J. Phys. Chem. C 2007, 111, 5667-5672
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Colloidal Crystals of Thermosensitive, Core/Shell Hybrid Microgels Daisuke Suzuki,†,‡ Jonathan G. McGrath,‡ Haruma Kawaguchi,† and L. Andrew Lyon*,‡ Faculty of Science & Technology, Keio UniVersity, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan, and School of Chemistry and Biochemistry & Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: December 12, 2006
We present the assembly of colloidal crystals composed of various thermoresponsive core/shell microgels and their hybrid microgel counterparts that contain localized Au nanoparticles. To obtain nanostructured microgels, we conducted a three-stage polymerization. First, core microgels composed of cross-linked poly(N-isopropylacrylamide) (pNIPAm) were synthesized by precipitation polymerization. Using these microgels as cores or “seeds”, a copolymer shell of pNIPAm was added to these core particles using a functional comonomer. Finally, a second shell consisting solely of cross-linked pNIPAm was added onto the preformed core/shell microgels by using this seeded polymerization technique. With use of these core/shell/shell microgels as templates, Au nanoparticles were synthesized in situ, using the cationic sites in the inner shell to nucleate particle growth. To control the optical properties of the Au nanoparticles, electroless Au plating was carried out with use of the preformed Au nanoparticles in the microgel as seeds. These hybrid microgels retained their thermoresponsive properties and possessed a high degree of size uniformity as confirmed by dynamic light scattering and were assembled into three-dimensional colloidal crystals by using thermal annealing processes. The resulting colloidal crystals were characterized by optical microscopy and reflectance spectroscopy. This synthetic design for producing distinctive, functional building blocks could be expanded to enable the creation of complex tunable optical materials containing refractive index periodicity on multiple length scales.
Introduction Colloidal crystalline assemblies have been of considerable interest due to the brilliant optical properties that evolve as the refractive index periodicity approaches the wavelength of visible light.1 These materials are investigated not only for the purpose of creating materials that possess unique properties such as omnidirectional photonic band gaps,2-5 but also for understanding the fundamental condensed matter and optical physics of such structures.6-10 The building blocks used to assemble colloidal crystals are diverse, and range from hard spheres such as polystyrene and silica particles to soft polymer microgels. Our group11-17 and others18-21 have explored the synthesis and assembly of such microgels. In many cases, the microgels used in these studies have been composed of poly(N-isopropylacrylamide) (pNIPAm), which is a typical thermosensitive polymer having a phase transition temperature around 31 °C.22 Microgels such as these were first synthesized by Pelton and Chibante,23,24 and their tunability and synthetic accessibility have made them attractive as colloidal building blocks. For example, in contrast to the techniques used in the assembly of hard spheres, such as slow sedimentation,25 gentle solvent evaporation,26 or filtration,27 soft pNIPAm microgels can be assembled into three-dimensional ordered structures via their volume phase transition. The optical properties of the obtained soft colloidal crystals can be tuned by varying the center-to-center distance of the microgel components. This objective can be achieved by simply choosing microgels of different sizes, by controlling their concentration, * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Keio University. ‡ Georgia Institute of Technology.
or by changing the particle size through thermal deswelling. We have also demonstrated that microgels can be synthesized to contain more complex nanostructures. Core/shell structured microgels have been synthesized to exhibit multiple responsiveness28 and core- or shell-specific functionalities.29,30 Another example of a complex nanostructured microgel is the hybrid microgel. In this case, pNIPAm-based microgels having functional groups inside the particle can act as reactors for the synthesis of inorganic nanoparticles.31-38 The size of inorganic nanoparticles inside the microgel can also be tuned by changing microgel compositions. These kinds of hybrid microgels are attractive candidates as components of functional materials because they combine the “softness” of microgels (i.e., stimuli responsiveness, etc.)39 with the optical, electronic, and magnetic properties of inorganic nanoparticles,40 while preserving their structural integrity due to the inherent stability of each. We have reported on hybrid microgels with Au nanoparticles synthesized in situ using several structured microgels as templates that exhibit tunable properties. One such hybrid microgel exhibits color changes by using the thermosensitive property of the microgels.31,33 Another template enabled the fabrication of a hybrid microgel in which a stable Au nanoshell was formed in situ at the surface of a rigid core.32 We have also found that such inorganic nanoparticles located within the microgel can be modified and grown because they are stabilized by the microgel network.32,33 In this paper, we present the assembly of colloidal crystals composed of various core/shell microgels and their hybrid microgel counterparts that contain localized Au nanoparticles. Our main purpose in this study is to demonstrate that complex, nanostructured microgels that are the product of multiple
10.1021/jp068535n CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007
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SCHEME 1: Synthesis of Hierarchal Hybrid Microgels and Their Crystals
synthetic steps can be crystallized into highly ordered 3D crystals. Notably, these microgels can be assembled as such without fractionation or purification aimed at narrowing the size polydispersity. All particles are obtained with a high degree of size uniformity and can be crystallized as prepared. To obtain nanostructured microgels, we conducted a three-stage polymerization. First, core microgels composed of cross-linked pNIPAm were synthesized by free-radical precipitation polymerization. By using these microgels as cores or “seeds”, a shell of pNIPAm copolymerized with a cationic comonomer was added by seeded polymerization, which provided a functional site for Au nanoparticle nucleation. Finally, another pNIPAm shell was formed onto the preformed core/shell microgels. This second shell was added to maintain colloidal stability and prevent aggregation during Au nanoparticle synthesis and crystallization. With use of these hierarchal core/shell/shell microgels as templates, Au nanoparticles were synthesized in situ, using the cationic sites in the inner shell to nucleate particle growth. To control the optical properties of the Au nanoparticles, electroless Au plating was carried out with the preformed Au nanoparticles in the microgel as seeds. Each type of microgel was assembled into three-dimensional colloidal crystals by centrifugation and thermal annealing, as described previously.11-16 The colloidal crystals that were obtained were characterized by optical microscopy and reflectance spectroscopy. These results suggest that this approach could be expanded to enable the creation of complex tunable optical materials containing refractive index periodicity on multiple length scales. Experimental Section Materials. All reagents were purchased from Sigma-Aldrich unless noted otherwise. The monomer N-isopropylacrylamide (NIPAm) was recrystallized from hexanes and dried in vacuo prior to use. The cross-linker N,N′-methylenbisacrylamide (BIS), the cationic comonomer N-(3-aminopropyl)methacrylamide hydrochloride (APMA), the surfactant dodecyltrimethyl ammonium bromide (DTAB), azobis-amidinopropane dihydrochloride (V-50), hydrogen tetrachloroaurate(III) trihydrate, sodium borohydride (NaBH4), and hydroxylamine hydrochloride (NH2OH‚HCl) were all used as received. Water for all reactions, solution preparation, and polymer purification was first distilled
then deionized to a resistance of 18 MΩ (Barnstead E-Pure system) and finally filtered through a 0.2 µm filter to remove particulate matter. Microgel Synthesis. Core/shell/shell type microgels were synthesized by aqueous free radical precipitation polymerization (Scheme 1). For a core synthesis, a mixture of NIPAm (1.553 g), BIS (0.043 g), DTAB (0.062 g), and water (199 mL) was poured into a 500-mL three-neck, round-bottom flask equipped with a magnetic stir-bar, a condenser, thermometer, and nitrogen gas inlet. Under a stream of nitrogen to purge oxygen and with constant stirring, the solution was heated to 70 °C. After stabilizing the solution for 1 h, V-50 initiator (0.054 g) dissolved in 1 mL of water was added to the flask for initiation of polymerization, which then continued for 4 h. After polymerization, the dispersion was cooled to room temperature and used without purification for the shell synthesis. For the first shell synthesis, a core dispersion (80 mL) and water (109 mL) were placed in a 500-mL three-neck, roundbottom flask equipped with a magnetic stir-bar, a condenser, thermometer, and nitrogen gas inlet. Under a stream of nitrogen to purge oxygen and with constant stirring, the solution was heated to 70 °C. After 55 min, a 10 mL solution of NIPAm (0.096 g), APMA (0.009 g), and BIS (0.016 g) was added to the flask. After 5 min, polymerization was initiated by injecting 1 mL of V-50 (0.055 g) solution. The reaction was kept at 70 °C for 4 h. The obtained core/shell microgel was purified by centrifugation/resuspension with water four times, using a relative centrifugal force (RCF) of 15 422 × g for approximately 90 min. For the second shell synthesis, a mixture of the core/shell microgel (0.2 g, which is determined by freeze-drying), NIPAm (0.222 g), BIS (0.006 g), DTAB (0.031 g) and water (50 mL) was poured into a 100-mL three-neck, round-bottom flask equipped with a magnetic stir-bar, a condenser, thermometer, and nitrogen gas inlet. Under a stream of nitrogen to purge oxygen and with constant stirring, the solution was heated to 70 °C. After stabilizing the solution for 1 h, V-50 initiator (0.014 g) dissolved in 1 mL of water was added to the flask for initiation of polymerization, which then continued for 4 h. The obtained core/shell/shell microgels were purified by the same centrifugation method described above.
Colloidal Crystals of Hybrid Microgels Hybrid Microgel Synthesis. In situ synthesis of Au nanoparticle seeds inside the core/shell/shell microgel was carried out as described in our previous report (Scheme 1).31-33 The core/shell/shell microgel (100 mg, which is determined by freeze-drying) and 1 wt % of HAuCl4 solution (0.5 mL) was stirred in 30 mL of aqueous medium at room temperature for 4 h. Afterward, excess HAuCl4 was removed by centrifugation of the particles, decantation, and washing with aqueous medium (pH 3, HCl) to prevent extraneous Au nanoparticle formation outside of the microgel. A total of 15 mL of water (pH 3, HCl) containing the microgels was poured into a 30-mL glass vial at 4 °C. NaBH4 (1.0 mg) was dissolved in 1 mL of water and added dropwise to the vial. After the addition of NaBH4, the mixture was stirred for 30 min. These hybrid microgels with Au seeds (core/Au@shell/shell microgels) were then purified by four cycles of centrifugation (20 800 × g for 15 min), decantation, and resuspension with water. Next, Au nanoparticle growth from Au nanoparticle seeds in the microgel was carried out by electroless plating, following a similar procedure as described previously.32,33 A 30 mL aqueous solution containing the hybrid microgel (50 mg) was stirred at 4 °C. After 10 min of stirring, a mixture of HAuCl4 solution (1 wt %, 100 µL) and NH2OH‚HCl solution (40 mM, 500 µL) was added into the vial. The reaction continued for 15 min, after which the hybrid microgels (core/Au+Au@shell/shell microgels) were purified by four cycles of centrifugation (20 800 × g for 5 min), decantation, and resuspension with water. Colloidal Crystal Assembly. Colloidal crystals were fabricated by using the nonhybrid and hybrid microgels described above. A 1.5 mL sample of the microgel dispersions was placed into a centrifuge tube and centrifuged at 25 °C at a relative centrifugal force of 16 100 × g using different amounts of time, since introduction of Au increases the mass of the particles (30 min for the core, core/shell, core/shell/shell microgels; 15 min for the core/Au@shell/shell microgels; 7 min for the core/ Au+Au@shell/shell microgels). The clear, colorless supernatant solutions were removed to isolate the microgel pellet. The resulting particle concentrations yielded polymer volume fractions consistent with those used for colloidal crystalline assemblies of pNIPAm particles.11-14 After agitating the microgel pellet to homogenize the slurry, the condensed dispersions were transferred into Vitrotube borosilicate rectangular capillaries (0.1 mm × 2.0 mm) by capillary action. The samples were heated and kept at 34 °C for 10 min, after which the samples were cooled to room temperature at a rate of approximately 0.2 deg/ min to obtain bulk crystallization. Characterization. Microgel sizes and polydispersities were determined by dynamic light scattering (DLS, Protein Solutions Inc.), as previously reported.12,28 Diluted microgel dispersions were analyzed in a three-sided quartz cuvette. The samples were allowed to equilibrate at the desired temperature for 10 min before data collection. Scattered light was collected at 90° by a single-mode optical fiber coupled to an avalanche photodiode detector. Data were analyzed with Dynamics Software Version 5.25.44 (Protein Solutions, Inc.). Absorption spectra of the microgel dispersions were collected on a Shimadzu UV-1601 spectrophotometer. For analysis of the hybrid microgels, the particles were dried on a carbon-coated copper grid (Okenshoji Co., Ltd.) and observed by field emission transmission electron microscopy (TEM; TECNAI F20, Philips Electron Optics Co., operated at 200 kV). Differential interference contrast (DIC) images were taken with an Olympus IX-70 inverted microscope, using standard DIC optics and a high numerical aperture objective (100×, N.A. ) 1.3). Reflectance spectra of crystal
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5669 samples were collected with a fiber optic spectrophotometer (Ocean Optics, Inc., USB2000 with integrated OFLV-3 detector), using OOIBase32 software. The reflectance probe (R4007-VIS/NIR) possessed bifurcated source and detector optical fibers such that it could be positioned at near-normal incidence above the sample. A dark spectrum was obtained by covering the reflectance probe with the probe cap, and a 100% reflectance reference was obtained by using a diffuse reflectance standard. Results and Discussion Preparation and Characterization of Core/Shell/Shell Hybrid Microgels. Cross-linked pNIPAm microgels prepared by free radical precipitation polymerization undergo a temperature-induced volume change around 32 °C, just slightly higher than the intrinsic lower critical solution temperature (LCST) of pNIPAm polymer (30.8 °C).22 Figure 1 shows the temperature dependence of the hydrodynamic diameter for all microgels synthesized in this study. As shown in Figure 1a, comparing the core and core/shell/shell microgels, we observe increases in the size both below and above the LCST. This indicates that material has indeed been added to the core microgels. However, the core/shell microgel diameter almost exactly corresponds to the core diameter. Only a small increase in particle size is expected since a very low monomer concentration was used in the polymerization of the first shell (5 mM). However, complexities in the comparison of these particle sizes can arise due to the introduction of the APMA functional groups, since the volume phase transition behavior may be dependent on the addition of comonomers, cross-linker type, and concentration.41,42 On the other hand, the addition of another hydrogel layer onto the preformed core microgel compresses core swelling.43-45 As a result, this correspondence is probably due to core compression after shell addition. Although the existence of this shell is not clearly discernible by direct comparison of the core and core/shell particle sizes, evidence of this functional shell can be observed with DLS, using buffered media of pH 6 or 10 (ionic strength: 0.001 M) to disperse the core/shell or core/shell/shell microgels (Figure 1b,c). Since these core/shell microgels contain amine groups in the first shell, the volume phase transition demonstrates noticeable pH dependence. At pH 10, the diameter of core/shell microgels below the volume phase transition temperature (VPTT) is smaller than that observed at pH 6 (Figure 1b). This arises from APMA deprotonation at pH 10 and concomitant Coulombic deswelling of the network. Even after another pNIPAm shell is added, the resultant core/shell/ shell microgel shows the same behavior (Figure 1c). Further evidence of first shell addition is that with increasing monomer concentration in the shell synthesis, the hydrodynamic diameter becomes larger over the entire temperature range from 25 to 45 °C (see the Supporting Information). These data suggest that the core/shell/shell-structured microgel containing cationic species in the inner shell can be synthesized successfully. Most importantly for this study, each dispersion of structured microgels was determined to be monodisperse, thereby allowing for assembly into colloidal crystals (calculated polydispersities from DLS files were between 10% and 20% for all batches of microgels at all temperatures). Next, Au nanoparticles were synthesized in situ by using the core/shell/shell microgels as templates. Our previous work has shown that Au nanoparticles can be formed where functional groups for binding the AuCl4- ion are localized.31-33 In this study, after synthesizing Au nanoparticles inside the microgels, the obtained (core/Au@shell/shell) microgels did not show the strong surface plasmon absorption that is inherent to Au
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Figure 1. Deswelling curves for all types of microgels used in this study, as measured by dynamic light scattering: (A) core (open circle), core/shell (open square), and core/shell/shell (open triangle) microgels at pH 6; (B) core/shell microgels at both pH 6 (open square) and 10 (solid square); (C) core/shell/shell microgels at both pH 6 (open triangle) and 10 (solid triangle); and (D) core/shell/shell (black triangle), core/Au@shell/ shell (red triangle), and core/Au+Au@shell/shell (blue triangle) microgels at pH 6. Error bars represent one standard deviation about the average value of five measurements.
Figure 2. UV-visible absorption spectra for core/Au@shell/shell (dashed line) and core/Au+Au@shell/shell (solid line) microgels in deionized water at room temperature.
Figure 3. TEM images of the core/Au+Au@shell/shell microgels.
nanoparticles, as determined by UV-vis spectroscopy (Figure 2). However, the color of the dispersion became very slightly pink after this initial Au synthesis. These results indicate that the synthesized Au nanoparticles were so small (
