pubs.acs.org/Langmuir © 2009 American Chemical Society
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Selective Fusion, Solvent Dissolution, and Local Symmetry Effects in Inversion of Colloidal Crystals to Ordered Porous Films Ting Zhang,†,^ Jun Qian,†,§,^ Xinlin Tuo,‡ and Jun Yuan*,†, †
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Beijing National Center for Electron Microscopy and Laboratory for Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China, ‡Laboratory for Advanced Materials, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P. R. China, § Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, and Department of Physics, University of York, Heslington, York, YO10 5DD, U.K. ^ These authors contributed equally to this research Received August 26, 2009. Revised Manuscript Received October 22, 2009 Polystyrene-methacrylic core-shell nanospheres, self-assembled into face-centered-cube-like colloidal crystals with their (001) planes parallel to the substrate, have been transformed into ordered pore structures by a toluene treatment. Detailed analysis by transmission electron microscopy reveals that the morphological transformation is preceded by an internal neck formation due to selective fusion of the polystyrene-rich core material, at the contacts between the nanoparticles, followed by the selective dissolution of the polystyrene-rich cores. We have demonstrated the importance of local symmetry and compactness of the nanospheres assembly in determining the nature of the neck formation and the existence of multiscale ordered pore structures in the square facing colloidal crystals. The pseudo layer-by-layer nature of the selective dissolution of square arranged nanosphere multilayers is responsible for the observed threedimensional pore structures.
1. Introduction Micro/mesoporous structures are important because of their potential applications in heterogeneous catalysis,1 high-speed separation,2 sensing,3 tunable photonic crystal materials,4 nanoreactors,5 and as templates to prepare other materials.6 Lithography,7 infiltration,8 and multiple template methods are popular in preparing such pore structures.9 Recently, an alternative method based on an in situ structure inversion through selective dissolution of colloidal crystals assembled by polymeric latex particles with a core-shell structure also becomes attractive.10 In this process, the colloidal crystal structure self-reorganizes into an ordered porous film after exposure to a solvent that can selectively dissolve the polymer component rich in the core of the latex nanospheres. This method is simple, straightforward, and without the need for any post-treatment. However, this method has only been demonstrated for the commonest nanosphere arrangement, *Corresponding author. E-mail:
[email protected] or yuanjun@tsinghua. edu.cn. (1) El-Nafaty, U. A.; Mann, R. Chem. Eng. Sci. 1999, 54, 3475–3484. (2) Minakuchi, H.; Nakanishi, K.; Soga, N.; Ishizuka, N.; Tanaka, N. J. Chromatogr. A 1997, 762, 135–146. (3) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829–832. (4) Villaescusa, L. A.; Mihi, A.; Rodriguez, I.; Garcia-Bennett, A. E.; Miguez, H. J. Phys. Chem. B 2005, 109, 19643–19649. (5) Urban, M.; Konya, Z.; Mehn, D.; Zhu, J.; Imre, K. PhysChemComm 2002, 5, 138–141. (6) Wang, Z.; Li, F.; Ergang, N. S.; Stein, A. Chem. Mater. 2006, 18, 5543–5553. (7) (a) Xia, Y. N.; Whiteside, G. M. Angew. Chem., Int. Ed. 1998, 37, 551–575. (b) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whiteside, G. M. Chem. Rev. 1999, 99, 1823– 1848. (c) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F. Nature 1998, 391, 667–669. (8) (a) Deutsch, M.; Vlasov, Y. A.; Norris, D. J. Adv. Mater. 2000, 12, 1176– 1180. (b) Lee, K. B.; Lee, S. M.; Cheon, J. Adv. Mater. 2001, 13, 517–520. (9) (a) Antonietti, M.; Berton, B.; Goeltner, C.; Hentze, H. P. Adv. Mater. 1998, 10, 154–159. (b) Sun, F. Q.; Cai, W. P.; Li, Y.; Jia, L.; Ch, Lu. F. Adv. Mater. 2005, 17, 2872–2877. (10) (a) Chen, Y. Y.; Ford, W. T.; Materer, N. F.; Teeters, D. J. Am. Chem. Soc. 2000, 122, 10472–10473. (b) Rugge, A.; Ford, W. T.; Tolbert, S. H. Langmuir 2003, 19, 7852–7861.
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namely hexagonal close packing of nanospheres.11 After a recent successful demonstration of a template-free method for simply preparing large-scale colloidal crystal films whose surfaces show square arrangements of latex nanospheres,12 we demonstrate in this paper the production of an ordered porous structure from such colloidal films. Until now, owing to the lack of full understanding of the physical mechanism underpinning the transformation process, many details of the inversion process have not been thoroughly elucidated even in colloidal crystal films with the common hexagonal arrangements of nanospheres at the surface. We will present evidence that morphological transformation in square open packing structure is preceded by previously unknown internal neck formation due to the fusion of core materials. This neck formation gives not only the structures rigidity but also additional complex details in the inversion processes. For example, together with the high fraction of the empty space in the open packing square structure, this has resulted in additional morphological configurations involving new multiscale nested pore structures. A selective swollen-fusion-dissolution model is proposed to describe the inversion process and to account for the local symmetry effect observed and the three-dimensional pore morphology. We believe that a multitude of morphological configurations observed in the inversion of square symmetry facing colloidal films provides rich possibilities to produce complex nanostructured surfaces for many applications, such as the template for fabrication of ordered two-dimension nanostructures and the positioning of catalysts for growth of ordered nanowire arrays.13,14 (11) Zhang, T.; Qian, J.; Tuo, X. L.; Yuan, J.; Wang, X. G. Colloids Surf., A 2009, 335, 202–206. (12) Zhang, T.; Tuo, X. L.; Yuan, J. Langmuir 2009, 25, 820–824. (13) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701–1703. (14) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105–1107.
Published on Web 12/08/2009
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2. Experimental Section The highly monodispersed poly(styrene-co-methacrylic acid) (PSMA) colloidal spheres, which contain styrene (St) in their cores and methacrylic acid (MAA) in their shells, were synthesized by a surfactant-free emulsion polymerization method.15 The weight ratio of styrene to methacrylic acid was 9:1; the Z-average mean diameter of the colloidal spheres in solution was 290 nm, as measured with dynamic laser light scattering (Zetasizer 3000 HS of Malvern Instruments Ltd. with a 633 nm light source and a detector angle of 90). The PSMA colloidal spheres arrays with square arrangement at its surface were fabricated by a modified flow-controlled vertical deposition (FCVD) method.12,16 In the experiment, the film substrate, a silicon wafer, typically of 1.5 mm in width, was first treated in a mixture of H2SO4/H2O2 (98% H2SO4:30% H2O2 = 3:2, v/v) for 8 h and then ultrasonically cleaned in acetone, ethanol, and deionized water for three times. The treated hydrophilic silicon wafer was then attached to a piece of clean hydrophobic aluminum block and partially immersed in the colloidal suspension (about 1 wt %) with a tilt angle of 10 from the liquid surface normal. The flow pumping velocity was set at a constant rate of 0.002 mL/min. As we have reported in ref 12, the formation of such square packing patterns is related to the shape of the meniscus at the edges of the silicon wafer used as the film substrate. The shape of the meniscus at the edges can be controlled by varying the width of the wafer substrate or the wetting properties of the backing support for the wafer. Toluene is known to be a good solvent for styrene but a bad solvent for methacrylic acid.10 To convert self-assembled colloidal crystals into pore structures, silicon wafers covered with the colloidal crystal films were placed in toluene at room temperature for specified periods. The wafers were then washed with acetone for a few seconds in order to fix the morphological transformation more precisely, reproducibly, and uniformly. No other effects on morphology were observed with the addition of acetone. The colloidal films were finally dried with an air stream and placed under vacuum at 30 C for a further 12 h before characterization were carried out. Scanning electron microscopy (SEM) (JEOL JSM6301F) and transmission electron microscopy (TEM) (JEM-200CX) were used to characterize the resulting morphology of the colloidal samples. In order to perform the SEM inspection, the samples were coated with a thin gold film as customary to prevent charging effects. The tapping mode of a Nanoscope IIIa atomic force microscope (AFM) was used to characterize the surface morphology of the colloidal sample, and silicon tips with the tip curvature radius less than 10 nm were used in the high-resolution topography measurements. In general, we expect the sphere sizes to be smaller when measured in the dehydrated state in SEM and AFM compared with the measurement in solution.
3. Results 3.1. Inversion of Square Pattern Facing Colloidal Crystals. Figure 1a shows the resulting square arrangement of nanospheres obtained by manipulating an edge meniscus effect.12 The insert in the top right corner is its Fourier transformation, confirming a high degree of the long-range order in the square periodicity of the colloidal array. Figure 1c shows the corresponding surface topographic image by AFM. The height profile along the white marked line indicates that the mean spacing between the spheres is about 312 nm, slightly bigger than that of the original spheres in the solution. This is consistent with the more open packing nature of the square arrangement of spheres at the exposed surface. (15) Eshuis, A.; Leendertse, H. J.; Thoenes, D. Colloid Polym. Sci. 1991, 269, 1086–1089. (16) Zhou, Z. Ch.; Zhao, X. S. Langmuir 2004, 20, 1524–1526.
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Figure 1. (a) A SEM image of the square colloidal array. (b) A SEM image of the square porous array. (c) A surface topography image of the square colloidal array. (d) A surface topography image of the square porous array. (e) A SEM image of the hexagonal colloidal array. (f) A SEM image of the hexagonal porous array.
We have succeeded in converting the colloidal crystals with square surface arrangements to porous films. Figure 1b gives the SEM image of the colloidal array after it was dipped in toluene for about 85 s, and the inset in the top right corner is its Fourier transformation. Clearly, a porous film with the same square periodicity as the starting array is observed. The voids among the original four adjacent colloidal spheres have been totally filled up and joined to form an intact continuous film. Figure 1d gives the surface topography image of such a pore structure from another similar colloidal crystal that has been toluene treated for about 75 s. Pore structures similar to that found in the SEM image are clearly seen in comparable toluene treatment times. Pore walls could be distinguished clearly, and the height profile taken along the white marked line shows that the average distance between the centers of the two adjacent pores is 320 nm, a little bigger than the average spacing between the assembled nanospheres in the colloidal crystals shown in Figure 1c. The average inner diameter of the opening of the pores is 240 nm, and the average pore depth is 135 nm, with the thickness of the pore wall estimated at 80 nm. We will call these primary pores because they show one-to-one correspondence with the nanospheres in the starting crystals. For comparison, we have also shown the SEM image of the primary pore structure more commonly found when inverting the colloidal films with a hexagonally close packing of spheres (Figure 1e). As we have reported in ref 11, the selective dissolution method is a good way to convert such colloidal array into a porous film. The image in Figure 1f shows the resulting porous film after the colloidal spheres array has been dipped in toluene for about 40 s. The averaging spacing between the centers of the nanospheres is 290 nm, consistent with the close-packing nature DOI: 10.1021/la903158z
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of the self-assembled structure. Interestingly, the average pore size is narrower at 210 nm, but the pore is deeper at 200 nm. 3.2. Nested Pore Structure Formation. More interestingly, we have found that the primary square pore structure is just one of a series of morphological transformations observed as a function of different toluene immersion times. By comparison, nothing similar is observed in the hexagonal close-packing structured nanosphere array. We will introduce the intermediate multiscale pore structures found during the transformation from nanosphere arrays with square open packing structured surface. Additional three-dimensional (3D) pore structures are also found after the prolonged toluene treatment that is necessary for primary pore structure formation. They are summarized and discussed in the Supporting Information.17 A nested architecture consisting of two sets of pores of different shapes and diameters is obtained when the square patterned colloidal arrays are dipped in toluene for a very short time. When the dipping time is only about 15 s, a precursor state to this nested architecture is observed, with a square array of small holes at the same density as the colloidal spheres in the original arrays (see the SEM image in Figure 2a). Closer inspection of the SEM image contrast shows an array of large and round gray features with a size similar to that of the original colloidal particles. This indicates that the small pores correspond to the pre-existing voids within the original self-assembled colloidal spheres and the large gray features are the colloidal nanospheres themselves. We call these small holes secondary to distinguish them from the primary pores eventually formed at the center of the nanospheres (Figure 1b). The reduced SEM contrast for the nanospheres is related to the filling of the grooves between the nanospheres by the released styrene polymer segments.18 This nested architecture is fully developed when the toluene treatment time is increased to 25 s, consisting of two sets of interlaced square porous arrays as shown in Figure 2b. One set of the porous arrays can be mapped onto the colloidal spheres whose positions are partially outlined with white dotted circles. They can be identified with the primary holes as shown in Figure 1b and are due to the selective dissolution of the polystyrene core materials.18 The others are the secondary pores discussed earlier. Formation of these two sets of pores indicates that the selective dissolution of square colloidal arrays could double the pore density compared with the hexagonal porous arrays. The AFM is used to measure the inner diameter and depth of these two kinds of pores. The average inner diameters of these two kinds of pores are 140 and 90 nm, respectively, while the average depths are 31 and 13 nm, respectively (Figure 2e). When the treatment time is further increased to 65 s, there is a change in the sizes of pores in such a nested architecture (Figure 2c). The average inner diameter of the primary pore is about 250 nm, as measured by SEM, but the mean inner diameter of the secondary pore structure is only 80 nm. Obviously, the pore sizes of the primary pores are now much larger than those of secondary pores. When the colloidal crystal is dipped in toluene for 87 s, the secondary pores disappear and porous square-symmetry arrays with just one set of primary pores form. As shown in Figure 2d, the voids within the colloids have completely joined. This indicates that varying the dipping time in toluene is a good way to control the pore sizes in the nested structure. Figure 2f gives the relationship between the pore sizes and the dipping time. Obviously, as the toluene treatment time increases, the average diameter of the primary pores always (17) See Supporting Information. (18) Qian, J.; Yuan, J.; Tuo, X. L.; LI, M. Z.; Wang, X. G. J. Chin. Electron Microsc. Soc. 2007, 26, 124–129.
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Figure 2. (a) A SEM image of the square porous array with small pores. (b) A SEM image of the dual-size square porous array with comparable sizes. (c) A SEM image of the dual-size square porous array. (d) A SEM image of square array of large primary pores. (e) A surface topography image of the dual-size square porous array and the height profiles along the colored lines indicated in the surface topography image. (f) Relationship between the primary and secondary pore sizes as a function of dipping time in toluene.
increases, whereas the average diameter of the secondary pores increases first and then reduces to zero. 3.3. Hidden Neck Formation. In order to clearly observe the details in structural transformation of the core-shell colloidal particles, we have stained colloidal spheres and their packed arrangement with phosphotungstic acid which preferentially binds to the benzene groups in the St units.18 A TEM image of the stained polystyrene-methacrylic core-shell nanospheres is shown in Figure 3a, with the darkest regions located at the outer boundary of the polystyrene core material. Usually, the more phosphotungstic acid materials stain, the darker materials appear in the TEM image. The outer bright rings are thus attributed to coronas dominated with MA units. If diffusion is not a limiting issue, then we would expect the centers of the stained nanospheres to show the darkest contrast. We interpreted the smaller than expected contrast at the core of the nanospheres as due to limited Langmuir 2010, 26(5), 3690–3694
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Figure 3. (a) TEM image of a colloidal sphere after a phosphotungstic acid staining treatment. (b) TEM image of the square colloidal array after a short toluene treatment.
diffusion of the phosphotungstic acid inside the core of the colloidal particles. Obviously, the spherical structures of colloids are intact and have no sign of any damage or transformation due to the staining treatment. When a group of squarely packed nanospheres (on amorphous carbon film for the TEM observation) is treated with toluene, we find although the spheres touch each other to form a close-packing structure, the outline of each nanoparticle is still spherical, and the diameter increases slightly when compared with original spheres. This accords with the solvent swelling model put forward by the Ford’s group, who studied the selective dissolution of close-packed monodispersed poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) by a toluene vapor treatment.19 It is believed that the transformation in their case is due to the mobility of styrene whose glass transition temperature was lowered to below room temperature owing to the absorption of the vapor.10 We believe that a similar selective solvent swelling mechanism operates here, although additional processes are involved. An important new observation is the fusion effect in which the styrene cores form an internal neck structure accompanying the change of the shape of the internal styrene core volume. Important for the subsequent porous pattern formation, this internal shape change of the core materials also deforms the outer shells of the close-packed colloidal nanospheres. This gives rises to the pseudo-rhombic shape of the interstitial voids in the colloidal films. At this stage, we can see that there is still no rupture of the outer shell of the nanospheres. The dark linear features bisecting the contacts between the nanospheres in Figure 3b probably resulted from the redeposition of styrene monomers from the surface of the nanospheres at the contacts between the nanospheres to minimize the nearly divergent curvatures there. Such effects have not been reported by Ford’s group for their vaportreated colloidal films.19 This may be due to the different amounts of toluene absorbed by the core styrene materials.
4. Discussion 4.1. Selective Swelling-Fusion-Dissolution Model. On the basis of the above observations, we can now understand that the formation of multisize pore structures is the result of the fusion of the swelling cores followed by a pattern directed selective dissolution of spheres. The hidden neck formation plays a key role in shaping this pattern formation process. The proposed transformation steps involved in the formation of ordered square pore structures are schematically shown in Figure 4. After the immersion in toluene, the colloidal spheres first swell due to the permeation of the solvent into the core area.19 This accounts (19) Chen, Y. T.; Ford, W. T.; Materer, N. F.; Teeters, D. Chem. Mater. 2001, 13, 2697–2704.
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Figure 4. Schematic depiction of the transformation sequences from square colloidal sphere arrays to ordered square porous arrays following a short toluene treatment: (a) the core-shell nanoparticles in the square arrangement; (b) the selective swollen-fusion of styrene core materials in the V2 region; (c) the start of dissolution of styrene core and closing up of the void area (V1); (d) the closure of the void area and complete dissolution of styrene core materials, leading to an inverted porous structure.
for the increase in the average diameters. At the same time, the adsorption of toluene by the core, which is more rapid in the toluene soaking experiment here than in Ford’s toluene vapor treatment, is going to lower the glass transition temperature of styrene from 100 C to the ambient temperature or below,19 inducing a significant increase in the mobility of styrene as well as the pressure due to swelling. At the point of contact between the swollen nanospheres, the outer shells are pressured by the mobile styrenes and become ruptured first, leading to the formation of the internal styrene-rich necks between the cores of neighboring nanospheres (Figure 4b). The reduced viscosity also means that the shape of the polystyrene-rich core, with linking necks, can be easily modified to minimize the interfacial area. This results in the nonspherical deformation seen in Figure 3b. Further swelling of the core materials leads to the crack of the poly-MA shells. The mobile styrene components in the cores now can flow out to fill up the voids between the colloidal spheres and to engulf the minor MA-rich shells. The large void in the square arrangement means that the priority is to cover the areas rich in the MA phase for minimizing the surface area per volume. Therefore, the voids are left as small square-shaped pores, leading to the existence of a hierarchical arrangement of dual-size pores (Figure 4c). When the nanosphere array is in direct contact with the toluene, the removal of styrene usually is more efficient once the outer shell is ruptured. This is a major factor for the formation of the deep holes at the center of spheres, compared with the relatively shallow pores reported by Ford et al.10 The square-shaped secondary pores formed are due to the local symmetry of the four neighboring spheres connected by formed neck and deformed because of the interfacial energy minimization. For this reason, we conclude that the shape of the smaller holes is directed by local symmetry in the pre-existing structure. As more and more styrene flows out of the center of the original nanospheres, the primary pore size increases monotonically. As for the secondary pore size, it would increase initially as the particle size collapses a little bit because the pressure of styrene is reduced by the rupture of the outer shell. However, this trend is quickly masked by the arrival of the outflowing styrene, which covers the toluene-averse MA-rich outer DOI: 10.1021/la903158z
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of the styrene core (V20 , in Figure 5 and V10 = 0.33V20 ), means that its filling by the leaked styrene is also relatively easy to accomplish for such small nanospheres (Figure 5c). In comparison, the corresponding volume ratio of the styrene core and the voids to be filled in the square arrangement is V1/V2 = 0.9 (see Figure 4a for the corresponding definition of V’s). Thus, we may understand why the multiscale ordered pore structure is not readily observed in the close-packed structures, although its possibility cannot be ruled out completely, for example, for kinetics reasons in the scaled-up version of the colloidal films studied here. Figure 5. (a) TEM image of colloidal nanoparticles in the closepacking hexagonal arrangement, after a short toluene treatment. (b) Schematics of the colloidal nanoparticles in a hexagonal closepacking arrangement.
shell to minimize the energy of the outer surface in the presence of toluene. This thermodynamics-driven process leads to the closing up of these secondary pores as shown in Figure 4d. 4.2. Local Symmetry Effects. The difference between the simple pore structures observed in hexagonal arrangement of spheres and the rich pore structures observed in square arrangement can be understood in terms of the local symmetry effect and geometrical factors. TEM observation is also used to characterize the movement of the styrene core materials in the hexagonal arrangement of spheres. Figure 5a shows TEM image of phosphotungstic acid-stained colloidal nanoparticles with a hexagonal arrangement, after toluene treatment. The fusion effect is also observed in this case, with the styrene channels formed between three adjacent sphere cores and the corresponding shape change. However, the channel is not formed along the line linking the nanospheres but points toward the center of the void between the three closest spheres. This is a somewhat unexpected result, but it would explain the reason why we and others do not observe the formation of a multiscale ordered pore structure in the inversion of hexagonally arranged nanospheres.10 We believe that the capillary driving force for styrene filling the voids between the contacting nanospheres in a hexagonal arrangement is much stronger due to large curvature existing between spheres in a close-packing arrangement than that for the nanospheres in a square arrangement. The smaller volume of the voids (V10 in Figure 5) in the nanoparticle assembly, compared with the volume
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5. Conclusion In conclusion, we have shown that multiscale pore structures can be formed by inversion of the colloidal films with square surface arrangement of nanospheres. We have observed the fusion phenomena of the swollen styrene-rich core materials prior to the rupture of the MA shells and overflowing of the styrene from the nanospheres, which is a general phenomenon in the selective solution of the core materials of the core-shell nanoparticles and crucial to the understanding of the existence of the rhombic shape of the secondary pore structure found in colloidal surfaces with square surface arrangements. The attraction of a colloidal film system with a square arrangement of nanospheres at its surface is the new flexibility for designing the core-shell structure in terms of the overall size of colloidal spheres, its swelling behavior, and its array arrangement to control the resulting pore structure. The sequential nature of the selective dissolution process may also open up the possibility for targeted drug or chemical release. Acknowledgment. We thank the Ministry of Science and Technology (MOST) of China for financial support (Basic Science Research Grant 2002CB613501). Supporting Information Available: This provides additional experimental evidence about the three dimensional pore structure created by prolonged selective dissolution of similar square facing multilayer colloidal close packing films, following the mechanism outlined in the paper. This material is available free of charge via the Internet at http://pubs.acs. org.
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