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Langmuir 2008, 24, 3532-3536
Superparamagnetic Fe3O4/SiO2 Nanocomposites: Enabling the Tuning of Both the Iron Oxide Load and the Size of the Nanoparticles Maria Stjerndahl,† Martin Andersson,† Holly E. Hall,† Daniel M. Pajerowski,‡ Mark W. Meisel,‡ and Randolph S. Duran*,† Department of Chemistry and Department of Physics, UniVersity of Florida, GainesVille, Florida 32611 ReceiVed NoVember 14, 2007 Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.
Introduction Superparamagnetic materials do not retain any magnetization in the absence of an externally applied magnetic field.1 Due to this property, superparamagnetic nanoparticles are of great interest for several biomedical applications, such as magnetic resonance imaging (MRI), hyperthermia, separation and purification of biomolecules, and drug delivery.1-4 For such applications, precautions have to be taken to ensure the stability of the particles, avoiding aggregation and biodegradation. In addition, the particle surface needs to allow the attachment of ligands for functionalization. Encapsulating the superparamagnetic iron oxide crystals (SPIOs) in silica yields a protective, biocompatible, inert, and hydrophilic surface with excellent anchoring points for derivatizing molecules.5,6 When using superparamagnetic particles for targeted purposes, such as targeted drug delivery or as targeted contrast agents for MRI, it is crucial that each particle contains enough magnetic material to generate a detectable response to an external magnetic field.7,8 Several methods for the formation of iron oxide containing superparamagnetic silica nanocomposites have been reported including the use of aerosol pyrolysis,9,10 emulsions,11,12 microemulsions,13-17 active silica,18 and reactions performed under Sto¨ber conditions.19-22 Many of these studies show the formation of silica nanocom* Corresponding author:
[email protected], Phone: (352) 392 2011, Fax: (352) 392 7426. † Department of Chemistry. ‡ Department of Physics. (1) Lu, A. H.; Salabas, E. L.; Schueth, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (2) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33. (3) Tartaj, P.; Morales, M. d. P.; Veintemillas-Verdaguer, S.; Gonzalez-Carreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, R182. (4) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (5) Sharma, P.; Brown, S.; Walter, G.; Santra, S.; Moudgil, B. AdV. Colloid Interface Sci. 2006, 123-126, 471. (6) Lai, W.; Garino, J.; Ducheyne, P. Biomaterials 2002, 23, 213. (7) Artemov, D.; Mori, N.; Okollie, B.; Bhujwalla, Z. M. Magn. Reson. Med. 2003, 49, 403. (8) Xu, H.; Cui, L.; Tong, N.; Gu, H. J. Am. Chem. Soc. 2006, 128, 15582. (9) Tartaj, P.; Gonzalez-Carreno, T.; Bomati-Miguel, O.; Serna, C. J.; Bonville, P. Phys. ReV. B: Condens. Matter 2004, 69. (10) Tartaj, P.; Gonzalez-Carreno, T.; Serna, C. J. Langmuir 2002, 18, 4556. (11) Im, S. H.; Herricks, T.; Lee, Y. T.; Xia, Y. Chem. Phys. Lett. 2005, 401, 19. (12) Xu, Z. Z.; Wang, C. C.; Yang, W. L.; Fu, S. K. J. Mater. Sci. 2005, 40, 4667.
posites containing only one or a few SPIOs. If the number of targets is limited, for example cell receptors, low amounts of magnetic material per particle, and hence per target, will give too weak a response when subjected to a magnetic field.11 It is therefore of interest to achieve high loads of superparamagnetic material in each particle, keeping in mind that the particle should not be too large. At present there are a limited number of studies where particles have been formed with both a high load of magnetic material and a relatively small particle size, i.e., ∼100 nm. Tartaj et al. have used aerosol pyrolysis9,10 and water-in-oil (w/o) microemulsions16 to prepare nanocomposites with several iron oxide crystallites evenly distributed in particles having diameters of 150 and 50 nm, respectively. The iron oxide crystallites were formed from either iron nitrate or iron chloride salts. Xu et al. utilized an inversed miniemulsion to incapsulate magnetite nanoparticles in silica.8 Im et al. used another approach, combining a commercial ferrofluid and an emulsion system.11 By varying the SPIO concentration and the solvent, nanocomposites with diameters between 100 and 700 nm were obtained. Magnetization measurements revealed that the iron oxide crystals occupied about 0.5 wt % of the nanocomposites. In the present work, a ferrofluid was used as the SPIO source and the aim was to prepare nanocomposites as fully loaded with iron oxide crystallites as possible, and to find ways of tuning their overall size and loading. The experimental method used to include the magnetite crystallites in silica was based on the hydrolysis and condensation of silicon alkoxide in w/o microemulsions.23 The change in loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes were investigated by systematically varying the water content of the (13) Lu, C.-W.; Hung, Y.; Hsiao, J.-K.; Yao, M.; Chung, T.-H.; Lin, Y.-S.; Wu, S.-H.; Hsu, S.-C.; Liu, H.-M.; Mou, C.-Y.; Yang, C.-S.; Huang, D.-M.; Chen, Y.-C. Nano Lett. 2007, 7, 149. (14) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Langmuir 2001, 17, 2900. (15) Tartaj, P.; Serna, C. J. Chem. Mater. 2002, 14, 4396. (16) Tartaj, P.; Serna, C. J. J. Am. Chem. Soc. 2003, 125, 15754. (17) Yi, D. K.; Lee, S. S.; Papaefthymiou, G. C.; Ying, J. Y. Chem. Mater. 2006, 18, 614. (18) Iler, R. K., US Patent, No. 2885366, 1959. (19) Barnakov, Y. A.; Yu, M. H.; Rosenzweig, Z. Langmuir 2005, 21, 7524. (20) Deng, Y. H.; Wang, C. C.; Shen, X. Z.; Yang, W. L.; An, L.; Gao, H.; Fu, S. K. Chem. Eur. J. 2005, 11, 6006. (21) Lu, Y.; Yin, Y. D.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 183. (22) Sun, Y. K.; Duan, L.; Guo, Z. R.; Yun, D. M.; Ma, M.; Xu, L.; Zhang, Y.; Gu, N. J. Magn. Magn. Mater. 2005, 285, 65.
10.1021/la7035604 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008
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Table 1. Water Content and Amount of TEOS Added to Samples A through J, Together with the Diameter and Standard Deviation of the Obtained Nanocompositesa H2O (wt %) TEOS (mg) diameter (nm) a
A
B
C
D
E
F
G
H
I
J
4.2 10 -
6.6 10 82 ( 38
8.6 10 56 ( 17
11.5 10 34 ( 14
15.4 10 -
19.4 10 -
26.9 10 -
6.6 4 52 ( 19
6.6 7 78 ( 23
6.6 16 120 ( 27
(-) indicates that no discrete particles were obtained.
microemulsion as well as the silica precursor concentration. Utilizing this method, particles of sizes between 30 and 120 nm were formed, that ranged from containing one or a few SPIOs to being highly loaded. Experimental Procedures All chemicals were obtained from commercial sources and were used without further purification. The water based Ferrofluid, EMG 304, contained 4.5 vol % magnetite and was obtained from Ferrotec (Nashua, NH). Triton X-100 [t-octylphenoxypolyethoxyethanol], 1-hexanol, and tetraethylorthosilicate (TEOS) were purchased from Aldrich (Milwaukee, WI). Cyclohexane and ammonium hydroxide (29.6 wt %) came from Fisher Scientific Co. (Pittsburgh, PA). All water used was obtained from a milli-Q water purification system. Phase Studies. In order to investigate the concentration boundaries of the microemulsion used for the formation of the nanocomposites, a ternary phase diagram was constructed according to the following procedure. First, a mixture of the surfactant and the co-surfactant was made, namely, Triton X-100 and 1-hexanol, 54 and 46 wt %, respectively. Then samples along the binary cyclohexane/ surfactant axis were prepared, each sample containing 4 g in total. Each sample was then titrated with water, typically 25 mg per addition. After each water addition, the samples were vortexed and observed with the naked eye after 30 min. Samples that were transparent were identified as microemulsions, and the titration was continued. However, if the sample scattered light, it was left over night, since the system seemed to need some time to equilibrate. More water was added to samples that eventually turned transparent. Samples that remained whitish were classified as two phase systems. In some instances, the transition from microemulsion to emulsion went through a translucent bluish state. The phase studies were performed at room temperature. Synthesis of the Nanocomposites. Water (150 µL) and aqueous ferrofluid (300 µL, EMG 304) were mixed by sonication and added to an organic surfactant solution of cyclohexane (5.8 g), Triton X-100 (1.5 g), and 1-hexanol (1.3 g) to form a w/o microemulsion containing SPIOs. The microemulsion was vortexed, sonicated, and then left still to allow the excess SPIOs to sediment. After 2 days, the sediment was removed and the microemulsion was sonicated briefly. If necessary, changes to the water content were performed either by addition of water or organic surfactant solution. Next, TEOS, in varied amounts, was added to 1.5 mL of the microemulsion and the resulting mixture was vortexed and sonicated for 10 min followed by 10 min of additional shaking. Last, ammonium hydroxide (21 µL) was added, and the sample was vortexed and then shaken for 2 days at room temperature. To separate the particles, ethanol was added to break the microemulsion. The resulting nanocomposites were washed repeatedly by centrifugation (2×ethanol, 1×ethanol: water (1:1), 1×water) and were then magnetically separated using a simple bar magnet. The amounts of TEOS, water, and iron oxide were varied according to Table 1. Samples with lower SPIO concentration in the water phase were prepared by diluting the microemulsion 1:10 and 1:100 with a microemulsion of the same total water content, but without ferrofluid, before the addition of TEOS. Transmission Electron Microscopy (TEM). TEM studies were performed using a Hitachi H-7000 microscope operated at 75 kV. Samples were prepared by placing a drop of the purified particles (23) Osseo-Asare, K. Silica. Hydrolysis of silicon alkoxides in microemulsions. In Fine particles: synthesis, characterization, and mechanisms of growth; Sugimoto, T., Ed.; Marcel Dekker: New York, 2000; Vol. 92; 147.
suspended in water onto a Formvar carbon coated copper grid followed by drying at room temperature. Size determinations were performed manually, by measuring the diameter and the shell thickness of at least 70 particles. The extension of the core was calculated by subtracting the surrounding shell from the overall diameter of the particle. X-ray Diffraction. X-ray diffraction (XRD) patterns were obtained on a Philips APD 3720 X-ray diffractometer equipped with a Cu KR radiation source (λ ) 1.54 Å). Scans were collected on dry nanoparticles deposited on scotch tape in the 2θ range of 29-65°. Measurements were performed at room temperature. Superconduction Quantum Interference Device (SQUID) Magnetometry. Magnetic properties of the dry nanoparticles were studied using a commercial Quantum Design MPMS SQUID magnetometer. All samples were mounted using the tops of two gelatin capsules. The background of the straw and capsules were independently measured, and the contribution was negligible. The hysteresis loops were run at 300 K, then at 5 K, and then at 300 K again. The maximum field strength was 7 T. The superconducting coil was not degaussed between runs so the magnetization could not be measured reliably below 35 G, thereby placing an experimental constraint on our ability to identify potential magnetic remanance. There was no change in the measured magnetization after cycling down to 5 K and returning to 300 K. All results were normalized against the sample weight, which was between 2.3 and 5.6 mg.
Results and Discussion This work presents a procedure for preparing superparamagnetic Fe3O4/SiO2 nanocomposites enabling the tuning of both the iron oxide load and the size of the nanoparticles. This tuning was accomplished by changing the compositions of a waterin-oil microemulsion system. Hence, as a starting point, the concentration ranges that permit the establishment of the microemulsion were determined and the conditions for the particle syntheses were then based upon this information. A pseudo-ternary phase diagram of the Triton X-100 + 1-hexanol/cyclohexane/water system was constructed, see Figure 1. Microemulsions are characterized as being optically transparent, thermodynamically stable isotropic dispersions with low viscosity.24,25 As a consequence, transparent samples were considered to be microemulsions, while samples that scattered light were considered to have two phases, which in this system indicates the presence of an ordinary emulsion (not thermodynamically stable). In some instances, the system also showed translucent, slightly bluish samples as an intermediate state between the microemulsion and the two-phase regions of the phase diagram. These bluish samples indicated the presence of an emulsion with somewhat larger water droplets than those of the typical microemulsion. Sometimes these systems are referred to as miniemulsions.25 The particle syntheses were performed at a constant surfactant-to-oil ratio that was in the middle of the microemulsion region, with respect to these two components. This approach assured that small changes in conditions, due to the presence of the SPIOs or the evolution of the reaction, would (24) Handbook of microemulsion science and technology; Kumar, P.; Mittal, K., Eds.; Marcel Dekker: New York, 1999. (25) Ganguli, D.; Ganguli, M. Inorganic Particle Synthesis Via Macro and Microemulsions. A Micrometer to Nanometer Landscape; Springer, 2003.
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Figure 1. The microemulsion-containing section of the pseudoternary phase diagram of the surfactant/ co-surfactant mixture, Triton X-100 and 1-hexanol, water and cyclohexane at room temperature. (b) correspond to microemulsions, (4) mark the two phase regions, and (0) label the slightly bluish miniemulsion samples. The stars (f) correspond to the different compositions A-G at which the syntheses of the nanocomposites were performed, with the water concentration increasing going from A to G.
Figure 2. TEM micrographs showing typical architectures of the nanocomposites obtained. (a-c) were synthesized within the microemulsion region, and (d) was synthesized in the two phase region. The darker spots are the SPIO crystallites. (a) shows sample B, (b) sample I, (c) sample C, and (d) sample F.
not be likely to break the microemulsion. At the surfactant-to-oil ratio chosen, the microemulsion region extended from 4 to 15 wt % water, bordered by two-phase regions at both higher and lower water concentrations, see Figure 1. The water content in the reaction system was varied in order to study how it influenced the outcome of the particle synthesis. In Figure 1, it can be seen that the different compositions, represented by red stars, used for the syntheses followed a dilution line, through the microemulsion region and into the two phase region. Figure 2 a-c shows representative transmission electron micrographs of nanocomposites obtained from syntheses performed at water concentrations within the microemulsion region. The particles ranged from being densely packed with SPIOs to having only a few per particle. All SPIOs were individually dispersed in the silica matrix, and the nanocomposites had shells consisting only of silica. Table 1 and Figure 3a show how the particle dimensions were affected when the amount of water in the microemulsion was changed. As the amount of water increased, the overall size of the particles decreased, the silica shells thinned and the number of SPIOs in each particle generally
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Figure 3. The overall particle diameter obtained from a fit to a log-normal distribution (9), as well as the average diameter of the SPIO-containing core (2) determined from TEM micrographs. (a) shows samples B, C, and D and the effect of increasing the amount of water at 10 mg TEOS. (b) shows samples H, I, B, and J and the effect of increasing the amount of TEOS at 6.6 wt % water. Full size distributions are available in Supporting Information.
decreased, i.e., the extension of the core (the portion of the particles containing SPIOs) decreased. At 8.6 wt % water, the particles were similar to those in Figure 2c, with just one or a few SPIOs per particle. At 6.6 wt % water, particles similar to those in Figure 2a,b were obtained. Pure silica nanoparticles, prepared from the same microemulsion system at 8.6 wt % water and 10 mg TEOS, but without the SPIOs added, were spheres with an approximate diameter of 50 nm. As can be seen in Figure 3a, the inclusion of iron oxide crystallites did not significantly affect the mean size of the resulting composite (56 nm). However, the polydispersity increased and it is also evident from Figure 2a-c that the contour of the silica in the nanocomposites is no longer spherical, but exhibits distortions reflecting the arrangement of the SPIOs, which has been reported elsewhere.26 When the syntheses were performed at higher water concentrations within the two phase region, non-discrete particles were obtained; see Figure 2d. At water concentrations below 4 wt %, the system was unable to disperse the SPIO crystallites. An inverse relationship between particle size and water content of the microemulsion is also observed in the synthesis of pure silica particles using microemulsions.27,28 This trend has also been observed in the growth of silica shells on single iron oxide crystals.17 The dependence of particle size on the amount of TEOS is shown in Figure 3b. As the amount of TEOS increased, so did the number of SPIOs in each particle and the overall size of the particles; the increase in the particle size was mainly due to an increase in the thickness of the silica shell. The architectures of the particles obtained were similar to those shown in Figure 2a-c. The trend of increased particle size with increased amount of silica precursor has also been found in the use of TEOS to coat FePt crystals.17,29 (26) Vestal, C. R.; Zhang, Z. J. Nano Lett. 2003, 3, 1739. (27) Bagwe, R. P.; Yang, C.; Hilliard, L. R.; Tan, W. Langmuir 2004, 20, 8336. (28) Chang, C. L.; Fogler, H. S. Langmuir 1997, 13, 3295.
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Figure 4. TEM dark field (a) and bright field (b) images of a cluster of nanocomposite particles with the dark field micrograph showing the crystalline iron oxide as bright spots due to the diffraction of electrons. The scale bars in both micrographs are 100 nm.
Figure 5. Powder X-ray diffractograms of the ferrofluid and sample B. The Bragg peaks are identified as those of magnetite, indicated by the intensity bars in the figure.
The SPIOs, each one surrounded by a thin layer of silica, are always present in the center of the composites, embedded in a smooth outer layer of pure silica. This observation, together with previouslypublishedworkonnanoparticlesandnanocomposites,30-32 suggests that the synthesis proceeds as follows. The SPIOs are dispersed in the water phase of the microemulsion, and based on the fact that a typical microemulsion droplet is about 5-20 nm in diameter,25 it is likely that a water droplet contains no more then one SPIO. When TEOS is added it is first dissolved in the continuous oil phase, then hydrolyzed at the water-oil interface. The partly or fully hydrolyzed silica precursor is then solubilized within the water droplets. The Fe3O4 crystallites, already present within the microemulsion droplets, act as seeds at the surface of which the silica precursor condenses in accordance with a seeded growth mechanism.21 Provided that the surface area of the iron oxide crystallites is large enough, the formation of new silica particles can be completely suppressed.33 When a thin layer of silica has formed, the crystallites seem to aggregate. The number of crystallites in each agglomerate varies depending on the microemulsion composition. Low water concentrations, as well as high TEOS concentrations, promote the formation of larger agglomerates, while microemulsions containing more water tend to give agglomerates consisting of only a few SPIOs. The aggregates of iron oxide crystallites are then subject to silica deposition that results in an outer wall of (29) Lee, D. C.; Mikulec, F. V.; Pelaez, J. M.; Koo, B.; Korgel, B. A. J. Phys. Chem. B 2006, 110, 11160. (30) Yan, Q. Y.; Purkayastha, A.; Kim, T.; Kroger, R.; Bose, A.; Ramanath, G. AdV. Mater. 2006, 18, 2569. (31) Chen, S. L.; Dong, P.; Yang, G. H.; Yang, J. J. J. Colloid Interface Sci. 1996, 180, 237. (32) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 211, 210. (33) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693.
Figure 6. The field dependent magnetization at 5 K (a) and 300 K (b). Ferro denotes SPIOs extracted from ferrofluid. Nanocomposites I and B have different ratios of SiO2 to Fe3O4; I was made from 7 mg of TEOS, B from 10 mg, both with the same amount of iron oxide.
smooth silica, with the thickness governed by the amount of TEOS added. As mentioned above, the formation of new silica particles can be suppressed by choosing the right TEOS to SPIO ratio; this was found to hold true for this system. The prepared samples, B-D and I-J, did not contain any silica particles without iron oxide in the core. However, when the SPIO concentration was reduced to 1/10 or 1/100 of the original concentration, the total surface area of the iron oxide crystallites became too low and as a result formation of pure silica particles was observed. The nanocomposites formed alongside the pure silica particles were of the same size and SPIO content as those formed using the higher initial concentration of iron oxide. Reactions performed at water concentrations too high to allow for the establishment of a microemulsion resulted in non-discrete particles. Performing the reaction under Sto¨ber conditions,34 i.e. in an alcoholic medium without the presence of surfactants, most commonly results in the formation of nanocomposites with one iron oxide crystallite per particle.21 Preliminary trials using Sto¨ber conditions to coat SPIOs resulted in the presence of excess iron oxide crystallites without any silica layer, together with pure silica particles and some silica covered SPIOs after reaction. To prove that the darker spots in the particles, as seen by TEM, were in fact the iron oxide crystallites, and to ensure that the crystalline structure of the iron oxide was retained after the (34) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
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embedment in silica, TEM dark field imaging and X-ray powder diffraction (XRD) analyses were performed. Figure 4a shows the dark field version of the bright field micrograph in Figure 4b. In the dark field image, the dark spots from the bright field image show as bright spots due to the diffraction of electrons by the crystallites. Figure 5 shows that the pattern of Bragg peaks of both the ferrofluid and the nanocomposites correlated well to that of magnetite, Fe3O4. Hence, the synthesis procedure does not affect the SPIOs. Furthermore, the widths of the peaks remain unchanged after the nanocomposite formation, indicating that the crystal size is retained. In order to assess the magnetic properties of the nanocomposites, their response when subjected to an external magnetic field was investigated. The magnetic characterization was performed using a commercial magnetometer and Figure 6a,b shows the field-dependent magnetization of the ferrofluid itself and nanocomposites B and I at 300 and 5 K, respectively. Both nanocomposites contained several SPIOs per particle (see Figure 2a,b); however, Sample I has a lower silica-to-magnetite ratio than Sample B. The curves are completely reversible with no hystereses, meaning that when the magnetic field is removed, the sample exhibits no remanence; this shows that all samples were superparamagnetic.1,20 As seen in the figures, the saturation magnetization, Ms, per gram of sample decreased when the SPIOs were embedded in silica. The decrease in the silica-to-magnetite ratio going from Sample B to Sample I was clearly reflected in the decrease in Ms value. Assuming that the magnetic properties of the iron oxide crystals are maintained within the nanocomposites, the amount of magnetite can be estimated from the magnetization analysis.11 For example, the decrease in Ms from 62 emu/g (ferrofluid) to 11 emu/g (sample B), revealed that sample B (Figure 1a) contained 17 wt % magnetite, which corresponds to ca. 40 SPIOs per particle, based on the assumption
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that all particles have dimensions corresponding to the average diameter (for calculation, see SI).
Summary This work has shown how the sizes and iron oxide loads of superparamagnetic Fe3O4/SiO2 nanocomposites can be tuned by changing the amount of water in a water-in-oil microemulsion system, and how the shell thickness can be tuned by varying the amount of TEOS. Taken together, these controls imply that both the size and the loading can be varied independently using this technique. The resulting nanocomposites with high loading of SPIOs make perfect candidates for biomedical applications based on specific targeting. In these applications the number of biological targets is generally limited. Being able to include large numbers of SPIOs per particle, as presented here, will greatly reduce the detection limits of these methods. Finally, the methodology presented herein is not limited to the use of SPIOs as core constituents, so extensions to other materials may yield new nanocomposites. Acknowledgment. Technical assistance was provided by Karen Kelley at the Electron Microscopy Core Laboratory, Biotechnology Program and Valentin Craciun at the Major Analytical Instrumentation Center, Department of Materials Science and Engineering, both at University of Florida. Financial support came from the Knut and Alice Wallenberg Foundation (MS and MA), the NHMFL internal research program (RD), NSF DMR-0305371 and DMR-0701400 (MM and DP). Supporting Information Available: Particle size distributions and calculations. This material is available free of charge via the Internet at https://pubs.acs.org. LA7035604
