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Article Cite This: ACS Omega 2018, 3, 6158−6165
High Reusability of Catalytically Active Gold Nanoparticles Immobilized in Core−Shell Hydrogel Microspheres Takuma Kureha,† Yasuhisa Nagase,† and Daisuke Suzuki*,†,‡ †
Graduate School of Textile Science & Technology and ‡Division of Smart Textiles, Institute for Fiber Engineering, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan S Supporting Information *
ABSTRACT: The reusability of hybrid core−shell microgels, whose core surfaces were decorated with gold nanoparticles, was investigated in terms of catalysis activity. Hybrid core−shell microgels composed of a rigid core and water-swollen gel shell endowed the immobilized gold nanoparticles with a high dispersion stability, which resulted in excellent catalytic activity. In contrast to free Au nanoparticles and conventional hybrid microgels, where the Au nanoparticles are randomly distributed over the entire microgel templates, the hydrogel shell part of the hybrid core−shell microgels suppressed the aggregation between the microgels and Au nanoparticles in individual microgels, which improved the reusability for the catalysis reaction. The results of this study should help to develop advanced catalyst systems that require high reusability even when the chemical reactions occur in aqueous solution and external stimuli are applied.
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INTRODUCTION Metallic nanoparticles represent fascinating aspects such as flexible assembly behavior, which is interesting in the context of materials science, the behavior of the individual particles, and their electronic, magnetic, and optical properties, as well as their applications in catalysis and biology.1−3 To improve the colloidal stability and endow the metallic nanoparticles with other properties in aqueous solution, colloidal carriers such as polymeric microspheres are usually applied,4−6 as they exhibit a large specific surface area, fast response, the potential to be used in mass production, and high diffusibility, as well as ease of recovery and handling.7−9 Hydrogel microspheres (microgels) are one of the most extensively studied classes of carriers, and they exhibit a three-dimensional cross-linked structure that swells in water.10−17 Hybrid microgels represent attractive prospective components for functional materials given that they combine the desirable features of water-swollen microgels (e.g., high colloidal stability and stimuli-responsive properties) with the attractive properties of metallic nanoparticles.18−27 So far, a variety of metallic nanoparticles immobilized in microgels have been developed for applications in, for example, sensors,28−31 bioimaging,32,33 and catalysis.34−36 Especially, the catalytic activity of metallic nanoparticles can be controlled by using stimuli-responsive microgels as templates. For example, control over the catalytic activity of metallic nanoparticles in polystyrene core/thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm) shell microspheres was achieved via the volume transition of pNIPAm.23,26,37 Moreover, thermoresponsive Au-pNIPAm yolk shell hybrid microspheres are effective catalysts for the reduction of p© 2018 American Chemical Society
nitrophenol and nitrobenzene in water, given that the selectivity of the catalysis can be enhanced by changing the temperature.38 More recently, hybrid microgel-based catalysts have been developed for applications in Heck/Suzuki coupling reactions or photocatalysis in aqueous media, which represent a novel type of green catalysts that are operative under mild conditions.39−41 However, to the best of our knowledge, only very few studies have been reported on the reusability of the hybrid microgels as catalysts during the reaction.42 In this situation, aggregation may occur between the metallic nanoparticles in individual microgels or the hybrid microgels, which leads to a decreased catalytic activity as a result of the decreasing specific surface area of the metallic nanoparticles. Such aggregation is likely to occur during catalysis and/or centrifugation (recovery), as the recovery and reuse of precious metals is mandatory nowadays, regardless of the type of carrier. Although various methods for the recovery of metallic nanoparticles based on filtration,43 dialysis,44 or the application of magnetic fields45 have been reported, centrifugation is the most common recovery method for colloidal particles, given its speed and convenience regardless of the presence or absence of metallic nanoparticles. It is accordingly very important to examine and reveal the aggregation behavior to develop adequate strategies that allow the suppression of aggregation during the reactions and Received: April 25, 2018 Accepted: May 25, 2018 Published: June 7, 2018 6158
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ACS Omega Table 1. Template Microspheres Used in This Study core microspheres NG core NGN1 NGN2
shell monomer
NIPAm (g)
GMA (g)
BIS (g)
V-50 (g)
water (mL)
0.2
2.2
0.04
0.02
100
core (g)
monomer concentration (mM)
NIPAm (g)
BIS (g)
V-50 (g)
water (mL)
0.5 0.5
25 40
0.28 0.38
0.006 0.010
0.02 0.02
100 100
Dh (nm)
PDI
25 °C 70 °C
25 °C
269 376 572
259 284 295
0.021 0.025 0.033
Figure 1. Hydrodynamic diameters (Dh) of (a) NG, (b) NGN1, and (c) NGN2 microgels, which represent different states (unmodified, aminofunctionalized, and hybrid), as a function of the temperature. TEM images of nonfunctionalized (a) NG, (b) NGN1, and (c) NGN2 microgels dried on a carbon-coated copper grid are displayed. The coefficient of variation for each microsphere is also shown (N = 30).
used in this study are relatively low and the obtained core−shell microgels were purified by centrifugation, suggesting that the influence of such secondary microgels on the catalytic reaction should be negligible. The resulting NG core−pNIPAm shell microgels (denoted as NGN1 and NGN2; NG and N refer to core NG microgels and the pNIPAm shell, respectively; 1 and 2 indicate shorter and longer shell thickness, respectively) were also uniform (Figure 1b,c). With increasing NIPAm monomer concentration during the seeded polymerization, the shell thickness of pNIPAm and the hydrodynamic diameters (Dh) increased (NG: Dh ≈ 269 nm; NGN1: Dh ≈ 376 nm; NGN2: Dh ≈ 572 nm; all at 25 °C; Table 1). In addition, the NGN1 and NGN2 microgels exhibited thermoresponsive behavior because of the presence of pNIPAm (Figure 1b,c), although Dh of the core NG microgels did not significantly change with temperature, which indicates that the seeded polymerization proceeded successfully. To introduce positive charges for the NG and NGN microspheres, which lead to a localization of the metallic nanoparticles on the surface of NG core, the reactive GMA moieties in the core were modified using 2-aminoethanethiol (2-AET), that is, the reaction between the thiol and epoxy groups.19,20 The incorporation of the amino groups was confirmed by the slightly increased Dh of the tested microgels synthesized in this study relative to those prior to the incorporation of the amino groups at the individual temperatures (Figure 1). The swelling behavior was attributed predominantly to the osmotic pressure and to repulsion between the amino groups. In this case, the increments of size are small because the amino groups are localized at the surface of core NG microspheres. It should be noted that the amine groups may potentially react with epoxides and esters to form secondary amines and amides, respectively, suggesting that 2-AET could work as a cross-linker. However, the facile staining of amino-functionalized NG and NGN microgel pastes after centrifugation confirms the successful introduction of
centrifugation, which would be highly attractive for advanced catalytic applications. To circumvent this obstacle, we hypothesized that metallic nanoparticles could be localized on the core surface of microgels composed of a rigid core and a swollen gel shell to suppress the contact between the metallic nanoparticles even during reaction and recovery. This should ensure both high dispersion stability and control over the diffusivity of the reaction substrates to the metallic nanoparticles from the bulk solution. In this study, we therefore immobilized metallic nanoparticles in the core−shell microgels and examined their catalytic activity and the reusability of these hybrid microgels in terms of the carrier structures.
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RESULTS AND DISCUSSION Synthesis and Characterization of the Hybrid Microgels. To distribute the metallic nanoparticles at the center of core−shell microgels, poly(NIPAm-co-glycidyl methacrylate) (NG) microspheres (rigid core) were prepared by free-radical polymerization in water (Table 1) and transmission electron microscopy (TEM) images (Figure 1a) confirmed their uniformity. Because of the difference in the reactivity ratio between N-isopropylacrylamide (NIPAm) (0.39) and glycidyl methacrylate (GMA) (2.69), the latter is consumed faster than the former.46 Thus, the exterior of the NG microspheres is rich in pNIPAm chains,19,20 which are formed by seeded precipitation polymerization, that can attach easily to pNIPAm on the surface of the NG core to grow the gel shell. Furthermore, seeded precipitation polymerizations were conducted in the presence of core NG microgels to cover the rigid core with gel layer. Here, the deswollen NG microspheres served as seeds for further polymerizations, which resulted in the preferential growth of existing particles relative to nucleation. It should be noted that the nucleation occurred in solution and that secondary (or individual) pNIPAm microgels could thus be formed. However, the monomer concentrations 6159
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was synthesized using the amino-functionalized NGN2 microgels as a template, where the removal of the excess of gold precursor ions was not carried out19 prior to the reduction. As a result, different from NGN1-Au and NGN2-Au microgels, the Au nanoparticles were localized randomly in the NGN2 microgels (NGN2-Au(R); Figure 2d) and the weight percentage of the Au nanoparticles in NGN2-Au(R) (0.98 wt %) was slightly higher than that in the NGN2-Au microgels (0.77 wt %). Reusability Capacity of the Hybrid Microgels in Catalytic Reactions. To characterize the reusability capacity of these hybrid microgels, we examined the reduction of pnitrophenol with an excess of NaBH4 as a model reaction.47,48 In this reaction, the consumption of p-nitrophenol can be easily monitored via UV−vis spectroscopy via the decrease of the strong adsorption of the p-nitrophenolate anion at 400 nm, leading directly to the rate constant, whereby the side reaction is not related to the main reaction.47−49 The evaluation method for the reusability capacity of the hybrid microgels is schematically illustrated in Figure 3a. Here, the reduction rates were assumed to be independent of the concentration of NaBH4 because a large excess compared to p-nitrophenol was used.42,50,51 Therefore, the kinetic data were fitted using a firstorder rate law. Moreover, the apparent rate constant (kapp) was assumed to be proportional to the surface of the Au nanoparticles present in the system
amino groups. After the reaction, the shape of the dried microspheres on the substrate did not change significantly as evident from the TEM images (Figure S2). In this study, we synthesized Au nanoparticles using these amino-functionalized NG, NGN1, and NGN2 microspheres as templates. Given that the dispersions of the template microspheres and the Au precursor anions were mixed for 1 h at 25 °C at low pH values, that is, with hydrated pNIPAm shells and the gold precursor ions, [AuCl4]−, should be localized at the core NG microspheres together with the amino groups. Subsequently, the microspheres were collected by centrifugation to remove any excess of anions with the dispersion medium. Immediately after the addition of NaBH4, the dispersion turned red. Figure 2 shows the TEM images of
−
dCt = kappCt = k1SAuCt dt
(1)
where Ct is the concentration of p-nitrophenol at any given time (t). Furthermore, kapp is strictly proportional to the total surface area of the Au nanoparticles (SAu).52,53 In this system, kapp can be easily obtained from monitoring the absorbance of the p-nitrophenolate ions at 400 nm as shown in Figures 3b and 4. The generation of p-nitrophenolate ions occurs immediately after the addition of NaBH4. Indeed, the reaction becomes stationary and follows a first-order rate law (blue line), which displays the linear section, from which kapp can be obtained. In addition, there is an induction time (t0), during which no reduction occurs. For the catalytic reduction of p-nitrophenol with a variety of catalysts, it is not unusual to observe induction periods of minutes.42,54,55 The concentration of [BH4]− has little effect on the induction. Conversely, the concentration of nitrophenol greatly affects the induction.56,57 Thus, it has been concluded that the induction time is related to a surface restructuring necessary to activate the metal nanoparticle catalysts. Chen et al. have demonstrated that a time scale of minutes may well be caused by processes related to a dynamic restructuring of the surface of the nanoparticles.58,59 They found a time scale for spontaneous surface restructuring of 60−250 s. For the reduction of nitrophenol in this study, we also determined the range of the time scale of t0 (Figures 3b and 4). However, the relation between t0 and the surface restructuring has been derived solely from kinetic data, and direct evidence from structural studies remains elusive. Moreover, in this study, the total surface areas of the Au nanoparticles used in the reaction were identical (1 × 1015 nm2), which allows comparing all cases by kapp. Figure 5a shows the rate constant of the tested hybrid microgels, including the free Au nanoparticles, as a function of reaction steps. The free Au nanoparticles, obtained from a reduction using citric acid, whereby the particles were coated with carboxylic acid, aggregated and precipitated during the
Figure 2. Schematic illustration, TEM images, and size distribution of Au nanoparticles within the template microspheres: (a) NG, (b) NGN1, and (c,d) NGN2 microgels. (d) Au nanoparticles are randomly distributed within the NGN2 microgel templates as the excess of anions was not removed. The ICP-AES-derived weight percentage of immobilized Au nanoparticles on each template is also shown.
the thus obtained hybrid microgels (NG-Au, NGN1-Au, and NGN2-Au), which indicate that the Au nanoparticles are located at the cores in agreement with our previous studies.19,20,28 The size of these immobilized Au nanoparticles (∼3 nm) and the narrow size distribution are shown in the histogram in Figure 2. The weight percentages of the Au nanoparticles in NG-Au (0.69 wt %), NGN1-Au (0.84 wt %), and NGN2-Au (0.77 wt %) were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). It should be noted that to confirm the effect of the distribution of the Au nanoparticles in the template microgels on the reusability in the catalysis, another type of Au nanoparticles 6160
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Figure 3. (a) Schematic illustration of experimental method for the reduction of p-nitrophenol with NaBH4. (b) Representative time dependence of the absorption of the p-nitrophenolate ions at 400 nm. The blue line displays the linear section, from which kapp was derived. The induction time (t0) is marked with the black arrow. These data were obtained using the NGN2-Au system as shown in Figure 4.
Figure 4. Time dependence of the absorption of the p-nitrophenolate ions at 400 nm during the reaction using the tested free and immobilized Au nanoparticles in each microsphere template. The photographs show each dispersion after the reaction. The blue lines represent the linearly fitted data for kapp.
thus effectively diminishing the surface area of the Au nanoparticles during the reaction and recovery. In contrast, the NGN2-Au hybrid microgels, where the longer pNIPAm shell was present on the surface of the NG core (Figures 1c and 2c), exhibited excellent reusability behavior (Figure 5a). At the first reaction, kapp was comparable to that of the tested free Au nanoparticles, as well as to those of the NG-Au and NGN1-Au microgels, indicating that the pNIPAm shell does not disturb the diffusion of p-nitrophenol. Therefore, in the NGN microgel systems in this study that have an open structure, p-nitrophenol can diffuse in the pNIPAm shell and reach the Au nanoparticles on the core more quickly. Moreover, the recovery and redispersion of NGN2-Au were easily accomplished and lead to adequate results for the second
reaction (Figure 4). The NG-Au hybrid microspheres also aggregated during the reaction. Therefore, it is desirable that the Au nanoparticles are not present on the surface of the carriers. For the NGN1-Au hybrid microgels, where the pNIPAm shell is presented on the surface of the core NG microspheres (Figures 1b and 2b), aggregation was not observed at the first reaction and the subsequent recovery of the hybrid microgels was successful. However, when the second reaction was conducted, kapp diminished to less than 50% under concomitant aggregation (Figures 4 and 5a). Indeed, after the first reaction, the size and size distribution of the Au nanoparticles localized on the core−shell NGN1 microgels increased from 2.7 ± 1.3 to 3.8 ± 2.1 nm (Figure 5b), which suggested that the Au nanoparticles were close to each other, 6161
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Figure 5. (a) Rate constant kapp for the tested hybrid microgels and free Au nanoparticles as a function of the reaction steps. The raw data for the reduction of p-nitrophenol and the photographs for the aggregation are shown in Figure S3. TEM images of the (b) NGN1-Au and (c) NGN2-Au hybrid microgels before and after the catalytic reaction. The average size and size distribution of the Au nanoparticles within the NGN1 and NGN2 core−shell microgel templates are also displayed (N = 300).
and third reaction, that is, kapp did not change significantly, and the aggregation behavior was not observed (Figure 5a). Furthermore, we were able to confirm the reuse of NGN2Au more than three times. Here, as evident from the TEM images observed after the third reaction, the average size and size distribution were close to that observed before the first reaction (Figure 5c). Therefore, the pNIPAm shell offers the benefits of the Au nanoparticles, that is, high dispersion stability and diffusion passage of the substrate. Note that NGN1-Au microgels aggregated at the second reaction as mentioned above (Figures 4 and 5), although the pNIPAm shell was also presented on the NG core. Indeed, Dh values of NGN1 and NGN2 microgels were decreased in the presence of pnitrophenol, where their concentration of 0.1 mM was used for the reaction (Figure S3). It suggested that the osmotic pressure in the gel shell was increased and the hydrophobic interaction occurred between pNIPAm and nitrophenol because the Dh values were further decreased with increasing concentration of nitrophenol near the room temperature. Thus, in the case of NGN1-Au microgels, the gel shell may not cover the Au nanoparticles completely during the catalytic reaction, and the NGN2-Au microgels have a sufficient gel thickness in the presence of reaction substrates. On the other hand, kapp of the control NGN2-Au(R) hybrid microgels, where the Au nanoparticles were randomly distributed over the entire NGN2 template (Figure 2d), was the highest for all tested hybrid microgels for the first reaction (Figure 5a), suggesting that when the Au nanoparticles are immobilized in the swollen gel shell, where the reactive substrates can diffuse and react with the Au nanoparticles in the gels similar to the free nanoparticles, NGN2-Au(R) hybrid
microgels exhibited high catalytic activity compared to those microgels that contained the Au nanoparticles on the surface of the core (NGN1-Au and NGN2-Au). However, the NGN2Au(R) hybrid microgels aggregated during the first reaction (Figure 4), indicating that they are not sufficient to fix the nanoparticles in the gels; indeed, they need to be located at the core surface and covered by a gel shell.
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CONCLUSIONS We applied hybrid core−shell microgels, wherein Au nanoparticles were localized only at the surface of the core, to the catalytic reduction of p-nitrophenol. The hydrogel shell layer can offer many advantages compared to conventional Au nanoparticles: (i) the reactive substrate, in this case pnitrophenol, can diffuse undisturbedly through the waterswollen gel shell to the Au nanoparticles and (ii) the gel shell prevents aggregations between hybrid microgels during the reaction, which is often observed for free or immobilized Au nanoparticles on the surface of whole carriers. We thus demonstrated design guidelines for nanoreactors with high reusability for applications in catalysis. Such hybrid microgel systems should be applicable to a variety of metallic nanoparticles and may thus lead to the development of new catalysts for applications that are aggregation sensitive.
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EXPERIMENTAL DETAILS
Materials. NIPAm (98%), N,N′-methylenebis(acrylamide) (BIS) (97%), GMA (95%), azobis(amidinopropane) dihydrochloride (V-50, 95%), and p-nitrophenol (Nit, 99%) were purchased from Wako Pure Chemical Industries (Japan) and 6162
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below. An aqueous dispersion (10 mL) containing the particles was poured into a glass vial (30 mL) at 25 °C. An aqueous solution (1 mL) containing NaBH4 (0.25 mg) was added dropwise, and the mixture was stirred for 1 h. Then, these hybrid microgels containing Au nanoparticles were purified by centrifugation/redispersion (water; RCF: 20 000g. The overall process for the synthesis of the hybrid microgels is illustrated in Figure S1. Characterization of the Hybrid Microgels. The hydrodynamic diameters of the tested microgels were measured by dynamic light scattering (Malvern Instruments Ltd., ZetasizerNanoS). The intensity autocorrelation was calculated for an average of 15 measurements with an acquisition time of 30 s. The concentrations of the tested microspheres for the measurements were ∼0.001 wt % in NaCl aqueous solution (1 mM). The samples were allowed to equilibrate thermally at the desired temperature for 10 min prior to the measurements. To calculate the hydrodynamic diameters, the measured diffusion coefficients were used in the Stokes−Einstein equation (Zetasizer software v6.12). Microsphere morphologies in the dried state were visualized by TEM (JEOL2010, operated at 200 kV). For TEM observations, the microsphere dispersions, which were purified by centrifugation, were dried on a carbon-coated copper grid (Okenshoji Co., Ltd., Japan). A quantitative determination of Au was carried out by ICPAES (SPS3100, SII NanoTechnology Inc., Tokyo, Japan). To prepare the calibration curve for Au, dilutions of Au standard solutions (Wako, 1000 μg/mL) were measured, which afforded linear calibration plots in the range of 0.1−10 μg/mL. The emission intensity was measured at 242.795 nm. For the ICPAES experiments, the tested hybrid microgels (concentrations ∼0.01 wt %), which were purified by centrifugation, were injected into the chamber. Characterization of the Catalytic Reaction. As a model reaction, we chose the reduction of p-nitrophenol to paminophenol using NaBH4.47,48 In a typical run, a specific amount of Au nanoparticles was added to a solution of pnitrophenol (0.1 mmol/L). After mixing these solutions, a specific amount of NaBH4 (10 mmol/L) was added to start the reduction. The kinetic process of the reduction was monitored by UV−vis spectroscopy (JASCO, V-630iRM) by measuring the extinction of the solution at 400 nm as a function of time. In this study, the total surface area (SAu) of Au nanoparticles localized on the tested microspheres was calculated using SAu = 6/Dρ, where D represents the average size of Au nanoparticles, which was obtained from TEM images, whereas ρ refers to the density of the Au nanoparticles (constant). Under the applied reaction conditions, the SAu values for the free or hybrid microgels were constant (0.5 m2). Thus, for catalytic reaction, the concentrations of hybrid microgels were 0.1 wt % (NGAu), 0.07 wt % (NGN1-Au), 0.085 wt % (NGN2-Au), and 0.11 wt % (NGN2-Au(R)). Prior to the addition of NaBH4, the solutions containing the Au nanoparticles were allowed to stabilize for a minimum of 10 min. Then, NaBH4 was added directly into the solution to start the reaction. For the characterization of the reusability of the Au nanoparticles, the reaction mixture was transferred into a centrifuge tube (SC0200, Ina-Optika Co., Ltd.). The solution was centrifuged (RCF: 20 000g) to pack the hybrid microgels at the bottom of the tube. The supernatant was removed from the tubes without disturbing the hybrid microgels at the bottom. Thereafter, the pellets of each hybrid microgel were redispersed in a fresh
used as received. AET (95%) was purchased from Tokyo Kasei Kogyo Co., Ltd. (Japan) and used as received. Chloroauric acid (HAuCl4, 99%) and sodium borohydride (NaBH4, 95%) were purchased from Junsei Chemical Co., Ltd. (Japan) and used as received. A gold nanoparticle suspension (5 nm; stabilizer: proprietary surfactant) as the control was purchased from Sigma-Aldrich (USA) and used as received. Water for all reactions, the preparation of solutions, and the purification of polymers was distilled and subsequently ion-exchanged (EYELA, SA-2100E1). Synthesis of NG and NG Core−pNIPAm Shell Microgels. The template NG microspheres, which are composed of NIPAm, GMA, and BIS, were copolymerized in water using water-soluble V-50 as an initiator (Table 1) according to a previously reported study.19,20 Polymerizations were carried out in a three-necked round-bottom flask (200 mL) equipped with a mechanical stirrer, condenser, and nitrogen gas inlet. A mixture of NIPAm (0.2 g), BIS (0.04 g), and GMA (2.2 g) was dissolved in water (95 mL) and heated to 70 °C under constant stirring (250 rpm) and a stream of nitrogen. After 30 min to stabilize the solution temperature, V-50 (0.02 g) dissolved in water (5 mL) was injected to the flask to initiate the polymerization. The solutions were stirred for 6 h, and after the completion of the polymerization, the NG microsphere dispersion was cooled to room temperature. The microspheres were purified via two cycles of centrifugation/redispersion in water using a relative centrifugal force (RCF) of 50 000 g. For the NG core−pNIPAm shell microgels, seeded precipitation polymerizations were conducted in the presence of NG microspheres. In this study, two types of core−shell microgels with different shell thicknesses were synthesized by tuning the shell monomer concentrations (25 and 40 mM). Mixtures of NG microspheres (0.5 g), NIPAm (0.28 g/25 mM or 0.38 g/40 mM), and BIS (0.006 g/25 mM or 0.01 g/40 mM) were deposited in a three-necked round-bottom flask (200 mL) equipped with a mechanical stirrer, condenser, and nitrogen gas inlet. The solutions were heated to 70 °C under a stream of nitrogen and constant stirring (250 rpm) and allowed to stabilize for a period of at least 30 min prior to initiation. An aqueous solution (5 g) containing V-50 (0.02 g) was added to the flask to initiate the polymerization, which was then continued for 2 h. The obtained microgels (NGN1 and NGN2) were purified via four cycles of centrifugation/ redispersion in water. For the modification of the epoxy groups of the inner NG or NGN microgels, 2-AET was added to introduce amino groups to the tested microgels. A mixture of NG or NGN microgels (0.4 g), 2-AET (0.78 g), and water (70 g) was deposited in a glass vial (100 mL) and stirred for 24 h at room temperature and pH = 11. The thus obtained microgels were purified by centrifugation using the aforementioned method, followed by 1 week of dialysis. Synthesis of the Hybrid Microgels. The synthesis of Au nanoparticles within the prepared microgels is described in detail elsewhere.19,20 A mixture of template particles (10 mg) and HAuCl4 (1.0 mg) was stirred for 24 h in an aqueous medium (10 mL; pH = 2). Afterward, any excess of HAuCl4 was removed by centrifugation, decantation, and washing with water (pH = 3) to prevent the formation of extraneous Au nanoparticles on the outside of the microgels. For a random distribution of Au nanoparticles over the entire NGN2 microgels, the washing process was not executed and the subsequent reduction of HAuCl4 was conducted as described 6163
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ACS Omega aqueous solution at 25 °C, and the dispersions were subsequently mixed for 1 h using a thermomixer (Thermomixer R, Eppendorf) at 25 °C. The redispersed hybrid microgels were also used for catalysis. The overall process is illustrated in Figure 3a.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00819. Information on the scheme for the microgel preparation; TEM images of amino-functionalized microgel templates; and thermoresponsive behavior of the microgels in the presence of nitrophenol (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (D.S.). ORCID
Takuma Kureha: 0000-0003-4680-6440 Daisuke Suzuki: 0000-0003-0444-156X Present Address
Shinshu University, 3-15-1, Tokida Ueda 386-8567, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We kindly acknowledge Emeritus Prof. Haruma Kawaguchi (Keio University) for permission to continue the research of metallic nanoparticle composite microgels. For financial support, D.S. acknowledges a Grant-In-Aid for Scientific Research on Innovative Areas (16H00760) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). T.K. acknowledges a Grant-In-Aid for JSPS Research Fellows (15J11533).
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REFERENCES
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