods2.1.MaterialsTetraethoxysilane(TEOS),Span80,and liquid paraffin were all purchased from Sinopharm Chemical Reagent Co.,Ltd.Absolute ethanol and methylene blue were acquired from Wuxi Yasheng Chemical Co.,Ltd.(Wuxi,China)and Shanghai SSS Reagent Co., Ltd.(Shanghai,China),respectively.Fatty acid methyl esters (FAMEs)were prepared in lab by transesterification of cottonseed oil with methanol in a microreactor with the assistance of catalyst KOH[50].Castor oil was bought from Wuxi Zhanwang Chemical Co.,Ltd.(Wuxi,China).2.2.The microfluidic deviceA microneedle and a PTFE tube were vertically arranged and sandwiched between two PMMA plates.The needle with110l m i.d.and2cm length was used for transferring the dispersed phase, and the PTFE tube with500l m i.d.and4cm length was used for collecting the microdroplets.The continuous-phase was intro-duced from an inlet of the PMMA plate perpendicular to the needle (Fig.1).The solvent-assisted thermal press technique was used to bond the two PMMA plates.Prior to use,the bonded microchip was cleaned by distilled water.Schematic illustration of the microfluidic device and the preparationM.Ju et al./Chemical Engineering Journal250(2014)112–1181132.4.Preparation of silica microspheresMonodisperse silica sol microdropletsthe above mentioned microfluidic device uous oil phase containing liquid paraffin a dispersed phase containing the silica sol glass capillaries through two syringe 5mL h À1and 0.5mL h À1,respectively.were collected in a 50mL polypropylene or other certain extraction solvent.During lets were solidified into the precursor and water in the droplets were extracted.ing was positioned at different depths in the structure of the obtained shown in Fig.1.Finally,the precursor formed to silica microspheres by 5h.The air flow rate was 12mL min À11°C min À1.Table 1lists the preparation of silica microspheres.2.5.CharacterizationMicrodroplets and precursor microspheres were observed by optical microscope (Olympus CX31).Scanning electron microscope (SEM,Philips Quanta 200and Hitachi-S4800)analyses were used to observe the morphology of silica microspheres.The samples were coated with Au.The typical acceleration voltage was 5–15kV.3.Results and discussionThe preparation of silica microspheres through the solvent extraction is based on shrinkage/hardening of the silica sol micro-droplets by a sol–gel process.Probably,difference in diffusion rate of the solvent out of the microdroplets and silica nanoparticles in the microdroplets determines the formation of solid or hollow microspheres.Therefore,efforts were made to adjust the solvent diffusion rate by choosing suitable extraction solvent and opera-tion mode.In the beginning,FAME was used as the extraction solvent.The silica sol microdroplets containing water,silica nanoparticles and ethanol from hydrolysis of TEOS were prepared in the microfluidic device.The optical microscope observation (Fig.2a)shows their quite uniform particle size of ca.320l m.The coefficient variation (CV)value was calculated to be 3%,indicating the formation of monodisperse silica sol microdroplets.The droplets were collected by submerging the outlet tubing in the FAME solution.The end of the tubing was about 4cm below the liquid surface.Under such a circumstance,the microdroplets did not float up to the surface.During the collecting process,the paraffin floated up quickly and dissolved in FAME completely at the same time,while the droplets floated up for about 1.5cm first and then settled down to the bot-tom of the beaker.This phenomenon can be ascribed to the lowerdensity of liquid paraffin (0.84g cm À3)and to the increased density of the microdroplets resulting from diffusion of the inside solvent (ethanol)to the extraction solvent.The obtained precursor micro-spheres (sample 1)are ca.100l m in particle size (Fig.2b).The size of these microspheres is much smaller than that of original micro-droplets,resulting from shrinkage of the microdroplets.Consistent transparency under optical microscope indicates homogenous internal structure.After calcination,the precursor microspheres transformed to the silica microspheres and further shrank to ca.90l m in diameter (Fig.2c).The SEM picture of a broken silica microsphere shows solid homogenous internal structure of the microspheres (Fig.2d),consistent with the observation by the opti-cal microscope.When the end of the outlet tubing was positioned 2cm below the surface of the FAME solvent,the liquid paraffin floated up and reached to the surface of the FAME.The interface between paraffin and FAME was not very distinct because of the dissolution of liquid paraffin and FAME.A few microdroplets floated up to the surface of FAME along with paraffin.Close observation revealed that these floating droplets stayed at the interface between FAME and paraffin,with part immersed in FAME and the other in paraffin.After about 1s,the droplets began to settle down to the bottom of the beaker.The optical microscope picture of the precur-sor microspheres (sample 2,Fig.3a)shows spherical morphology with ca.106l m in diameter.While the particle size of the calcined silica microspheres are ca.95l m (Fig.3b).Most of the precursor microspheres transmit light uniformly,while shadows are observed on several precursor microspheres obviously (arrows in Fig.3a).The broken silica microsphere in Fig.3c reveals that there are some cavities inside the silica microspheres.Thus,the shadows observed by the optical microscope indicate the internal hollowTable 1Properties and synthesis parameters of silica microspheres from the microdroplets with ca.320l m in diameter.Sample T (°C)Extraction solvent Diameter of the precursor microspheres (l m)Diameter of silica microspheres (l m)b Diameter of the cavity (l m)120FAME 9890–220FAME 10695–320FAME 11011060460FAME 135********Castor oil138133656a 60Castor oil +20wt%DMC 172165957a 60Castor oil +35wt%DMC 187185125860Castoroil +50wt%DMC118108–a Hollow silica microspheres with a hole on the shell.bAll the silica microspheres are shrunk by 13±3%in diameter compared with the precursor microspheres.Fig.2.The optical microscope pictures (a,b)and SEM pictures (c,d)of microdro-plets (a),precursor microspheres (b)and silica microspheres (c,d)of sample 1.114structure in the pared with sample1,the silica microspheres of sample2are a little bit larger,probably resulting from their loose internal structure.When the end of the outlet tubing was positioned right at the surface of FAME,almost all the droplets could stay at the interface between the FAME and liquid paraffin before they settled down to the bottom of the beaker after about1s.The collected precursor microspheres and the corresponding silica microspheres(sample 3)are quite uniform in size,with mean sizes of ca.120and 110l m,respectively(Fig.4).The shadow on each precursor micro-sphere can also be observed(Fig.4a),implying the internal hollow structure.The corresponding silica microspheres exhibit integral spherical structure(Fig.4b).However,one big cavity(ca.60l m) inside the microsphere can be seen from the SEM picture of a bro-ken silica microsphere(Fig.4c).Thus,the observations by SEM andoptical microscope are consistent.The cavity is not in the center of the microsphere,and the thinnest shell of the silica sphere is about l m in thickness.The sizes of both the precursor microspheres and the shadows for sample3are larger than those of sample2. These results indicated that the internal structure of the micro-sphere can be tuned by adjusting the diffusion rate of the solvent inside the droplets.To further increase the diffusion rate of solvent,the tempera-ture of FAME was raised from previous room temperature to 60°C.The end of the outlet tubing was still positioned right at the surface of FAME.On this occasion,these microdroplets stayed at the interface between the FAME and liquid paraffin for less than one second before they settled down quickly.The obtained precursor microspheres(sample4)exhibit a much bigger shadow than sample3on each microsphere,as observed by the optical microscope(Fig.5a).This indicates the enlargement of the cavities. The SEM picture of the corresponding silica microspheres(Fig.5b) still shows integral spherical structure with a uniform particle size distribution.The diameters of the precursor and silica micro-spheres are about135and123l m,respectively.Some of the silica microspheres possess a hole on the surface,probably resulting from damage of shell by calcination.The SEM picture of a broken silica microsphere(Fig.5c)clearly shows a hollow internal struc-ture with small wall thickness.The diameter of the cavity is ca. 90l m,larger than that in the silica microspheres of sample3.Previous work have demonstrated that mass diffusion of the solvent encapsulated in a polymeric shell could open up a hole on the shell,leading to formation of a hollow polymer microsphere with a single hole[26–36].By keeping this in mind,we further raised the FAME temperatures to80and90°C,and found that the formed microdroplets settled down quickly.This phenomenon may be attributed to the enhancement of the diffusion rate of eth-anol because of boiling of ethanol at this temperature.Thus the residence time of microdroplets on the interface between liquid paraffin and FAME was shortened,and the expected hole on the shell did not appear yet.To keep the microspheres on the interface for a longer time,we substituted castor oil for FAME as the density of the castor oil is higher and ethanol is easily solved in castor oil.Furthermore,it can form a distinct interfacial surface with liquid paraffin because of their immiscibility.The temperature of the castor oil was also kept at60°C.Under such a circumstance,the droplets could stay for about7s before they settled down.The optical microscope picture of the collected precursor microspheres(sample5, Fig.6a)shows a spherical-like morphology with a dark-spot(ca.Fig.3.The optical microscope picture(a)and SEM pictures(b,c)of precursor microspheres(a)and silica microspheres(b,c)of sample2.Fig.4.The optical microscope picture(a)and SEM pictures(b,c)of precursor microspheres(a)and silica microspheres(b,c)of sample3.Fig.5.The optical microscope picture(a)and SEM picture(b,c,d)of precursor microspheres(a)and silica microspheres(b,c,d)of sample4.Journal250(2014)112–118115(Fig.6d).The diameter of the cavity is ca.65l m.The thickness of the thinnest shell is ca.100nm,much thinner than that of sample 4.These results suggest that formation of the hollow microsphereswith a hole is quite possible when the droplets can stay longer at the interface before they settle down.This means the solvent inside the droplets can be extracted in a fast and complete way.To further enhance the solvent diffusion rate,we thus added certain amount of dimethyl carbonate(DMC)into the castor oil because of its higher density(1.069g cmÀ3)and higher solubility for ethanol and water[51].Therefore,addition of DMC in castor oil can prolong the residence time of microdroplets at the interface and increase the diffusion rate for ethanol and water.The droplets stayed at the interface between liquid paraffin and the extractant for ca.7s before they settled down.Fig.7a and b shows,respec-tively,the optical microscope picture of collected precursor micro-spheres and the SEM picture of the silica microspheres obtained by adding20wt%DMC in castor oil(sample6).Both of them show spherical morphology,with diameters of188and165l m,respec-tively.The photopermeability of this sample is different from those of other samples,exhibiting a shadow on the shell and a circle on the sphere.There is a hole(ca.35l m)on the surface of the silica microspheres(Fig.7c).Silica microspheres also exhibit a hollow structure,with a big cavity of ca.95l m(Fig.7d).Fig.8a and c shows,respectively,the optical microscope picture of the precursor microspheres and SEM picture of the correspond-ing silica microspheres obtained with35wt%DMC in castor oil (sample7).Similar photopermeability to that of sample6was observed.The diameters of the precursor and silica microspheres are200and185l m,respectively.The diameter of the hole on the microsphere is ca.80l m,a little bit larger than that on sample 6.The size of the inside cavity is ca.125l m(Fig.8e).Thus,by add-ing certain amount of DMC in castor oil,hollow silica microspheres with a hole on the surface can be prepared.In addition,the diam-eters of the hole and the cavity can also be easily adjusted by con-trolling the amounts of DMC.When50wt%DMC was added into castor oil,the droplets could stay at the interface between paraffin and the extraction solvent for a longer time and did not settle down until the precursor microspheres were obtained.The optical microscope picture of the precursor microspheres(sample8, Fig.8b)shows uniform photopermeability,suggesting solid inter-nal structure.However,silica microspheres with asymmetric spherical shape,filbert-like morphology,were formed(Fig.8d). They are comprised of an integrated hemisphere and a1/5sphere with a distinct boundary between them.The diameters of the hemisphere for the precursor and silica microspheres are120 and110l m,respectively,and those of the1/5sphere for the pre-cursor and silica microspheres are160and140l m,respectively.Fig.6.The optical microscope picture(a)and SEM pictures(b,c,d)of precursor microspheres(a)and silica microspheres(b,c,d)of sample5.Fig.7.The optical microscope picture(a)and SEM pictures(b,c,d)of precursor microspheres(a)and silica microspheres(b,c,d)of sample6.Fig.8.The optical microscope picture(a,b)and SEM pictures(c,d,e,f)of precursor microspheres(a,b)and silica microspheres(c,d,e,f)of sample7(a,c,e)and8(b, Journal250(2014)112–118can be proposed,as shown in Fig.9.Once the droplet is collected in the extractant,the extracting process starts with diffusion of etha-nol in droplets to the extractant,leading to shrinkage of the droplet surface.At the same time,the silica nanoparticles in the droplets migrate to the surface of the droplet driven by the diffusion of eth-anol.Also,they exchange for volume phase and epiphase[52], leading to the inward migration of the nanoparticles.If the diffu-sion rate of ethanol is fast enough,this inward migration of the precursor nanoparticles can be ignored.Thus,the nanoparticles are considered to move to the surface of the droplet.In the case that the droplets are totally immersed in FAME,ethanol in droplets diffuses to FAME from all directions uniformly,resulting in the uni-form shrinkage of the droplets and formation of silica solid micro-spheres.Consequently,the diameter of the resultant precursor microsphere is much less than that of the droplets(row1,Fig.9). This process is the same as those for the preparation of TiO2and SiO2solid microspheres[40,41].When the droplet is collected on the surface of FAME,the lower part of the droplet is immersed in FAME and the upper part is covered by liquid paraffin.On this occasion,ethanol in the lower part diffuses to FAME from mainly the lower part,leading to the accumulation of the precursor nano-particles at the lower part of the droplet.This will decrease the par-ticle concentration in the upper part,resulting in heterogeneous concentration of the silica nanoparticles in the droplet.Conse-quently,microspheres with inconsonant wall thickness and the eccentric hollow structure are formed after the droplet settles down in FAME.Raising the temperature of the extraction solvent can accelerate the diffusion rate of ethanol and decreases the den-sity of the extraction solvent,leading to the increase in the area of the lower part immersed in FAME and the decrease in the silica nanoparticle concentration in the upper area covered by liquid par-affin.This results in the formation of thin wall in the upper part and large cavity in the microspheres.Additionally,the faster diffu-sion of ethanol makes the precursor nanoparticles easier to gelati-nize,resulting in the production of the larger silica microspheres (row2,Fig.9).This can explain the increase in the diameters of further reduced,facilitating the formation of silica microspheres with very thin wall.Under certain conditions,the formed wall in the liquid paraffin-covered upper area is so thin that it cannot stand itself and collapses to form a hole on the surface(row3, Fig.9).In fact,we observed the inward bending of the wall around the hole in some silica microspheres of sample6(Fig.S1in the sup-porting information).When a large amount of DMC is added into castor oil,the droplet can stay at the interface between the extract-ant and paraffin for a long time(more than7s).Ethanol is extracted in a fast and complete way,leading to fast shrinkage of the droplet which results in the formation of a small microsphere with solid internal.Even under this circumstance,the diffusing dif-ference between the liquid paraffin-covered upper part and the lower part immersed in the extraction solvent is still obvious,lead-ing to the formation of asymmetrical microspheres(row4,Fig.9).Based on the above proposed mechanism,we correlated the diameters of the silica microspheres and the cavity with thefloat-ing state of the droplet on the interface based on force balance (supporting information).We chose samples5,6and7for the cor-relation because the preparation conditions for these three sam-ples are almost the same and cavity is formed in these samples. We thus obtained linear relationship between the diameter of the precursor microspheres(d p)and the diameter of the cavity (d c)with the height of the silica sol microdroplet immersed in the extractant(h)with high correlation coefficient as follows.D p¼À1:65hþ257d c¼À1:84hþ1974.ConclusionSilica microspheres with various structures(solid,hollow, hollow with a hole andfilbert-like solid)were prepared based on solvent extraction of silica sol microdroplets formed in a simple microfluidic device.The type of the extractant and the extracting temperature determined the structure and morphology of the obtained silica microspheres.By immersing the droplets in the extractant with low density,solid silica microspheres were formed. Keeping the droplets on the interface between this extractant and liquid paraffin at a high temperature led to the formation of hollow silica microspheres.This process could be further used to prepare hollow silica microspheres with a hole on the surface orfilbert-like silica solid microspheres by substituting the extractant with high density and high ethanol and water solubility for the one used before.The above formation process was related to the diffusion rate and amount of the solvent in the droplets to the extractant, and can be controlled through adjusting the time for droplets stayed at the interface between the extractant and liquid paraffin. The size of the microspheres has linear relation with the height of the droplet immersed in the extractant.This preparation method is versatile to adjust the morphology and the internal structure and the particle size of the silica microspheres by changing the extrac-tion condition and the channel size of the microfluidic device.It can be easily scaled up by numbering up the microfluidic chips and is expected to prepare other kinds of microspheres(such as TiO2)with similar inner structure and morphology. AcknowledgementsThis work is supported by Natural Science Key Project of the Jiangsu Higher Education Institutions(12KJA530002),the Priority Academic Program Development of Jiangsu Higher Education Institutions,and the Research and Innovation Program for College Postgraduates of Jiangsu Province(No.CXZZ11_0356).Fig.9.The proposed formation mechanism of different structures of the silicamicrospheres.M.Ju et al./Chemical Engineering Journal250(2014)112–118117Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at /10.1016/j.cej.2014.04.008. References[1]W.T.Hu,B.C.Liu,Q.Wang,Y.Liu,Y.X.Liu,P.Jing,S.L.Yu,L.X.Liu,J.Zhang,Amagnetic double-shell microsphere as a highly efficient reusable catalyst for catalytic applications,mun.49(2013)7596–7598.[2]Z.C.Zhang,L.M.Zhang,L.Chen,L.G.Chen,Q.H.Wan,Synthesis of novel porousmagnetic silica microspheres as adsorbents for isolation of genomic DNA, Biotechnol.Prog.22(2006)514–518.[3]P.Y.Cai,W.Bai,L.C.Zhang,H.J.Song,Y.Y.Su,Y.Lv,Hierarchical hollowmicrosphere andflower-like indium oxide:controllable synthesis and application as H2S cataluminescence sensing materials,Mater.Res.Bull.47 (2012)2212–2218.[4]W.Wei,G.H.Ma,G.Hu,D.Yu,T.Mcleish,Z.G.Su,Z.Y.Shen,Preparation ofhierarchical hollow CaCO3particles and the application as anticancer drug carrier,J.Am.Chem.Soc.130(2008)15808–15810.[5]L.Chevallier,A.Bauer,S.Cavaliere,R.Hui,J.Rozière,D.J.Jones,Mesoporousnanostructured Nb-doped titanium dioxide microsphere catalyst supports for PEM fuel cell electrodes,ACS Appl.Mater.Interfaces4(2012)1752–1759. [6]W.Stöber,A.Fink,E.J.Bohn,Controlled growth of monodisperse silica spheresin the micron size range,J.Colloid Interface Sci.26(1968)62–69.[7]J.Bahadur,D.Sen,S.Mazumder,P.U.Sastry,B.Paul,H.Bhatt,S.G.Singh,One-step fabrication of thermally stable TiO2/SiO2nanocomposite microspheres by evaporation-induced self-assembly,Langmuir28(2012)11343–11353.[8]M.M.Titirici, A.Thomas,M.Antonietti,Replication and coating of silicatemplates by hydrothermal carbonization,Adv.Funct.Mater.17(2007)1010–1018.[9]M.Peer, A.Qajar,R.Rajagopalan,H.C.Foley,On the effects of emulsionpolymerization of furfuryl alcohol on the formation of carbon spheres and other structures derived by pyrolysis of polyfurfuryl alcohol,Carbon51(2013) 85–93.[10]I.Yamaguchi,M.Watanabe,T.Shinagawa,M.Chigane,M.Inaba,A.Tasaka,M.Izaki,Preparation of core/shell and hollow nanostructures of cerium oxide by electrodeposition on a polystyrene sphere template,ACS Appl.Mater.Interfaces5(2009)1070–1075.[11]J.M.Gu,S.H.Li,M.L.Ju,E.B.Wang,In situ carbon template-based strategy tofabricate ferrite hollow spheres and their magnetic property,J.Cryst.Growth 320(2011)46–51.[12]J.U.Park,H.J.Lee,W.Cho,C.Jo,M.Oh,Facile synthetic route for thickness andcomposition tunable hollow metal oxide spheres from silica-templated coordination polymers,Adv.Mater.28(2011)3161–3164.[13]H.G.Yang,H.C.Zeng,Preparation of hollow anatase TiO2nanospheres viaOstwald ripening,J.Phys.Chem.B11(2004)3492–3495.[14]M.Mo,J.C.Yu,L.Zhang,S.-K.A.Li,Self-assembly of ZnO nanorods andnanosheets into hollow microhemispheres and microspheres,Adv.Mater.6 (2005)756–760.[15]Z.G.Teng,Y.D.Han,J.Li,F.Yan,W.S.Yang,Preparation of hollow mesoporoussilica spheres by a sol–gel/emulsion approach,Micropor.Mesopor.Mater.127 (2010)67–72.[16]S.Shi,M.Wang, C.Chen, F.Lu,X.Zheng,J.Gao,J.Xu,Preparation ofhydrophobic hollow silica nanospheres with porous shells and their application in pollutant removal,RSC Adv.3(2013)1158–1164.[17]L.Yu,Y.C.Pan,C.Q.Wang,L.X.Zhang,A two-phase segmented microfluidictechnique for one-step continuous versatile preparation of zeolites,Chem.Eng.J.219(2013)78–85.[18]P.F.Dong,J.H.Xu,H.Zhao,G.S.Luo,Preparation of10l m scale monodispersedparticles by jettingflow in coaxial microfluidic device,Chem.Eng.J.214(2013) 106–111.[19]J.Fidalgo,Â.Dias,A.M.Mendes,F.D.Magalhães,Production of monodispersemultivesiculated polyester particles with a T-junction microfluidic device, Chem.Eng.J.2323(2013)323–330.[20]Y.H.Li,D.G.Yamane,S.N.Li,S.Biswas,R.K.Reddy,J.S.Goettert,K.Nandakumar,C.S.S.R.Kumar,Geometric optimization of liquid–liquid slugflow in aflow-focusing millifluidic device for synthesis of nanomaterials,Chem.Eng.J.217 (2013)447–459.[21]X.Chen,M.Arruebo,K.L.Yeung,Flow-synthesis of mesoporous silicas andtheir use in the preparation of magnetic catalysts for Knoevenagel condensation reactions,Catal.Today204(2013)140–147.[22]T.H.Eun,S.H.Kim,W.J.Jeong,S.J.Jeon,S.H.Kim,S.M.Yang,Single-stepfabrication of monodisperse TiO2hollow spheres with embedded nanoparticles in microfluidic devices,Chem.Mater.21(2009)201–203. [23]C.X.Zhao,A.P.J.Middelberg,One-step fabrication of titania hollow spheres bycontrolled interfacial reaction in a droplet-based microfluidic system, Microfluid.Nanofluid.14(2013)703–709.[24]D.Y.Li,Z.C.Guan,W.H.Zhang,X.Zhou,W.Y.Zhang,Z.X.Zhuang,X.R.Wang,C.J.Yang,Synthesis of uniform-size hollow silica microspheres through interfacial polymerization in monodisperse water-in-oil droplets,ACS Appl.Mater.Interfaces10(2010)2711–2714.[25]Y.C.Pan,M.H.Ju,C.Q.Wang,L.X.Zhang,N.P.Xu,Versatile preparation ofmonodisperse poly(furfuryl alcohol)and carbon hollow spheres in a simple microfluidic device,mun.46(2010)3732–3734.[26]H.Zhao,J.H.Xu,P.F.Dong,G.S.Luo,A novel microfluidic approach formonodispersed chitosan microspheres with enhanced autofluorescence, Chem.Eng.J.215–216(2013)784–790.[27]H.Zhao,J.H.Xu,n,T.Wang,G.S.Luo,Microfluidic production of porouschitosan/silica hybrid microspheres and its Cu(II)adsorption performance, Chem.Eng.J.229(2013)82–89.[28]T.Zhang,X.F.Zhang,X.J.Yan,L.Y.Kong,G.C.Zhang,H.O.Liu,J.S.Qiu,K.L.Yeung,Synthesis of Fe3O4@ZIF-8magnetic core–shell microspheres and their potential application in a capillary microreactor,Chem.Eng.J.228(2013) 398–404.[29]W.Wang,M.J.Zhang,R.Xie,X.J.Ju,C.Yang,C.L.Mou,D.A.Weitz,L.Y.Chu,Hole-shell microparticles from controllably evolved double emulsions,Angew.Chem.Int.Ed.52(2013)8084–8087.[30]J.Han,G.P.Song,R.Guo,Synthesis of polymer hollow spheres with holes intheir surfaces,Chem.Mater.19(2007)973–975.[31]S.H.Im,U.Jeong,Y.N.Xia,Polymer hollow particles with controllable holes intheir surfaces,Nat.Mater.4(2005)671–675.[32]U.Jeong,S.H.Im,P.H.C.Camargo,J.H.Kim,Y.N.Xia,Microscalefish bowls:anew class of latex particles with hollow interiors and engineered porous structures in their surfaces,Langmuir23(2007)10968–10975.[33]H.N.Yow,A.F.Routh,Colloidal buckets formed via internal phase separation,Soft Matter4(2008)2080–2085.[34]G.J.Guan,Z.P.Zhang,Z.Y.Wang,B.H.Liu,D.M.Gao,C.G.Xie,Single-hole hollowpolymer microspheres toward specific high-capacity uptake of target species, Adv.Mater.19(2007)2370–2374.[35]M.W.Chang,E.Stride,M.Edirisinghe,A new method for the preparation ofmonoporous hollow microspheres,Langmuir7(2010)5115–5121.[36]M.Li,J.M.Xue,Facile route to synthesize polyurethane hollow microsphereswith size-tunable single holes,Langmuir27(2011)3229–3232.[37]H.Minami,H.Kobayashi,M.Okubo,Preparation of hollow polymer particleswith a single hole in the shell by SaPSeP,Langmuir13(2005)5655–5658. [38]C.L.Mou,X.H.He,X.J.Ju,R.Xie,Z.Liu,L.Liu,Z.B.Zhang,L.Y.Chu,Change in sizeand structure of monodisperse poly(N-isopropylacrylamide)microcapsules in response to varying temperature and ethyl gallate concentration,Chem.Eng.J.210(2012)212–219.[39]H.J.Zhang,J.Wu,L.P.Zhou, D.Y.Zhang,L.M.Qi,Facile synthesis ofmonodisperse microspheres and gigantic hollow shells of mesoporous silica in mixed water–ethanol solvents,Langmuir23(2007)1107–1113.[40]Y.T.Lim,J.K.Kim,Y.W.Noh,M.Y.Cho, B.H.Chung,Multifunctional silicananocapsule with a single surface hole,Small3(2009)324–328.[41]X.Y.Fu,X.H.He,Y.Wang,Facile preparation of silica hollow microspheres byprecipitation-phase separation method,Colloid Surf.A380(2011)241–249.[42]X.Y.Fu,X.H.He,X.Hu,Preparation of single-hole silica hollow microspheresby precipitation-phase separation method,Colloid Surf.A396(2012)283–291.[43]S.J.Ding,T.Q.Lin,Y.M.Wang,X.J.Lu,F.Q.Huang,New facile synthesis of TiO2hollow sphere with an opening hole and its enhanced rate performance in lithium-ion batteries,New.J.Chem.37(2013)784–789.[44]D.Schunk,S.Hardt,H.Wiggers,F.Marlow,Monodisperse titania microspheresvia controlled nanoparticle aggregation,Phys.Chem.Chem.Phys.14(2012) 7490–7496.[45]A.P.Fang,C.Gosse,C.Gaillard,X.Zhao,J.Davy,Tuning silica particle shape atfluid interfaces,Lab Chip12(2012)4960–4963.[46]I.Lee,Y.Yoo,Z.D.Cheng,H.K.Jeong,Generation of monodisperse mesoporoussilica microspheres with controllable size and surface morphology in a microfluidic device,Adv.Funct.Mater.18(2008)4012–4014.[47]N.J.Carroll,S.B.Rathod,E.Derbins,S.Mendez,D.A.Weitz,D.N.Petsev,Droplet-based microfluidics for emulsion and solvent evaporation synthesis of monodisperse mesoporous silica microspheres,Langmuir24(2008)658–661.[48]W.C.Jeong,M.Choi,C.H.Lim,S.M.Yang,Microfluidic synthesis of atto-literscale double emulsions toward ultrafine hollow silica spheres with hierarchical pore networks,Lab Chip12(2012)5262–5271.[49]K.Xu,J.H.Xu,Y.C.Lu,G.S.Luo,Extraction-derived self-organization of colloidalphotonic crystal particles within confining aqueous droplets,Cryst.Growth Des.13(2013)926–935.[50]P.Y.Sun,B.Wang,J.F.Yao,L.X.Zhang,N.P.Xu,Fast synthesis of biodiesel athigh throughput in microstructured reactors,Ind.Eng.Chem.Res.3(2010) 1259–1264.[51]E.Rondeau,J.J.Cooper-White,Biopolymer microparticle and nanoparticleformation within a microfluidic device,Langmuir24(2008)6931–6937. [52]G.Derkachov,K.Kolwas,D.Jakubczyk,M.Zientara,M.Kolwas,Drying of amicrodroplet of water suspension of nanoparticles:from surface aggregates to microcrystal,J.Phys.Chem.C112(2008)16919–16923.118M.Ju et al./Chemical Engineering Journal250(2014)112–118。
