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Size-controlled synthesis of polymer hollow nanoparticles using emulsion templates prepared by tandem acoustic emulsification
Ultrasonics - Sonochemistry 54 (2019) 250–255
Contents lists available at ScienceDirect
Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Size-controlled synthesis of polymer hollow nanoparticles using emulsion templates prepared by tandem acoustic emulsification Miharu Koshinoa, Yukihide Shiraishib, Mahito Atobea,b, a b
T
⁎
Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan Graduate School of Science and Engineering, Yokohama National University, Yokohama 240-8501, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Tandem acoustic emulsification Size-controlled synthesis Polymer hollow nanoparticle
We have developed a new emulsion template method for the synthesis of poly(methylmethacrylate) (PMMA) hollow nanoparticles with different sizes. This synthetic method involves sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) for acoustic emulsification of a water-insoluble fluorous solvent such as perfluoromethylcyclohexane (PFMCH) in an aqueous medium, followed by monomer (methylmethacrylate (MMA)) adsorption on the surface of the PFMCH emulsion droplets and photopolymerization of the adsorbed MMA in the obtained emulsion solution. Since the size of the PFMCH droplet templates can be tuned according to the number of steps of tandem acoustic emulsification, the obtained PMMA particle size can also be controlled. The subsequent removal of the core fluorous solvent by the heat treatment yielded size-controlled PMMA hollow nanoparticles and monodisperse PMMA hollow nanoparticles of different sizes. Furthermore, we confirmed that the substances could go in or go out of the hollow particles through the shells. Such a nice permeability is important for applications such as nanoreactors and drug delivery systems.
1. Introduction In recent years, polymer hollow particles with nanometer to micrometer dimensions have attracted intense research attention because of their wide range of applications, such as drug delivery systems, catalysts, cosmetics, coating materials, nano- and micro-reactors, and so on [1–5]. For most of these applications, size control is of key importance because the required size varies depending on each application. In order to meet the requirement for size, various approaches have been demonstrated for producing size-controlled polymer hollow particles in the past decades [6–8]. Until now, the best-established and most commonly used method for size-controlled synthesis of hollow particles is the template method. The template method is further classified into two categories, namely, the hard template method and the soft template method [9]. The hard template method is more straightforward and more common for producing size-controlled polymer hollow particles, but this method requires after treatments such as calcination at high temperature and dissolution with an organic solvent to remove template particles. On the other hand, templates can be easily removed in the soft template method because emulsion droplets or bubbles are used as templates. However, there are a relatively smaller number of reports on the soft template method due to poor monodispersity of the obtained hollow particles and difficulties in the ⁎
polymer coating on template surfaces. Moreover, in the soft template method, the use of surfactants is usually indispensable to avoid aggregation between soft templates [10,11]. However, the presence of surfactants might cause several problems such as increase in the cost and waste, remaining undesired contaminants in the end product. Therefore, it is more preferable to use the surfactant-free emulsions or bubble dispersions as templates for the synthesis of size-controlled polymer hollow particles. On the other hand, it is well known that ultrasonication provides stable emulsions without using any surfactants simply by mechanical dispersion forces, which arise at the liquid/liquid phase boundaries [12–14]. This has been termed as “acoustic emulsification”. The acoustic emulsification is regarded as a powerful tool for rapid and environment-friendly emulsion production, and nowadays stable emulsions generated with ultrasound are used in the textile, cosmetic, pharmaceutical and food industries [15]. Recently, we reported a new technique for the preparation of a highly transparent emulsified aqueous solution containing water-immiscible organic droplets with diameters of less than a few hundred nanometers under surfactant-free conditions using several ultrasonic devices having different frequencies [16–19]. In these demonstrations, the droplet size could be reduced clearly by the sequential ultrasonic operations. This novel technique, tandem acoustic emulsification, was found to be adequate for
Corresponding author at: Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan. E-mail address: [email protected] (M. Atobe).
https://doi.org/10.1016/j.ultsonch.2019.01.032 Received 29 October 2018; Received in revised form 22 January 2019; Accepted 23 January 2019 Available online 24 January 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
Ultrasonics - Sonochemistry 54 (2019) 250–255
M. Koshino et al.
composed of PFMCH core and PMMA shell. An Hg-Xe lamp (Sanei Electric Co. Supercure-205S, 1.5 W cm−2 at 365 nm) was used as a UV light source for the polymerization. In order to make hollow particles, PFMCH (b.p. 76 °C) core was removed by heating at 80 °C with a water bath for 2 h. Then, the resultant colloidal dispersion was ultracentrifuged at 15,000 rpm (20400 G) for 90 min. By this operation, almost PMMA solid particles precipitated, while the hollow particles still existed in the supernatant. Hence, the supernatant was then recovered by decantation. Finally, the hollow particles were separated from the supernatant solution by filtration (220 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. About a few tens of gram of the hollow particles could be obtained by the above series of processes.
producing emulsion nanodroplets with desired sizes. Under these backgrounds, we envisioned that surfactant-free emulsions prepared by tandem acoustic emulsification would be ideal templates for the synthesis of size-controlled polymer hollow particles. We report here the realization of this idea. 2. Experimental 2.1. Materials Methylmethacrylate (MMA, analytical grade) was purchased from Tokyo Chemical Industry Co., Ltd. and passed through a basic alumina column for the removal of polymerization inhibitor (6-tert-butyl-2,4xylenol). Aluminum oxide 90 active basic (0.063–0.200 mm) (activity stage I) for column chromatography was purchased from MERCK. 2,2′Azobis(2-methylpropionamidine)dihydrochloride (AAPH) and perfluoromethylcyclohexane (PFMCH) were purchased from Tokyo Chemical Industry Co., Ltd. and used without further purification. Sudan III, 0.1 mol L−1 silver nitrate (AgNO3) solution, and sodium tetrahydroborate (NaBH4) were purchased from Wako Pure Chemical Co. and used without further purification. Distilled and deionized water was used as a solvent for the acoustic emulsification and polymerization of MMA.
2.5. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy and energy dispersive spectroscopy (TEMEDS) Dried down PMMA hollow particles were redispersed in deionized water. The hollow particle samples for the FE-SEM and TEM-EDS analyses were prepared by dropping the hollow nanoparticle dispersion on glass plate and TEM grid, respectively, and then naturally dried in air. After this process, the samples were subjected to FE-SEM (JSM-7001F, JEOL Co.) for the observation of surface morphology of the particle. The accelerating voltage of SEM was 20 kV. On the other hand, the samples were also analyzed by TEM-EDS (TEM, JEM-2100F (JEOL Co.); EDS, EDAX Genesis XM2 (AMETEK)) for the confirmation of hollow structure of the particles. The accelerating voltage of TEM was 200 kV.
2.2. Tandem acoustic emulsification PFMCH (0.50 mL) was added to 20 mL of deionized water in Rosette Cooling Cell (Branson Ultrasonics Co.). Then, the 20 kHz ultrasonication to the PFMCH/water mixture was conducted with an ultrasonic stepped horn (13 mm diameter, titanium alloy) connecting with a 20 kHz oscillator (23 W cm−2, SONIFIER-250D, Branson Ultrasonics Co.) for 7 min. The sequential ultrasonication with 500 kHz treatment after 20 kHz was carried out using an ultrasonic transducer (63 W cm−2, Honda Electric Co.) connected with a Pyrex grass cylindrical tube (diameter, 24 mm; length 75 mm) for 15 min. Subsequently, the sequential ultrasonication with 1.6 MHz treatment after 20 kHz and 500 kHz was carried out using an ultrasonic transducer (16 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min. The further sequential ultrasonication with 2.4 MHz treatment after 20 kHz, 500 kHz and 1.6 MHz was carried out using an ultrasonic transducer (8 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min. Finally, the last sequential ultrasonication with 5.0 MHz treatment after 20 kHz, 500 kHz,1.6 MHz and 2.4 MHz was conducted by an ultrasonic transducer (19 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min.
2.6. Preparation of hollow particles with encapsulated Ag In order to check whether or not the substances can go in the hollow particles through the PMMA shells, we prepared hollow particles encapsulating Ag in reference to the literature [5]. Firstly, the hollow nanoparticles were dispersed in 0.1 mM AgNO3 ethanol solution for 20 min to make AgNO3 enter the cavity of the hollow particles via 28 kHz ultrasonication (Ultrasonic Multi Cleaner W-113, Honda Electronics Co., Ltd.). Then, the particles were separated from the solution by filtration (100 nm-pore-size membrane filter), washed with water for several times to remove the AgNO3 on the outer surface of the particles. Subsequently, they were redispersed in 0.1 mM NaBH4 ethanol solution with 28 kHz ultrasonication for 20 min. When NaBH4 enters the hollow core of the particles, a redox reaction takes place, and therefore, the encapsulated Ag should be yielded. After this process, the particles were separated from the solution by filtration (100 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. To confirm the formation of Ag inside the particles, the dried down PMMA hollow particles were finally subjected to TEM-EDS (TEM: JEM-2100F (JEOL Co.), EDS: EDAX Genesis XM2 (AMETEK)). The accelerating voltage of TEM was 200 kV.
2.3. Measurement of droplet size and distribution PFMCH droplet sizes and their distribution were determined by the dynamic light scattering (DLS) method at 25 °C with light scattering photometer (nano-ZS ZEN 3600, Sysmex Co.) with diluting the mixture. The minimal measurement time of 30 sec was required for setting and stabilizing of the sample before the first data point was taken.
3. Results and discussion In the present work, we chose PFMCH as a template substance due to its unique properties. PFMCH, which is one of the fluorous solvents, is quite hydrophobic and immiscible with water and almost all organic solvents at room temperature [20]. For the preparation of PMMA hollow particles by the template method, MMA monomer should not be dissolved in PFMCH templates, but should be adsorbed around PFMCH droplet templates. Therefore, we firstly checked whether or not MMA monomer adsorbs around the PFMCH droplets in an aqueous medium. As mentioned in the above, since PFMCH is immiscible with water, it exists as a droplet in an aqueous media, as shown in Fig. 2(a). Subsequently, MMA colored with a red dye (Sudan III) was added to this aqueous phase, and this was then shaken by hand and left to stand for about 30 min. After this operation, we confirmed that MMA monomer
2.4. Template preparation of PMMA hollow nanoparticles The experiment procedure for the template preparation of PMMA hollow nanoparticles is shown in Fig. 1. MMA monomer (0.1 mL) and AAPH initiator (2.7 mg) were added to PFMCH emulsified aqueous solutions obtained at each ultrasonication steps in the tandem operation. Polymerization of MMA adsorbed on PFMCH droplets was started by UV irradiation and carried out with mechanical stirring at 300 rpm under ambient temperature for 30 min in cylindrical glass cell (SV-50A, Nichiden-Rika Glass Co., Ltd.) to give composite nanoparticles 251
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Fig. 1. Schematic illustration of the experimental procedure for synthesis PMMA hollow nanoparticles.
Table 1 Mean size of perfluoromethylcyclohexane (PFMCH) droplets. Emulsification condition
Mean size of PFMCH droplets/nm
20 kHz 20 kHz → 500 kHz 20 kHz → 500 kHz → 1.6 MHz 20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz 20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5 MHz
175–311 224–430 342 306 158
From this experiment, it can be stated that PFMCH is a suitable template substance for the preparation of PMMA hollow particles by the template method. In the next, we prepared size-controlled PFMCH emulsion templates by using the tandem acoustic emulsification. Sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5 MHz) for acoustic emulsification of a water-insoluble PFMCH was carried out in
Fig. 2. Photographs of a PFMCH droplet and MMA colored with a red dye in aqueous solution mixture (a) before and (b) after shaking by hand.
effectively adsorbed around the PFMCH droplet and did not dissolve into the PFMCH droplet, as shown in Fig. 2(b). This may be ascribed that both MMA and PFMCH are immiscible with each other and water.
Fig. 3. Photographic observations of tandem acoustic emulsification of PFMCH in aqueous solution. Photograph (a) represents the original PFMCH in aqueous solution mixture. Emulsification conditions were (b) 20 kHz, 7 min; (c) 20 kHz, 7 min → 500 kHz, 15 min; (d) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min; (e) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min → 2.4 MHz; and (f) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min → 2.4 MHz, 15 min → 5 MHz, 15 min. 252
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Fig. 4. FE-SEM images of PMMA nanoparticles prepared by using (a) 0.05 mL, (b) 0.10 mL, and (c) 0.20 mL of MMA.
Fig. 5. TEM images (a, b) and EDS spectrum (c) of the 400 nm particle (a, c) before and (b) after the heat treatment.
Fig. 7. FE-SEM images (a, c, e) and TEM images (b, d, f) of PMMA hollow particles prepared from PFMCH emulsion templates obtained by (a, b) three, (c, d) four, and (e, f) five ultrasonication steps in the tandem operation.
ultrasonication with 5.0 MHz treatment after the 20 kHz, 500 kHz, 1.6 MHz, and 2.4 MHz treatments (Fig. 3f). To estimate the droplets size quantitatively, we measured the size distribution of the PEMCH droplets after the acoustic emulsification treatments by DLS (Table 1). The emulsion samples obtained by a single ultrasonication step (20 kHz) and two ultrasonication steps (20 kHz → 500 kHz) were unstable and exhibited polydisperse size distribution, as shown in Table 1. On the other hand, a single peak in the number-mode was observed at 342 nm by sequential ultrasonication with 1.6 MHz treatment after the 20 kHz and 500 kHz. Further sequential ultrasonication with 2.4 MHz resulted in a peak shift to 306 nm. Finally, ultrasonication with 5.0 MHz after 20 kHz, 500 kHz, 1.6 MHz, and 2.4 MHz led to a peak shift to 158 nm. Thus, sequential ultrasonication at a higher frequency could break up the larger droplets to form smaller droplets. From these measurements, it can be stated that the size of the
Fig. 6. FE-SEM images of PMMA hollow particles after centrifugation and filteration.
an aqueous medium (Fig. 3). A milky white solution was obtained after 20 kHz ultrasonic treatment for 7 min (Fig. 3b). This appearance was not so changed by sequential ultrasonication with 500 kHz treatment after the 20 kHz (Fig. 3c). However, subsequent ultrasonication with 1.6 MHz gave a clearer emulsified solution (Fig. 3d), and a further clear and transparent solution was obtained by further sequential 253
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Fig. 8. TEM images of PMMA hollow nanoparticles encapsulating Ag deposits.
ultracentrifuged at 15,000 rpm for 90 min. By this operation, almost aggregates precipitated, while the hollow particles still existed in the supernatant. The supernatant was therefore recovered by decantation. Finally, the hollow particles were separated from the supernatant solution by filtration (220 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. About a few tens of gram of the hollow particles could be obtained by these operations. Fig. 6 shows FE-SEM images of PMMA hollow particles purified. Only the hollow particles were observed in FE-SEM images of the purified samples, while no small particle aggregate was seen in the images. To obtain the hollow particles with various sizes, we then used the PEMCH emulsion templates obtained by three (20 kHz → 500 kHz → 1.6 MHz), four (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz), and five (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) ultrasonication steps in the tandem operation. Fig. 7 shows FE-SEM and TEM images of PMMA hollow particles prepared from PFMCH emulsified aqueous solutions obtained at each ultrasonication steps in the tandem operation. The size of the particles prepared from the emulsion templates obtained at three, four, and five ultrasonication steps were found to be about 420, 390, and 230 nm, respectively (see Fig. 7(a), (c), and (e)). These particle sizes are indeed reasonable compared with the corresponding PEMCH droplet template diameter indicated in Table 1. In addition, from the TEM images of the particle samples after the heat treatment, it was also confirmed that all particle samples possessed a hollow structure, as shown in Fig. 7 (b), (d), and (f). Moreover, the thickness of hollow core shell for all tested samples was ranging from 40 to 50 nm. On the other hand, some particles shown in TEM images seem to have a non-spherical shape. This may be ascribed that the heat treatment for removing PFMCH core distorted the particle shape. For the application of hollow particles in nanoreactor and drug delivery systems, it is required that the substances can go in or go out of the particles through the shells. Therefore, in the last stage of investigation, the hollow particles prepared from the emulsion template obtained at three ultrasonication steps were applied as nanoreactors so as to attest to their values in practical applications. To realize this subject, silver (Ag) was synthesized inside PMMA hollow particles, and PMMA-encapsulated Ag composite particles could be thus be prepared. In the first, the hollow particles were dispersed in a 0.10 mM AgNO3 ethanol solution to make AgNO3 enter the cavity of the particles via 28 kHz sonication. AgNO3 thus penetrated inside the hollow particles through PMMA shells. The AgNO3 on the outer surface of the spheres was removed by rinsing the particles with water. Then, they were dispersed in a 0.10 mM NaBH4 ethanol solution, and 28 kHz ultrasonicated for 20 min. By this operation, NaBH4 penetrated into the hollow core of the particles, and consequently a redox reaction between AgNO3 and NaBH4 took place inside the hollow particles to yield Ag. Fig. 8 shows TEM images of PMMA hollow nanoparticles encapsulating Ag deposits. As shown in Fig. 8(a), Ag deposits could be clearly observed in a TEM
PFMCH nanodroplets was controlled intentionally by selecting the number of ultrasonication steps in the tandem operation. Especially, the PEMCH emulsions obtained by three (20 kHz → 500 kHz → 1.6 MHz), four (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz), and five (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) ultrasonication steps seems to be suitable template for the preparation of size-controlled PMMA hollow particles because of their monodisperse size distribution. In the next, 0.05, 0.10, or 0.20 mL of MMA monomer and 2.7 mg of AAPH initiator were added to PFMCH emulsified aqueous solution obtained at three ultrasonication steps (20 kHz → 500 kHz → 1.6 MHz) in the tandem operation. MMA adsorbed on PFMCH droplets was then polymerized under UV irradiation to produce PMMA shells surrounding PFMCH droplets. Fig. 4 shows the FE-SEM image of PMMA particles prepared by adding 0.05, 0.10, and 0.20 mL of MMA. In the case with 0.05 mL of MMA, about 80 nm and 200 nm PMMA particles were observed in the corresponding sample (Fig. 4(a)). Probably the 80 nm particles were generated not on template surfaces but in water phase. By considering the size of PFMCH emulsion template obtained at three ultrasonication steps (average droplet size: 342 nm), it may be assumed that the 200 nm particles also were not generated on the emulsion template surfaces, but rather formed by agglomeration between small PMMA particles in water phase. Therefore, 0.05 mL of MMA would be insufficient to adsorb fully on PFMCH droplet surfaces. On the other hand, in the cases of 0.10 and 0.20 mL MMA addition, the 400 nm particles other than small particle aggregates could be observed, as shown in Fig. 4(b) and (c). By taking their size into consideration, the 400 nm particles are expected to be produced via PMMA shell formation on PFMCH droplet surfaces. To confirm the structure of the produced particles, we then analyzed the particle samples prepared with 0.10 mL MMA addition by using TEM-EDS. Fig. 5 (a) and (b) show TEM images of the 400 nm particle before and after the heat treatment, respectively. On the other hand, Fig. 5(c) shows EDS spectrum of the 400 nm sample before the heat treatment. As shown in Fig. 5(a), jet-black core and pale-gray shell parts could be clearly seen before the heat treatment. The EDS spectrum of this sample revealed that a large amount of F element was contained in the core portion (Fig. 5(c)). Therefore, it can be expected that the PFMCH template still remained in the core portion before the heat treatment. In addition, a trace amount of titanium was also detected on the spectrum. This may be ascribed to Ti contamination arised from cavitation erosion of ultrasonic horn (Ti alloy) in 20 kHz ultrasonication. On the other hand, the hollow structure was clearly observed in TEM image of the sample after the heat treatment (Fig. 5(b)). This result indicates that the PFMCH core template can be removed by the heat treatment to provide the PMMA hollow particles. To obtain only the PMMA hollow particles, we then remove the small particle aggregates from the polymerized sample. The sample after the heat treatment was redispersed in deionized water and 254
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image when the center of the spherical particle was focused. However, as shown in Fig. 8(b), the image of Ag deposits became blurred when the focus point was shifted up from the center of the particle. This fact suggested that Ag deposits were indeed formed inside the hollow core. For the application of hollow particles in drug delivery systems, a permeability test of hollow nanoparticles with the larger organic molecules is in progress.
[3] [4] [5]
[6]
4. Conclusions
[7]
We successfully synthesized size-controlled PMMA hollow particles using PFMCH emulsion templates prepared by tandem acoustic emulsification under a surfactant-free condition. Thus, tandem acoustic emulsification offers a “green” method and is clearly a powerful tool for the preparation of size-controlled PFMCH droplet templates. The subsequent MMA adsorption on the surface of the PFMCH emulsion droplets, photopolymerization of MMA adsorbed on PEMCH templates, and removal of the core PFMCH by the heat treatment yielded sizecontrolled PMMA hollow particles. In addition, the prepared hollow particles could be applied as nanoreactors. To demonstrate this subject, AgNO3 and NaBH4 could be diffused into the hollow core of the particles across their sells, where a redox reaction could take place, yielding Ag inside the hollow particles.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements The authors are grateful to Dr. Masashi Kondo of the Instrumental Analysis Center at Yokohama National University, Japan for his kind assistance with the TEM analyses. This work was financially supported by JSPS KAKENHI Grant Number JP15H0384310, Japan.
[17]
[18]
[19]
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Contents lists available at ScienceDirect
Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Size-controlled synthesis of polymer hollow nanoparticles using emulsion templates prepared by tandem acoustic emulsification Miharu Koshinoa, Yukihide Shiraishib, Mahito Atobea,b, a b
T
⁎
Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan Graduate School of Science and Engineering, Yokohama National University, Yokohama 240-8501, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Tandem acoustic emulsification Size-controlled synthesis Polymer hollow nanoparticle
We have developed a new emulsion template method for the synthesis of poly(methylmethacrylate) (PMMA) hollow nanoparticles with different sizes. This synthetic method involves sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) for acoustic emulsification of a water-insoluble fluorous solvent such as perfluoromethylcyclohexane (PFMCH) in an aqueous medium, followed by monomer (methylmethacrylate (MMA)) adsorption on the surface of the PFMCH emulsion droplets and photopolymerization of the adsorbed MMA in the obtained emulsion solution. Since the size of the PFMCH droplet templates can be tuned according to the number of steps of tandem acoustic emulsification, the obtained PMMA particle size can also be controlled. The subsequent removal of the core fluorous solvent by the heat treatment yielded size-controlled PMMA hollow nanoparticles and monodisperse PMMA hollow nanoparticles of different sizes. Furthermore, we confirmed that the substances could go in or go out of the hollow particles through the shells. Such a nice permeability is important for applications such as nanoreactors and drug delivery systems.
1. Introduction In recent years, polymer hollow particles with nanometer to micrometer dimensions have attracted intense research attention because of their wide range of applications, such as drug delivery systems, catalysts, cosmetics, coating materials, nano- and micro-reactors, and so on [1–5]. For most of these applications, size control is of key importance because the required size varies depending on each application. In order to meet the requirement for size, various approaches have been demonstrated for producing size-controlled polymer hollow particles in the past decades [6–8]. Until now, the best-established and most commonly used method for size-controlled synthesis of hollow particles is the template method. The template method is further classified into two categories, namely, the hard template method and the soft template method [9]. The hard template method is more straightforward and more common for producing size-controlled polymer hollow particles, but this method requires after treatments such as calcination at high temperature and dissolution with an organic solvent to remove template particles. On the other hand, templates can be easily removed in the soft template method because emulsion droplets or bubbles are used as templates. However, there are a relatively smaller number of reports on the soft template method due to poor monodispersity of the obtained hollow particles and difficulties in the ⁎
polymer coating on template surfaces. Moreover, in the soft template method, the use of surfactants is usually indispensable to avoid aggregation between soft templates [10,11]. However, the presence of surfactants might cause several problems such as increase in the cost and waste, remaining undesired contaminants in the end product. Therefore, it is more preferable to use the surfactant-free emulsions or bubble dispersions as templates for the synthesis of size-controlled polymer hollow particles. On the other hand, it is well known that ultrasonication provides stable emulsions without using any surfactants simply by mechanical dispersion forces, which arise at the liquid/liquid phase boundaries [12–14]. This has been termed as “acoustic emulsification”. The acoustic emulsification is regarded as a powerful tool for rapid and environment-friendly emulsion production, and nowadays stable emulsions generated with ultrasound are used in the textile, cosmetic, pharmaceutical and food industries [15]. Recently, we reported a new technique for the preparation of a highly transparent emulsified aqueous solution containing water-immiscible organic droplets with diameters of less than a few hundred nanometers under surfactant-free conditions using several ultrasonic devices having different frequencies [16–19]. In these demonstrations, the droplet size could be reduced clearly by the sequential ultrasonic operations. This novel technique, tandem acoustic emulsification, was found to be adequate for
Corresponding author at: Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan. E-mail address: [email protected] (M. Atobe).
https://doi.org/10.1016/j.ultsonch.2019.01.032 Received 29 October 2018; Received in revised form 22 January 2019; Accepted 23 January 2019 Available online 24 January 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.
Ultrasonics - Sonochemistry 54 (2019) 250–255
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composed of PFMCH core and PMMA shell. An Hg-Xe lamp (Sanei Electric Co. Supercure-205S, 1.5 W cm−2 at 365 nm) was used as a UV light source for the polymerization. In order to make hollow particles, PFMCH (b.p. 76 °C) core was removed by heating at 80 °C with a water bath for 2 h. Then, the resultant colloidal dispersion was ultracentrifuged at 15,000 rpm (20400 G) for 90 min. By this operation, almost PMMA solid particles precipitated, while the hollow particles still existed in the supernatant. Hence, the supernatant was then recovered by decantation. Finally, the hollow particles were separated from the supernatant solution by filtration (220 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. About a few tens of gram of the hollow particles could be obtained by the above series of processes.
producing emulsion nanodroplets with desired sizes. Under these backgrounds, we envisioned that surfactant-free emulsions prepared by tandem acoustic emulsification would be ideal templates for the synthesis of size-controlled polymer hollow particles. We report here the realization of this idea. 2. Experimental 2.1. Materials Methylmethacrylate (MMA, analytical grade) was purchased from Tokyo Chemical Industry Co., Ltd. and passed through a basic alumina column for the removal of polymerization inhibitor (6-tert-butyl-2,4xylenol). Aluminum oxide 90 active basic (0.063–0.200 mm) (activity stage I) for column chromatography was purchased from MERCK. 2,2′Azobis(2-methylpropionamidine)dihydrochloride (AAPH) and perfluoromethylcyclohexane (PFMCH) were purchased from Tokyo Chemical Industry Co., Ltd. and used without further purification. Sudan III, 0.1 mol L−1 silver nitrate (AgNO3) solution, and sodium tetrahydroborate (NaBH4) were purchased from Wako Pure Chemical Co. and used without further purification. Distilled and deionized water was used as a solvent for the acoustic emulsification and polymerization of MMA.
2.5. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy and energy dispersive spectroscopy (TEMEDS) Dried down PMMA hollow particles were redispersed in deionized water. The hollow particle samples for the FE-SEM and TEM-EDS analyses were prepared by dropping the hollow nanoparticle dispersion on glass plate and TEM grid, respectively, and then naturally dried in air. After this process, the samples were subjected to FE-SEM (JSM-7001F, JEOL Co.) for the observation of surface morphology of the particle. The accelerating voltage of SEM was 20 kV. On the other hand, the samples were also analyzed by TEM-EDS (TEM, JEM-2100F (JEOL Co.); EDS, EDAX Genesis XM2 (AMETEK)) for the confirmation of hollow structure of the particles. The accelerating voltage of TEM was 200 kV.
2.2. Tandem acoustic emulsification PFMCH (0.50 mL) was added to 20 mL of deionized water in Rosette Cooling Cell (Branson Ultrasonics Co.). Then, the 20 kHz ultrasonication to the PFMCH/water mixture was conducted with an ultrasonic stepped horn (13 mm diameter, titanium alloy) connecting with a 20 kHz oscillator (23 W cm−2, SONIFIER-250D, Branson Ultrasonics Co.) for 7 min. The sequential ultrasonication with 500 kHz treatment after 20 kHz was carried out using an ultrasonic transducer (63 W cm−2, Honda Electric Co.) connected with a Pyrex grass cylindrical tube (diameter, 24 mm; length 75 mm) for 15 min. Subsequently, the sequential ultrasonication with 1.6 MHz treatment after 20 kHz and 500 kHz was carried out using an ultrasonic transducer (16 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min. The further sequential ultrasonication with 2.4 MHz treatment after 20 kHz, 500 kHz and 1.6 MHz was carried out using an ultrasonic transducer (8 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min. Finally, the last sequential ultrasonication with 5.0 MHz treatment after 20 kHz, 500 kHz,1.6 MHz and 2.4 MHz was conducted by an ultrasonic transducer (19 W cm−2, Honda Electric Co.) connected with a Pyrex glass cylindrical tube (diameter, 24 mm; length, 75 mm) for 15 min.
2.6. Preparation of hollow particles with encapsulated Ag In order to check whether or not the substances can go in the hollow particles through the PMMA shells, we prepared hollow particles encapsulating Ag in reference to the literature [5]. Firstly, the hollow nanoparticles were dispersed in 0.1 mM AgNO3 ethanol solution for 20 min to make AgNO3 enter the cavity of the hollow particles via 28 kHz ultrasonication (Ultrasonic Multi Cleaner W-113, Honda Electronics Co., Ltd.). Then, the particles were separated from the solution by filtration (100 nm-pore-size membrane filter), washed with water for several times to remove the AgNO3 on the outer surface of the particles. Subsequently, they were redispersed in 0.1 mM NaBH4 ethanol solution with 28 kHz ultrasonication for 20 min. When NaBH4 enters the hollow core of the particles, a redox reaction takes place, and therefore, the encapsulated Ag should be yielded. After this process, the particles were separated from the solution by filtration (100 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. To confirm the formation of Ag inside the particles, the dried down PMMA hollow particles were finally subjected to TEM-EDS (TEM: JEM-2100F (JEOL Co.), EDS: EDAX Genesis XM2 (AMETEK)). The accelerating voltage of TEM was 200 kV.
2.3. Measurement of droplet size and distribution PFMCH droplet sizes and their distribution were determined by the dynamic light scattering (DLS) method at 25 °C with light scattering photometer (nano-ZS ZEN 3600, Sysmex Co.) with diluting the mixture. The minimal measurement time of 30 sec was required for setting and stabilizing of the sample before the first data point was taken.
3. Results and discussion In the present work, we chose PFMCH as a template substance due to its unique properties. PFMCH, which is one of the fluorous solvents, is quite hydrophobic and immiscible with water and almost all organic solvents at room temperature [20]. For the preparation of PMMA hollow particles by the template method, MMA monomer should not be dissolved in PFMCH templates, but should be adsorbed around PFMCH droplet templates. Therefore, we firstly checked whether or not MMA monomer adsorbs around the PFMCH droplets in an aqueous medium. As mentioned in the above, since PFMCH is immiscible with water, it exists as a droplet in an aqueous media, as shown in Fig. 2(a). Subsequently, MMA colored with a red dye (Sudan III) was added to this aqueous phase, and this was then shaken by hand and left to stand for about 30 min. After this operation, we confirmed that MMA monomer
2.4. Template preparation of PMMA hollow nanoparticles The experiment procedure for the template preparation of PMMA hollow nanoparticles is shown in Fig. 1. MMA monomer (0.1 mL) and AAPH initiator (2.7 mg) were added to PFMCH emulsified aqueous solutions obtained at each ultrasonication steps in the tandem operation. Polymerization of MMA adsorbed on PFMCH droplets was started by UV irradiation and carried out with mechanical stirring at 300 rpm under ambient temperature for 30 min in cylindrical glass cell (SV-50A, Nichiden-Rika Glass Co., Ltd.) to give composite nanoparticles 251
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Fig. 1. Schematic illustration of the experimental procedure for synthesis PMMA hollow nanoparticles.
Table 1 Mean size of perfluoromethylcyclohexane (PFMCH) droplets. Emulsification condition
Mean size of PFMCH droplets/nm
20 kHz 20 kHz → 500 kHz 20 kHz → 500 kHz → 1.6 MHz 20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz 20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5 MHz
175–311 224–430 342 306 158
From this experiment, it can be stated that PFMCH is a suitable template substance for the preparation of PMMA hollow particles by the template method. In the next, we prepared size-controlled PFMCH emulsion templates by using the tandem acoustic emulsification. Sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5 MHz) for acoustic emulsification of a water-insoluble PFMCH was carried out in
Fig. 2. Photographs of a PFMCH droplet and MMA colored with a red dye in aqueous solution mixture (a) before and (b) after shaking by hand.
effectively adsorbed around the PFMCH droplet and did not dissolve into the PFMCH droplet, as shown in Fig. 2(b). This may be ascribed that both MMA and PFMCH are immiscible with each other and water.
Fig. 3. Photographic observations of tandem acoustic emulsification of PFMCH in aqueous solution. Photograph (a) represents the original PFMCH in aqueous solution mixture. Emulsification conditions were (b) 20 kHz, 7 min; (c) 20 kHz, 7 min → 500 kHz, 15 min; (d) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min; (e) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min → 2.4 MHz; and (f) 20 kHz, 7 min → 500 kHz, 15 min → 1.6 MHz, 15 min → 2.4 MHz, 15 min → 5 MHz, 15 min. 252
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Fig. 4. FE-SEM images of PMMA nanoparticles prepared by using (a) 0.05 mL, (b) 0.10 mL, and (c) 0.20 mL of MMA.
Fig. 5. TEM images (a, b) and EDS spectrum (c) of the 400 nm particle (a, c) before and (b) after the heat treatment.
Fig. 7. FE-SEM images (a, c, e) and TEM images (b, d, f) of PMMA hollow particles prepared from PFMCH emulsion templates obtained by (a, b) three, (c, d) four, and (e, f) five ultrasonication steps in the tandem operation.
ultrasonication with 5.0 MHz treatment after the 20 kHz, 500 kHz, 1.6 MHz, and 2.4 MHz treatments (Fig. 3f). To estimate the droplets size quantitatively, we measured the size distribution of the PEMCH droplets after the acoustic emulsification treatments by DLS (Table 1). The emulsion samples obtained by a single ultrasonication step (20 kHz) and two ultrasonication steps (20 kHz → 500 kHz) were unstable and exhibited polydisperse size distribution, as shown in Table 1. On the other hand, a single peak in the number-mode was observed at 342 nm by sequential ultrasonication with 1.6 MHz treatment after the 20 kHz and 500 kHz. Further sequential ultrasonication with 2.4 MHz resulted in a peak shift to 306 nm. Finally, ultrasonication with 5.0 MHz after 20 kHz, 500 kHz, 1.6 MHz, and 2.4 MHz led to a peak shift to 158 nm. Thus, sequential ultrasonication at a higher frequency could break up the larger droplets to form smaller droplets. From these measurements, it can be stated that the size of the
Fig. 6. FE-SEM images of PMMA hollow particles after centrifugation and filteration.
an aqueous medium (Fig. 3). A milky white solution was obtained after 20 kHz ultrasonic treatment for 7 min (Fig. 3b). This appearance was not so changed by sequential ultrasonication with 500 kHz treatment after the 20 kHz (Fig. 3c). However, subsequent ultrasonication with 1.6 MHz gave a clearer emulsified solution (Fig. 3d), and a further clear and transparent solution was obtained by further sequential 253
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Fig. 8. TEM images of PMMA hollow nanoparticles encapsulating Ag deposits.
ultracentrifuged at 15,000 rpm for 90 min. By this operation, almost aggregates precipitated, while the hollow particles still existed in the supernatant. The supernatant was therefore recovered by decantation. Finally, the hollow particles were separated from the supernatant solution by filtration (220 nm-pore-size membrane filter), washed with water for several times, and naturally dried in air. About a few tens of gram of the hollow particles could be obtained by these operations. Fig. 6 shows FE-SEM images of PMMA hollow particles purified. Only the hollow particles were observed in FE-SEM images of the purified samples, while no small particle aggregate was seen in the images. To obtain the hollow particles with various sizes, we then used the PEMCH emulsion templates obtained by three (20 kHz → 500 kHz → 1.6 MHz), four (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz), and five (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) ultrasonication steps in the tandem operation. Fig. 7 shows FE-SEM and TEM images of PMMA hollow particles prepared from PFMCH emulsified aqueous solutions obtained at each ultrasonication steps in the tandem operation. The size of the particles prepared from the emulsion templates obtained at three, four, and five ultrasonication steps were found to be about 420, 390, and 230 nm, respectively (see Fig. 7(a), (c), and (e)). These particle sizes are indeed reasonable compared with the corresponding PEMCH droplet template diameter indicated in Table 1. In addition, from the TEM images of the particle samples after the heat treatment, it was also confirmed that all particle samples possessed a hollow structure, as shown in Fig. 7 (b), (d), and (f). Moreover, the thickness of hollow core shell for all tested samples was ranging from 40 to 50 nm. On the other hand, some particles shown in TEM images seem to have a non-spherical shape. This may be ascribed that the heat treatment for removing PFMCH core distorted the particle shape. For the application of hollow particles in nanoreactor and drug delivery systems, it is required that the substances can go in or go out of the particles through the shells. Therefore, in the last stage of investigation, the hollow particles prepared from the emulsion template obtained at three ultrasonication steps were applied as nanoreactors so as to attest to their values in practical applications. To realize this subject, silver (Ag) was synthesized inside PMMA hollow particles, and PMMA-encapsulated Ag composite particles could be thus be prepared. In the first, the hollow particles were dispersed in a 0.10 mM AgNO3 ethanol solution to make AgNO3 enter the cavity of the particles via 28 kHz sonication. AgNO3 thus penetrated inside the hollow particles through PMMA shells. The AgNO3 on the outer surface of the spheres was removed by rinsing the particles with water. Then, they were dispersed in a 0.10 mM NaBH4 ethanol solution, and 28 kHz ultrasonicated for 20 min. By this operation, NaBH4 penetrated into the hollow core of the particles, and consequently a redox reaction between AgNO3 and NaBH4 took place inside the hollow particles to yield Ag. Fig. 8 shows TEM images of PMMA hollow nanoparticles encapsulating Ag deposits. As shown in Fig. 8(a), Ag deposits could be clearly observed in a TEM
PFMCH nanodroplets was controlled intentionally by selecting the number of ultrasonication steps in the tandem operation. Especially, the PEMCH emulsions obtained by three (20 kHz → 500 kHz → 1.6 MHz), four (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz), and five (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz → 5.0 MHz) ultrasonication steps seems to be suitable template for the preparation of size-controlled PMMA hollow particles because of their monodisperse size distribution. In the next, 0.05, 0.10, or 0.20 mL of MMA monomer and 2.7 mg of AAPH initiator were added to PFMCH emulsified aqueous solution obtained at three ultrasonication steps (20 kHz → 500 kHz → 1.6 MHz) in the tandem operation. MMA adsorbed on PFMCH droplets was then polymerized under UV irradiation to produce PMMA shells surrounding PFMCH droplets. Fig. 4 shows the FE-SEM image of PMMA particles prepared by adding 0.05, 0.10, and 0.20 mL of MMA. In the case with 0.05 mL of MMA, about 80 nm and 200 nm PMMA particles were observed in the corresponding sample (Fig. 4(a)). Probably the 80 nm particles were generated not on template surfaces but in water phase. By considering the size of PFMCH emulsion template obtained at three ultrasonication steps (average droplet size: 342 nm), it may be assumed that the 200 nm particles also were not generated on the emulsion template surfaces, but rather formed by agglomeration between small PMMA particles in water phase. Therefore, 0.05 mL of MMA would be insufficient to adsorb fully on PFMCH droplet surfaces. On the other hand, in the cases of 0.10 and 0.20 mL MMA addition, the 400 nm particles other than small particle aggregates could be observed, as shown in Fig. 4(b) and (c). By taking their size into consideration, the 400 nm particles are expected to be produced via PMMA shell formation on PFMCH droplet surfaces. To confirm the structure of the produced particles, we then analyzed the particle samples prepared with 0.10 mL MMA addition by using TEM-EDS. Fig. 5 (a) and (b) show TEM images of the 400 nm particle before and after the heat treatment, respectively. On the other hand, Fig. 5(c) shows EDS spectrum of the 400 nm sample before the heat treatment. As shown in Fig. 5(a), jet-black core and pale-gray shell parts could be clearly seen before the heat treatment. The EDS spectrum of this sample revealed that a large amount of F element was contained in the core portion (Fig. 5(c)). Therefore, it can be expected that the PFMCH template still remained in the core portion before the heat treatment. In addition, a trace amount of titanium was also detected on the spectrum. This may be ascribed to Ti contamination arised from cavitation erosion of ultrasonic horn (Ti alloy) in 20 kHz ultrasonication. On the other hand, the hollow structure was clearly observed in TEM image of the sample after the heat treatment (Fig. 5(b)). This result indicates that the PFMCH core template can be removed by the heat treatment to provide the PMMA hollow particles. To obtain only the PMMA hollow particles, we then remove the small particle aggregates from the polymerized sample. The sample after the heat treatment was redispersed in deionized water and 254
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image when the center of the spherical particle was focused. However, as shown in Fig. 8(b), the image of Ag deposits became blurred when the focus point was shifted up from the center of the particle. This fact suggested that Ag deposits were indeed formed inside the hollow core. For the application of hollow particles in drug delivery systems, a permeability test of hollow nanoparticles with the larger organic molecules is in progress.
[3] [4] [5]
[6]
4. Conclusions
[7]
We successfully synthesized size-controlled PMMA hollow particles using PFMCH emulsion templates prepared by tandem acoustic emulsification under a surfactant-free condition. Thus, tandem acoustic emulsification offers a “green” method and is clearly a powerful tool for the preparation of size-controlled PFMCH droplet templates. The subsequent MMA adsorption on the surface of the PFMCH emulsion droplets, photopolymerization of MMA adsorbed on PEMCH templates, and removal of the core PFMCH by the heat treatment yielded sizecontrolled PMMA hollow particles. In addition, the prepared hollow particles could be applied as nanoreactors. To demonstrate this subject, AgNO3 and NaBH4 could be diffused into the hollow core of the particles across their sells, where a redox reaction could take place, yielding Ag inside the hollow particles.
[8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements The authors are grateful to Dr. Masashi Kondo of the Instrumental Analysis Center at Yokohama National University, Japan for his kind assistance with the TEM analyses. This work was financially supported by JSPS KAKENHI Grant Number JP15H0384310, Japan.
[17]
[18]
[19]
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