Article pubs.acs.org/Langmuir
One-Step Interfacial Thiol−Ene Photopolymerization for Metal Nanoparticle-Decorated Microcapsules (MNP@MCs) Dandan Liu, Xuesong Jiang,* and Jie Yin School of Chemistry and Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: We herein reported a one-step strategy to prepare the noble metal nanoparticle-decorated microcapsules (MNP@MCs) through the interfacial thiol−ene photopolymerization. In the presence of amphiphlic polyhedral oligomeric silsesquioxane (POSS) containing thiol groups (PTPS) as a reactive surfactant and trimethylolpropane triacrylate (TMPTA) as a cross-linker, the oil phase of toluene dissolved with a photoinitiator was emulsified into a water phase containing a metal precursor to form an oil-in-water (O/W) emulsion. Upon irradiation of ultraviolet (UV) light, the thiol−ene photoploymerization and photoreduction at the interface of toluene/water lead to the formation of the cross-linked wall and metal nanoparticles, respectively. A series of gold, silver, and platinum nanoparticledecorated microcapsules (AuNP@MC, AgNP@MC, and PtNP@MC) were prepared through this one-step interfacial thiol−ene photopolymerization and were characterized carefully by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results revealed that the obtained MNP@MCs were 2.2−2.7 μm in diameter with a wall of 40−70 nm in thickness, which was covered with the metal nanoparticles. The size and amount of metal nanoparticles increased with the increasing concentration of the metal precursor in water. Furthermore, the catalyst performance of AuNP@MC was studied by reduction of aromatic nitro compounds and exhibited the enhanced catalytic activity and good stability in the reduction of hydrophobic nitrophenol. It is believed that this robust, convenient, simple strategy based on the onestep interfacial thiol−ene photopolymerization will provide an important alternative to fabricate the functional metal nanoparticle-modified microcapsules.
■
INTRODUCTION Metal nanoparticle-decorated microcapsules (MNP@MCs) are of both scientific and technological importance because of the wide range of potential applications,1−13 such as catalyst, controlled delivery of drugs, sensor, and imaging. The incorporation of metal nanoparticles in polymer microcapsules is an effective way to combine the unique optical, electrical, and catalytic properties of metal nanoparticles14−21 with the novel encapsulation of microcapsules. Furthermore, the synergistic effect between metal nanoparticles and polymer microcapsules can overcome the shortcomings derived from nanoparticles and polymer materials and brings some positive impacts to the resulting [email protected]−26 The metal nanoparticle-decorated MNPs were shown to improve the mechanical and thermal performance of MCs, while MCs as the platform can enhance the stability and amphiphilicty of MNPs, which significantly decreases the known aggregation of MNPs.11,27 For example, the Parak group incorporated both gold and Fe2O3 nanoparticles into the wall of a polyelectrolyte capsule to synthesize the light- and magnetic-responsive microcapsules, which was well-suited for in vitro delivery of drugs inside cells.28 Because of the outstanding performance in catalysis, stability to © XXXX American Chemical Society
oxidation, and surface plasmon resonance (SPR), noble metal nanoparticles, such as gold, silver, and platinum, are widely decorated on polymer microcapsules for the desired function. The remote control of bioreaction in the microcapsule modified by AgNPs or AuNPs was demonstrated by the Skirtach group.29 As a result, much effort has been devoted to fabricate MNP@ MCs. Among the established methodologies, the typical way is usually composed of two steps: microcapsules were first prepared, and then metal nanoparticles were incorporated into microcapsules through in situ reduction or electrostaticdriven adsorption. Through the layer-by-layer (LBL) assembly, the Tsukruk group30 and the Caruso group31 prepared the polyelectrolyte microcapsules, which can adsorb the precursor of Au, and then embedded AuNPs into the wall of microcapsules through in situ reduction. Using AuNP with carboxy groups on the surface as a building block, Geest fabricated the stimuli−responsive multilayer AuNP/polyelecReceived: April 21, 2014 Revised: May 23, 2014
A
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Scheme 1. (a) Chemical Structure of the Reactive Surfactant (PTPS), Acrylate Cross-linker (TMPTA), and Photoinitiator System (ITX/I907) and (b) Whole Strategy To Fabricate the Microcapsules Containing Metal Nanoparticles (MNP@MCs)
trolyte microcapsules through LBL.32 These novel studies really enriched the technologies for the fabrication of the MNPdecorated microcapsules. Quite a few steps involved in the preparation of MNP@MCs, however, might limit the application to some extent. To obtain MNP@MCs through the efficient, convenient, and easy way, we developed here a one-step approach based on the interfacial thiol−ene photopolymerization, in which microcapsules and metal nanoparticles are generated simultaneously by irradiation of ultraviolet (UV) light (Scheme 1). The in situ photoreduction of metal ions can minimize the number of processing steps in fabrication of metal nanoparticles.33,34 Using thiol groups containing POSS (PTPS) as a reactive surfactant, we recently fabricated the uniform-sized microcapsules through the one-step toluene/water interfacial thiol− ene photopolymerization.35 This emulsion-based approach is very appealing for the large-scale fabrication of microcapsules. Through adding a metal precursor into the water phase, this approach can be successfully modified to prepare MNP@MCs, which were fully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The obtained MNP@MCs can be used as a catalyst for the chemical reduction of nitrobenzene compounds.
form sulfur radicals, which then initiate polymerization of TMPTA to form the cross-linked wall. Simultaneously, metal ions diffusing to the interface and adsorbed by the thiol groups of PTPS can be in situ reduced by the radicals to generate nanoparticles in the wall. Thus, MNPs located at the outer layer of the wall of the microcapsule are covered by the hydrophilic PEG chains, while the thiol−ene cross-linked network of TMPTA forms the hydrophobic inner layer of the wall of the microcapsule. In this strategy, the emulsion droplets can serve as templates and guide the thiol−ene photopolymerization and photoreduction on their surface, resulting in the formation of MNP@MCs. Taking the formulation PTPS/TMPTA/toluene/HAuCl4 = 1:0.85:48:5.0 as an example, the resulting AuNP@MC-1 aqueous solution turned wine-red and exhibited the characteristic plasmonic band absorption at around 526 nm, suggesting the formation of AuNPs (Figure 1).6 SEM images revealed that
■
RESULTS AND DISCUSSION The whole strategy for the fabrication of MNP@MCs is illustrated in Scheme 1. Toluene containing multifunctional acrylate TMPTA and photoinitiator I907/ITX serves as the oil phase, while the metal precursor aqueous solution is used as the water phase. In the presence of PTPS as a reactive surfactant, the oil phase of toluene is emulsified into a water phase containing a metal precursor to obtain an oil-in-water (O/W) emulsion. As the reactive surfactant, amphiphilic PTPS is composed of the POSS skeleton, hydrophobic alkyl chains, hydrophilic poly(ethylene glycol) (PEG) chains, and thiol groups, which can participate in the thiol−ene polymerization. The star structure and high molecular weight provide amphiphilic PTPS as the excellent surfactant, which distributed at the interface of toluene/water. Upon exposure of 365 nm UV light, the radicals generated from photoinitiator system I907/ ITX can be trapped quickly by the thiol groups of PTPS to
Figure 1. UV−vis spectrum of AuNP@MC-1 aqueous solution. The inset picture is the photograph of AuNP@MC-1 aqueous solution.
AuNP@MC-1 took the typical morphology of the microcapsule and nanoparticles distributed at the surface of the microcapsule (Figure 2a). The average size of AuNP@MC-1 is 2.2 ± 0.2 μm in diameter, with a low polydispersity index (PDI) of 0.10, and the wall is about 70 nm in thickness, which is determined by SEM analysis. The aspect ratio between the diameter of AuNP@MC-1 and the thickness of the wall is as high as around 30, indicating the thin wall. Generally, microcapsules with a thin wall always collapse and deform under the extreme vacuum B
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 2. Characterization of the gold nanoparticle-decorated microcapsule (AuNP@MC-1). (a) SEM image of AuNP@MC-1 with analysis of the thickness of the wall. (b) Size distribution histogram of AuNP@MC-1 determined by SEM images. A total of 50 microcapsules were counted for the distribution. (c) Size distribution of AuNP@MC-1 in THF determined by DLS. (d) AFM image of AuNP@MC-1. (e) TEM image of AuNP@MC1. (f) TEM image of the gold nanoparticles decorated on the microcapsule. (g) Size distribution histogram of AuNPs determined by TEM images. A total of 100 gold nanoparticles were counted for the distribution.
Figure 3. (a) FTIR spectra of AuNP@MC-1 exposed under a 365 nm UV light for different times. (b) UV−vis kinetics curves of the preparation of AuNP@MC-1 under irradiation by 365 nm UV light.
conditions of SEM measurement.36 The typical microcapsule morphology of AuNP@MC-1 is further supported by the atomic force microscopy (AFM) image. Even under the experiment condition of AFM without vacuum, AuNP@MC1 took the deformed morphology of microcapsules, which might be ascribed to the thin wall and high aspect ratio between the diameter and the thickness of the wall. Dynamic light scattering (DLS) was also used to check the size distribution of AuNP@MC-1 in tetrahydrofuran (THF). As shown in Figure 2c, only one peak related to the microscale appeared in DLS curves, indicating that there are no free AuNPs in the solution and all AuNPs are fixed on the surface of microcapsules. The Zaverage diameter of AuNP@MC-1 determined by DLS is around 1.5 μm, with a low PDI of 0.05. DLS experiments revealed that AuNP@MC-1 is uniform, which is in good agreement with SEM observation. The size of AuNP@MC-1
determined by DLS is smaller than that determined by SEM, which might be explained by the deformed and flattened morphology of microcapsules in the SEM measurement. The typical morphology of the microcapsule decorated with nanoparticles was further confirmed by TEM images. As shown in panels e−g of Figure 2, the deformed microcapsules are covered by Au nanoparticles with a size of 25 ± 2 nm. No nanoparticles appear in the background of the TEM image, and all nanoparticles are completely fixed in the microcapsules, which is in good agreement with DLS results. This might be ascribed to the advantages of in situ photoreduction of Au ions in the wall of the microcapsule.33,34 Because radicals are produced only in the toluene phase and trapped by PTPS at the interface, ions cannot be reduced in the water phase and metal nanoparticles are generated only in the wall of microcapsules. It should be noted that, although the thickness of the wall is thin, C
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Scheme 2. Proposed Mechanism Involving in the Processes for Generation of MNP@MCs: (a) Generation of Free Radicals from the Photoinitiator System I907/ITX, (b) Radical Mediated Thiol−Ene Polymerization for the Formation of the CrossLinked Wall of Microcapsules, and (c) Generation of Metal Nanoparticles through the Reduction of Metal Ions by Free Radicals
Table 1. Size in Diameter of the MNP@MCs and MNPs and Wall Thickness of the Obtained Microcapsules Containing Metal Nanoparticles (MNP@MCs) feed ratioa
sizeb
number
PTPS/TMPT/toluene/metal precursor
size (μm)
size of NPs (nm)
wall thickness (nm)
AuNP@MC-1 [email protected] [email protected] [email protected]
1:0.85:48:5.0 1:0.85:48:2.5 1:0.85:48:2.5 1:0.85:48:2.5
2.4 2.2 2.4 2.7
25.2 5.9 13.2 3.1
70 50 45 40
a
The feed ratio refers to the weight ratio of PTPS/TMPT/toluene/metal precursor. The total concentration of the mixture of PTPS/TMPTA/ toluene in water is kept for 10 mg/mL. bThe diameter and wall thickness were obtained by statistical analysis from SEM images, in which 50 MNP@ MCs and 100 MNPs were counted.
AuNP@MC-1 kept the well-defined morphology and was not destroyed, even under the extreme vacuum conditions in SEM and TEM measurements. During the formation of AuNP@MC-1, the thiol−ene photopolymerization of TMPTA and the photoreduction of Au ions were traced by Fourier transform infrared spectroscopy (FTIR) and UV−vis spectra, respectively (Figure 3). After irradiation of 365 nm of UV light for 2 h, the peak of 1636 cm−1 assigned to CC of TMPTA almost disappeared, indicating that the cross-linked wall of the microcapsule can form in 2 h. As shown in Figure 3b, the characteristic plasmonic bond absorption of AuNPs at 526 nm did not appear at the beginning of UV irradiation for 1 h and then increased gradually with the continuing irradiation in the following 5 h. The in situ photoreduction of Au ions is obviously slower than the thiol−ene photopolymerization. This can be explained by the fact that the active radicals generated by irradiation first participated in thiol−ene photopolymerization to produce the cross-linked wall then reduced Au ions to generate AuNP. The possible mechanism for the preparation of Au@MC-1 via one step involving the thiol−ene polymerization and photoreduction was proposed in Scheme 2. As the good type II photoinitiator and photosensitizer, ITX was excited to the excited state by 365 nm UV light irradiation, which can produce radicals through two approaches (Scheme 2a). In the presence of the hydrogen donor, such as amino and thiol groups, the excited state of ITX can abstract hydrogen to produce the active radical derived from amine, thiol, and another ketyl radical from ITX. Because of the steric hindrance and delocalization of the unpaired electron, the ketyl radical is usually not reactive to the vinyl monomer and cannot initiate
polymerization of TMPTA. The type I photoinitiator I907 can also be sensitized by the excited state of ITX to produce two types of active radicals. Except for ketyl radicals, all active radicals can be quickly trapped by −SH of PTPS (Scheme 2b). The thiol−ene reaction proceeds only at the interface via a radical step-growth polymerization manner, and the excess acrylate continues to polymerize through homopolymerization. After almost all acrylate groups are polymerized, the active radicals as well as ketyl radicals reduce Au ions to produce AuNPs (Scheme 2c). The reduction happens at the water/ toluene interface, where the Au ions in the water phase can diffuse to the interface because of the complexity between sulfur of PTPS in the interface and Au ions. Although the ketyl radical of ITX is not active, Galian et al.34,37,38 have proven that it can reduce noble metal ions and change back into the ketone. As for this point, ITX can diffuse to the interface and act as a catalyst for the photogeneration of AuNPs. Therefore, both photopolymerization and photoreduction take place only at the water/toluene interface, which is the key factor to the successful preparation of AuNP@MCs through a one-step approach. To verify the feasibility of this one-step strategy, we prepared a series of MNP@MCs through the interfacial thiol−ene photopolymerization. Through changing the concentration of HAuCl4 or replacing HAuCl4 with AgNO3 or H2PtCl6, microcapsules decorated with AgNP, PtNP, and the smaller AuNP can be obtained. As for all MNP@MCs, the formulation, size in diameter and thickness of the wall for the microcapsule, and size of MNP were summarized in Table 1. As shown in Figure 4, both the representative SEM and TEM images show that the typical morphology of the microcapsule is covered with the nanoparticles and all metal nanoparticles are fixed on the D
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 4. (a) SEM image and (b) TEM image of the microcapsule containing metal nanoparticles [email protected], [email protected], and PtNP@ MC-0.5, respectively. (c) TEM image and (d) size distribution histogram and photograph of MNPs decorated on [email protected], [email protected], and [email protected], respectively.
Figure 5. (a) UV−vis absorption spectra of the CF3Nip solution as a function of time during the reduction catalyzed by AuNP@MC-1 at 25 °C. (b) Time-conversion plot of the reduction of Nip and CF3NiP catalyzed by AuNP@MC-1 at 25 °C. (c) Proposed scheme of the reduction of Nip and CF3NiP by NaBH4 in the presence of AuNP@MC-1. (d) Recycle experiments of the AuNP@MC-1 catalyst for the reduction of CF3NiP.
surface of microcapsules. The diameter of AuNP and the thickness of the wall for [email protected] are 5.9 ± 0.3 and
about 50 nm, respectively. In comparison to AuNP@MC-1, the less and smaller AuNP for [email protected] should be ascribed E
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
and 0.25 min−1, respectively. The higher kapp for CF3Nip might be ascribed to the encapsulation of the hydrophobic CF3Nip by AuNP@MC-1 in aqueous solution. As proposed in Figure 5c, the encapsulation of CF3Nip can increase the local concentration of CF3Nip and accelerate its adsorption on the surface of AuNPs, resulting in the enhancement of the reduction speed. The reusability of AuNP@MC-1 as a microreactor was also evaluated. As shown in Figure 5d, AuNP@MC-1 could be recovered by simple centrifugation and kept the good catalytic activity after 6 cycles. The excellent catalytic stability might be explained by the fact that AuNPs are well-decorated in the wall of the microcapsule, which can decrease their aggregation during reaction.
to the low concentration of HAuCl4 in preparation, which is also reflected by the thinner wall of [email protected]. This indicated that the size and amount of AuNP on the surface of the microcapsule might be tunable by this strategy. The solutions of AuNP@MC-1 and [email protected] are wine-red and purple-red, respectively, suggesting the different sizes of AuNP (see Figure S1 of the Supporting Information). As for [email protected], the diameter of AgNP and the thickness of the wall are 13 ± 0.9 and 45 nm, respectively. Upon exposure of UV light, the solution of [email protected] became orange and exhibited the characteristic SPR of Ag nanoparticles, suggesting the generation of AgNP (Figure 4 and Figure S2 of the Supporting Information). The size of PtNP in diameter and the thickness of the wall for [email protected] are 3.1 ± 0.1 and 40 nm, respectively. Because of the weak SPR of PtNP, the color of [email protected] solution is yellow and does not change obviously with the irradiation of UV light (see Figure S3 of the Supporting Information). The size dependence of MNPs upon the different metals might be ascribed to the difference in the ability of the sulfur radicals at the interface to reduce different metal ions. The other factor might be that the diffusion of metal ions from the water phase to the interface is dependent upon the type of metal ions or the numbers of MNPs in each capsule. These factors might mainly cause the different sizes of various metal nanoparticles. X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) experiments were also carried out to determine the chemical composition of the obtained MNP@MCs (see Figures S4 and S5 of the Supporting Information). The signals assigned to Au, Ag, and Pt observed in the XPS spectra of [email protected], [email protected], and [email protected] confirm the presence of metal nanoparticles. In comparison to AuNP@ MC-0.5, the higher weight retention in the TGA curve of AuNP@MC-1 suggested the high content of Au nanoparticles, which is in good agreement with the observation in SEM and TEM images. Because of the hydrophilic outer layer of PEG, the obtained MNP@MC can be well-dispersed in aqueous solution. For example, no obvious aggregation occurred in AuNP@MC-1 aqueous solution when it was kept for 1 month at room temperature. Motivated by these novel characteristics of MNP@MCs, such as the facile preparation of one step, amphiphilicity, and hybrid microstructure with nanoscale nanoparticles, we tried to use AuNP@MC-1 as a catalyst because of its combination of the catalysis property of metal nanoparticles and the encapsulation of the microcapsule.39 To test the catalysis performance of AuNP@MC-1, the reduction of nitrophenol was chosen as the model reaction.40−42 UV−vis spectra were used to monitor the reduction of 4-nitrophenol (Nip) and the more hydrophobic 4-nitro-3-(trifluoromethyl)phenol (CF3Nip) in the presence of NaBH4. As shown in Figure 5a, the strong absorption peak of CF3Nip at 394 nm decreased gradually after adding AuNP@MC-1, while simultaneously, a new peak appeared at 317 nm, which should be assigned to the product 4-amino-3-(trifluoromethyl)phenol. Because the concentration of sodium borohydride largely exceeds the concentration of nitrophenol, first-order rate kinetics with regard to the nitrophenol concentration could be used to evaluate the catalytic rate.41 The conversion of the catalytic reduction process can be obtained by the ratio of the respective absorption A/A0, and the apparent rate constant kapp is calculated from the linear plot of ln(A/A0) versus reduction time. As shown in Figure 5b, kapp for CF3Nip and Nip are 0.29
■
CONCLUSION We demonstrated a one-step strategy to prepare the noble metal nanoparticle-decorated microcapsules (MNP@MCs) through the interfacial thiol−ene photopolymerization. The oil-in-water (O/W) emulsion droplets can serve as templates and guide the thiol−ene photopolymerization and photoreduction on their surface, resulting in the formation of MNP@ MCs. The obtained MNP@MCs were covered with the metal nanoparticles, whose size and density increased with the increasing concentration of the metal precursor in water. Because of the synergistic effect of the encapsulation of MC and the catalytic activity of AuNP, AuNP@MC exhibited enhanced catalytic activity and good stability in the reduction of hydrophobic nitrophenol. It is believed that this robust, convenient, and simple strategy based on the one-step interfacial thiol−ene photopolymerization will provide an important alternative to fabricate the functional metal nanoparticle-modified microcapsules.
■
EXPERIMENTAL SECTION
Preparation of the Hybrid Microcapsule-Containing Metal Nanoparticles (MC@MNPs). The photoinitiators I907 (1% of TMPTA and PTPS) and ITX (0.5% of TMPTA and PTPS) were dissolved in a 200 mg mixture of PTPS, TMPTA, and toluene (w/w/w = 1:0.85:48) in a 25 mL vial. Then, 20 mL of the designed concentration of HAuCl4·4H2O (or AgNO3 and H2PtCl6·6H2O) aqueous solution was dropped into the mixture with stirring. After that, the solution was stirred for 1 h, followed by ultrasonic for 1 h to obtain a stable milky emulsion. The obtained emulsion was exposed under an UV light-emitting diode (LED) lamp at 365 nm (the intensity is about 8.4 mW/cm2) with constant stirring (400 rpm). After fully photo-cross-linking and complete reduction, the hybrid microcapsule-containing gold nanoparticles were obtained by centrifugation/redispersion in THF 3 times. The number and size of the obtained MNP@MCs are summarized in Table 1. Catalytic Experiment of AuNP@MC-1. At room temperature, 1.0 mL of NaBH4 solution (30 mmol/L) was added to 1.75 mL of Nip or CF3Nip solution (0.17 mmol/L) that was contained in a glass cuvette. Then, 0.2 mL of AuNP@MCs solution ([Au] = 2.4 × 10−3 mol/L) was added, and UV−vis spectra of the sample were taken every 2 min in the range of 250−550 nm. The rate constant of the reaction was determined by measuring the change of the absorption peak intensity at 400 nm with time. In the catalyst recycle studies, the UV−vis spectroscopy was taken to make sure that all CF3Nip had been consumed, thus signaling that the first cycle was over. After that, the previous aqueous solution was centrifuged to obtain the used AuNP@MC-1, and the catalyst was then washed with deionized water, followed by vacuum drying. The obtained AuNP@MC-1 from the first cycle was used for the second catalytic reduction cycle. Similarly, we repeated the recycle 6 times. Measurements. FTIR spectra measurements were carried out with a Spectrum 100 Fourier transformation infrared absorption F
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Insulation for their financial support. Xuesong Jiang is supported by the NCET-12-3050 Project.
spectrometer (PerkinElmer, Inc., Waltham, MA). The samples were prepared by dropping the polymer solution onto a KBr film and dried below an infrared lamp. The SEM images were obtained using a NOVA NanoSEM 230 (FEI Company) field emission scanning electron microscope operated at an acceleration voltage of 5 kV. The samples were prepared by dropping the MNP@MCs solution onto silica wafers and air-drying the silica wafers at room temperature. Then, the samples were sputter-coated with gold before examination. The TEM images were obtained using a JEM-2100 (JEOL, Ltd., Japan) transmission electron microscope operated at an acceleration voltage of 200 kV. The sample was prepared by dropping MNP@MCs solution onto copper grids coated with a thin carbon film, while the excess solution was removed by filter paper and air-dried at room temperature. AFM images were taken by SII Nanonavi E-sweep under ambient conditions. AFM was operated in tapping mode using silicon cantilevers with a force constant of 40 N m−1. The samples were prepared by dropping the MNP@MCs solution onto a mica sheet. The DLS measurements were performed in the PTPST microcapsules solution using a ZS90 Zetasizer Nano ZS instrument (Malvern Instruments, Ltd., U.K.) equipped with a multi-τ digital time correlation and a 4 mW He−Ne laser (λ = 633 nm) at an angle of 90°. Regularized Laplace inversion (CONTIN algorithm) was applied to analyze the obtained autocorrelation functions. The concentration of MNP@MCs solution was 1 g/L. XPS experiments were conducted on a PHI-5000C ESCA system (PerkinElmer) with Al Kα radiation (hν = 1486.6 eV). The X-ray anode was run at 250 W, and the high voltage was 14.0 kV, with a detection angle at 54°. To ensure the sufficient sensitivity, the pass energy was fixed at 46.95 eV. The base pressure of the analyzer chamber was about 5 × 10−8 Pa. The sample was pressed to a selfsupported disk (10 × 10 mm), mounted on a sample holder, and transferred to the analyzer chamber. The whole spectra (0−700 eV) of all elements were recorded with high resolution. TGA was carried out on a TA Q5000IR thermogravimetric analyzer under nitrogen with the temperature range of 40−800 °C at a heating rate of 20 °C min−1. The UV−vis spectra of MNP@MCs were carried out with a UV2550 spectrophotometer (Shimadzu, Japan).
■
■
(1) Alvarez-Paino, M.; Marcelo, G.; Munoz-Bonilla, A.; FernandezGarcia, M. Catecholic chemistry to obtain recyclable and reusable hybrid polymeric particles as catalytic systems. Macromolecules 2013, 46 (8), 2951−2962. (2) He, J.; Liu, Y.; Hood, T. C.; Zhang, P.; Gong, J.; Nie, Z. Asymmetric organic/metal(oxide) hybrid nanoparticles: Synthesis and applications. Nanoscale 2013, 5 (12), 5151−5166. (3) Li, X.; Guo, J.; Asong, J.; Wolfert, M. A.; Boons, G.-J. Multifunctional surface modification of gold-stabilized nanoparticles by bioorthogonal reactions. J. Am. Chem. Soc. 2011, 133 (29), 11147− 11153. (4) Song, J.; Zhou, J.; Duan, H. Self-Assembled plasmonic vesicles of SERS-encoded amphiphilic gold nanoparticles for cancer cell targeting and traceable intracellular drug delivery. J. Am. Chem. Soc. 2012, 134 (32), 13458−13469. (5) Samanta, B.; Yang, X. C.; Ofir, Y.; Park, M.-H.; Patra, D.; Agasti, S. S.; Miranda, O. R.; Mo, Z.-H.; Rotello, V. M. Catalytic microcapsules assembled from enzyme−nanoparticle conjugates at oil−water interfaces. Angew. Chem., Int. Ed. 2009, 48 (29), 5341−5344. (6) Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. Plasmonic vesicles of amphiphilic gold nanocrystals: Self-assembly and external-stimuli-triggered destruction. J. Am. Chem. Soc. 2011, 133 (28), 10760−10763. (7) Chen, L.; Hu, J.; Qi, Z.; Fang, Y.; Richards, R. Gold sanoparticles intercalated into the walls of mesoporous silica as a versatile redox catalyst. Ind. Eng. Chem. Res. 2011, 50 (24), 13642−13649. (8) Laudenslager, M. J.; Schiffman, J. D.; Schauer, C. L. Carboxymethyl shitosan as a matrix material for platinum, gold, and silver nanoparticles. Biomacromolecules 2008, 9 (10), 2682−2685. (9) Han, J.; Liu, Y.; Guo, R. Facile synthesis of highly stable gold nanoparticles and their unexpected excellent catalytic activity for Suzuki−Miyaura cross-coupling reaction in water. J. Am. Chem. Soc. 2009, 131 (6), 2060−2061. (10) Brust, M.; Gordillo, G. J. Electrocatalytic hydrogen redox chemistry on gold nanoparticles. J. Am. Chem. Soc. 2012, 134 (7), 3318−3321. (11) Städler, B.; Price, A. D.; Zelikin, A. N. A critical look at multilayered polymer capsules in biomedicine: Drug carriers, artificial organelles, and cell mimics. Adv. Funct. Mater. 2011, 21 (1), 14−28. (12) Becker, A. L.; Johnston, A. P. R.; Caruso, F. Layer-by-layerassembled capsules and films for therapeutic delivery. Small 2010, 6 (17), 1836−1852. (13) Esser-Kahn, A. P.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Triggered release from polymer capsules. Macromolecules 2011, 44 (14), 5539−5553. (14) Xu, H.; Xu, J.; Jiang, X.; Zhu, Z.; Rao, J.; Yin, J.; Wu, T.; Liu, H.; Liu, S. Thermosensitive unimolecular micelles surface-decorated with gold nanoparticles of tunable spatial distribution. Chem. Mater. 2007, 19 (10), 2489−2494. (15) Zhang, Q.; Xie, J.; Yang, J.; Lee, J. Y. Monodisperse icosahedral Ag, Au, and Pd nanoparticles: Size control strategy and superlattice formation. ACS Nano 2008, 3 (1), 139−148. (16) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. Toward functional nanocomposites: Taking the best of nanoparticles, polymers, and small molecules. Chem. Soc. Rev. 2013, 42 (7), 2654− 2678. (17) Selvan, T.; Spatz, J. P.; Klok, H.-A.; Möller, M. Gold− polypyrrole core−shell particles in diblock copolymer micelles. Adv. Mater. 1998, 10 (2), 132−134. (18) Pradhan, S.; Sun, J.; Deng, F.; Chen, S. Single-electron transfer in nanoparticle solids. Adv. Mater. 2006, 18 (24), 3279−3283. (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seeding growth for size control of 5−40 nm diameter gold nanoparticles. Langmuir 2001, 17 (22), 6782−6786.
ASSOCIATED CONTENT
S Supporting Information *
Materials and results of UV−vis spectrum of [email protected] aqueous solution (Figure S1), UV−vis spectrum of AgNP@ MC-0.5 aqueous solution (Figure S2), UV−vis spectrum of [email protected] aqueous solution (Figure S3), XPS of AuNP@ MC-0.5 and [email protected] (Figure S4), and TGA curves of AuNP@MC-1, [email protected], [email protected], and PtNP@ MC-0.5, which were conducted under nitrogen with a heating rate of 20 °C min−1 (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-21-54743268. Fax: +86-21-54747445. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of China (21174085, 21274088, and 51373098), the Education Commission of Shanghai Municipal Government (12ZZ020), and the Shanghai Key Laboratory of Polymer and Electrical G
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(20) Hu, J.; Wu, T.; Zhang, G.; Liu, S. Effcient synthesis of single gold nanoparticle hybrid amphiphilic triblock copolymers and their controlled self-assembly. J. Am. Chem. Soc. 2012, 134 (18), 7624− 7627. (21) Mieszawska, A. J.; Mulder, W. J. M.; Fayad, Z. A.; Cormode, D. P. Multifunctional gold nanoparticles for diagnosis and therapy of disease. Mol. Pharmaceutics 2013, 10 (3), 831−847. (22) Mercato, L. L.; Gonzalez, E.; Abbasi, A. Z.; Parak, W. J.; Puntes, V. Synthesis and evaluation of gold nanoparticle-modified polyelectrolyte capsules under microwave irradiation for remotely controlled release for cargo. J. Mater. Chem. 2011, 21 (31), 11468−11471. (23) Hickey, R. J.; Haynes, A. S.; Kikkawa, J. M.; Park, S.-J. Controlling the self-assembly structure of magnetic nanoparticles and amphiphilic block-copolymers: From micelles to vesicles. J. Am. Chem. Soc. 2011, 133 (5), 1517−1525. (24) Abbasi, A. Z.; Gutiérrez, L.; Mercato, L. L.; Herranz, F.; Chubykalo-Fesenko, O.; Veintemillas-Verdaguer, S.; Parak, W. J.; Morales, M. P.; González, J. M.; Hernando, A.; Presa, P. Magnetic capsules for NMR imaging: Effect of magnetic nanoparticles spatial distribution and aggregation. J. Phys. Chem. C 2011, 115 (14), 6257− 6264. (25) Anandhakumar, S.; Vijayalakshmi, S. P.; Jagadeesh, G.; Raichur, A. M. Silver nanoparticle synthesis: Novel route for laser triggering of polyelectrolyte capsules. ACS Appl. Mater. Interfaces 2011, 3 (9), 3419−3424. (26) Li, Y.; Smith, A. E.; Lokitz, B. S.; McCormick, C. L. In situ formation of gold-“decorated” vesicles from a RAFT-synthesized, thermally responsive block copolymer. Macromolecules 2007, 40 (24), 8524−8526. (27) Tian, J.; Yuan, L.; Zhang, M.; Zheng, F.; Xiong, Q.; Zhao, H. Interface-directed self-assembly of gold nanoparticles and fabrication of hybrid hollow capsules by interfacial cross-linking polymerization. Langmuir 2012, 28 (25), 9365−9371. (28) Ochs, M.; Carregal-Romero, S.; Rejman, J.; Braeckmans, K.; De Smedt, S. C.; Parak, W. J. Light-addressable capsules as caged compound matrix for controlled triggering of cytosolic reactions. Angew. Chem., Int. Ed. 2013, 52 (2), 695−699. (29) Kreft, O.; Skirtach, A. G.; Sukhorukov, G. B.; Möhwald, H. Remote control of bioreactions in multicompartment capsules. Adv. Mater. 2007, 19 (20), 3142−3145. (30) Kozlovskaya, V.; Kharlampieva, E.; Chang, S.; Muhlbauer, R.; Tsukruk, V. V. pH-Responsive layered hydrogel microcapsules as gold nanoreactors. Chem. Mater. 2009, 21 (10), 2158−2167. (31) Angelatos, A. S.; Radt, B.; Caruso, F. Light-responsive polyelectrolyte/gold nanoparticle microcapsules. J. Phys. Chem. B 2005, 109 (7), 3071−3076. (32) De Geest, B. G.; Skirtach, A. G.; De Beer, T. R. M.; Sukhorukov, G. B.; Bracke, L.; Baeyens, W. R. G.; Demeester, J.; De Smedt, S. C. Stimuli−responsive multilayered hybrid nanoparticle/polyelectrolyte capsules. Macromol. Rapid Commun. 2007, 28 (1), 88−95. (33) McGilvray, K. L.; Decan, M. R.; Wang, D.; Scaiano, J. C. Facile photochemical synthesis of unprotected aqueous gold nanoparticles. J. Am. Chem. Soc. 2006, 128, 15980−15981. (34) Marin, M. L.; McGilvray, K. L.; Scaiano, J. C. Photochemical strategies for the synthesis of gold nanoparticles from Au(III) and Au(I) using photoinduced free radical generation. J. Am. Chem. Soc. 2008, 130, 16572−16584. (35) Liu, D. D.; Yu, B.; Jiang, X. S.; Yin, J. Responsive hybrid microcapsules by the one-step interfacial thiol−ene photopolymerization. Langmuir 2013, 29 (17), 5307−5314. (36) Yu, B.; Jiang, X. S.; Qin, N.; Yin, J. Thiol−ene photocrosslinked hybrid vesicles from co-assembly of POSS and poly(ether amine) (PEA). Chem. Commun. 2011, 47 (44), 12110−12112. (37) Galian, R. E.; Guardia, M.; Pérez-Prieto, J. Size reduction of CdSe/ZnS core−shell quantum dots photosensitized by benzophenone: Where does the Cd(0) go? Langmuir 2011, 27 (5), 1942−1945. (38) Consuelo Cuquerella, M.; Pocoví-Martínez, S.; Pérez-Prieto, J. Photocatalytic coalescence of functionalized gold nanoparticles. Langmuir 2009, 26 (3), 1548−1550.
(39) Wang, R.; Jiang, X. S.; Yu, B.; Yin, J. Stimuli−responsive microgels formed by hyperbranched poly(ether amine) decorated with platinum nanoparticles. Soft Matter 2011, 7 (18), 8619−8627. (40) Xia, B.; He, F.; Li, L. Preparation of bimetallic nanoparticles using a facile green synthesis method and their application. Langmuir 2013, 29 (15), 4901−4907. (41) Baruah, B.; Gabriel, G. J.; Akbashev, M. J.; Booher, M. E. Facile synthesis of silver nanoparticles stabilized by cationic polynorbornenes and their catalytic activity in 4-nitrophenol reduction. Langmuir 2013, 29 (13), 4225−4234. (42) Dhar, J.; Patil, S. Self-assembly and catalytic activity of metal nanoparticles immobilized in polymer membrane prepared via layerby-layer approach. ACS Appl. Mater. Interface 2012, 4 (3), 1803−1812.
H
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
One-Step Interfacial Thiol−Ene Photopolymerization for Metal Nanoparticle-Decorated Microcapsules (MNP@MCs) Dandan Liu, Xuesong Jiang,* and Jie Yin School of Chemistry and Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: We herein reported a one-step strategy to prepare the noble metal nanoparticle-decorated microcapsules (MNP@MCs) through the interfacial thiol−ene photopolymerization. In the presence of amphiphlic polyhedral oligomeric silsesquioxane (POSS) containing thiol groups (PTPS) as a reactive surfactant and trimethylolpropane triacrylate (TMPTA) as a cross-linker, the oil phase of toluene dissolved with a photoinitiator was emulsified into a water phase containing a metal precursor to form an oil-in-water (O/W) emulsion. Upon irradiation of ultraviolet (UV) light, the thiol−ene photoploymerization and photoreduction at the interface of toluene/water lead to the formation of the cross-linked wall and metal nanoparticles, respectively. A series of gold, silver, and platinum nanoparticledecorated microcapsules (AuNP@MC, AgNP@MC, and PtNP@MC) were prepared through this one-step interfacial thiol−ene photopolymerization and were characterized carefully by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results revealed that the obtained MNP@MCs were 2.2−2.7 μm in diameter with a wall of 40−70 nm in thickness, which was covered with the metal nanoparticles. The size and amount of metal nanoparticles increased with the increasing concentration of the metal precursor in water. Furthermore, the catalyst performance of AuNP@MC was studied by reduction of aromatic nitro compounds and exhibited the enhanced catalytic activity and good stability in the reduction of hydrophobic nitrophenol. It is believed that this robust, convenient, simple strategy based on the onestep interfacial thiol−ene photopolymerization will provide an important alternative to fabricate the functional metal nanoparticle-modified microcapsules.
■
INTRODUCTION Metal nanoparticle-decorated microcapsules (MNP@MCs) are of both scientific and technological importance because of the wide range of potential applications,1−13 such as catalyst, controlled delivery of drugs, sensor, and imaging. The incorporation of metal nanoparticles in polymer microcapsules is an effective way to combine the unique optical, electrical, and catalytic properties of metal nanoparticles14−21 with the novel encapsulation of microcapsules. Furthermore, the synergistic effect between metal nanoparticles and polymer microcapsules can overcome the shortcomings derived from nanoparticles and polymer materials and brings some positive impacts to the resulting [email protected]−26 The metal nanoparticle-decorated MNPs were shown to improve the mechanical and thermal performance of MCs, while MCs as the platform can enhance the stability and amphiphilicty of MNPs, which significantly decreases the known aggregation of MNPs.11,27 For example, the Parak group incorporated both gold and Fe2O3 nanoparticles into the wall of a polyelectrolyte capsule to synthesize the light- and magnetic-responsive microcapsules, which was well-suited for in vitro delivery of drugs inside cells.28 Because of the outstanding performance in catalysis, stability to © XXXX American Chemical Society
oxidation, and surface plasmon resonance (SPR), noble metal nanoparticles, such as gold, silver, and platinum, are widely decorated on polymer microcapsules for the desired function. The remote control of bioreaction in the microcapsule modified by AgNPs or AuNPs was demonstrated by the Skirtach group.29 As a result, much effort has been devoted to fabricate MNP@ MCs. Among the established methodologies, the typical way is usually composed of two steps: microcapsules were first prepared, and then metal nanoparticles were incorporated into microcapsules through in situ reduction or electrostaticdriven adsorption. Through the layer-by-layer (LBL) assembly, the Tsukruk group30 and the Caruso group31 prepared the polyelectrolyte microcapsules, which can adsorb the precursor of Au, and then embedded AuNPs into the wall of microcapsules through in situ reduction. Using AuNP with carboxy groups on the surface as a building block, Geest fabricated the stimuli−responsive multilayer AuNP/polyelecReceived: April 21, 2014 Revised: May 23, 2014
A
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Scheme 1. (a) Chemical Structure of the Reactive Surfactant (PTPS), Acrylate Cross-linker (TMPTA), and Photoinitiator System (ITX/I907) and (b) Whole Strategy To Fabricate the Microcapsules Containing Metal Nanoparticles (MNP@MCs)
trolyte microcapsules through LBL.32 These novel studies really enriched the technologies for the fabrication of the MNPdecorated microcapsules. Quite a few steps involved in the preparation of MNP@MCs, however, might limit the application to some extent. To obtain MNP@MCs through the efficient, convenient, and easy way, we developed here a one-step approach based on the interfacial thiol−ene photopolymerization, in which microcapsules and metal nanoparticles are generated simultaneously by irradiation of ultraviolet (UV) light (Scheme 1). The in situ photoreduction of metal ions can minimize the number of processing steps in fabrication of metal nanoparticles.33,34 Using thiol groups containing POSS (PTPS) as a reactive surfactant, we recently fabricated the uniform-sized microcapsules through the one-step toluene/water interfacial thiol− ene photopolymerization.35 This emulsion-based approach is very appealing for the large-scale fabrication of microcapsules. Through adding a metal precursor into the water phase, this approach can be successfully modified to prepare MNP@MCs, which were fully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The obtained MNP@MCs can be used as a catalyst for the chemical reduction of nitrobenzene compounds.
form sulfur radicals, which then initiate polymerization of TMPTA to form the cross-linked wall. Simultaneously, metal ions diffusing to the interface and adsorbed by the thiol groups of PTPS can be in situ reduced by the radicals to generate nanoparticles in the wall. Thus, MNPs located at the outer layer of the wall of the microcapsule are covered by the hydrophilic PEG chains, while the thiol−ene cross-linked network of TMPTA forms the hydrophobic inner layer of the wall of the microcapsule. In this strategy, the emulsion droplets can serve as templates and guide the thiol−ene photopolymerization and photoreduction on their surface, resulting in the formation of MNP@MCs. Taking the formulation PTPS/TMPTA/toluene/HAuCl4 = 1:0.85:48:5.0 as an example, the resulting AuNP@MC-1 aqueous solution turned wine-red and exhibited the characteristic plasmonic band absorption at around 526 nm, suggesting the formation of AuNPs (Figure 1).6 SEM images revealed that
■
RESULTS AND DISCUSSION The whole strategy for the fabrication of MNP@MCs is illustrated in Scheme 1. Toluene containing multifunctional acrylate TMPTA and photoinitiator I907/ITX serves as the oil phase, while the metal precursor aqueous solution is used as the water phase. In the presence of PTPS as a reactive surfactant, the oil phase of toluene is emulsified into a water phase containing a metal precursor to obtain an oil-in-water (O/W) emulsion. As the reactive surfactant, amphiphilic PTPS is composed of the POSS skeleton, hydrophobic alkyl chains, hydrophilic poly(ethylene glycol) (PEG) chains, and thiol groups, which can participate in the thiol−ene polymerization. The star structure and high molecular weight provide amphiphilic PTPS as the excellent surfactant, which distributed at the interface of toluene/water. Upon exposure of 365 nm UV light, the radicals generated from photoinitiator system I907/ ITX can be trapped quickly by the thiol groups of PTPS to
Figure 1. UV−vis spectrum of AuNP@MC-1 aqueous solution. The inset picture is the photograph of AuNP@MC-1 aqueous solution.
AuNP@MC-1 took the typical morphology of the microcapsule and nanoparticles distributed at the surface of the microcapsule (Figure 2a). The average size of AuNP@MC-1 is 2.2 ± 0.2 μm in diameter, with a low polydispersity index (PDI) of 0.10, and the wall is about 70 nm in thickness, which is determined by SEM analysis. The aspect ratio between the diameter of AuNP@MC-1 and the thickness of the wall is as high as around 30, indicating the thin wall. Generally, microcapsules with a thin wall always collapse and deform under the extreme vacuum B
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 2. Characterization of the gold nanoparticle-decorated microcapsule (AuNP@MC-1). (a) SEM image of AuNP@MC-1 with analysis of the thickness of the wall. (b) Size distribution histogram of AuNP@MC-1 determined by SEM images. A total of 50 microcapsules were counted for the distribution. (c) Size distribution of AuNP@MC-1 in THF determined by DLS. (d) AFM image of AuNP@MC-1. (e) TEM image of AuNP@MC1. (f) TEM image of the gold nanoparticles decorated on the microcapsule. (g) Size distribution histogram of AuNPs determined by TEM images. A total of 100 gold nanoparticles were counted for the distribution.
Figure 3. (a) FTIR spectra of AuNP@MC-1 exposed under a 365 nm UV light for different times. (b) UV−vis kinetics curves of the preparation of AuNP@MC-1 under irradiation by 365 nm UV light.
conditions of SEM measurement.36 The typical microcapsule morphology of AuNP@MC-1 is further supported by the atomic force microscopy (AFM) image. Even under the experiment condition of AFM without vacuum, AuNP@MC1 took the deformed morphology of microcapsules, which might be ascribed to the thin wall and high aspect ratio between the diameter and the thickness of the wall. Dynamic light scattering (DLS) was also used to check the size distribution of AuNP@MC-1 in tetrahydrofuran (THF). As shown in Figure 2c, only one peak related to the microscale appeared in DLS curves, indicating that there are no free AuNPs in the solution and all AuNPs are fixed on the surface of microcapsules. The Zaverage diameter of AuNP@MC-1 determined by DLS is around 1.5 μm, with a low PDI of 0.05. DLS experiments revealed that AuNP@MC-1 is uniform, which is in good agreement with SEM observation. The size of AuNP@MC-1
determined by DLS is smaller than that determined by SEM, which might be explained by the deformed and flattened morphology of microcapsules in the SEM measurement. The typical morphology of the microcapsule decorated with nanoparticles was further confirmed by TEM images. As shown in panels e−g of Figure 2, the deformed microcapsules are covered by Au nanoparticles with a size of 25 ± 2 nm. No nanoparticles appear in the background of the TEM image, and all nanoparticles are completely fixed in the microcapsules, which is in good agreement with DLS results. This might be ascribed to the advantages of in situ photoreduction of Au ions in the wall of the microcapsule.33,34 Because radicals are produced only in the toluene phase and trapped by PTPS at the interface, ions cannot be reduced in the water phase and metal nanoparticles are generated only in the wall of microcapsules. It should be noted that, although the thickness of the wall is thin, C
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Scheme 2. Proposed Mechanism Involving in the Processes for Generation of MNP@MCs: (a) Generation of Free Radicals from the Photoinitiator System I907/ITX, (b) Radical Mediated Thiol−Ene Polymerization for the Formation of the CrossLinked Wall of Microcapsules, and (c) Generation of Metal Nanoparticles through the Reduction of Metal Ions by Free Radicals
Table 1. Size in Diameter of the MNP@MCs and MNPs and Wall Thickness of the Obtained Microcapsules Containing Metal Nanoparticles (MNP@MCs) feed ratioa
sizeb
number
PTPS/TMPT/toluene/metal precursor
size (μm)
size of NPs (nm)
wall thickness (nm)
AuNP@MC-1 [email protected] [email protected] [email protected]
1:0.85:48:5.0 1:0.85:48:2.5 1:0.85:48:2.5 1:0.85:48:2.5
2.4 2.2 2.4 2.7
25.2 5.9 13.2 3.1
70 50 45 40
a
The feed ratio refers to the weight ratio of PTPS/TMPT/toluene/metal precursor. The total concentration of the mixture of PTPS/TMPTA/ toluene in water is kept for 10 mg/mL. bThe diameter and wall thickness were obtained by statistical analysis from SEM images, in which 50 MNP@ MCs and 100 MNPs were counted.
AuNP@MC-1 kept the well-defined morphology and was not destroyed, even under the extreme vacuum conditions in SEM and TEM measurements. During the formation of AuNP@MC-1, the thiol−ene photopolymerization of TMPTA and the photoreduction of Au ions were traced by Fourier transform infrared spectroscopy (FTIR) and UV−vis spectra, respectively (Figure 3). After irradiation of 365 nm of UV light for 2 h, the peak of 1636 cm−1 assigned to CC of TMPTA almost disappeared, indicating that the cross-linked wall of the microcapsule can form in 2 h. As shown in Figure 3b, the characteristic plasmonic bond absorption of AuNPs at 526 nm did not appear at the beginning of UV irradiation for 1 h and then increased gradually with the continuing irradiation in the following 5 h. The in situ photoreduction of Au ions is obviously slower than the thiol−ene photopolymerization. This can be explained by the fact that the active radicals generated by irradiation first participated in thiol−ene photopolymerization to produce the cross-linked wall then reduced Au ions to generate AuNP. The possible mechanism for the preparation of Au@MC-1 via one step involving the thiol−ene polymerization and photoreduction was proposed in Scheme 2. As the good type II photoinitiator and photosensitizer, ITX was excited to the excited state by 365 nm UV light irradiation, which can produce radicals through two approaches (Scheme 2a). In the presence of the hydrogen donor, such as amino and thiol groups, the excited state of ITX can abstract hydrogen to produce the active radical derived from amine, thiol, and another ketyl radical from ITX. Because of the steric hindrance and delocalization of the unpaired electron, the ketyl radical is usually not reactive to the vinyl monomer and cannot initiate
polymerization of TMPTA. The type I photoinitiator I907 can also be sensitized by the excited state of ITX to produce two types of active radicals. Except for ketyl radicals, all active radicals can be quickly trapped by −SH of PTPS (Scheme 2b). The thiol−ene reaction proceeds only at the interface via a radical step-growth polymerization manner, and the excess acrylate continues to polymerize through homopolymerization. After almost all acrylate groups are polymerized, the active radicals as well as ketyl radicals reduce Au ions to produce AuNPs (Scheme 2c). The reduction happens at the water/ toluene interface, where the Au ions in the water phase can diffuse to the interface because of the complexity between sulfur of PTPS in the interface and Au ions. Although the ketyl radical of ITX is not active, Galian et al.34,37,38 have proven that it can reduce noble metal ions and change back into the ketone. As for this point, ITX can diffuse to the interface and act as a catalyst for the photogeneration of AuNPs. Therefore, both photopolymerization and photoreduction take place only at the water/toluene interface, which is the key factor to the successful preparation of AuNP@MCs through a one-step approach. To verify the feasibility of this one-step strategy, we prepared a series of MNP@MCs through the interfacial thiol−ene photopolymerization. Through changing the concentration of HAuCl4 or replacing HAuCl4 with AgNO3 or H2PtCl6, microcapsules decorated with AgNP, PtNP, and the smaller AuNP can be obtained. As for all MNP@MCs, the formulation, size in diameter and thickness of the wall for the microcapsule, and size of MNP were summarized in Table 1. As shown in Figure 4, both the representative SEM and TEM images show that the typical morphology of the microcapsule is covered with the nanoparticles and all metal nanoparticles are fixed on the D
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Figure 4. (a) SEM image and (b) TEM image of the microcapsule containing metal nanoparticles [email protected], [email protected], and PtNP@ MC-0.5, respectively. (c) TEM image and (d) size distribution histogram and photograph of MNPs decorated on [email protected], [email protected], and [email protected], respectively.
Figure 5. (a) UV−vis absorption spectra of the CF3Nip solution as a function of time during the reduction catalyzed by AuNP@MC-1 at 25 °C. (b) Time-conversion plot of the reduction of Nip and CF3NiP catalyzed by AuNP@MC-1 at 25 °C. (c) Proposed scheme of the reduction of Nip and CF3NiP by NaBH4 in the presence of AuNP@MC-1. (d) Recycle experiments of the AuNP@MC-1 catalyst for the reduction of CF3NiP.
surface of microcapsules. The diameter of AuNP and the thickness of the wall for [email protected] are 5.9 ± 0.3 and
about 50 nm, respectively. In comparison to AuNP@MC-1, the less and smaller AuNP for [email protected] should be ascribed E
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
and 0.25 min−1, respectively. The higher kapp for CF3Nip might be ascribed to the encapsulation of the hydrophobic CF3Nip by AuNP@MC-1 in aqueous solution. As proposed in Figure 5c, the encapsulation of CF3Nip can increase the local concentration of CF3Nip and accelerate its adsorption on the surface of AuNPs, resulting in the enhancement of the reduction speed. The reusability of AuNP@MC-1 as a microreactor was also evaluated. As shown in Figure 5d, AuNP@MC-1 could be recovered by simple centrifugation and kept the good catalytic activity after 6 cycles. The excellent catalytic stability might be explained by the fact that AuNPs are well-decorated in the wall of the microcapsule, which can decrease their aggregation during reaction.
to the low concentration of HAuCl4 in preparation, which is also reflected by the thinner wall of [email protected]. This indicated that the size and amount of AuNP on the surface of the microcapsule might be tunable by this strategy. The solutions of AuNP@MC-1 and [email protected] are wine-red and purple-red, respectively, suggesting the different sizes of AuNP (see Figure S1 of the Supporting Information). As for [email protected], the diameter of AgNP and the thickness of the wall are 13 ± 0.9 and 45 nm, respectively. Upon exposure of UV light, the solution of [email protected] became orange and exhibited the characteristic SPR of Ag nanoparticles, suggesting the generation of AgNP (Figure 4 and Figure S2 of the Supporting Information). The size of PtNP in diameter and the thickness of the wall for [email protected] are 3.1 ± 0.1 and 40 nm, respectively. Because of the weak SPR of PtNP, the color of [email protected] solution is yellow and does not change obviously with the irradiation of UV light (see Figure S3 of the Supporting Information). The size dependence of MNPs upon the different metals might be ascribed to the difference in the ability of the sulfur radicals at the interface to reduce different metal ions. The other factor might be that the diffusion of metal ions from the water phase to the interface is dependent upon the type of metal ions or the numbers of MNPs in each capsule. These factors might mainly cause the different sizes of various metal nanoparticles. X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) experiments were also carried out to determine the chemical composition of the obtained MNP@MCs (see Figures S4 and S5 of the Supporting Information). The signals assigned to Au, Ag, and Pt observed in the XPS spectra of [email protected], [email protected], and [email protected] confirm the presence of metal nanoparticles. In comparison to AuNP@ MC-0.5, the higher weight retention in the TGA curve of AuNP@MC-1 suggested the high content of Au nanoparticles, which is in good agreement with the observation in SEM and TEM images. Because of the hydrophilic outer layer of PEG, the obtained MNP@MC can be well-dispersed in aqueous solution. For example, no obvious aggregation occurred in AuNP@MC-1 aqueous solution when it was kept for 1 month at room temperature. Motivated by these novel characteristics of MNP@MCs, such as the facile preparation of one step, amphiphilicity, and hybrid microstructure with nanoscale nanoparticles, we tried to use AuNP@MC-1 as a catalyst because of its combination of the catalysis property of metal nanoparticles and the encapsulation of the microcapsule.39 To test the catalysis performance of AuNP@MC-1, the reduction of nitrophenol was chosen as the model reaction.40−42 UV−vis spectra were used to monitor the reduction of 4-nitrophenol (Nip) and the more hydrophobic 4-nitro-3-(trifluoromethyl)phenol (CF3Nip) in the presence of NaBH4. As shown in Figure 5a, the strong absorption peak of CF3Nip at 394 nm decreased gradually after adding AuNP@MC-1, while simultaneously, a new peak appeared at 317 nm, which should be assigned to the product 4-amino-3-(trifluoromethyl)phenol. Because the concentration of sodium borohydride largely exceeds the concentration of nitrophenol, first-order rate kinetics with regard to the nitrophenol concentration could be used to evaluate the catalytic rate.41 The conversion of the catalytic reduction process can be obtained by the ratio of the respective absorption A/A0, and the apparent rate constant kapp is calculated from the linear plot of ln(A/A0) versus reduction time. As shown in Figure 5b, kapp for CF3Nip and Nip are 0.29
■
CONCLUSION We demonstrated a one-step strategy to prepare the noble metal nanoparticle-decorated microcapsules (MNP@MCs) through the interfacial thiol−ene photopolymerization. The oil-in-water (O/W) emulsion droplets can serve as templates and guide the thiol−ene photopolymerization and photoreduction on their surface, resulting in the formation of MNP@ MCs. The obtained MNP@MCs were covered with the metal nanoparticles, whose size and density increased with the increasing concentration of the metal precursor in water. Because of the synergistic effect of the encapsulation of MC and the catalytic activity of AuNP, AuNP@MC exhibited enhanced catalytic activity and good stability in the reduction of hydrophobic nitrophenol. It is believed that this robust, convenient, and simple strategy based on the one-step interfacial thiol−ene photopolymerization will provide an important alternative to fabricate the functional metal nanoparticle-modified microcapsules.
■
EXPERIMENTAL SECTION
Preparation of the Hybrid Microcapsule-Containing Metal Nanoparticles (MC@MNPs). The photoinitiators I907 (1% of TMPTA and PTPS) and ITX (0.5% of TMPTA and PTPS) were dissolved in a 200 mg mixture of PTPS, TMPTA, and toluene (w/w/w = 1:0.85:48) in a 25 mL vial. Then, 20 mL of the designed concentration of HAuCl4·4H2O (or AgNO3 and H2PtCl6·6H2O) aqueous solution was dropped into the mixture with stirring. After that, the solution was stirred for 1 h, followed by ultrasonic for 1 h to obtain a stable milky emulsion. The obtained emulsion was exposed under an UV light-emitting diode (LED) lamp at 365 nm (the intensity is about 8.4 mW/cm2) with constant stirring (400 rpm). After fully photo-cross-linking and complete reduction, the hybrid microcapsule-containing gold nanoparticles were obtained by centrifugation/redispersion in THF 3 times. The number and size of the obtained MNP@MCs are summarized in Table 1. Catalytic Experiment of AuNP@MC-1. At room temperature, 1.0 mL of NaBH4 solution (30 mmol/L) was added to 1.75 mL of Nip or CF3Nip solution (0.17 mmol/L) that was contained in a glass cuvette. Then, 0.2 mL of AuNP@MCs solution ([Au] = 2.4 × 10−3 mol/L) was added, and UV−vis spectra of the sample were taken every 2 min in the range of 250−550 nm. The rate constant of the reaction was determined by measuring the change of the absorption peak intensity at 400 nm with time. In the catalyst recycle studies, the UV−vis spectroscopy was taken to make sure that all CF3Nip had been consumed, thus signaling that the first cycle was over. After that, the previous aqueous solution was centrifuged to obtain the used AuNP@MC-1, and the catalyst was then washed with deionized water, followed by vacuum drying. The obtained AuNP@MC-1 from the first cycle was used for the second catalytic reduction cycle. Similarly, we repeated the recycle 6 times. Measurements. FTIR spectra measurements were carried out with a Spectrum 100 Fourier transformation infrared absorption F
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Insulation for their financial support. Xuesong Jiang is supported by the NCET-12-3050 Project.
spectrometer (PerkinElmer, Inc., Waltham, MA). The samples were prepared by dropping the polymer solution onto a KBr film and dried below an infrared lamp. The SEM images were obtained using a NOVA NanoSEM 230 (FEI Company) field emission scanning electron microscope operated at an acceleration voltage of 5 kV. The samples were prepared by dropping the MNP@MCs solution onto silica wafers and air-drying the silica wafers at room temperature. Then, the samples were sputter-coated with gold before examination. The TEM images were obtained using a JEM-2100 (JEOL, Ltd., Japan) transmission electron microscope operated at an acceleration voltage of 200 kV. The sample was prepared by dropping MNP@MCs solution onto copper grids coated with a thin carbon film, while the excess solution was removed by filter paper and air-dried at room temperature. AFM images were taken by SII Nanonavi E-sweep under ambient conditions. AFM was operated in tapping mode using silicon cantilevers with a force constant of 40 N m−1. The samples were prepared by dropping the MNP@MCs solution onto a mica sheet. The DLS measurements were performed in the PTPST microcapsules solution using a ZS90 Zetasizer Nano ZS instrument (Malvern Instruments, Ltd., U.K.) equipped with a multi-τ digital time correlation and a 4 mW He−Ne laser (λ = 633 nm) at an angle of 90°. Regularized Laplace inversion (CONTIN algorithm) was applied to analyze the obtained autocorrelation functions. The concentration of MNP@MCs solution was 1 g/L. XPS experiments were conducted on a PHI-5000C ESCA system (PerkinElmer) with Al Kα radiation (hν = 1486.6 eV). The X-ray anode was run at 250 W, and the high voltage was 14.0 kV, with a detection angle at 54°. To ensure the sufficient sensitivity, the pass energy was fixed at 46.95 eV. The base pressure of the analyzer chamber was about 5 × 10−8 Pa. The sample was pressed to a selfsupported disk (10 × 10 mm), mounted on a sample holder, and transferred to the analyzer chamber. The whole spectra (0−700 eV) of all elements were recorded with high resolution. TGA was carried out on a TA Q5000IR thermogravimetric analyzer under nitrogen with the temperature range of 40−800 °C at a heating rate of 20 °C min−1. The UV−vis spectra of MNP@MCs were carried out with a UV2550 spectrophotometer (Shimadzu, Japan).
■
■
(1) Alvarez-Paino, M.; Marcelo, G.; Munoz-Bonilla, A.; FernandezGarcia, M. Catecholic chemistry to obtain recyclable and reusable hybrid polymeric particles as catalytic systems. Macromolecules 2013, 46 (8), 2951−2962. (2) He, J.; Liu, Y.; Hood, T. C.; Zhang, P.; Gong, J.; Nie, Z. Asymmetric organic/metal(oxide) hybrid nanoparticles: Synthesis and applications. Nanoscale 2013, 5 (12), 5151−5166. (3) Li, X.; Guo, J.; Asong, J.; Wolfert, M. A.; Boons, G.-J. Multifunctional surface modification of gold-stabilized nanoparticles by bioorthogonal reactions. J. Am. Chem. Soc. 2011, 133 (29), 11147− 11153. (4) Song, J.; Zhou, J.; Duan, H. Self-Assembled plasmonic vesicles of SERS-encoded amphiphilic gold nanoparticles for cancer cell targeting and traceable intracellular drug delivery. J. Am. Chem. Soc. 2012, 134 (32), 13458−13469. (5) Samanta, B.; Yang, X. C.; Ofir, Y.; Park, M.-H.; Patra, D.; Agasti, S. S.; Miranda, O. R.; Mo, Z.-H.; Rotello, V. M. Catalytic microcapsules assembled from enzyme−nanoparticle conjugates at oil−water interfaces. Angew. Chem., Int. Ed. 2009, 48 (29), 5341−5344. (6) Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. Plasmonic vesicles of amphiphilic gold nanocrystals: Self-assembly and external-stimuli-triggered destruction. J. Am. Chem. Soc. 2011, 133 (28), 10760−10763. (7) Chen, L.; Hu, J.; Qi, Z.; Fang, Y.; Richards, R. Gold sanoparticles intercalated into the walls of mesoporous silica as a versatile redox catalyst. Ind. Eng. Chem. Res. 2011, 50 (24), 13642−13649. (8) Laudenslager, M. J.; Schiffman, J. D.; Schauer, C. L. Carboxymethyl shitosan as a matrix material for platinum, gold, and silver nanoparticles. Biomacromolecules 2008, 9 (10), 2682−2685. (9) Han, J.; Liu, Y.; Guo, R. Facile synthesis of highly stable gold nanoparticles and their unexpected excellent catalytic activity for Suzuki−Miyaura cross-coupling reaction in water. J. Am. Chem. Soc. 2009, 131 (6), 2060−2061. (10) Brust, M.; Gordillo, G. J. Electrocatalytic hydrogen redox chemistry on gold nanoparticles. J. Am. Chem. Soc. 2012, 134 (7), 3318−3321. (11) Städler, B.; Price, A. D.; Zelikin, A. N. A critical look at multilayered polymer capsules in biomedicine: Drug carriers, artificial organelles, and cell mimics. Adv. Funct. Mater. 2011, 21 (1), 14−28. (12) Becker, A. L.; Johnston, A. P. R.; Caruso, F. Layer-by-layerassembled capsules and films for therapeutic delivery. Small 2010, 6 (17), 1836−1852. (13) Esser-Kahn, A. P.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Triggered release from polymer capsules. Macromolecules 2011, 44 (14), 5539−5553. (14) Xu, H.; Xu, J.; Jiang, X.; Zhu, Z.; Rao, J.; Yin, J.; Wu, T.; Liu, H.; Liu, S. Thermosensitive unimolecular micelles surface-decorated with gold nanoparticles of tunable spatial distribution. Chem. Mater. 2007, 19 (10), 2489−2494. (15) Zhang, Q.; Xie, J.; Yang, J.; Lee, J. Y. Monodisperse icosahedral Ag, Au, and Pd nanoparticles: Size control strategy and superlattice formation. ACS Nano 2008, 3 (1), 139−148. (16) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. Toward functional nanocomposites: Taking the best of nanoparticles, polymers, and small molecules. Chem. Soc. Rev. 2013, 42 (7), 2654− 2678. (17) Selvan, T.; Spatz, J. P.; Klok, H.-A.; Möller, M. Gold− polypyrrole core−shell particles in diblock copolymer micelles. Adv. Mater. 1998, 10 (2), 132−134. (18) Pradhan, S.; Sun, J.; Deng, F.; Chen, S. Single-electron transfer in nanoparticle solids. Adv. Mater. 2006, 18 (24), 3279−3283. (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seeding growth for size control of 5−40 nm diameter gold nanoparticles. Langmuir 2001, 17 (22), 6782−6786.
ASSOCIATED CONTENT
S Supporting Information *
Materials and results of UV−vis spectrum of [email protected] aqueous solution (Figure S1), UV−vis spectrum of AgNP@ MC-0.5 aqueous solution (Figure S2), UV−vis spectrum of [email protected] aqueous solution (Figure S3), XPS of AuNP@ MC-0.5 and [email protected] (Figure S4), and TGA curves of AuNP@MC-1, [email protected], [email protected], and PtNP@ MC-0.5, which were conducted under nitrogen with a heating rate of 20 °C min−1 (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-21-54743268. Fax: +86-21-54747445. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of China (21174085, 21274088, and 51373098), the Education Commission of Shanghai Municipal Government (12ZZ020), and the Shanghai Key Laboratory of Polymer and Electrical G
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
(20) Hu, J.; Wu, T.; Zhang, G.; Liu, S. Effcient synthesis of single gold nanoparticle hybrid amphiphilic triblock copolymers and their controlled self-assembly. J. Am. Chem. Soc. 2012, 134 (18), 7624− 7627. (21) Mieszawska, A. J.; Mulder, W. J. M.; Fayad, Z. A.; Cormode, D. P. Multifunctional gold nanoparticles for diagnosis and therapy of disease. Mol. Pharmaceutics 2013, 10 (3), 831−847. (22) Mercato, L. L.; Gonzalez, E.; Abbasi, A. Z.; Parak, W. J.; Puntes, V. Synthesis and evaluation of gold nanoparticle-modified polyelectrolyte capsules under microwave irradiation for remotely controlled release for cargo. J. Mater. Chem. 2011, 21 (31), 11468−11471. (23) Hickey, R. J.; Haynes, A. S.; Kikkawa, J. M.; Park, S.-J. Controlling the self-assembly structure of magnetic nanoparticles and amphiphilic block-copolymers: From micelles to vesicles. J. Am. Chem. Soc. 2011, 133 (5), 1517−1525. (24) Abbasi, A. Z.; Gutiérrez, L.; Mercato, L. L.; Herranz, F.; Chubykalo-Fesenko, O.; Veintemillas-Verdaguer, S.; Parak, W. J.; Morales, M. P.; González, J. M.; Hernando, A.; Presa, P. Magnetic capsules for NMR imaging: Effect of magnetic nanoparticles spatial distribution and aggregation. J. Phys. Chem. C 2011, 115 (14), 6257− 6264. (25) Anandhakumar, S.; Vijayalakshmi, S. P.; Jagadeesh, G.; Raichur, A. M. Silver nanoparticle synthesis: Novel route for laser triggering of polyelectrolyte capsules. ACS Appl. Mater. Interfaces 2011, 3 (9), 3419−3424. (26) Li, Y.; Smith, A. E.; Lokitz, B. S.; McCormick, C. L. In situ formation of gold-“decorated” vesicles from a RAFT-synthesized, thermally responsive block copolymer. Macromolecules 2007, 40 (24), 8524−8526. (27) Tian, J.; Yuan, L.; Zhang, M.; Zheng, F.; Xiong, Q.; Zhao, H. Interface-directed self-assembly of gold nanoparticles and fabrication of hybrid hollow capsules by interfacial cross-linking polymerization. Langmuir 2012, 28 (25), 9365−9371. (28) Ochs, M.; Carregal-Romero, S.; Rejman, J.; Braeckmans, K.; De Smedt, S. C.; Parak, W. J. Light-addressable capsules as caged compound matrix for controlled triggering of cytosolic reactions. Angew. Chem., Int. Ed. 2013, 52 (2), 695−699. (29) Kreft, O.; Skirtach, A. G.; Sukhorukov, G. B.; Möhwald, H. Remote control of bioreactions in multicompartment capsules. Adv. Mater. 2007, 19 (20), 3142−3145. (30) Kozlovskaya, V.; Kharlampieva, E.; Chang, S.; Muhlbauer, R.; Tsukruk, V. V. pH-Responsive layered hydrogel microcapsules as gold nanoreactors. Chem. Mater. 2009, 21 (10), 2158−2167. (31) Angelatos, A. S.; Radt, B.; Caruso, F. Light-responsive polyelectrolyte/gold nanoparticle microcapsules. J. Phys. Chem. B 2005, 109 (7), 3071−3076. (32) De Geest, B. G.; Skirtach, A. G.; De Beer, T. R. M.; Sukhorukov, G. B.; Bracke, L.; Baeyens, W. R. G.; Demeester, J.; De Smedt, S. C. Stimuli−responsive multilayered hybrid nanoparticle/polyelectrolyte capsules. Macromol. Rapid Commun. 2007, 28 (1), 88−95. (33) McGilvray, K. L.; Decan, M. R.; Wang, D.; Scaiano, J. C. Facile photochemical synthesis of unprotected aqueous gold nanoparticles. J. Am. Chem. Soc. 2006, 128, 15980−15981. (34) Marin, M. L.; McGilvray, K. L.; Scaiano, J. C. Photochemical strategies for the synthesis of gold nanoparticles from Au(III) and Au(I) using photoinduced free radical generation. J. Am. Chem. Soc. 2008, 130, 16572−16584. (35) Liu, D. D.; Yu, B.; Jiang, X. S.; Yin, J. Responsive hybrid microcapsules by the one-step interfacial thiol−ene photopolymerization. Langmuir 2013, 29 (17), 5307−5314. (36) Yu, B.; Jiang, X. S.; Qin, N.; Yin, J. Thiol−ene photocrosslinked hybrid vesicles from co-assembly of POSS and poly(ether amine) (PEA). Chem. Commun. 2011, 47 (44), 12110−12112. (37) Galian, R. E.; Guardia, M.; Pérez-Prieto, J. Size reduction of CdSe/ZnS core−shell quantum dots photosensitized by benzophenone: Where does the Cd(0) go? Langmuir 2011, 27 (5), 1942−1945. (38) Consuelo Cuquerella, M.; Pocoví-Martínez, S.; Pérez-Prieto, J. Photocatalytic coalescence of functionalized gold nanoparticles. Langmuir 2009, 26 (3), 1548−1550.
(39) Wang, R.; Jiang, X. S.; Yu, B.; Yin, J. Stimuli−responsive microgels formed by hyperbranched poly(ether amine) decorated with platinum nanoparticles. Soft Matter 2011, 7 (18), 8619−8627. (40) Xia, B.; He, F.; Li, L. Preparation of bimetallic nanoparticles using a facile green synthesis method and their application. Langmuir 2013, 29 (15), 4901−4907. (41) Baruah, B.; Gabriel, G. J.; Akbashev, M. J.; Booher, M. E. Facile synthesis of silver nanoparticles stabilized by cationic polynorbornenes and their catalytic activity in 4-nitrophenol reduction. Langmuir 2013, 29 (13), 4225−4234. (42) Dhar, J.; Patil, S. Self-assembly and catalytic activity of metal nanoparticles immobilized in polymer membrane prepared via layerby-layer approach. ACS Appl. Mater. Interface 2012, 4 (3), 1803−1812.
H
dx.doi.org/10.1021/la501531g | Langmuir XXXX, XXX, XXX−XXX
