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Biomimetic PEGylation of carbon nanotubes through surface-initiated RAFT polymerization
Materials Science and Engineering C 80 (2017) 404–410
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Biomimetic PEGylation of carbon nanotubes through surface-initiated RAFT polymerization Yingge Shi a,c,1, Guanjian Zeng a,1, Dazhuang Xu a, Meiying Liu a, Ke Wang b, Zhen Li b, Lihua Fu c, Qingsong Zhang b, Xiaoyong Zhang a,⁎, Yen Wei b a b c
Department of Chemistry and Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, PR China Department of Chemistry and the Tsinghua Center for Frontier Polymer Research, Tsinghua University, Beijing 100084, PR China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 8 May 2017 Received in revised form 24 May 2017 Accepted 15 June 2017 Available online 16 June 2017 Keywords: Carbon nanotubes Surface-initiated polymerization Mussel inspired chemistry Biomedical applications
a b s t r a c t Carbon nanotubes (CNTs) are a type of one-dimensional carbon nanomaterials that possess excellent physicochemical properties and have been potentially utilized for a variety of applications. Surface modification of CNTs with polymers is a general route to expand and improve the performance of CNTs and has attracted great research interest over the past few decades. Although many methods have been developed previously, most of these methods still showed some disadvantages, such as low efficiency, complex experimental procedure and harsh reaction conditions etc. In this work, we reported a practical and novel way to fabricate CNTs based polymer composites via the combination of mussel inspired chemistry and reversible addition fragmentation chain transfer (RAFT) polymerization. First, the amino group was introduced onto the surface of CNTs via self-polymerization of dopamine. Then, chain transfer agent can be immobilized on the amino groups functionalized CNTs to obtain CNT-PDA-CTA, which can be utilized for surface-initiated RAFT polymerization. A water soluble and biocompatible monomer poly(ethylene glycol) monomethyl ether methacrylate (PEGMA) was adopted to fabricate pPEGMA functionalized CNTs through RAFT polymerization. The successful preparation of CNTs based polymer composites (CNT-pPEGMA) was confirmed by transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy in details. The CNTpPEGMA showed good dispersibility and desirable biocompatibility, making them highly potential for biomedical applications. More importantly, a large number of CNTs based polymer composites could also be fabricated through the same strategy when different monomers were used due to the good monomer adaptability of RAFT polymerization. Therefore, this strategy should be a general method for preparation of various multifunctional CNTs based polymer composites. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Since they were officially recognized and named in 1991, carbon nanotubes (CNTs) have drawn much attention for their outstanding mechanical, thermal and electrical properties [1–3]. Therefore, they have been widely applied in structure materials, thermal conductor, environmental adsorption, energy storage and conversion, chem/biosensors and biomedical applications [4–16]. Even though CNTs own remarkable properties, it is wiser to make a combination with others, such as metal, inorganic substances, polymers and so on [17–20]. CNTs based polymer composites were considered as one of the significant parts, which were devisable and can maximize the potential of both polymers and CNTs [21]. So far, many studies have been done to ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.msec.2017.06.009 0928-4931/© 2017 Elsevier B.V. All rights reserved.
fabricate CNTs based polymer composites and achieved certain success. For example, solution mixing, melt blending and in situ polymerization were developed from the point of practical application and theoretical research [22]. Compared with pure polymers or CNTs, these CNTs based polymer composites may overcome the defects such as poor designability, low conductivity and weak intensity [21,23]. In the process of preparation, dispersion and interfacial adhesion were two critical factors to create excellent composites. Because of the small size, high specific surface areas and aspect ratio, strong hydrophobic interaction was existed between CNTs, which lead to the serious aggregation of these pristine CNTs [24]. From the aspect of the micro, this behavior was resulted from electronic efficient and van der Waals attraction. Chemical functionalization, based on covalent linkages, was employed to exclude the electronic efficient and endow reactive sites on the surface of CNTs [25–28]. On the other hand, physical functionalization, based on non-covalent self-assembly, was also used to enhance interfacial adhesion. The later mainly depended on van der Waals attraction
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and π-π stacking between polymers and CNTs [29–31]. Nonetheless, the pretreatment of covalent linkages may cause irreparable damage to the structure of CNTs and negative impact on the thermal, mechanical or electrical properties. In addition, the strength of π-π stacking is relative unstable in some extent and only limited polymers can be choose for non-covalent strategies. The limitation of these conventional surface modification strategies will largely impede the further applications of CNTs. Therefore, it was still necessary to design and explore novel and efficient methods for fabrication of CNTs based polymer composites. Mussel inspired chemistry is an emerged surface modification strategy, which has attracted increasing attention in the past few years [32, 33]. It has been demonstrated that dopamine (DA), a crucial component of mussel inspired chemistry, would be self-polymerized into PDA under alkaline environment. The resulting PDA films can not only strongly adhere onto almost any materials, but also provide a number of active sites for further reactions [34,35]. Because of these unique characteristics, mussel inspired chemistry have becoming the research focus for diverse applications [36–42, 44–48, 50]. For example, Lee et al. connected hydroxyapatite on the surface of CNTs by the aid of mussel inspired chemistry [49]. In the process, DA not only plays the role of adhesion but significantly weakened the toxicity of CNTs and enhanced the biocompatibility. On the other hand, we have demonstrated that amino groups functionalized CNTs can be facilely fabricated via the combination of mussel inspired chemistry and Michael addition reaction. These functionalized CNTs showed obviously enhanced adsorption capability toward Cu2 + [50]. Furthermore, surface modification of graphene oxide with heparin and BSA was also demonstrated by Zhao and Cheng et al. [51] They confirmed that surface modification of graphene oxide with these biomacromolecules could effectively improve water dispersibility and biocompatibility of graphene oxide. Surface modification of CNTs with synthetic polymers through in situ surface polymerization methods have demonstrated to be efficient routes to prepare CNTs based polymer composites [52–54]. Most of these strategies were mainly relied on the surface oxidation of CNTs. However, the procedure for surface oxidation of CNTs is rather complex and inefficient and will inevitably destroy the structure of CNTs. Therefore, some studies for surface modification of CNTs via the combination of mussel inspired chemistry and atom transfer radical polymerization (ATRP) and single-electron living radical polymerization (SET-LRP) have been demonstrated recently [55,56]. However, both ATRP and SET-LRP are required using copper ions as catalysts, which are toxic to living organisms and may influence the final properties of composites. Reversible addition fragmentation chain transfer (RAFT) polymerization is another controlled living polymerization method, which can be used for preparation of well controlled polymers in the absence of metal catalysts [57,58]. To the best of our knowledge, the preparation of CNTs based polymer composites through the combination of mussel inspired chemistry and surface-initiated RAFT polymerization has not reported thus far. In this contribution, we describe a practical and efficient method to fabrication of CNTs based polymer composites, that combination of mussel inspired chemistry and RAFT polymerization. The pristine CNTs were first coated with PDA, which was used for surface immobilization of chain transfer agent (CTA) to initiate RAFT polymerization (Scheme 1). The successful preparation of CNTs based polymer composites (CNT-pPEGMA) were confirmed by a number of characterization techniques. Cell viability evaluation demonstrated that CNT-pPEGMA showed great cytocompatibility, indicating that these CNTs based polymer composites are potential for biomedical applications. 2. Experiment 2.1. Materials and characterization All of the chemicals were directly used without further purification. CNTs were purchased from Sinonano (Beijing, China). Dopamine
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Scheme 1. Schematic representation for the preparation of CNT-pPEGMA via the combination of mussel inspired chemistry and RAFT polymerization.
hydrochloride was purchased from Sangon Biotech. Co., Ltd. (Shanghai, China). N-(3-Dimethylaminopropy)-N-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), Tris-(hydroxymethyl) aminomethane (Tris) and poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mw: 950 Da, 98%) and polyethylenimine (PEI, Mw: 1800 Da) were obtained from Aladdin (Shanghai, China) without further purification. 2-Morpholinoethanesulfonic acid (MES) was supplied by Heowns (Tianjin, China). Chain transfer agent (CTA) was synthesized according to our previous report [29]. The other chemical agents were all of commercially available and analytical grade. The water involved in the experiment was deionized water (D.I. water). The Fourier transform infrared (FT-IR) spectra were gained in a transmission mode on a Nicolet 5700 spectrometer (Waltham, MA, USA). Thermal gravimetric analysis (TGA) was tested on a TA instrument Q50 with a heating rate of 20 °C min−1 using crucibles of aluminum. The X-ray photoelectron spectra (XPS) were measured on a VGESCALAB 220-IXL spectrometer using an A1 Ka X-ray source (1486.6 eV). Transmission electron microscopy (TEM) images were recorded on a Hitachi 7650B microscope operated at 80 Kv. The TEM specimens were made by placing a drop nanoparticle ethanol suspension on a carbon-coated copper grid. 2.2. Preparation of CNT-PDA-NH2 CNT-PDA-NH2 was synthesized as described in the following procedure. 150 mg of pristine CNTs were added into 50 mL of Tris solution (10 mM L−1, pH = 8.5) and sonicated for 10 min. Then, 150 mg of DA was also put into and the mixture was stirred at room temperature for 2 h. The mixture (CNT-PDA) was separated from the reaction solution through centrifugation. After that, 150 mg of PEI was added to react with CNT-PDA for further 2 h. The PEI can be immobilized on PDA coatings through Michael addition reaction between PDA and amino groups. On the other hand, the surplus amino groups that immobilized on the surface of CNTs can be further reacted with carboxyl group of CTA to facility the immobilization of CTA on CNTs. The CNT-PDA-NH2 was got after centrifuging and washing at 8000 rpm for 10 min per time until suspension was clarified. Last, the product was dried under vacuum for further use. 2.3. Preparation of CNT-PDA-CTA CNT-PDA-CTA was synthesized in MES buffer solution (25 mM, pH = 6). First, 10 mg of CTA was added into 10 mL of MES solution. And then, 4.5 mg of NHS and 7.5 mg of EDC were added in the CTA contained solution. After 10 min, 100 mg of CNT-PDA-NH 2 was added and further reacted overnight at room temperature. The resulting CNT-PDA-CTA was separated from MES solution by centrifugation at 8000 rpm for 10 min and vigorously washing with
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acetone to remove residual reactants. The obtained product was dried under vacuum for further use. 2.4. PEGylation of CNTs via surface-initiated RAFT polymerization CNT-pPEGMA was synthesized via RAFT polymerization using PEGMA as monomer under the protection of N2. 150 mg of PEGMA, 50 mg of CNT-PDA-CTA, 0.6 mg of AIBN and 5 mL methylbenzene were added into reaction flask, which was sealed and vacuum for 10 min to remove any air from the reaction system. The mixture was put into an oil bath maintained at 70 °C for 10 h. In the end, the products were obtained by centrifugation and washing repeatedly. 2.5. Biocompatibility evaluation of CNT-pPEGMA The biocompatibility evaluation was conducted based on our previous report [59,60]. Cells were seeded in 96-well microplates at a density of 5 × 104 cells per mL in 160 μL of the respective media containing 10% FBS. After 24 h of cell attachment, the HeLa cells and A549 cells were incubated with 10, 20, 40, 80, 100 μg mL−1 of CNT-pPEGMA for 24 h. Then the cells were washed with PBS for three times to remove the uninternalized nanoparticles. After that, 10 μL of CCK-8 dye and 100 μL of Dulbecco's Modified Eagle's Medium (DMEM) cell culture media was added to each well and incubated for 2 h at 37 °C. Plates were then analyzed with a microplate reader (VictorIII, Perkin-Elmer). Measurements of dye absorbance were carried out at 450 nm, with the reference wavelength at 620 nm. The values were proportional to the number of live cells. The percent reduction of WST was compared to the control (cells not exposed to nanoparticles), which represented 100% WST reduction. Three replicate wells were used for each control and test concentrations per microplate, and the experiment was repeated three times. Cell survival was expressed as absorbance relative to that of untreated controls. Results are presented as mean ± standard deviation (SD). 3. Results and discussion To extend the applications of CNTs, surface modification of CNTs with polymers to improve dispersion of CNTs based composites and enhance the interfacial adhesion CNTs and polymer matrix was incredibly important. Although many efforts have been made in the fabrication of CNTs based polymer composites, simple, effective and universal strategies to prepare CNTs based polymer composites are still rarely developed. One of the most adopted strategies for fabrication of polymers functionalized CNTs is through surface-initiated polymerization such as ATRP, RAFT etc. [53,61] However, surface oxidation is often required to introduce oxygen contained functional groups on the surface of CNTs for immobilization of polymerization initiators [53,62]. This procedure
is rather complex, time consuming and relative low efficiency. Mussel inspired chemistry has been demonstrated to be a very promising surface functionalization method for the strong and universal adhesion of PDA toward different material surfaces [32,63]. The combination of mussel inspired chemistry and controlled living polymerization such as (ATRP and SET-LRP) have recently demonstrated our group and some other groups [55,64]. However, both of the two polymerization methods (ATRP, SET-LRP) are needed using metal catalysts, which should be completely removed from the final products. As compared with ATRP and SET-LRP, RAFT polymerization is another important controlled living polymerization method with good polymerization properties [65,66]. However, to the best of our knowledge, the preparation of CNTs based polymer composites through combination of mussel inspired chemistry and surface-initiated RAFT polymerization has not demonstrated before. In this work, the PEGylated CNTs have been prepared using the strategy as displayed in Scheme 1. And the final product (CNT-pPEGMA) was characterized by a number of techniques in detail. As shown in Fig. 1, both CNT-PDA-CTA (Fig. 1A) and CNT-pPEGMA (Fig. 1B) still maintained their fiber like morphology. However, some blurry films can be also found around the CNTs (as indicated by the red arrows). The enlarged TEM images of CNT-PDA-CTA and CNT-pPEGMA further confirmed that the CNTs were functionalized by polymer films. As evidenced by Fig. S1, rather uniform polymer films can be observed. The thickness of the PDA films is about 5 nm. As compared with the CNT-PDA-CTA, the thickness of CNT-pPEGMA was increased to about 8 nm. These results implied that PDA and pPEGMA have successfully modified CNTs via mussel inspired chemistry and RAFT polymerization, respectively. The mass percentage of polymers coated on CNTs was determined by TGA under N2 environment. As shown in Fig. 2, the loss of pristine CNT was about 2.6% except free water, while the loss increased to 10.32% after introducing PDA. It can be calculated that the weight percentage of PDA and PEI on CNTs is about 7.7%. The CNT-PDA-CTA began to lose weight at about 170 °C and had about 16.36% (without free water) residue weight to 600 °C. The significant weight loss between CNT-PDA-NH2 and CNT-PDA-CTA suggested that the CTA has immobilized on the CNTs successfully. Based on the TGA results, the mass percentage of CTA immobilized on the CNTs is greater than 6%. After polymerization, the weight loss of CNT-pPEGMA was further increased to 84.73%. These obvious weight loss clearly evidenced that pPEGMA has successfully grafted onto the CNTs through RAFT polymerization. The significant weight loss was occurred at the temperature between 350 and 420 °C, which is well consistent with the weight loss of pPEGMA. Based on the experimental conditions (weight percentage of PEGMA is about 75%) and TGA curves, we could speculate that almost all the PEGMA monomer was grafted onto CNTs. The DTA curves of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA were displayed in Fig. S3. It can be seen that almost no peaks were found in the sample
Fig. 1. Representative TEM images of CNT-PDA-CTA (A) and CNT-pPEGMA (B); scale bar = 200 nm. It can be seen that some blurry films around the CNTs were observed, implying the successful coating CNTs with PDA and pPEGMA.
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of pristine CNTs below 600 °C, indicating the good thermal stability of CNTs (Fig. S3A). As compared with CNTs, broad endothermic peaks with max values about 300 °C was observed in the samples of CNTPDA-NH2 and CNT-PDA-CTA, that can be ascribed to the decomposition of PDA and CTA (Figs. S3B and S3C). These rather broad endothermic peaks suggested that the compositions of PDA are rather complex. However, the max values of endothermic peaks of CNT-PDA-NH2 and CNTPDA-CTA also showed some difference. For example, the max value of CNT-PDA-NH2 is 309.6 °C. However, the max value was shifted to 284.8 °C for the sample of CNT-PDA-CTA. The difference should be ascribed to the decomposition of CTA is different from PDA. And the decomposition temperature of CTA is lower than 300 °C. These results suggested that CTA has indeed immobilized on CNTs. Two endothermic peaks were observed in the sample of CNT-pPEGMA, which was located at 302.9 and 429.9 °C, respectively. The emerged peak at 429.9 °C should be ascribed to the decomposition of pPEGMA. All of these results further confirmed that CNTs based polymer composites can be facilely prepared via combination of mussel inspired chemistry and RAFT polymerization. Fig. 3 gave the FT-IR spectra of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA. The peaks at 3200, 1400 and 1250 cm− 1 just corresponded to N\\H stretching vibration, N\\H flexural vibration and C\\N stretching vibration, respectively. More importantly, the signals in the region 1600–1400 cm−1 derived from stretching vibration of aromatic ring in DA. Therefore, DA had been introduced onto the surface of CNTs successfully. The peak at 1708 cm−1, assigned to C_O, indicated the graft of CTA as expected. By contrast, after polymerization
with pPEGMA, the strong peaks at 2867, 1711 and 1135 cm−1 were observed, that can be ascribed to characteristic functional groups, C\\H (methylene), C_O (ester) and C\\O (ether), confirmed the successful surface modification of CNTs with pPEGMA. The elements and chemical information of all samples were investigated by XPS spectra (Fig. 4). In the XPS spectrum of pristine CNTs, two obvious signals, C 1s and O 1s were appeared at about 285 and 533 eV, which just corresponded with previously reports [67]. The successful introduction of PDA can be confirmed by the emergence of the N 1s signals at a binding energy at about 400 eV in the wide scan XPS spectrum of CNT-PDA-CTA. Furthermore, the successful conjugation of CTA with CNT-PDA-NH2 can be evidenced by the emergence of S 2p peak in the XPS spectrum of CNT-PDA-CTA. As compared with CNTPDA-CTA, the N 1s intensity was obviously decreased while the O 1s intensity was significantly increased in the sample of CNT-pPEGMA, implying that pPEGMA has been grafted on the surface of CNTs through RAFT polymerization successfully. Fig. S4 showed the XPS spectra of C 1s, O 1s, N 1s and S 2p, respectively. As shown in Fig. S4A, only a major peak located at 285 eV can be observed in the sample of pristine CNTs, which should be attributed to the C\\C and C_C bonds. After surface coated with PDA and linked with CTA, a small peak with higher binding energy was emerged, indicating that PDA and CTA were linked onto CNTs. After surface polymerized with PEGMA, a small peak with lower binding energy was appeared, indicating the pPEGMA was grafted onto CNT-PDA-CTA. The successful immobilization of PDA and CTA on the CNTs was also confirmed by N 1s and S 2p spectra. It can be seen that obvious peaks of N 1s and S 2p were only found in the sample of CNT-PDA-CTA. These results confirmed that CTA was successfully immobilized onto CNTs. On the other hand, significant decrease intensity of N 1s and S 2p in the sample of CNT-pPEGMA demonstrated that the polymer (pPEGMA) has been grafted onto CNT-PDA-CTA through RAFT polymerization. As shown in Fig. 5A, the C 1s core level spectrum of CNTs can be fitted into three peaks with BEs at 284.9, 285.9 and 290.45 eV. These peaks can be attributed to Sp2 C_C/Sp3 C\\C, C\\O and O\\COO species, respectively. After introducing PDA and CTA, the C 1s spectrum of CNT-PDA-CTA can be deconvoluted into six peaks (Fig. 5B) which were C\\C (284.67 eV), C\\H (285.06 eV), C\\N (285.74 eV), C\\O/ C\\S (286.65 eV), O_C\\O (288.19 eV) and π-π* (290.91 eV). The emergence of C\\N and C\\S bonds further indicated that PDA and CTA were linked on the CNTs through mussel inspired chemistry. In the curve fitted C 1s core level spectrum of CNT-pPEGMA, four peaks assigned to C\\H (285 eV), C\\C (285.5 eV), C\\O (286.4 eV) and \\COO (289.5 eV) were emerged. As compared with other samples, the C\\O signal at 286.4 eV was significantly increased in the sample
Fig. 3. FT-IR spectra of CNT, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA.
Fig. 4. Survey scans XPS spectra of CNTs, CNT-PDA-CTA and CNT-pPEGMA.
Fig. 2. TGA curves of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA.
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Fig. 5. C 1s XPS spectra of CNT (A), CNT-PDA-CTA (B) and CNT-pPEGMA (C).
of CNT-pPEGMA, indicating that the surface of CNT-pPEGMA had been fully covered by pPEGMA coating (Fig. 5C). Furthermore, the contents of C, N, O and S in the sample of CNTs, CNT-PDA-CTA and CNT-pPEGMA were displayed in Table 1. It can be seen that the pristine CNTs are majorly composited with C (97.13%) with a small part of O (2.87%). However, two new elements N and S were emerged in sample of CNT-PDA-CTA with percentages of 7.62% and 1.36%, respectively. Based on the content of S, the percentage of CTA immobilized on CNT-PDA-CTA is about 7.45%, which is well consistent with the TGA results. After surface modified with pPEGMA, the percentage of C was significantly decreased to 64.41%, while the percentage of O was significantly increased to 34.74%. On the other hand, the percentages of N and S were also significantly decreased to 0.68% and 0.17%, respectively. These results clearly confirmed that pPEGMA was successfully grafted onto CNT-PDA-CTA through RAFT polymerization. The pPEGMA on CNTs was calculated to be 76.58% based on the results of XPS spectra, which are well consistent with the result of TGA. The dispersity of CNTs, CNT-PDA-CTA and CNT-pPEGMA was shown in Fig. 6. It can be seen that the CNTs (bottle 1) and CNT-PDA-CTA (bottle 2) were quickly deposited at the bottom of bottles within 10 min. As compared with CNTs and CNT-PDA-CTA, no aggregation was found in the sample of CNT-pPEGMA even they were deposited for more than 24 h. The excellent water dispersibility also reflected successful modification of CNTs with pPEGMA through RAFT polymerization. Moreover, CNT-pPEGMA also had outstanding dispersion in some organic solvents such as dimethyl sulfoxide (DMSO), N,N-Dimethylmethanamide (DMF) and acetone. CNT-pPEGMA can be dispersed in dichloromethane (DCM) (bottle 7) for short time, however, it will also deposit within 2 h. The possible reason is poor dispersibility of pPEGMA in DCM. The good dispersibility of CNT-pPEGMA in aqueous solution and many organic solvents is very useful for their applications.
It is well known that PEG is a biocompatible and hydrophilic polymer, which has been widely used for surface modification of nanomaterials. It has been demonstrated that surface PEGylation can not only enhance the water solution of nanomaterials, but also improve their pharmacokinetics behavior and biocompatibility in vivo. Therefore, the cytocompatibility of CNT-pPEGMA to HeLa cells and A549 cells was evaluated using CCK-8 assay. As shown in Fig. 7, all the cell viability values of CNT-pPEGMA to HeLa cells and A549 cells for 24 h are still greater than 91% even the concentration of is CNT-pPEGMA as high as 100 μg mL− 1. As compared with our previous reports, the PEGylated CNTs exhibited much better biocompatibility to cells. The possible reason is likely due to the anti-oxidation effect of PDA and camouflage effect of CNT-pPEGMA. The excellent biocompatibility and well water dispersity of CNT-pPEGMA implied their potential for biomedical applications. The CNTs have high photothermal conversion capability and the PEGMA is a desirable biological monomer. More importantly,
Table 1 The surface elemental compositions of CNTs, CNT-PDA-CTA and CNT-pPEGMA. Samples
C 1s
N 1s
O 1s
S 2p
CNTs CNT-PDA-CTA CNT-pPEGMA
97.13 83.85 64.41
0 7.62 0.68
2.87 7.17 34.74
0 1.36 0.17
Fig. 6. Dispersion of CNT (1), CNT-PDA-CTA (2) and CNT-pPEGMA (3–7) at 10 min (A); 2 h (B); 24 h (C). From 1 to 3, the solvent was water. From 4 to 7, the solvents were DMSO, DMF, acetone and DCM, respectively.
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Fig. 7. Cell viability evaluation of CNT-pPEGMA to HeLa cells and A549 cells. The concentrations of CNT-pPEGMA were ranged from 10 to 100 μg mL−1.
the end hydroxyl groups of the grafted PEGMA polymer can further react with various functional compounds. Therefore, the CNT-pPEMGA is suitable used for various biomedical applications, such as intracellular delivery biological active agents and photothermal cancer treatment. Moreover, due to the universality of mussel inspired chemistry and good applicability of RAFT, this method developed in this work should also be potentially adopted for fabrication of many other polymeric nanocomposites with great potential for controlled drug delivery, cancer photothermal treatment etc. [68–77]. 4. Conclusion In summary, we have developed a novel and effective strategy for surface modification of CNTs based on mussel inspired chemistry and surface-initiated RAFT polymerization. As compared with the conventional surface-initiated polymerization, the strategy described in this work need not oxidize CNTs first. Therefore the structure and physicochemical properties of CNTs can be well preserved. On the other hand, due to the versatility of mussel inspired chemistry and general applicability of RAFT polymerization, various polymer composites can be fabricated through the strategy developed in this work. Furthermore, different from the ATRP and SET-LRP, no metal catalysts were required for RAFT polymerization, thus additional purification procedure can be avoided for the finally polymer composites. It is therefore, the strategy combination of mussel inspired chemistry and RAFT polymerization should be of great research interest for fabrication polymer composites for various applications. Acknowledgements This research was supported by the National Science Foundation of China (nos. 51363016, 21474057, 21564006, 21561022, 21644014), Natural Science Foundation of Jiangxi Province in China (nos. 20161BAB203072, 20161BAB213066) and the National 973 Project (Nos. 2011CB935700). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2017.06.009. References [1] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Iijima, Nat. Mater. 5 (2006) 987–994. [2] K. Jiang, Q. Li, S. Fan, Nature 419 (2002) 801.
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Biomimetic PEGylation of carbon nanotubes through surface-initiated RAFT polymerization Yingge Shi a,c,1, Guanjian Zeng a,1, Dazhuang Xu a, Meiying Liu a, Ke Wang b, Zhen Li b, Lihua Fu c, Qingsong Zhang b, Xiaoyong Zhang a,⁎, Yen Wei b a b c
Department of Chemistry and Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, PR China Department of Chemistry and the Tsinghua Center for Frontier Polymer Research, Tsinghua University, Beijing 100084, PR China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 8 May 2017 Received in revised form 24 May 2017 Accepted 15 June 2017 Available online 16 June 2017 Keywords: Carbon nanotubes Surface-initiated polymerization Mussel inspired chemistry Biomedical applications
a b s t r a c t Carbon nanotubes (CNTs) are a type of one-dimensional carbon nanomaterials that possess excellent physicochemical properties and have been potentially utilized for a variety of applications. Surface modification of CNTs with polymers is a general route to expand and improve the performance of CNTs and has attracted great research interest over the past few decades. Although many methods have been developed previously, most of these methods still showed some disadvantages, such as low efficiency, complex experimental procedure and harsh reaction conditions etc. In this work, we reported a practical and novel way to fabricate CNTs based polymer composites via the combination of mussel inspired chemistry and reversible addition fragmentation chain transfer (RAFT) polymerization. First, the amino group was introduced onto the surface of CNTs via self-polymerization of dopamine. Then, chain transfer agent can be immobilized on the amino groups functionalized CNTs to obtain CNT-PDA-CTA, which can be utilized for surface-initiated RAFT polymerization. A water soluble and biocompatible monomer poly(ethylene glycol) monomethyl ether methacrylate (PEGMA) was adopted to fabricate pPEGMA functionalized CNTs through RAFT polymerization. The successful preparation of CNTs based polymer composites (CNT-pPEGMA) was confirmed by transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy in details. The CNTpPEGMA showed good dispersibility and desirable biocompatibility, making them highly potential for biomedical applications. More importantly, a large number of CNTs based polymer composites could also be fabricated through the same strategy when different monomers were used due to the good monomer adaptability of RAFT polymerization. Therefore, this strategy should be a general method for preparation of various multifunctional CNTs based polymer composites. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Since they were officially recognized and named in 1991, carbon nanotubes (CNTs) have drawn much attention for their outstanding mechanical, thermal and electrical properties [1–3]. Therefore, they have been widely applied in structure materials, thermal conductor, environmental adsorption, energy storage and conversion, chem/biosensors and biomedical applications [4–16]. Even though CNTs own remarkable properties, it is wiser to make a combination with others, such as metal, inorganic substances, polymers and so on [17–20]. CNTs based polymer composites were considered as one of the significant parts, which were devisable and can maximize the potential of both polymers and CNTs [21]. So far, many studies have been done to ⁎ Corresponding author. E-mail address: [email protected] (X. Zhang). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.msec.2017.06.009 0928-4931/© 2017 Elsevier B.V. All rights reserved.
fabricate CNTs based polymer composites and achieved certain success. For example, solution mixing, melt blending and in situ polymerization were developed from the point of practical application and theoretical research [22]. Compared with pure polymers or CNTs, these CNTs based polymer composites may overcome the defects such as poor designability, low conductivity and weak intensity [21,23]. In the process of preparation, dispersion and interfacial adhesion were two critical factors to create excellent composites. Because of the small size, high specific surface areas and aspect ratio, strong hydrophobic interaction was existed between CNTs, which lead to the serious aggregation of these pristine CNTs [24]. From the aspect of the micro, this behavior was resulted from electronic efficient and van der Waals attraction. Chemical functionalization, based on covalent linkages, was employed to exclude the electronic efficient and endow reactive sites on the surface of CNTs [25–28]. On the other hand, physical functionalization, based on non-covalent self-assembly, was also used to enhance interfacial adhesion. The later mainly depended on van der Waals attraction
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and π-π stacking between polymers and CNTs [29–31]. Nonetheless, the pretreatment of covalent linkages may cause irreparable damage to the structure of CNTs and negative impact on the thermal, mechanical or electrical properties. In addition, the strength of π-π stacking is relative unstable in some extent and only limited polymers can be choose for non-covalent strategies. The limitation of these conventional surface modification strategies will largely impede the further applications of CNTs. Therefore, it was still necessary to design and explore novel and efficient methods for fabrication of CNTs based polymer composites. Mussel inspired chemistry is an emerged surface modification strategy, which has attracted increasing attention in the past few years [32, 33]. It has been demonstrated that dopamine (DA), a crucial component of mussel inspired chemistry, would be self-polymerized into PDA under alkaline environment. The resulting PDA films can not only strongly adhere onto almost any materials, but also provide a number of active sites for further reactions [34,35]. Because of these unique characteristics, mussel inspired chemistry have becoming the research focus for diverse applications [36–42, 44–48, 50]. For example, Lee et al. connected hydroxyapatite on the surface of CNTs by the aid of mussel inspired chemistry [49]. In the process, DA not only plays the role of adhesion but significantly weakened the toxicity of CNTs and enhanced the biocompatibility. On the other hand, we have demonstrated that amino groups functionalized CNTs can be facilely fabricated via the combination of mussel inspired chemistry and Michael addition reaction. These functionalized CNTs showed obviously enhanced adsorption capability toward Cu2 + [50]. Furthermore, surface modification of graphene oxide with heparin and BSA was also demonstrated by Zhao and Cheng et al. [51] They confirmed that surface modification of graphene oxide with these biomacromolecules could effectively improve water dispersibility and biocompatibility of graphene oxide. Surface modification of CNTs with synthetic polymers through in situ surface polymerization methods have demonstrated to be efficient routes to prepare CNTs based polymer composites [52–54]. Most of these strategies were mainly relied on the surface oxidation of CNTs. However, the procedure for surface oxidation of CNTs is rather complex and inefficient and will inevitably destroy the structure of CNTs. Therefore, some studies for surface modification of CNTs via the combination of mussel inspired chemistry and atom transfer radical polymerization (ATRP) and single-electron living radical polymerization (SET-LRP) have been demonstrated recently [55,56]. However, both ATRP and SET-LRP are required using copper ions as catalysts, which are toxic to living organisms and may influence the final properties of composites. Reversible addition fragmentation chain transfer (RAFT) polymerization is another controlled living polymerization method, which can be used for preparation of well controlled polymers in the absence of metal catalysts [57,58]. To the best of our knowledge, the preparation of CNTs based polymer composites through the combination of mussel inspired chemistry and surface-initiated RAFT polymerization has not reported thus far. In this contribution, we describe a practical and efficient method to fabrication of CNTs based polymer composites, that combination of mussel inspired chemistry and RAFT polymerization. The pristine CNTs were first coated with PDA, which was used for surface immobilization of chain transfer agent (CTA) to initiate RAFT polymerization (Scheme 1). The successful preparation of CNTs based polymer composites (CNT-pPEGMA) were confirmed by a number of characterization techniques. Cell viability evaluation demonstrated that CNT-pPEGMA showed great cytocompatibility, indicating that these CNTs based polymer composites are potential for biomedical applications. 2. Experiment 2.1. Materials and characterization All of the chemicals were directly used without further purification. CNTs were purchased from Sinonano (Beijing, China). Dopamine
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Scheme 1. Schematic representation for the preparation of CNT-pPEGMA via the combination of mussel inspired chemistry and RAFT polymerization.
hydrochloride was purchased from Sangon Biotech. Co., Ltd. (Shanghai, China). N-(3-Dimethylaminopropy)-N-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), Tris-(hydroxymethyl) aminomethane (Tris) and poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mw: 950 Da, 98%) and polyethylenimine (PEI, Mw: 1800 Da) were obtained from Aladdin (Shanghai, China) without further purification. 2-Morpholinoethanesulfonic acid (MES) was supplied by Heowns (Tianjin, China). Chain transfer agent (CTA) was synthesized according to our previous report [29]. The other chemical agents were all of commercially available and analytical grade. The water involved in the experiment was deionized water (D.I. water). The Fourier transform infrared (FT-IR) spectra were gained in a transmission mode on a Nicolet 5700 spectrometer (Waltham, MA, USA). Thermal gravimetric analysis (TGA) was tested on a TA instrument Q50 with a heating rate of 20 °C min−1 using crucibles of aluminum. The X-ray photoelectron spectra (XPS) were measured on a VGESCALAB 220-IXL spectrometer using an A1 Ka X-ray source (1486.6 eV). Transmission electron microscopy (TEM) images were recorded on a Hitachi 7650B microscope operated at 80 Kv. The TEM specimens were made by placing a drop nanoparticle ethanol suspension on a carbon-coated copper grid. 2.2. Preparation of CNT-PDA-NH2 CNT-PDA-NH2 was synthesized as described in the following procedure. 150 mg of pristine CNTs were added into 50 mL of Tris solution (10 mM L−1, pH = 8.5) and sonicated for 10 min. Then, 150 mg of DA was also put into and the mixture was stirred at room temperature for 2 h. The mixture (CNT-PDA) was separated from the reaction solution through centrifugation. After that, 150 mg of PEI was added to react with CNT-PDA for further 2 h. The PEI can be immobilized on PDA coatings through Michael addition reaction between PDA and amino groups. On the other hand, the surplus amino groups that immobilized on the surface of CNTs can be further reacted with carboxyl group of CTA to facility the immobilization of CTA on CNTs. The CNT-PDA-NH2 was got after centrifuging and washing at 8000 rpm for 10 min per time until suspension was clarified. Last, the product was dried under vacuum for further use. 2.3. Preparation of CNT-PDA-CTA CNT-PDA-CTA was synthesized in MES buffer solution (25 mM, pH = 6). First, 10 mg of CTA was added into 10 mL of MES solution. And then, 4.5 mg of NHS and 7.5 mg of EDC were added in the CTA contained solution. After 10 min, 100 mg of CNT-PDA-NH 2 was added and further reacted overnight at room temperature. The resulting CNT-PDA-CTA was separated from MES solution by centrifugation at 8000 rpm for 10 min and vigorously washing with
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acetone to remove residual reactants. The obtained product was dried under vacuum for further use. 2.4. PEGylation of CNTs via surface-initiated RAFT polymerization CNT-pPEGMA was synthesized via RAFT polymerization using PEGMA as monomer under the protection of N2. 150 mg of PEGMA, 50 mg of CNT-PDA-CTA, 0.6 mg of AIBN and 5 mL methylbenzene were added into reaction flask, which was sealed and vacuum for 10 min to remove any air from the reaction system. The mixture was put into an oil bath maintained at 70 °C for 10 h. In the end, the products were obtained by centrifugation and washing repeatedly. 2.5. Biocompatibility evaluation of CNT-pPEGMA The biocompatibility evaluation was conducted based on our previous report [59,60]. Cells were seeded in 96-well microplates at a density of 5 × 104 cells per mL in 160 μL of the respective media containing 10% FBS. After 24 h of cell attachment, the HeLa cells and A549 cells were incubated with 10, 20, 40, 80, 100 μg mL−1 of CNT-pPEGMA for 24 h. Then the cells were washed with PBS for three times to remove the uninternalized nanoparticles. After that, 10 μL of CCK-8 dye and 100 μL of Dulbecco's Modified Eagle's Medium (DMEM) cell culture media was added to each well and incubated for 2 h at 37 °C. Plates were then analyzed with a microplate reader (VictorIII, Perkin-Elmer). Measurements of dye absorbance were carried out at 450 nm, with the reference wavelength at 620 nm. The values were proportional to the number of live cells. The percent reduction of WST was compared to the control (cells not exposed to nanoparticles), which represented 100% WST reduction. Three replicate wells were used for each control and test concentrations per microplate, and the experiment was repeated three times. Cell survival was expressed as absorbance relative to that of untreated controls. Results are presented as mean ± standard deviation (SD). 3. Results and discussion To extend the applications of CNTs, surface modification of CNTs with polymers to improve dispersion of CNTs based composites and enhance the interfacial adhesion CNTs and polymer matrix was incredibly important. Although many efforts have been made in the fabrication of CNTs based polymer composites, simple, effective and universal strategies to prepare CNTs based polymer composites are still rarely developed. One of the most adopted strategies for fabrication of polymers functionalized CNTs is through surface-initiated polymerization such as ATRP, RAFT etc. [53,61] However, surface oxidation is often required to introduce oxygen contained functional groups on the surface of CNTs for immobilization of polymerization initiators [53,62]. This procedure
is rather complex, time consuming and relative low efficiency. Mussel inspired chemistry has been demonstrated to be a very promising surface functionalization method for the strong and universal adhesion of PDA toward different material surfaces [32,63]. The combination of mussel inspired chemistry and controlled living polymerization such as (ATRP and SET-LRP) have recently demonstrated our group and some other groups [55,64]. However, both of the two polymerization methods (ATRP, SET-LRP) are needed using metal catalysts, which should be completely removed from the final products. As compared with ATRP and SET-LRP, RAFT polymerization is another important controlled living polymerization method with good polymerization properties [65,66]. However, to the best of our knowledge, the preparation of CNTs based polymer composites through combination of mussel inspired chemistry and surface-initiated RAFT polymerization has not demonstrated before. In this work, the PEGylated CNTs have been prepared using the strategy as displayed in Scheme 1. And the final product (CNT-pPEGMA) was characterized by a number of techniques in detail. As shown in Fig. 1, both CNT-PDA-CTA (Fig. 1A) and CNT-pPEGMA (Fig. 1B) still maintained their fiber like morphology. However, some blurry films can be also found around the CNTs (as indicated by the red arrows). The enlarged TEM images of CNT-PDA-CTA and CNT-pPEGMA further confirmed that the CNTs were functionalized by polymer films. As evidenced by Fig. S1, rather uniform polymer films can be observed. The thickness of the PDA films is about 5 nm. As compared with the CNT-PDA-CTA, the thickness of CNT-pPEGMA was increased to about 8 nm. These results implied that PDA and pPEGMA have successfully modified CNTs via mussel inspired chemistry and RAFT polymerization, respectively. The mass percentage of polymers coated on CNTs was determined by TGA under N2 environment. As shown in Fig. 2, the loss of pristine CNT was about 2.6% except free water, while the loss increased to 10.32% after introducing PDA. It can be calculated that the weight percentage of PDA and PEI on CNTs is about 7.7%. The CNT-PDA-CTA began to lose weight at about 170 °C and had about 16.36% (without free water) residue weight to 600 °C. The significant weight loss between CNT-PDA-NH2 and CNT-PDA-CTA suggested that the CTA has immobilized on the CNTs successfully. Based on the TGA results, the mass percentage of CTA immobilized on the CNTs is greater than 6%. After polymerization, the weight loss of CNT-pPEGMA was further increased to 84.73%. These obvious weight loss clearly evidenced that pPEGMA has successfully grafted onto the CNTs through RAFT polymerization. The significant weight loss was occurred at the temperature between 350 and 420 °C, which is well consistent with the weight loss of pPEGMA. Based on the experimental conditions (weight percentage of PEGMA is about 75%) and TGA curves, we could speculate that almost all the PEGMA monomer was grafted onto CNTs. The DTA curves of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA were displayed in Fig. S3. It can be seen that almost no peaks were found in the sample
Fig. 1. Representative TEM images of CNT-PDA-CTA (A) and CNT-pPEGMA (B); scale bar = 200 nm. It can be seen that some blurry films around the CNTs were observed, implying the successful coating CNTs with PDA and pPEGMA.
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of pristine CNTs below 600 °C, indicating the good thermal stability of CNTs (Fig. S3A). As compared with CNTs, broad endothermic peaks with max values about 300 °C was observed in the samples of CNTPDA-NH2 and CNT-PDA-CTA, that can be ascribed to the decomposition of PDA and CTA (Figs. S3B and S3C). These rather broad endothermic peaks suggested that the compositions of PDA are rather complex. However, the max values of endothermic peaks of CNT-PDA-NH2 and CNTPDA-CTA also showed some difference. For example, the max value of CNT-PDA-NH2 is 309.6 °C. However, the max value was shifted to 284.8 °C for the sample of CNT-PDA-CTA. The difference should be ascribed to the decomposition of CTA is different from PDA. And the decomposition temperature of CTA is lower than 300 °C. These results suggested that CTA has indeed immobilized on CNTs. Two endothermic peaks were observed in the sample of CNT-pPEGMA, which was located at 302.9 and 429.9 °C, respectively. The emerged peak at 429.9 °C should be ascribed to the decomposition of pPEGMA. All of these results further confirmed that CNTs based polymer composites can be facilely prepared via combination of mussel inspired chemistry and RAFT polymerization. Fig. 3 gave the FT-IR spectra of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA. The peaks at 3200, 1400 and 1250 cm− 1 just corresponded to N\\H stretching vibration, N\\H flexural vibration and C\\N stretching vibration, respectively. More importantly, the signals in the region 1600–1400 cm−1 derived from stretching vibration of aromatic ring in DA. Therefore, DA had been introduced onto the surface of CNTs successfully. The peak at 1708 cm−1, assigned to C_O, indicated the graft of CTA as expected. By contrast, after polymerization
with pPEGMA, the strong peaks at 2867, 1711 and 1135 cm−1 were observed, that can be ascribed to characteristic functional groups, C\\H (methylene), C_O (ester) and C\\O (ether), confirmed the successful surface modification of CNTs with pPEGMA. The elements and chemical information of all samples were investigated by XPS spectra (Fig. 4). In the XPS spectrum of pristine CNTs, two obvious signals, C 1s and O 1s were appeared at about 285 and 533 eV, which just corresponded with previously reports [67]. The successful introduction of PDA can be confirmed by the emergence of the N 1s signals at a binding energy at about 400 eV in the wide scan XPS spectrum of CNT-PDA-CTA. Furthermore, the successful conjugation of CTA with CNT-PDA-NH2 can be evidenced by the emergence of S 2p peak in the XPS spectrum of CNT-PDA-CTA. As compared with CNTPDA-CTA, the N 1s intensity was obviously decreased while the O 1s intensity was significantly increased in the sample of CNT-pPEGMA, implying that pPEGMA has been grafted on the surface of CNTs through RAFT polymerization successfully. Fig. S4 showed the XPS spectra of C 1s, O 1s, N 1s and S 2p, respectively. As shown in Fig. S4A, only a major peak located at 285 eV can be observed in the sample of pristine CNTs, which should be attributed to the C\\C and C_C bonds. After surface coated with PDA and linked with CTA, a small peak with higher binding energy was emerged, indicating that PDA and CTA were linked onto CNTs. After surface polymerized with PEGMA, a small peak with lower binding energy was appeared, indicating the pPEGMA was grafted onto CNT-PDA-CTA. The successful immobilization of PDA and CTA on the CNTs was also confirmed by N 1s and S 2p spectra. It can be seen that obvious peaks of N 1s and S 2p were only found in the sample of CNT-PDA-CTA. These results confirmed that CTA was successfully immobilized onto CNTs. On the other hand, significant decrease intensity of N 1s and S 2p in the sample of CNT-pPEGMA demonstrated that the polymer (pPEGMA) has been grafted onto CNT-PDA-CTA through RAFT polymerization. As shown in Fig. 5A, the C 1s core level spectrum of CNTs can be fitted into three peaks with BEs at 284.9, 285.9 and 290.45 eV. These peaks can be attributed to Sp2 C_C/Sp3 C\\C, C\\O and O\\COO species, respectively. After introducing PDA and CTA, the C 1s spectrum of CNT-PDA-CTA can be deconvoluted into six peaks (Fig. 5B) which were C\\C (284.67 eV), C\\H (285.06 eV), C\\N (285.74 eV), C\\O/ C\\S (286.65 eV), O_C\\O (288.19 eV) and π-π* (290.91 eV). The emergence of C\\N and C\\S bonds further indicated that PDA and CTA were linked on the CNTs through mussel inspired chemistry. In the curve fitted C 1s core level spectrum of CNT-pPEGMA, four peaks assigned to C\\H (285 eV), C\\C (285.5 eV), C\\O (286.4 eV) and \\COO (289.5 eV) were emerged. As compared with other samples, the C\\O signal at 286.4 eV was significantly increased in the sample
Fig. 3. FT-IR spectra of CNT, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA.
Fig. 4. Survey scans XPS spectra of CNTs, CNT-PDA-CTA and CNT-pPEGMA.
Fig. 2. TGA curves of CNTs, CNT-PDA-NH2, CNT-PDA-CTA and CNT-pPEGMA.
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Fig. 5. C 1s XPS spectra of CNT (A), CNT-PDA-CTA (B) and CNT-pPEGMA (C).
of CNT-pPEGMA, indicating that the surface of CNT-pPEGMA had been fully covered by pPEGMA coating (Fig. 5C). Furthermore, the contents of C, N, O and S in the sample of CNTs, CNT-PDA-CTA and CNT-pPEGMA were displayed in Table 1. It can be seen that the pristine CNTs are majorly composited with C (97.13%) with a small part of O (2.87%). However, two new elements N and S were emerged in sample of CNT-PDA-CTA with percentages of 7.62% and 1.36%, respectively. Based on the content of S, the percentage of CTA immobilized on CNT-PDA-CTA is about 7.45%, which is well consistent with the TGA results. After surface modified with pPEGMA, the percentage of C was significantly decreased to 64.41%, while the percentage of O was significantly increased to 34.74%. On the other hand, the percentages of N and S were also significantly decreased to 0.68% and 0.17%, respectively. These results clearly confirmed that pPEGMA was successfully grafted onto CNT-PDA-CTA through RAFT polymerization. The pPEGMA on CNTs was calculated to be 76.58% based on the results of XPS spectra, which are well consistent with the result of TGA. The dispersity of CNTs, CNT-PDA-CTA and CNT-pPEGMA was shown in Fig. 6. It can be seen that the CNTs (bottle 1) and CNT-PDA-CTA (bottle 2) were quickly deposited at the bottom of bottles within 10 min. As compared with CNTs and CNT-PDA-CTA, no aggregation was found in the sample of CNT-pPEGMA even they were deposited for more than 24 h. The excellent water dispersibility also reflected successful modification of CNTs with pPEGMA through RAFT polymerization. Moreover, CNT-pPEGMA also had outstanding dispersion in some organic solvents such as dimethyl sulfoxide (DMSO), N,N-Dimethylmethanamide (DMF) and acetone. CNT-pPEGMA can be dispersed in dichloromethane (DCM) (bottle 7) for short time, however, it will also deposit within 2 h. The possible reason is poor dispersibility of pPEGMA in DCM. The good dispersibility of CNT-pPEGMA in aqueous solution and many organic solvents is very useful for their applications.
It is well known that PEG is a biocompatible and hydrophilic polymer, which has been widely used for surface modification of nanomaterials. It has been demonstrated that surface PEGylation can not only enhance the water solution of nanomaterials, but also improve their pharmacokinetics behavior and biocompatibility in vivo. Therefore, the cytocompatibility of CNT-pPEGMA to HeLa cells and A549 cells was evaluated using CCK-8 assay. As shown in Fig. 7, all the cell viability values of CNT-pPEGMA to HeLa cells and A549 cells for 24 h are still greater than 91% even the concentration of is CNT-pPEGMA as high as 100 μg mL− 1. As compared with our previous reports, the PEGylated CNTs exhibited much better biocompatibility to cells. The possible reason is likely due to the anti-oxidation effect of PDA and camouflage effect of CNT-pPEGMA. The excellent biocompatibility and well water dispersity of CNT-pPEGMA implied their potential for biomedical applications. The CNTs have high photothermal conversion capability and the PEGMA is a desirable biological monomer. More importantly,
Table 1 The surface elemental compositions of CNTs, CNT-PDA-CTA and CNT-pPEGMA. Samples
C 1s
N 1s
O 1s
S 2p
CNTs CNT-PDA-CTA CNT-pPEGMA
97.13 83.85 64.41
0 7.62 0.68
2.87 7.17 34.74
0 1.36 0.17
Fig. 6. Dispersion of CNT (1), CNT-PDA-CTA (2) and CNT-pPEGMA (3–7) at 10 min (A); 2 h (B); 24 h (C). From 1 to 3, the solvent was water. From 4 to 7, the solvents were DMSO, DMF, acetone and DCM, respectively.
Y. Shi et al. / Materials Science and Engineering C 80 (2017) 404–410
Fig. 7. Cell viability evaluation of CNT-pPEGMA to HeLa cells and A549 cells. The concentrations of CNT-pPEGMA were ranged from 10 to 100 μg mL−1.
the end hydroxyl groups of the grafted PEGMA polymer can further react with various functional compounds. Therefore, the CNT-pPEMGA is suitable used for various biomedical applications, such as intracellular delivery biological active agents and photothermal cancer treatment. Moreover, due to the universality of mussel inspired chemistry and good applicability of RAFT, this method developed in this work should also be potentially adopted for fabrication of many other polymeric nanocomposites with great potential for controlled drug delivery, cancer photothermal treatment etc. [68–77]. 4. Conclusion In summary, we have developed a novel and effective strategy for surface modification of CNTs based on mussel inspired chemistry and surface-initiated RAFT polymerization. As compared with the conventional surface-initiated polymerization, the strategy described in this work need not oxidize CNTs first. Therefore the structure and physicochemical properties of CNTs can be well preserved. On the other hand, due to the versatility of mussel inspired chemistry and general applicability of RAFT polymerization, various polymer composites can be fabricated through the strategy developed in this work. Furthermore, different from the ATRP and SET-LRP, no metal catalysts were required for RAFT polymerization, thus additional purification procedure can be avoided for the finally polymer composites. It is therefore, the strategy combination of mussel inspired chemistry and RAFT polymerization should be of great research interest for fabrication polymer composites for various applications. Acknowledgements This research was supported by the National Science Foundation of China (nos. 51363016, 21474057, 21564006, 21561022, 21644014), Natural Science Foundation of Jiangxi Province in China (nos. 20161BAB203072, 20161BAB213066) and the National 973 Project (Nos. 2011CB935700). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2017.06.009. References [1] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Iijima, Nat. Mater. 5 (2006) 987–994. [2] K. Jiang, Q. Li, S. Fan, Nature 419 (2002) 801.
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