Crystal Plane- and Size-Dependent Protein Adsorption on Nanozeolite

Sep 24, 2009 - In this study, nanozeolites LTL with different exposed crystal planes and sizes were .... with changing pH from 3.0 to 11.0 were detect...
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Crystal Plane- and Size-Dependent Protein Adsorption on Nanozeolite Yuan-Yuan Hu, Ya-Hong Zhang,* Nan Ren, and Yi Tang* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: April 30, 2009; ReVised Manuscript ReceiVed: July 31, 2009

In this study, nanozeolites LTL with different exposed crystal planes and sizes were synthesized as excellent model material to study the effects of the crystal plane and size of the nanozeolite on the protein adsorption behaviors. A larger protein adsorption amount is observed on the smaller nanocrystals due to their larger surface area. More importantly, it is found that the (001) crystal plane with a 12-membered ring channel array has a larger contribution for protein adsorption on zeolite LTL nanocrystals than the other two dimensions with a very small pore opening (1.5 Å). It is proposed that the difference of protein adsorption on various crystal planes could be attributed to abundant exposed pore openings on the top (bottom) surface and curved surface of the side surface in columned nanozeolites LTL. This fact shows that the nanoscaled topography is also an important factor determining protein adsorption on the surface of nanozeolites, and efforts in the future should be focused on the synthesis of nanozeolites LTL with abundant exposed (001) planes. This observation will provide a new view for bioapplication and design of crystalline nanomaterials. 1. Introduction Zeolitic-type materials are widely used as catalysts, ionexchangers, and adsorbents due to their well-defined microporosity with molecular size as well as their controlled active sites.1 However, when the size of zeolite crystals is decreased to nanoscale, their external surface areas increase dramatically so that their surface-related properties are remarkably exhibited, such as adjustable surface charge, hydrophilicity/hydrophobicity, and surface ion-exchangeability. These rich surface properties would provide attractive parameters for the exploration of adsorption and reaction of bulky biomolecules.2 In our previous studies, nanozeolite particles were found to possess amazing protein and peptide adsorption amounts, and different nanozeolite materials displayed different adsorption capacities due to their various surface characteristics.3–5 Moreover, the interactions between nanozeolites and protein molecules could be well adjusted by varying the frameworks and surface properties of nanozeolites,3–5 which can be supported by a big family of more than 150 zeolite types with different frameworks and morphologies. Therefore, by adsorbing enzymes on the surface of appropriate nanozeolites, the immobilized enzyme microdevice with high-sensitive biocatalytic function and long-life biosensor property could be constructed.4,5 Furthermore, different from mesoporous silica materials,6 nanozeolites exhibited a multilayer protein adsorption mechanism on the their surface,4b which will endow nanozeolite particles with larger protein adsorption capacity. In addition, high chemical and environmental stability of zeolite materials will also favor their applications under some harsh conditions compared to other nanomaterials. These facts trigger us to thoroughly investigate the effects of surface structures (shape, size, crystal plane, and so on) and compositions of nanocrytals on their performance, which will be very important for the reasonable application of nanozeolite materials. * To whom correspondence should be addressed. E-mail: zhangyh@ fudan.edu.cn (Y.-H.Z.), [email protected] (Y.T.). Fax: (86-21)65641740. Tel: (86-21)5566-4125.

Zeolite LTL is a crystalline aluminosilicate with hexagonally arranged cancrinite cages along the c direction.7 As a result, zeolite LTL normally exists as an approximate cylinder morphology with 12-membered ring (12MR) one-dimensional channels of 7.1 Å on the (001) direction (c axis) and nonplanar 8MR with openings of about 1.5 Å on the other two directions (Figure 1a).7,8 This structure provides a visible difference of crystal plane between the c direction and the other two dimensions. More importantly, the synthesis of LTL with controlled size and facet has been reported by several groups.9,10 Therefore, nanozeolite LTL is apparently a good candidate as an adsorption support to study the effects of size and crystal plane on the performance of nanozeolites. In addition, the interest in the nanozeolite LTL is also based on its wonderful enzyme immobilization and adsorption capacity compared to other nanozeolites.4,11 Focused on the effects of crystal plane and size of nanozeolite on the protein adsorption, in the present study, nanozeolites LTL with different exposed crystal planes and sizes were synthesized and studied. It is found that the (001) crystal plane with a 12MR channel array has a larger contribution for the protein adsorption than the other two dimensions with a very small pore opening (1.5 Å). External surface area, surface charge density, as well as a special pore-mouth effect and surface curvature are proposed to explain the results. This observation will provide a new view for the application and design of crystalline nanomaterials. 2. Experimental Method 2.1. Materials. Fumed silica (Aerosil400, Shanghai Chlorine Alkali Industry), aluminum foil (g99.5%, Sinopharm Chemical Reagent Co., Ltd.), KOH (g82%, Shanghai Reagent Factory), and distilled water were employed for the preparation of nanozeolite LTL. All the proteins (cytochrome c from a horse heart, myoglobin from a rat, and transferrin from a human) for the adsorption were obtained from Sigma. 2.2. Preparation of Nanozeolites LTL. All the referred nanozeolite samples with different sizes and exposed crystal planes are hydrothermally synthesized in the precursor solution

10.1021/jp903989p CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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Figure 1. Top and side views of zeolite LTL with a cylinder morphology (a). SEM and TEM (inset) images of as-prepared zeolite LTL nanocrystals with different sizes (length × diameter, l × d, nm × nm): 170 × 80 (b), 120 × 60 (c), and 40 × 30 (d).

of 4-5.5K2O/10SiO2/0.5-1A12O3/140-170H2O in the presence of microwave irradiation.10 The summary of the synthesis conditions and compositions of the starting gel is shown in the Supporting Information as Table S1. The microwave-assisted synthesis procedure was divided into two steps according to the hydrothermally treated temperature. The synthesis solution was first treated at 80 °C for 90 min under microwave irradiation, and then the synthesis temperature was increased to facilitate the crystal growth. The resulting nanocrystals were cooled to room temperature and then rinsed with distilled water four to five times by centrifugation at 15 000 rpm. The size of all the as-prepared nanozeolites was determined by sizecalculated software on the basis of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. 2.3. Adsorption of Proteins on the Nanozeolites LTL. The adsorption was carried out for 1.5 h to reach the equilibrium at room temperature by the incubation of protein (0.1 mg ml-1) and nanozeolite (0.1 mg ml-1) mixture solutions at phosphate buffer solutions (PBS, 20 mM, 0.5 mL) with different pHs changing from 3.0 to 11.0. After centrifugation, the UV-vis absorption value (220 nm) of the supernatant solution was measured to calculate the amounts of protein adsorbed on the nanozeolites under different pHs. To calculate the adsorption amounts accurately, a standard curve at λ ) 220 nm was done by using a series of protein solutions with different concentrations. 2.4. Characterization. SEM and TEM images were obtained on Philips XL 30 and JEOL JEM-2010 instruments, respectively. Powder X-ray diffraction (XRD) patterns were taken on a Rigaku D/MAX-rB diffractometer with Cu KR radiation at 40 kV and 60 mA. UV absorbance of the protein solution was recorded on a Shimadzu UV-2450 spectrophotometer. ζ potentials of nanozeolites (0.1 mg ml-1) dispersed in PBS (20 mM) with changing pH from 3.0 to 11.0 were detected by a MALVERN Nano-ZS90 instrument at 20 °C. 3. Results and Discussion 3.1. Characterization of Nanozeolites LTL. Nanozeolites LTL with different sizes and exposed crystal planes are controllably synthesized by adjusting synthesis time and temperature as well as gel compositions under microwave irradia-

tion,10 and detailed synthesis conditions are shown in Table S1 (Supporting Information). Figures 1b-d and 2 display the morphologies of all the as-prepared nanozeolites with different sizes and exposed crystal planes. Obviously, all the nanozeolites possess a uniform nanosize and cylinder-like morphology, which provides perfect candidates for further study of protein adsorption behavior. Their mean sizes are calculated by measuring at least 100 nanoparticles, and the standard deviations of size distribution of all the samples are shown in Table S2 (Supporting Information). Their XRD patterns (Figure S1, Supporting Information) reliably confirm their framework of zeolite LTL crystals. 3.2. Effect of Size of Nanozeolite LTL on the Protein Adsorption. Three model proteins with different isoelectric points (pI) (cytochrome c, pI ) 10; myoglobin, pI ) 7.0; and transferrin, pI ) 5.9) are used to study adsorption characteristics of nanozeolites LTL with different sizes in 20 mM PBS with various pH values. Figure 3a shows the adsorption amount of cytochrome c on nanozeolite LTL with different sizes under various pH values. Obviously, nanozeolite possesses a high protein adsorption capacity in a proper pH range, and the adsorption amount increases with decreasing size. The other two proteins display the same size-dependent adsorption trend, although their changes of adsorption behavior with pH value are different from that of cytochrome c due to the different properties (e.g., pI) of these proteins (Figure 3b,c). The adsorption that occurred on the material is generally believed to be governed by its surface area and surface property.12 The electrostatic interaction plays a dominant role in the adsorption process, and adsorption free energy mainly depends on the ζ potential of materials.12 Figure 3d displays ζ potentials of nanozeolites LTL with different sizes under various pH values, and all the nanozeolite particles exhibit negatively charged surface in the pH range from 3.0 to 11.0. Table 1 summarizes the external surface area of one zeolite nanoparticle (sparticle) and total zeolite nanoparticles (Stotal) as well as their surface ζ potential, surface charge (q), and surface charge density (q/ sparticle) to investigate their relationship with the protein adsorption amount at pH 7.0. All the calculations of surface area of nanozeolites are referred to the equations in the following section

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Figure 2. SEM and TEM (inset) images of as-prepared zeolite LTL nanocrystals with different l × d (nm × nm): 230 × 120 (a), 230 × 90 (b), 200 × 120 (c), and 200 × 90 (d).

Figure 3. Protein adsorption amounts on different nanozeolites LTL (length × diameter, l × d) under different pHs: cytochrome c (a), transferrin (b), and myoglobin (c). ζ potentials (d) of the corresponding nanozeolites LTL.

(section 3.3). Surface charge q can be obtained from the ζ potential by eq 1:13

q ) 4π · ε · r0 · ζ

(1)

where r0 is the equivalent spherical radius corresponding to each nanozeolite with a cylinder-like morphology14 and ε is the dielectric constant of the solution. With the decrease of particle size, the total external surface (Stotal) of nanozeolites LTL in

the solution increases, which provides a larger surface for the protein adsorption. However, although decreasing particle size leads to a decrease of ζ potential and surface charge of each nanoparticle, surface charge per unit surface area of nanozeolite (charge density, q/sparticle) only slightly increases with the decrease of particle size (Table 1). Accordingly, the larger protein adsorption amount of nanozeolite with smaller size mainly is attributed to its large external surface area. Comparison with those of other three nanozeolites with larger size, the apparently higher surface charge density of the nanozeolite with

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scylinder ) sside + stop + sbottom ) sside + 2stop )

the smallest size of 40 × 15 nm (length × radius, l × r) might be attributed to its abundant surface defects due to its extreme small size, which further enhance the protein adsorption amount of the smallest nanozeolite particles (Figure 3a-c). 3.3. Effect of Crystal Plane of Nanozeolite LTL on the Protein Adsorption. Crystallinity is one trait of nanozeolite materials distinguished from other amorphous silica materials. Thus, when an enzyme is immobilized on them, the anisotropic crystal planes of nanozeolites could bring different interactions with protein molecules and thereby exhibit some different effects on the adsorption or immobilization of the proteins. Herein, four nanozeolites LTL with either different diameters but same length or different lengths but same diameter are designedly employed to study their protein adsorption behavior. The changes of protein adsorption amount with pH value and ζ potential of these nanozeolite particles are shown in Figure 4. It is worthy recalling that all the nanozeolites LTL have the same mass concentration (mg ml-1) during protein adsorption; therefore, all the adsorption solutions comprise the same mass of nanozeolite although the numbers of nanoparticle in the different solutions are different due to their size difference. Considering this fact and approximate cylinder morphology of nanozeolites LTL, the relationship between mass (m) and particle numbers (n) of nanozeolites in the adsorption solution can be written as follows (eqs 2 and 3):

m ) F · V ) n · F · V ) n · F · πr2l n)

2πrl + 2πr2 (4) in which sside and stop are side and top (or bottom) areas of each cylinder. Therefore, the total surface area (Stotal) of all the nanozeolite particles in the solution can be expressed by n and the surface area of one nanozeolite particle (eq 5):

Stotal ) Sside + 2Stop ) n(sside + 2stop) ) V 1 1 (2πrl + 2πr2) ) 2V + r l πr2l

(

(5)

Sside )

2V r

(6)

2Stop )

2V l

(7)

Sside and Stop correspond to the total areas of the side and top (bottom) of the nanozeolites LTL in the adsorption solution, which can be written as eqs 6 and 7, according to eq 5, respectively. That is to say, when two nanozeolite samples comprise the crystals with the same radius, that is, r1 ) r2 (Figure 5, view b), they will possess the same total side area (Sside). In this instance, the total surface area difference (∆S) of the two samples with the same radius is that of the total top and bottom areas (2∆Stop), as shown below (eq 8):

(2)

V πr2l

)

(3)

(

∆S ) 2V where V is total volume of all the nanozeolites in solution and V is that of one nanocrystal. F is the density of nanozeolites, and r and l represent the radius and length of nanozeolites with a cylinder-like morphology, respectively. For a cylinder, its surface area could be expressed by eq 4:

)

1 1 ) 2∆Stop l2 l1

(8)

In contrast, when two samples are composed of nanozeolites particles with the same length, that is, l1 ) l2 (Figure 5, view a), they have the same total top and bottom area (2Stop), and

TABLE 1: Summary of External Surface Area, ζ Potential, and Surface Charge Density of Nanozeolites LTL with Different Sizes (Length × Radius, l × r) sample size (l × r, nm × nm)

Stotala (cm2 g-1)

ζ (mV) pH ) 7

qparticle (C) pH ) 7

sparticleb (cm2 particle-1)

q/sparticle (C cm-2)

230 × 60 170 × 40 120 × 30 40 × 15

4.2V × 105 6.18V × 105 8.33V × 105 18.3V × 105

-40.3 -31.5 -29.1 -19.6

13.74πε × 10-9 7.42πε × 10-9 5.04πε × 10-9 1.48πε × 10-9

34.8π × 10-11 16.80π × 10-11 9.00π × 10-11 1.65π × 10-11

39.52ε 44.15ε 55.96ε 89.80ε

a Stotal and V are the total surface area and volume of nanozeolites in the adsorption solution, respectively. b sparticle is the surface area of one nanozeolite particle.

TABLE 2: Adsorption Amount Difference of Myoglobin on the Nanozeolites LTL with Different Exposed Crystal Planes at pH 7.0 sample size (l × r, nm × nm)

adsorption amount (A, mg g-1)

230 × 45 200 × 45 230 × 60 200 × 60

145 195 110 155

sample size (l × r, nm × nm)

A (mg g-1) pH ) 7.0

200 × 60 200 × 45 230 × 60 230 × 45

155 195 110 145

∆A (mg g-1)

2∆Stop (cm2 g-1)

∆A/2∆Stop (mg cm-2)

50

1.30 × 104V

38.5 × 10-4 V-1

45

1.30 × 104V

34.6 × 10-4 V-1

∆A (mg g-1)

∆Sside (cm2 g-1)

∆A/∆Sside (mg cm-2)

40

1.11 × 105V

3.63 × 10-4 V-1

35

1.11 × 105V

3.15 × 10-4 V-1

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Figure 4. Protein adsorption amounts on the different nanozeolites LTL (length × radius, l × r) under different pHs: transferrin (a), myoglobin (b), and cytochrome c (c). ζ potentials (d) of the corresponding nanozeolites LTL.

the ∆S of the two samples is the difference of their total side areas (∆Sside, eq 9):

(

∆S ) 2V

)

1 1 ) ∆Sside r2 r1

(9)

Thus, the differences of protein adsorption capacities on different crystal planes could be easily calculated according to the external surface areas and the protein adsorption amounts of the samples with different exposed crystal planes. Table 2 displays the difference of the myoglobin adsorption amount (calculated from Figure 4) on the nanozeolites with different r (45 or 60 nm) and l (200 or 230 nm) at pH 7.0. It is found that the change of adsorption amount per square centimeter over the change of the top and bottom area (∆A/ 2∆Stop) is larger than that for the side area (∆A/∆Sside), and the former is even 10 times of the latter. This fact indicates that the top (bottom) surface [(001) crystal plane] of nanozeolites LTL has a larger contribution for protein adsorption on them; that is, the protein adsorption capacity of the (001) plane with 12 MR channels is much larger than that of the other two dimensions with 8MR openings. Taking the protein property into account, the adsorption behaviors of three model proteins (cytochrome c, myoglobin, and transferrin) are also considered under other pH values of 5.0 and 9.0 (Tables S3-S5, Supporting Information), and all the results are summarized in Figure 6. Notably, for the proteins with different pIs, the results under pH 5.0 and 9.0 are similar to that of myoglobin under pH 7.0, implying the reliability of the above observation.

Figure 5. Scheme of two regular partitions of zeolite LTL with a cylinder morphology.

What is the reason that the (001) crystal plane displays high protein adsorption capacity? The surface charge density of different crystal planes of a single nanocrystal is first considered because the protein adsorption process is normally driven by electrostatic interactions. According to eq 4, the surface area difference of two nanozeolite particles with a different l or r can be easily deduced. For two nanozeolite particles with the same r, their surface area difference can be described as eq 10:

∆s ) 2πr(l2 - l1) ) ∆sside

(10)

However, for two nanozeolites with the same l, their surface area difference can be calculated by eq 11:

∆s ) 2πr2l + 2πr22 - 2πr1l - 2πr21 ) 2πl(r2 - r1) + 2π(r22 - r21) ) ∆sside+2top

(11)

In this way, the changes of surface charge over the surface area difference of two nanozeolites with different exposed crystal

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Figure 6. Summary of protein adsorption amount change per square centimeter of the top area change (∆A/2∆Stop) and that per square centimeter of the side area change (∆A/∆Sside) under different pHs.

TABLE 3: Surface Charge Change per Unit Area Difference between Two Nanozeolites LTL with Different Exposed Crystal Planes at pH 7.0 sample size (l × r, nm × nm)

ζ (mV)

230 × 45 200 × 45 230 × 60 200 × 60

-35.8 -34.3 -40.3 -36.7

sample size (l × r, nm × nm)

ζ (mV)

200 × 60 200 × 45 230 × 60 230 × 45

-36.7 -34.3 -40.3 -35.8

∆qparticle (C)

∆sside (cm2)

∆q/∆sside (C cm-2)

8.62πε × 10-10

2.70π × 10-11

31.94ε

17.99πε × 10-10

3.60π × 10-11

49.97ε

∆qparticle (C)

∆sside+2top (cm2)

∆q/∆sside+2top (C cm-2)

27.31πε × 10-10

9.15π × 10-11

29.85ε

36.68πε × 10-10

10.05π × 10-11

36.50ε

planes at pH 7.0 are listed in Table 3. It is surprising that the side surface with 1.5 Å openings displays a larger change of charge density (∆q/∆sside) compared with the (001) surface with abundant 12MR channels because the changes of surface charge density (∆q/∆sside+2top) do not increase but decrease when the contribution of the top (bottom) area is introduced into the surface area difference. In other words, the contribution of the top (bottom) face to the change of surface charge density of a nanozeolite particle is actually smaller than that of the side surface, which seems to be inconsistent with the fact that the protein adsorption capacity of the (001) plane is much larger than that of the other two dimensions (Table 2 and Figure 6). Similarly, the surface charge changes over the surface area difference of nanozeolite particles with different exposed crystal planes under the pH of 5.0 and 9.0 show the same trend with that under pH 7.0 (Tables S6 and S7, Supporting Information). These results imply that some other factors are also influencing the protein adsorption behaviors of the (100) crystal plane and side surface besides electrostatic interaction. Surface geometry is then considered as one important feature of nanomaterials to influence their adsorption properties. In comparison with other two-dimension directions, there exists abundant exposed pore openings (7.1 Å) on the (001) crystal plane of nanozeolites LTL. The lower surface charge density change of the (001) crystal plane could be assigned to these exposed pore mouths (channels). Similarly, these exposed pore

mouths are recently believed to influence catalytic performance of zeolite materials by insertion of the part or branch groups of the large reactant molecules into them,15 and this effect will become more and more dominant with the decreasing zeolite size.16 Moreover, Curcio et al.17 have found that the porous surface can decrease the nucleation free energy of proteins and thereby promote the deposition of protein molecules on it. Therefore, we can speculate that abundant exposed pore openings on the (001) crystal plane could induce the aggregation of proteins on it, which might favor the stabilization of the native conformation of adsorbed proteins. Both the aggregation of protein and the retention of its native conformation will promote effective protein adsorption. In addition, for the nanozeolite LTL with a cylinder morphology, its side surface is a curved one. The earlier results proved that surface curvature of nanomaterials played a major role in protein packing density on the material surface;18,19 that is, such surface curvature of nanozeolites (diameter of 90-120 nm) will disorder the native conformation of proteins,19 and so reduces adsorbed proteins on its surface. 4. Conclusions In conclusion, the protein adsorption on zeolite LTL nanocrystals is strongly dependent upon the size and crystal plane of the nanocrystals. A larger protein adsorption amount is observed on the surface of smaller nanocrystals and the (001)

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crystal plane. Larger surface area promotes protein adsorption on the smaller nanocrystals. However, the difference of protein adsorption on the different crystal planes can be attributed to abundant exposed pore openings on the top (bottom) surface and curved surface of the side surface in columned nanozeolites LTL. This means that, besides external surface area and surface charge density, topography (crystal plane and surface curvature) at the nanoscale is also an important factor determining protein adsorption on the surface of nanozeolites. Clearly, new efforts should be focused on the synthesis of nanozeolites LTL with abundant exposed (001) planes, toward a high immobilized amount of enzyme and high stabilization of protein native conformation. Although only a LTL nanocrystal is described as a model, this observation is expected to be applied to other crystalline materials, such as other types of zeolites and metal nanoparticles. Acknowledgment. The authors greatly appreciate the support of NSFC (20721063, 30828010, 20873025, and 20890122), STCSM (08DZ2270500), and major state basic research development program (2009CB930403 and 2009CB623506). Supporting Information Available: XRD patterns of asprepared samples with different sizes, standard deviation of sample size, adsorption amount differences of proteins on nanozeolites LTL with different exposed crystal planes at different pHs, and surface charge change per unit area difference between two nanozeolites LTL with different exposed crystal planes at different pHs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Breck, D. W. Zeolite Molecular SieVes, Structure, Chemistry and Use; Wiley: New York, 1974. (b) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (2) (a) Zhang, Y. H.; Ren, N.; Tang, Y. In Ordered Porous SolidsRecent AdVances and Prospects; Valtchev, V., Mintova, S., Tsapatsis, M., Eds.; Elsevier B.V.: Amsterdam, The Netherlands, 2009; Chapter 17. (b) Valtchev, V. P.; Tosheva, L. Chem. Mater. 2005, 17, 2494. (3) (a) Zhang, Y. H.; Wang, X. Y.; Shan, W.; Wu, B. Y.; Fan, H. Z.; Yu, X. J.; Tang, Y.; Yang, P. Y. Angew. Chem., Int. Ed. 2005, 44, 615. (b)

Hu et al. Huang, Y.; Shan, W.; Liu, B. H.; Liu, Y.; Zhang, Y. H.; Zhao, Y.; Lu, H. J.; Tang, Y.; Yang, P. Y. Lab Chip 2006, 6, 534. (4) (a) Yu, T.; Zhang, Y. H.; You, C. P.; Zhuang, J. H.; Wang, B.; Liu, B. H.; Kang, Y. J.; Tang, Y. Chem.sEur. J. 2006, 12, 1137. (b) Zhang, Y. H.; Liu, Y.; Kong, J. L.; Yang, P. Y.; Tang, Y.; Liu, B. H. Small 2006, 2, 1170. (5) (a) Zhou, X. Q.; Yu, T.; Zhang, Y. H.; Kong, J. L.; Tang, Y.; Marty, J. L.; Liu, B. H. Electrochem. Commun. 2007, 9, 1525. (b) Ji, J.; Zhang, Y. H.; Zhou, X. Q.; Kong, J. L.; Tang, Y.; Liu, B. H. Anal. Chem. 2008, 80, 2457. (6) (a) Vinu, A.; Murugesan, V.; Hartmann, M. J. Phys. Chem. B 2004, 108, 7323. (b) Vinu, A.; Murugesan, V.; Tangermann, O.; Hartmann, M. Chem. Mater. 2004, 16, 3056. (c) Hudson, S.; Magner, E.; Cooney, J.; Hodnett, B. K. J. Phys. Chem. B 2005, 109, 19496. (7) Meier, W. M.; Olsen, D. H.; Ba¨rlocher, C. Atlas of Zeolite Structure Types; Elsevier: Amsterdam, The Netherlands, 1996. (8) (a) Barrer, R. M.; Villiger, H. Z. Kristallogr. 1969, 128, 352. (b) Newsam, J. M. J. Phys. Chem. 1989, 93, 7689. (9) (a) Lovallo, M. C.; Tsapatsis, M. In AdVanced Catalysts and Nanostructured Materials: Modern Synthetic Methods; Moser, W. R., Ed.; Academic Press: San Diego, CA, 1996; Chapter 13. (b) Megelski, S.; Calzaferri, G. AdV. Funct. Mater. 2001, 11, 277. (c) Larlus, O.; Valtchev, V. P. Chem. Mater. 2004, 16, 3381. (d) Lee, Y. J.; Lee, J. S.; Yoon, K. B. Microporous Mesoporous Mater. 2005, 80, 237. (10) Hu, Y. Y.; Liu, C.; Zhang, Y. H.; Ren, N.; Tang, Y. Microporous Mesoporous Mater. 2009, 119, 306. (11) Cao, J.; Hu, Y. Y.; Shen, C. P.; Yao, J.; Wei, L. M.; Yang, F. Y.; Nie, A. Y.; Wang, H.; Shen, H. L.; Liu, Y. K.; Zhang, Y. H.; Tang, Y.; Yang, P. Y. Proteomics 2009, DOI: 10.1002/pmic.200800877. (12) Zhang, R.; Somasundaran, P. AdV. Colloid Interface Sci. 2006, 123126, 213. (13) Myers, D. Surface, Interface, and Colloid: Principles and Applications, 2nd ed.; Wiley: New York, 1999; Chapter 5. (14) Rawle, A. Basic principles of particle size analysis; Malvern Instruments Limited: Worcestershire, U.K., 2001. (15) (a) Schenk, M.; Smit, B.; Vlugt, T. J. H.; Maesen, T. L. M. Angew. Chem., Int. Ed. 2001, 40, 736. (b) Denayer, J. F.; Ocakoglu, A. R.; Huybrechts, W.; Martens, J. A.; Thybaut, J. W.; Marin, G. B.; Baron, G. V. Chem. Commun. 2003, 1880. (c) Degnan, T. F., Jr. J. Catal. 2003, 216, 32. (16) Sugimoto, M.; Katsuno, H.; Takatsu, K.; Kawata, N. Zeolites 1989, 7, 503. (17) (a) Curcio, E.; Fontananova, E.; Di Profio, G.; Drioli, E. J. Phys. Chem. B 2006, 110, 12438. (b) Troger, J.; Lunkwitz, K.; Burger, W. J. Colloid Interface Sci. 1997, 194, 281. (18) (a) Roach, P.; Farrar, D.; Perry, C. C. J. Am. Chem. Soc. 2006, 128, 3939. (b) Roach, P.; Farrar, D.; Perry, C. C. J. Am. Chem. Soc. 2005, 127, 8168. (19) Vertegel, A. A.; Siegel, R. W.; Dordick, J. S. Langmuir 2004, 20, 6800.

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