Preparation of Monodispersed Uniform Silica Spheres with Large Pore

Apr 1, 2010 - to synthesize the uniform silica spheres with large pore structures for the ... The reversible-thermosentive property of methylcellulose...
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Preparation of Monodispersed Uniform Silica Spheres with Large Pore Size for Fast Adsorption of Proteins Zheng Zhai, Yujun Wang, Yangcheng Lu, and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China

In this paper, a “temperature-induced gelation” method, combined with microfluidic flows, has been developed to synthesize the uniform silica spheres with large pore structures for the fast adsorption of bovine serum albumin (BSA). The reversible-thermosentive property of methylcellulose (MC) and auxiliary function of poly(ethylene glycol) (PEG) were introduced to realize a fast gelation in a coaxial microfludic device. The effects of temperature and MC concentration on the gelation process were investigated by monitoring the change of viscosity of sol system over time. The silica spheres with different morphologies of internal structure were synthesized by adjusting the concentration of MC. The prepared silica spheres had large pore volumes (>2.0 mL/g) and an average mesopore diameter of >12 nm. Meanwhile, a large amount of macropores existed in the silica spheres. The bimodal mesopore-marcopore structure resulted in a high protein adsorption capacity (590 mg/g) and a fast adsorption rate (reaching equilibrium within 9 h). 1. Introduction The fast and efficient separation and purification of proteins are very important in the biomedical and pharmaceutical industries, especially in the fields of enzymatic catalysis, biosensors, and disease diagnostics.1,2 The adsorption of proteins on inorganic materials can improve the stability of proteins.3 Particularly, mesoporous silica materials, with high surface area, large pore volume, hydrophilic character, chemical and thermal stability, suitable particle morphologies, and toxicological safety, are viewed as the ideal candidates for hosts of proteins.4,5 Recently, the adsorption and separation of proteins on siliceous molecular sieve have attracted the interest of many researchers;6-12 especially, some mesoporous silica materials were used in chromatography for protein purification.13-17 However, the adsorption rate of protein is still relatively slow and the adsorption capacity is also not very high, because of the unsuitable pore structure of the mesoporous materials.5,7,8 Generally, the adsorption capacity is dependent on the pore size of the adsorbent and the amount of adsorption sites on the surface of the material. A pore size slightly larger than the hydrodynamic radius of protein is essential to obtain the high capacities.9,10 The ideal pore structure is a bimodal macroporousmesoporous structure, where the macropores provide the passage ways for proteins to enter and transfer to the adsorption location and the mesopores provide many adsorption locations. Usually, the macroporous-mesoporous structure was prepared by a double-template route, the surfactants were used as a mesoporous template and gas, small droplets, or particles were used as the macroporous templates.18-22 On the other hand, the morphology of mesoporous silica materials is also important for large-scale applications. Silica spheres with large diameters and uniform sizes are desired. Therefore, controllable preparation technologies for silica spheres have been required, especially for spheres with a large spherical diameter, large pore size and pore volume, and bimodal macroporous-mesoporous structures. * To whom correspondence should be addressed. Tel.: +86 1062783870. Fax: +86 1062780304. E-mail address: gsluo@ tsinghua.edu.cn.

Several methods (e.g., cross-flow membrane, vibrating nozzles, microfluid, etc.) could be used to synthesize the narrow-sizedistribution particles. Among these methods, microfluidic devices have recently emerged as the promising tools for synthesis of polymer particles. Over the conventional processes, the microfluidic-assisted technologies allowed the production of polymer particles with an improved control over their sizes, size distributions, morphologies, and compositions.23-27 In our previous work, the micrometer-sized mesoporous silica spheres with uniform and optionally adjustable diameter were successfully synthesized using a gelation process based on polymerization; in a sedimentation bath, a microfluidic device was used to control the shape of droplets, and the silica spheres prepared exhibited excellent separation performance.28-30 Nevertheless, the method must be developed further. In that preparation process, the droplet solidification was accomplished by the assistance of the acrylamide polymerization reaction. The polymerization conditions decreased the ability to control pore structure and size. Besides, the alkali (trioctylamine, TOA) used as one component of the sedimentation bath may destroy the pore structure of silica spheres. In this work, a new method using a temperature-induced gelation has been proposed to prepare the spheres with bimodal structure in a microfluidic device. Methylcellulose (or methyl cellulose, MC) is a chemical compound derived from cellulose, which possesses a reversible thermosentive property. In a certain range of degree of methyl group substitution (1.4-2.0), MC is water-soluble, and it can form a thermo-reversible hydrogel in an aqueous solution, which is a three-dimensional network of polymer chains cross-linked via either physical or chemical bonds. The sol-gel transition of MC in aqueous solutions appeared when the temperature varied.31-40 The characteristics of MC were used to prepare the size-controllable zeolite nanocrystals by controlling the growth rate.41 Some researchers found that, in the sol-gel process, poly(ethylene glycol) (PEG) could be adsorbed on the silica sols in an acidic aqueous solution42 and cause a phase separation by the bridging flocculation.43 Therefore, PEG could accelerate the coagulation and precipitation of colloidal silica particles by a temperature-induced bridging flocculation. Zhang et al44 syn-

10.1021/ie9014815  2010 American Chemical Society Published on Web 04/01/2010

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Figure 1. Schematic of the experimental device.

thesized silica particles with a diameter of 3-10 µm, good dispersibility, and a smooth outer surface when the suitable amount of PEG and MC were simultaneously added. PEG with a high molecular weight (PEG 20000) accelerated the gelation process.45 Until now, the use of MC and PEG as the gelation or solidification agents and pore structure control agents has not been found, especially integrated with a microfludic apparatus. Based on the special functions of MC and PEG in the sol-gel process, we tried to introduce PEG and MC into the prehydrolysis solution of tetraethyl orthosilicate (TEOS) to realize fast gelation, instead of a polymerization reaction. More importantly, a bimodal pore structure in micrometer-sized silica spheres at a relative high temperature (90 °C) was formed. A coaxial microdevice was used to actualize the initial microdispersion and primary gelation process. In the microdevice, the temperature could be accurately controlled. In additional, the viscosity of sol system will increase in the sol-gel process. To investigate the effects of temperature and MC concentration on the gelation speed, the change of viscosity of sol system over time was monitored by measuring the viscosity of silica sol. Eventually, the protein adsorption performance of prepared micrometer-sized silica spheres was determined. 2. Experimental Section 2.1. Materials. Bovine serum albumin (BSA) (purity of g98%) was purchased from Biodee Biotechnology Co., Ltd. (Beijing, PRC). Methyl cellulose (MC) was provided by Sigma-Aldrich Corporation (USA). Tetraethyl orthosilicate (TEOS) (purity of g99.95%) was produced by Xilong Chemical Co., Ltd. (Shantou, PRC). Sorbitan trioleate (Span 85) (purity of g96%) was obtained from China Medicine (Group) (Shanghai Chemical Reagent Corporation, Shanghai, PRC). Hydrochloric acid (HCl, 36.5 wt %) was produced by Beijing Chemical Company (Beijing, PRC). Poly(ethylene glycol) (PEG 20000) was obtained from Yili Fine Chemical Co., Ltd. (Beijing, PRC). All chemicals were used as received. 2.2. Synthesis of Silica Spheres. A novel coaxial microfluidic device was designed to synthesize the silica spheres.

Table 1. Synthetic Conditions and Dimensions of Silica Spheres Dimensionsa

Silica Sol sample

MC (g)

TEOS (g)

PEG20000 (g)

0.01 M HCl solution (g)

ds (µm)

CV (%)

S1 S2

0.5 0.25

2.5 2.5

1.0 1.0

5 5

490 490

2.1 2.5

a The symbol ds represents the average diameter of silica spheres; CV represents the coefficient of variation, which is defined as the ratio of the standard deviation σ to the mean µ (CV ) σ/µ).

Figure 1 shows the experimental scheme and the geometric structures of the coaxial microdevice. This coaxial microdevice was fabricated on two poly(methyl methacrylate) (PMMA) sample plates (40 mm × 40 mm × 3 mm). These two PMMA plates were incised by the ball end mills and then sealed together under an ultrasonic field.46 The main channel (channel 1) was 2.0 mm in diameter, and a long polytetrafluoroethylene (PTFE) tube (∼2.5 m in length, 1.5 mm ID × 2.0 mm OD) was embedded in the channel. A needle (0.2 mm ID × 0.7 mm OD) was inserted into the PTFE tubing coaxially as the inlet of the dispersed aqueous phase. The two side channels (channels 2 and 3) were 1.6 mm in diameter with two needles (1.3 mm ID × 1.6 mm OD) were embedded in them. They were fixed perpendicularly to the main channel as the inlets of the continuous oil phase. The positions of three channels are illustrated in Figure 1. The dispersed aqueous phase was silica sol, which was prepared as follows: 0.5 g MC (sample 1, S1) or 0.25 g MC (sample 2, S2) and 1.0 g of PEG 20000 were dissolved in 5.0 g of 0.01 M HCl aqueous solution while stirring; then, 2.5 g of TEOS was added. The resultant mixture was stirred for 3 days at room temperature to obtain a clear silica sol. The continuous oil phase was a liquid paraffin solution containing 2 wt % Span 85. The synthesis conditions of corresponding samples are listed in Table 1. The average diameter and coefficients of variation of silica spheres was measured according to the scanning electron microscopy (SEM) images.

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Figure 2. Viscosity of the silica sol system with a specific composition (samples S1 and S2) at different temperatures.

Figure 3. “Gelation time” at different temperatures and linear fit results.

The flow rate of the dispersed phase was 0.005 mL/min, and that of the continuous phase was 0.200 mL/min. The aqueous solution was dispersed into the continuous oil phase to form monodispersed droplets. A water bath (90 °C) was used to heat the long PTFE tubing to allow the gelation of droplets. The solidified organic-inorganic hybrid spheres were collected at the exit of the PTFE tubing. The spheres were subsequently treated at 100 °C for 24 h in an autoclave to complete gelation. Finally, the solid product was washed with acetone, air-dried at 80 °C overnight, and then calcined at 550 °C for 6 h. 2.3. Characterization of Silica Spheres. Scanning electron microscopy (SEM) observations were performed on a JEOL Model JSM 7401F microscope operating at 1.0 kV. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Quantachrome Autosorb-1-C Chemisorption-Physisorption Analyzer. Before measurement, the silica spheres were outgassed at 200 °C for 40 min. The (Brunauer-Emmett-Teller) BET surface area was calculated from the adsorption branches in the relative pressure range of 0.05-0.25, and the total pore volume was evaluated at a relative pressure of ∼0.995. The pore size distributions were calculated from the adsorption branches using the Barrett-Joyner-Halenda (BJH) method.47 The gelation process of silica sol was characterized by the change of viscosity over time. A Model NDJ-5S viscometer (Shanghai Jingtian Co. Ltd., of China) was used to determine the viscosity. The experiments were conducted in a flask connected with a water bath. The composition of silica sol was the same as the dispersed aqueous phase (sample S1 or S2). The amount of silica sol used in the measurement process was 70 g. A No. 2 spindle in the Model NDJ-5S viscometer was used at a shear rate of 30 r/min. When viscosity increased to a high value, a slower shear rate (6 r/min) was used. 2.4. Protein Adsorption. The batch adsorption experiments were performed by contacting 50 mg of silica spheres with 50

mM of HAc-NaAc buffer (pH 5.0) with the protein concentration of 10 mg/mL, the mass of buffer solution was 10.0 g. The silica spheres and solution were shaken in a HZS-H Environmental Incubator Shaker (Harbin Donglian Electronic & Technology Development Co. Ltd., of China) at 160 rpm and 25 °C until equilibrium was reached. The protein concentrations in the solution before and after adsorption were analyzed using a UV spectrophotometer (Model HP 8453, Agilent) at 280 nm and the mass conservation law was applied to calculate the amount of protein adsorbed on the silica spheres. 3. Results and Discussion 3.1. Gelation Kinetics. In the gelation process, the main feature was the fast increase of viscosity of the system. Investigation of viscosity was used to semiquantitatively analyze the factors influencing the gelation process. The changing of viscosity of the silica sol system over time at different temperatures in the gelation processes is shown in Figure 2. At the beginning, the viscosity increased slowly; after a while, the viscosity increased sharply at the critical point, indicating the gelation of silica sol. The time corresponding to the critical point of viscosity was defined as the “gelation time”. The “gelation time” at different temperatures is plotted in Figure 3. It can be seen that, for both samples S1 and S2, temperature had a great influence on the rate of gelation process; the gelation time became short with the increase of temperature. For sample S1, when the temperature was 94 °C (367 K), 70 g of silica sol formed a gel within 20 min. Comparing sample S1 with sample S2, sample S1 needed less time to form a gel, because of its higher concentration of MC (see Table 1). With the increase in MC concentration, the probability of collision and cross-linking between the molecular chains became larger, leading to a faster apparent gelation rate.

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Figure 4. SEM images of spheres, external surface, and internal structure of samples S1 and S2 (denoted in Table 1): (a) the image of sample S1, (b) the image of sample S2, (c) external surface of sample S1, (d) external surface of sample S2, (e) internal structure of sample S1, and (f) internal structure of sample S2 (images in panels e and f are observed from the cross-section, by cutting the spheres).

Experiments have also been conducted to verify that, without MC, the silica sol could not be gelated under all the temperature used in our research. Therefore, MC was one of the major factors influencing the gelation formation. Experimental data of ln(1/t) was plotted versus 1/T, which is shown in Figure 3. Arrhenius-type kinetic behavior was observed from the plot, obeying a linear relationship between ln(1/t) and 1/T (1/t is proportional to the apparent gelation rate, and then is proportional to the rate constant k): ln

Ea

( 1t ) ) ln(A′) - RT

(1)

where A′ is a constant related to the pre-exponential factor A, Ea the apparent activation energy, and t the gelation time.

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The slopes of the linear fit line for samples S1 and S2 are 12 477 and 12 521, respectively. The average apparent activation energy Ea of this process was calculated as 1503 J/mol. The results above verify that silica sol formed a gel at high temperature, which was assisted by MC and PEG. 3.2. Characterization of the Silica Spheres. The SEM images of two representative samples are shown in Figure 4. It was determined that the spheres prepared were monodispersed with diameters of ∼490 µm. Figures 4c and 4d show the external surface of the silica spheres, from which the porous structure can be observed, but the porous structure on the surface of sample S1 was denser than that of sample S2. The homogeneous macroporous interior structure is visible in Figures 4e and 4f. Many large macropores (∼1 µm) existed in sample S1, and there was a significant difference between the external surface and the internal structure of sample S1 (see Figures 4c and 4e). For sample S2, the macropore size (∼0.1 µm) was smaller and the external surface and the internal structures of sample S2 were more homogeneous (see Figures 4d and 4f). The gelation process was very complicated. Heat transfer and mass transfer happen simultaneously, and the gelation speed depends on both the composition of solution (especially the concentration of MC) and the temperature. On the one hand, when MC concentration increased (sample S1), the gelation process would be completed faster. This phenomenon depended on the thermodynamics, which is shown in Figure 2. Meanwhile, when the gelation speed was fast, the concentration gradient of sol particles in the droplet was large. The sol particles would diffuse to the outside, aggregate and then the gelation process happened on the surface of the droplet. Therefore, for sample S1, the final structure was more asymmetric. However, for lower MC concentrations (sample S2), the gelation speed was slow. The gradient of sol particles along the radius was small, so the difference between the interior and outer structure was small. Therefore, the gelation process depended not only on thermodynamics but also mass transfer. On the other hand, when MC concentration increased, MC occupied more space in the spheres. After removal of MC, the reticular structure in the interior of sample S1 was much more obvious than that of sample S2. The detailed mesopore structure of silica spheres was determined by nitrogen adsorption-desorption measurements. The nitrogen adsorption-desorption isotherms of two representative samples are plotted in Figure 5 and physicochemical properties are listed in Table 2. Both had large pore volumes (>2.0 mL/g), which were quite suitable for the adsorption of large biomolecules. The pore size distribution calculated by the

Figure 5. Nitrogen adsorption-desorption isotherm of samples S1 and S2, and its pore-size distribution calculated by the BJH method using the adsorption branches (inset). (Legend: (b) the adsorption isotherm and (O) the desorption isotherm.)

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Table 2. Physicochemical Properties of Representative Spheresa sample SBET (m2/g) Vtotal (mL/g) DA (nm) DBJHA (nm) DBJHD (nm) S1 S2

309 390

2.14 2.00

13.8 20.5

11.5 31.8

8.9 17.7

Table 4. Amount of BSA Adsorbed onto S1 and Adsorbed BSA Occupied Volume

sample

BSA adsorbed (mg/g)

volume occupied, VBSA (mL/g)

Vtotal (mL/g)

VBSA/Vtotal (%)

S1 S2

590 420

0.60 0.43

2.14 2.00

28 22

SBET ) multipoint BET surface area; Vtotal ) total pore volume; DA ) average pore diameter; DBJHA ) BJH method adsorption pore diameter; and DBJHD) BJH method desorption pore diameter. a

Figure 6. BSA adsorption kinetics on (9) sample S1 and (b) sample S2. (Conditions: pH 5.0, 25 °C, [BSA]0 ) 10 mg/mL.)

BJH method using the adsorption branch showed a broad peak at ∼11.5 nm of sample S1 and ∼31.8 nm of sample S2 (seen in the inset), verifying that mesopores exist in the framework structure. Combining the results from Figure 4 and Table 2, we can clearly see the bimodal mesopore-marcopore structure in the synthesized silica spheres. The pore volume of sample S1 was larger than that of sample S2, which meant the pore volume increased with the increase of MC concentration. This result could be explained by the fact that, with higher MC concentration, more space appeared after calcinations. 3.3. Discussion of the Mechanism. In the silica sol, the MC molecular chains randomly intertwist with weak interactions. In the gelation process, when the temperature increased up to 90 °C, the gel formed. It could be assumed that MC, PEG, and primary silica particles all contributed to the preliminary gelation of the sol particles. MC was the only substance with a reversible-thermosensitive property, so it was an indispensable and important component. The mechanisms of thermogelation of aqueous MC solutions were previously proposed.20,48,49 Generally, when the sol was heated, the gelation process can be divided into three steps: (1) The hydrogen bond interaction between polymer and water molecule became weaker; (2) The interaction between the hydrophobic groups of side or terminal chain of MC molecule and chain of PEG enhanced, and the hydrophobic junctions were intensified; and

(3) When the amount of the molecular clusters exceeded the critical value, the gel was formed. MC was assumed to have two important functions: (i) MC was the main substance accomplishing the gelation or the solidification process, and (ii) it formed a three-dimensional network structure inside the silica spheres; the network was helpful for the gelation process. As for the role of PEG, PEG with a high molecular weight could be adsorbed on silica sols. Therefore, it accelerated the coagulation and precipitation of colloidal silica particles by a temperature-induced bridging flocculation.42,43 Moreover, during the hydrothermal process, MC molecules interacted with the PEG molecules. The molecular chains of MC and PEG twisted together and accelerated the gelation of silica sol. The mixture of PEG and MC had a cooperative effect on the morphology of silica spheres. 3.4. Protein Adsorption. To investigate the adsorption capacity of large biomolecules onto the silica spheres, BSA was used as a model protein. BSA is a large protein with the molecular weight of 69 kDa, dimensions of 4 nm × 4 nm × 14 nm, and an isoelectric point of 4.9.50 Samples S1 and S2 were used for the protein adsorption experiments. BSA adsorption kinetics at pH 5.0 with 50 mM of acetate buffer for 96 h with an initial BSA concentration of 10 mg/mL are shown in Figure 6. The pH value used was close to the isoelectric point of BSA (pI ) 4.9); under these conditions, the net charge of BSA was very low, so the repulsive electrostatic interaction was minimal. Therefore, the highest adsorption amount was achieved. An adsorption capacity as high as 590 mg/g on S1 was obtained (see Figure 6). Meanwhile, the adsorption rate was fast. The adsorption saturation of BSA was reached in 9 h for S1. The comparisons with other results are listed in Table 3. From ref 5, it can be seen that MCM-41 with a very small pore diameter could only adsorb BSA on the external surface. The SBA-15 shown in refs 6-8, with larger pore diameters, adsorbed more BSA. However, the adsorption capacity was not high enough, and the adsorption rate was slow. Besides, the SBA15 carriers used in other experiments were small particles, which was not suitable for large-scale processes. In our previous work,29 the equilibrium time for adsorption was 24 h, which was slower than the results shown in this work. Theoretical volume occupied by BSA in the silica spheres has been calculated assuming the BSA molecule to be an ellipsoid with dimensions of 4 nm × 4 nm × 14 nm,50 and the results are listed in Table 4. It can be seen that BSA molecules occupied ∼28% of the total volume in sample S1 and ∼22%

Table 3. Adsorption Results of BSA on Mesoporous Siliceous Molecular Sieve ref

type

BSA adsorbed (mg/g)

time (h)

particle size

SBET (m2/g)

Vtotal (mL/g)

DBJHA (nm)

6 7 8 5

SBA-15 SBA-15 SBA-15 MCM-41

20 450 482 150

146 ∼30 24

spherical particles, 4-10 µm fiberlike, ∼30 µm in length unshaped particles, ∼400 µm nanometer

768 570 1104 890

1.63 2.28 3.99 0.79

12.7 24 16.7 1.88

520

24

612

2.57

32.1

29

spheres, ∼300 µm

* SBET ) multipoint BET surface area; Vtotal ) total pore volume; and DBJHA ) BJH method adsorption pore diameter.

remarks

adsorpted on the external surface

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Figure 7. Nitrogen adsorption-desorption isotherms of samples S1 and S2 before and after BSA adsorption and its pore size distributions calculated by the BJH method using the adsorption branches (see insets; the upper line: before BSA adsorption; the lower line: after BSA adsorption). Solid symbols represent the point on the adsorption isotherm, whereas open symbols represent the point on the desorption isotherm ((b, O) before BSA adsorption and (9, 0) after BSA adsorption). Table 5. Physicochemical Properties of Samples S1 and S2 before and after BSA Adsorptiona sample S1 S1 S2 S2

(before adsorption) (after adsorption) (before adsorption) (after adsorption)

SBET (m2/g)

Vtotal (mL/g)

DA (nm)

DBJHA (nm)

DBJHD (nm)

309 169 390 247

2.14 1.15 2.00 1.24

13.8 27.4 20.5 20.1

11.5 23.2 31.8 31.8

8.9 17.7 17.7 17.7

a SBET ) multipoint BET surface area; Vtotal ) total pore volume; DA ) average pore diameter; DBJHA ) BJH method adsorption pore diameter; and DBJHD ) BJH method desorption pore diameter.

in sample S2. It was a relatively high utilization ratio of the pore volume. To investigate whether BSA molecules enter the pores of spheres or just adsorb on the external surface of silica spheres, nitrogen adsorption-desorption isotherms of samples before and after BSA adsorption are shown in Figure 7, and the physicochemical properties of samples before and after BSA adsorption are compared in Table 5. After adsorption, total pore volume decreased from 2.14 mL/g to 1.15 mL/g for sample S1; from 2.00 mL/g to 1.24 mL/g for sample S2. The decrease of total pore volume and surface area after BSA adsorption evidently demonstrated that BSA molecules occupied part of pore volume. Since the external surface area of the spheres was very low (400 µm in size) with a bimodal macroporous-mesoporous structure in

a microfluidic device. The reversible-thermosensitive property of methylcellulose (MC) and auxiliary function of poly(ethylene glycol) (PEG) were introduced to the gelation process of droplets. Besides, MC and PEG were also considered as macropore structure templates. The silica spheres with different morphology of internal structure were synthesized by adjusting the concentration of MC. The prepared silica spheres had a large pore volume (>2.0 mL/ g), a homogeneous structure, and a large amount of macropores. A high protein adsorption capacity (590 mg/g) and fast adsorption rate (9 h) to bovine serum albumin (BSA) were achieved. The high content of texture pores on the surface and the bimodal macroporous-mesoporous structures of the interior were the key factors influencing the adsorption rate and capacity. Acknowledgment We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 20676066, 20976096, and 20525622) and National Basic Research Program of China (No. 2007CB714302) on this work. Literature Cited (1) Martin, B. D.; Gaber, B. P.; Patterson, C. H.; Turner, D. C. Direct Protein Microarray Fabrication Using a Hydrogel Stamper. Langmuir 1998, 14, 3971. (2) Inglis, W.; Sanders, G. H.; Williamsan, P. M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. A Simple Method for Biocompatible Polymer Based Spatially Controlled Adsorption of Blood Plasma Proteins to a Surface. Langmuir 2001, 17, 7402. (3) Klibanov, A. M. Immobilized Enzymes and Cells as Practical Catalysts. Science 1983, 219, 722. (4) Hartmann, M. Ordered Mesoporous Materials for Bioadsorption and Biocatalysis. Chem. Mater. 2005, 17, 4577. (5) Katiyar, A.; Ji, L.; Smirniotis, P. G.; Pinto, N. G. Adsorption of Bovine Serum Albumin and Lysozyme on Silicious MCM-41. Microporous Mesoporous Mater. 2005, 80, 311. (6) Katiyar, A.; Yadav, S.; Smirniotis, P. G.; Pinto, N. G. Synthesis of Ordered Large Pore SBA-15 Spherical Particles for Adsorption of Biomolecules. J. Chromatogr. A 2006, 1122, 13. (7) Katiyar, A.; Ji, L.; Smirniotis, P. G.; Pinto, N. G. Protein Adsorption on the Mesoporous Molecular Sieve Silicate SBA-15: Effects of pH and Pore Size. J. Chromatogr. A 2005, 1069, 119. (8) Nguyen, T. P. B.; Lee, J.-W.; Shim, W. G.; Moon, H. Synthesis of Functionalized SBA-15 with Ordered Large Pore Size and Its Adsorption Properties of BSA. Microporous Mesoporous Mater. 2008, 110, 560. (9) Deere, J.; Magner, E.; Wall, J. G.; Hodnett, B. K. Adsorption and Activity of Cytochrome C on Mesoporous Silicates. Chem. Commun. 2001, 5, 465.

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ReceiVed for reView October 3, 2009 ReVised manuscript receiVed January 30, 2010 Accepted March 18, 2010 IE9014815