Terminating Effects of Organosilane in the Formation of Silica Cross

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Terminating Effects of Organosilane in the Formation of Silica Cross-Linked Micellar Core-Shell Nanoparticles Fangli Chi, Buyuan Guan, Bin Yang, Yunling Liu, and Qisheng Huo* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China Received March 4, 2010. Revised Manuscript Received April 19, 2010 One advanced synthesis strategy for monodisperse silica cross-linked micellar core-shell nanoparticles (SCMCSNs) involves the use of organosilane termination agent RnSi(OR0 )4 - n. In this study, we investigated the effects of the organosilane termination agent in the formation of SCMCSNs. Experimental data (synthesis results, 29Si MAS NMR, molecule probe fluorescence spectra, etc.) from a synthesis system with Pluronic F127 as the template indicate that organosilane either covers or reacts with the surface Si-OH groups of nanoparticles. The reduction of reactive surface Si-OH groups helps to stabilize nanoparticles by avoiding aggregation. The terminating behavior of organosilane is determined by its molecular structure, including (1) the value of n, (2) the length of hydrocarbon chain R, and (3) the charge of R. Effective organosilane termination agents are also applicable to other synthesis mixtures such as the systems using Si(OC2H4OH)4 as the silica source or F108 or Brij 700 as the template. Furthermore, we can obtain monodisperse nanoparticles by using the trisodium salt of triacetic acid N-(trimethoxysilylpropyl)ethylenediamine (TANED), which acts not only as a termination agent for the successful synthesis of SCMCSNs but also as a functional group to improve the performance of SCMCSNs in potential applications.

Introduction Monodisperse nanoparticles have attracted a great amount of attention in the last two decades1-3 because they are ideal candidates for applications in a broad range of fields such as nanomedicine,4,5 biology,6,7 catalysis,8-10 optoelectronics,11 chemical sensing,12,13 and so forth. Among various nanoparticle materials, monodisperse inorganic-organic hybrid nanoparticles are especially attractive because they have the combined advantages of inorganic (e.g., rigidity and stability) and organic (e.g., flexibility) materials. Moreover, monodisperse core-shell nanoparticles, especially a polymer core surrounded by a silica shell, lead to materials with promising properties. Many researchers are devoted to creating monodisperse polymer-silica core-shell nanoparticles with a controlled size, a narrow size distribution, and various surface properties.14 Several *Corresponding author. Phone: þ86-431-85168602. Fax: þ86-431-85168624. E-mail: [email protected]. (1) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630. (2) Plush, S. E.; Woods, M.; Zhou, Y.-F.; Kadali, S. B.; Wong, M. S.; Sherry, A. D. J. Am. Chem. Soc. 2009, 131, 15918. (3) Lin, Y. S.; Hung, Y.; Lin, H. Y.; Tseng, Y. H.; Chen, Y. F.; Mou, C. Y. Adv. Mater. 2007, 19, 577. (4) Kim, J.; Park, S.; Lee, J. E.; Jin, S. M.; Lee, J. H.; Lee, I. S.; Yang, I.; Kim, J. S.; Kim, S. K.; Cho, M. H.; Hyeon, T. Angew. Chem., Int. Ed. 2006, 45, 7754. (5) McCarthy, J. R.; Kelly, K. A.; Sun, E. Y.; Weissleder, R. Nanomedicine 2007, 2, 153. (6) Ow, H.; Larson, D. R.; Srivastava, M.; Baird, B. A.; Webb, W. W.; Wiesner, U. Nano Lett. 2005, 5, 113. (7) Fuller, J. E.; Zugates, G. T.; Ferreira, L. S.; Ow, H. S.; Nguyen, N. N.; Wiesner, U. B.; Langer, R. S. Biomaterials 2008, 29, 1526. (8) Shylesh, S.; Schweizer, J.; Demeshko, S.; Schunemann, V.; Ernst, S.; Thiela, W. R. Adv. Synth. Catal. 2009, 351, 1789. (9) Bell, A. T. Science 2003, 299, 1688. (10) Zhao, Y.; Jiang, L. Adv. Mater. 2009, 21, 3621. (11) Zhang, L. H.; Liu, B. F.; Dong, S. J. J. Phys. Chem. B 2007, 111, 10448. (12) Braun, G.; Lee, S. J.; Dante, M.; Nguyen, T. Q.; Moskovits, M.; Reich, N. J. Am. Chem. Soc. 2007, 129, 6378. (13) Jun, Y. W.; Lee, J. H.; Cheon, J. Angew. Chem., Int. Ed. 2008, 47, 5122. (14) Jovanovic, A. V.; Underhill, R. S.; Bucholz, T. L.; Duran, R. S. Chem. Mater. 2005, 17, 3375.

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synthesis methods have been reported in the literature for the preparation of polymer-silica core-shell nanoparticles; among them hard and soft template methods have attracted growing interest in recent years. Polystyrene beads are often used as one of the hard templates for the synthesis of monodisperse nanoparticles. The silica precursors used as shells are often prepared by the classical or improved St€ober method,15 which can provide silica particles with high monodispersity. Graf et al.16 described a general method of coating amphiphilic polymer PVP-adsorbed polystyrene colloidal particles with silica in a solution of ammonia in ethanol. Xia et al.17 reported the synthesis of hybrid spherical colloids composed of polystyrene cores and silica shells. The surfaces of polystyrene beads functionalized with amine groups are essentially neutral or slightly positively charged, which causes silica sols to be easily nucleated on the surfaces of polystyrene beads and eventually to merge and grow into a uniform shell. Micelles from AB, ABC, and ABA copolymers are often used as soft templates. Armes et al.18 used AB diblock copolymer micelles comprising cationic poly(2-(dimethylamino)ethyl methacrylate) (PDMA) coronas and hydrophobic poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) cores as nanosized templates for the deposition of silica from aqueous solution at pH 7.2. Khanal19 used ABC triblock copolymer micelles having a core-shellcorona architecture as templates for the fabrication of hollow silica nanospheres. Silica species were deposited on poly(styrene-b2-vinyl pyridine-b-ethylene oxide) (PS-PVP-PEO) micelles by the sol-gel method. (15) Werner, S.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62. (16) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (17) Lu, Y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, 3464. (18) Yuan, J. J.; Mykhaylyk, O. O.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2007, 129, 1717. (19) Khanal, A.; Inoue, Y.; Yada, M.; Nakashima, K. J. Am. Chem. Soc. 2007, 129, 1534.

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We have synthesized monodisperse SCMCSNs20 by using Pluronic F127, an ABA triblock copolymer (PEO-PPO-PEO), micelles as templates, with the strong interactions of TEOS and PEO under extremely acidic conditions to form the silica shell. The key difference from previous syntheses is the use of diethoxydimethylsilane (DEDMS) as a termination agent to inhibit the condensation of nanoparticles. The use of a termination agent has been utilized by other groups since then. Yu et al.21synthesized well-dispersed silica nanoparticles with a uniform particle size of 24.7 nm by using F108 as the template, trimethylbenzene (TMB) as the swelling agent, and dimethoxydimethylsilane as the termination agent. Liu et al.22 fabricated a family of magnetic nanoparticles using termination agent DEDMS based on silica crosslinked Pluronic F127 block copolymer micelles loaded with nanosized iron oxide. The termination agent mentioned here refers to one kind of organosilane, a silane coupling agent RnSi(OR0 )4 - n having at least one reactive group such as a methoxy, ethoxy, or silanol hydroxy group. The silane coupling agent is commonly used to modify the surface of silica to make particles functional23,24 or change the surface properties (hydrophobic, hydrophilic, or amphiphilic).25 The hydrolysis and condensation reactions of the organosilane coupling agent have been well studied using NMR, IR, and fluorescence spectra.26-30 However, the effects of the termination agent in the synthesis of SCMCSNs has not been investigated in detail. Here we have systematically studied the effects of the termination agent in the formation of silica cross-linked monodisperse nanoparticles by using TEM, DLS, 29Si MAS NMR, and fluorescence spectra. The effect of the value of n in organosilane coupling agent RnSi(OR0 )4 - n, the length of the hydrocarbon chain, and the charge on R have been examined. The termination mechanism is proposed and is supported by the synthesis of SCMCSNs using Si(OC2H4OH)4 as the silica source or different surfactants such as F127, F108, or Brij700 as the template. We also made the functionalized SCMCSNs by using the silane coupling agent with carboxyl groups that can coordinate with metal ions such as Co(II).

Experimental Section Chemicals. Pluronic F127, F108, Brij700, pyrene, methyltriethoxysilane (MeSi(OEt)3, MTES), diethoxydimethylsilane (Me2Si(OEt)2, DEDMS), and trimethylethoxysilane (Me3SiOEt, TMES) were purchased from Sigma-Aldrich. Tetraethoxysilane (TEOS) was purchased from Fluka. Aminopropyltriethoxysilane(APTES), 3-aminopropylmethyldiethoxysilane(APMDS), (3-trimethoxysilylpropyl)diethylenetriamine (AEPTMS), and N-(trimethoxysilylpropyl)ethylenediamine triacetic acid, trisodium salt (45% in water) (TANED) were purchased from Gelest. Vinyltriethoxysilane (VTES), phenyltrimethoxysilane (PTMS), diphenyldimethoxysilane (20) Huo, Q. S.; Liu, J.; Wang, L. Q.; Jiang, Y. B.; Lambert, T. N.; Fang, E. J. Am. Chem. Soc. 2006, 128, 6447. (21) Zhu, J.; Tang, J.; Zhao, L.; Zhou, X.; Wang, Y.; Yu, C. Small 2010, 6, 276. (22) Liu, Z.; Ding, J.; Xue, J. New J. Chem. 2009, 33, 88. (23) Antochshuk, V.; Jaroniec, M. Chem. Mater. 2000, 12, 2496. (24) Bradley, C. A.; Yuhas, B. D.; McMurdo, M. J.; Tilley, T. D. Chem. Mater. 2009, 21, 174. (25) Jungmann, N.; Schmidt, M.; Ebenhoch, J.; Weis, J.; Maskos, M. Angew. Chem., Int. Ed. 2003, 42, 1714. (26) Xu, Y.; Wu, D.; Sun, Y.; Gao, H.; Yuan, H.; Deng, F. J. Sol.-Gel Sci. Technol. 2007, 42, 13. (27) Stangar, U. L.; Sassi, A.; Venzo, A.; Zattin, A.; Japelj, B.; Orel, B.; Gross, S. J. Sol.-Gel Sci. Technol. 2009, 49, 329. (28) Yokoi, T.; Yoshitake, H.; Tatsumi, T. J. Mater. Chem. 2004, 14, 951. (29) Dong, H. J.; Lee, M.; Thomas, R. D.; Zhang, Z. P.; Reidy, R. F.; Mueller, D. W. J. Sol.-Gel Sci. Technol. 2003, 28, 5. (30) de Monredon-Senani, S.; Bonhomme, C.; Ribot, F.; Babonneau, F. J. Sol.-Gel Sci. Technol. 2009, 50, 152.

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(DPDMS), propyltriethoxysilane (PTES), and octyltriethoxysilane (OTES) were purchased from Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd. All of the chemicals were used without further purification. Synthesis of Nanoparticles. TEOS (0.5 g) was added to an F127 acidic aqueous solution (5 g; 0.5-2 M HCl and 5.4 wt % F127). The mixture was stirred for 0.5 h, and then a certain amount of termination agent was added. After another 0.5 h of stirring, the solution was kept at room temperature. Characterization of Nanoparticles. Transmission electron microscopy (TEM) was carried out by using JEOL 2100F and a Hitachi H-800 operated at 200 kV. The TEM samples were prepared by placing a small drop of the solution containing the particles on a TEM grid and removing the excess water. Dynamic light scattering (DLS) was recorded with the Malvern Zetasizer Nano-Series instrument at room temperature. The steady-state fluorescence of pyrene was measured using an Ocean Optics USB4000 spectrometer and a Thermo Scientific PI95035 3UV ultraviolet lamp (302 nm) as the light source. The UV-vis characterization was carried out with a UV-2450 spectrophotometer (Shimadzu, Japan). 29Si MAS NMR spectroscopy was conducted on a Varian Infinity Plus 400 NMR spectrometer operated at a spectral frequency of 79.407 MHz. Scans (900012 000) were acquired with a 3 s pulse delay to achieve sufficient signal intensity.

Results and Discussions Terminating Effect and Organosilane Molecular Structures. The synthesis of SCMCSNs is characterized by using a DEDMS termination agent and a Pluronic F127 template.20 By using a termination agent, stable, uniform nanoparticles can be synthesized and the syntheses can be accomplished easily at high concentration on a large scale. The termination agent plays a crucial role in the synthesis process. To investigate the effects of the termination agent, we extended our study. Eleven RnSi(OR0 )4 - n silane coupling agents are used in this study (Chart 1). They have various molecular structures: different values of n (n = 1, 2, 3), different hydrocarbon chain lengths, and a chain R with a chargeable amino group. We first visually evaluated the effects of the termination agent after samples had been kept for at least 7 days. A precipitate or a cloudy solution indicates that the termination agent is not effective; a clear transparent solution indicates that SCMCSNs have formed and that the termination agent is effective. The Tyndall effect is initially used to determine the foramtion of nanoparticles. The Tyndall effect can be easily seen by using a laser pointer. To confirm the synthesis of the results further, TEM and DLS technique are used. Two successful examples shown in Figure 1a,b are monodisperse spherical nanoparticles obtained with DEDMS or TMES as the termination agent. Their sizes as determined from TEM images are 11-13 nm. DLS data (Figure 1c) shows very narrow size distributions, indicating that the nanoparticles are uniform. The particle size from the DLS measurement is larger than that from TEM (Figure 1a,b). The reason is that the light-scattering measurement includes PEO chains stretching out into the aqueous solution, which cannot be observed by TEM because of the low contrast of the PEO polymers.22,31 Effect of the Value n. To demonstrate the effect of the termination agent with different values of n, MTES (n = 1), DEDMS (n = 2), and TMES (n = 3) were used for comparison. Figure 2 shows the formation domain diagram of SCMCSNs with the three termination agents. The terminating effect is in the order (31) Lam, Y. M.; Grigorieff, N.; Goldbeck-Wood, G. Phys. Chem. Chem. Phys. 1999, 1, 3331.

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Figure 1. TEM images of SCMCSNs with (a) DEDMS as the termination agent and (b) TMES as the termination agent at 0.5 M HCl. (c) DLS of SCMCSNs with DEDMS (red) and TMES (blue). The molar ratio of the termination agent to TEOS is 0.11:1. Chart 1. Chemical Structures of Termination Agents

of TMES > DEDMS > MTES. Through these formation domain diagrams, we can observe that (1) as the value of n increases, the chance of successfully synthesizing SCMCSNs increases. This may be attributed to the fact that more methyl can cover more Si-OH on the surface and block the attack of other Si-OH and hence reduce the chance of the aggregation of nanoparticles. (2) SCMCSNs are favored at low [Hþ] and a high amount of the termination agent; otherwise, precipitation or a cloudy solution is obtained. There are two possible reasons: (i) [Hþ] is proportional to the reaction rates of the hydrolysis and condensation of organosilane.32 At high [Hþ], it is difficult for the termination agent to stop the reaction. (ii) A larger amount of the termination agent can cover and react with more Si-OH, which favors the formation of SCMCSNs. We also observed that when the amount of termination agent used reached a certain level under the same [Hþ], the solution in

which SCMCSNs formed stayed clear. Below that level, we observed either precipitation or a cloudy solution (Figure 2). This is easily seen on the DEDMS plane at [Hþ] = 1.0 M in Figure 2. This indicates that more termination agent molecules can cover or react with more silanols on the surfaces of nanoparticles. Effect of the Length of the Hydrocarbon Chain. To understand the effect of the hydrocarbon chain length, the silane coupling agent with methyl (MTES), vinyl (VTES), propyl (PTMS), or octyl (OTES) was used. The effect of the termination agent is shown in Figure 3. Under the same synthesis experimental condition, PTMS has a better terminating effect because the propyl group is longer than methyl and vinyl groups. The intermediate species of PTMS in the hydrolysis requires more space during condensation,33 and the propyl groups hinder certain Si-OH groups on the surface of nanoparticle from participating in the condensation reaction.34 Hence, longer chains

(32) Brinker, C. J.; Scherer, G. W Sol-Gel Science; Academic Press: San Diego, 1990.

(33) Thiesen, P. H.; Beneke, K.; Lagaly, G. J. Mater. Chem. 2002, 12, 3010. (34) Vidinha, P.; Barreiros, S.; Cabral, J. M. S.; Nunes, T. G.; Fidalgo, A.; Ilharco, L. M. J. Phys. Chem. C 2008, 112, 2008.

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Figure 4. TEM image of SCMCSNs with AEPTMS as a termination agent.

Figure 2. Formation domain diagrams of SCMCSNs with different termination agents (stars, SCMCSNs; triangles, cloudy solution; diamonds, precipitation).

Figure 3. Formation domain diagram of a termination agent with a different number of carbons under 1 M [Hþ] (stars, SCMCSNs; triangles, cloudy solution, diamonds, precipitation).

can better block the attack of Si-OH. However if the number of carbons is eight as shown for OTES, the terminating effect becomes weaker. One possible reason is that the organosilane with an apolar long hydrocarbon chain is not easy to react with Si-OH located in the hydrophilic region. The organosilane molecules may form micellelike aggregates in a manner analogous to typical surfactant in bulk water. The propyl chain in PTMS is not excessive long, and the intermediate species of PTMS in the hydrolysis homogeneously disperses in solution.35 PTMS can terminate the condensation of nanoparticles, but OTES cannot. Effect of Charge. Although OTES cannot stop the aggregation of the nanoparticles, AEPTMS with an amino group has a good terminating effect. The TEM image of SCMCSNs with AEPTMS is shown in Figure 4. The average nanoparticle size determined by TEM is 11 nm, and the nanoparticles are monodisperse. The effective region of AEPTMS under different [Hþ] conditions was also studied (Supporting Information, Figure S1). Although AEPTMS and OTES have similar chain lengths, they show different termination performance. The difference results from their different molecular structures and properties. In acidic solution, the amino group in AEPTMS has two distinct effects. It makes organosilane with a long hydrocarbon chain hydrophilic, and AEPTMS reacts easily with silanol on the nanoparticle surface. The amino group also produces positive charges located on the nanoparticle surface. Together, the positive charges from (35) Shen, S. K.; Hu, D. D. J. Phys. Chem. B 2007, 111, 7963. (36) Ascah, T. L.; B. F J. Chromatogr. 1990, 506, 357.

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the amino group and protonated silanol species36 enhance the repulsion between nanoparticles. Possible Termination Mechanism. To understand fully the effect of the termination agent on the stabilization of nanoparticles, we chose silane coupling agents MenSi(OEt)4 - n {MTES (n = 1), DEDMS (n = 2), and TMES (n = 3)} as termination agents and studied their behaviors with 29Si MAS NMR. They are good candidates for this study because they have distinct terminating effects and simple structures. 29Si MAS NMR is the most powerful technique for identifying the chemical environment of silicon. To make sure the spectra truly reflect the features of the particles, we also took steps to reduce the possible interference of the oligomers of silica to a nonsignificant level. One important step was to remove the impacts of self-condensation. When the termination agent is added to the reaction mixture, self-condensation and cross-condensation may happen simultaneously. For example, TMES can provide a hydrolyzed derivative (Me3SiOH), a dimer species (Me3SiOSiMe3), and a silicon species grafted onto the SCMCSN surface (Me3Si-O-[SiO2]).30 We dialyzed all SCMCSN samples against water for 2 days to remove dimer species and other possible oligomers before NMR measurements. In our 29Si MAS NMR spectra (Figure 5), the resonances at approximately -90, -101, and -110 ppm are assigned to Q2, Q3, and Q4 species, respectively,37 where Q2 represents the Si(OSi)2(OH)2 group, Q3 represents the Si(OSi)3OH group, and Q4 represents the Si(OSi)4 group. Additional signals appearing at 11.5 (Figure 5a), -22 (Figure 5b), and -65 ppm (Figure 5c) correspond to M-Q units (M1, Me3Si-O-Si),38 D-Q units (Dx, Me2Si(OSi)x(OH)2 - x), and T-Q units (Tx, MeSi(OSi)x(OH)3 - x), respectively.30 The percentages of M, D, and T with respect to the total silica species (21.4, 4.7, and 1.6%, respectively) indicate that the order of the termination agent connected to the nanoparticles is TMES > DEDMS > MTES. Hence, compared to MTES and DEDMS, TMES has a greater propensity to react with Si-OH and cover the surface of nanoparticles. The cross-condensation of silane coupling agents often happens on the surfaces of silica nanoparticles.39 However, the condensation ability with surface Si-OH is different among various agents. Scheme 1 shows the possible forms of TMES and MTES on the nanoparticle surface in this study. As shown in Scheme 2, when TMES is in contact with a nanoparticle, it reacts with surface Si-OH. More importantly, the three methyl groups play steric roles40 in blocking the attack of surface Si-OH from (37) Pouxviel, J. C.; Boilot, J. P. J. Non-Cryst. Solids 1987, 89, 345. (38) Backer, M.; Grimmer, A. R.; Auner, N.; John, P.; Weis, J. Solid State Nucl. Magn. Reson. 1997, 9, 241. (39) Xu, Y.; Liu, R. L.; Wu, D.; Sun, Y. H.; Gao, H. C.; Yuan, H. Z.; Deng, F. J. Non-Cryst. Solids 2005, 351, 2403. (40) Rao, A. V.; Kulkarni, M. M. Mater. Res. Bull. 2002, 37, 1667.

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Figure 6. Emission spectra of pyrene (2.4  10-5 M, λex = 302 nm) in Figure 5. 29Si NMR spectra for SCMCSNs synthesized by the termination agent: (a) TMES, (b) DEDMS, and (c) MTES.

the SCMCSNs using TMES as a termination agent with different molar ratios of TMES to TEOS (purple for 0, red for 0.06, and green for 0.14). For comparison, the spectrum intensities were normalized at 396 nm.

Scheme 1. Most Likely Coverage Models for Methyl Termination Agents (TMES and MTES) on the Surfaces of Nanoparticles

other nanoparticles. Compared to TMES, although one MTES molecular may react with three Si-OH groups, it can happen only when their locations are suitable. This probability can be extremely rare. In most cases, MTES may react with one or two intrinsic Si-OH groups, but this advantage is reduced by providing additional Si-OH groups, which increases the chance of selfcondensation and condensation with other silanols. Ultimately, it is not very effective. On the basis of our study, we have drawn the conclusion that the steric factor in the termination agent plays an important role in stabilizing nanoparticles. Note, though, that the increased steric effect caused by replacing one or two methyls by phenyls (PTMS or DPDMS) does not give SCMCSNs because of the high hydrophobic property of the phenyl group. To confirm the above mechanism, molecular probe fluorescence spectrum measurements are used. The fluorescence technique is more attractive because it offers important information, especially in microenvironments on the molecular scale. Pyrene or pyrenylsubstituted organosilane41 as a fluorescent probe molecule is widely used to monitor the sol-gel process of silane coupling agents. Pyrene excimer emission (470-490 nm) reflects the variation of its local concentration. It is known that the excimer emission is due to the interaction between an excited pyrene species and a pyrene molecule in the ground state. This interaction depends on both the orientation of and distance between pyrene molecules.35 The intensity of the excimer band increases with increased concentration of pyrene. We used pyrene that is covalently connected to a silane coupling agent (compound 1)42 to measure the relative number of active Si-OHs on the nanoparticle surface. A higher concentration of Si-OH increases the chance of excimer pyrene formation. This makes it possible to use the fluorescence property of excimer pyrene as an index to measure the concentration of active Si-OH on the nanoparticle surface. As (41) Chambers, R. C.; Haruvy, Y.; Fox, M. A. Chem. Mater. 1994, 6, 1351. (42) Rampazzo, E.; Bonacchi, S.; Montalti, M.; Prodi, L.; Zaccheroni, N. J. Am. Chem. Soc. 2007, 129, 14251.

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Figure 7. UV-vis spectra of EDTA-functionalized SCMCSNCo(II) (red) and normal SCMCSN-Co(II) (black) complexes in water before (-) and after (- 3 -) dialysis. (Inset) Digital photograph of EDTA-functionalized SCMCSN-Co(II) (left) and SCMCSNCo(II) (right).

shown in Figure 6, the excimer emission band (470-490 nm) decreases as the amount of TMES increases. This indicates that an increased amount of TMES reduces the active Si-OH on the particle surface. By combining the NMR results with the fluorescence spectra, we can deduce that the termination agent can react with or cover some Si-OHs on the nanoparticle surface. Extend the Termination Agent Concept to Similar Synthesis Systems and Functionalized Nanoparticles. Experiments with another silica source (Si(OC2H4OH)4, (Supporting Information, Figure S2) and other surfactant templates F108 and Brij 700 (Supporting Information, Figure S3) were conducted on the basis of the termination concept. The results are supportive. We conclude that termination agent method is effective for synthesizing SCMCSNs under certain reaction conditions. With an understanding of the termination agent, we now can selectively choose different termination agents to realize a specific functionalization. As an example, we functionalized SCMCSNs with TANED containing the EDTA complex group. It is well known that EDTA can coordinate greatly with heavy metal and can be applied in water treatment,43 metal detection, and so (43) Koehler, F. M.; Rossier, M.; Waelle, M.; Athanassiou, E. K.; Limbach, L. K.; Grass, R. N.; Gunther, D.; Stark, W. J. Chem. Commun. 2009, 4862.

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forth.44 In this scenario, TANED acts not only as a termination agent but also as a functional group of SCMCSN. The effective synthesis conditions are shown in Figure S4 (Supporting Information). EDTA-functionalized SCMCSN can coordinate with many metals. In this study, we used Co(II) as an example. The UV spectra of EDTA-functionalized SCMCSN-Co(II) and normal SCMCSN-Co(II) are shown is Figure 7. The absorption band at ∼375 nm is characteristic of EDTA-Co(II) complexes,45,46 and that at ∼512 nm is characteristic of Co(II) in water. EDTA-functionalized SCMCSN-Co(II) absorbs strongly at 375 nm, which confirms the EDTA coordination with Co(II), whereas normal SCMCSNCo(II) does not absorb at all. Moreover, after dialysis, Co(II) in a normal SCMCSN-Co(II) sample escapes completely and the sample becomes colorless whereas the absorption of EDTAfunctionalized SCMCSN-Co(II) remains. This result indicates that EDTA-functionalized SCMCSN-Co(II) is stable against dialysis. The favorable sorption properties may be useful.

The proposed termination mechanism is that the organosilane agent can cover or react with Si-OH on the nanoparticle surface to avoid the aggregation of nanoparticles. This mechanism is supported by the following experimental results: (1) An agent with a larger number of methyls has a stronger steric hindrance, which improves the terminating effect. The effective order of termination agents is TMES > DEDMS > MTES. (2) An agent with a longer hydrocarbon chain better blocks attacks from other nanoparticles. Accordingly, the terminating effect of PTMS is better than that of VTES and MTES. (3) A charged agent causes silica nanoparticles to be well dispersed. (4) An agent can be used in similar synthesis systems, such as Si(OC2H4OH)4 as the silica source and F108 or Brij 700 as the template. Additionally, the termination agent can functionalize SCMCSNs.

Conclusions

Supporting Information Available: Domain-formation diagram of SCMCSNs with an AEPTMS termination agent. TEM image of SCMCSNs with Si(OC2H4OH)4 as the silica source. DLS of the size distribution of SCMCSNs with different surfactants. DLS size distribution of SCMCSNs functionalized with TANED (left) and domain-formation diagram of SCMCSNs with TANED as the termination agent. This material is available free of charge via the Internet at http://pubs.acs.org.

We have systematically studied the effects of termination agent RnSi(OR0 )4 - n in the formation of silica cross-linked monodisperse nanoparticles. The terminating effect is dependent on the value of n, the length of the hydrocarbon chain, and the charge. (44) Rieter, W. J.; Taylor, K. M. L.; Lin, W. B. J. Am. Chem. Soc. 2007, 129, 9852. (45) Xue, Y.; Traina, S. J. Environ. Sci. Technol. 1996, 1975. (46) B€urgisser, C. S.; Stone, A. T. Environ. Sci. Technol. 1997, 31, 2656.

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Acknowledgment. We gratefully acknowledge financial support from the National Nature Science Foundation of China (grant nos. 20788101 and 20671041)

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