Hollow polymer microspheres containing a gold nanocolloid core

Hollow polymer microspheres containing a gold nanocolloid core adsorbed on the inner surface as a catalytic microreactor

Xiaoxu Wang ?Hongfen Ji ?Xu Zhang ?

Han Zhang ?Xinlin Yang

Received:8November 2009/Accepted:5April 2010/Published online:17April 2010óSpringer Science+Business Media,LLC 2010

Abstract Hollow polymer microspheres with different polarity and functional group for the shell layer containing gold nanocolloid cores adsorbed on the inner surface were prepared by selective removal of sandwiched silica layer from the corresponding gold/silica/polydivinylbenzene (Au/SiO 2/PDVB),Au/SiO 2/poly(ethyleneglycol dimethacrylate)(Au/SiO 2/PEGDMA),and Au/SiO 2/poly(ethyleneglycol dimethacrylate-co -methacrylic acid)(Au/SiO 2/P(EGDMA-co -MAA)tri-layer microspheres,respectively.The tri-layer microspheres were synthesized by distillation precipitation polymerizations of divinylbenzene (DVB),ethyleneglycol dimethacrylate (EGDMA),EGDMA together with metha-crylic acid (MAA)in presence of 3-(methacryloxy)pro-pyltrimethoxysilane (MPS)-modi?ed gold/silica (Au/SiO 2)core–shell particles as seeds,which were prepared by coating of a layer of silica onto the surface of Au nano-colloids with the aid of polyvinylpyrrolidone (PVP)via a

modi?ed Sto

¨ber method.The catalytic property and sta-bility as a microreactor of the hollow polymer micro-spheres with Au nanocolloid cores adsorbed on the inner surface were studied by the reduction of 4-nitrophenol (4-NP)to 4-aminophenol (4-AnP)with sodium borohy-dride (NaBH 4)as reductant.Transmission electron micros-copy (TEM)and Fourier transform infrared spectra (FT-IR)were used for characterizing the morphology and structure of the resultant microspheres.

Introduction

Colloidal nanoparticles with unique optical,electrical,and magnetic properties have been an important branch of chemical research and technical applications [1].However,the high aggregation tendency of the nanocolloids prevents them from wide applications.The most essential role of the stabilizing polymer is to protect the nanoparticles from coagulation.Although soluble metallic nanocolloids stabilized by polymer have high activity,stability,and selectivity in homogeneous reactions [2–4],the recovery of the catalyst from products by conventional techniques,such as ?ltration and ultracentrifugation,is neither eco-nomic nor convenient.

Various physical and chemical methods have been uti-lized for the encapsulation of metallic nanoparticles with either an inorganic or organic shell layers [5–7].The solid Au/SiO 2core–shell particles were conveniently prepared by a modi?ed sol–gel procedure with the surface protection from polyvinylpyrrolidone (PVP)and the Au/SiO 2yolk–shell nanostructures were further formed via a surface-protected etching processing with sodium hydroxide for the utilization as a nanoscale reactor for catalytic reactions [5].Spherical hollow colloids of polybenzyl methacrylate (PBzMA)containing cores of Au nanoparticles adsorbed on the inner surface were synthesized via three major steps,which included the coating of gold nanoparticles with uniform shells of amorphous silica by hydrolysis of tetra-ethyl orthosilicate (TEOS),surface atom transfer radical polymerization (ATRP)of benzyl methacrylate (BzMA)and the selective removal of sandwiched silica layer in aqueous hydro?uoric acid (HF)[6].Hollow zirconia spheres with gold nanocolloids adsorbed on the inner sur-face were prepared by removing the silica mid-layer in 1N aqueous sodium hydroxide solution from the corresponding

X.Wang áH.Ji áX.Zhang áH.Zhang áX.Yang (&)

Key Laboratory of Functional Polymer Materials,The Ministry of Education,Institute of Polymer Chemistry,Nankai University,Tianjin 300071,China e-mail:xlyang88@https://www.wendangku.net/doc/2e885186.html,

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J Mater Sci (2010)45:3981–3989DOI 10.1007/s10853-010-4470-z

Au/SiO2/ZrO2tri-layer microspheres,which were used as catalysts for the oxidation of CO[7].Hollow microspheres have attracted increasing attention due to their unique properties,such as low density,high speci?c surface area, good?ow ability,and surface permeability.They have found wide applications in many?elds,such as catalysis, controlled drug delivery systems,arti?cial cells,?llers, pigments,light-weight structural materials,nanoreactors, low dielectric constant materials,acoustic insulation,and photonic crystals[8–12].An important concern of hollow microspheres is to accommodate guest nanoparticles in their cavity,which leads to an interesting structure as hollow microspheres with movable cores and guest nano-particles different from those of host hollow microsphere-shell and guest nanoparticles.Nanoparticles such as gold [5–7,13,14],silver[15],tin[16],silica[17],and iron oxide[18,19],have been successfully incorporated into the interior of the hollow microspheres by various techniques.

Hollow polymer microspheres have combined charac-teristics of precisely controllable size and shell thickness, superior monodispersity and permeability,high structural strength and?ne?ow ability.All of these properties enable the hollow polymer microspheres ideal shell to encapsulate the metallic nanocolloids to greatly enhance their stability against coalescence while maintaining their catalytic activ-ity[5,14,20].Bearing these in mind,the present work describes a facile route for the synthesis of hollow polymer microspheres with polymer shells having different polarity and functional groups together with movable gold nanocol-loid cores via the selective removal of silica mid-layer from the corresponding Au/SiO2/polymer tri-layer microspheres, which are prepared by distillation precipitation polymeri-zation of divinylbenzene(DVB),ethyleneglycol dimethac-rylate(EGDMA),EGDMA together with methacrylic acid (MAA)in presence of3-(methacryloxy)propyl trimethox-ysilane(MPS)modi?ed Au/SiO2core–shell composites as seeds.

Experimental

Materials

Tetrachloro auric acid trihydrate(HAuCl4á3H2O)was obtained from Shenyang Research Institute of Nonferrous Metals,China.Poly(N-vinylpyrrolidone)with average molar masses of10kg/mol(PVP-10)was purchased from Tianjin Chemical Engineering Institute.Tetraethyl ortho-silicate(Si(OC2H5)4,TEOS)and EGDMA were obtained from Aldrich and used without any further puri?cation. MPS was provided by Aldrich and distilled under vacuum. Divinylbenzene(DVB,80%of DVB isomers)was supplied as technical grade by Shengli Chemical Technical Factory,Shandong,China,which was washed with5wt%aqueous sodium hydroxide and water,then dried over anhydrous magnesium sulfate prior to use.Methacrylic acid(MAA) and hydro?uoric acid(containing40wt%of HF)were purchased from Tianjin Chemical Reagents II Co.2,20-Azobisisobutyronitrile(AIBN)was bought from Tianjin Chemical Reagents III Co.4-Nitrophenol(4-NP,Tianjin Chemical Reagent Factory)was recrystallized from petro-leum ether and ethyl acetate.Acetonitrile(analytical grade, Tianjin Chemical Reagents Co.)was dried over calcium hydride and puri?ed by distillation before utilization.All the other reagents were analytical grade and used without any further treatment.

Synthesis of MPS-modi?ed Au/SiO2core–shell microspheres

Gold/silica(Au/SiO2)core–shell microspheres were pre-pared by the method reported in the literature[21].The Au nanocolloids were synthesized via a standard sodium cit-rate reduction method:A10mL of1wt%sodium citrate solution was added to90mL of HAuCl4aqueous solution containing20mg of HAuCl4under the boiling state with vigorous stirring.The reduction was performed further for 30min without exposure to light.Then the Au nanocol-loids were cooled down to room temperature immediately. The PVP solution(0.4g PVP-10in15mL of water)was added to the Au nanocolloid solution and stirred for12h at room temperature to guarantee the complete adsorption of PVP-10onto the Au nanocolloids.Subsequently,the solution was centrifugated and redispersed in a solution of isopropanol and water with the mass ratio of4:1.Then 0.30mL of TEOS and2.0mL of ammonia(29.3wt%of NH3in water)was added to the solution and stirred for 12h to coat the PVP-modi?ed Au nanocolloids with silica shell.Further,the MPS-modi?ed Au/SiO2core–shell microspheres were made by coupling silica alcosol parti-cles via hydrolysis of MPS:Excess MPS of0.20mL was introduced into the Au/SiO2suspension under stirring and the coating process lasted for48h at room temperature. The resultant MPS-modi?ed Au/SiO2nanocolloids were puri?ed by three cycles of centrifugation,decantation,and resuspension in ethanol with ultrasonic irradiation and ?nally dried in a vacuum oven at50°C till constant weight.

Preparation of Au/SiO2/polymer tri-layer microspheres The as-prepared MPS-modi?ed Au/SiO2microspheres were used as seeds for the further-stage distillation pre-cipitation polymerization to afford Au/SiO2/polymer tri-layer microspheres.In a dried50mL two-necked?ask, 0.01g of MPS-modi?ed Au/SiO2seeds was ultrasonically

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suspended in40mL of acetonitrile as a red suspension. Then0.05g of MAA and0.05g of EGDMA(total as 0.25wt%of the reaction system)and AIBN(2.0910-3g, 2wt%relative to the monomer)were dissolved in the sus-pension.The two-necked?ask was attached with a frac-tionating column,Liebig condenser and receiver and submerged in a heating mantle.The reaction mixture was heated from ambient temperature till the boiling state for 10min,and the reaction system was kept under the re?uxing state for a further10min.The polymerization was ended after20mL of acetonitrile was distilled off the reaction system within70min and the red color turned into pink during the heating process.After the polymerization,the resultant Au/SiO2/P(EGDMA-co-MAA)tri-layer compos-ites were puri?ed by repeating centrifugation,decantation, and resuspension in acetone with ultrasonic irradiation for3 times.The tri-layer composite nanoparticles were then dried in a vacuum oven at50°C till constant weight.

A series of parallel synthesis using different monomers including DV

B and EGDMA were carried out following the same pattern by distillation precipitation polymeriza-tion in the presence of MPS-modi?ed Au/SiO2seeds to obtain Au/SiO2/polymer tri-layer microspheres having poly-mer shells with different polarity,in which the monomers used were set as0.10g,respectively.

The reproducibility of the distillation precipitation polymerization for the synthesis of Au/SiO2/polymer tri-layer microspheres was con?rmed through several dupli-cate and triplicate experiments.

Synthesis of hollow polymer microspheres containing gold nanocolloid cores adsorbed on the inner surface

The Au/SiO2/polymer tri-layer microspheres were treated with40wt%hydro?uoric acid solution for2h at room temperature to remove the silica mid-layer.Then,the hollow polymer microspheres with Au nanocolloid cores adsorbed on the inner surface were puri?ed by several ultracentrifugation/washing cycles in water till pH7.The resultant hollow polymer microspheres with Au nanocol-loid cores adsorbed on the inner surface were then dried in a vacuum oven at50°C till constant weight.

Catalytic reduction of4-nitrophenol to4-aminophenol

To investigate the catalytic activity and stability of the hollow polymer microspheres with Au nanocolloid cores adsorbed on the inner surface,the reduction of 4-nitrophenol(4-NP)to4-aminophenol(4-AnP)in the presence of NaBH4as reductant was performed in aqueous solution at room temperature as a model reaction.1mL of 4-NP aqueous solution(0.1mmol/L,2910-4mol)and 0.1mL suspension of hollow PEGDMA microspheres with Au nanocolloid cores adsorbed on the inner surface (1.0g/L,containing6910-8mol Au,with a PEGDMA shell thickness of31nm)were subsequently introduced into the quartz cell with gentle shaking.The catalytic activity was monitored by a UV–vis spectrometer with a decrease of the intensity of the peak at400nm attributing to the typical absorption of4-NP.

The catalyst was recycled via centrifugation,decanta-tion,washing and drying.After that,1mL of4-NP aque-ous solution(0.1mmol/L)and2mL of freshly prepared NaBH4aqueous solution(0.10mol/L)were added to test the recycling activity of the catalyst for3times.

To compare the catalytic property of the hollow struc-tures with pure Au nanoparticles,sodium citrate stabilized Au nanoparticles aqueous solution with diameter of23nm was prepared according to literature[15].Typically, 100mL of0.25mM HAuH4aqueous solution was heated to boiling state with mechanical stirring.Then,1mL of5 wt%sodium citrate aqueous solution was quickly added. The reaction was maintained till the color of the reaction mixture changes to wine red.The wine red mixture was cooled to room temperature and centrifuged for40min at 8000rpm to obtain concentrated Au nanoparticles aqueous solution.Then,75%of the colorless supernatant was removed and the remained solution was redispersed under gentle shaking.To study the catalytic of the as-obtained Au nanoparticles,three drops of such concentrated aqueous solution(containing3.6910-8mol of Au atoms)were added into2mL of0.1mM4-NP(2910-7mol)aqueous solution with gentle shaking.Then,4mL of0.1M NaBH4 (4910-4mol)aqueous solution was introduced into such mixture with gentle shaking.The catalytic reaction was detected by UV–vis spectroscopy.

Characterization

The morphology,size,and size distribution of Au nanoparticles,Au/SiO2seeds,Au/SiO2/polymer tri-layer microspheres,and the corresponding hollow polymer microspheres with Au cores adsorbed on the inner surface were characterized by transmission electron microscopy (TEM,Technai G220S-TWIN).All the size and size distribution re?ect the averages about100particles each, which are calculated according to the following formula:

D w?

X k

i?1

n i D4

i

=

X k

i?1

n i D3

i

D n?

X k

i?1

n i D i=

X k

i?1

n i

U?D w=D n

where U is the polydispersity index,D n is the number-average diameter,D w is the weight-average diameter,and D i is the particle diameters of the determined microparticles.

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Fourier transform infrared spectra(FT-IR)were scanned over the range of400–4000cm-1with potassium bromide plate on a Bio-Rad FTS135FTIR spectrometer.

UV–vis spectra were performed on a Cary100conc spectrometer ranging from200to600nm with water as background.

Results and discussion

Scheme1illustrates the synthesis of hollow polymer microspheres containing Au nanocolloid cores adsorbed on the inner surface via the selective removal of the silica mid-layer in hydro?uoric acid from the corresponding Au/SiO2/polymer tri-layer microspheres,which are pre-pared by distillation precipitation polymerization of DVB, EGDMA,EGDMA together with MAA to afford the poly-mer shell layer with different polarity and functional groups.

Preparation of MPS-modi?ed Au/SiO2composites

It is dif?cult to directly perform the polymerization on the surface of the gold nanocolloids for the synthesis of the hybrids containing noble metallic Au cores adsorbed on the inner surface and polymer shells due to the lack of an appropriate interaction between Au nanoparticles and the monomer.To solve such a problem,inserting an inter-layer between the Au nanoparticles and the polymer is necessary, which would have suitable interaction between the Au nanoparticles and the polymer components.Coating the Au nanoparticles with a silica layer accomplishes such a requirement.There are many reactive hydroxyl groups on the surface of silica layer,which enable the Au/SiO2 composites to well disperse in acetonitrile and act as ideal cores for the further-stage polymerization affording Au/ SiO2/polymer tri-layer microspheres.

The Au/SiO2core–shell particles were synthesized with the aid of amphiphilic poly(vinylpyrrolidone)according to the method as described in the reference[22].The TEM micrograph of gold nanoparticles in Fig.1a indicates that the Au particles prepared by the sodium citrate reduction of HAuCl4had spherical shapes with average diameter of 28nm and narrow dispersity index(U)of1.044as sum-marized in Table1.A critical step for the silica-coating procedures is the transfer of Au nanocolloids to be well dispersed from aqueous solution to alcoholic solvent, where the classical Sto¨ber process would be performed.For this,the PVP-stabilized Au nanocolloids were separated from the aqueous dispersion and redispersed in the mixture of isopropanol/water(4/1,V/V)for the hydrolysis of TEOS.In the present work,isopropanol/water(4/1,V/V) mixed solvent was used as a medium for the encapsulation of PVP-stabilized Au nanocolloids via a Sto¨ber method to prevent the formation of the secondary-initiated

silica V

P

P

V

P

32

Polymerization

PVP

Scheme1Synthesis of Au/SiO2/polymer tri-layer microspheres and the corresponding hollow polymer microspheres with Au cores

Fig.1TEM micrographs:a Au nanocolloids prepared by sodium citrate reduction,b Au/SiO2core–shell microspheres via a sol–gel process with the aid of amphiphilic PVP

123

particles,which was some different from the procedure for coating of Au nanocolloids with silica layer in ethanol[21]. This was much different from the method to encapsulate the citrate-stabilized gold nanoparticles with silica layer using the silane coupling agent(2-aminopropyl)trimeth-oxysilane(APS)as a primer to reach a higher af?nity of the gold surface to silica[14,23].The amphiphilic character of PVP adsorbed onto the Au nanocolloids enables the af?nity of the gold surface to silica suf?ciently high enough for encapsulation of silica layer without any coupling agent, such as APS.The typical TEM micrograph of Au/SiO2 composites in Fig.1b had core–shell structures with spherical shapes and smooth surfaces,although few of the Au nanocolloids were not coated by silica layer and some of the Au/SiO2core–shell particles contained two or more Au nanoparticles.The size of the Au/SiO2core–shell microspheres was signi?cantly increased from28nm of the Au nanocolloid cores to228nm with narrow dispersity

index(U)of1.026as shown in Table1.This meant that the silica layer with thickness of100nm was successfully encapsulated over the Au nanoparticles via a modi?ed Sto¨ber procedure with the aid of the amphiphilic PVP-10.

Narrow disperse MPS-modi?ed Au/SiO2microspheres were prepared by the hydrolysis of MPS via self-conden-sation reaction between the hydroxyl groups of silica component to incorporate the reactive vinyl groups for further-stage polymerization,which was much similar to the modi?cation of silica particles with MPS in the previ-ous works[23,24].The modi?cation of the Au/SiO2 microspheres via hydrolysis of MPS was con?rmed by FT-IR spectrum as shown in Fig.2a,which displayed the bands at1632and1714cm-1attributing to the stretching vibrations of the vinyl and carbonyl groups of MPS com-ponent,respectively.

Preparation of Au/SiO2/polymer tri-layer microspheres The residual reactive vinyl groups on the surface of polydivinylbenzene(PDVB)microspheres were essential for the growth of the polymer particles[25]and the formation of monodisperse core–shell microspheres via two-stage distillation precipitation polymerization[26],in which the newly formed oligomers and monomers were captured by these reactive vinyl groups in absence of any secondary-initiated particles.In the present work,MPS-modi?ed Au/SiO2particles with vinyl groups on the sur-faces were used as seeds in the further-stage distillation precipitation polymerization for the formation of the polymer shell layer to afford monodisperse Au/SiO2/ polymer tri-layer microspheres as shown in Scheme1.

The TEM micrograph of Au/SiO2/PDVB tri-layer microspheres had spherical shapes with cauli?ower-like surfaces as shown in Fig.3a,which may be due to the rigidity of the PDVB network.The polarity of the polymer shell layer is an important factor in many applications for the resultant multi-layer composite microspheres.The typical TEM micrograph of Au/SiO2/P(EGDMA-co-MAA) tri-layer microspheres was shown in Fig.3c,in which the di-and triple particles were observed originating from the higher reactivity of EGDMA than that of DVB[27]. Polymer microspheres with various functional groups have many applications in many?elds,including solid carriers

Table1The size,size distribution,shell thickness,yield,and the fraction of shell layer of the Au/SiO2/polymer tri-layer microspheres Entry D n(nm)D w(nm)U b Shell thickness(nm)Conversion(%)c

Au2829 1.044––

Au/SiO2a228234 1.02610025

Au/SiO2/P(MAA-co-EGDMA)289293 1.0153112

Au/SiO2/PEGDMA289293 1.0113113

Au/SiO2/PDVB256261 1.016146

a Au/silica particles

b U=D

w

/D n

c Conversion=(M

Au/silica/polymer

-M Au/silica)/M monomer9100%

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for the immobilization of biological substances,such as enzymes,antibodies,etc.[28].The TEM micrograph of Au/SiO 2/PEGDMA in Fig.3e demonstrated that the tri-layer microspheres had spherical shapes with smooth and non-segmented surfaces.All the TEM micrographs in Fig.3a,c,e indicated that the resultant Au/SiO 2/polymer microspheres had typical tri-layer structures in absence of any secondary-initiated particles,in which the gold nano-colloids (deepest contrast)were located in the center of the particles together with a sandwiched silica layer (deeper contrast)and an outer polymer shell layer (light contrast).The reaction conditions,size,size distribution,and the conversion of the resultant Au/SiO 2/polymer tri-layer microspheres for distillation precipitation polymerizations in presence of MPS-modi?ed Au/SiO 2particles as seeds were summarized in Table 1,in which all the monomers used were set at 10/1as mass ratio to MPS-modi?ed Au/SiO 2seeds in order to guarantee the identical

monomer

Fig.3TEM micrographs:a Au/SiO 2/PDVB,b hollow PDVB microspheres containing Au cores adsorbed on the inner surface,c Au/SiO 2/P(EGDMA-co -MAA),d hollow P(EGDMA-co -MAA)

microspheres containing Au cores adsorbed on the inner surface,e Au/SiO 2/PEGDMA,f hollow PEGDMA

microspheres containing Au cores adsorbed on the inner surface

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concentrations for these polymerizations.The diameters of both Au/SiO2/PEGDMA and Au/SiO2/P(EGDMA-co-MAA) tri-layer microspheres were signi?cantly increased from 228nm of MPS-modi?ed Au/SiO2seeds to289nm,while the size of the Au/SiO2/PDVB tri-layer particles was slightly grown to256nm.All the resultant Au/SiO2/poly-mer tri-layer microspheres retained monodispersion with polydispersity index(U)lower than1.016as shown in Table1.These results meant that the shell thicknesses of PDVB,PEGDMA,P(EGDMA-co-MAA)layer were14, 31,and31nm,respectively.The much thinner thickness of the PDVB than those of PEGDMA and P(EGDMA-co-MAA)shell layer was originated from the much lower reactivity of DVB than those of EGDMA and MAA,which was consistent with the results reported in our previous works[26,27].The different reactivity of DVB,EGDMA, and MAA were also con?rmed further by the conversion of the(co)monomers to the corresponding polymer shell layer as illustrated in Table1,in which the yields of Au/SiO2/ PDVB,Au/SiO2/PEGDMA,Au/SiO2/P(EGDMA-co-MAA) tri-layer microspheres were6,12,and13%,respectively.

The successful encapsulation of MPS-modi?ed Au/SiO2 with polymer shell layer having different polarity and carboxylic acid group was proven further by FT-IR spectra as shown in Fig.2.The FT-IR spectrum in Fig.2b of Au/ SiO2/PDVB had a weak peak at710cm-1corresponding to the typical adsorption of the phenyl group of PDVB component.In FT-IR spectrum of Fig.2c for Au/SiO2/ P(EGDMA-co-MAA),the absorption peak at1473and 1397cm-1was corresponding to the methyl in the ester group on the MAA molecule and the peak at761cm-1was assigning to the absorption of two methylene groups on the main-chain for PEGDMA component.Figure2d of the FT-IR spectrum for Au/SiO2/PEGDMA showed the absorption peak at1734and757cm-1contributing to the vibration of the carbonyl unit in ester group and the vibration of the connecting two methylene groups on the PEGDMA network.

Synthesis of hollow polymer microspheres with Au cores adsorbed on the inner surface

The sandwiched silica layer of Au/SiO2/polymer tri-layer microspheres were selectively removed from the particles by the etching process with hydro?uoric acid via formation of SiF4gas to result in the corresponding hollow polymer microspheres with Au cores adsorbed on the inner surface. The typical TEM micrographs of the hollow polymer microspheres with different shell layer containing the Au cores adsorbed on the inner surface were shown in Fig.3b, d,f,respectively.The results in Fig.3b indicated that the hollow PDVB microspheres had partially collapsed parti-cles,as the PDVB shell layer with thickness of14nm was not thick enough to support the cavities formed during the selective etching process of silica mid-layers.Most of the Au nanoparticles were on the wall of hollow PDVB microspheres with an obvious cavity structures for the resultant hollow PDVB particles in Fig.3b,while some of the gold nanoparticles in hollow PDVB microspheres were leached out possibly due to the occurrence of the breaking of the polymer shell during the formation of the hollow structures with much thinner(14nm)and rigid PDVB shell layer.The hollow PEGDMA and P(EGDMA-co-MAA) microspheres with polymer shell thicknesses of31nm in Fig.3d,f had convincing hollow sphere structures with the presence of circular rings of non-aggregated spheres and a cavity in the interior,in which the gold nanoparticles were attached onto the inner wall of PEGDMA and P(EGDMA-co-MAA)shell.In other words,the gold nanoparticles were movable if the cavity of the hollow polymer microspheres was full of suitable solvent,which provide the possibility to utilize these hollow polymer microspheres as a microre-actor as discussed further in this paper.

Hollow polymer microspheres with gold nanocolloid cores adsorbed on the inner surface as catalytic reactor The catalytic reduction of4-nitrophenol(4-NP)to4-ami-nophenol(4-AnP)with NaBH4as reductant was set as a model reaction to test the activity and stability of the hollow polymer microspheres with Au nanocolloid cores adsorbed on the inner surface as a microreactor.An aqueous solution of4-NP has a yellow color and a distinct UV–vis spectrum pro?le with the absorption maximum at around400nm.When4-NP aqueous solution was mixed with NaBH4only,the yellow color of the solution did not change,indicating the absence of the reduction.After the hollow P(EGDMA-co-MAA)microspheres with Au cores adsorbed on the inner surface were introduced into the mixture of4-NP and NaBH4,the yellow color of the solution faded gradually with a signi?cant decrease of the peak at400nm in UV–vis spectra as shown in Fig.4, indicating the successful conversion of4-NP to4-AnP.The catalytic activity were estimated from the reaction time for the complete disappearance of the UV–vis absorption at 400nm of4-NP in Fig.4,which was indicated by the disappearance of the peak at400nm of the UV–vis absorption after31min.In such a case,the hollow P(EGDMA-co-MAA)microspheres with Au cores adsor-bed on the inner surface acted as a catalytic reactor to transfer the electron from NaBH4to4-NP for the formation of4-AnP in aqueous solution.Since Au nanoparticles were located in the cavity of the crosslinked P(EGDMA-co-MAA)hollow microspheres and the hydrophilic nature of the polymer shell layer permitted4-NP,NaBH4,4-AnP molecules in aqueous solution to pass through the polymer

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shell layer,the cavity in the hollow P(EGDMA-co-MAA) microspheres with Au cores adsorbed on the inner surface acted as the catalytic microreactor in the aqueous solution. In the present work,the hollow P(EGDMA-co-MAA) microsphere with Au cores adsorbed on the inner surface as a heterogeneous catalytic microreactor was facilely isolated from the reaction system by simple ultracentrifu-gation and decantation.The heterogeneous microreactor was recovered,washed,dried and then recycled for the reduction of4-NP to4-AnP.The catalytic activity of the hollow P(EGDMA-co-MAA)microspheres with Au cores adsorbed on the inner surface during the recycling was quantitatively determined by the reaction time for the complete disappearance of the UV–vis absorption at 400nm corresponding to4-NP as summarized in Table2. The results demonstrated that the catalytic microreactor retained high activity during the recycling,which was referred by the reaction time keeping at around31min even after three cycles.All these results implied that the hollow P(EGDMA-co-MAA)microspheres with Au cores adsorbed on the inner surface acted as an ef?cient and recyclable catalytic microreactor for the reduction of4-NP to4-AnP in aqueous solution.

To compare the catalytic property of the hollow struc-tures with pure Au nanoparticles,sodium citrate stabilized Au nanoparticles with diameter of23nm were chosen as the comparative catalyst.It should be pointed out that the total surface area of the sodium citrate stabilized gold particles in the catalytic reaction approximately equals to that of Au nanoparticles adsorbed on the inner surface of hollow polymer microspheres used above.Three parallel reactions were processed and the mean reaction time was 420s,which indicated that the catalytic activity of the sodium citrate stabilized23nm Au nanoparticles is better than that(1860s)of the hollow microspheres with Au nanoparticle functionalized movable cores in the?rst cat-alytic cycle of the reduction.This is probably because the outer shell layer blocks the permeation of4-NP molecule to the Au nanoparticles on the surface of movable cores. Despite of this,the Au nanoparticles adsorbed on the inner surface of hollow polymer microspheres still can be fac-ilely separated from the reaction system by centrifugation while the pure Au nanoparticles can hardly be recovered, which showed better recycle-catalytic property after sev-eral reaction times as discussed above.On the other hand, the pure Au nanoparticles would severely aggregate after one time of catalytic reaction according to the results from Yin’s group[29],in which the aggregation would cause sharp decrease of the catalytic property.The hollow polymer microspheres with different polar and functional shell layer containing the metallic cores adsorbed on the inner surface provide the possibility for the utilization of these hollow particles as an ef?cient catalytic microreactor in different environment from non-polar to strong polar system.The investigation of the property and stability of these hollow functional polymer microspheres containing metallic core adsorbed on the inner surface as micro-reactor under different environments is now being carried out in our group.

Conclusion

Au/silica/polymer tri-layer microspheres with different polar and functional shell layer were prepared by distilla-tion precipitation polymerization of DVB,EGDMA,EG-DMA,and MAA in the presence of MPS-modi?ed Au/ SiO2particles as seeds in neat acetonitrile without any additive,in which the inorganic Au/SiO2seeds were pre-pared by the coating of a layer of SiO2onto the gold nanoparticles via a modi?ed sol–gel process with the aid of amphiphilic PVP.The hollow PDVB,PEGDMA, P(EGDMA-co-MAA)microspheres containing Au cores adsorbed on the inner surface were prepared by the selec-tive etching of the sandwiched silica layer with hydro?u-oric acid from the corresponding Au/SiO2/polymer tri-layer microspheres.The hollow P(EGDMA-co-MAA)micro-spheres with Au cores adsorbed on the inner surface were

Table2Reaction time for the complete disappearance of4-NP in the UV–vis spectra during the catalytic reaction after each recycling Recycling no.Reaction time(min)

131

231

331

123

used as a recyclable catalytic microreactor for the effective reduction of4-NP to4-AnP with NaBH4as reductant in aqueous solution.

Acknowledgement This work was supported by the National Foundation of China with project No.:20874049.

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