Article pubs.acs.org/IECR
Size-Controlled Synthesis of Monodispersed Poly(3mercaptopropylsilsesquioxane) Microspheres by a Two-Step Sol−Gel Method Xin Lu, Yuhui Hou, Jie Zha, and Zhong Xin* State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, PO BOX 545, Meilong Road 130, Shanghai 200237, People’s Republic of China ABSTRACT: A two-step acid−base catalyzed sol−gel method has been used to prepare monodispersed poly(3mercaptopropylsilsesquioxane) (PMPSQ) microspheres. Initially, the hydrolysis of 3-mercaptopropyltrimethoxysilane was catalyzed by hydrochloric acid in an aqueous medium, and then the addition of ammonium hydroxide initiated the condensation of hydrolysate. The formation process of PMPSQ microspheres was characterized by field emission scanning electron microscopy and 29Si NMR. The formation mechanism of the monodispersed microspheres was also discussed. Through this method, monodispersed PMPSQ microspheres with a high yield were achieved and the particle size could be adjusted by changing the precursor concentration.
1. INTRODUCTION The fabrication and application of polysilsesquioxane (PSQ) particles have attracted much attention recently.1−3 Organoalkoxysilane precursors with different functional groups could be easily polymerized into PSQ spherical particles.4−8 One type of functional PSQ particles, poly(3-mercaptopropylsilsesquioxane) (PMPSQ) possesses thiol functional groups which allow for the linkage of biomolecules and organic fluorescent dyes. Thus, PMPSQ particles have potential applications in bioanalysis.9,10 The sol−gel method has been widely employed to synthesize PSQ particles. Various functional PSQ particles have been synthesized by a modified Stöber method.11−15 Silanization of presynthesized silica nanoparticles with mercapto-trialkoxysilane has been developed to prepare thiol-functionalized silica particles which could be applied for the immobilization of oligonucleotide.16 The co-condensation of tetraethyl orthosilicate (TEOS) and organotrialkoxysilane was an another method for fabricating functional PSQ particles.17,18 PSQ particles could also be produced directly from organotrialkoxysilane precursors.19,20 Nakamura et al.9,21,22 have prepared PMPSQ nanoparticles from mercapto-trialkoxysilane with ammonium hydroxide acting as catalyst in an alcohol/water mixture or in entirely aqueous solvent. The yield of PMPSQ particles was only 54% after 24 h of reaction even at the reaction temperature of 100 °C when the MPTMS concentration was 0.1 mol/L.22 Johnston et al. have described a two-step sol−gel method for producing PMPSQ particles.23,24 First, an oil-in-water emulsion formed during the hydrolysis and condensation of MPTMS in an HCl solution. Second, these emulsion droplets were cross-linked after the addition of triethylamine, resulting in the formation of polydispersed PMPSQ microspheres. In order to obtain organosilica particles with narrow size distribution, Vogel et al.25 modified the above two-step sol−gel approach. The oil-in-water emulsion formed in the first step was centrifuged, and then triethylamine was © 2014 American Chemical Society
added to the diluted supernatant to start the base-catalyzed condensation. We have reported a facile acid-catalyzed hydrolysis and basecatalyzed condensation method for producing PMPSQ microspheres, and the resultant microspheres demonstrated powerful adsorption capability of silver ions.26 This method has several advantages. First, it was a simple and reproductive synthetic method to prepare monodispersed PMPSQ microspheres. Second, compared with traditional Stöber method, water was used as the reaction solvent instead of alcohol. Third, PMPSQ microspheres could be obtained with high yield at room temperature through this method. Despite the above advantages, the formation mechanism of monodispersed PMPSQ microspheres has not been fully understood yet. In this paper, we studied the formation process by 29Si NMR and field emission scanning electron microscopy (FE-SEM) and described the reasons for the uniform particle size. Furthermore, the effect of precursor concentration on the particle size has been examined.
2. EXPERIMENTAL SECTION 2.1. Materials. 3-Mercaptopropyl trimethoxysilane (MPTMS) was purchased from Diamond Advanced Material of Chemical Inc. and was distilled under vacuum before use. Hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) were purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. 2.2. Synthesis of PMPSQ Microspheres. A two-step sol− gel method was used to synthesize PMPSQ microshperes (Figure 1).26 Various amounts of MPTMS precursor were mixed with 100 mL of water by stirring to obtain samples with MPTMS at concentrations of 0.10, 0.25, 0.50, 0.75, and 1.00 Received: Revised: Accepted: Published: 14659
May 27, 2014 August 8, 2014 August 21, 2014 September 12, 2014 dx.doi.org/10.1021/ie502146k | Ind. Eng. Chem. Res. 2014, 53, 14659−14663
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Figure 1. Synthesis process of PMPSQ microspheres.
mol/L. Then, 0.1 mL of 1 mol/L HCl was added to the mixture. After 1 h of the acid-catalyzed hydrolysis, 0.2 mL of 10% NH4OH was mixed with the hydrolysate to initiate the condensation. Afterward, the mixture was kept static. PMPSQ microspheres were separated from the mixture by centrifugation, rinsed thoroughly using ethanol, and dried in a vacuum oven. 2.3. Characterizations. 29Si NMR spectra were acquired using a Bruker Avance 500 NMR spectrometer. Liquid-state 29 Si NMR samples were dissolved in acetone-d6 with tetramethylsilane (TMS) as reference. FE-SEM analysis was conducted on a FEI Nova NanoSEM 450. The samples were sputtered with platinum before observation. Image-Pro Plus software was used to calculate the number-average diameter (dn) and the coefficient of variance (CV) by measuring about 200 particles.
3. RESULTS AND DISCUSSION 3.1. Acid-Catalyzed Hydrolysis Reaction. PMPSQ microspheres were obtained through a two-step sol−gel method in an aqueous solution. The hydrolysis of MPTMS containing three methoxy groups occurred rapidly under acidic conditions.27 When MPTMS mixed with water, an emulsion was generated because of the immiscibility of the MPTMS precursor. As the precursor hydrolyzed to a more soluble silanol, the emulsion gradually became clear. Figure 2 shows the 29Si NMR spectra of MPTMS precursor and the reaction solution after acid-catalyzed hydrolysis. The nonhydrolyzed precursor showed a T0 peak at −42.7 ppm (Figure 2a).19 In Figure 2b, the peaks at −40.1 and −49.5 ppm belonged to the hydrolyzed silanol (T0) and the dimer (T1), respectively.27 There are an additional two weak peaks in the 29 Si NMR spectrum after 5h hydrolysis (Figure 2c) which were due to the formation of a slight amount of the trimer. The presence of T3 was not observed in Figure 2c, indicating that the condensation reaction had not proceeded under acidic conditions. The results of 29Si NMR confirmed the completion of hydrolysis reaction after 1h. Hence, we adopted 1 h hydrolysis time for further experiments. 3.2. Base-Catalyzed Condensation Reaction. NH4OH was added to the transparent hydrolysate after the hydrolysis reaction. The reaction solution turned opaque in several minutes. The rapid condensation of silanol in an ammonia solution was caused by the attack of deprotonated silanol (SiO−) on the Si atom of Si−OH.28 To investigate the formation process of PMPSQ microspheres, the yields of the solid particles were measured at different times during the base-catalyzed condensation reaction. As shown in Figure 3, the yields were 84.1% after 15 min, 91.6% after 1 h, and 97.2% after 3 h. These results indicated that the condensation reaction occurred rapidly under basic conditions.
Figure 2. 29Si NMR spectra of (a) MPTMS precursor and the reaction solutions following (b) 1 h and (c) 5 h of hydrolysis reaction (MPTMS concentration, 1.00 mol/L).
The growth of PMPSQ microspheres was also observed using FE-SEM. As shown in Figure 4, the micron-sized nuclei with vague edges were observed at 15 min after the addition of ammonia solution. Then, abundant soft particles formed quickly after 30 min. As the condensation reaction continued, these particles gradually grew to be spherical and turned to uniformly smooth microspheres after 3 h of condensation. Figure 5 represents the solid-state 29Si NMR spectrum of PMPSQ microspheres. The peaks at −58.7 and −67.7 ppm are due to T2 and T3 species, respectively.26 No T0 or T1 species 14660
dx.doi.org/10.1021/ie502146k | Ind. Eng. Chem. Res. 2014, 53, 14659−14663
Industrial & Engineering Chemistry Research
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Figure 5. Solid-state 29Si NMR spectrum of PMPSQ microspheres. Figure 3. Effect of condensation time on the yields of PMPSQ microspheres (MPTMS concentration, 1.00 mol/L).
solution for more than 8 h formed an oil-in-water emulsion. The droplets composed of different length oligomers condensed to generate polydispersed solid microspheres after the addition of base catalyst. The formation mechanism may be similar to the emulsion polymerization mechanism. Vogel et al.25 have demonstrated that the PMPSQ microspheres with narrow size distribution were achieved when the aqueous phase was separated from the solution with centrifugation after 8 h of acid-catalyzed hydrolysis and condensation. The addition of triethylamine to the supernatant solution initiated the rapid condensation to produce monodispersed microspheres. Generally, it is necessary to control the reactive conditions to ensure the quick generation of nuclei and then allow the nuclei to grow at the same speed to form monodispersed colloid spheres.29 In this paper, we controlled the acid-catalyzed hydrolysis time to avoid phase separation. The hydrolyzed mixtures became a homogeneous solution after 1 h of hydrolysis and consisted of fully hydrolyzed monomer and dimer according to the 29Si NMR results. Then, the addition of NH4OH initiated a rapid condensation, and the originally transparent hydrolyzed solution turned opaque in several minutes. Numerous nuclei formed at the initial stage of condensation, which indicated the formation of supersaturated solution of oligomers after the addition of the base. When the concentration of nuclei overtook the saturation concentration in the reaction solution, these nuclei would precipitate from solution by way of homogeneous nucleation. These nuclei could further grow to stable microspheres until the condensation process was complete. The results of FE-SEM (Figure 4) confirmed that PMPSQ microspheres grew without forming new particles. The formation process of microspheres are illustrated in Figure 6. According to this formation mechanism, all PMPSQ particles formed at the same time after the addition of base catalyst and subsequently grew at the same rate. Hence, this two-step sol−gel method paved a facile way for the preparation of monodispersed polysilsesquioxane microspheres. 3.4. Effect of MPTMS Concentration on Particle Size. The concentration of organosilane precursor is a key factor for controlling the size of PSQ microspheres.6 Table 1 shows the properties of PMPSQ microspheres prepared with different MPTMS concentration. The results showed that the particle size increased with the increase of precursor concentration. When the concentration of the precursor was adjusted, various PMPSQ microspheres with average particle size ranging from 0.92 to 3.29 μm were obtained. In cases where the same concentration of condensation catalyst is adopted, the condensation rate will be in proportion to the hydrolysate
Figure 4. FE-SEM images of PMPSQ microspheres synthesized with different condensation time: (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 3 h, (f) 5 h, (g) 7 h, and (h) 15 h (MPTMS concentration, 1.00 mol/L).
were identified in this 29Si NMR spectrum, indicating that the condensation reaction was nearly complete. 3.3. Formation Mechanism of Monodispersed PMPSQ Microspheres. On the basis of the process reported by Miller et al.,27 the hydrolysis and condensation of MPTMS in HCl 14661
dx.doi.org/10.1021/ie502146k | Ind. Eng. Chem. Res. 2014, 53, 14659−14663
Industrial & Engineering Chemistry Research
Article
Figure 6. Formation mechanism of monodispersed PMPSQ microspheres.
The morphologies of PMPSQ microspheres synthesized with various MPTMS concentrations were observed by FE-SEM (Figure 7). These FE-SEM images showed that the obtained PMPSQ microspheres were very smooth and uniform in size.
Table 1. Properties of PMPSQ Microspheres Prepared Using Different MPTMS Concentrations MPTMS concentration (mol/L)
dn (μm)
CV (%)
yields (%)
0.10 0.25 0.50 0.75 1.00
0.92 1.15 1.97 2.08 3.29
7 9 3 5 4
96.6 98.4 99.7 99.5 97.0
4. CONCLUSIONS Monodispersed PMPSQ microspheres have been successfully prepared by a two-step sol−gel method. The acid-catalyzed hydrolysis of MPTMS was precisely controlled to avoid phase separation, and it was proven that 1 h was sufficient for completing hydrolysis of MPTMS. The addition of NH4OH could initiate the rapid base-catalyzed condensation of silanol, resulting in monodispersed polysilsesquioxane microspheres. With a careful study of the synthesis process, we considered that the formation of the monodispersed particles may be due to the homogeneous nucleation process. When the precursor concentration was changed, the particle size could be adjusted in the range from 0.92 to 3.29 μm.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-21-64252972. Fax: +86-21-64240862. E-mail: xzh@ ecust.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Project21006025) and the Fundamental Research Funds for the Central Universities.
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REFERENCES
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Figure 7. FE-SEM images of PMPSQ microspheres synthesized with various concentrations of MPTMS: (a) 0.10 mol/L, (b) 0.25 mol/L, (c) 0.50 mol/L, (d) 0.75 mol/L, and (e) 1.00 mol/L.
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