Silk fibroin biomaterial-functionalized carbon nanotubes for high water dispersibility and promising biomedical applications

Abstract

The dispersion of carbon nanotubes in solvents is crucial for their practical applications. In this study, multi-walled carbon nanotubes (MWCNTs) functionalized with water-soluble silk fibroin (SF) were prepared via chemical modification on the carbon nanotube surface in water solution. These fillers are low in toxicity and environmentally friendly. The SF-functionalized MWCNTs greatly improved dispersion in H₂O and methanoic acid. Additionally, a methyl thiazolyl tetrazolium assay was performed to assess biocompatibility and the results indicated the favorable biocompatible properties of MWCNTs due to SF functionalization. Therefore, SF-functionalized MWCNTs have potential applications in waterborne polymer-based nanocomposites, especially in the biomedical field.

Keywords

silk fibroin carbon nanotubes dispersity waterborne biomaterials

Recently, carbon nanotubes (CNTs) have attracted considerable attention due to their unique structural and outstanding electrical, mechanical and optical properties.^1–4 For instance, CNTs show great potential in polymer composites and in biological and biomedical fields.^3–8 However, the smooth surface of nanotubes, the lack of interfacial bonding across the interface of a nanotube/polymer and the level of dispersion of nanotubes in the matrix are critical factor for the fabrication of the reinforced composites.^9–12 Thus, numerous methods for the chemical functionalization of CNTs have already been reported.^1,9 Chemical functionalization of CNTs can maintain stable chemical covalent bonding between CNTs and the material of interest.^13–15 Amino-functionalized CNTs have been researched because the amino group exhibits high reactivity.^16–18 However, the functionalization of most amino groups is through organic solvents, which are not environmentally friendly and sometimes toxic.^9,19 In recent years, many researchers have explored waterborne polymer-based composites because they are non-toxic.^20,21 Therefore, CNTs with high water dispersibility possess great advantages in the preparation of waterborne polymer-based composites.^22–24 Furthermore, much effort has been devoted to demonstrating the potential of CNTs for a wide variety of opportunities and applications (due to their unique structural and other properties) in biological and biomedical systems and devices.^25–27 However, the high toxicity of CNTs should be immediately addressed.^28,29 The high water dispersibility of CNTs is beneficial for applications in biological and biomedical systems and devices.^30–32

Biomass materials, which exhibit remarkable biocompatibility with several cell types, promoting adhesion, growth and functionality are promising raw materials to modify CNTs and obtain low-toxicity functionalized CNTs.^33,34 Silk fibroin (SF) is a traditional biomaterial that displays high biocompatibility.^35–39 SF molecules are amino-terminated and are reasonably reactive with the carboxyl group.^16,33

In this study, raw multi-walled CNTs (MWCNTs) (Raw-Ms) were first prefunctionalized through acid oxidation (to gain oxidized MWCNTs), followed by water-soluble SF (owning –NH₂) modification in water solution and mild reaction conditions. In the process of modifying the acid-oxidized MWCNTs (AO-Ms), no toxicant was used or introduced. Moreover, the modification generated only a small amount of polluted air or wastewater. The chemical reactions for the functionalization of MWCNTs are illustrated in Figure 1. A series of tests was carried out to characterize the properties of such functionalized CNTs. These properties included thermal properties, dispersibility and cytotoxicity.

Figure 1.

Schematic of the reaction scheme to form multi-walled carbon nanotubes with amino functionalization.

Materials and methods

Materials and preparation of functionalized MWCNTs

Water-soluble SF (powder, 0.1–2 µm diameter, bulk density = 0.2 g·cm⁻³, Mn = 12,000) was prepared in our laboratory (SF obtained by “0.05 M (w/v) Na₂CO₃ aqueous solution at approximately 120℃ under 0.24 MPa for 2 h” was used).⁴⁰ N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC, 98%) and N-hydroxysuccinimide (NHS, 98%) were purchased from Aladdin (Shanghai, China). MWCNTs (98%, 10–20 nm outer diameters and 10–20 µm lengths) were purchased from Chengdu Organic Chemistry Limited Company (Sichuan, Chengdu, China). All the reagents were purchased from commercially available sources and used without further purification unless otherwise stated.

Raw-Ms were oxidized with a mixture of H₂SO₄ (98%)/HNO₃ (68%) (3:1, v/v) at 60℃ for 3 h. The carboxylated MWCNTs were filtered, washed with deionized water until pH 7 was reached and dried in a vacuum oven. The as-prepared AO-Ms (100 mg) were suspended in 10 ml of deionized water by sonicating the mixture for 30 min. Subsequently, 4.5 mmol EDC and 4.5 mmol NHS were added to the above suspension and mixed sufficiently. After stirring the mixture for 24 h, SF (1.98 g, to obtain a saturated solution) was added. The reaction was allowed to stir for another 36 h. After centrifugation, sonication and repeated washing, the SF-functionalized MWCNTs (SF-Ms) were obtained. The control sample, which was prepared following the same steps but without adding catalyst (EDC/NHS) as per the reaction was in Figure 1, was called SF/M (mixture of SF and MWCNTs).

Characterization

Functionalization of MWCNTs was confirmed by FTIR spectra recorded on a Nicolet 5700 spectrophotometer (Thermo Fisher Scientific, USA). X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific) was conducted using an Al Ká X-ray source at 250 W. The binding energy scale was calibrated with respect to the C1s peak of hydrocarbon contamination fixed at 285.0 eV. The peak at around 400 eV corresponded to N 1s, which was derived from the C–N bond. The structure was analyzed via transmission electron microscopy (TEM; JEOL JEM-2100, Japan). Thermal properties were determined with TG209F1 (NETZSCH, Germany).

Dispersibility experiment

After mixing 5 mg of MWCNTs in 10 ml of three kinds of solvents (H₂O, methanoic acid and methanol) and sonicating with a KQ-400DE sonicator for 1 h, we took optical micrographs of the samples in these kinds of solvents obtained 6 h after the solution had been sonicated for 1 h. Furthermore, the dispersions of MWCNTs were determined by a UV–vis spectrophotometer (TU-1901 spectrophotometer, Pgeneral, China) and conducted quantitative analysis for the as-prepared samples in the same three kinds of solvents at a concentration of 1 mg/10 ml taken 6 h after the solution had been sonicated for 1 h.

MTT assay

The cytotoxicity of the as-prepared samples was further evaluated via the MTT assay. Murine preosteoblastic MC3T3-E1 cells were grown in a 96-well plate containing α-MEM cell culture media at 37℃ in a 5% CO₂ incubator. After culturing for 1 day, the culture medium was removed and incubated with supernatant (100 µl), followed by ultrasonication for 30 min. After additional incubation for 1 day, 20 µl of MTT (MTT concentration = 5 mg/ml) was added to each well. The cultures were then incubated for 4 h at 37℃ and the supernatants were removed from the wells. About 150 µl of dimethyl sulfoxide was added to each well and 100 µl of mixed liquid from each well was transferred to a 96-well plate. Finally, the plate was read in an automated microplate spectrophotometer with 490 nm as the reference. Every 48 h, the same test was performed as mentioned above until 168 h had passed.

Results and discussion

FTIR spectroscopy

Figure 2(a) shows the FTIR spectra for the as-received raw-M, AO-M, SF/M and SF-M samples. The peak at 1631 cm⁻¹ in spectrum B was attributed to the C = O stretching frequency of the carboxylate (COO-) group, which was stronger than the one in spectrum A contributing to the acid oxidation, and there was no other evident difference between spectra B and A. The peak at 1631 cm⁻¹ without any red or blue shift (spectrum C) confirmed that the oxidized MWCNT did not graft with SF. The difference between spectra C and B may be caused by the SF attaching to the MWCNT (around 1534 cm⁻¹ and 1250 cm⁻¹, attributed to amide II and amide III bands of SF, see spectra C). Furthermore, the small vibration at 1700 cm⁻¹ may have arisen from the reacted SF. The FTIR spectrum of the SF-M samples in spectrum D indicated that the C = O stretching frequencies shifted from 1631 cm⁻¹ of COO- to 1636 cm⁻¹ (which was obviously stronger) assigned to the amide I, arising from amide functional group. Moreover, the presence of stronger bands at 1535 cm⁻¹, 1256 cm⁻¹ and 3283 cm⁻¹ corresponding to amide II, amide III and N-H bond stretching, respectively, further confirmed the presence of the amide functional group, namely, the SF functional group. A previous study reported similar results.^9,16,40

Figure 2.

(a) FTIR spectra of raw-Ms (spectrum A), AO-Ms (spectrum B), SF/Ms (spectrum C) and SF-Ms (spectrum D). (b) TGA curves of raw-Ms, AO-Ms, SF/Ms and SF-Ms in N₂ at 10℃/min.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) measurements of SF-Ms were conducted for the quantitative evaluation of the amino grafted onto MWCNTs (Figure 2(b)). TGA traces of raw-Ms illustrated minimal weight loss, which was 3% below 700℃. The AO-M sample demonstrated a small amount of weight loss: 8%, which can be attributed to the evaporation of the adsorbed water and the hydroxyl and carboxyl on the oxidized MWCNTs through the oxidation, in order to obtain enough carboxyl groups, so the acid treatment was stronger than usual.⁴¹ In contrast, the curves of SF/Ms and SF-Ms both displayed noticeable weight loss. The weight loss of SF/Ms may be due to the SF attached on the surface of MWCNTs, whereas the weight loss of the SF-Ms was attributed to the SF grafted and attached on the MWCNT surfaces. Therefore, the weight loss in the SF-M sample was about 16% higher than that of the SF/Ms. Thermal analysis indicated that the surfaces of SF-grafted MWCNTs were coated with a large amount of SF and SF/Ms displayed low coating efficiency.^9,33

XPS analysis

XPS analysis patterns for raw-Ms, AO-Ms, SF/Ms and SF-Ms are shown in Figure 3 to determine the chemical species introduced by the modification. Samples were degassed overnight within the XPS chamber (10⁻³ mbar) prior to analysis. As shown in Figure 3(a), unmodified MWCNTs displayed a single peak at 285 eV because of carbon. The peaks around 284.6 and 532.7 eV (Figure 3(b) to (d)) were attributed to carbon and oxygen, respectively. In addition, the peak at 400.2 eV in Figure 3(d) was due to nitrogen, which agreed with the results of FTIR analysis and confirmed that the surfaces of MWCNTs were grafted with a certain amount of SF. SF/M and SF-M samples were washed several times and most of the SF on the surfaces of the SF/Ms was washed away due to the weak combination. Therefore, there was no N1 peak in Figure 3(c). However, the combination of SF on the surfaces of SF-Ms and SF-Ms was much stronger so that there was an N1 peak in Figure 3(d) despite the washing. The atomic ratio of nitrogen was 6.287%,⁹ as reported in previously published reports, and the steric effect may influence the grafting efficiency. Moreover, the Mn of the SF used in this study was 12,000, which was much higher than common amines, thereby resulting in a strong steric effect.¹⁶

Figure 3.

XPS spectra of (a) raw-Ms, (b) AO-Ms, (c) SF/Ms and (d) SF-Ms.

TEM analysis

The morphology and nanostructures of raw-Ms, AO-Ms, SF/Ms and SF-Ms were observed by TEM, and the results are shown in Figure 4. The detailed morphology is presented in Figure 4(a) to (d). The unevenness of the SF-M surface differed from the other three surfaces. In EDS analysis, the appearance of a nitrogen peak in Figure 4(c) could be ascribed to the SF attaching on MWCNTs, and the stronger nitrogen peak could be attributed to the modification. To further prove the effect of SF modification, SF/Ms and SF-Ms were compared (Figure 4(e) to (h)). The representative fields were marked as clearly displayed in Figure 4(f). The MWCNT surface was notably coated with SF, which may be attributed to chemical processing. The grafted SF may result in more quantities of coated SF because of the interaction between SF molecules. Meanwhile, SF/Ms showed minimal changes, namely, in the difference in quantity between these two samples, which agreed with the previous images.³³ The difference between SF/Ms and SF-Ms in Figure 4 reflect the different coating efficiencies of these two kinds of materials, and those between the SF-Ms and SF-Ms in Figure 4 result from different magnifications and different areas.

Figure 4.

Nanostructures of (a) raw-Ms, (b) AO-Ms, (c) SF/Ms and (d) SF-Ms, and TEM images of (e) SF/Ms, (g) magnified SF/Ms, (f) SF-Ms and (h) magnified SF-Ms.

Evaluation of dispersibility

Table 1 and Figure 5(a) show the dispersibility of MWCNTs in three solvents, with mixing of 5 mg of MWCNTs in 10 ml of these solvents and sonicating with a KQ-400DE sonicator for 1 h. Furthermore, the dispersions of MWCNTs were determined by a UV–vis spectrophotometer (TU-1901 spectrophotometer, Pgeneral, China) and the obtained quantitative analysis for the as-prepared samples is illustrated in Figure 5(b). The MWCNT dispersibilities were changed significantly by the modification. SF-Ms displayed high dispersibility, whereas SF/Ms showed similar dispersibility to AO-Ms. However, according to the attaching SF on the MWCNT surfaces, the absorbance of SF/Ms was slightly higher than that of AO-Ms. The dispersion behavior of SF-Ms for methanoic acid was better than in H₂O, which could be attributed to the superior dissolubility of SF in methanoic acid. The remarkably enhanced dispersibility in H₂O and methanoic acid resulted from the grafted SFs on the surface of the MWCNTs. The dispersion activity for methanol was also studied. AO-Ms and SF/Ms were relatively easily dispersed in methanol, thereby suggesting that the attaching SF hardly affected the dispersibility of SF/Ms, which agreed with the results for the other two solvents. In addition, the poor dispersion of SF-Ms in methanol may be ascribed to the poor dissolubility of SF in methanol. The dispersibility for methanol further confirmed the change resulting from the modification. Given the novel SF-M dispersibility and the excellent biocompatibility for SF, studies on reactions with this kind of amino functionalization are of great importance because such functionalized MWCNTs are believed to be highly promising for the preparation of composite materials and biological technologies.⁴²

Table 1.

Dispersion of multi-walled carbon nanotubes in different solvents after various times (23 ± 2℃)

Solvent	Time	raw-M	AO-M	SF/M	SF-M
H₂O	8 h	D	B	A	A
Methanoic acid	8 h	D	B	A	A
Methanol	8 h	C	A	A	C
H₂O	20 h	D	C	B	A
Methanoic acid	20 h	D	C	B	A
Methanol	20 h	D	A	A	D
H₂O	3 weeks	D	D	D	A
Methanoic acid	3 weeks	D	D	D	A
Methanol	3 weeks	D	A	C	D

A: well dispersed; AO-M: acid-oxidized multi-walled carbon nanotubes; B: mainly dispersed; C: partly precipitated; D: fully precipitated; MWCNT: multi-walled carbon nanotube; SF/M: mixture of silk fibroin and MWCNTs; SF-M: silk fibroin-functionalized MWCNTs.⁹

Figure 5.

(a) Optical micrographs of the samples in (A1) H₂O, (A2) methanoic acid and (A3) methanol obtained 6 h after the solution had been sonicated for 1 h. The concentration was 5 mg/10 ml; (b) UV–vis spectra of the samples in (B1) H₂O, (B2) methanoic acid and (B3) methanol at a concentration of 1 mg/10 ml taken 6 h after the solution had been sonicated for 1 h. (c) MTT results of raw-M, AO-M, SF/M, SF-M and control samples.

Cytotoxicity analysis

The cytotoxicities of the raw-M, AO-M, SF/M, and SF-M samples were evaluated by MTT assay. The control group, in which cells underwent the same process but without the addition of any MWCNTs, provided a comparison. As shown in Figure 5(c), all samples displayed a certain amount of cytotoxicity and restricted cell viability on the first day of incubation. Further comparison revealed that SF-Ms demonstrated high cell viability, suggesting enhanced biocompatibility. The following results also showed a similar trend, which may be due to two aspects. On the one hand, the SF-M sample possessing better dispersion (as shown in Figure 5(a) and (b)) may prevent the occurrence of sectional toxicity. On the other hand, the favorable biocompatibility of SF could play a critical role as a positive factor.⁴³

Conclusion

In this study, SF-Ms were prepared by carboxylation and grafting. The SF-functionalized CNTs displayed outstanding dispersibility in H₂O and methanoic acid, which may result from the excellent solubility of SF for H₂O and methanoic acid, indicating the modification of the MWCNTs. This kind of functionalization is believed to offer nanotube derivatives suitable for a number of applications such as waterborne polymer/CNT composites. Moreover, this low-toxicity and environmentally friendly filler meets the requirements for several applications, especially in the field of biomaterials. Cytotoxicity analysis was conducted and good proliferation for SF-Ms was found. Thus, functionalization of nanotubes dramatically improved the biocompatibility of these nanomaterials. Future work will focus on the biomedical applications and other properties of SF-M composites.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant Number 51473147) and the Zhejiang Sci-Tech University graduate innovation foundation (Grant Number YCX15005).

References

Siddiqui

Marom

et al.

Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review. Compos Part A Appl Sci Manuf 2010; 41: 1345–1367.

Sun

Lin

et al.

Functionalized carbon nanotubes: properties and applications. Accounts Chem Res 2002; 35: 1096–1104.

Prato

Kostarelos

Bianco

. Functionalized carbon nanotubes in drug design and discovery. Accounts Chem Res 2007; 41: 60–68.

Battigelli

Ménard-Moyon

Da Ros

et al.

Endowing carbon nanotubes with biological and biomedical properties by chemical modifications. Adv Drug Deliver Rev 2013; 65: 1899–1920.

White

Best

Kinloch

. Hydroxyapatite–carbon nanotube composites for biomedical applications: a review. Int J Appl Ceram Tec 2007; 4: 1–13.

Sun

Shao

Xiang

et al.

Poly (dopamine)-modified carbon nanotube multilayered film and its effects on macrophages. Carbon 2017; 113: 176–191.

Vorobei

Pokrovskiy

Ustinovich

et al.

Preparation of polymer–multi-walled carbon nanotube composites with enhanced mechanical properties using supercritical antisolvent precipitation. Polymer 2016; 95: 77–81.

Jackson

Laibinis

Collins

et al.

Development and thermal properties of carbon nanotube-polymer composites. Compos Part B Eng 2016; 89: 362–373.

Shen

Huang

et al.

Study on amino-functionalized multiwalled carbon nanotubes. Mater Sci Eng A Struct Mater 2007; 464: 151–156.

10.

Sahoo

Rana

Cho

et al.

Polymer nanocomposites based on functionalized carbon nanotubes. Prog Polym Sci 2010; 35: 837–867.

11.

Shen

Huang

et al.

The reinforcement role of different amino-functionalized multi-walled carbon nanotubes in epoxy nanocomposites. Compos Sci Technol 2007; 67: 3041–3050.

12.

Yazdani

Smith

Hatami

. Multi-walled carbon nanotube-filled polyvinyl chloride composites: Influence of processing method on dispersion quality, electrical conductivity and mechanical properties. Compos Part A Appl Sci Manuf 2016; 82: 65–77.

13.

Balasubramanian

Burghard

. Chemically functionalized carbon nanotubes. Small 2005; 1: 180–192.

14.

Kabbani

Tiwary

Som

et al.

A generic approach for mechano-chemical reactions between carbonnanotubes of different functionalities. Carbon 2016; 104: 196–202.

15.

González-Gaitán

Ruiz-Rosas

Morallón

et al.

Effects of the surface chemistry and structure of carbon nanotubes on the coating of glucose oxidase and electrochemical biosensors performance. RSC Adv 2017; 7: 26867–26878.

16.

Ramanathan

Fisher

Ruoff

et al.

Amino-functionalized carbon nanotubes for binding to polymers and biological systems. Chem Mater 2005; 17: 1290–1295.

17.

Liu

Cui

Luo

et al.

Synthesis, characterization and application of amino-functionalized multi-walled carbon nanotubes for effective fast removal of methyl orange from aqueous solution. RSC Adv 2014; 4: 55162–55172.

18.

Vatanpour

Esmaeili

Farahani

MHDA

. Fouling reduction and retention increment of polyethersulfone nanofiltration membranes embedded by amine-functionalized multi-walled carbon nanotubes. J Membrane Sci 2014; 466: 70–81.

19.

Awasthi

Singh

et al.

Attachment of biomolecules (protein and DNA) to amino-functionalized carbon nanotubes. New Carbon Mater 2009; 24: 301–306.

20.

Patel

Bastioli

Marini

et al.

Life-cycle assessment of bio-based polymers and natural fiber composites. Biopolymers 2005, pp. 10.

21.

Azizi

Ahmad

Ibrahim

et al.

Cellulose nanocrystals/ZnO as a bifunctional reinforcing nanocomposite for poly (vinyl alcohol)/chitosan blend films: fabrication, characterization and properties. Int J Mol Sci 2014; 15: 11040–11053.

22.

Zhang

Liu

Zhang

et al.

Interaction of tannic acid with carbon nanotubes: enhancement of dispersibility and biocompatibility. Toxicol Res 2015; 4: 160–168.

23.

Shi

Deng

Zhang

et al.

Synthesis of highly water-dispersible polydopamine-modified multiwalled carbon nanotubes for matrix-assisted laser desorption/ionization mass spectrometry analysis. ACS Appl Mater Inter 2013; 5: 7770–7776.

24.

Liu

Hughes

Muir

et al.

Water-dispersible magnetic carbon nanotubes as T2-weighted MRI contrast agents. Biomaterials 2014; 35: 378–386.

25.

Mundra

Sauer

et al.

Nanotubes in biological applications. Curr Opin Biotech 2014; 28: 25–32.

26.

Kumar

Rani

Dilbaghi

et al.

Carbon nanotubes: a novel material for multifaceted applications in human healthcare. Chem Soc Rev 2017; 46: 158–196.

27.

Sun

Müllen

Yin

. Water-soluble perylenediimides: design concepts and biological applications. Chem Soc Rev 2016; 45: 1513–1528.

28.

Lanone

Andujar

Kermanizadeh

et al.

Determinants of carbon nanotube toxicity. Adv Drug Deliver Rev 2013; 65: 2063–2069.

29.

Cha

Shin

Annabi

et al.

Carbon-based nanomaterials: multifunctional materials for biomedical engineering. ACS Nano 2013; 7: 2891–2897.

30.

Mehra

Jain

. Pharmaceutical and biomedical applications of surface engineered carbon nanotubes. Drug Discov Today 2015; 20: 750–759.

31.

Sharma

Mehra

Jain

et al.

Biomedical applications of carbon nanotubes: a critical review. Curr Drug Deliv 2016; 13: 796–817.

32.

Wan

Tian

Liu

et al.

Mussel inspired preparation of highly dispersible and biocompatible carbon nanotubes. RSC Adv 2015; 5: 25329–25336.

33.

Dionigi

Posati

Benfenati

et al.

A nanostructured conductive bio-composite of silk fibroin–single walled carbon nanotubes. J Mater Chem B 2014; 2: 1424–1431.

34.

Aznar-Cervantes

Martínez

Bernabeu-Esclapez

et al.

Fabrication of electrospun silk fibroin scaffolds coated with graphene oxide and reduced graphene for applications in biomedicine. Bioelectrochemistry 2016; 108: 36–45.

35.

Koh

Cheng

Teng

et al.

Structures, mechanical properties and applications of silk fibroin materials. Prog Polym Sci 2015; 46: 86–110.

36.

Kundu

Rajkhowa

Kundu

et al.

Silk fibroin biomaterials for tissue regenerations. Adv Drug Deliver Rev 2013; 65: 457–470.

37.

Melke

Midha

Ghosh

et al.

Silk fibroin as biomaterial for bone tissue engineering. Acta Biomaterialia 2016; 31: 1–16.

38.

Sun

Luo

Dong

et al.

Shape memory and mechanical properties of silk fibroin/poly (ɛ-caprolactone) composites. Mater Lett 2017; 193: 26–29.

39.

Zhao

Heusler

Jones

et al.

Decoration of silk fibroin by click chemistry for biomedical application. J Struct Biol 2014; 186: 420–430.

40.

Wang

Dong

et al.

Silk fibroin powder prepared by nontoxic low-sodium salt system. Mater Lett 2017; 206: 5–8.

41.

Datsyuk

Kalyva

Papagelis

et al.

Chemical oxidation of multiwalled carbon nanotubes. Carbon 2008; 46: 833–840.

42.

Unger

Graham

Kreupl

et al.

Electrochemical functionalization of multi-walled carbon nanotubes for solvation and purification. Curr Appl Phys 2002; 2: 107–111.

43.

Xiao

Zhou

Wang

et al.

Electro-active shape memory properties of poly (ɛ-caprolactone)/functionalized multiwalled carbon nanotube nanocomposite. ACS Appl Mater Inter 2010; 2: 3506–3514.