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Controlled Microsynthesis of Silica Particles in a Microfluidic Setup
Available online at www.sciencedirect.com
Procedia Chemistry 1 (2009) 377–380
Procedia Chemistry www.elsevier.com/locate/procedia
Proceedings of the Eurosensors XXIII conference
Controlled Microsynthesis of Silica Particles in a Microfluidic Setup J.Wacker, V.K.Parashar, M.A.M.Gijs * Laboratory of Microsystems, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
Abstract
We report on a novel microsynthesis method for easy controlling the size of silica micro- and nanoparticles. Colloids are synthesized in a microfluidic chamber by layer-wise adsorption of silica to precipitated nuclei. We prepare samples of spherical particles with different numbers of coatings and find that particles with less than 15 coating layers are uniform in size. Particles with more coatings are disperse. This finding is attributed to peptization of nanoparticles and independent growth of secondary nanoparticles. For all numbers of layers, the maximum particle size increases with the number of coatings. Keywords: microfluidic synthesis; silica nanoparticle; lamination
1. Introduction Due to their chemical inertness and good biocompatibility, silica nanoparticles find application in diverse fields of research, e.g. in oncology1, analysis of chemical compunds2 or drug delivery3. In the classical Stöber process silica (SiO2) particles are synthesized by hydrolysis of tetraethylorthosilicate (TEOS) in a basic solution with subsequent condensation of silicon hydroxide4. This process is, however, capricious and particle size as well as size distribution vary in a wide range from one synthesis to the next. To overcome these problems, particles can be synthesized in templates, e.g. in droplets5 or micelles6. Template synthesis is, however, more complex than a process in which a predetermined particle size is the intrinsic result of the synthesis itself. Miniaturization is one way of gaining control over a chemical reaction: mass and heat transfer are faster than on a macroscopic scale, and enhanced surface-to-volume ratios increase the reactivity of chemical compounds. Here we use a microfluidic setup to synthesize silica nanoparticles. By repeated injection of reactants in a microfluidic mixing chamber, we grow the particles in a layer-wise manner. This provides an easy way of control over particle size.
*
Corresponding author. Tel.: +41-21-693-6734; fax: +41-21-693-5950. E-mail address: [email protected]
1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.094
378
J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
2. Materials and Methods Microsynthesis of silica nanoparticles is carried out using an experimental setup, as depicted in fig. 1. TEOS (from Sigma Aldrich) and a hydrolyzing mixture (HM: 8 mL deionized water, 2.4 mL ethanol and 9.6 mL ammonia [the latter two from Sigma Aldrich]) are injected periodically into a reaction chamber, where they are mixed using a linearly moving stirring magnet. The reaction chamber is made of PDMS bonded to glass. Two syringe pumps that are controlled with a LabView script enable autonomous operating of the experimental system. Each injection consists of 0.33x µL TEOS plus 0.67x µL HM, where x=500 µL/(number of layers). In this paper we present particles with 9, 15 and 20 coatings. First four doses are applied after 0, 5, 9 and 12 minutes total reaction time. After that the two solutions are brought to the reaction chamber every two minutes until the final number of coatings is reached. Finally, the reaction chamber is opened, the particles are washed with acetone and dried at ambient air. Analysis is done using a Philips XL30-FEG scanning electron microscope (SEM). Particle diameter is evaluated using imageJ and histograms are plotted with MATLAB. For investigation on a transmission electron microscope (TEM) (Philips/FEI CM20), samples are prepared as follows: particles are embedded in Epofix™, which is heat-cured over-night. Sample thickness is reduced by polishing with diamond lapping paper and ion milling. Acceleration voltage on the TEM is 200kV.
Figure 1: Experimental setup of microsynthesis of silica particles. HM and TEOS are cyclically brought to a reaction chamber in which they are mixed with a linearly moving stirring magnet. Dimensions of the chamber are given in mm. Inset: schematic of the injection cycles for particles with 15 coatings.
J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
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Figure 2: Size distribution of SiO2 particles. Particles with 9 coatings are monodisperse. Secondary particles form by re-nucleation in samples with 15 coatings. Particles with 20 coatings are polydisperse, probably due to continuous re-nucleation and disintegration of big particles. Scale bars in the insets are 2 µm.
3. Results and Discussion Generally, particle growth is explained with the “two-stage synthesis mechanism”7. According to this model, primary particles nucleate when the hydrous oxide reaches a critical concentration (here assisted by increasing the pH). These nuclei aggregate to form particles. After nucleation the concentration of the oxide decreases and in a closed system re-nucleation is improbable. As can be seen in the SEM images of fig. 2, for each number of coatings spherical particles are produced. While the spherical shape of particles has been proven in the classical Stöber process4, the mechanisms that lead to this form are not yet fully understood7. Probably they involve the rearrangement of newly adsorbed material to reduce surface free energy of the particle. More injection cycles lead to bigger particles (mean diameters of particles with 9, 15 and 20 coatings are: 0.41 µm, 0.67 µm and 0.94 µm). This finding can be easiest explained with layer-wise growth: each injection leads to adsorption of a new layer of material on preformed particles. This idea is corroborated by a cross-sectional TEM image (fig. 3) where different layers can be distinguished. Particles with 9 coatings are fairly monodisperse (single standard deviation: 45 nm), while more injections lead to a broader size distribution: in samples with 15 coatings, two main diameters are distinguishable (524±83 nm and 791±90 nm); 20 injection cycles induce a broad variety of particle sizes (single standard deviation: 300 nm). In the light of the two-stage synthesis mechanism, this means that particles with 9 coatings stem from nuclei that formed
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J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
Figure 3: TEM image of a particle with 20 coatings. Lamination inside the particle is clearly visible (arrows point at boundaries between layers). Scale bar is 100 nm.
during the first injection cycle. Material that was injected in the following cycles aggregated to already existing particles. Since nucleation is an instantaneous process we can expect that particles stemming from the same nucleation event have a narrow size distribution (they are all given the same time to grow). Between 9 and 15 injections TEOS again reaches the critical concentration for particle formation and re-nucleation takes place. These newly formed particles grow simultaneously with the first generation particles. In samples with 20 coatings specific diameters are not distinguishable. Therefore, we speculate that nucleation is continuously going on. Concurrently, particles are growing in the discussed layer-wise manner and they may also be disintegrated via peptization. Pieces of particles can again serve as nuclei thereafter. 4. Conclusion We have synthesized silica nanoparticles in a microfluidic setup. The choice of a miniaturized reaction chamber reduces chemical waste and increases control over process parameters (e.g. fast mixing of reactants). The fact that the setup is working autonomously gives additional repeatability of synthesis. Further control over the characteristics of particles is given by the layer-wise manner of growth. The layered structure of particles can be an interesting option when e.g. engineering nano-carriers for controlled drug release. References 1. Wang J, Liu GD, Engelhard MH, Lin YH. Sensitive immunoassay of a biomarker tumor necrosis factor-alpha based on poly(guanine) functionalized silica nanoparticle label. Analytical Chemistry 2006;78(19):6974-9. 2. Deng YH, Deng CH, Yang D, Wang CC, Fu SK, Zhang XM. Preparation, characterization and application of magnetic silica nanoparticle functionalized multi-walled carbon nanotubes. Chemical Communication 2005;44:5548-50. 3. Linong M, France B, Bradley KA, Zink JI. Anitmicrobial Activity of Silver Nanocrystals Encapsulated in Mesoporous Silica Nanoparticles. Advanced Materials 2009;21(17):1684-9. 4. Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in micron size range. Journal of Colloid and Interface Science 1968;26(1):62-9. 5. Khan SA, Gunther A, Schmidt MA, Jensen KF. Microfluidic Synthesis of Colloidal Silica. Langmuir 2004;20(20):8604-11. 6. Nandiyanto ABD, Kim SG, Iskandar F, Okuyama K. Synthesis of spherical mesoporous silica nanoparticles with nanometer-sized controllable pores and outer diameters. Microporous and Mesoporous Materials 2009;120(3):447-53. 7. Privman V. Mechanisms of Diffusional Nucleation of Nanocrystals and Their Self-Assembly into Uniform Colloids. Interdisciplinary Transport Phenomena: Fluid, Thermal, Biological, Materials, and Space Sciences 2009;1161:508-25
Procedia Chemistry 1 (2009) 377–380
Procedia Chemistry www.elsevier.com/locate/procedia
Proceedings of the Eurosensors XXIII conference
Controlled Microsynthesis of Silica Particles in a Microfluidic Setup J.Wacker, V.K.Parashar, M.A.M.Gijs * Laboratory of Microsystems, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
Abstract
We report on a novel microsynthesis method for easy controlling the size of silica micro- and nanoparticles. Colloids are synthesized in a microfluidic chamber by layer-wise adsorption of silica to precipitated nuclei. We prepare samples of spherical particles with different numbers of coatings and find that particles with less than 15 coating layers are uniform in size. Particles with more coatings are disperse. This finding is attributed to peptization of nanoparticles and independent growth of secondary nanoparticles. For all numbers of layers, the maximum particle size increases with the number of coatings. Keywords: microfluidic synthesis; silica nanoparticle; lamination
1. Introduction Due to their chemical inertness and good biocompatibility, silica nanoparticles find application in diverse fields of research, e.g. in oncology1, analysis of chemical compunds2 or drug delivery3. In the classical Stöber process silica (SiO2) particles are synthesized by hydrolysis of tetraethylorthosilicate (TEOS) in a basic solution with subsequent condensation of silicon hydroxide4. This process is, however, capricious and particle size as well as size distribution vary in a wide range from one synthesis to the next. To overcome these problems, particles can be synthesized in templates, e.g. in droplets5 or micelles6. Template synthesis is, however, more complex than a process in which a predetermined particle size is the intrinsic result of the synthesis itself. Miniaturization is one way of gaining control over a chemical reaction: mass and heat transfer are faster than on a macroscopic scale, and enhanced surface-to-volume ratios increase the reactivity of chemical compounds. Here we use a microfluidic setup to synthesize silica nanoparticles. By repeated injection of reactants in a microfluidic mixing chamber, we grow the particles in a layer-wise manner. This provides an easy way of control over particle size.
*
Corresponding author. Tel.: +41-21-693-6734; fax: +41-21-693-5950. E-mail address: [email protected]
1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.094
378
J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
2. Materials and Methods Microsynthesis of silica nanoparticles is carried out using an experimental setup, as depicted in fig. 1. TEOS (from Sigma Aldrich) and a hydrolyzing mixture (HM: 8 mL deionized water, 2.4 mL ethanol and 9.6 mL ammonia [the latter two from Sigma Aldrich]) are injected periodically into a reaction chamber, where they are mixed using a linearly moving stirring magnet. The reaction chamber is made of PDMS bonded to glass. Two syringe pumps that are controlled with a LabView script enable autonomous operating of the experimental system. Each injection consists of 0.33x µL TEOS plus 0.67x µL HM, where x=500 µL/(number of layers). In this paper we present particles with 9, 15 and 20 coatings. First four doses are applied after 0, 5, 9 and 12 minutes total reaction time. After that the two solutions are brought to the reaction chamber every two minutes until the final number of coatings is reached. Finally, the reaction chamber is opened, the particles are washed with acetone and dried at ambient air. Analysis is done using a Philips XL30-FEG scanning electron microscope (SEM). Particle diameter is evaluated using imageJ and histograms are plotted with MATLAB. For investigation on a transmission electron microscope (TEM) (Philips/FEI CM20), samples are prepared as follows: particles are embedded in Epofix™, which is heat-cured over-night. Sample thickness is reduced by polishing with diamond lapping paper and ion milling. Acceleration voltage on the TEM is 200kV.
Figure 1: Experimental setup of microsynthesis of silica particles. HM and TEOS are cyclically brought to a reaction chamber in which they are mixed with a linearly moving stirring magnet. Dimensions of the chamber are given in mm. Inset: schematic of the injection cycles for particles with 15 coatings.
J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
379
Figure 2: Size distribution of SiO2 particles. Particles with 9 coatings are monodisperse. Secondary particles form by re-nucleation in samples with 15 coatings. Particles with 20 coatings are polydisperse, probably due to continuous re-nucleation and disintegration of big particles. Scale bars in the insets are 2 µm.
3. Results and Discussion Generally, particle growth is explained with the “two-stage synthesis mechanism”7. According to this model, primary particles nucleate when the hydrous oxide reaches a critical concentration (here assisted by increasing the pH). These nuclei aggregate to form particles. After nucleation the concentration of the oxide decreases and in a closed system re-nucleation is improbable. As can be seen in the SEM images of fig. 2, for each number of coatings spherical particles are produced. While the spherical shape of particles has been proven in the classical Stöber process4, the mechanisms that lead to this form are not yet fully understood7. Probably they involve the rearrangement of newly adsorbed material to reduce surface free energy of the particle. More injection cycles lead to bigger particles (mean diameters of particles with 9, 15 and 20 coatings are: 0.41 µm, 0.67 µm and 0.94 µm). This finding can be easiest explained with layer-wise growth: each injection leads to adsorption of a new layer of material on preformed particles. This idea is corroborated by a cross-sectional TEM image (fig. 3) where different layers can be distinguished. Particles with 9 coatings are fairly monodisperse (single standard deviation: 45 nm), while more injections lead to a broader size distribution: in samples with 15 coatings, two main diameters are distinguishable (524±83 nm and 791±90 nm); 20 injection cycles induce a broad variety of particle sizes (single standard deviation: 300 nm). In the light of the two-stage synthesis mechanism, this means that particles with 9 coatings stem from nuclei that formed
380
J.Wacker et al. / Procedia Chemistry 1 (2009) 377–380
Figure 3: TEM image of a particle with 20 coatings. Lamination inside the particle is clearly visible (arrows point at boundaries between layers). Scale bar is 100 nm.
during the first injection cycle. Material that was injected in the following cycles aggregated to already existing particles. Since nucleation is an instantaneous process we can expect that particles stemming from the same nucleation event have a narrow size distribution (they are all given the same time to grow). Between 9 and 15 injections TEOS again reaches the critical concentration for particle formation and re-nucleation takes place. These newly formed particles grow simultaneously with the first generation particles. In samples with 20 coatings specific diameters are not distinguishable. Therefore, we speculate that nucleation is continuously going on. Concurrently, particles are growing in the discussed layer-wise manner and they may also be disintegrated via peptization. Pieces of particles can again serve as nuclei thereafter. 4. Conclusion We have synthesized silica nanoparticles in a microfluidic setup. The choice of a miniaturized reaction chamber reduces chemical waste and increases control over process parameters (e.g. fast mixing of reactants). The fact that the setup is working autonomously gives additional repeatability of synthesis. Further control over the characteristics of particles is given by the layer-wise manner of growth. The layered structure of particles can be an interesting option when e.g. engineering nano-carriers for controlled drug release. References 1. Wang J, Liu GD, Engelhard MH, Lin YH. Sensitive immunoassay of a biomarker tumor necrosis factor-alpha based on poly(guanine) functionalized silica nanoparticle label. Analytical Chemistry 2006;78(19):6974-9. 2. Deng YH, Deng CH, Yang D, Wang CC, Fu SK, Zhang XM. Preparation, characterization and application of magnetic silica nanoparticle functionalized multi-walled carbon nanotubes. Chemical Communication 2005;44:5548-50. 3. Linong M, France B, Bradley KA, Zink JI. Anitmicrobial Activity of Silver Nanocrystals Encapsulated in Mesoporous Silica Nanoparticles. Advanced Materials 2009;21(17):1684-9. 4. Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in micron size range. Journal of Colloid and Interface Science 1968;26(1):62-9. 5. Khan SA, Gunther A, Schmidt MA, Jensen KF. Microfluidic Synthesis of Colloidal Silica. Langmuir 2004;20(20):8604-11. 6. Nandiyanto ABD, Kim SG, Iskandar F, Okuyama K. Synthesis of spherical mesoporous silica nanoparticles with nanometer-sized controllable pores and outer diameters. Microporous and Mesoporous Materials 2009;120(3):447-53. 7. Privman V. Mechanisms of Diffusional Nucleation of Nanocrystals and Their Self-Assembly into Uniform Colloids. Interdisciplinary Transport Phenomena: Fluid, Thermal, Biological, Materials, and Space Sciences 2009;1161:508-25