Covalent Functionalization of Single-Walled Carbon Nanotubes with

May 17, 2012 - ... Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China ... made available by participants in Crossref's Cited-by Linking...
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Covalent Functionalization of Single-Walled Carbon Nanotubes with Thermoresponsive Core Cross-Linked Polymeric Micelles Yonggui Li,† Duanguang Yang,† Alex Adronov,*,§ Yong Gao,† Xujun Luo,† and Huaming Li*,†,‡ †

College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, and Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University, Xiangtan 411105, P. R. China § Department of Chemistry and the Brockhouse Institute for Materials Research (BIMR), McMaster University, Hamilton, Ontario, Canada L8S 4M1 ‡

S Supporting Information *

ABSTRACT: A facile method for the covalent functionalization of single-walled carbon nanotubes (SWNTs) with thermoresponsive core cross-linked (CCL) polymeric micelles is presented. This method is based on SWNT functionalization as well as polymer self-assembly. The copolymer, poly(N,Ndimethylacrylamide)-b-poly(N-isopropylacrylamide-co-N-acryloxysuccinimide) (PDMA-b-P(NIPAM-co-NAS)), bearing an azide group at the PDMA end, was first prepared by reversible addition−fragmentation chain transfer (RAFT) polymerization. Covalent functionalization of SWNTs with well-defined, azide-derivatized PDMA-b-P(NIPAM-co-NAS) was then accomplished by a nitrene addition reaction. The copolymer-functionalized SWNTs (f-SWNTs) consisted of about 26 wt % copolymer and exhibited relatively high solubility in water. Subsequently, the coassembly of the copolymer and f-SWNT blends was carried out in aqueous solution. It was found that the copolymer chains grafted onto the surface of SWNTs were coassembled with the free chains in solution, leading to thermoresponsive polymeric micelles that adhered to the surface of nanotubes. Upon cross-linking, the copolymer aggregates were stabilized and covalently anchored to SWNTs (SWNT-micelle). The resulting assembled nanostructures were still soluble in water and were characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM).

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INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima,1 CNTs have been the subject of intense interest because of their unique structure, exceptional thermal stability, and remarkable mechanical and electronic properties.2−4 These lead to a myriad of actual and anticipated applications in areas such as molecular electronics,5 sensors,6 field-emission devices,7 biological systems,8 solar cell,9 and high-performance composites.10 However, the inherent insolubility of CNTs in all solvents due to the strong van der Waals interaction between nanotubes hinders their solution phase processing and manipulation,11 as is required for many applications. Recently, two main approaches for the functionalization of CNTs, including covalent sidewall coupling reactions12−18 and noncovalent exohedral interactions,19−22 have been developed to overcome the solubility limitations. Among these approaches, modification of CNTs with polymeric structures has shown promise in improving the solubility of nanotubes.23−25 Furthermore, the versatility of polymer chemistry allows for control over the final properties of the functionalized tubes, which are dictated by the chemical and physical characteristics of the polymeric modifier. For example, the attachment of functional polymers with controlled architectures may provide opportunities for the preparation of hybrid materials capable of © 2012 American Chemical Society

environmental responsiveness and, potentially, supramolecular organization. So far, the polymeric modifiers used in the covalent and noncovalent functionalization of CNTs include linear,14,15,21,22,26 star-shaped,27 dendronized,28 dendrimerized,29 hyperbranched,30 and other architectural polymers.31 Recently, polymeric aggregates were also used in the noncovalent functionalization of CNTs. Current methods for the preparation of CNT−aggregate hybrids include “encapsulation” as well as “adsorption” methods. The former approach attempts to encapsulate the nanotubes within cross-linked polymeric micelles or vesicles. In this case, block copolymer molecules self-assemble around the nanotubes, generating aggregates in which the nanotubes are encapsulated after cross-linking.32−34 For example, block copolymers including PS-b-PAA,32 poly(ethylene glycol)-b-poly(acrylic acid)-b-polystyrene (PEG-bPAA-b-PS),33 PEG-b-polybutadiene,33 H-shaped copolymer based on poly(3-(trimethoxysilyl)propyl methacrylate), and PEG34 have been successfully used to encapsulate CNTs. On the other hand, polymeric aggregates such as spherical micelles Received: March 2, 2012 Revised: May 3, 2012 Published: May 17, 2012 4698

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can be directly absorbed onto the surface of CNTs.35−39 For example, amphiphilic block copolymer micelles physically adhered to the surface of CNTs have been demonstrated, both theoretically35−37 and experimentally,38,39 based on Pluronic block copolymers and polystyrene-b-poly(4-vinylpyridine). Additionally, the absorption of positively charged poly(methyl methacrylate) particles onto the surface of negatively charged CNTs has also been reported.38 In these cases, micelles have played a particularly important role in solubilization of CNTs due to their large loading capacity. Considering that block copolymer micelles are potential carriers for drugs, flavorings, dyes, and other active agents,40,41 this functionalization strategy can potentially endow new properties that will lead to new applications for the CNTs. However, the stability of the formed micelles as well as CNT− micelle hybrids is an important issue in their practical applications since the assembly process is fully reversible in most cases. To address the stability issue in polymeric micelles, two strategies, including core cross-linking (CCL)42−45 and shell cross-linking (SCL),46−49 have been developed, leading to structurally stable nanostructures. Therefore, in order to obtain a stable and biocompatible CNT−micelle system, we have prepared CNT−micelle hybrids through covalent attachment of cross-linked polymeric micelles to the surface of CNTs. This method is based on the covalent functionalization of CNTs with thermoresponsive copolymer, (poly(N,N-dimethylacrylamide)-b-poly(N-isopropylacrylamideco-N-acryloxysuccinimide) (PDMA-b-P(NIPAM-co-NAS)). The copolymer-functionalized single-walled carbon nanotubes (f-SWNTs) and the free copolymers were then coassembled in aqueous solution. Upon cross-linking, the thermoresponsive cross-linked polymeric micelles were covalently anchored onto the external walls of SWNT. To our knowledge, there are no literature reports describing the covalent functionalization of SWNTs with thermoresponsive CCL polymeric micelles. This copolymer, containing both a permanently hydrophilic DMA block and a “smart” NIPAM-co-NAS block, is capable of reversibly forming micelles in response to changes in temperature, thereby exhibiting thermoresponsive solubility. The transition temperature and the diblock copolymer micellar size can be controlled by adjusting the NIPAM-co-NAS block length or the polymer architecture.50 Copolymers with an appropriate NIPAM-co-NAS block length and/or an appropriate NAS content exhibit a transition temperature near human body temperature, showing potential applications in biotechnology and medicine.51−53 In addition, it can be used to prepare thermoreversible separators, thermoresponsive soft actuators, automatic gel valves, and smart, reusable catalysts.54−58 Therefore, nanotubes will be endowed with new properties and new potential applications if the thermosensitive cross-linked polymeric micelles are covalently attached onto their surface.

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recrystallized twice from methanol. All other reagents were used as received unless otherwise stated. Measurements. Gel permeation chromatography (GPC) measurements were performed on a Waters 1515 GPC setup equipped with a Waters 2414 differential refractive index detector in DMF at 45 °C with a flow rate of 1.0 mL/min. Narrow polydispersity polystyrene standards in the range of 0.5−1000.0 kDa were used for calibration. 1H NMR spectra were recorded with a 400 MHz Bruker AV-400 NMR spectrometer, and chemical shifts were recorded in ppm units with TMS as the internal standard. Dynamic light scattering (DLS) studies were performed on a BI-200SM Brookhaven instrument equipped with a 100 mW adjustable solid-state laser emitting at 532 nm, a BI200SM goniometer, and a BI-9000 digital correlator. The laser was adjusted to 43 mW. A BI-TCD temperature controller was utilized to precisely adjust the solution temperature. The light scattering intensity and hydrodynamic diameter were recorded at an angle of 90°. To ensure that DLS measurements were not affected by dust, all sample solutions were filtered through 200 nm filters. The solutions were heated in steps and stabilized for 15 min before recording data. Distribution averages and particle size distributions were computed using cumulants analysis and CONTIN routines. All data were averaged over three measurements. Fourier transform infrared spectroscopy (FTIR) spectra in KBr pellets were recorded on a PE Spectrum One FTIR spectrophotometer. Laser Raman spectroscopy was performed on a LabRam-1B micro-Raman spectrometer using an Ar laser with 632.8 nm excitation at room temperature. Thermogravimetric analysis (TGA) was carried out on a STA 449C instrument with a heating rate of 5 °C/min under a nitrogen flow rate of 60 mL/ min. Ultraviolet−visible (UV−vis) spectra were measured using a PE Lambda 20 spectrophotometer. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-3010 electron microscope and JEOL 100CX operating at 100 kV. Samples for TEM observations were prepared by placing 10 μL of the sample at a concentration of 0.1 g/L on copper grids coated with thin films of Formvar and carbon successively. No staining was required. Atomic force microscopy (AFM) images were collected in tapping mode using a SI-DF 20 silicon microcantilever (spring constant 16 N/m and resonance frequency 138 kHz) (Nano Navi SII) on a Dimension 3000 scanning probe microscope (Digital Instruments). Preparation of Micelles and CCL Micelles. Three PDMA-bP(NIPAM-co-NAS) copolymers, P2−P4, with different molecular

Table 1. Molecular Weight and Polydispersity Data for Polymers sample

polymer

Mn (g/mol)

Mw/Mn

P1 P2 P3 P4

DMA160 DMA160-b-(NIPAM0.89-co-NAS0.11)183 DMA160-b-(NIPAM0.84-co-NAS0.16)122 DMA160-b-(NIPAM0.68-co-NAS0.32)144

16 000 38 000 31 000 35 000

1.06 1.35 1.33 1.35

weights and block ratios (Table 1) were prepared as shown in the Supporting Information. P2 (3.0 mg) was dissolved in double distilled water (3.0 mL) at room temperature, and the solution was filtered through a 200 nm filter. A 10.0 mL vial containing 2.0 mL of this solution was placed in a 50 °C water bath. The solution was equilibrated for 30 min to allow the formation of uniform micelles. At the same time, a solution of ethylenediamine (51 μg) in 3.0 mL of double distilled water was placed in a water bath at 50 °C and allowed to equilibrate for 30 min. Then 2.0 mL of the ethylenediamine solution was added to the diblock copolymer solution. The final copolymer concentration was 0.5 mg/mL, and the ethylenediamine/ NAS molar ratio was kept at 1:2. The solution was stirred at 50 °C for 5 h. Functionalization of SWNTs with Thermoresponsive Diblock Copolymer. In a typical experiment, pristine SWNTs (10.0 mg) and N-methyl-2-pyrrolidinone (NMP, 5.0 mL) were placed in a 25 mL flask fitted with a condenser. The mixture was sonicated in an

EXPERIMENTAL SECTION

Materials. Single-walled carbon nanotubes (SWNTs, 90%) were purchased from the Chengdu Organic Chemicals Co., Chinese Academy of Science. N,N-Dimethylacrylamide (DMA) and Nisopropylacrylamide (NIPAM) were purchased from Alfa-Aesar. NAcryloxysuccinimide (NAS) monomer was synthesized according to a literature procedure59 involving direct esterification of the Nhydroxysuccinimide (NHS) with acryloyl chloride in the presence of triethylamine (TEA). 2-Azidoethyl-2-(ethylthiocarbonothioylthio)-2methylpropanoate (EMP-N3) were prepared according to procedures outlined in the literature.18 2,2′-Azoisobutyronitrile (AIBN) was 4699

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to hydrolysis.61 In order to fabricate CCL micelles, NAS was therefore incorporated into the NIPAM block. The copolymerization of NAS with other vinyl monomers via RAFT has already been reported although the homopolymerization of NAS by RAFT is uncontrolled.61−63 Such polymer precursors have also been used as active sites in the fabrication of crosslinked micelles.64 Our results for the RAFT copolymerization of NIPAM with NAS in dioxane showed slightly high polydispersity index (i.e., below 1.35). To obtain the copolymer compositions, the NAS moieties in the copolymer were reacted with excess amounts of n-butylamine in aqueous solution. After the reaction was completed (ca. 90 min), the UV−vis absorbance of the produced N-hydroxysuccinimide (NHS) anion was measured at 259 nm.65 A calibration curve was constructed by measuring the absorbance of NHS standard solutions containing n-butylamine. The NAS content in the copolymer was calculated (Table 1) using the calibration curve (Figure S1 in the Supporting Information). Although the NAS homopolymer is hydrophobic, results of our preliminary experiments indicate that P(NIPAM-co-NAS) remains watersoluble if the NAS content is below 25%. In the current case, all copolymers except P4 in Table 1 are sufficiently hydrophilic to dissolve molecularly in cold water, forming homogeneous aqueous solutions. Functionalization of SWNTs with Thermoresponsive Diblock Copolymer. In the present study, functionalization of SWNTs with thermoresponsive diblock copolymers was accomplished by nitrene chemistry.15,66−68 As mentioned previously, all the synthesized copolymers except P4 dissolve molecularly in cold water. Therefore, the thermoresponsive diblock copolymer with the lowest NAS content (P2) was chosen to modify SWNTs in order to obtain high water solubility of the resulting polymer functionalized SWNTs. The [2 + 1] cycloaddition reaction between azide-terminated diblock copolymer and SWNTs was performed in NMP at 160 °C, resulting in the formation of f-SWNTs as shown in Scheme 1. Before carrying out the coupling reaction, nitrogen

ultrasonic bath (40 kHz) for 1 h, and then N2 was bubbled through it for 50 min. The suspension was heated to 160 °C, and deaerated P2 (2.0 g in 5.0 mL of NMP) was added dropwise. The reaction mixture was maintained at 160 °C in a nitrogen atmosphere under constant stirring for 60 h. After cooling down to room temperature, the product was vacuum filtered on a 0.20 μm Teflon membrane. The obtained SWNTs were then subjected to dispersion by sonication and rinsed five times (50 mL each) with DMF and acetone, respectively, until no white floccules appeared when the filtrate was dropped into diethyl ether. The resultant SWNTs were dried under vacuum at room temperature overnight. The SWNTs functionalized with the thermoresponsive diblock copolymers are designated f-SWNTs. The amount of dried f-SWNTs obtained by this procedure was 11.5 mg. Functionalization of SWNTs with Thermoresponsive CCL Micelles. A saturated aqueous solution of f-SWNTs (f-SWNTs concentration greater than 2.0 mg/mL) was sonicated for 10 min at 40 kHz and then centrifuged at 3000g for 20 min, followed by standing undisturbed overnight prior to UV−vis absorption measurements, which were performed on the supernatant of the resulting sample. A specific extinction coefficient of 0.0103 L mg−1 cm−1 was used to estimate the nanotube concentrations.60 P2 was dissolved in water at a concentration of 5.0 mg/mL (25 °C). The aqueous solution containing P2 (1.0 mL, 5.0 mg/mL), the f-SWNTs solution (1.0 mL, 50 mg/L), and water (5.0 mL) was heated to 50 °C. After equilibration for 30 min at 50 °C, 3.0 mL of an aqueous solution of ethylenediamine (17 mg/L) preheated to 50 °C was injected within 30 min. The ethylenediamine/NAS molar ratio was kept constant at 1:2. The mixture was stirred at 50 °C for 5 h to stabilize the micelles on the surface of f-SWNTs. After cooling to room temperature, the micellefunctionalized SWNTs were separated from free micelles by centrifugation. The suspension was subjected to high-speed centrifugation at 7000g for 30 min. The supernatant was discarded, and the same volume of deionized water was added to the pellet. This procedure was repeated five times. The resulting purified SWNT− micelle hybrids were redispersed in pure water with minimal sonication. In a different set of experiments the copolymer/fSWNTs ratio was modified by keeping the concentration of the fSWNTs constant (5.0 mg/L) while varying the copolymer concentration (0.5, 1.0, and 5.0 mg/mL) and keeping the concentration of the copolymer constant (0.5 mg/mL) while varying the final f-SWNTs concentration (5.0, 15, and 25 mg/L). As a control experiment, the concentration of P2 and pristine SWNTs aqueous suspension was maintained at 0.5 and 15 mg/L, respectively. The temperature of the mixed suspension containing the two components was raised above 50 °C. After the cross-linking reaction, the resulting suspension was allowed to cool to room temperature and subjected to TEM observation.

Scheme 1. Covalent Functionalization of SWNTs with PDMA-b-P(NIPAM-co-NAS) by Nitrene Chemistry

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RESULTS AND DISCUSSION Synthesis of Thermoresponsive Diblock Copolymer. The general approach employed for the preparation of PDMAb-P(NIPAM-co-NAS) diblock copolymers is shown in the Supporting Information. A series of azide-terminated diblock copolymers bearing an azide group at the PDMA end were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization of the corresponding monomers in dioxane at 60 °C using 2-azidoethyl-2-(ethylthiocarbonothioylthio)-2-methylpropanoate (EMP-N3) as the RAFT agent and 2,2′-azobis(isobutyronitrile) (AIBN) as the free radical initiator. Azide decorated RAFT agent was prepared by coupling 2-(ethylthiocarbonothioylthio)-2-methylpropanoic acid with 2-azidoethanol in the presence of 1,3-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine in dry CH2Cl2.18 All polymerizations proceeded with control, as evidenced by relatively low product polydispersity (Table 1). NAS was chosen as the active monomer due to its enhanced activity toward primary amines and relatively low susceptibility

bubbling for several minutes is necessary to prevent the highly reactive intermediates from reacting with oxygen.69 Since the pristine SWNTs dispersed poorly in NMP, it is necessary to sonicate the mixture in a bath sonicator for 1 h to achieve adequate dispersion. In addition, thermal activation of the azide precursor also yielded some azo and heterocyclic byproducts, causing the reaction solution to become dark brown. After reaction, the mixture was separated by ultrafiltration. The final f-SWNT product was obtained after repeated washing with DMF and acetone. Raman spectroscopy was utilized not only to verify the structural integrity of the modified SWNTs but also to gather information regarding the degree of nanotube functionalization. Figure 1 illustrates the Raman spectra of pristine SWNTs and fSWNTs. The typical Raman spectrum for SWNTs shows two 4700

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Figure 3. TGA curves for (a) pristine SWNTs, (b) f-SWNTs, and (c) P2.

Figure 1. Raman spectra of (a) pristine SWNTs and (b) f-SWNTs.

prominent bands around 1320 cm−1 (D band) and 1580 cm−1 (G band). The disorder mode band (D band) is related to the presence of defects in the nanotubes. The strong G band is associated with the vibration of sp2-bonded carbon atoms in a graphitic layer. The intensity ratio between Raman D and G bands, ID/IG, is typically taken as a measure of surface defects in SWNTs. As shown in Figure 1, the ID/IG ratio increases from 0.081 for the pristine SWNTs to 0.247 for f-SWNTs, which is an indication of the increase in the defects in the nanotube lattice after functionalization, suggesting that a large number of sp2-hybridized carbons have been converted to sp3-hybridization. This result is in good agreement with previous studies.70 FTIR spectroscopy provided information about the structures appended to the surface of the SWNTs, which is not available from the Raman data. Figure 2 depicts the FTIR

of impurities, suggesting that the thermal stability as well as the purity of the nanotubes was high. Interestingly, the copolymer showed a two-step thermal degradation, which might be caused by the loss of amide/ester bonds followed by backbone decomposition. Since the pure copolymer can be completely decomposed at a temperature of 500 °C, as indicated in Figure 3, the mass loss from the f-SWNT sample at 500 °C is used to estimate the amount of copolymer that is covalently attached to the SWNT surface. A mass loss of about 26 wt %, due to polymer decomposition, was observed to occur between temperatures of 300 and 500 °C for f-SWNTs. Functionalization of SWNTs with Thermoresponsive CCL Micelles. The primary motivation of this study is to prepare CCL polymeric micelle−SWNT conjugates. In order to fabricate CCL micelles with a thermoresponsive core from the PDMA-b-P(NIPAM-co-NAS) diblock copolymers, the thermally induced self-assembly behavior of these copolymers in dilute aqueous solution was initially studied. To obtain copolymer micelles, an aqueous solution of the P2 at a concentration of 0.5 mg/mL was heated from room temperature to its micellization temperature. The expected transition from molecularly dissolved unimers at low temperatures to aggregated micelles above a critical micelle temperature (CMT) was observed by DLS measurements. Above the CMT, the NIPAM-co-NAS segment became dehydrated due to an entropy gain resulting from the release of water molecules upon association of the isopropyl groups.71 Thus, micelles were formed in aqueous solution with a hydrophobic core and a hydrophilic shell. Furthermore, it is found that the CMT and the micelle diameter are sensitive to the concentration of the diblock copolymer. For example, the CMTs of the P2 aqueous solutions at concentration of 0.5, 1.0, 1.5, 2.5, and 5.0 mg/mL are approximately 28, 27, 27, 26, and 26 °C, respectively, while the hydrodynamic diameters (Dh) of the formed micelles are around 89, 82, 80, 76, and 75 nm, respectively (Figure 4a). As mentioned previously, the NAS monomer and its corresponding homopolymer are hydrophobic in nature. When NAS is copolymerized with NIPAM, the copolymer will remain thermoresponsive provided the NAS content is kept low. For example, P2 and P3 are sufficiently hydrophilic to dissolve molecularly in aqueous solution and show a CMT at 28 and 23 °C, respectively, at a concentration of 0.5 mg/mL as confirmed by DLS measurements (Figure S8). When the NAS content is increased further (i.e., P4), the diblock copolymer does not dissolve molecularly. Instead, a complex, micelle-like structure is formed. This behavior will be the subject of future

Figure 2. FTIR spectra of (a) pristine SWNTs, (b) f-SWNTs, and (c) P2. The inset shows a magnified view of the azide group on P2.

spectra of pristine SWNTs, f-SWNTs, and copolymer. Upon [2 + 1] cycloaddition reactions, the FTIR spectrum of the product (spectrum b) bears the characteristic signals for copolymer, indicating that polymer has been grafted. Additionally, the disappearance of the azide stretch at 2106 cm−1 after the coupling reaction indicates that the free polymer can be completely removed by ultrafiltration and prolonged washing with water. To gain a more quantitative picture of the extent of nanotube functionalization, TGA analysis was performed on the reaction product (Figure 3). The weight loss of pristine SWNTs was less than 1.0 wt % at 500 °C, which might arise from decomposition 4701

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slightly smaller than that of the non-cross-linked micelles. This was reasonable considering that the micelle core may contract somewhat upon cross-linking. AFM was utilized to image the CCL micelles. The sample was prepared by spin-coating CCL micellar aqueous solution onto freshly cleaved mica substrates at 2500 rpm. Figure 5 shows an AFM image of the CCL micelles prepared from P2. It can be seen that the micelles are relatively uniform, with diameters of ∼100 nm, which is slightly larger than observed by light scattering in aqueous solution at 25 °C (88 nm, as shown in Figure 4b). AFM analysis also shows that the height of the CCL micelles is only about 15 nm (Figure 5, three-dimensional image), indicating that some flattening of the partially swollen (loose) CCL micelles occurs when placed on the mica substrate. This clearly confirmed that the CCL was successful as un-cross-linked micelles would be expected to dissociate into unimers upon cooling, leading to the formation of a thin film on the mica upon coating. The successful fabrication of the thermoresponsive CCL micelles prompted us to develop a SWNTs functionalization strategy with CCL micelles through covalent attachment to their surface. As mentioned above, this methodology is based on covalent functionalization of SWNTs with a thermoresponsive diblock copolymer (i.e., preparation of f-SWNTs) together with coassembly of f-SWNTs and the identical copolymer in aqueous solution. The as-fabricated hybrid nanostructures could then be stabilized by cross-linking, leading to cross-linked polymeric micelles covalently bound to SWNTs as shown in Scheme 2. In the current study, an aqueous solution of f-SWNTs was prepared, and its concentration was evaluated spectrophotometrically. All samples were typically sonicated for 10 min and then centrifuged at 3000g for 20 min, followed by standing undisturbed overnight prior to UV−vis absorption measurements, which were performed on the supernatant of the resulting sample. A specific extinction coefficient of 0.0103 L mg−1 cm−1 was used to estimate the nanotube concentrations.60 In the first set of experiments, the concentration of f-SWNTs in aqueous solution was maintained at 5.0 mg/L, while the concentration of P2 was systematically varied from 0.5 to 5.0 mg/mL. The cross-linking reaction was carried out at 50 °C for 5 h using ethylenediamine as the crosslinker. AFM was again used to image the stabilized hybrid structures. As shown in Figure 6a, cross-linked micelles were indeed attached to the nanotube wall after adding P2 (0.5 mg/ mL) into the coassembly and cross-linking systems, indicating that the copolymer chains grafted onto the surface of SWNTs coassembled with the free chains in the solution, leading to

Figure 4. (a) Temperature dependence of average hydrodynamic diameter (D h ) obtained for P2 aqueous solution with the concentration of 0.5, 1.0, 1.5, 2.5, and 5.0 mg/mL. (b) Temperature dependence of the Dh and the scattered light intensity obtained for 0.5 mg/mL aqueous solution of CCL micelles prepared from P2.

studies; however, we focus here on P2 for subsequent investigations of thermoresponsive micellization with fSWNTs and CCL. Recently, McCormick et al.46 and Liu et al.45 have separately prepared SCL and CCL micelles starting from a NAScontaining triblock copolymer in the presence of a difunctional primary amine. In the present study, we employ ethylenediamine as a cross-linker to fabricate CCL micelles from P2. At 50 °C, this diblock copolymer forms P(NIPAM-co-NAS)-core micelles stabilized by PDMA coronas. Five hours after the addition of ethylenediamine, we can apparently tell that the core cross-linking is successful, as revealed by the fact that a bluish tinge persists after the micellar solution is cooled back to 20 °C (Figure S9). The formation of CCL micelles was also evident from the DLS analysis. As illustrated in Figure 4b, the scattered light intensity exhibited no appreciable changes in the temperature range of 15−55 °C. This clearly confirmed that the CCL was successful. Otherwise, the micelles will dissociate into unimers upon cooling, leading to a large reduction of scattered intensity. On the other hand, in the temperature range of 28− 30 °C, Dh exhibited a significant decrease. It then stabilized above 38 °C, remaining constant at round 80 nm. This is due to the thermosensitive swelling/deswelling of the cross-linked core, exhibiting a critical phase transition temperature of 28 °C. Below this critical temperature, the cross-linked P(NIPAM-coNAS) core was solvated and led to swelling of the core. Additionally, the Dh value of the CCL micelle above CMT was

Figure 5. Typical AFM height and three-dimensional images for 0.5 mg/mL aqueous solution of CCL micelles prepared from P2. 4702

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Scheme 2. Coassembly and Cross-Linking of f-SWNT and P2 Blends in Aqueous Solution

polymeric micelles that adhered to the surface of nanotubes upon cross-linking. It is worth noting that the micelles are relatively uniform in diameter, around 100 nm, in agreement with what was observed in Figure 5. In addition, polymeric micelles that were not bound to SWNTs (free micelles) are still observed, suggesting that unbound micelles cannot be completely removed by centrifugation. When the concentration of P2 was increased to 1.0 mg/mL, free micelles as well as SWNT-bound micelles with a diameter of 95 nm were observed, as depicted in Figure 6b. In this case, micelles were densely attached onto the surface of the SWNTs, with the amount of SWNT-bound micelles having increased due to the intermicellar cross-linking. Further increasing the P2 concentration to 5.0 mg/mL resulted in the formation of micellar agglomerates, where nanotubes were buried in the intermicellar cross-linked structures and cannot be seen (Figure 6c). In the second set of experiments, the concentration of P2 in aqueous solution was maintained at 0.5 mg/mL, while the concentration of f-SWNTs was systematically varied from 5.0 to 25 mg/L. The temperature of the mixed solutions containing 15 mg/L f-SWNTs was raised above 50 °C to coassemble the structures. After the cross-linking reaction, the resulting solution was allowed to cool to room temperature. AFM was also utilized to characterize the fabricated hybrid nanostructures. As shown in Figure 6d, free micelles as well as SWNTbound micelles can be observed in a similar way. The micelles are relatively uniform in diameter, around 100 nm. However, more SWNTs can be observed throughout the surface due to the increase in f-SWNT concentration. In addition, micelles that are bound to the surface of SWNTs are clearly seen in the magnified AFM images (Figure 6f). Further increasing f-SWNT

concentration to 25 mg/L resulted in the formation of tangled nanotube networks (Figure 6e). These results indicated that the concentrations of f-SWNTs and free copolymer should be maintained at an appropriate range for coassembly. To complement the AFM data, high-resolution transmission electron microscopy (TEM) analysis of the coassembled product was performed by placing a single drop of the aqueous solution onto a holey carbon-coated copper grid. Figures 7a and 7b depict representative features showing necklace-like structures in which micelles are attached to the surface of SWNTs, although their shapes remain somewhat irregular. Figure 7c depicts a representative TEM image for the P2functionalized SWNTs. Clearly, a discontinuous polymer coating is observed on the sidewall of the nanotube under higher magnification. As a control experiment, the concentration of P2 and pristine SWNT aqueous suspension was maintained at 0.5 mg/mL and 15 mg/L, respectively. The temperature of the mixed suspension was raised above 50 °C. After the cross-linking reaction, the resulting suspension was allowed to cool to room temperature and subjected to TEM observation. In this case, the nanotube surface is practically featureless, with no micelles attached to the surface (Figure 7d). Additionally, it was found that the SWNTs precipitated within 10 min during the crosslinking reaction. Therefore, any direct micelle absorption onto the nanotube surface can be ruled out under our experimental conditions.

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CONCLUSION In summary, we demonstrated a facile method for the covalent functionalization of SWNTs with thermoresponsive CCL 4703

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Figure 6. AFM height images of SWNT-micelle formed from coassembly and cross-linking of f-SWNT and P2 blends in aqueous solution: (a−c) fSWNTs = 5 mg/L, P2 = 0.5, 1.0, 5.0 mg/mL, respectively; (d, e) P2 = 0.5 mg/mL, f-SWNTs = 15, 25 mg/L; (f) the magnified AFM image for (d) showing SWNT−micelle conjugates.

polymeric micelles based on SWNT functionalization as well as polymer self-assembly. In this method, the copolymer PDMAb-P(NIPAM-co-NAS) bearing an azide group at the PDMA end was first prepared by RAFT polymerization. Covalent functionalization of SWNTs with well-defined, azide-derivatized copolymer was then accomplished by a nitrene addition reaction. The coassembly of the copolymer and f-SWNT blends was carried out in aqueous solution. It was found that the copolymer chains grafted onto the surface of SWNTs were coassembled with the free chains in solution, leading to thermoresponsive polymeric micelles that adhered to the surface of nanotubes. Upon cross-linking, the copolymer aggregates were stabilized and covalently anchored to SWNTs (SWNT−micelle). The resulting assembled structures were still soluble in water and characterized by AFM and TEM. Our method is potentially useful when combined with a variety of applications of cross-linked polymeric micelles, which have been already well recognized.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. (a, b) TEM images of SWNT-micelle formed from coassembly and cross-linking of f-SWNT (5.0 mg/L) and P2 (0.5 mg/ mL) blends in aqueous solution. (c) TEM image of f-SWNTs. (d) TEM image of SWNT/micelle composites formed from assembly and cross-linking of P2 (0.5 mg/mL) aqueous solution in the presence of pristine SWNTs (5.0 mg/L).

Diblock copolymer synthesis, copolymer compositions, 1H NMR, 13C NMR, GPC, and FTIR data for polymer, DLS data for P3 and P4 with the concentration of 0.5, 1.0, 1.5, 2.5, and 5.0 mg/mL, DLS data for P2−P4 with the concentration of 0.5 mg/mL and digital photographs for 0.5 mg/mL aqueous solution of P2 micelles before and after CCL. This material is available free of charge via the Internet at http://pubs.acs.org. 4704

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AUTHOR INFORMATION

Corresponding Author

*Fax +86 731 58293264; Ph +1 905 521 2773; e-mail [email protected] (H.L.), [email protected] (A.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Program for NSFC (51072172), International Joint Research Program of Hunan Province (2010WK2009), and Open Project of Hunan Provincial University Innovation Platform (10K066) is greatly acknowledged.

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