Synthesis,structural characterization,and tensile properties of fructose functionalized multi-walled carbon nanotubes/chitosan nanocomposite films

Abstract

In this study, novel nanocomposite films of chitosan with fructose modified multi-walled carbon nanotube were prepared by solution casting technique. The fructose modified multi-walled carbon nanotube and the resulting bionanocomposite films were characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, atomic force microscopy and transmission electron microscopy. The transmission electron microscopy showed that the presence of the functional moieties helped to improve the dispersion and compatibility of the multi-walled carbon nanotubes in the chitosan matrix. The tensile strength of bionanocomposite films was increased from 32 MPa for the pure chitosan film up to 62.96 MPa for the nanocomposite with 7 wt% addition of fructose modified multi-walled carbon nanotube.

Keywords

Bionanocomposite films chitosan multi-walled carbon nanotube fructose

Introduction

Recently, using biodegradable polymers has received considerable attention because they are derived from natural resources. These polymers possess a great potential commercial value to substitute petroleum-based synthetic film materials.^1–3

Among various biopolymers, chitosan is a polysaccharide obtained from the deacetylation of chitin.⁴ Chitosan has been considered one of the most promising materials for future because of its excellent biodegradability, biocompatibility, antimicrobial activity, economic and nontoxic advantages.^5–7 Also, chitosan can be simply extracted from the exoskeletons of crabs and shrimps.^8,9 Chitosan has been used in producing plastic films due to its unique structure, multidimensional properties and the wide range of applications in biomedical and other industrial areas.¹⁰ This polymer is used in different applications such as food packaging, drug delivery systems, wastewater treatment and thermoplastic films.^11,12

Bionanocomposite (BNC) membrane is made of natural renewable resources and an inorganic filler viewed as an environmentally friendly material.¹³ In BNC fabrication, the dispersion of fillers is crucial in maintaining the quality of the resultant materials.¹⁴ Among different fillers, carbon nanotubes (CNTs) have been applied in various applications due to their remarkable properties such as: mechanical, electrical, thermal and optical properties, high surface area and high flexibility. Their potential applications have been reported in many fields; for instance, in nanoelectronic devices,¹⁵ catalyst supports,¹⁶ chemical sensors¹⁷ and biomedical materials.¹⁸ Besides, CNTs act also as a high tensile strength fiber facilitating the reinforcement of mechanical properties for polymers such as chitosan.¹⁹ It has been observed that the good interaction between chitosan and CNTs can improve polymer mechanical strength.^20,21 In addition, chitosan/CNT composite can be used for different applications such as removing heavy metals,²² adsorption of Congo red,²³ biosensors for different molecules, such as cholesterol,²⁴ and hydrogen peroxide.²⁵ In nanocomposite fabrication, the affinity (hydrophobicity or hydrophilicity) of CNT plays a key role in determining the dispersibility of CNT in a polymeric matrix. However, this condition is difficult to achieve with unmodified CNTs, which are well known for their chemical inertness and the tendency to agglomerate into a stabilized bundle. Therefore, attaching specific functional moieties on the CNTs is necessary to inhibit the van der Waals interactions and help the CNTs interact with the polymer. Also, by acid treatment, CNT can be further functionalized and grafted with a desired molecule.^26–29

Therefore, in the present study, the multi-walled carbon nanotube (MWCNT) was incorporated into chitosan films with different concentrations. To increase their dispersion and compatibility in chitosan matrix, fructose was chosen as a modifier. Both fructose functionalized MWCNTs (MWCNT-Fr) and chitosan that contain hydroxyl groups were expected to provide a pathway to improve the interaction between chitosan and fructose modified multi-walled carbon nanotube (MWCNT-Fr). Moreover, a simple ultrasonication assisted solution casting technique was proposed to prepare the BNC films.

Experimental

Materials

Carboxyl-modified MWCNT (diameter 8–15 nm, length ∼50 mm, carboxyl content 2.56 wt% and purity >95 wt%), which has been manufactured by a thermal chemical vapor deposition process, was purchased from Neutrino Co. (Tehran, Iran). Medium molecular weight chitosan powder with Mw = 1.095 × 10⁶g mol^–1 and DD = 75–85% were purchased from Aldrich Chemical Co. (Milwaukee, WI). Glacial acetic acid was purchased from Merck chemical Co. (Germany). N,N′-carbonyldiimidazole (CDI) was used as received without further purification and purchased from Merck Chemical Co. (Germany). Fructose was purchased from Science Kit Inc Tonawanda, N.Y. 14150.

Modification of MWCNT

The carboxylated-MWNTs were first functionalized with fructose. Briefly, 50 mg of MWCNT-COOH was completely dispersed in a freshly prepared 10-mL aqueous solution of CDI (10 mg/mL) and stirred at room temperature for 2 h. Then, 100 mg of fructose was added to the solution and this solution was kept under magnetic stirring at room temperature for 2 h and ultrasonicated for 1 h. At the end, the mixture was decanted and separated by centrifugation. Then the MWCNT-Fr was washed thoroughly with water and dried under vacuum to give the product of MWCNT-Fr.³⁰

Preparation of chitosan BNC Films

The chitosan/MWCNT-Fr (CMF) BNC films were prepared through solution casting method. The pure chitosan was dissolved in an aqueous acetic acid solution (2% (v/v)) using a magnetic stirrer for 3 h. Afterward, 3, 5 and 7 wt% of surface-modified MWCNT-Fr was uniformly dispersed in water and then slowly added in the chitosan solution. After that, the suspension was stirred for 24 h and sonicated for 1 h. The BNC suspensions were poured into a Petri dish and placed at room temperature to allow water to evaporate and form a film.

All sonication processes were carried out with MISONIX ultrasonic liquid processor, XL-2000 SERIES. Ultrasound was a wave of frequency 2.25 × 10⁴Hz and the power of 100 W. The sample beaker was placed inside an ice-water bath during sonication to control the mixture temperature rise.

Characterization

The stretching frequencies of samples were examined by Fourier transform infrared spectroscopy (FT-IR) (Jasco-680 (Japan). The vibrational transition frequencies were reported in the range of 4000–400 cm^–1.

The X-ray diffraction (XRD) patterns were recorded on a Philips X’PERT MPD (Netherlands) with a copper target operating at the current of 100 mA and a voltage of 45 kV, and some Cu Ka radiation (λ = 0.1542 nm) in the range of 10–80° and at the speed of 0.05°/min.

The thermogravimetric analysis (TGA) was achieved using a STA503 TA (Germany) with scan range from 25℃ to 800℃ at 20℃ min^–1 and continuous argon flow. The BNC films were dried in vacuum at 100℃ for 8 h to remove the residue solvent.

The dispersion morphology of the samples was characterized using field emission scanning electron microscopy (FE-SEM). Atomic force microscopy (AFM) topographic images were evaluated using Nano S1/1 (BRUKER, Germany, noncontact mode) and transmission electron microscopy (TEM) images were also obtained using a Philips CM 120 microscope (Germany) with an accelerating voltage of 150 kV.

Tensile testing was performed at room temperature on a Testometric Universal Testing Machine M350/500 (Mainz, Germany), according to ASTM D 882 (standards). Tests were carried out with a cross-head speed of 12.5 mm/min. The dimensions of the test specimens were 35 × 2 × 0.04 mm. Property values reported here represent an average of the results for tests run on at least three specimens. Tensile strength, tensile modulus and strain were obtained from these measurements.

Results and discussion

Chitosan BNC films preparation

The CNTs are promising nanomaterials with great potential in biological and biomedical applications. Such applications are fundamentally dependent on the compatibility between the polymer and the nanofiller surface. Therefore, the CNT modification generated surface functional groups which enhance the CNT dispersion in the polymer matrix. To improve the BNC interfacial morphology, the MWCNTs were modified with fructose. Then, BNCs were prepared by adding 3, 5 and 7 wt% of MWCNT-Fr in chitosan solution via vigorous stirring and utrasonication. The fructose functional groups on the MWCNT surface interacted with the polymer chains and improved the interfacial interactions and MWCNT dispersibility in the matrix. Possible interactions between hydroxyl groups (-OH) of MWCNT-Fr and hydroxyl or amino groups of chitosan chains are illustrated in Scheme 1.

Structure of chitosan BNC films

The FT-IR spectra of carboxylated MWCNT, fructose and MWCNT-Fr are shown in Figure 1. For the pure fructose, the absorption peaks at 3376 and 2930 cm^–1 could be attributed to the stretching vibration of OH and C-H stretching vibration. The C = O group of fructose was observed at 1720 and 1641 cm^–1.

Figure 1 shows that the characteristic absorption bands of MWNT-COOH at 3433 cm^–1 could be related to the stretching vibration of O-H bands of carboxylic acid moieties. On the other hand, those at 2923 cm^–1 and 1713 cm^–1 could be ascribed to aliphatic sp³ C-H of MWCNTs and C = O stretching vibration of the carboxylic acid groups, respectively. Also, the peaks at 1620 and 1433 cm⁻¹ could be associated with the stretching of the carbon nanotube backbone. The new absorption bands at 1771 cm^–1 appeared in the spectra of the MWCNT-Fr, thereby indicating that fructose was grafted onto the MWCNT surface.

Figure 2 shows FT-IR spectra for pure chitosan and CMF BNC films between 400–4000 cm⁻¹. For the pure chitosan, as shown in Figure 2(a), the absorption peak at 3450 cm⁻¹ could be ascribed to the stretching vibration of OH and NH₂ groups, as shown in the spectrum of BNC films 3 wt% (Figure 2b), were broadened compared with that of chitosan. This indicated that MWCNT-Fr interacted with chitosan matrix, and the hydrogen bonding between chitosan chains was partially destructed. The peak at 1637 cm⁻¹ was due to residual acetamido groups remaining after the deacetylation of chitin during the production process of chitosan. The absorption band between 1220 and 1020 cm^–1 represented the C-O-C in the six-member ring. The peak at 1156 cm⁻¹ could be assigned to the special broad peak of β (1–4) glucosidic band in the polysaccharide unit.

After the formation of CMF BNC films, it was found that the characteristic peaks at 3450, 2982 and 1637 cm^–1 of chitosan were shifted to 3436, 2882 and 1655 cm^–1, respectively. These changes could be attributed to hydrogen bonding and electrostatic interactions between chitosan and MWCNT-Fr. These FT-IR spectra proved that chitosan BNCs were prepared successfully. Table 1 summarizes the absorption bands of FT-IR spectra.

Table 1.

Characteristic absorption bands (cm^–1) in FT-IR.

Assignment	chitosan	CMF BNCs
-OH and –NH₂ stretching	3450	3422--3351
-CH stretching	2982	2915--2882
C = O stretching	1637	1655--1653
C-O stretching	1080	1086--1076

BNC: bionanocomposite; CMF: chitosan/fructose modified multi-walled carbon nanotube.

XRD analysis

Figure 3 shows the XRD patterns of carboxylated MWCNTs (MWCNT-COOH), MWCNT-Fr, chitosan and CMF BNC films. For MWCNTs, two peaks appeared at 2θ = 26° and 44°, which are typically associated with the (002) and (100) diffractions of the hexagonal graphite structure.³¹ The MWCNTs-Fr showed very few changes in the XRD pattern. It could be seen that the XRD pattern was very similar to that of MWCNT-COOH. The MWCNTs-Fr still had the same cylinder wall structure as raw MWCNTs.

The XRD pattern of the neat chitosan film showed a characteristic peak located at 20.93°. The sharp peak around 20.93° indicated the existence of the crystalline structure.³² Compared with neat chitosan, the CMF BNC films almost showed similar XRD patterns although the intensity of the diffraction peaks at 20.93° became a little weaker and wider, especially for the CMF BNC 7 wt%. This indicated that the crystalline structure of chitosan was changed after the introduction of MWCNT-Fr. It could be due to the intermolecular and intramolecular hydrogen bonding between the amine groups in chitosan and hydroxyl groups in both chitosan and MWCNT-Fr. At the same time, no characteristic diffraction peak for MWCNT-Fr was detected in the BNC films, probably due to the low loading MWCNT-Fr.

Morphological image analysis

Figure 4 shows the FE-SEM micrographs of the unmodified and modified MWCNTs. Figure 4(a) also displays the FE-SEM of MWCNT-COOH. The FE-SEM image of the MWCNT-COOH surface was approximately smooth, but after modification by fructose, it became rough and debundled (Figure 4b).

Figure 5 shows the FE-SEM images of chitosan and CMF BNC films with the MWCNT-Fr weight ratio of 3, 5 and 7 wt%. The neat chitosan film FE-SEM images exhibited a smooth and tight fracture surface. From BNC films micrographs, one sees that composites with different MWCNT-Fr concentrations showed different dispersion states. It is known that the homogeneous distribution of the nanofiller is the most critical step in performing BNCs. In the case of MWCNT-Fr, this was almost observed throughout the chitosan matrix as shown in Figure 5, except for BNC 5 wt%. For the CMF BNC 5 wt%, the morphology (Figure 5e and f) exhibited the presence of the small aggregates of MWCNT-Fr, which could be attributed to the small interaction of chitosan with MWCNT. Figure 5 also shows that there were some cavities which could be attributed to the strong interfacial interaction between MWCNT-Fr and chitosan.

AFM was used to characterize the surface topographic feature of the films. Figure 6 shows FM images of MWCNT-Fr, chitosan and BNC 5 wt%. The one- and two-dimensional photos of the MWCNT-Fr clearly indicate that the MWCNT had a cylindrical-shaped tubular morphology (Figure 6b). Figure 6(a) and (c) shows that pristine chitosan had a smooth surface with small bulges, while CMF BNC 5 wt% had a surface rougher than chitosan film. This revealed that chitosan chains had been successfully interacted onto the surface of MWCNT-Fr.

Figure 7(a) and (b) shows the TEM images of the MWCNT-COOH and MWCNT-Fr. These images indicate that the MWCNT-Fr had a nanotubular shape. Figure 7(c) to (f) shows the TEM micrographs of CMF BNC with 5 wt% MWCNT-Fr. The nanotubes were dispersed uniformly in the chitosan matrix, except for some small aggregates of MWCNT-Fr on the bottom of the image (Figure 7e).

Mechanical properties of the reinforced chitosan films

In general, the good dispersion of MWNTs in the chitosan matrix and the strong interfacial adhesion between the two phases are required to obtain BNC films with suitable mechanical properties. The tensile testing provides an indication of the strength and elasticity of the films. Table 2 shows the tensile strength and the strain-at-break of different samples. The variation of tensile strength with MWCNTs content exhibited different tendencies. The tensile strength and Young’s modulus increased with increasing MWCNTs content from 0 to 7 wt%.

Table 2.

Tensile properties of chitosan and composites.

MWCNT-Fr content (%)	Tensile strength (MPa)	Strain (%)	Young’s modulus (GPa)
0	32.5	4.9	7.7
3	44.3	3.4	12.8
5	38.5	2.5	15.4
7	62.9	4.9	15.5

Due to the possibility of the interaction between the functional groups of chitosan (OH and NH₂) and OH on the surface of MWCNT-Fr, mechanical properties of the films were improved. As can be seen in Table 2, the tensile strength and Young’s modulus of chitosan were 32.5 MPa and 7.72 GPa. These increased to 44.3 MPa and 12.84 GPa for BNC 3 wt% and 62.96 MPa and 15.5 GPa for 7 wt%, respectively. These results indicated that the BNC films had higher tensile strength than the pure chitosan film and MWCNT-Fr showed good compatibility with the chitosan matrix. Therefore, good interfacial interactions and the load transfer of MWCNT-Fr with chitosan improved the mechanical properties.

Thermal properties

Thermal degradation of the material is very important and it can, in many cases, determine the functional groups. Figure 8 depicts the TGA results for the MWCNT, MWCNT-COOH and the functionalized MWCNT with fructose. The pristine MWCNT curve showed insignificant (less than 6%) weight loss between 25 and 800℃. The TGA curve of MWCNT-COOH also exhibited a small degradation (10%) in the range of 0–550℃. The amount of the carboxyl group of the MWCNT-COOH was determined based on MWCNT and MWCNT-COOH char yield differences at 800℃. The calculated carboxyl content was found to be about 4 wt%. The mass loss of MWCNT-Fr was observed to be below ∼250℃ (52%), due to the release of moisture and the decomposition of the associated organic groups. Comparing the results one concludes that the weight ratio of the grafted fructose on MWCNTs was approximately 42%.

Figure 9 shows the thermal stability and thermal decomposition of chitosan and CMF BNC films as examined by TGA. The two-step degradation was observed at 100–200℃ and 200–400℃ for chitosan and BNC films. The first weight loss step (T_d1) is attributed to the evaporation of water absorbed in the chitosan and the second one (T_d2) corresponds to the degradation and deacetylation of chitosan. This was similar to the results reported by other researchers.³³ It is noteworthy that the MWCNT-Fr increased the T_d1 of BNC films as compared to the chitosan film. The interaction of many OH and NH₂ hydrophilic groups of the chitosan with OH groups of MWCNT-Fr is regarded as the main reason for low moisture content of these BNC films. The second step, which starts over 200℃, was related to depolymerization of glucosamine chains. All BNC films showed a decrease in the T_d2. The reduction in T_d2 could be due to the loss of interpolymer chain interactions in film formation and the decomposition of fructose moieties and other functional groups of MWCNT-Fr. It is known that the chitosan is a semi-crystalline polymer due to intra or intermolecular hydrogen bonding between chitosan chains. The interaction of MWCNT-Fr with chitosan, therefore, weakened the interchain interactions and decreased the crystallinity and T_d2.

Conclusions

In the present study, we employed fructose as a modifier and incorporated it into MWCNTs to increase their dispersion and compatibility in the chitosan matrix. The CDI catalyzed the esterification between MWCNT and fructose. The modification was confirmed by FT-IR spectroscopy, TGA and electron microscopic techniques such as FE-SEM, AFM and TEM. The combined effect of biopolymer chitosan and MWCNT-Fr content on the properties of the obtained BNC films was investigated. The XRD analysis of CMF BNC films showed that the crystallinity of chitosan decreased due to hydrogen bonding interaction of MWCNT-Fr with chitosan. The FE-SEM, AFM and TEM showed that the presence of the functional moieties helped to improve the dispersion and compatibility of the MWCNTs in the chitosan matrix. Also, incorporating MWCNT-Fr increased the tensile strength compared with the pure chitosan films.

Footnotes

Declaration of conflicting interest

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

Funding

This work was supported by the Research Affairs Division of Isfahan University of Technology (IUT), National Elite Foundation (NEF), and Center of Excellency in Sensors and Green Chemistry Research (IUT).

Shadpour Mallakpour, organic polymer chemist, graduated from chemistry department, University of Florida (UF), Gainesville, Florida, USA in 1984. He spent two years as post-doc at UF. He joined the department of chemistry, Isfahan University of Technology (IUT), Iran, in 1986. He held several positions such as chairman of Department of Chemistry and Deputy of Research, Department of Chemistry at IUT. From 1994 to 1995 he worked as visiting professor, University of Mainz, Germany, and from 2003 to 2004 as visiting professor, Virginia Tech, Blacksburg, USA. Now he has published more than 550 journal papers and more than 400 conference papers and received more than 34 awards. The most important award was for the selection of first laureate on fundamental research, at 21st Khwarizmi International award in 2008. He is listed in the Top 1% Scientists in Chemistry in ISI Essential Science Indicators since 2003. He was selected as academic guest of the 59th Meeting of Nobel Prize Winners in Chemistry, 2009, at Lindau, Germany. He presented many lectures as invited or keynote speaker in different national and international conferences or universities. He was member of organizing and scientific committees for many national and international conferences. He was also the chairperson of many national and international meetings. In recent years he focused on the preparation and characterization of polymers containing chiral amino acid moieties under green conditions using ionic liquids and microwave irradiation as new technology and bringing these aspects towards nanotechnology for the preparation of novel chiral bionanocomposite polymers.

Maryam Madani is a PhD student. She received her MSc degree from the Isfahan University of Technology, Isfahan, Iran, in Organic Polymer Chemistry. Her research is focused on the synthesis of different nanocomposite materials and their characterization for specific applications.

References

Sudhakar

Trishul

Doble

. Biofouling and biodegradation of polyolefins in ocean waters. Polym Degrad Stabil 2007; 92: 1743–1752.

Bonhomme

Cuer

Delort

. Environmental biodegradation of polyethylene. Polym Degrad Stabil 2003; 81: 441–452.

Miyazaki

Arai

Shibata

. Study on biodegradation mechanism of novel oxo-biodegradable polypropylenes in an aqueous medium. Polym Degrad Stabil 2012; 97: 2177–2184.

Singh

Dutta

. Preparation and properties of highly soluble chitosan–l-glutamic acid aerogel derivative. Carbohydr Polym 2009; 76: 188–195.

Rutnakornpituk

Ngamdee

Phinyocheep

. Preparation and properties of polydimethylsiloxane-modified chitosan. Carbohydr Polym 2006; 63: 229–237.

Yeng

Husseinsyah

Ting

. Modified corn cob filled chitosan biocomposite films. Polym Plast Technol Eng 2013; 52: 1496–1502.

Najeeb

Chang

Lee

J-H

. Preparation of semiconductor-enriched single-walled carbon nanotube dispersion using a neutral pH water soluble chitosan derivative. J Colloid Interface Sci 2011; 354: 461–466.

Fernández-Saiz

Lagaron

. Chitosan for film and coating applications. In: Plackett D (ed.) Biopolymers – new materials for sustainable films and coatings, Chichester, UK: John Wiley & Sons, Ltd, 2011, pp. 87–105.

Carson

Hibbert

Akindoju

. Synthesis, characterization and stability of chitosan and poly(methyl methacrylate) grafted carbon nanotubes. Spectrochimica Acta A Molecul Biomolecul Spectrosc 2012; 96: 380–386.

10.

Kim SK. Marine cosmeceuticals: trends and prospects. Taylor & Francis, CRC Press, 2011.

11.

Salmah

Azieyanti

. Properties of recycled polyethylene/chitosan composites: the effect of polyethylene-graft-maleic anhydride. J Reinf Plast Compos 2011; 30: 195–202.

12.

Navarro

Moreno

Arizmendi-Morquecho

. Preparation and tribological properties of chitosan/hydroxyapatite composite coatings applied on ultra high molecular weight polyethylene substrate. J Plast Film Sheet 2012; 28: 279–297.

13.

Ong

Ahmad

Zein

SHS

. A review on carbon nanotubes in an environmental protection and green engineering perspective. Brazilian J Chem Eng 2010; 27: 227–242.

14.

Abdolmohammadi

Wan Yunus

WMZ

AB Rahman

. Effect of organoclay on mechanical and thermal properties of polycaprolactone/chitosan/montmorillonite nanocomposites. J Reinf Plast Compos 2011; 30: 1045–1054.

15.

Tsukagoshi

Yoneya

Uryu

. Carbon nanotube devices for nanoelectronics. Physica B: Condensed Matter 2002; 323: 107–114.

16.

Planeix

Coustel

Coq

. Application of Carbon Nanotubes as Supports in Heterogeneous Catalysis. J Am Chem Soc 1994; 116: 7935–7936.

17.

Hobbs

Patel

Kim

. Glucose determination in beverages using carbon nanotube modified biosensor: an experiment for the undergraduate laboratory. J Chem Edu 2013; 90: 1222–1226.

18.

Armentano

Dottori

Fortunati

. Biodegradable polymer matrix nanocomposites for tissue engineering: a review. Polym Degrad Stabil 2010; 95: 2126–2146.

19.

Wang

S-F

Shen

Zhang

W-D

. Preparation and mechanical properties of chitosan/carbon nanotubes composites. Biomacromolecules 2005; 6: 3067–3072.

20.

Moulton

Minett

Murphy

. Biomolecules as selective dispersants for carbon nanotubes. Carbon 2005; 43: 1879–1884.

21.

Shieh

Y-T

Yang

Y-F

. Significant improvements in mechanical property and water stability of chitosan by carbon nanotubes. Eur Polym J 2006; 42: 3162–3170.

22.

Salam

Makki

MSI

Abdelaal

MYA

. Preparation and characterization of multi-walled carbon nanotubes/chitosan nanocomposite and its application for the removal of heavy metals from aqueous solution. J Alloys Compound 2011; 509: 2582–2587.

23.

Chatterjee

Lee

Woo

. Adsorption of congo red by chitosan hydrogel beads impregnated with carbon nanotubes. Bioresource Technol 2010; 101: 1800–1806.

24.

Solanki

Kaushik

Ansari

. Multi-walled carbon nanotubes/sol–gel-derived silica/chitosan nanobiocomposite for total cholesterol sensor. Sensors Actuat B Chem 2009; 137: 727–735.

25.

Yang

Huang

. A new hydrogen peroxide biosensor based on gold nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film-modified electrode. Colloid Surf A Physicochem Eng Aspects 2009; 340: 50–55.

26.

Balasubramanian

Burghard

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

27.

Mallakpour

Zadehnazari

. One-pot synthesis of glucose functionalized multi-walled carbon nanotubes: dispersion in hydroxylated poly(amide-imide) composites and their thermo-mechanical properties. Polymer 2013; 54: 6329–6338.

28.

Abdolmaleki

Mallakpour

Borandeh

. Amino acid-functionalized multi-walled carbon nanotubes for improving compatibility with chiral poly(amide-ester-imide) containing l-phenylalanine and l-tyrosine linkages. Appl Surf Sci 2013; 287: 117–123.

29.

Mallakpour

Soltanian

. Chemical modification of MWCNTs with 5-aminoisophthalic acid and its effects on the thermal and morphological properties of chiral poly (ester-imide)/MWCNT nanocomposites having N-trimellitylimido-L-isoleucine moieties. J Polym Res 2014; 21: 1–9.

30.

Mallakpour S, Abdolmaleki A and Rostami M. Morphology and thermal properties of environmental friendly nanocomposites using biodegradable poly(amide–imide) based on N-trimellitylimido-S-valine matrix reinforced by fructose-functionalized multi-walled carbon nanotubes. Colloid Polym Sci 2014; 293: 545--553.

31.

Pérez-Cabero

Rodríguez-Ramos

Guerrero-Ruíz

. Characterization of carbon nanotubes and carbon nanofibers prepared by catalytic decomposition of acetylene in a fluidized bed reactor. J Catalysis 2003; 215: 305–316.

32.

Yen

M-T

Yang

J-H

Mau

J-L

. Physicochemical characterization of chitin and chitosan from crab shells. Carbohydr Polym 2009; 75: 15–21.

33.

López

Mercê

ALR

Alguacil

. A kinetic study on the thermal behaviour of chitosan. J Therm Anal Calorim 2008; 91: 633–639.