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Preparation and Characterization of Oxidized Multi-Walled Carbon Nanotubes and Glycine FunctionalizedMulti-Walled Carbon NanotubesM. Deboraha, A. Jawahara, T. Mathavanb, M. Kumara Dhasb & A. Milton Franklin Benialba Department of Chemistry, NMSSVN College, Madurai-625 019, Tamil Nadu, Indiab Department of Physics, NMSSVN College, Madurai-625 019, Tamil Nadu, IndiaAccepted author version posted online: 01 Jul 2014.Published online: 25 Sep 2014.

To cite this article: M. Deborah, A. Jawahar, T. Mathavan, M. Kumara Dhas & A. Milton Franklin Benial (2015) Preparationand Characterization of Oxidized Multi-Walled Carbon Nanotubes and Glycine Functionalized Multi-Walled Carbon Nanotubes,Fullerenes, Nanotubes and Carbon Nanostructures, 23:7, 583-590, DOI: 10.1080/1536383X.2014.899212

To link to this article: http://dx.doi.org/10.1080/1536383X.2014.899212

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Preparation and Characterization of Oxidized Multi-WalledCarbon Nanotubes and Glycine FunctionalizedMulti-Walled Carbon Nanotubes

M. DEBORAH1, A. JAWAHAR1, T. MATHAVAN2, M. KUMARA DHAS2, and A. MILTON FRANKLIN BENIAL2

1Department of Chemistry, NMSSVN College, Madurai-625 019, Tamil Nadu, India2Department of Physics, NMSSVN College, Madurai-625 019, Tamil Nadu, India

Received 9 January 2014; accepted 24 February 2014

In this work, oxidized multi-walled carbon nanotubes (MWCNTs) and glycine functionalized MWCNTs were synthesized andcharacterized using ultraviolet-visible (UV-Vis), fourier transform infrared (FT-IR), electron paramagnetic resonance (EPR),scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques.. UV-Vis spectroscopic study shows acharacteristic peak at 276 nm for oxidized MWCNTs, and the red shift was observed for glycine-functionalized MWCNTs at296 nm. The appearance of another peak at around »350 nm is attributed to the formation of glycine-functionalized MWCNTs.FT-IR spectral study confirms that glycine molecules have bonded to MWCNTs. EPR study reveals that the line shape is a Gaussianline shape and the electron spin concentration decreases with increasing concentration of glycine, which indicates that the unpairedelectrons undergo a reduction process in the glycine-functionalized MWCNTs. The g-value indicates that the systems are isotropic innature. The surface morphology of the samples was observed from SEM images. The EDX analysis shows the high purity of bothoxidized and glycine-functionalized MWCNTs.

Keywords: multi-walled carbon nanotubes, functionalization, chemical synthesis, electron paramagnetic resonance (EPR), electronmicroscopy (SEM)

1. Introduction

Carbon nanotubes (CNTs) are synthetic nanomaterials,which belong to the fullerene family (1). They have uniquemechanical, electrical, thermal, and chemical properties. Dueto these unique properties, CNTs are considered distinctivematerials having various applications in the field of nanoelec-tronics, nanotechnology, and medicinal chemistry (2–5).Generally, CNTs are not soluble in solvents; therefore, tomake CNTs soluble in solvents, certain modifications ofCNTs are necessary. CNTs can be functionalized by i) cova-lent functionalization of CNT side walls by oxidation andthen converting into derivatives such as amides, ii) non-cova-lent functionalization can be done by wrapping the CNTswith surfactants or by polymers, and iii) chemical functionali-zation of the side walls of the CNTs by the addition reactionwith direct fluorination, followed by nucleophilic substitu-tion, and the addition of radicals formed by reduction (6–8).

The functionalization process results in the exfoliation oflarge bundles of CNTs and enhances sensitivity. The func-tionalization of CNTs increases the biocompatibility andchanges the chemical, structural, and optical properties of thenanotubes, which also leads to modification of the CNTs.The functionalization of CNTs improves their solubility,which makes them suitable for chemical and biological appli-cations (9, 10). Haiqing et al. (11) found that the functionali-zation of CNTs provides sites for the attachment ofbiologically active moieties like amino acids, peptides, andproteins to the CNTs for drug delivery and biosensor applica-tions as CNTs can cross the cell membrane. Besides, CNTsare of a size where cells do not recognize them as harmfulintruders and they can translocate into the cytoplasam or thenucleus of a cell through its membrane without causing animmunogenic response and toxic effects (12–14).

Amino acids (AA) are the basic unit for biomolecules and,therefore, they can reflect the common properties of complexbiomolecules. The interaction between the CNTs and AAs isessential for understanding the mechanism of interactionbetween the CNTs and biomolecules (4). Functionalizationof CNTs with different amino groups on the surface of theCNTs provides greater dispersibility in different solvents andopens a broad prospect for their biomedical applications (15,16). L-cysteine and histidine functionalized multi-walledCNTs have been used as selective sorbents for the separation

Address correspondence to A. Milton Franklin Benial AssociateProfessor, Department of Physics, NMSSVN College, Nagama-lai, Madurai-625019, Tamilnadu, India. E-mail: miltonfranklin@yahoo.comColor versions of one or more figures in this article can be foundonline at www.tandfonline.com/lfnn.

Fullerenes, Nanotubes and Carbon Nanostructures (2014) 23, 583–590Copyright © Taylor & Francis Group, LLCISSN: 1536-383X print / 1536-4046 onlineDOI: 10.1080/1536383X.2014.899212

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and preconcentration of heavy metals in both biological andenvironmental samples (17, 18). In this work, the simplestAA, glycine, is used, which is the only AA that is notoptically active and it is an important model compound inchemical physics, biophysics, and biochemistry (19, 20). The–N–O–C(O)– structure, in glycine, is a fundamental buildingblock of a-AAs and proteins (21). The glycine-functionalizedMWCNTs has been researched because glycine is essentialfor the biosynthesis of nucleic acids, and has the ability to actas inhibitory neurotransmitters in the central neurosystem(22). Glycine inhibits glutamate-evoked depolarization anddepresses firing of neurons. The binding of glycine to itsreceptor produces a large increase in Cl¡ conductance, whichcauses membrane hyper-polarization. The aim of the presentwork is to develop a relatively simple and effective process offunctionalizing MWCNTs and carry out a characterizationstudy. In this study, oxidized MWCNTs and glycine-func-tionalized MWCNTs (0.3, 0.6, and 0.9 M concentrations ofglycine) were characterized by using ultraviolet-visible (UV-Vis), Fourier transform infrared (FT-IR), electron paramag-netic resonance (EPR), scanning electron microscopy (SEM),and energy dispersive X-ray (EDX) techniques.

2. Materials and Methods

The multi-walled CNTs was purchased from Aldrich Chemi-cal Co, St. Louis, MO, USA. Glycine, H2SO4, and HNO3

were purchased fromMerck, Germany.

2.1 Sample Preparation

Oxidized MWCNTs

Pristine MWCNTs were mixed in a mixture of 3:1 concen-trated sulfuric and nitric acid and sonicated for 3 hours at40�C in an ultrasonic bath to introduce carboxylic acidgroups on the surface of MWCNTs (23). After sonication,the mixture was added dropwise to cold distilled water and

the resulting samples, oxidized MWCNTs, were filtered anddried in vacuum at 80�C for 4 hours.

Glycine-functionalized MWCNTs

The oxidized MWCNTs samples were mixed with 0.3 M gly-cine suspension and sonicated for approximately 1 hour atroom temperature. After sonication, the oxidizedMWCNTs/glycine suspension was directly filtered and thesolid sample was dried in a vacuum for approximately16 hours at room temperature (4). In a similar way, glycine-functionalized MWCNTs (0.6 and 0.9 M concentrations ofglycine) were also prepared. Figure 1 shows the scheme forthe synthesis of oxidized MWCNTs and glycine-functional-ized MWCNTs.

2.2 Spectral Measurements

UV-Vis Measurements

A Shimadzu UV-3600 UV-Vis-NIR spectrophotometer (Shi-madzu Scientific Instruments, Columbia, MD) was used forabsorption spectra measurements in the wavelength range of200–600 nm.

FT-IR Measurements

The FT-IR spectra of the samples dispersed in the potassiumbromide matrix were recorded in the wave number range of400–4000 cm¡1 at 64 scans per spectrum at 4 cm¡1 resolutionusing a computerized Bruker Optik GmbH FT-IR spectro-photometer. Spectra were corrected for the moisture and car-bon dioxide in the optical path.

EPRMeasurements

EPR spectra of the samples were recorded at room tempera-ture using a Bruker EMX plus spectrometer with 100 kHzfield modulation frequency and phase-sensitive detection.The ESR spectra were recorded by varying the magnetic fieldin the range of 337–362 mT with the following spectrometer

Fig. 1. Scheme for the synthesis of glycine-functionalized MWCNTs (a) pristine MWCNTs were mixed with (H2SO4/HNO3 (3:1),sonicated for 3 hours, and the mixture was added to cold distilled water. The samples were filtered and dried in vacuum at 80�C for4 hours. (b) The oxidized MWCNTs was mixed with glycine suspension and sonicated for 1 hour at room temperature. The sampleswere filtered and dried in vacuum at room temperature for 16 hours.

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settings: field modulation amplitude 0.6 mT; conversion time,30 ms; radio-frequency power, 5.0 mW; receiver gain, 2000;sweep width, 25 mT; sweep time, 30 s; number of scans, 16;1024 k resolution; and radiofrequency, 9.86 GHz. The tem-perature was controlled using a controller with water as acoolant.

SEM Imaging

The surface morphological studies of the samples were car-ried out by SEM (SEM-VEGA3 TESCAN, USA).

EDXMeasurements

The chemical composition of oxidized MWCNTs and gly-cine-functionalized MWCNTs was characterized by anenergy-dispersive spectrometer (Bruker Nano, Germany).

3. Results and Discussion

3.1 UV-Vis Analysis

The UV-Vis spectra of oxidized MWCNTs and glycine-func-tionalized MWCNTs were shown in Figure 2 and their

corresponding spectral data are given in Table 1. The charac-teristic peak appeared at 276 nm for oxidized MWCNTs,which is in good agreement with the value reported in the lit-erature (24, 25). The red shift was observed for glycine-func-tionalized MWCNTs around »288 and »296 nm. (0.6 and0.9 M concentrations of glycine). The appearance of anotherpeak around »350 nm is attributed to the formation of acharge-transfer complex, glycine-functionalized MWCNTs.

3.2 FT-IR Analysis

The FT-IR spectra of oxidized MWCNTs and glycine-func-tionalized MWCNTs are shown in Figure 3 and their corre-sponding FT-IR assignments are listed in Table 2. Thecarbonyl stretching mode of carboxylic acid group appearedaround »1706 cm¡1, which indicates the existence of a car-boxyl group on the surface of MWCNTs (4, 26, 27). Anotherpeak in the region around »1633 cm¡1 is due to the carbonylstretching mode of quinone type units along the side walls ofthe CNT (26). Functionalization of glycine on the oxidizedMWCNTs, which resulted in the formation of a secondary

Fig. 2. UV-Vis spectra of (a) pure, (b) 0.3, (c) 0.6, and (d) 0.9 Mglycine-functionalized MWCNTs.

Table 1. UV-Vis spectral data for pure MWCNTs and glycinefunctionalized MWCNTs

SampleAbsorbance

λ1 (nm)Absorbance

λ2 (nm)

Pure MWCNT 276 —Glycine functionalised

MWCNTsConcentration

of glycine276 350

0.3 M0.6M 288 3500.9 M 296 347

Fig. 3. FT-IR spectra of (a) pure, (b) 0.3, (c) 0.6, and (d) 0.9 Mglycine-functionalized MWCNTs.

Table 2. FTIR spectral assignments of pure MWCNTs andGlycine functionalized MWCNTs

Glycine concentration

Assignment Pure MWCNTs 0.3 M 0.6 M 0.9 M

CHO stretching 1710 1704 1706 1707CHO stretching 1632 1633 1632 1633C��H bending 1434 1433 1430 1434O��H bending 1374 1384 1382 1381C��N stretching — 1224 1222 1222C��O stretching 1023 1021 1023 1023N��H stretching — 874 875 880C��OH stretching 669 664 666 668

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Fig. 4. EPR spectra of (a) pure, (b) 0.3, (e) 0.6, and (f) 0.9 M glycine-functionalized MWCNTs. (c), (d), (g), and (h) are the corre-sponding absorption spectra and Gaussian fit.

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amide on the MWCNTs, is confirmed by the characteristicpeak around»1222 cm¡1 which is assigned to C-N stretchingand another peak in the region around»877 cm¡1, assignedto the N-H stretching mode (4, 22), which confirms that gly-cine molecules bonded to the MWCNTs. The O-H bendingand C-OH stretching modes in carboxylic acid were observedaround »1384 cm¡1 and »666 cm¡1 respectively (27, 28).The C-H bending mode appeared around »1430 cm¡1 (29).The observed peak around »1023 cm¡1 is assigned to the C-O stretching mode (30).

3.3 EPR Analysis

The EPR spectra of oxidized MWCNTs and glycine-func-tionalized MWCNTs (0.3, 0.6, and 0.9 M concentrationsof glycine) and their corresponding absorption spectra areshown in Figure 4. The EPR parameters are listed inTable 3. EPR parameters such as line width, line shape,g-factor, signal intensity, and spin concentration arereported for oxidized MWCNTs and glycine-functional-ized MWCNTs.

Line Shape

The lineshape analysis was carried out using Origin 8 soft-ware, which reveals that the EPR absorption spectra have aGaussian lineshape. According to previous studies, oxidizedMWCNTs showed a sharp EPR signal, which is attributableto the generation of many unpaired electrons on the surfaceof the CNTs (7, 31). The EPR line shape is usually describedby Lorentzian and Gaussian line shapes. The EPR absorp-tion spectral data was found to be the best fit for the Gaussianfunction. The EPR absorption spectra for the oxidizedMWCNTs and glycine-functionalized MWCNTs have aGaussian lineshape. The dipolar broadening generally produ-ces Gaussian-shaped lines.

Line Width and g-Factor

The full width at half-maximum (FWHM) line-width valuesfor the EPR spectra of oxidized MWCNTs and glycine-func-tionalized MWCNTs were obtained from the Gaussian fit.The line-width value increases with increasing concentrationof glycine, which reveals that the dipole–dipole interactionincreases with increasing concentration of glycine. The g-value was calculated using the magnetic field B0, which is

Table 3. EPR parameters of pure MWCNTs and glycine functionalized MWCNTs

SamplesR2 from

Gaussian fitSpin concentration

from Gaussian fit (a.u)FWHM(mT) g-Factor

Pure MWCNTs 0.96443 9.61 £ 109 43.79 2.0072Glycine functionalized MWCNTs Glycine concentration 0.97634 2.84 £ 109 51.47 2.0072

0.3M0.6 M 0.905 2.73 £ 109 55.63 2.00720.9 M 0.9523 2.43 £ 109 64.05 2.0072

Fig. 5. SEM images of (a) oxidized MWCNTs and (b) glycine-functionalized MWCNTs.

Glycine Functionalized MWCNTs 587

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obtained from the central position of the EPR spectral line.EPR lines are seen with the g-value of 2.0072, which is veryclose to the free electron g-value. The g-value indicates theisotropic nature of the system (32), which reveals that theproperties of the system remains same with respect to thedirection of the magnetic field.

Signal Intensity and Spin Concentration

The EPR signal intensity decreases with increasing concen-tration of glycine, as shown in Figure 4, which implies thatmore unpaired electrons obtained by acid treatment have suc-cessfully reacted with glycine. From the Gaussian fit of EPRabsorption data, spin concentration values were obtained,which are shown in Table 3. The spin concentration valuedecreases with increasing concentration of glycine, whichreveals that the unpaired electrons undergo a reduction pro-cess in glycine-functionalized MWCNTs.

3.4 SEM and EDX Analysis

Both oxidized MWCNTs and glycine-functionalizedMWCNTs are characterized by using SEM. The SEMimages of oxidized and glycine-functionalized MWCNTs(Figure 5) show CNTs with uniform size, shape, and asmooth surface. The SEM image of oxidized MWCNTs was

observed with an average diameter of »80 nm. After thefunctionalization of a glycine molecule on the oxidizedMWCNTs, there resulted an increase of the average diameterof »100 nm due to the adsorption of glycine on the sidewallsof oxidized MWCNTs (4, 25). The glycine molecules wereadsorped on the surface of oxidized MWCNTs by polarinteractions, p–p stacking, hydrogen bonding, and covalentbonding (4). Agglomeration of the MWCNTs was notobserved by SEM images, which indicates higher dispersingability of CNTs in solvents (16, 33, 34). Figure 6 shows theEDX spectra of i) oxidized MWCNTs and ii) glycine-func-tionalized MWCNTs. The EDX elemental microanalysis(wt.%) of oxidized and glycine-functionalized MWCNTswere listed in Table 4. The EDX elemental microanalysisconfirmed that elemental nitrogen existed in glycine-function-alized MWCNTs along with carbon and oxygen. The spectralline corresponding to elemental nitrogen did not appeared inthe EDX spectrum (Fig 6a) of oxidized MWCNTs (35),which is also is evidence of the adsorption of glycine mole-cules on oxidized MWCNTs. EDX quantitative analysisshows the high purity of the samples.

4. Conclusion

The spectroscopic studies on oxidized MWCNTs and gly-cine-functionalized MWCNTs were carried out using UV-Vis, FT-IR, EPR, SEM, and EDX spectroscopic techniques.From the UV-Vis study, the characteristic peak was observedfor oxidized MWCNTs and the same peak was red-shifted by20 nm for glycine-functionalized MWCNTs. The appearanceof another peak around »350 nm is attributed to the forma-tion of glycine-functionalized MWCNTs. FT-IR study con-firms the presence of functional groups of oxidizedMWCNTs and glycine-functionalized MWCNTs. EPR anal-ysis reveals that the EPR absorption spectral data was found

Fig. 6. EDX spectra of (a) oxidized MWCNTs and (b) glycine-functionalized MWCNTs.

Table 4. EDX elemental micro analysis (wt.%) of oxidized andglycine functionalized

Normalized wt%

Samples C O N

Oxidized MWCNTs 97.67 2.33 —Glycine functionalized MWCNTs 94.59 3.03 2.38

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to be the best fit for the Gaussian lineshape. EPR study showsthat the EPR line width increases with increasing concentra-tions of glycine. The g-values indicate that the system wasfound to be isotropic in nature. SEM and EDX analyses con-firm that glycine molecules are functionalized on oxidizedMWCNTs.

Acknowledgment

The authors thank management team of their institution fortheir encouragement and permission to carry out this work.

Funding

This work was supported by the UGC Research AwardScheme, New Delhi (F. No. 30-35/2011 (SA-II)).

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