Article pubs.acs.org/Langmuir
Multipetal-Structured and Dumbbell-Structured Gold−Polymer Composite Particles with Self-Modulated Catalytic Activity Mingmeng Zhang,† Todd P. Otanicar,‡ Patrick E. Phelan,† and Lenore L. Dai*,† †
School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85281, United States Department of Mechanical Engineering, University of Tulsa, Tulsa, Oklahoma 74104, United States
‡
S Supporting Information *
ABSTRACT: A simple synthesis route for gold−polymer composite particles with controlled structure (multipetal structure and dumbbell structure) is developed. It is intriguing to observe that by controlling the reaction time and size of gold nanoparticles (AuNPs), tetrapetal-, tripetal-, and dumbbell-structured gold−polystyrene composite are obtained via seeded polymerization. The average number of petals on a single AuNP increases with the AuNP diameter. These particles show potential applications as building blocks for advanced ordered and hierarchical supracolloidal materials. Further, with the incorporation of poly(N-isopropylacrylamide) (PNIPAm), “smart” thermoresponsive dumbbell-structured gold−PNIPAm/polystyrene composite particles are formed. Significant size variation is validated for particles with 83 and 91 wt % PNIPAm content around lower critical solution temperature (LCST), which results in self-modulated catalytic activity. photonic bandgap, as predicted 20 years ago.2 In addition, particles with complex structure (e.g., multipetal structure) provide high surface roughness and large specific surface area. These unique properties might lead to potential applications such as superhydrophobic and superhydrophilic surfaces.3,4 For dumbbell-structured composite particles, applications such as photonic band gap materials, nanopatterns by colloidal lithography, and catalyst supports has been reported.5,6 To date, reports for synthesis of multipetal-structured composite particles are quite rare. Ravaine and co-workers successfully synthesize silica-core/polystyrene-petal multipetal composite particles via seeded-dispersion polymerization.7,8 The morphology is found to be controlled by adjusting the density of polymerizable groups on silica surfaces, and the number of polystyrene petals on a single silica core increases with reaction time. For multipetal-structured particles composed by organic molecules only, Pine and co-workers apply a four-step route (formation of polystyrene clusters, polystyrene clusters with amidine patches, polystyrene clusters with biotin patches, polystyrene clusters with DNA patches) to assemble polystyrene microclusters functionalized with DNA patches.1 The complementary DNA patches show high affinity to each other, allowing particles to self-assemble into supracolloidal particles with specific directional bonding, which imitates hybridized atomic orbitals.1 To the best of our knowledge, synthesis of organic−inorganic hybrid multipetal-structured composite particles has so far been limited to the utilization of
1. INTRODUCTION Nanoscale particles with complex structures have drawn an increasing interest in recent years. Particles can be synthesized with different structures (e.g., dumbbell structure, multipetal structure, etc., as shown in Scheme 1), composition, and Scheme 1. Illustration of Composite Particles with Dumbbell and Multipetal Structures
functionality, enabling them to serve as building blocks for advanced ordered and hierarchical materials. For instance, Pine’s group successfully design and assemble polystyrene microclusters functionalized with DNA patches.1 These colloidal particles possess a variety of three-dimensional structures with specific directional bondings, imitating hybridized atomic orbitals (including sp, sp2, sp3 sp3d, sp3d2, and sp3d3). The success suggests the possibility of assembling micron-sized particles in a way which mimics molecular structures. These particles with three-dimensional structure could serve as potential building blocks for a diamond lattice (fcc dielectric structure), which has a three-dimensional © XXXX American Chemical Society
Received: June 25, 2015 Revised: October 6, 2015
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DOI: 10.1021/acs.langmuir.5b02333 Langmuir XXXX, XXX, XXX−XXX
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water (HPLC grade, Acros Organics) are utilized in the polymerization process without further purification. 2.2. Particle Synthesis. Typically, a AuNP dispersion (0.03−0.1 g), 15 mL of water, styrene (0.05, 0.1, 0.2, 0.3 g), and NIPAAm (0.5, 1 g) are sonicated by a VCX 500 ultrasound sonicator to form the emulsion. The emulsion is then immediately transferred to a threeneck 15 mL flask and degassed with nitrogen for 15 min. After the solution is heated to 65 °C, 0.035 g of VA-086 initiator in 1.1 mL of water solution is injected. 1−2 mL samples are taken out during the reaction at 1, 2, and 3 h. The reaction lasts for 4 h. A vacuum oven is used for product yield measurements. For measuring the yield of dumbbell-structured gold polystyrene composite particles, we apply DLS analysis to obtain the number percentage of dumbbell-structured particles for yield calculation. The yield and conversion measurements are under the reaction condition of 1.2 wt % initial styrene concentration and 0.0104 wt % initial AuNP concentration. Before characterization, the synthesized sample is washed with water by three centrifugation−redispersion cycles to remove unreacted monomers/ oligomers. 2.3. Characterization Techniques. Particle size distributions of the gold−PNIPAm/polystyrene composite particles are obtained by NICOMP 380 ZLS using the dynamic light scattering (DLS) technique. The transmission electron microscopy (TEM) images are obtained via an environmental TEM Tecnai F20. The TEM specimen is prepared by placing one droplet of the solution onto the TEM grids then air-drying the specimen. The energy-dispersive X-ray spectroscopy (EDX) spectra are obtained by utilizing a Philips CM-200 FEG TEM’s adjunct X-ray detector. The UV−vis extinction properties of the sample are analyzed via a Cary 300 Bio UV−vis spectrophotometer. The temperature control function is utilized to study the temperature transition property of NIPAAm-incorporated particles. Extinction intensity is taken from a Cary 300 Bio UV−vis spectrophotometer under a laser wavelength of 639 nm. Scattering intensity data are taken from the NICOMP 380 DLS machine with laser wavelength of 639 nm. 2.4. Catalytic Study of Gold−PNIPAm/Polystyrene Composite Particles. Typically, 5 mL of Rhodamine B (RhB) solution (4 × 10−5 mol/L) is mixed with 2 × 10−3 g of NaBH4. The dumbbellstructured gold−PNIPAm/polystyrene composite particles are added as the catalyst. UV−vis analysis is employed to perform a time study on the reaction. The cuvette filled with freshly prepared solution of NaBH4 and RhB is immediately put into spectrometer cell after the addition of the particle catalysts. The reaction time is counted from the addition of particle catalysts to the point where the RhB peak at 552 nm disappears completely. The temperature control function of UV− vis spectrometer is utilized for a range of temperature from 16 to 51 °C.
silica nanoparticle as the core. For dumbbell-structured composite particles, several synthesis techniques have been developed, such like seeded emulsion polymerization,9,10 surface templating,11 and microfluid techniques.12 Currently, synthesis methods capable of controlling the particle structure from multipetal structure to dumbbell structure are seldom reported. During the process of our work, Wang et al. reported a simultaneous synthesis of multipetal- and dumbbellstructured composite particles, where particles containing polytrimethoxysilane−styrene derivatives are synthesized via a combination of hydrolytic condensation process with radiation seeded emulsion polymerization.13 Utilization of gold nanoparticles (AuNPs) as cores to synthesize multipetal-structured composite particles has not been reported. AuNPs are well-known for their catalytic properties14 and superior surface plasmon resonance (SPR) effect.15 Utilization of AuNPs for synthesis of multipetal and dumbbell composite particles is anticipated to provide advanced features for catalytic and optical applications. Here we demonstrate a one-step seeded polymerization route for synthesis of gold−polymer composite particles, with controlled structure from multipetal structure to dumbbell structure. The synergetic combination of “smart” polymers and inorganic particles has been drawing increasing research interest recently. In response to temperature16−21 or pH,22−24 the stimuli-responsive polymers are capable to smartly modulate inorganic particles’ optical,16−19 catalytic,20,21 and transport properties.22,23 In particular, the thermoresponsiveness of poly(N-isopropylacrylamide) (PNIPAm) enables the polymer a sharp phase transition around lower critical solution temperature (LCST).25 AuNPs demonstrate superior catalytic properties toward homogeneous reaction and heterogeneous reaction,14,26 since the discovery by Haruta et al. and Hutching et al.27,28 The possibility of self-controlling the catalytic properties of AuNPs is promising. Currently, Wu et al. report that tunable catalytic activities are exhibited by York-shell structured gold−PNIPAm composite particles.29 These particles are synthesized via a three-step method utilizing Au−silica core−shell particles, in which the silica layer is removed to form a hollow PNIPAm microsphere encapsulating a movable AuNP core. Liz-Marźan’s group demonstrates modulatable catalytic activities of AuNPs encapsulated PNIPAm microgel.30 These particles are synthesized by surface grafting of PNIPAm on vinyl-functionalized AuNPs. Lu et al. report that polystyrene− PNIPAm−gold microgel composite particles are successfully synthesized with tunable catalytic activities, where AuNPs are immobilized into PNIPAm microgel networks via complexation of Au ions by the nitrogen atoms of PNIPAm.31,32 Different from their particle structures and synthesis routes, here we demonstrate a one-step seeded polymerization route for synthesis of dumbbell-structured gold-centered gold−PNIPAm/polystyrene composite particles. The self-modulated catalytic activity properties are experimentally demonstrated.
3. RESULTS AND DISCUSSION 3.1. Multipetal- and Dumbbell-Structured Gold− Polystyrene Composite Particles. Multipetal- and dumbbell-structured gold−polystyrene composite particles are successfully synthesized via seeded polymerization. Figure 1 shows the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the sample synthesized particles with 125 nm AuNPs after 1 h and with 80 nm AuNPs after 1 and 4 h reaction time, respectively. In Figures 1a and 1b, each AuNP center is surrounded with 3−5 polystyrene “petals” in nonspecific direction, showing a flowerlike structure. Similar results are shown in Figures 1c and 1d. Close observation shows most petals of multipetal-structured particles are distributed in three dimensions. The TEM and SEM images in Figures 1e and 1f show that the synthesized particles have dumbbell structure. It is intriguing to find that for most of the dumbbell-structured particles the two polystyrene petals are aligned with a near-perfect 180° angle. The observed particle sizes of multipetal- and dumbbell-structured composite
2. EXPERIMENTAL SECTION 2.1. Materials. The organic spherical AuNPs are from Nanopartz Inc. with the following specifications: spherical AuNPs, 80 and 125 nm, and 3.0 wt % in isopropanol with a capping agent Nsol (alkyl acrylate polymer, 1−2 nm). The SPR peaks are 550 and 580 nm for 80 and 125 nm AuNPs, respectively. Other materials inluce styrene monomer (99.9%, Fisher), N-isopropylacrylamide monomer (NIPAAm, 97%, Aldrich), nonionic initiator VA-086 (98%, 2,2-azobis(2methyl-N-(2-hydroxyethyl)propionamide), Wako Chemicals), and B
DOI: 10.1021/acs.langmuir.5b02333 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Multipetal- and dumbbell-structured gold−polystyrene composite particles. (a, b) Particles with tetrapetal structure. (particles synthesized with 125 nm AuNPs, 1 h reaction time). (c, d) Tripetal-structured particles (particles synthesized with 80 nm AuNPs, 1 h reaction time). (e, f) Dumbbell-structured particles. (particles synthesized with 80 nm AuNPs, 4 h reaction time).
monomers/oligomers are growing on the surface of AuNP surface (Figure 2a−c). As shown by the TEM images of the 15 min sample, three small petals are observed on AuNP surface. As time increases to 45 min, polystyrene petals grow bigger and a spherical shape is formed. This phenomenon is consistent with what we find in our previous study about the formation mechanism of asymmetric gold−polystyrene particles.33,34 Therefore, the seeded growth formation mechanism is proposed. It is possible that during the seeded-growth formation styrene monomers/oligomers dispersed in water phase are able to provide π electrons from aromatic rings to interact with AuNPs in water phase.33,35,36 The seeded growth formation mechanism is further confirmed by Figures 2d and 2e, which demonstrate the influence of AuNP concentration on the size of composite particles. From Figures 2d and 2e, it is observed that for both multipetal (1 h reaction time) and dumbbell (4 h reaction time, discussed in the next paragraph) samples particle size decreases with the increase of AuNP concentration under the same styrene concentration. This indicates that for each polystyrene petal a decreased amount of styrene monomers is participating on its growth when increasing amounts of AuNPs are applied. This decreasing trend of particle size influenced by increasing amount of AuNPs
particles from TEM images match well with dynamic light scattering (DLS) results (362, 324, and 563 nm for the tetrapetal-, tripetal-, and dumbbell-structured composite particles, respectively). For multipetal composite particles (1 h reaction time), the yields are about 21% conversion at 1 h reaction when using both 80 nm AuNPs and 125 nm. For dumbbell composite particles, the yields are 83% (for 80 nm AuNPs) and 89% (for 125 nm AuNPs) at 4 h reaction time. To the best of our knowledge, utilization of AuNP for synthesis of multipetal-structured gold−polystyrene composite particles is first reported here. Previously, utilization of silica nanoparticle for synthesis of multipetal-structured silica−polystyrene composite particles are reported by Reculusa et al., where 170 nm silica nanoparticles serve as seeds for seeded dispersion polymerization.7 The multipetal- and dumbbell-structured gold−polystyrene composite particles could potentially serve as building blocks for advanced hierarchical and supracolloidal materials. 3.2. Formation Mechanisms of Multipetal- and Dumbbell-Structured Gold−Polystyrene Composite Particles. In order to understand the formation mechanisms, we perform a time study of the reaction products, sampling at 15 min, 45 min, and 1 h during the early stage of reaction. It is found as the polymerization reaction starts, polystyrene C
DOI: 10.1021/acs.langmuir.5b02333 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. Seeded-growth formation of multipetal-structured gold-polystyrene composite particles. (a−c) TEM images of gold−polystyrene composite particles sampling at the very early stage (15 min, 45 min, and 1 h reaction time) and AuNP size of 80 nm. (d, e) Influence of AuNP concentration on the size of gold−polystyrene composite particles for 80 nm AuNPs and 125 nm AuNPs, respectively. The columns of dark cyan color and light cyan color represent samples of 4 and 1 h reaction time, respectively. Styrene concentration is kept at 1.2 wt %.
further confirms that AuNPs serve as the seeds for the growth of styrene monomers/oligomers. The structure of the gold−polystyrene composite particles is found to be controlled by reaction time and AuNP size. Scheme 2 illustrates the proposed formation pathway of multipetal- and dumbbell-structured gold−polystyrene composite particles. For particles synthesized with 125 nm AuNPs, the structure of particles gradually changes from tetrapetal structure to dumbbell for 1, 2, and 4 h samples (Figure 3a−c). The average number of polystyrene petals on each AuNP center gradually decreases from 3.9 to 2.8 to 1.9 for the 1, 2, and 4 h samples, as shown in statistical histograms along with the figures of samples for each reaction time. For the 4 h samples, we find nearly all the composite particles are dumbbell-structured. In addition, we observe many bare polystyrene particles with the same size of polystyrene petals. DLS analysis shows the number-weighted percentage of these bare polystyrene particles increases with reaction time (see Supporting Information Figure S1). About 10% and 45% of the particles are bare polystyrene particles for samples at 1 and 4 h reaction time, respectively. It is reasonable to propose that the structure change is due to the fact that as the reaction proceeds, the growing of polystyrene petals gives more steric hindrance; this causes the polystyrene petals to drop off the AuNP center, and eventually, the dumbbellstructured particles are formed with an 180° angle of petal distribution for the most kinetic preference. The same trend is shown for gold−polystyrene composite particle system utilizing 80 nm AuNPs (Figure 3g−i). From the statistical histograms (Figures 3d and 3j), we can tell at the early stage of the reaction, as the diameter of AuNP increases from 80 to 125 nm, the number of polystyrene petals on each AuNP increases (from tripetal structure to tetrapetal structure). This is because
AuNPs with larger size provide more sites for seeded-growing polystyrene petals and reduce the possibility of petals’ dropping caused by steric hindrance. For the 1 h sample of 125 nm AuNP system, we even observe a small amount (
