Hard-templating synthesis of mesoporous carbon spheres with controlled particle size

Materials Chemistry and Physics 129 (2011) 1035–1041

Contents lists available at ScienceDirect

Materials Chemistry and

Physics

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m a t c h e m p h y

s

Hard-templating synthesis of mesoporous carbon spheres with controlled particle size and mesoporous structure for enzyme immobilization

Jitong Wang,Qingjun Chen,Xiaojun Liu,Wenming Qiao,Donghui Long ?,Licheng Ling

State Key Laboratory of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China

a r t i c l e

i n f o

Article history:

Received 16March 2011

Received in revised form 4May 2011Accepted 20May 2011

Keywords:

Nanostructures Sol–gel growth Adsorption Biomaterials

a b s t r a c t

Mesoporous carbon spheres (MCSs)with controlled particle size and pore structure were synthe-sized via a combined hard templating and sol–gel processing within water-in-oil emulsions,using resorcinol–formaldehyde polymer as carbon precursor and colloidal silica nanoparticles as hard tem-plates.The addition of silica nanoparticles in polymer sol not only served as the pore structure reagents but also shortened the gelation time,making it easy to control the emulsion process.The sphere size of MCSs can be controlled in the range from 10to 500?m by changing the emulsi?cation conditions.The pore structure of MCSs can be tuned by adjusting the mass ratio of resorcinol–formaldehyde polymer to silica nanoparticles and the diameter of silica nanoparticles.The as-prepared MCSs possessed large surface area (>600m 2g ?1),large pore volume (>1cm 3g ?1)and a narrow pore size distribution replicated from the silica nanoparticles used.These MCSs exhibited extraordinary high adsorption capacities (ca.1100mg g ?1)for ?-Chymotrypsin (Chy)in solution.Due to their well-developed pore structure and the controllable pore size,as well as the unique shape and good af?nity to biomolecules,the as-prepared MCSs should have a good potential in enzyme immobilization.

? 2011 Elsevier B.V. All rights reserved.

1.Introduction

Enzyme immobilization has attracted continuous attention in the ?elds of ?ne chemistry,biomedicine,and biosensor [1].The use of immobilized enzymes can allow repeated use,facile sepa-ration from reaction mixtures,and prevent enzyme contamination in products as well as enhance the stability [2,3].Numerous sup-ports have been employed for enzyme immobilization,including polymers and resins,silica and silica–alumina composites,and car-bonaceous materials [4–6].Among these supports,porous carbon materials as carrier matrices exhibit some signi?cant advantages,such as biocompatibility,highly developed pores and chemical sta-bility as well as low cost and accessible porosity [7,8].It has been shown that the pore size and pore size distributions of porous car-bon materials have a strong effect on the immobilization of bulky biochemical molecules [9].Generally,mesopores are required to host enzymes and allow an easy transport of substrate into its active site,and importantly,create an environment most favorable for the expression of enzyme activity [10–13].A considerable amount of research on the immobilization of enzymes and adsorption of biomolecules on mesoporous carbon materials can be found in the literature [14–16].

?Corresponding author.Tel.:+862164252924;fax:+862164252914.E-mail address:longdh@https://www.sodocs.net/doc/6a3996890.html, (D.Long).

The ability to control pore size and characters of mesoporous carbons is of immense advantage since these factors can exhibit direct impacts on activity and stability of immobilized enzymes.As such,tremendous recent interests have been triggered in the controllable synthesis of mesoporous carbons.Three main ways are effective to introduce mesopores in carbon materials,includ-ing activation to high burn-off degrees or combination of physical and chemical activation [17–19],replica synthesis with templates [20–22],and carbonization of organic gels issued from sol–gel processing [23,24].By employing the ?rst method,it is dif?-cult to achieve mesoporous carbons with strictly controlled pore structures.The template approach has been proved to be very fea-sible to prepare mesoporous carbons with well controlled pores [25–28].However,the resulting mesoporous carbons are gener-ally in the form of ?ne powder,demonstrating poor mechanical integrity.Sol–gel processing could be effective to introduce 3-D interconnected mesopores and keep high mechanical properties;nevertheless,the furry and complicated supercritical drying makes the resulting materials high cost and limited availability.Recently,several new synthesis routes for the preparations of mesoporous carbons using colloidal silicas as templates and resorcinols as carbon precursors have been demonstrated [29,30].Such surface-templating routes normally lead to carbon replicas with large pore volumes and multi-modal pore sizes.Unfortunately,the pores of these mesoporous carbons are mostly randomly distributed,and that their adsorptive properties are not well understood.Therefore,a facile synthesis procedure for mesoporous carbon with controlled

0254-0584/$–see front matter ? 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2011.05.085

1036J.Wang et al./Materials Chemistry and Physics129 (2011) 1035–1041

pore structures and accessible pore is much in demand for biomed-ical applications.

For enzyme immobilization,the morphology and size of meso-porous carbons are also important factors to be considered.The mesoporous carbon powders cannot be used in most biomedical applications due to dif?culties of separating them from the bio?uid. The spherical morphology with appropriate pore size is an attrac-tive feature to open up possibilities for the creation of supports that are expected to?nd practical applications in enzyme immobiliza-tion and biomedical materials[31].Thus,many synthesis strategies have been developed to form mesoporous carbon spheres so far. Horikawa et al.[32]prepared spherical carbon aerogels by an emulsion polymerization and carbon dioxide supercritical drying method.Lei et al.[33]reported the synthesis of spherical meso-porous carbons by chemical vapor deposition(CVD)method using ferrocene as the concurrent iron/carbon precursor and silica arrays as template.More recently,carbon microspheres with a uniform size of1?m were prepared through a nanocasting technique using uniform spherical mesoporous silicas as sacri?cial templates[34]. However,how to control and tailor the shape of mesoporous car-bons and their pore structures for biomedical applications is still a challenging work.

In this paper,we combined the advantages of hard templating and sol–gel processing and developed a robust method to pre-pare mesoporous carbon spheres(MCSs).This method employed the inverse emulsion sol–gel polymerization of RF monomers and colloidal silica in a paraf?n oil medium,followed by the ambi-ent drying,carbonization and silica template removal.The primary advantages for this method included giving up the troublesome supercritical drying and preserving the3-D rigid carbon network in spherical shape.Moreover,the addition of colloidal silica nanopar-ticles into resorcinol–formaldehyde sol caused the gelation time to be very short(ca.5min at85?C),making it very easy to con-trol the particle size by changing the emulsi?cation conditions. The pore structure of MCSs was reversely replicated from the silica network,which can be tuned by changing the mass ratio of resorcinol–formaldehyde polymer/silica nanoparticles and the diameter of silica nanoparticles.To the best of our knowledge,it was the?rst trial that the mesoporous carbons with controlled spherical shape were prepared via a cost effective hard-templating method.A preliminary study on the physical adsorption of?-Chymotrypsin suggested that the as-prepared MCSs have a good potential in enzyme immobilization.Also this kind of MCSs can be used in other application?elds such as column packing,?llers, adsorption/separation and catalyst support.

2.Experimental

2.1.Preparation of MCSs

The MCSs were prepared via a combined hard templating and sol–gel method within water-in-oil emulsions with resorcinol–formaldehyde polymer as carbon source and colloidal silica sol(LUDOX?SM-30colloidal silica,30wt.%,DuPont Co.) as hard template.In a typical synthesis,4.85g of resorcinol and7.16g of forma-lin(37wt.%formaldehyde)were dissolved in15ml deionized water.Next,25g of colloidal silica sol was dropwise added to the above solution under stirring.The solution was then diluted to50ml.After pre-polymerization at45?C for20min,the solution was transferred to a500ml glass reactor containing a mixture of300ml paraf?n oil and1ml sorbitan monooleate(Span80).Then,the mixture was stirred with a speed of200rpm at85?C for30min and aged at85?C for2days.The as-made products,hybrid hydrogel spheres,were separated from the solvent by?ltration and washed with ethanol and water.The spheres were dried in an oven at80?C for24h to obtain hybrid silica–polymer spheres.The silica–carbon spheres were obtained by pyrolysis of the silica–polymer spheres at800?C for3h in the nitrogen atmo-sphere.After carbon-silica spheres were immersed in10wt.%HF solutions for24h, silica was removed and crake-free MCSs were obtained.

In this work,to control particle size of MCSs,we changed the emulsi?cation con-ditions such as the pre-polymerization period from0to40min,the concentration of Span80from0.3to10g L?1and the stirring speed from100to500rpm.Three different mass ratios of resorcinol–formaldehyde polymer to silica(R)and three kinds of colloidal silica particles with average particle sizes of7,12and22nm were used to control the pore structure under the same emulsion conditions(20min of pre-polymerization time,200rpm of stirring speed and1ml of Span80).The obtained MCSs are denoted as MCS(x)-R,where x is the silica particle size used and R is the mass ratio of resorcinol–formaldehyde polymer to silica.Some hybrid hydro-gel spheres(7)-20/15were dried under supercritical petroleum ether(boiling range 30–60?C)conditions operating at240?C and7.0MPa[35].

2.2.Characterization

The particle size distributions of MCSs were measured with a dynamic light scat-tering analyzer(DLS,Malvern Mastersizer2000)covering a wide size range of0.02 and2000?m.The DLS was operated under the following experimental conditions: dispersant of water,particle concentration of0.0634vol%,temperature of25?C. The morphologies of MCSs were characterized by a scanning electron microscopy (FEI,Qunta300).The microstructures were observed under transmission electron microscopy(TEM,JEOL2100F)operated at200kV.The samples were prepared by dispersing the powder products as slurry in ethanol,which was then deposited and dried on a holy carbon?lm on a Cu grid.Nitrogen adsorption/desorption isotherms were measured at77K with a Micromeritcs ASAP2020analyzer.Before the measurements,the samples were degassed in vacuum at473K for12h.The Brunauer–Emmett–Teller(BET)method was utilized to calculate the speci?c sur-face areas.The total pore volume was estimated from the adsorbed amount at a relative pressure of P P?10=0.985.The micropore volume was calculated by the t-plot method.The pore size distributions were derived from desorption branch by using the Barrett–Joyner–Halenda(BJH)model.

2.3.Adsorption experiment

?-Chymotrypsin(Chy)was purchased from Merck Company.The molecular dimension and molecular weight for Chy are about2.9nm×2.0nm×4.0nm and 25,000,respectively.The isoelectric point(p I)of Chy is8.3.Ultrapure water(18M station Millipore Milli-Q Plus)was used for all adsorption experiments.The pH of solution was adjusted using different buffer solutions.

To determine the adsorption rate,test samples(500mg)were suspended in 250ml of400mg L?1Chy aqueous solution at the initial pH value of8.3(Tris–HCl buffer)and6.1(sodium phosphate buffer),respectively.The mixture was contin-uously shaken in a water bath with a speed of160rpm at25?C.Three milliliter samples was taken from the reactor at pre-determined time intervals,including equilibrium time.The adsorption isotherms were measured at25?C by shaking 100ml of each Chy solution at different initial concentrations for48h with50mg of the carbons.The suspensions were?ltered,and the?ltrates were analyzed by a UV–vis spectrometer(UV-3150,Purkinje General Instrument Co.Ltd.).Cali-bration experiments were done separately before each set of measurements.The wavelength of spectrometer for Chy is280nm.The adsorption capacities(q)were determined according to the following formula:

q=(C0

?C)×V

m

wherein C0is the initial concentration,C is the residual concentration,V is the volume of the solution and m is the mass of the adsorbent.

3.Results and discussion

3.1.Particle size evolution during the preparation process

MCSs were synthesized by a combined hard templating and sol–gel processing within water-in-oil emulsions,which involved two intermediates named the silica–polymer spheres and the silica–carbon spheres.To understand the morphology and parti-cle size evolution during the preparation process,Fig.1gives the typical SEM images and particle size distributions for the three dif-ferent forms of spheres.It can be seen that all spheres are uniform in size and retain the high monodispersity.Moreover,their surfaces are smooth without obvious cracks or contaminations,suggesting the effectivity of this preparation method.The particle size distri-butions of the three spheres derived from DLS are shown in Fig.1d. The average size decreases gradually from336to168and then to 117?m during the preparation process.Huge shrinkage(ca.50%) takes place during the carbonization,as a result of the formation of compact carbon network[36].The spheres experience ca.30% shrinkage in diameter after the silica removal because of the col-lapse of their nanoparticle network arising from the large capillary pressures during the drying.

J.Wang et al./Materials Chemistry and Physics129 (2011) 1035–1041

1037

Fig.1.SEM images of silica–polymer spheres(a),silica–carbon spheres(b)and mesoporous carbon spheres(c)and their DLS particle size distributions.Volume(%)is the volume frequency of particles.

3.2.Particle size control of MCSs

Particle shape and particle size are critically important for effec-tive application of mesoporous carbons.In this work,the colloidal silica sol not only acted as a hard template but also provided a basic solution for the polymerization of resorcinol and formaldehyde, resulting in a shorter gelation time(ca.5min at85?C)than the conventional method(30min or more)without using the colloidal silica sol[37].Therefore,it was very easy to control the particle size during the emulsi?cation process.Herein,three factors,including the pre-polymerization time,the stirring speed and the amount of surfactant were investigated to control the particle size of as-prepared MCSs.

Firstly,a set of MCSs were prepared at different pre-polymerization times,keeping other conditions constant.Fig.2 illustrates the resulting DLS particle size distributions of the obtained MCSs.It reveals that the average particle size exhibits a systematic increase with increasing pre-polymerization time. The smallest particle in this series(35?m)is obtained for a pre-polymerization period of0min,while the particle diame-ters increase to about490?m for the longest pre-polymerization period.This could be interpreted as follows.The sol solution was dispersed into a continuous hot oil phase as primary emulsion droplets by the combined action with steric stabilizer and agi-tation.During the emulsi?cation process,the primary droplets underwent continuous breakage and coalescence and then were stabilized by the fast gelation reaction into hydrogel spheres.The ?nal sphere size depended on the kinetic balance between breakage and coalescence[32].Pre-polymerization could greatly increase the initial viscosity of the sol solution,which was unfavorable for breakage.Therefore,the obtained sphere size increases with the pre-polymerization time extending.

Secondly,the stirring speed was changed from100to400rpm, while holding other conditions invariable.As shown in Fig.3,with the increase of stirring speed,the average particle size decreases from475to19?m.This is apparent because increased stirring speed caused high shear rate in the continuous phase,which was favorable for breakage and splitting of the large droplets into smaller ones[38].

Lastly,the amount of surfactant was also found to determine the particle size and its distribution,as shown in Fig.4.According to Santini et al.[39],all the concentrations studied are above the critical micellar concentration(0.18g L?1)of Span80.By increas-ing the concentration of surfactant from0.3to10g L?1,the mean size of resultant MCSs decreases from770to40?m.Under

contin-

Fig.2.Particle size distributions of MCSs at different pre-polymerization times (200rpm of stirring speed,1ml of Span80).Volume(%)is the volume frequency of particles.

1038J.Wang et al./Materials Chemistry and Physics 129 (2011) 1035–

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Fig.3.Particle size distributions of MCSs at different stirring speeds (20min of pre-polymerization time,1ml of Span 80).Volume (%)is the volume frequency of particles.

uous stirring,the surfactant prevented the formation of deformed spheres by controlling the sol-solvent interfacial tension.Increas-ing the surfactant concentration led to a reduction of the interfacial surface tension,as a result,smaller droplets sizes were usually pro-duced (under the same stirring conditions).

3.3.Pore structure control of MCSs

Colloidal silica sol is a nanometric particle size solution of silica particles in water.The polymerization rate of silica nanoparti-cles is low at room temperature.However,when resorcinol and formaldehyde were added into the colloidal silica solution,the balance would be broken.The hydrogen ions from the surface of colloidal silica tended to dissociate in aqueous solution,facilitat-ing the polymerization of resorcinol and formaldehyde as well as colloidal silica nanoparticles.After sol–gel transformation,the 3-D hybrid silica–polymer network formed in the water-in-oil emul-sion.The network of hybrid hydrogel spheres would be collapsed completely during the ambient drying and pyrolysis.Therefore,after silica removal,the obtained MCSs had a porosity fully repli-cated from the silica network [40].By changing the initial mass ratio of polymer to silica (R )and the silica nanoparticle size,the mesoporous structure of MCSs could be tailored in a wide range.

Fig.5shows the N 2adsorption–desorption isotherms and the resulting BJH mesopore size distributions of the MCSs prepared at different R .All MCSs exhibit similar type-IV isotherms with cap-illary condensation steps occurring at relative wide pressures of 0.7–0.9,indicating their similar mesoporous structures.The high R causes decrease in the amount of adsorbed N 2,revealing

the

Fig.4.Particle size distributions of MCSs at different surfactant concentrations (20min of pre-polymerization time,200rpm of stirring speed).Volume (%)is the volume frequency of

particles.

Fig.5.The nitrogen adsorption–desorption isotherms (a)and the BJH pore size distributions (b)of MCSs prepared at different mass ratios of polymer to

silica.

Fig.6.The nitrogen adsorption–desorption isotherms (a)and the BJH pore size distributions (b)of MCSs templated from silica nanoparticles with different sizes.

J.Wang et al./Materials Chemistry and Physics129 (2011) 1035–10411039

Fig.7.SEM and TEM images of MCSs prepared by the supercritical drying and the ambient drying.(a,c)SEM images of MCSs prepared by the supercritical drying with low and high magni?cation;(b,d)SEM images of MCSs prepared by the ambient drying with low magni?cation and high magni?cation;(e)TEM image of MCSs prepared by the supercritical drying;(f)TEM image of MCSs prepared by the ambient drying.

Table1

Porous properties of as-prepared MCSs.

Samples S BET a(m2g?1)S ext b(cm3g?1)V mic c(cm3g?1)V t d(cm3g?1)D p e(nm)

MCS(7)-10/159317490.09 2.18.1

MCS(7)-15/158344820.18 1.38.0

MCS(7)-20/156704080.14 1.08.0

MCS(12)-20/156095290.10 1.811.9

MCS(22)-20/154762980.09 2.322.5

MCS(7)-20/15-S f9328130.08 3.820.5

MCS(0)-200.22––––

a BET surface area.

b External surface area calculated by t-plot method.

c Micropore volume calculate

d by t-plot method.

d Total por

e volume.

e BJH desorption average pore diameter.

f Mesoporous carbon spheres prepared by the supercritical drying.

1040J.Wang et al./Materials Chemistry and Physics129 (2011) 1035–1041

decrease of pore volume.The pore parameters are summarized in Table1.The BET speci?c surface areas and pore volume decrease from931to670m2g?1and2.1to1.0cm3g?1,respectively,with the change of R from10/15to20/15.As the same silica nanopar-ticle used,the mesopore size distributions of the obtained MCSs are similar as shown in Fig.5b.Their mean mesopore sizes are ca.

8.0nm,approximately equal to the average diameter of the silica nanoparticle(7nm).These results demonstrate that the pore vol-ume of MCSs can be effectively tuned without causing a change of pore size.

Fig.6shows the N2adsorption–desorption isotherms corre-sponding to the MCSs synthesized using silica templates of different sizes(7,12,and22nm).It can be observed that the starting point of the hysteresis loop shifts to high relative pressure with increas-ing silica particle size,suggesting the increase of mesopore size of MCSs.The resultant BJH pore size distributions further con-?rm it.The average pore sizes of as-prepared MCSs are consistent with the respective silica nanoparticle size.Moreover,MCSs sample obtained without silica nanoparticles(MCS(0)-20)almost exhibit no pore with the BET speci?c surface area of0.22m2g?1.It can be concluded that the pores of MCSs are formed from the removal of silica and effectively replicated from the nanoparticles.Due to the same amount of silica used,the smaller silica particle should supply higher pore volume.However,the pore volume presents a reverse trend,increasing from1.0to2.3cm3g?1with increasing silica par-ticle size.The smaller silica particles have higher surface energy and activity,which result in the easy aggregation of small silica particles into large ones.In addition,the small pores may be eas-ily collapsed because of high interfacial surface during the drying process,which is also one possible reason[33].

3.4.The effect of drying method on sphere size and pore structure

During the ambient drying,the network of gel is completely collapsed due to the strong capillary force.To alleviate collapse of the network,it can use the supercritical drying to decrease the capillary force.Herein,we employed both kinds of drying tech-nology to obtain MCSs.For the MCSs prepared by the ambient drying and supercritical drying,the mean particle sizes from DLS are210and300?m,respectively.Obviously,the supercritical dry-ing could effectively restrain the shrinkage of the gel network.Fig.7 compares the morphology of MCSs prepared from two drying tech-nologies.Both samples exhibit similar sphericity.The particle size of MCSs prepared by the supercritical drying is signi?cantly larger than that prepared by the ambient drying,in agreement with the result of DLS.High magni?cation images reveal that the surfaces of MCSs have different features:the former from the supercritical drying has a typical3-D open structure inherited from the hydrogel network,while the latter has relatively dense porosity only origi-nated from the silica network.The microstructures of two samples were further observed by TEM.Both samples consist of highly ram-i?ed and?brous amorphous carbon frameworks with a disordered mesoporous structure.However,the network of MCSs from the supercritical drying is obviously loose than that from the ambient drying,which exhibits a relatively regular and uniform mesoporous structure.

The porosity structures of the two samples were characterized by N2adsorption.As shown in Fig.8,both samples exhibit type-IV curves with an H2-type hysteresis loop,re?ecting their mesoporous characteristic.On comparison with MCSs from the ambient dry-ing,the pore size of MCSs from the supercritical drying is a little larger,and its distribution gets wider,suggesting the supercriti-cal drying could inherit partial network of hydrogel.Therefore,this sample has higher speci?c surface area of932m2g?1and larger pore volume of3.8cm3g?1

.Fig.8.The nitrogen adsorption–desorption isotherms(a)and the BJH pore size dis-tributions(b)of MCSs prepared by the supercritical drying(MCS(7)-20/15-S)and the ambient drying(MCS(7)-20/15).

3.5.Typical adsorption performances of MCSs

The adsorption kinetic of Chy onto MCS-10/15was investigated at two pH values of6.1and8.3using buffered solution,as shown in Fig.9.The adsorbed amount is signi?cantly larger at pH8.3,which is the isoelectric point(p I)of Chy.Under this condition,the net charge of Chy is low and the Coulombic repulsive force between Chy molecules is minimal.Consequently,closer packing of Chy molecules is possible and thus the adsorbed protein concentration increases steadily for the?rst15h and quickly attains its equilib-rium capacity.At pH of6.1,the behavior is remarkably different. Since the protein changes conformation or orientation on the sur-face at pH6.1,there is an intermediate plateau between10and 20h.The reconformation/reorientation process is slower than the initial protein–surface interaction,and is driven by an increase in entropy.Thus,the adsorption capacity begins to increase again after 20h[41]

.

Fig.9.Rate curves of?-Chemotropism at different pH for MCS-10/15.

J.Wang et al./Materials Chemistry and Physics129 (2011) 1035–1041

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Fig.10.?-Chemotropism adsorption isotherms at298K.

The adsorption isotherms of Chy in the pores of MCS(7)-10/15 and MCS(7)-20/15were measured with respect to the equi-librium concentration,as shown in Fig.10.The isotherms are characterized by a sharp initial rise at lower?nal solution con-centration,suggesting a high af?nity between the biomolecules and the carbon surface.Finally,the isotherms slowly increase and reach a plateau.These isotherms basically obey a Langmuir-type isotherm,suggesting monomer coverage of biomolecules on the surface of the host[42].The solid lines in the?gures rep-resent a?t of the experimental data employing the Langmuir model.The equilibrium adsorption capacities of1100mg g?1and 800mg g?1carbon are deduced for MCS(7)-10/15and MCS(7)-20/15,respectively.Note that these MCSs show superior Chy adsorption capacity as compared to other porous carbons and silica adsorbents,such as carbon aerogels(232.5mg g?1)[35], and SBA-15(147mg g?1)[43]under similar conditions.This result suggests that the MCSs with the pore size of8nm could effectively accommodate the Chy molecule with a dimension of 2.9nm×2nm×4.0nm.The superior adsorption performance con-?rms that the as-prepared MCSs exhibit great potential in enzyme immobilization.

4.Conclusion

A facile route has been demonstrated to fabricate meso-porous carbon spheres via combining hard templating and sol–gel process.The particle size of spheres could be tai-lored by the emulsion conditions,the pre-polymerization time, the stirring rate and the concentration of surfactant,whereas the mesostructure was well controlled by the mass ratio of resorcinol–formaldehyde polymer/silica nanoparticles and the diameter of silica source.Owing to their well-developed pore structure and unique shape,as well as the control-lable pore size and good af?nity to biomolecules,these MCSs are promising candidates as hosts for enzyme immo-bilization.These materials are also expected to be properly controlled for purposed applications such as selective adsorp-tion,chromatography,catalyst supports and electrochemical applications.Acknowledgements

This work was supported by National Science Foundation of China(No.50672025)and by Science and Technology Commission of Shanghai Municipality(Nos.065258033and06DZ22003). References

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随着******城市建设规模的不断扩大，******热力公司现在每年冬季供热用煤需要4万多吨，为了保证冬季的正常供暖，秋季储存煤就非常关键，但是热力公司的场地有限，无法大量储存煤，加之供热产生的炉渣占地面积也很大。 为此双方约定由我公司申请30亩土地，其中一半无偿作为热力公司储煤场地，一半用于我公司建设轻集料小型空心砌块生产线使用，同时供热产生的炉渣及时运送到本厂作为生产原料。 充分体现了双方互利互惠的原则。 综上所述，建设轻集料小型空心砌块生产线，即符合国家产业政策，同时为确保******冬季正常供暖，热力公司秋季储煤的问题也得到了解决。 因此，恳请******建设局审批30亩土地作为************有限责任公司建设轻集料小型空心砌块生产线和******热力公司储煤场地为盼。 ************有限责任公司 20**年月日 向政府写申请书范文：向政府申请资金请示范文 县政府： XX镇政府办公楼建于X年X月，迄今XX年，由于该楼建筑时间长，加之建筑质量不好等原因，部分房间的墙体出现裂缝，虽小有修缮但仍存在屋顶掉块、墙围脱落等现象，该楼已存在安全隐患，不适宜继续办公，必须进行修缮。 经多方论证，修缮费用预算为XX万元，因镇政府资金短缺，特向县政府申请修缮办公楼经费，我们一定加强对招投标和工程质量的管理，指派专人负责办公楼修缮事宜，做到专款专用，严格质量、严格纪律，请予以支持为盼。 当否，请批示。 附：XX镇政府办公楼修缮预算开支一览表。 X县X镇人民政府

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申请离职书范文6篇 小泰温馨提示：辞职报告是个人离开原来的工作岗位时向单位领导或上级组织提请批准的一种申请书，是解除劳动合同关系的实用文体。本文档根据辞职报告内容要求展开说明，具有实践指导意义，便于学习和使用，本文下载后内容可随意修改调整及打印。 本文简要目录如下：【下载该文档后使用Word打开，按住键盘Ctrl键且鼠标单击目录内容即可跳转到对应篇章】 1、篇章1：申请离职书范文 2、篇章2：申请离职书范文 3、篇章3：申请离职书范文 4、篇章4：酒店离职申请范文 5、篇章5：酒店离职申请范文 6、篇章6：酒店离职申请范文 为离职写一份离职申请书，本文是小泰为大家整理的申请离职书范文，仅供参考。 篇章1:申请离职书范文

您好!首先感谢您在百忙之中抽出时间阅读我的离职信。 我是怀着十分复杂的心情写这封离职信的。自我进入公司之后，由于您对我的关心、指导和信任，使我获得了很多机遇和挑战。经过这段时间在公司的工作，我在酒店领域学到了很多知识，尤其是办公室合规的相关方面，积累了一定的经验，对此我深表感激。 由于自身存在很多尚不完善的地方，想通过继续学习来 进一步加强自己的能力。为了不因为我个人原因而影响公司的工作，决定辞去目前的工作。我知道这个过程会给公司带来一定程度上的不便，对此我深表歉意。 我会尽快完成工作交接，以减少因我的离职而给公司带 来的不便。为了尽量减少对现有工作造成的影响，我请求在公司的员工通讯录上保留我的手机号码一段时间，在此期间，如果有同事对我以前的工作有任何疑问，我将及时做出答复。 非常感谢您在这段时间里对我的教导和照顾。在平安的 这段经历于我而言非常珍贵。将来无论什么时候，我都会为自己曾经是平安公司的一员感到荣幸。我确信这段工作经历将是我整个职业生涯发展中相当重要的一部分。

企业向政府申请书范文

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在每年冬季供热用煤需要4万多吨，为了保证冬季的正常供暖，秋季储存煤就非常关键，但是热力公司的场地有限，无法大量储存煤，加之供热产生的炉渣占地面积也很大。为此双方约定由我公司申请30亩土地，其中一半无偿作为热力公司储煤场地，一半用于我公司建设轻集料小型空心砌块生产线使用，同时供热产生的炉渣及时运送到本厂作为生产原料。充分体现了双方互利互惠的原则。 综上所述，建设轻集料小型空心砌块生产线，即符合国家产业政策，同时为确保******冬季正常供暖，热力公司秋季储煤的问题也得到了解决。因此，恳请******建设局审批30亩土地作为 ************有限责任公司建设轻集料小型空心砌块生产线和******热力公司储煤场地为盼。 ************有限责任公司 2011年? 月? 日

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对领导尽好最后的责任。要离开企业的这一刻，我衷心向您说声谢谢!也感谢全体同事对我无微不至的关怀，对此我表示诚挚的谢意，也同时对我的离去给企业带来的不便表示深深地歉意。在离开之前我仍将按往常一样尽力将自己的工作做好。 祝企业领导及同事们前程似锦，鹏程万里! 此致 敬礼! 申请人： 申请日期： 岗位辞职申请书2尊敬的领导： 您好! 怀着复杂的心情，我提出辞职的请求。屈指算来，我到公司已有两年多时间了。在这段时间里，虽然我的工作并不能尽善尽美，但在公司同事们的指导下，我尽量严格要求自己尽心尽职.按时按量完成了公司分配的销售任务。 鉴于两年多来我在公司的发展与期望有些距离，加之近来的工作中我常常觉得力不从心，故遗憾的提出辞职。 非常感谢在这段时间里公司对我的培育和关怀，在公司的这段经历对我而言弥足珍贵。将来无论什么时候，我都会为自己曾经是公司的一员而感到荣幸，在公司的这段工作经历也将是我整个职业生涯中相当重要的一部分。 祝公司领导和所有同事身体健康、工作顺利!

Synthesis essay托福作文

Shirley Zhang Mr. Michael Crachiolo AP Language 2 16 October 2016 Ever since the industrial revolution, science and technology have developed at a high speed and exert a profound and significant impact of people’s lives. The introduction of the steam powered manufacture machine has increased the productivity of industry drastically. The production output gradually fulfills people’s basic needs. However, people’s wants can not be satisfied. In today’s society, people become more materialistic. It is human nature that we do not get satisfied for just getting the basic material for living. According to Henry David Thoreau, after guarantee “the grand necessity—warmth, rich food, houses, clothing,” people will turn to the product that can bring them a higher level of satisfaction—luxury ( Economy P474-476). Luxury can serve as a symbol of middle-class or higher social status and a symbol of identity. Therefore, it is able to provide the processer with the emotional satisfaction he needs. And gradually, people focus their needs for spiritual demand on material things, and thus, become more and more materialistic as the demand increase. As Phyllis Rose suggests “We Americans are beyond a simple, possessive materialism.”The writer points out some interesting phenomena in the article Shopping and Other Spiritual Adventures in America Today that shopping has been functioning as a form of therapy.

辞职申请书理由 精选十篇

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此致 敬礼 申请人： XXXX年XX月XX日 辞职申请书理由范文二 尊敬的领导您好: 我进xx已经有几个月了，由于我个人的原因。经过深思熟虑地考虑，我决定辞去我目前在公司所担任的职位。 我非常重视在xx公司内这段经历，也很荣幸成为xx的一员，特别是xx的处事风范及素质使我倍感钦佩。在xx这几个月所学到的知识也是我一生宝贵的财富。也祝所有xx成员在工作和活动中取得更大的成绩及收益! 望领导批准我的辞职申请，并请协助办理相关离职手续(本人在2xx年x月x日离职)。在正式离开之前我将认真继续做好目前的每一项工作。 愿祝xx生意兴隆! 敬请领导同意并批复! 此致 敬礼 申请人： XXXX年XX月XX日 辞职申请书理由范文三