Surface modification of carbon nanotubes by using iron-mediated activators generated by electron transfer for atom transfer radical polymerization

Herein, a surface-initiated activator generated by electron transfer for an atom transfer radical polymerization (AGET ATRP) system was developed on the surface of multiwall carbon nanotubes (MWCNTs) by using FeCl3·6H2O as the catalyst, tris-(3,6-dioxoheptyl) amine (TDA-1) as the ligand and ascorbic acid (AsAc) as the reducing agent. A wide range of polymers, such as polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMA), were successfully grafted onto the surfaces. The core–shell structure of MWCNTs@PS was observed by TEM. Both Raman spectra and the results of hydrolysis of MWCNTs@PS (after extraction by THF) confirmed that the PS chains were covalently tethered onto the surfaces of the MWCNTs. Due to superior biocompatibility of the iron catalyst, the strategy of modification of MWCNTs via iron-mediated AGET ATRP provided a promising method for the controllable and biocompatible modification of nanomaterials.


Introduction
Since carbon nanotubes (CNTs) were discovered by Iijima in 1991, 1 it has opened up a new chapter in the development of carbon science. Their unique structure and physicochemical properties have drawn people's attention and brought us into a new era of nanotechnology. 2 Due to their small size, high mechanical strength, large specic surface, high conductivity and strong interface effect, CNTs have special mechanical, physical and chemical properties. However, since the CNTs cannot dissolve in solvents, and are not easy to disperse in most polymers, their practical application is limited and the properties cannot be fully demonstrated. Therefore, the chemical modication of CNTs has attracted great attention from researchers.
CNTs have special specic surface area and interstitial structure. The incomplete coordination of the atoms on the surface of the CNTs leads to an increase of the active sites on the surfaces, which also provides available strategies for their surface modication. For the purpose of improving processability and expanding applications, there are many surface modication methods for CNTs, including almost all available chemical reactions. 3 The chemical modication methods reported so far mainly involve in the introduction of carboxylic acid groups on the surface of the CNTs by carboxylic acid treatment, followed by chlorination, alcoholization or amination, thereby introducing a polymer molecular layer on the surface of the CNTs. 4 In general, polymers can be graed onto the surface of CNTs by both direct graing and in situ graing. 5 So far, there have been a variety of polymers including PS, PMMA and their copolymers successfully graed onto the surface of CNTs. Shaffer et al. graed PS on multi-walled carbon nanotubes (MWCNTs) using in situ free radical polymerization for the rst time. 6 With anionic polymerization employed, PS and poly(N-vinyl carbazole) (PVK) chains were also successfully graed onto the surface of MWCNTs. However, in order to gra polymer chains on CNTs conveniently and controllably, there is an urgent need for more efficient graing methods. Living radical polymerization (LRP), especially atom transfer radical polymerization (ATRP) has been proven to be a good controllable method for the surface modication of solid materials. Zhu and Cheng's group carried out some works about graing modication on various solid surfaces, including surface functionalization, 7 synthesis of magnetic nanomaterials 8 and so on. 9 LRP can effectively control the molecular weight and its distribution of the graing polymers, which graed on the surface of solid matrix such as silicon, carbon black, Fe 3 O 4 and other nanoparticles. Kong et al. used ATRP method to gra amphiphilic block copolymer brushes onto the surface of silicon. 10 Yang et al. synthesized four kinds of well-dened polymers using 4-hydroxyl-2,2,6,6-tetramethylpi-peridin-1-oxyl (HTEMPO)-mediated radical polymerization. 11 These polymers were graed onto the surface of carbon black using radical trapping method. Dispersion experiments demonstrated that the carbon black graed with polymers could be well dispersed in most of organic solvents. In addition, if the carbon black graed with poly(4-vinylpyridine) was quarternized with iodomethane, it can become hydrophilic material, which has a good application prospects in sensor materials eld. Wang et al. synthesized Fe 3 O 4 magnetic nanoparticles (MNP) graed with styrene and acrylic acid, using reversible addition fragmentation chain transfer (RAFT) method. 12 Well dened polymers was obtained and characterized by gel permeation chromatography (GPC). Transmission electron microscopy (TEM) images showed that the product had a core-shell structure.
In recent years, modied CNTs has a good application on the photoelectric materials. Wei et al. synthesized ZnS/carbon nanotube nanocables by a two-step vapor deposition method. 13 The product has good conductance as well as obvious light response. Vannikov et al. investigated the effect of cyanine dye additives on the photoelectric and photorefractive properties of polyvinyl carbazole composites based on closed single walled carbon nanotubes. 14 In addition, the biomedical applications are also very promising. 15 For example, Pan et al. utilized polyamide dendrimers modied CNTs as gene carriers and investigated the effect of dendrimer's algebra on the performance of gene vectors. 16 Lay et al. put the anticancer drug paclitaxel on poly(ethylene glycol) (PEG) graed CNTs and studied its application in treatment of cancer. They found that the delivery system can efficiently kill HeLa and MCF-7 cancer cells. 17 Vannikova et al. found that the biocompatibility and low cytotoxicity of CNTs are attributed to size, dose, duration, testing systems, and surface functionalization. 18 They functionalized CNTs to improve its solubility and biocompatibility and reduce its cytotoxic effects.
Considering the biological toxicity of copper and the superior biocompatibility of iron catalyst, 19 we employed the ironcatalyzed AGET ATRP for the surface modication of MWCNTs, which provided a viable method for the synthesis of biomedical materials.

Acidication of MWCNTs
5.0 g MWCNTs and 100 mL HNO 3 (60%) were added in a 250 mL single-necked ask and it was placed in an ultrasonic bath for 30 min. Then, the reaction ask was transferred into an oil bath and heated to reux (120 C) with vigorous stirring. Aer keeping the reux for 72 h, the mixture was cooled to room temperature and diluted with 200 mL deionized water. Then the diluted solution was ltered with a Buchner funnel and the ltrate was repeatedly washed with deionized water until the pH of the ltrate nearly neutral. 4.1 g MWCNTs-COOH was obtained aer drying in a vacuum oven.

Synthesis of MWCNTs-OH
Added 100 mL SOCl 2 into a 250 mL single-necked ask with 2.0 g MWCNTs-COOH as well as a magnetic stir bar and put it into an ultrasonic bath for 30 min. Then, the reaction ask was transferred into an oil bath under 65 C and kept stirring for 48 h. The solid product was ltered and washed with dry THF. Aer that, the washed solid product was dried under vacuum at room temperature and 1.92 g MWCNTs-COCl was obtained.
Subsequently, in a 100 mL single-necked ask with a magnetic stir bar, 60 mL ethylene glycol and 1.2 g MWCNTs-COCl were added. Aer ultrasonic dispersion for 30 min, the ask was placed in a 120 C oil bath and stirring for 48 h. The solid product was still ltered through a Buchner funnel, and washed repeatedly with THF. 1.11 g MWCNTs-OH was obtained aer dried under vacuum at room temperature.

Synthesis of MWCNTs-Br initiator
1.0 g MWCNTs-OH, 30 mL dry CHCl 3 , 0.073 g DMAP and 0.76 g dry TEA were added into a 100 mL three-necked ask with a magnetic stir bar and the mixture was placed in an ultrasonic bath for 30 min. Slowly added acyl bromide solution (0.96 g 2bromoisobutyryl bromide and 13 mL dry CHCl 3 ) into the ask under ice bath and Ar atmosphere for about 1 h. Then the mixture was stirred at 0 C for 3 h and then at room temperature for 48 h. The solid product was ltered with a Buchner funnel, and washed repeatedly with CHCl 3 . Aer that, the initial product was dissolved in 30 mL CHCl 3 and placed in an ultrasonic bath and ltered. Washed repeatedly with CHCl 3 until no residual 2-bromoisobutyryl bromide. 0.94 g MWCNTs-Br initiator was obtained aer dried under vacuum. In a dried 5 mL ampule with a magnetic stir bar, MWCNTs-Br (25.0 mg), FeCl 3 $6H 2 O (13.0 mg), TDA-1 (46.6 mg) and DMF (1.0 mL) were added. The mixture was placed in an ultrasonic bath for 15 min. Aer that, St (0.275 mL) and AsAc (8.4 mg) were added in the mixture and then the ampule was bubbled thoroughly with Ar for 20 min to eliminate the dissolved oxygen in the reaction system and ame-sealed. The ampule was transferred into an oil bath keeping it at 110 C. Aer the desired polymerization time, the ampule was diluted with 10 mL of CHCl 3 , precipitated with methanol and ltered. The product was dried under vacuum to a constant weight. In order to remove the homopolymer from the product, it was extracted with THF using a Soxhlet extractor for 72 h. The nal product was redispersed with 5 mL CHCl 3 , precipitated with methanol and ltered, and MWCNTs@PS were obtained aer vacuum drying. The polymerization procedures of other monomers were the same as mentioned above.

Hydrolysis of MWCNTs@PS
In order to obtain the graing polymers for GPC analysis, 40 mg MWCNTs@PS were dispersed in 40 mL THF and then adding 10 mL 1 M KOH/ethanol solution and reuxing for 72 h with stirring. The mixture was centrifuged at 1500 rpm for 10 min to obtain de-functionalized MWCNTs (bottom of centrifuge tube) and PS dissolved in THF (supernatant) hydrolyzed from MWCNT@PS. The supernatant was precipitated with a large amount of methanol ($500 mL), let stand overnight and then ltered with a Buchner funnel. Aer that, graing PS hydrolyzed from MWCNTs@PS were obtained aer vacuum drying.

Characterization
1 H NMR spectra were recorded on an INOVA 400 MHz nuclear magnetic resonance spectrometer using CDCl 3 as a solvent and tetramethylsilane (TMS) as an internal standard. Transmission Electron Microscopy (TEM) was performed using TecnaiG220 with an acceleration voltage of 200 kV. Infrared spectroscopic analysis was measured by a KBr pellet using Nicolet 1300. Thermal Analysis (TGA) using the SDT 2960 and the heating rate was 10 C min À1 under N 2 atmosphere. Raman spectroscopy was recorded by HR800. The molecular weight (M n,GPC ) and molecular weight distribution (M w /M n ) of the resultant polymers were determined using a Waters 1515 gel permeation chromatography (GPC) equipped with refractive index detector (Waters 2414), using HR1, HR2 and HR3 columns (7.8 Â 300 mm) with measurable molecular weights ranging from 10 2 to 5 Â 10 5 g mol À1 . THF was employed as the eluent at a ow rate of 1.0 mL min À1 and 30 C. GPC samples were injected using a Waters 717 plus autosampler. The graing PS molecular weights were calibrated with PS standards and graing PMMA were calibrated with PMMA standards, respectively.

Immobilization of initiator MWCNTs-Br
The route of immobilization of initiator MWCNTs-Br is shown in Scheme 1. Firstly, carboxyl groups are attached to the surface of MWCNTs by acidication with nitric acid. Secondly, carboxyl groups reacts with thionyl chloride and converted to acid chloride groups and then converted to hydroxy esters by reaction with ethylene glycol. Finally, aer the reaction with acyl bromide, initiating group 2-bromoisobutyrate is immobilized on the surface of the MWCNTs. In our experiments, the ltration and washing procedure is necessary in each step. Small molecules adsorbed on MWCNTs must be completely removed to ensure the purity of MWCNTs-Br.
Fourier transform infrared (FT-IR) spectroscopy is used to characterize the immobilization of ATRP initiator on MWCNTs. The results are shown in Fig. 1a-d. MWCNTs-OH (c), a product of ethylene glycol graing on MWCNTs, has an obvious C]O peak at 1730 cm À1 . As for MWNCTs-Br (d), the product of MWCNTs-OH reacts with 2-bromoisobutyryl bromide, its intensity of C]O peak is increased, which indicates the initiator was immobilized on the surface of MWCNTs.
In the synthesis of MWCNTs-Br, MWCNTs dispersion has undergone great changes. As is shown in Fig. 2, MWCNTs cannot be well dispersed in any solvents we used ( Fig. 2A). Aer acidication, the MWCNTs-COOH can partially disperse in water and gathered at the interface between CHCl 3 and H 2 O (Fig. 2B). MWCNTs-OH showed better dispersibility than MWCNTs-COOH in water and organic solvents (Fig. 2C). However, MWCNTs-Br shows very poor dispersibility in water, and relatively good dispersibility in organic solvents (Fig. 2D). This is mainly due to the good hydrophilicity of hydroxyl and carboxyl groups on the surface of MWCNTs while 2-bromo isobutyrate has poor hydrophilicity.

Surface-initiated AGET ATRP
To illustrate the versatility of iron-catalyzed AGET ATRP in surface modication, we select three typical monomers, St, MMA and PEGMA, which were initiated on the surfaces of MWCNTs-Br. In order to remove the homopolymer in the product, all samples were extracted by THF. As is shown in Fig. 1e-g, the characteristic peaks in PS (695 cm À1 , 755 cm À1 , 1495 cm À1 and 1600 cm À1 ), 20 PMMA (1380 cm À1 and 1730 cm À1 ) 21  In addition, considering that the methacrylates polymers cannot be perfectly hydrolyzed from the MWCNTs, we selected MWCNTs@PS as the graing polymers for the characterization of molecular weights; namely, the graing PS polymers were obtained by hydrolysis of MWCNTs@PS. The polymerization conditions and the results are shown in Table 1. As the weight ratio (R 1 ) of styrene to MWCNTs-Br gradually increases, the number average molecular weight of graed PS (M n,GPC ) increases, indicating the molecular weight of PS graed onto the surface of MWCNTs can be controlled by adjusting the feed ratio of monomer to initiator via iron-mediated AGET ATRP. At the same time, the resultant molecular weight distribution of graing PS is relatively broad (M w /M n $ 2.0) but narrower than that reported in the literatures (M w /M n $ 3.0). 3 The broad M w / M n values may be due to the following facts: MWCNTs are not uniform and even small amount of single-walled carbon nanotubes may exist, resulting in different surface conditions of MWCNTs. Even on the same MWCNTs, the wider molecular weight distribution can be caused by the different density of immobilized initiators due to different distributions of defects and different degrees of oxidation.

TGA and 1 H NMR characterization
TGA curve for MWCNTs@PS and homopolymer PS is shown in Fig. 3. The PS decomposition temperature (T d ) of MWCNTs@PS is close to 390-400 C, about 40-50 C higher than homopolymer PS (350 C). This is due to the synergistric effect of MWCNTs, which has high thermal stability, and the graing PS chains. It is noted that from the residual weight% of the MWCNTs@PS the functionality of MWCNTs seemed not much high, indicating that the functionalization has happened but not quantitatively and efficiently. Fig. 4 shows the 1 H NMR spectrum of MWCNTs@PS-2. It is obvious that the chemical shi at 6.5-7.2 ppm ((a) in Fig. 4) corresponds to the characteristic peak of benzene ring and the chemical shi at 1.0-2.0 ppm ((b) in Fig. 4) belongs to methylene and methine groups. According to the literature, electron absorption effect of chlorine atom at PS chain end will lead to a chemical shi of methine of PS chain end at 4.2-4.5 ppm. 23 When the molecular weight of PS is too high, the unique signal peaks become too weak to be easily observed. However, the presence of these peaks above demonstrates that PS is indeed graed onto the MWCNTs.

Raman and TEM characterization
It is well known that Raman spectroscopy can be used to characterize the presence and proportion of D-line (amorphous carbon and disordered induction line) and G-line in MWNTs. The Raman spectra of MWCNTs, MWCNTs-Br and MWCNTs@PS are shown in Fig. 5. Two peaks at 1320 cm À1 and 1580 cm À1     Fig. 6 shows a TEM image of the original MWCNTs and the modied MWCNTs. As can be clearly seen from Fig. 6a, the MWCNTs is not uniform in size and there is also a portion of amorphous carbon nanotube impurities present. Fig. 6c and d show the TEM images of MWCNTs@PS. We can clearly see that the outer wall of the MWCNTs is covered with a layer of polymer. Fig. 6e and f also show a core-shell structure, which means the corresponding polymers were successfully graed onto MWCNTs.

X-ray photoelectron spectroscopy (XPS) characterization
XPS is one of the most powerful tools for characterizing chemical structures and chemical compositions of solid surfaces. XPS wide scan, C 1s, and O 1s core-level spectra of the original MWCNTs are shown in Fig. 7a   MWCNTs@PS (c). The laser wavelength is 632.8 nm and the laser power used is 6 mW.    (Fig. 7d). 25 The appearance of a new Br 3d peak in the wide scan spectrum as well as binding energy at 70.2 eV and 71.3 eV in the Br 3d core-level spectra (Fig. 7g) indicate that the initiator 2-bromoisobutyrate has been successfully immobilized on the surface of MWCNTs. Binding energy at 286.2 eV corresponds to the covalently linked C-Br characteristic peak, 25 which further demonstrates the successful immobilization of the initiator (Fig. 7e). Moreover, the ve characteristic peaks (283.6, 284.9, 285.6, 286.9 and 288.4 eV respectively correspond to C]C, C-C, C-O, C]O and O]C-O) of original MWCNTs still exist, which indicates that the thickness of the initiator layer is less than the XPS detection depth (about 7.5 nm (ref . 27)). This is consistent with the fact that organic chemical reactions on solid surfaces oen generate monomolecular layers.
XPS wide scan, C 1s, and O 1s core-level spectra of MWCNTs@PS are shown in Fig. 7h-j. There is a strong new peak at 291.4 eV (Fig. 7i) belongs to p-p conjugated characteristic peak of benzene ring, 25 which demonstrates that PS successfully grafted on the surface of MWCNTs. In addition, since there is no oxygen atom in PS, if the surface of entire MWCNTs is covered by PS layer, a signal of O 1s should not be detected. However, the existence of O 1s signal ( Fig. 7h and j) and the ve characteristic peaks (Fig. 7i, 283

Conclusions
In summary, ATRP initiator was successfully immobilized on MWCNTs by a 4-step method, and different kinds of polymers were successfully graed by iron-catalyzed surface-initiated AGET ATRP. The core-shell structure of MWCNTs@PS was proved by TEM. The MWCNT@PS hydrolysis defunctionalization aer THF extraction and Raman spectroscopy both demonstrated that the modied MWCNTs and PS were linked by covalent bonds. Therefore, a promising method for the controllable, facile and biocompatible surface modication of nanomaterials was established.

Conflicts of interest
There are no conicts to declare.