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Langmuir 2008, 24, 1714-1720
Facile Preparation of Highly Monodisperse Small Silica Spheres (15 to >200 nm) Suitable for Colloidal Templating and Formation of Ordered Arrays Kurtis D. Hartlen, Aristidis P. T. Athanasopoulos, and Vladimir Kitaev* Chemistry Department, Wilfrid Laurier UniVersity, 75 UniVersity AVenue W, Waterloo, Ontario, Canada N2L 3C5 ReceiVed August 15, 2007. In Final Form: October 19, 2007 Highly monodisperse spherical silica nanoparticles with diameters ranging from ca. 15 to 200 nm were prepared using an environmentally friendly water-based synthesis. The size of the spheres can be precisely controlled by using a facile regrowth procedure in the same reaction media. Furthermore, these monodisperse silica spheres can be successfully used as seeds in the well-established Sto¨ber silica preparation. The regrowth approach allows for easy incorporation of functional additives. High monodispersity and charge stabilization renders these nanoparticles highly suitable for close-packed array formation and colloidal templating.
Introduction Monodisperse nanosized colloids are one of the cornerstones of nanoscience and nanotechnology due to their well-defined dimensions and functional properties.1,2 Silica is arguably the most versatile colloidal material3 given its ease of preparation, precursor availability, and low point of zero charge (pzc) in water at pH ) 2.4 The latter is important for imparting remarkable colloidal stability to silica due to its large negative surface charge at neutral and basic conditions. Given all these factors, silica features a myriad of diverse applications from formation of synthetic opals and their inverse5 to colloidal templating6 and silica coating for stabilization and protection of metal and semiconductor nanoparticles.7 Notably, silica was the first inorganic monodisperse colloidal particles prepared in solution and systematically characterized in 1968 by Sto¨ber et al.8 The synthetic procedure utilized ammonia-catalyzed hydrolysis of silicon alkoxides of varying chain length in water-ethanol solutions to produce particles in a size range of several hundred nanometers. Subsequently, the original method was streamlined to use readily available tetraethyl orthosilicate (TEOS) with the size control attained through variation of ammonia and TEOS concentrations.9 In an important development, seeded regrowth was adopted10,11 to improve monodispersity and size control by disentangling the less controllable nucleation as a separate stage of formation of small particles (seeds) and regrowing these seeds in well-defined conditions. Once the size of the seeds is determined by one of the standard techniques, the size of the final particles can be * Corresponding author. E-mail:
[email protected]. (1) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (2) Wang, X.; Li, Y. Chem. Commun. 2007, 2901. (3) Bergna, H. E. Surf. Sci. Ser. 2006, 131, 9. (4) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley: New York, 1979. (5) Stein, A.; Schroden, R. C. Curr. Opin. Solid State Mater. Sci. 2001, 5, 553. (6) Kuai, S.; Badilescu, S.; Bader, G.; Bruning, R.; Hu, X.; Truong, V.-V. AdV. Mater. 2003, 15, 73. (7) Liz-Marzan, L. M.; Mulvaney, P. Surf. Sci. Ser. 2006, 131, 665. (8) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (9) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F., IV J. Non-Cryst. Solids 1988, 104, 95. (10) Van Blaaderen, A.; van Geest, A.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481. (11) Giesche, H. J. Eur. Ceram. Soc. 1994, 14, 205.
controlled precisely through the amount of a precursor added to the seed dispersion.11 Modifications of the Sto¨ber process currently remain the dominant route for the synthesis of monodisperse silica particles. High monodispersity of better than 3% in standard deviation from the average size can be achieved for particle diameters larger than 200-250 nm,11 with the monodispersity improving proportionally with the particle size.11,12 The main limitation of the Sto¨ber process remains in preparation of smaller particles. The smallest particles that can be prepared have a size around 15-20 nm and very high polydispersity (typically >20%).11 As a result, monodispersity better than 4-5% is very challenging to achieve for sizes below ca. 120 nm using either a single-stage preparation or regrowth of smaller seeds. An alternative route to produce monodisperse silica spheres in a size range of several tens to a few hundred nanometers is chimie douce of microemulsions, where particles form in inverse micelles compartmentalized by a suitable surfactant in a nonpolar organic solvent.13 The microemulsion approach works particularly well for the size range from 30 to 60 nm, yielding silica spheres with superior average monodispersity compared to particles produced by the Sto¨ber method.14 At the same time, notable disadvantages of microemulsions include using large amounts of surfactants (typically several times more by weight than silica)15 which often necessitates extensive cleaning. Furthermore, microemulsions have inherent problems with sample nonhomogeneity arising from a challenge to sustain uniform micelle compartments in soft and sensitive phases. Recently, in an important breakthrough, Yokoi et al. reported preparation of monodisperse silica particles with the range of sizes from 12 to 23 nm.16 Their approach utilizes lysine as a base catalyst in aqueous media, while TEOS is delivered heterogeneously using a top organic layer. Previously, lysine has been demonstrated to be effective for the formation of silica particles17 (12) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589. (13) Pileni, M. P. Nat. Mater. 2003, 2, 145. (14) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 221, 210. (15) Finnie, K. S.; Bartlett, J. R.; Barbe, C.; Kong, L. Langmuir 2007, 23, 3017. (16) Yokoi, T.; Sakomoto, Y.; Terasaki, O.; Kubota, Y.; Okubo, T.; Tatsumi, T. J. Am. Chem. Soc. 2006, 128, 13664. (17) Davis, T. M.; Snyder, M. A.; Krohn, J. E.; Tsapatsis, M. Chem. Mater. 2006, 18, 5814.
10.1021/la7025285 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
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due to its electrostatic interactions with silicic acid. Yet the monodispersity of silica particles achieved by Yokoi et al.16 was quite unprecedented for such a small size. To control the silica particle size, D- and L-forms of lysine and their mixtures were used, which allowed size variation in a fairly limited range.16 Utilizing this effective approach16 as a starting point, we report facile preparation of highly monodisperse silica particles ranging from 15 nm to more than 200 nm using a regrowth approach and arginine as a base catalyst in aqueous media. Furthermore, the reported silica particles have been proven to be compatible with the Sto¨ber process and thus can be readily regrown further without any purification to arbitrarily larger sizes, while perfectly preserving their monodispersity. High monodispersity of the prepared silica spheres renders them perfectly suitable for colloidal array formation and colloidal templating. All of the above factors combined with easy incorporation of functional additives during regrowth enable a plethora of potential applications of these particles ranging from photonics18 to biological imaging,19 especially since produced silica dispersions do not contain hazardous chemicals and the particle surface is stabilized by a natural amino acid. Experimental Section Chemicals. Tetraethylorthosilicate (TEOS, 98%) was acquired both from Lancaster and Aldrich. l-Arginine (98%), cyclohexane (anhydrous, 99.5%), ammonium hydroxide (TraceSelect), and anhydrous ethanol (>99.5%) were supplied by Aldrich and used as received. High-purity deionized water (>18.4 MΩ‚cm) was produced using Millipore A10 Milli-Q. Seed Synthesis. A typical synthesis of the smallest silica seeds of the test series was performed in a 20 mL vial (VWR Scientific) and involved addition of 9.1 mg of L-arginine (0.052 mmol) to 6.9 mL of water while thoroughly mixing the solution. Then 0.45 mL of cyclohexane was added to the water-arginine solution and the reaction was heated to ∼60.0 ( 0.2 °C in a water bath under magnetic stirring using a Heidolph MR3004. Employing a standard Tefloncoated stirring bar with a length of 1 cm, the stirring rate was ca. 300 rpm to mix the aqueous solution well, while keeping the top layer relatively undisturbed. Once the solution reached 60 °C, 0.55 mL of TEOS (2.5 mmol) was added to the vial. The reaction was then kept at constant stirring and temperature for 20 h. The reaction parameters summarized above yielded reproducibly silica particles of 24 ( 3 nm in more than 40 independent runs. In several trials, the above reaction was scaled up 40 times in reagent amounts and performed in a 500 mL double-walled jacketed flask with overhead stirring at 55 rpm (Heidolph RZR 2021) using a large Teflon blade stirrer fitting the shape of the flask. Regrowth of Silica Seeds. To regrow silica seeds to a desired size, an appropriate portion of the seeds was taken and diluted with water so that the final arginine concentration was in the range of 1-2 mM. Subsequently, cyclohexane was added to ensure that the final volume ratio of total TEOS to cyclohexane remained at or slightly below 1:1. The mixture was then brought to 60 °C under constant stirring of ca. 300 rpm (Heidolph MR3004) and a required amount of TEOS was added to the solution at once. Representative reagent amounts for a typical regrowth of 24 nm seeds to a final particle size of 45 nm were as follows: 1 mL of as-prepared silica seeds (silica concentration of 0.315 M), 3.6 mL of water, 0.5 mL of cyclohexane, and 0.352 mL of TEOS. Once TEOS was added, the reaction was allowed to proceed for 30 h while maintaining the stirring and constant temperature of 60 °C. Further Regrowth Using Sto1 ber Method. In a typical regrowth reaction, 1 mL of 88 nm silica seeds (silica concentration of 0.34 M), which were to be regrown to a final size of 230 nm, was gently dispersed in 18 mL of ethanol solution containing water and ammonia with the total concentrations in the final mixture being 11 and 1.2 (18) Arsenault, A. C., et al. Nat. Mater. 2006, 5, 179. (19) Cohen, S. M. Curr. Opin. Chem. Biol. 2007, 11, 115.
Langmuir, Vol. 24, No. 5, 2008 1715 M, respectively. Upon uniform mixing, 4.4 mL of TEOS (21 mmol) was added over a 5 h period using a syringe pump (KDS 200, KD Scientific). Fluorophore Incorporation. Starting with 1 mL of 40 nm spheres (silica concentration of 0.275 M) obtained after the first regrowth step (as described above in the section Regrowth of the Silica Seeds), Rhodamine 6G (1 × 10-6 mol; 1:1000 molar ratio to silica in the final particles) was introduced as an aqueous solution. The physisorbed dye was capped by a silica shell in a second regrowth stage by adding 0.146 mL of TEOS (9.3 mmol) diluted with 0.14 mL of cyclohexane to yield the particles with the final size of 54 nm. To remove non-incorporated Rhodamine 6G and its complexes with soluble silicates, the dye-containing silica particles were cleaned by centrifugation and redispersion in water three times until no fluorescence could be detected in supernatant. After a period of several weeks, the cleaned dispersion was centrifuged to test for dye leaching, none of which could be detected by fluorescence. Characterization. Electron microscopy was performed using a Hitachi S-5200. Samples were prepared by directly depositing dilute dispersions onto copper grids coated with Formvar and a carbon layer (EMS Corp.). Accelerating voltage of 30 kV without conductive coating of the sample was used to image individual particles both in TEM and SEM modes. Lower accelerating voltages from 1 to 5 kV were used for imaging of close-packed silica arrays. The average size and standard deviation were determined from SEM and TEM images by averaging diameters of more than 100 particles. An Ocean Optic QE 65000 fiber-optic UV-vis spectrometer was used for monitoring the yield of dye incorporation into silica particles.
Results and Discussions Figure 1 shows a wide range of sizes of monodisperse silica spheres achieved in our work by using a modified synthesis of Yokoi et al.16 coupled with semi-continuous regrowth. In an initial stage of seed formation, tetraethoxysilane (TEOS) was hydrolyzed in aqueous solution catalyzed by arginine and delivered heterogeneously using a top layer of cyclohexane. TEOS was employed as the most common silica precursor, which has suitable hydrolysis rates in our reaction conditions. Its effective concentration, the same as the final silica concentration, was optimal in the range from 0.3 to 0.5 M. Higher concentrations led to noticeable deterioration of monodispersity due to an increasing number of rougher and irregular-shaped particles. TEOS concentrations lower than 0.3 M did not lead to noticeable improvement in monodispersity while lowering synthetic efficiency of the reaction. Influence of the Reaction Parameters. TEOS was delivered into the reaction medium in a form of a mixture with cyclohexane. The role of the cyclohexane is to slow TEOS release into the aqueous layer and to preserve TEOS prior to its partitioning into the reaction. Cyclohexane was used as an organic layer since it is readily available in high purity; it does not evaporate appreciably in the reaction conditions (yet it could be readily removed at low pressure with minimal water removal and hence drying of the dispersions). Optimal amounts of cyclohexane in the reaction were around 1:1 by volume (cyclohexane to TEOS) or slightly less (typically 0.45-0.55). Larger amounts of cyclohexane could be used, if necessary, since the organic solvent does not participate in reaction. In principle, any nonpolar solvent, which is immiscible with water and has a lower density than water, can be used. For instance, we were successful using a vegetable oil as a TEOS delivery medium to explore “green chemistry” modifications of the reaction (see below). Cyclohexane (as any solvent of the top organic layer) does not affect the chemistry of the reaction, yet it is important to physically slow down TEOS hydrolysis and its subsequent release into the aqueous phase. Without cyclohexane, TEOS is hydrolyzed at a faster rate, which tends to nucleate more particles and leads to
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Figure 1. Range of monodisperse silica spheres achieved imaged by electron microscopy: (a) and (b) 18.5 ( 0.8 and 26 ( 1 nm silica seeds, respectively, prepared by a single-stage reaction; (c) 45.5 ( 1.3 nm and (d) 71.5 ( 2.5 nm silica produced by one-stage regrowth process from the seeds; (e) 84 ( 2 nm, (f) 117 ( 2.5 nm, (g) 141 ( 2.5 nm, and (h) 167 ( 3 nm silica spheres synthesized by two, three, four, and five stages of regrowth, respectively. See Experimental Section for more details. The scale bar is 100 nm for (a) and (b) and 500 nm for (c)-(h).
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Figure 2. Electron microscopy images illustrating influence of the reaction parameters: (a) 26.5 ( 0.8 nm particles prepared at 6.6 mM arginine with the use of cyclohexane and (b) 11 ( 2 nm particles prepared at 6.6 mM arginine without cyclohexane; (c) 21.5 ( 1 nm silica prepared at 26.5 mM arginine without cosolvent; (d) 17.2 ( 1 nm silica prepared at 26.5 mM arginine with 7.7 vol % ethanol. The scale bar is 100 nm for all images.
appreciably smaller average particle sizes with lower reproducibility from run to run. Figure 2b demonstrates particles of smaller size and higher polydispersity, which were synthesized in the absence of cyclohexane. At the same time, the recent work by Snyder et al.20 utilized high concentrations of lysine (25-30 nM) in the presence of other solvents to produce monodisperse silica particles. Similarly, for higher arginine concentrations of 25-30 mM, the presence of cosolvents has minimal effect on the reaction. Figures 2c and 2d show a comparison of the silica particles prepared using 0.3 M TEOS and 26.5 mM arginine in the absence of any cosolvent and with 7.7 vol % ethanol present, respectively. It is reasonable to expect that as the arginine concentration increases, arginine interacts more strongly with the silica surface as well as accelerates TEOS hydrolysis. As a result, the presence of an organic layer or cosolvent has less relative influence in such systems. The reaction conditions corresponding to the image shown in Figure 2d closely match recently reported silica preparation using lysine,20 which yielded particles of ca. 18-20 nm particles with comparable polydispersity that suggests that at relatively high concentrations lysine and arginine can be used interchangeably. Arginine has been used previously as a base catalyst of silica hydrolysis and for silica particle stabilization;21 it was also suggested to be suitable for the preparation of small monodisperse particles.16 Lysine was used by Yokoi et al.,16 Davis et al.,17 and (20) Snyder, M. A.; Lee, A.; Davis, T. M.; Scriven, L. E.; Tsapatsis, M. Langmuir 2007, 23, 9924. (21) Patwardhan, S. V.; Clarson, S. J. J. Inorg. Organomet. Polym. 2003, 13, 49.
Snyder et al.,20 for preparation and stabilization of smaller silica particles and undoubtedly can be used for the regrowth procedure. At the same time we found that using arginine in a lower concentration range of 2-10 mM in combination with a top organic layer is more effective for a regrowth process. Arginine allows for better stability to the minor variations experimental conditions and as a result offers better size reproducibility in a seed-formation stage, which is likely due to its stronger basicity and interactions with a silica surface. For instance, the reaction parameters described in the Experimental Section were reproduced in more than 40 test runs yielding silica particles of 24 ( 3 nm with great reliability (the first number is the average of the average size and the second number is the standard deviation from this average not size polydispersity of individual runs). The reproducibility of the seed size is very important for the size control in subsequent regrowth reactions without the necessity to rely on electron microscopy for size monitoring. Furthermore, using lower arginine concentrations allows producing particles of a reasonable roughness. Note that no annealing steps16,20 were used for any of the silica shown in Figure 1 and elsewhere. The rate of TEOS hydrolysis was optimal at 60 °C to complete reaction under 24 h, so we did not vary the temperature relative to previously reported conditions.16 Furthermore, 60 °C is also the optimal temperature established in the Sto¨ber process for the preparation of the smallest silica particles.22 In principle, hexadecane can be used as an organic solvent to minimize evaporation of both organic layer and water to run the reaction at higher temperatures to achieve higher reaction rates, if required. (22) Philipse, A. P. Colloid Polym. Sci. 1988, 26, 1174.
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Figure 3. Electron microscopy images of silica spheres prepared in less than optimum conditions: (a) particles with bimodal distribution incurred as a result of secondary seeding at the higher than optimal arginine concentration of 6 mM during regrowth; (b) rough particles produced at a low arginine concentration of 0.2 mM during regrowth; (c) 48 ( 2 nm silica prepared in the regrowth reaction where the high total silica concentration of 2.0 M has been reached; (d) polydisperse particles produced by Sto¨ber regrowth of the over-aged seeds. See Experimental Section for more details. The scale bar is 100 nm for (b) and 500 nm for (a), (c), and (d).
Regrowth in the Same Reaction Media. Following the seed formation, larger monodisperse silica nanoparticles of a desired size have been conveniently prepared upon judicious tuning of the regrowth conditions in the same reaction media. In a typical regrowth step, a required portion of seeds was removed from the reaction and replaced by an aqueous arginine solution or pure water, after which a new portion of TEOS was added to achieve a desired size according to a straightforward formula:11
(
)
MassTEOS,added Diameterfinal 3 -1) Diameterseed MassTEOS,seed
The single most important parameter for the successful regrowth of the silica seeds was discovered to be the arginine concentration. We found that arginine concentration should be lowered to minimize secondary seed formation to an optimal 0.8-1.5 mM range for a typical regrowth first from ∼24 to 40-50 nm and then further to larger sizes. Figure 3a exemplifies a typical scenario of secondary seed nucleation with a characteristic bimodal distribution featuring well-defined sizes when arginine concentration remained at 6 mM during regrowth. For larger regrowth steps (greater than 2 times the original seed size) arginine concentration should be further lowered and, preferably, TEOS added in increments. By the same token, higher arginine concentrations can be used for the smaller regrowth steps, e.g., 2 mM for regrowth from 40 to 65 nm. We also found that the regrowth reaction works at arginine concentrations even as low as 0.2 mM, although particles become increasingly rough (Figure 3b).
The optimal regrowth temperature remains at 60 °C since it allows use of lower arginine concentrations and minimizes secondary seeding. Regrowth at room temperature is possible at arginine concentration higher than 10 mM. However, the resulting particles produced are substantially rougher, to the point of monodispersity impairment, and are similar to those shown in Figure 3b. TEOS concentration was typically kept in a range from 0.3 to 0.36 M for the regrowth to the final sizes smaller than 50 nm and in the range from 0.3 to 0.5 M for the regrowth of the particles between 50 and 120 nm. These TEOS concentrations were established by drawing upon our experience with Sto¨ber regrowth where using low TEOS (and hence final silica) concentration is essential to minimizing doublet formation.12 At the same time, it is noteworthy that for arginine concentrations higher than 0.1 M the final silica concentrations higher than 2 M can be achieved without noticeable doublet formation but with some decrease in monodispersity, as demonstrated in Figure 3c. This may become important for the large-scale synthesis of monodisperse small silica since dispersions with high concentrations are most cost-effective to produce. Using the described facile procedure to produce silica particles in the same reaction media works most effectively up to a size of 120 nm, using regrowth increments of about 1.5 times in size. After reaching 120 nm particle size, the regrowth increments have to be decreased to avoid secondary seed nucleation. Alternatively, in a more universal approach, the silica particles produced can be introduced as seeds into a well-established Sto¨ber silica formation reaction. The advantage of such regrowth is the
Highly Monodisperse Small Silica Spheres
Figure 4. Electron microscopy images of the larger monodisperse silica spheres produced by Sto¨ber regrowth of 80 nm silica seeds prepared using arginine as a catalyst: (a) 238 ( 2.2 nm; (b) 229 ( 2.2 nm. See Experimental Section for more details. The scale bar is 1 µm for all images.
ability to produce arbitrarily large silica particles while preserving excellent monodispersity, which cannot be achieved by the Sto¨ber process itself. The silica seeds produced by arginine regrowth can be used as is without any purification. Neither arginine nor traces of cyclohexane interfere with the Sto¨ber silica regrowth. However, we have found that the smaller the seed size, the more crucial it is to use the seeds for the regrowth promptly after their preparation. Twenty nanometer silica needs to be used within 1 day postsynthesis, while particles of ∼40 nm size have a window of roughly 3 days after preparation to be used as seeds. Eighty nanometer silica has a useful “shelf life” of at least a few weeks after their synthesis. It seems that aged particles partially aggregate upon addition of ammonia, yielding polydisperse samples upon regrowth, as illustrated in Figure 3d. Further Regrowth by Sto1 ber Method. Figure 4 shows several batches of silica particles regrown from ∼80 nm silica seeds (and aimed at 230 nm final sizes without prior TEM imaging of the seeds), which demonstrates the ease and convenience of the size control. This regrowth was performed in ethanol-water solution with a usual water concentration range of 8-10 M and ammonia as a catalyst.11,12 Ammonia concentration was kept at 1.0-1.5 M to sustain efficient regrowth rates at room temperature, which allowed adding TEOS over a relatively short period of 5-6 h and gave final silica concentration of 0.6 M. Suitable Applications. All the synthesized silica particles are stabilized electrostatically by virtue of the negatively charged silica surface counterbalanced by protonated arginine. These silica
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spheres can be easily cleaned by standard centrifugation procedures and re-dispersed in a range of polar solvents. We found that a minimum of two supernatant removals is sufficient to eliminate the majority of soluble arginine-silicate complexes. At the same time, it is worthwhile to note that when working with silica coating of nanoparticles produced in similar conditions, we could observe partial dissolution of the coatings (few nanometers) upon cleaning when the arginine concentration remained relatively high (0.7-3 mM range). By nature of its charge stabilization and high monodispersity, the prepared silica spheres readily formed close-packed colloidal arrays upon drying. The arrays could be produced even by assynthesized dispersions dried directly on TEM grids (Figure 5a). The packing quality could be further refined by cleaning the particles and utilizing evaporation-assisted vertical self-assembly23 (Figure 5b). Interestingly, water, used as deposition solvent, yielded higher quality arrays of the smaller size silica particles compared to the traditionally used ethanol.23 This observation can be tentatively rationalized by the fact that slower evaporation should work better for arrangement of smaller, more mobile, particles in the meniscus.24 It is also noteworthy that compared to closely packed silica particles produced by the Sto¨ber process, the arrays of small silica particles prepared in this work seem to have relatively few cracks. A plausible explanation for fewer cracks would be that the arginine-stabilized surface partially screens charge interactions and allows particles to come into closer contact during the initial stages of the array formation in the drying front.24 The reported regrowth procedure is highly versatile in terms of accommodating functional additives. To illustrate this, we performed silica labeling with a fluorescent dye, Rhodamine 6G, by incorporating it through physical adsorption by nature of its amino groups. The dye was introduced into the reaction after the first seed regrowth and subsequently capped with a pure silica outer shell.25,26 After removal of nonbound dye, the amount of incorporated Rhodamine 6G was determined to be 10% of the total amount of dye used for incorporation into silica, based on the comparison of adsorption intensities of dyed particles with the free dye in water. This value is reasonable considering that it is likely an underestimation since the dye incorporated inside the spheres will have less interactions with light and there is substantial scattering from the dispersion. Furthermore, no dye leaching into the solution could be detected by fluorescence following a period of several weeks after cleaning. The TEM image of the particles with the incorporated dye is shown in Figure 5c. Considering that the produced particles should be biologically compatible by virtue of their amino acid capping, the only potentially hazardous component remaining upon reaction completion is the hydrophobic solvent (cyclohexane). Since it is not essential for the reaction, we opted to replace it with a biocompatible substance in one of the test runs using canola oil as a typical vegetable oil. The use of polyunsaturated oils, e.g., sunflower, may be more problematic due to prolonged heating at 60 °C in the presence of air. The particles produced using canola oil are comparable in quality to ones prepared using cyclohexane, as can be seen in Figure 5d, while their TEM imaging in the as-prepared state is a bit more challenging due to unavoidable oil presence in the samples. Therefore, it can be (23) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (24) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (25) Van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921. (26) Kitaev, V.; Fournier-Bidoz, S.; Yang, S. M.; Ozin, G. A. J. Mater. Chem. 2002, 12, 966.
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Figure 5. Electron microscopy images of (a) and (b) close-packed colloidal arrays formed by small monodisperse silica spheres upon (a) drying 44.0 ( 1.2 nm particles taken directly from the reaction on TEM grids and (b) self-assembly of purified 52.0 ( 1.5 nm particles; (c) monodisperse 54.0 ( 2.2 nm silica particles with Rhodamine 6G incorporated into them; (d) 17 ( 1 nm silica particles prepared by using canola oil as a top organic layer instead of cyclohexane. See Experimental Section for more details. The scale bar is 200 nm for all images.
claimed that the reported silica preparation is compatible with green chemistry and the resulting particles can be used directly for diverse biological and medical applications.
Conclusions We have successfully developed a facile one-pot method to synthesize monodisperse silica spheres with the sizes ranging from 15 nm to >200 nm. The reaction parameters have been optimized to produce small silica seeds with high reproducibility and subsequently to regrow them in the same reaction media using arginine as a catalyst and size-determining agent. Regrown silica particles with the size range from 50 to 100 nm were demonstrated to be directly compatible with the well-established Sto¨ber silica regrowth. Such compatibility enabled us to produce a continuous range of silica spheres with better than 2% polydispersity, which is challenging to achieve for sub-200 nm particles by the Sto¨ber process alone.
Due to high monodispersity and charge-stabilized surface, reported silica particles are highly suitable for formation of closepacked arrays and colloidal templating. The facile regrowth process allows for convenient introduction of functional additives such as fluorescent dyes. The reaction can be run with an environmentally friendly vegetable oil as an organic solvent to ensure maximum biocompatibility for potential biological and medical applications. Acknowledgment. The authors gratefully acknowledge financial support by the Natural Science and Engineering Research Council of Canada, Canada Foundation for Innovation, Research Corporation (Cottrell Award), and Wilfrid Laurier University. WATLabs (University of Waterloo) and the Centre for Nanostructure Imaging (University of Toronto) are greatly appreciated for providing access to Electron Microscopy facilities. LA7025285
