Facile synthesis and size control of highly monodispersed hybrid silica spheres

Facile synthesis and size control of highly monodispersed hybrid silica spheres through a novel nuclei controlling methodQishu Qu a ,⁎,Ping Xu a ,Debby Mangelings b ,Chun Yang a ,Xiaoya Hu aa Jiangsu Key Laboratory of Environmental Materials and Environmental Engineering,College of Chemistry and Chemical Engineering,Yangzhou University,Yangzhou 225002,China bDepartment of Analytical Chemistry and Pharmaceutical Technology,Center for Pharmaceutical Research,Vrije Universiteit Brussel-VUB,Laarbeeklaan 103,B-1090Brussels,Belgiuma b s t r a c ta r t i c l e i n f o Article history:Received 16April 2010Received in revised form 2November 2010Available online 3December 2010Keywords:Monodisperse;Hybrid;Silica spheresMonodispersed hybrid silica spheres with particles sizes up to 4μm were obtained by controlling the number of nuclei generated in the initiation phase of particle formation using a two-step sol –gel method.This method has two advantages:(i)the particle size can be tuned easily just by variation of the amount ammonia added in the step of precursor hydrolysis,and (ii)the amount of precursor used to prepare particles with the same diameter is at least ten times lower than with traditional methods.The effects of stirring speed and stirring time on the resulting particle size were also studied.©2010Elsevier B.V.All rights reserved.1.IntroductionToday organic –inorganic hybrid materials are of great interest in the field of biosensors [1–6],drug delivery [7],luminescence [8],catalysis [9],and magnetic materials [10].Within the organic –inorganic hybrid materials,monodispersed spherical silica particles modi fied with various active organic groups such as vinyl,thiol,and amine were widely used.Two different methods can be employed to prepare silica based organic –inorganic hybrids,i.e.postgrafting and co-condensation [11].For postgrafting,pure silica particles are synthesized first using a classical Stöber method [12],or using other methods,such as the surfactant template route [13],an evaporation-driven self assembly method,[14],or a porous organic bead template method [15].The obtained silica particles are redispersed in reaction medium and a silane coupling agent is covalently linked onto the surface silanol groups of the silica particles [9,16].This method is time-consuming and the particle surfaces can only be partially covered.However,although the Stöber method is the most used method to prepare silica spheres and yields monodispersed spherical particles,it can only produce particles with an upper size limit of about 2μm.In contrast,the co-condensation method refers to the direct synthesis of functionalized organosilica species by co-condensation of tetraalkoxysilane and organosilane precursors [17,18].A higher and more homogeneous surface coverage of functional groups can be yielded by this method.Trau et al.simpli fied this method by using a functionalized silane monomer,3-mercaptopropyl trimethoxysilane (MPTMS)[19–21].In this study,thiol-functionalized silica microspheres were synthesized through a two-step process involving acid-catalyzed hydrolysis and base-catalyzed condensation of MPTMS in aqueous pared with the traditional Stöber method where large quantities of alcohol were employed,no alcohol was needed in this method and particles with diameter up to 5μm were obtained.Recently,Lee et al.[22]further simpli fied the direct synthesis method.With their method,thiol-functionalized monodispersed spherical silica particles with narrow size distribution were obtained by base-catalyzed condensation of MPTMS,which was dissolved in water in advance.Hybrid silica spheres with diameter ranging from 1to 4μm were obtained using various amounts of MPTMS as precursor.In this study,we report a facile two-step synthetic route to achieve size controlled monodispersed hybrid silica spheres with a narrow size distribution using a nuclei controlling pared to the method developed by Lee et al.in which MPTMS was dissolved first in pure water and then catalyzed by a high concentration of base,MPTMS was dissolved first in water containing a little amount of ammonia (first step)in our method and then the mixture was catalyzed further using a high concentration of ammonia (second step).Since the number of nuclei generated in the first step was limited by the small amount of base,further condensation of silica spheres occurred by particle growth on the limited quantity of nuclei.Thus,to obtain the particles with the same diameter,only 10%of the amount of MPTMS was needed compared to the methods developed by Trau et al.[19–21]or Lee et al.[22].In addition,hybrid silica spheres with diameters of up to 4μm can be obtained just by changing the amount of ammonia used in the first step of synthesis.Journal of Non-Crystalline Solids 357(2011)976–980⁎Corresponding author.Tel.:+8651487975590;fax:+8651487975244.E-mail address:quqishu@ (Q.Qu).0022-3093/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.jnoncrysol.2010.11.004Contents lists available at ScienceDirectJournal of Non-Crystalline Solidsj ou r n a l h o me pa g e :ww w.e l s ev i e r.c o m/l oc a t e /j no n c r ys o l2.Materials and methods 2.1.MaterialsMPTMS was purchased from Alfa Chemical (St.Louis,MO,USA).Ammonia was purchased from Chemical Reagent (Shanghai,China).Water used in all the experiments was doubly distilled and puri fied by a Milli-Q system (Millipore,Milford,MA,USA).2.2.SynthesisTwo grams of MPTMS and various amounts of NH 3⋅H 2O (1,2,and 3μL)were added into 100mL of H 2O under gentle stirring (115rpm)for 12h.Then,0.1mL NH 3⋅H 2O was added to the solution and the reaction was allowed to proceed at room temperature for 2h.After that,the precipitate was centrifuged three times in water.As a comparison,hybrid silica particles were also prepared using the method developed by Lee et al.Two grams of MPTMS was added in 100mL H 2O under vigorous stirring until the oil droplets were completely dissolved.Then 0.1mL NH 3⋅H 2O was added to the solution and the mixture was stirred for ~2h.2.3.CharacterizationThe morphology was evaluated using Environmental Scanning Electron Microscope (ESEM)micrographs under low variable water pressure (0–1Torr)on a Philips ESEM XL30.The average particlediameter was calculated from the diameters of 200particles in a SEM picture for each sample.3.ResultsFig.1shows the SEM images of hybrid silica spheres prepared using different amounts of ammonia (1to 3μL)in the first step to control the number of nuclei in the solution.It can be seen from Fig.1that the hybrid silica spheres obtained are monodispersed and micrometer-sized particles.As the amount of ammonia increased from 1to 3μL,the particle diameter presents a noticeable change from 3.8to 1.3μm.In all these experiments,only 2mL of MPTMS was used.Fig.2shows the SEM images of the particles obtained when a high concentration of ammonia was added into the solutions in the second step after stirring for different times.It can be seen in Fig.2that the particle size distribution narrows when the stirring time in the first step increases.Fig.3shows the effect of stirring speed on the particles sizes.It can be found that the diameter of the particles decreased with the increasing stirring speed.The average diameters of the particles are 1.22and 0.84μm when the stirring speed was 115and 140rpm,respectively.4.DiscussionConventional sol –gel synthesis of monodispersed spherical silica particles uses silicon alkoxides as precursor and ammonia as the catalyst in a water/ethanol solution.In this procedure,the rate ofFig.1.SEM micrographs of the monodispersed hybrid silica spheres prepared using different amounts of ammonia added in the first step.(a)1μL NH 3⋅H 2O.(b)2μL NH 3⋅H 2O.(c)3μL NH 3⋅H 2O.The graph represents obtained particle sizes as a function of the amount ammonia used.A linear regression line was fitted through the data.977Q.Qu et al./Journal of Non-Crystalline Solids 357(2011)976–980hydrolysis of silicon alkoxide,nuclei generation,and primary particle formation are all very fast when catalyzed by a suf ficient amount of ammonia.The number of nuclei is large is because the rate of nuclei generation is faster than that of primary particle formation.As a result,particles with smaller diameters are obtained.In the synthetic process developed by Lee et al.[22],MPTMS was first dissolved and fully hydrolyzed in pure water.Then a large amount of ammonia was added into the mixture,resulting in the formation of a lot of nuclei and inducing the growth of primary particles.Therefore,the particle sizes with this method are also small.It can be seen that in all the above mentioned methods the number of nuclei is uncontrollable which leads to the smaller particles.To obtain particles with large diameter,larger amounts of silicon alkoxide have to be used.Since the number of nuclei depends not only on the concentration of precursor but also on the concentration of the catalyst used in the synthetic process,particles with larger diameters can be obtained when small amounts of ammonia are used as catalyst in the initial hydrolysis process of the ing our method,monodispersed hybrid silica spheres with a diameter up to 3.8μm were obtained.In contrast,the method of Lee et al.was used to obtain the particles with a diameter of 3.8μm,at least 20mL MPTMS had to be used,which is 10times larger than in our method.In our experiments,it was found that the final obtained particle size increased observably with only a slight decrease in ammoniaFig.2.SEM pictures and the histogram displaying the effect of different stirring times on the particle size and distribution.(a)4h,(b)6h,(c)8h,(d)10h,(e)12h.Two microliter NH 3⋅H 2O was used in the first step.978Q.Qu et al./Journal of Non-Crystalline Solids 357(2011)976–980content used in thefirst step.These results indicate that the number of nuclei generated in thefirst step of hydrolysis mainly depends on the amount of ammonia used.When only a small amount of ammonia is used in thefirst step,the rate of hydrolysis of MPTMS,nuclei generation,and particle growth are all slower compared with othermethods[23,24].Since the used stirring speed is low,the hydrolysis rate of MPTMS is also slower.Thus,the gradually hydrolyzed MPTMS only produces a limited number of nuclei.The growth of the primary particles afterwards begins on the surface of these nuclei,resulting in particles with larger diameters.As the rate of hydrolysis of MPTMS,nuclei generation,and particle growth are all slower when very little amount of ammonia was added in thefirst step,the whole synthetic process would take a long time.If a large amount of ammonia is added into the solution in the second step when not all the MPTMS is hydrolyzed,particle sizes with a broad distribution will be obtained.Thus,when a large amount of ammonia is added into the solution before12h,the remaining un-hydrolyzed MPTMS will be hydrolyzed further,resulting in nuclei formation and the generation of more smaller particles.In order to obtain particles with a narrow size distribution,the stirring time should be long enough,i.e.at least12h.It is well known that during the particles synthesis process overconcentration of the precursor in some regions of the solution, leading to the generation of more nuclei,can be avoided by stirring at high speed.Therefore,in normal synthetic processes,the stirring speed was set to be fast enough to ensure equal dispersion of the precursor in the solution.However,the available amount of hydrolyzed MPTMS used as the source for nuclei generation increases with increasing stirring speed.As a result,the number of formed nuclei increases because a higher amount of monomer is hydrolyzed.in thefirst step was very little in our experiments,overconcentration of ammonia in the solution does not occur,even with a low stirring speed.The stirring speed was therefore kept at115rpm and the stirring time for thefirst step was12h.Afterwards,a large amount of ammonia(typically100μL)was added which induced the polycondensation of primary particles and resulted in the formation of stable and larger particles.5.ConclusionsIn summary,our results indicate that by adding a very little amount of ammonia in thefirst step of hydrolysis of MPTMS,it is possible to prepare monodispersed hybrid silica spheres up to4μm with a small amount of MPTMS.The diameter of the hybrid silica spheres can be easily tuned by changing the volume of ammonia.This simple one-pot synthetic method can be extended to produce different diameters of silica spheres functionalized with other organic groups.AcknowledgmentsWe are grateful for the support of the National Natural Science Foundation of China(20975090and20875081),the Qing Lan Project, and by a Bilateral Project between China and Belgium(BWS07/03).}Fig.2(continued).979Q.Qu et al./Journal of Non-Crystalline Solids357(2011)976–980References[1]K.Sarkar,K.Dhara,M.Nandi,P.Roy,A.Bhaumik,P.Banerjee,Adv.Funct.Mater.19(2009)223.[2]H.Tan,N.S.Liu,B.P.He,S.Y.Wong,Z.K.Chen,X.Li,J.Wang,mun.(2009)6240.[3]I.Sokolov,S.Naik,Small 4(2008)934.[4]Y.H.Jin,S.Lohstreter,D.T.Pierce,J.Parisien,M.Wu,C.Hall,J.X.J.Zhao,Chem.Mater.20(2008)4411.[5] rson,H.Ow,H.D.Vishwasrao,A.A.Heikal,U.Wiesner,W.W.Webb,Chem.Mater.20(2008)2677.[6]R.Kumar,I.Roy,T.Y.Hulchanskyy,L.N.Goswami,A.C.Bonoiu,E.J.Bergey,K.M.Tramposch,A.Maitra,P.N.Prasad,ACS Nano 2(2008)449.[7]L.Contessotto,E.Ghedini,F.Pinna,M.Signoretto,G.Cerrato,V.Crocella,Chem.Eur.J.15(2009)12043.[8]J.Font,P.de March,F.Busque,E.Casas,M.Benitez,L.Teruel,H.Garcia,J.Mater.Chem.22(2007)2336.[9]Q.H.Yang,J.Liu,L.Zhang,C.Li,J.Mater.Chem.19(2009)1945.[10] B.Julian-Lopez,C.Boissiere,C.Chaneac,D.Grosso,S.Vasseur,S.Miraux,E.Duguet,C.Sanchez,J.Mater.Chem.17(2007)1563.[11]M.H.Lim,A.Stein,Chem.Mater.11(1999)3285.[12]W.Stober,A.Fink,E.Bohn,J.Colloid Interface Sci.26(1968)62.[13] C.T.Kresge,M.E.Leonowicz,W.J.Roth,J.C.Vartuli,J.S.Beck,Nature 359(1992)710.[14]G.V.Rama Rao,G.P.López,J.Bravo,H.Pham,A.K.Datye,H.F.Xu,T.L.Ward,Adv.Mater.14(2002)1301.[15]U.Meyer,rsson,H.P.Hentze,R.A.Caruso,Adv.Mater.14(2002)1768.[16] B.Gonzalez,M.Colilla,C.L.de Laorden,M.Vallet-Regi,J.Mater.Chem.19(2009)9012.[17]Z.Meng,C.Y.Xue,Q.H.Zhang,X.H.Yu,K.Xi,X.D.Jia,Langmuir 25(2009)7879.[18]X.Zhuang,Q.F.Zhao,Y.Wan,J.Mater.Chem.20(2010)4715.[19] A.P.R.Johnston,B.J.Battersby,wrie,M.Trau,mun.(2005)848.[20] ler,R.Vogel,P.P.T.Surawski,S.R.Corrie,A.Rühmann,M.Trau,Chem.Commun.(2005)4783.[21] ler,R.Vogel,P.P.T.Surawski,K.S.Jack,S.R.Corrie,M.Trau,Langmuir 21(2005)9733.[22]Y.G.Lee,J.H.Park,C.Oh,S.G.Oh,Y.C.Kim,Langmuir 23(2007)10875.[23]T.Matsoukas,E.Gulari,J.Colloid Interface Sci.124(1988)252.[24]T.Matsoukas,E.Gulari,J.Colloid Interface Sci.132(1989)13.Fig.3.SEM micrographs of the monodispersed hybrid silica spheres prepared with 3μL NH 3⋅H 2O in the first step using a different stirring speed.(a)115rpm and (b)140rpm.980Q.Qu et al./Journal of Non-Crystalline Solids 357(2011)976–980。