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Spectroscopic studies on sidewall carboxylic acid functionalization of multi-walled carbon nanotubes with valine
Accepted Manuscript Spectroscopic studies on sidewall carboxylic acid functionalization of multiwalled carbon nanotubes with valine M. Deborah, A. Jawahar, T. Mathavan, M. Kumara Dhas, A. Milton Franklin Benial PII: DOI: Reference:
S1386-1425(14)01812-5 http://dx.doi.org/10.1016/j.saa.2014.12.041 SAA 13082
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
23 September 2014 3 December 2014 15 December 2014
Please cite this article as: M. Deborah, A. Jawahar, T. Mathavan, M. Kumara Dhas, A. Milton Franklin Benial, Spectroscopic studies on sidewall carboxylic acid functionalization of multi-walled carbon nanotubes with valine, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.12.041
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Spectroscopic studies on sidewall carboxylic acid functionalization of multi-walled carbon nanotubes with valine M. Deborah1, A. Jawahar1, T. Mathavan2, M. Kumara Dhas2, A. Milton Franklin Benial2*
*Author for correspondence Dr. A. Milton Franklin Benial Associate Professor, Department of Physics, NMSSVN College, Nagamalai, Madurai-625019, Tamilnadu, India. TEL: +91-9486468945. E-mail: [email protected] 1
Department of Chemistry, NMSSVN College, Madurai-625 019, Tamil Nadu, India.
2
Department of Physics, NMSSVN College, Madurai-625 019, Tamil Nadu, India.
Manuscript information Total Word Count
: 4082
Number of Text pages
: 19
Number of Figures
:8
Number of Tables
:4
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Abstract The valine functionalized multi-walled carbon nanotubes (MWCNTS) were prepared and characterized by using XRD, UV-Vis, FT-IR, EPR, SEM, and EDX, spectroscopic techniques. The enhanced XRD peak (002) intensity was observed for valine functionalized MWCNTs compared with oxidized MWCNTs, which is likely due to sample purification by acid washing. UV-Vis study shows the formation of valine functionalized MWCNTs. FT-IR study confirms the presence of functional groups of oxidized MWCNTs and valine functionalized MWCNTs. The ESR line shape analysis indicates that the observed EPR line shape is a Gaussian line shape. The g-values indicate that the systems are isotropic in nature. The morphology study was carried out for oxidized MWCNTs and valine functionalized MWCNTs by using SEM. The EDX spectra revealed that the high purity of oxidized MWCNTs and valine functionalized MWCNTs. The functionalization has been chosen because, functionalization of CNTs with amino acids makes them soluble and biocompatible. Thus, they have potential applications in the field of biosensors and targeted drug delivery. Key words: Characterization; Electron paramagnetic resonance; Functionalization; Multi-walled carbon nanotubes; Valine
2
1. Introduction Carbon nanotubes (CNTs) are a newly discovered form of crystalline carbon, which forms cylinders of carbon having a diameter in the nanometer scale and a variable length [1]. The functionalization of CNTs is an actively discussed topic in contemporary nanotube literature since the modification of CNTs properties is believed to open the road toward real nanotechnology applications [2–7]. Due to their exceptional combination of mechanical, thermal, chemical, and electronic properties, MWCNTs are considered as unique materials, with very promising future applications, especially in the field of nanotechnology, nanoelectronics, composite materials and as well as in medicinal chemistry [8–21]. The researchers are still far from considering CNTs as entities easy to integrate into organic, inorganic, or biological systems. One of the most powerful approaches to improve CNTs handling is the carboxylic acid functionalization of their sidewalls, which enables chemical bonding between the MWCNTs and the material of interest [22]. The mixture of sulfuric acid/nitric acid or nitric acid can be used to form carboxylic acid groups on the surface of CNTs. The introduction of carboxylic acid groups to CNTs by oxidative procedures gives useful sites for further modifications as they enable the covalent coupling of molecules through the creation of amide and ester bonds. The presence of carboxylic acid groups in the CNTs leads to a reduction of van der Waals interactions between the CNTs, which strongly facilitates the separation of CNTs bundles into individual nanotubes [4]. Functionalization of CNTs with the assistance of biological molecules remarkably improves the solubility of nanotubes in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine, and bioengineering [23]. The functionalized MWCNTs can have higher sensitivity and better response towards electrochemical detection than pristine MWCNTs [24]. Ilie et al. reported that MWCNTs functionalized with nicotinamide 3
leads to significant insulin production compared with individual administration of nicotinamide representing nanomediated treatment of diabetes mellitus [25]. Meng et al. reported that antitumor response can be increased by conjugating a tumor lysate protein to MWCNTs [26]. A novel biomaterial-MWCNT-chitosan-phycocyanin prepared by functionalizing MWCNTs with chitosan and conjugated to phycocyanin for photodynamic and photothermal cancer therapy were tested on breast and liver cancer cells. Liu et al. reported that the biomoleculefunctionalized MWCNTs are expected to be more selective than untreated and oxidizedMWCNTs for the solid-phase extraction of metal ions. Functionalized CNTs are promising novel materials for a variety of biomedical applications. The potential applications of MWCNTs are particularly enhanced by their ability to penetrate biological membranes [27]. Amino acids are components of peptides, indispensable compounds in the life process and can also reflect the common chemical properties of complicated biomolecules. So, the interaction between CNTs and amino acid is very important for understanding the interaction mechanism between CNTs and biomolecules [28,29]. Valine is an essential branched chain amino acid, important for smooth nervous system and cognitive functioning. Valine is particularly important for gall bladder and liver function, as well as balancing nitrogen levels in the body. Valine enhances energy, increase endurance, and aid in muscle tissue recovery and repair. Valine also lowers elevated blood sugar levels and increases growth hormone production [30,31]. Valine functionalized MWCNTs has been researched because Valine has a high reactivity and wealth of chemistry. The aim of the present work is to develop a relatively simple and effective process of functionalizing MWCNTs with amino acids and carry out the characterization study. Here we report, the preparation and characterization of oxidized MWCNTs and valine functionalized MWCNTs. The characterization work has been extensively 4
carried out by using X-ray diffraction (XRD), ultraviolet-visible (UV-Vis), fourier transform infrared (FT-IR), electron paramagnetic resonance (EPR), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. 2. Materials and methods The MWCNTs was purchased from Aldrich Chemical Co, St. Louis, MO, USA. Valine, H2SO4, and HNO3 were purchased from Merck, Germany. 2.1. Sample preparation 2.1.1. Oxidized MWCNTs Pristine MWCNTs were mixed with a mixture of 3:1 concentrated sulfuric and nitric acid and sonicated for 3 hours at 40ºC in an ultrasonic bath to introduce carboxylic acid groups on the surface of MWCNTs. After sonication the mixture was added dropwise to cold distilled water and the resulting samples, oxidized MWCNTs were filtered and dried in vacuum at 80ºC for 4h [32]. 1.1.2.
Valine functionalized MWCNTs The oxidized MWCNTs were mixed with 0.3 M valine suspension and sonicated for
about 1h at room temperature. After sonication the oxidized MWCNTs/valine suspension was directly filtered and the solid sample was dried in a vacuum for about 16 h at room temperature [33]. In the similar way, valine functionalized MWCNTs (0.6 and 0.9 M concentration of valine) were also prepared. Fig. 1 shows the scheme for the synthesis of oxidized MWCNTs and valine functionalized MWCNTs.
5
2.2 Spectral measurements 2.2.1. XRD analysis The X-ray diffraction technique was used to characterize the crystalline structure of the samples. XRD patterns were collected using a PANalytical (Eindhoven, The Netherlands) diffractometer with a copper target at the wave length of λ CuKα=1.54 Å, a tube voltage of 40 kV, and tube current of 30 mA. The scanning range was selected between10° to 80° of 2θ. For XRD studies, rectangular pellets prepared by compression molding were used. 2.2.2. UV-Vis measurements A
Shimadzu
UV-3600
UV-Vis-NIR
spectrophotometer
(Shimadzu
Scientific
Instruments, Columbia, MD) was used for absorption spectra measurements of the samples in the wavelength range of 200 - 600 nm. 2.2.3. FT-IR measurements The FT-IR spectra of the samples dispersed in the potassium bromide matrix were recorded in the wave number range of 400-4000 cm-1 at 64 scans per spectrum at 4 cm-1 resolution using a computerized Bruker Optik GmbH FT-IR spectrophotometer. Spectra were corrected for the moisture and carbon dioxide in the optical path. 2.2.4. EPR measurements EPR spectra of the samples were recorded at room temperature using a Bruker EMX plus spectrometer with 100 kHz field modulation frequency and phase sensitive detection. The ESR spectra were recorded by varying the magnetic field in the range of 338–358 mT with the 6
following spectrometer settings: field modulation amplitude 0.6 mT; conversion time, 30 ms; radio-frequency power, 5.0 mW; receiver gain, 2000; sweep width, 20 mT; sweep time, 30 s; number of scans, 8; 2048 k resolution and radio frequency, 9.77 GHz. The temperature was controlled using a controller with water as a coolant. 2.2.5. SEM imaging The surface morphological studies of the oxidized MWCNTs and valine functionalized MWCNTs were carried out by scanning electron microscopy (SEM-VEGA3 TESCAN, USA). 2.2.6. EDX measurements The chemical composition of oxidized MWCNTs and valine functionalized MWCNTs were characterized by energy dispersive spectrometer (Bruker Nano, Germany). 3. Results and discussion 3.1. XRD analysis The XRD patterns of the oxidized MWCNTs and valine functionalized MWCNTs were taken to reveal detailed information about the crystallographic structure of MWCNTs. Fig. 2 shows the X-ray diffraction patterns of oxidized MWCNTs and valine functionalized MWCNTs. For oxidized MWCNTs, two peaks appeared at 2θ = 26.54°, 42.43° and for valine functionalized MWCNTs, two peaks appeared at 2θ = 26.95º and 42.91º which were assigned to (002), (100) diffraction plane of MWCNTs [34]. These two diffraction peaks are attributed to the graphitic structure of MWCNTs. Valine functionalized MWCNTs showed very few changes in the XRD pattern. It could be seen that the XRD pattern of valine functionalized MWCNTs is much similar to the oxidized MWCNTs. which indicates that the valine functionalized MWCNTs still had the
7
same cylinder wall structure as oxidized MWCNTs. Thus, the structure of MWCNTs is protected even after the functionalization process. Interestingly, the intensity of a diffraction peak at (002) in valine functionalized MWCNTs was increased as compared with oxidized MWCNTs, which is likely due to sample purification by acid washing. This is an indication of the more ordered CNTs floss in the valine functionalized MWCNTs [35]. 3.2. UV-Vis analysis The UV-Vis spectra of oxidized MWCNTs and valine functionalized MWCNTs were shown in Fig. 3 and their corresponding spectral data were given in Table 1. The characteristic peak was appeared at 276 nm for oxidized MWCNTs, which is in good agreement with the literature value [36]. The blue shift was observed for valine functionalized MWCNTs at 263 nm (0.6 and 0.9 M concentration of valine). Another broad peak was observed at 325 nm for valine functionalized MWCNTs, which is attributed to the formation of charge transfer complex, valine functionalized MWCNTs. 3.3. FT-IR analysis FT-IR spectroscopy is a qualitative technique used for identifying the functional groups appended to the MWCNTs. The FT-IR Spectra of oxidized MWCNTs and valine functionalized MWCNTs were shown in Fig. 4 and their corresponding FT-IR assignments were listed in Table 2. The amino acid treated MWCNTs samples provided good evidence for the desired functionalities in contrast with the oxidized MWCNTs. The carbonyl stretching mode of carboxylic acid group appeared around ~1715 cm-1, which indicates the existence of carboxyl group on the surface of MWCNTs [37]. Another peak in the region around ~1635 cm-1 is due to the carbonyl stretching mode of quinone type units along the side walls of the CNTs [38]. 8
Functionalization of valine on the oxidized MWCNTs, which resulted in the formation of secondary amide on the MWCNTs is confirmed by the characteristic peak around ~1217 cm-1 which is assigned to C-N stretching and another peak in the region around ~885 cm-1 is assigned to the N-H stretching mode, which confirms that valine molecules have been bonded to CNTs [32]. The N-H groups observed on the valine functionalized samples indicates that the amidation reactions occurred between the amino groups of amino acid and the carboxyl groups on the surfaces of the MWCNTs [39]. The O-H bending and C-OH stretching modes in carboxylic acid were observed around ~1371 cm-1 and ~667 cm-1 respectively [40,41]. The C-H bending mode appeared around ~1433 cm-1 [42]. The observed peak around ~1034 cm-1 is assigned to the C-O stretching mode [37]. 3.4. EPR analysis EPR spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. EPR spectroscopy has been used to assess the quality of carbon nanotube. EPR measurements are sensitive to metallic impurities and dangling bond defects [43]. Fig. 5 shows the EPR spectra of oxidized MWCNTs and valine functionalized MWCNTs (0.3, 0.6 and 0.9 M concentration of valine). Fig. 6 shows the EPR absorption spectra and Gaussian fit of oxidized and valine functionalized MWCNTs (0.3, 0.6 and 0.9 M concentration of valine). The EPR parameters such as line width, g-factor, spin concentration and R2 from Gaussian fit were obtained for oxidized MWCNTs and valine functionalized MWCNTs were listed in Table 3.
9
3.4.1. Line shape The line shape analysis was carried out using Origin 8 software package, which reveals that the EPR absorption spectra have a Gaussian lineshape. Previous studies have revealed that the acid-oxidized MWCNTs exhibited strong EPR signal, which implied that many unpaired electrons were generated on the surface of the CNTs [44]. The EPR line shape is usually described by Lorentzian and Gaussian line shapes. The EPR absorption spectral data were found to be the best fit (correlation coefficient value [R2]>0.94) for the Gaussian function, f(x) = a*exp –((x-b)2/2c2), where a, b and c are real constants [45]. The EPR absorption spectra for the oxidized MWCNTs and valine functionalized MWCNTs have a Gaussian lineshape. The dipolar broadening generally produces Gaussian shaped lines. 3.4.2. Line width and g-factor The full width at half maximum (FWHM) line width values for the EPR spectra of oxidized MWCNTs and valine functionalized MWCNTs were obtained from the Gaussian fit. The line width broadening arises due to the dipolar interaction. Hence, the dipolar interaction term can be explained in terms of the FWHM values of Gaussian line shape. The FWHM line width value increases with increasing concentration of valine, which reveals that the dipoledipole interaction between the valine and MWCNTs increases with increasing concentration of valine. In valine functionalized MWCNTs, the amidation reaction takes place between the amino group of valine and the carboxyl groups on the side walls of the MWCNTs, which also leads to the increase in FWHM value.
10
The g-value was calculated using the magnetic field B0, which is obtained from the central position of the EPR spectral line. The ‘g’ value of ∼2.01–2.02 is assigned to the interaction between the conducting electrons in the CNTs trapped at defects or magnetic ions site. The more the deviation from the free electron ‘g’ value, the more is the localization due to defects [46]. In valine functionalized MWCNTs, EPR lines are seen with the g-value of 2.01112.0122, which is very close to the free electron g-value (2.0023). The g-value indicates the isotropic nature of the system, which reveals that the properties of the system remains the same with respect to the direction of the magnetic field. 3.4.3. Signal intensity and spin concentration The EPR Signal intensity decreases with increasing concentration of valine, which were shown in Fig. 5, which reveals that the MWCNTs successfully reacted with valine. The observed EPR signal is in the derivative mode, which is converted into the EPR absorption signal using Origin 8 software package. The EPR absorption spectral data were then fitted to the Gaussian function. The spin concentration values were obtained for oxidized MWCNTs and valine functionalized MWCNTs from the Gaussian fitting of EPR absorption spectral data, which were shown in Table 3. The spin concentration values were estimated from the area of the Gaussian line shape of the EPR spectral line. The spin concentration value decreases with increasing concentration of valine, which reveals that the unpaired electrons in the CNTs undergo reduction process in valine functionlized MWCNTs. 3.5. SEM and EDX analysis
11
The morphology of CNTs, their dimensions, and orientation can be easily revealed by SEM [47]. Both oxidized MWCNTs and valine functionalized MWCNTs were characterized by SEM. A representative image of valine functionalized MWCNTs, along with an image of oxidized MWCNTs, were shown in Fig. 7. The SEM image of oxidized MWCNTs was observed with an average diameter of ~85 nm. After the functionalization of a valine molecule on the oxidized MWCNTs, which resulted in an increase of the average diameter of ~120 nm due to the adsorption of valine molecules on the sidewalls of oxidized MWCNTs. After functionalization with valine the tubes are less isolated and distinct as compared to oxidized MWCNTs. The adsorption of valine molecules on the surface of oxidized MWCNTs was caused by polar interactions, π-π stacking, hydrogen bonding and covalent bonding [33]. Agglomeration of the MWCNTs was not observed by SEM images, which indicates the higher dispersing ability of MWCNTs in solvents [48]. Fig. 8 shows the EDX spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs. The EDX elemental micro analysis (wt.%) of oxidized MWCNTs and valine functionalized MWCNTs were listed in Table 4. The EDX elemental micro analysis confirms that the element nitrogen exists in valine functionalized MWCNTs along with carbon and oxygen. The spectral line corresponds to element nitrogen was not appearing in the EDX spectrum (Fig 8a) of oxidized MWCNTs, which is also evident that the adsorption of valine molecules on oxidized MWCNTs [33]. EDX quantitative analysis shows that the high purity of the samples. 4. Conclusion In the present work, sidewall carboxylic acid functionalization of MWCNTs with valine by a facile approach was demonstrated. The spectroscopic studies on oxidized MWCNTs and 12
valine functionalized MWCNTs were carried out using XRD, UV-Vis, FT-IR, EPR, SEM, and EDX spectroscopic techniques. XRD study concluded that the general structure of MWCNTs remained the same even after functionalization. From the UV-Vis study, the characteristic peak was observed at 276 nm for oxidized MWCNTs. The appearance of another peak is attributed to the formation of charge transfer complex, valine-functionalized MWCNTs. FT-IR study confirms the presence of functional groups of oxidized MWCNTs and valine functionalized MWCNTs. EPR analysis shows that the EPR absorption spectral data found to be best fit for the Gaussian lineshape. EPR study shows that the EPR line width increases with increasing concentration of valine, which reveals that the dipole-dipole interaction increases with increasing concentration of valine. EPR study also reveals that the spin concentration value decreases with increasing concentration of valine, which implies that the unpaired electron undergoes reduction process in valine functionalized MWCNTs. The SEM and EDX analysis confirm that valine molecules were functionalized on the oxidized MWCNTs. The amino acid functionalized CNTs, becomes relatively simple to use as a support for peptide synthesis by simply adding further amino acids in the desired order and quantity. The sidewall terminal carboxyl groups in the valine functionalized MWCNTs can be used for further functionalization and covalent binding with a variety of monomer and polymer matrices in the processing and fabrication of composites and fibers, ceramics for tissue engineering, and implants in orthopedics and dentistry. 4 ACKNOWLEDGMENTS The authors thank the management for encouragement and permission to carry out this work. This work was supported by the UGC Research Award scheme, New Delhi (F.No.30-35/2011 (SA-II). 13
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FIGURE CAPTIONS Fig. 1 Scheme for the synthesis of valine functionalized MWCNTs a) pristine MWCNTs was 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 for 4 hours. (b) The oxidized MWCNTs was mixed with valine suspension, sonicated for 1 hour at room temperature. The samples were filtered and dried in vacuum at room temperature for 16 hours. Fig. 2 XRD spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs. Fig. 3 UV-Vis spectra of a) oxidized MWCNTs b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 4 FT-IR spectra of a) oxidized MWCNTs b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 5 EPR of a) oxidized MWCNTs and b) 0.3 e) 0.6 and f) 0.9 M valine functionalized MWCNTs. Fig. 6 EPR absorption spectra and Gaussian fit (-----) of a) oxidized MWCNTs and b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 7 SEM images of a) oxidized MWCNTs and b) valine functionalized MWCNTs. Fig. 8 EDX spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs.
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TABLE CAPTIONS Table 1. UV-Vis spectral data for oxidized MWCNTs and valine functionalized MWCNTs. Table 2. FT-IR spectral assignments of oxidized MWCNTs and valine functionalized MWCNTs. Table 3. EPR parameters of oxidized MWCNTs and valine functionalized MWCNTs. Table 4. EDX elemental micro analysis (wt. %) of oxidized MWCNTs and valine functionalized MWCNTs.
19
20
21
22
23
24
25
26
27
Table 1. UV-Vis spectral data for oxidized MWCNTs and valine functionalized MWCNTs.
Absorbance
Absorbance
λ1(nm)
λ2(nm)
276
-
276
-
0.6M
263
325
0.9 M
263
325
Sample
Oxidized MWCNT
concentration of valine
0.3 M Valine functionalized MWCNTs
28
Table 2. FT-IR spectral assignments of oxidized MWCNTs and valine functionalized MWCNTs.
Valine concentration
Assignment
Oxidized MWCNTs Wavenumber (cm-1)
0.3M Wavenumber (cm-1)
0.6M Wavenumber (cm-1)
0.9M Wavenumber (cm-1)
C=O stretching
1710
1708
1714
1718
C=O stretching
1632
1633
1632
1637
C-H bending
1434
1430
1432
1439
O-H bending
1374
1376
1377
1360
C-N stretching
-
1222
1221
1208
C-O stretching
1023
1025
1027
1063
N-H stretching
-
884
881
889
C-OH stretching
669
668
668
665
29
Table 3. EPR parameters of oxidized MWCNTs and valine functionalized MWCNTs.
Samples
R2 from Gaussian fit
Spin concentration from Gaussian fit (a.u)
Oxidized MWCNTs
0.964
9.61 x 109
43.79
2.0072
0.3 M
0.946
1.72 x 109
48.79
2.0111
0.6 M
0.947
1.22 x 109
49.27
2.0122
0.9 M
0.957
9.81 x 108
50.92
2.0122
FWHM (mT)
g - Factor
Valine concentration
Valine functionalized MWCNTs
30
Table 4 EDX elemental micro analysis (wt.%) of oxidized MWCNTs and valine functionalized MWCNTs
Normalized wt % Samples
C
O
N
97.67
2.33
-
94.58
2.37
3.05
Oxidized MWCNTs
Valine functionalized MWCNTs
31
32
Highlights The convenient and simple method for sidewall carboxylic acid functionalization of MWCNTs with valine was demonstrated. The enhanced XRD peak (002) intensity was observed for valine functionalized MWCNTs The blue shift was observed in the UV-Vis spectra of valine functionalized MWCNTs The EPR absorption spectral data were found to be best fit for the Gaussian lineshape. SEM images show that the increase in the diameter of the MWCNTs was observed for valine functionalized MWCNTs
33
S1386-1425(14)01812-5 http://dx.doi.org/10.1016/j.saa.2014.12.041 SAA 13082
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
23 September 2014 3 December 2014 15 December 2014
Please cite this article as: M. Deborah, A. Jawahar, T. Mathavan, M. Kumara Dhas, A. Milton Franklin Benial, Spectroscopic studies on sidewall carboxylic acid functionalization of multi-walled carbon nanotubes with valine, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.12.041
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Spectroscopic studies on sidewall carboxylic acid functionalization of multi-walled carbon nanotubes with valine M. Deborah1, A. Jawahar1, T. Mathavan2, M. Kumara Dhas2, A. Milton Franklin Benial2*
*Author for correspondence Dr. A. Milton Franklin Benial Associate Professor, Department of Physics, NMSSVN College, Nagamalai, Madurai-625019, Tamilnadu, India. TEL: +91-9486468945. E-mail: [email protected] 1
Department of Chemistry, NMSSVN College, Madurai-625 019, Tamil Nadu, India.
2
Department of Physics, NMSSVN College, Madurai-625 019, Tamil Nadu, India.
Manuscript information Total Word Count
: 4082
Number of Text pages
: 19
Number of Figures
:8
Number of Tables
:4
1
Abstract The valine functionalized multi-walled carbon nanotubes (MWCNTS) were prepared and characterized by using XRD, UV-Vis, FT-IR, EPR, SEM, and EDX, spectroscopic techniques. The enhanced XRD peak (002) intensity was observed for valine functionalized MWCNTs compared with oxidized MWCNTs, which is likely due to sample purification by acid washing. UV-Vis study shows the formation of valine functionalized MWCNTs. FT-IR study confirms the presence of functional groups of oxidized MWCNTs and valine functionalized MWCNTs. The ESR line shape analysis indicates that the observed EPR line shape is a Gaussian line shape. The g-values indicate that the systems are isotropic in nature. The morphology study was carried out for oxidized MWCNTs and valine functionalized MWCNTs by using SEM. The EDX spectra revealed that the high purity of oxidized MWCNTs and valine functionalized MWCNTs. The functionalization has been chosen because, functionalization of CNTs with amino acids makes them soluble and biocompatible. Thus, they have potential applications in the field of biosensors and targeted drug delivery. Key words: Characterization; Electron paramagnetic resonance; Functionalization; Multi-walled carbon nanotubes; Valine
2
1. Introduction Carbon nanotubes (CNTs) are a newly discovered form of crystalline carbon, which forms cylinders of carbon having a diameter in the nanometer scale and a variable length [1]. The functionalization of CNTs is an actively discussed topic in contemporary nanotube literature since the modification of CNTs properties is believed to open the road toward real nanotechnology applications [2–7]. Due to their exceptional combination of mechanical, thermal, chemical, and electronic properties, MWCNTs are considered as unique materials, with very promising future applications, especially in the field of nanotechnology, nanoelectronics, composite materials and as well as in medicinal chemistry [8–21]. The researchers are still far from considering CNTs as entities easy to integrate into organic, inorganic, or biological systems. One of the most powerful approaches to improve CNTs handling is the carboxylic acid functionalization of their sidewalls, which enables chemical bonding between the MWCNTs and the material of interest [22]. The mixture of sulfuric acid/nitric acid or nitric acid can be used to form carboxylic acid groups on the surface of CNTs. The introduction of carboxylic acid groups to CNTs by oxidative procedures gives useful sites for further modifications as they enable the covalent coupling of molecules through the creation of amide and ester bonds. The presence of carboxylic acid groups in the CNTs leads to a reduction of van der Waals interactions between the CNTs, which strongly facilitates the separation of CNTs bundles into individual nanotubes [4]. Functionalization of CNTs with the assistance of biological molecules remarkably improves the solubility of nanotubes in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine, and bioengineering [23]. The functionalized MWCNTs can have higher sensitivity and better response towards electrochemical detection than pristine MWCNTs [24]. Ilie et al. reported that MWCNTs functionalized with nicotinamide 3
leads to significant insulin production compared with individual administration of nicotinamide representing nanomediated treatment of diabetes mellitus [25]. Meng et al. reported that antitumor response can be increased by conjugating a tumor lysate protein to MWCNTs [26]. A novel biomaterial-MWCNT-chitosan-phycocyanin prepared by functionalizing MWCNTs with chitosan and conjugated to phycocyanin for photodynamic and photothermal cancer therapy were tested on breast and liver cancer cells. Liu et al. reported that the biomoleculefunctionalized MWCNTs are expected to be more selective than untreated and oxidizedMWCNTs for the solid-phase extraction of metal ions. Functionalized CNTs are promising novel materials for a variety of biomedical applications. The potential applications of MWCNTs are particularly enhanced by their ability to penetrate biological membranes [27]. Amino acids are components of peptides, indispensable compounds in the life process and can also reflect the common chemical properties of complicated biomolecules. So, the interaction between CNTs and amino acid is very important for understanding the interaction mechanism between CNTs and biomolecules [28,29]. Valine is an essential branched chain amino acid, important for smooth nervous system and cognitive functioning. Valine is particularly important for gall bladder and liver function, as well as balancing nitrogen levels in the body. Valine enhances energy, increase endurance, and aid in muscle tissue recovery and repair. Valine also lowers elevated blood sugar levels and increases growth hormone production [30,31]. Valine functionalized MWCNTs has been researched because Valine has a high reactivity and wealth of chemistry. The aim of the present work is to develop a relatively simple and effective process of functionalizing MWCNTs with amino acids and carry out the characterization study. Here we report, the preparation and characterization of oxidized MWCNTs and valine functionalized MWCNTs. The characterization work has been extensively 4
carried out by using X-ray diffraction (XRD), ultraviolet-visible (UV-Vis), fourier transform infrared (FT-IR), electron paramagnetic resonance (EPR), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. 2. Materials and methods The MWCNTs was purchased from Aldrich Chemical Co, St. Louis, MO, USA. Valine, H2SO4, and HNO3 were purchased from Merck, Germany. 2.1. Sample preparation 2.1.1. Oxidized MWCNTs Pristine MWCNTs were mixed with a mixture of 3:1 concentrated sulfuric and nitric acid and sonicated for 3 hours at 40ºC in an ultrasonic bath to introduce carboxylic acid groups on the surface of MWCNTs. After sonication the mixture was added dropwise to cold distilled water and the resulting samples, oxidized MWCNTs were filtered and dried in vacuum at 80ºC for 4h [32]. 1.1.2.
Valine functionalized MWCNTs The oxidized MWCNTs were mixed with 0.3 M valine suspension and sonicated for
about 1h at room temperature. After sonication the oxidized MWCNTs/valine suspension was directly filtered and the solid sample was dried in a vacuum for about 16 h at room temperature [33]. In the similar way, valine functionalized MWCNTs (0.6 and 0.9 M concentration of valine) were also prepared. Fig. 1 shows the scheme for the synthesis of oxidized MWCNTs and valine functionalized MWCNTs.
5
2.2 Spectral measurements 2.2.1. XRD analysis The X-ray diffraction technique was used to characterize the crystalline structure of the samples. XRD patterns were collected using a PANalytical (Eindhoven, The Netherlands) diffractometer with a copper target at the wave length of λ CuKα=1.54 Å, a tube voltage of 40 kV, and tube current of 30 mA. The scanning range was selected between10° to 80° of 2θ. For XRD studies, rectangular pellets prepared by compression molding were used. 2.2.2. UV-Vis measurements A
Shimadzu
UV-3600
UV-Vis-NIR
spectrophotometer
(Shimadzu
Scientific
Instruments, Columbia, MD) was used for absorption spectra measurements of the samples in the wavelength range of 200 - 600 nm. 2.2.3. FT-IR measurements The FT-IR spectra of the samples dispersed in the potassium bromide matrix were recorded in the wave number range of 400-4000 cm-1 at 64 scans per spectrum at 4 cm-1 resolution using a computerized Bruker Optik GmbH FT-IR spectrophotometer. Spectra were corrected for the moisture and carbon dioxide in the optical path. 2.2.4. EPR measurements EPR spectra of the samples were recorded at room temperature using a Bruker EMX plus spectrometer with 100 kHz field modulation frequency and phase sensitive detection. The ESR spectra were recorded by varying the magnetic field in the range of 338–358 mT with the 6
following spectrometer settings: field modulation amplitude 0.6 mT; conversion time, 30 ms; radio-frequency power, 5.0 mW; receiver gain, 2000; sweep width, 20 mT; sweep time, 30 s; number of scans, 8; 2048 k resolution and radio frequency, 9.77 GHz. The temperature was controlled using a controller with water as a coolant. 2.2.5. SEM imaging The surface morphological studies of the oxidized MWCNTs and valine functionalized MWCNTs were carried out by scanning electron microscopy (SEM-VEGA3 TESCAN, USA). 2.2.6. EDX measurements The chemical composition of oxidized MWCNTs and valine functionalized MWCNTs were characterized by energy dispersive spectrometer (Bruker Nano, Germany). 3. Results and discussion 3.1. XRD analysis The XRD patterns of the oxidized MWCNTs and valine functionalized MWCNTs were taken to reveal detailed information about the crystallographic structure of MWCNTs. Fig. 2 shows the X-ray diffraction patterns of oxidized MWCNTs and valine functionalized MWCNTs. For oxidized MWCNTs, two peaks appeared at 2θ = 26.54°, 42.43° and for valine functionalized MWCNTs, two peaks appeared at 2θ = 26.95º and 42.91º which were assigned to (002), (100) diffraction plane of MWCNTs [34]. These two diffraction peaks are attributed to the graphitic structure of MWCNTs. Valine functionalized MWCNTs showed very few changes in the XRD pattern. It could be seen that the XRD pattern of valine functionalized MWCNTs is much similar to the oxidized MWCNTs. which indicates that the valine functionalized MWCNTs still had the
7
same cylinder wall structure as oxidized MWCNTs. Thus, the structure of MWCNTs is protected even after the functionalization process. Interestingly, the intensity of a diffraction peak at (002) in valine functionalized MWCNTs was increased as compared with oxidized MWCNTs, which is likely due to sample purification by acid washing. This is an indication of the more ordered CNTs floss in the valine functionalized MWCNTs [35]. 3.2. UV-Vis analysis The UV-Vis spectra of oxidized MWCNTs and valine functionalized MWCNTs were shown in Fig. 3 and their corresponding spectral data were given in Table 1. The characteristic peak was appeared at 276 nm for oxidized MWCNTs, which is in good agreement with the literature value [36]. The blue shift was observed for valine functionalized MWCNTs at 263 nm (0.6 and 0.9 M concentration of valine). Another broad peak was observed at 325 nm for valine functionalized MWCNTs, which is attributed to the formation of charge transfer complex, valine functionalized MWCNTs. 3.3. FT-IR analysis FT-IR spectroscopy is a qualitative technique used for identifying the functional groups appended to the MWCNTs. The FT-IR Spectra of oxidized MWCNTs and valine functionalized MWCNTs were shown in Fig. 4 and their corresponding FT-IR assignments were listed in Table 2. The amino acid treated MWCNTs samples provided good evidence for the desired functionalities in contrast with the oxidized MWCNTs. The carbonyl stretching mode of carboxylic acid group appeared around ~1715 cm-1, which indicates the existence of carboxyl group on the surface of MWCNTs [37]. Another peak in the region around ~1635 cm-1 is due to the carbonyl stretching mode of quinone type units along the side walls of the CNTs [38]. 8
Functionalization of valine on the oxidized MWCNTs, which resulted in the formation of secondary amide on the MWCNTs is confirmed by the characteristic peak around ~1217 cm-1 which is assigned to C-N stretching and another peak in the region around ~885 cm-1 is assigned to the N-H stretching mode, which confirms that valine molecules have been bonded to CNTs [32]. The N-H groups observed on the valine functionalized samples indicates that the amidation reactions occurred between the amino groups of amino acid and the carboxyl groups on the surfaces of the MWCNTs [39]. The O-H bending and C-OH stretching modes in carboxylic acid were observed around ~1371 cm-1 and ~667 cm-1 respectively [40,41]. The C-H bending mode appeared around ~1433 cm-1 [42]. The observed peak around ~1034 cm-1 is assigned to the C-O stretching mode [37]. 3.4. EPR analysis EPR spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. EPR spectroscopy has been used to assess the quality of carbon nanotube. EPR measurements are sensitive to metallic impurities and dangling bond defects [43]. Fig. 5 shows the EPR spectra of oxidized MWCNTs and valine functionalized MWCNTs (0.3, 0.6 and 0.9 M concentration of valine). Fig. 6 shows the EPR absorption spectra and Gaussian fit of oxidized and valine functionalized MWCNTs (0.3, 0.6 and 0.9 M concentration of valine). The EPR parameters such as line width, g-factor, spin concentration and R2 from Gaussian fit were obtained for oxidized MWCNTs and valine functionalized MWCNTs were listed in Table 3.
9
3.4.1. Line shape The line shape analysis was carried out using Origin 8 software package, which reveals that the EPR absorption spectra have a Gaussian lineshape. Previous studies have revealed that the acid-oxidized MWCNTs exhibited strong EPR signal, which implied that many unpaired electrons were generated on the surface of the CNTs [44]. The EPR line shape is usually described by Lorentzian and Gaussian line shapes. The EPR absorption spectral data were found to be the best fit (correlation coefficient value [R2]>0.94) for the Gaussian function, f(x) = a*exp –((x-b)2/2c2), where a, b and c are real constants [45]. The EPR absorption spectra for the oxidized MWCNTs and valine functionalized MWCNTs have a Gaussian lineshape. The dipolar broadening generally produces Gaussian shaped lines. 3.4.2. Line width and g-factor The full width at half maximum (FWHM) line width values for the EPR spectra of oxidized MWCNTs and valine functionalized MWCNTs were obtained from the Gaussian fit. The line width broadening arises due to the dipolar interaction. Hence, the dipolar interaction term can be explained in terms of the FWHM values of Gaussian line shape. The FWHM line width value increases with increasing concentration of valine, which reveals that the dipoledipole interaction between the valine and MWCNTs increases with increasing concentration of valine. In valine functionalized MWCNTs, the amidation reaction takes place between the amino group of valine and the carboxyl groups on the side walls of the MWCNTs, which also leads to the increase in FWHM value.
10
The g-value was calculated using the magnetic field B0, which is obtained from the central position of the EPR spectral line. The ‘g’ value of ∼2.01–2.02 is assigned to the interaction between the conducting electrons in the CNTs trapped at defects or magnetic ions site. The more the deviation from the free electron ‘g’ value, the more is the localization due to defects [46]. In valine functionalized MWCNTs, EPR lines are seen with the g-value of 2.01112.0122, which is very close to the free electron g-value (2.0023). The g-value indicates the isotropic nature of the system, which reveals that the properties of the system remains the same with respect to the direction of the magnetic field. 3.4.3. Signal intensity and spin concentration The EPR Signal intensity decreases with increasing concentration of valine, which were shown in Fig. 5, which reveals that the MWCNTs successfully reacted with valine. The observed EPR signal is in the derivative mode, which is converted into the EPR absorption signal using Origin 8 software package. The EPR absorption spectral data were then fitted to the Gaussian function. The spin concentration values were obtained for oxidized MWCNTs and valine functionalized MWCNTs from the Gaussian fitting of EPR absorption spectral data, which were shown in Table 3. The spin concentration values were estimated from the area of the Gaussian line shape of the EPR spectral line. The spin concentration value decreases with increasing concentration of valine, which reveals that the unpaired electrons in the CNTs undergo reduction process in valine functionlized MWCNTs. 3.5. SEM and EDX analysis
11
The morphology of CNTs, their dimensions, and orientation can be easily revealed by SEM [47]. Both oxidized MWCNTs and valine functionalized MWCNTs were characterized by SEM. A representative image of valine functionalized MWCNTs, along with an image of oxidized MWCNTs, were shown in Fig. 7. The SEM image of oxidized MWCNTs was observed with an average diameter of ~85 nm. After the functionalization of a valine molecule on the oxidized MWCNTs, which resulted in an increase of the average diameter of ~120 nm due to the adsorption of valine molecules on the sidewalls of oxidized MWCNTs. After functionalization with valine the tubes are less isolated and distinct as compared to oxidized MWCNTs. The adsorption of valine molecules on the surface of oxidized MWCNTs was caused by polar interactions, π-π stacking, hydrogen bonding and covalent bonding [33]. Agglomeration of the MWCNTs was not observed by SEM images, which indicates the higher dispersing ability of MWCNTs in solvents [48]. Fig. 8 shows the EDX spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs. The EDX elemental micro analysis (wt.%) of oxidized MWCNTs and valine functionalized MWCNTs were listed in Table 4. The EDX elemental micro analysis confirms that the element nitrogen exists in valine functionalized MWCNTs along with carbon and oxygen. The spectral line corresponds to element nitrogen was not appearing in the EDX spectrum (Fig 8a) of oxidized MWCNTs, which is also evident that the adsorption of valine molecules on oxidized MWCNTs [33]. EDX quantitative analysis shows that the high purity of the samples. 4. Conclusion In the present work, sidewall carboxylic acid functionalization of MWCNTs with valine by a facile approach was demonstrated. The spectroscopic studies on oxidized MWCNTs and 12
valine functionalized MWCNTs were carried out using XRD, UV-Vis, FT-IR, EPR, SEM, and EDX spectroscopic techniques. XRD study concluded that the general structure of MWCNTs remained the same even after functionalization. From the UV-Vis study, the characteristic peak was observed at 276 nm for oxidized MWCNTs. The appearance of another peak is attributed to the formation of charge transfer complex, valine-functionalized MWCNTs. FT-IR study confirms the presence of functional groups of oxidized MWCNTs and valine functionalized MWCNTs. EPR analysis shows that the EPR absorption spectral data found to be best fit for the Gaussian lineshape. EPR study shows that the EPR line width increases with increasing concentration of valine, which reveals that the dipole-dipole interaction increases with increasing concentration of valine. EPR study also reveals that the spin concentration value decreases with increasing concentration of valine, which implies that the unpaired electron undergoes reduction process in valine functionalized MWCNTs. The SEM and EDX analysis confirm that valine molecules were functionalized on the oxidized MWCNTs. The amino acid functionalized CNTs, becomes relatively simple to use as a support for peptide synthesis by simply adding further amino acids in the desired order and quantity. The sidewall terminal carboxyl groups in the valine functionalized MWCNTs can be used for further functionalization and covalent binding with a variety of monomer and polymer matrices in the processing and fabrication of composites and fibers, ceramics for tissue engineering, and implants in orthopedics and dentistry. 4 ACKNOWLEDGMENTS The authors thank the management for encouragement and permission to carry out this work. This work was supported by the UGC Research Award scheme, New Delhi (F.No.30-35/2011 (SA-II). 13
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FIGURE CAPTIONS Fig. 1 Scheme for the synthesis of valine functionalized MWCNTs a) pristine MWCNTs was 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 for 4 hours. (b) The oxidized MWCNTs was mixed with valine suspension, sonicated for 1 hour at room temperature. The samples were filtered and dried in vacuum at room temperature for 16 hours. Fig. 2 XRD spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs. Fig. 3 UV-Vis spectra of a) oxidized MWCNTs b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 4 FT-IR spectra of a) oxidized MWCNTs b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 5 EPR of a) oxidized MWCNTs and b) 0.3 e) 0.6 and f) 0.9 M valine functionalized MWCNTs. Fig. 6 EPR absorption spectra and Gaussian fit (-----) of a) oxidized MWCNTs and b) 0.3 c) 0.6 and d) 0.9 M valine functionalized MWCNTs. Fig. 7 SEM images of a) oxidized MWCNTs and b) valine functionalized MWCNTs. Fig. 8 EDX spectra of a) oxidized MWCNTs and b) valine functionalized MWCNTs.
18
TABLE CAPTIONS Table 1. UV-Vis spectral data for oxidized MWCNTs and valine functionalized MWCNTs. Table 2. FT-IR spectral assignments of oxidized MWCNTs and valine functionalized MWCNTs. Table 3. EPR parameters of oxidized MWCNTs and valine functionalized MWCNTs. Table 4. EDX elemental micro analysis (wt. %) of oxidized MWCNTs and valine functionalized MWCNTs.
19
20
21
22
23
24
25
26
27
Table 1. UV-Vis spectral data for oxidized MWCNTs and valine functionalized MWCNTs.
Absorbance
Absorbance
λ1(nm)
λ2(nm)
276
-
276
-
0.6M
263
325
0.9 M
263
325
Sample
Oxidized MWCNT
concentration of valine
0.3 M Valine functionalized MWCNTs
28
Table 2. FT-IR spectral assignments of oxidized MWCNTs and valine functionalized MWCNTs.
Valine concentration
Assignment
Oxidized MWCNTs Wavenumber (cm-1)
0.3M Wavenumber (cm-1)
0.6M Wavenumber (cm-1)
0.9M Wavenumber (cm-1)
C=O stretching
1710
1708
1714
1718
C=O stretching
1632
1633
1632
1637
C-H bending
1434
1430
1432
1439
O-H bending
1374
1376
1377
1360
C-N stretching
-
1222
1221
1208
C-O stretching
1023
1025
1027
1063
N-H stretching
-
884
881
889
C-OH stretching
669
668
668
665
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Table 3. EPR parameters of oxidized MWCNTs and valine functionalized MWCNTs.
Samples
R2 from Gaussian fit
Spin concentration from Gaussian fit (a.u)
Oxidized MWCNTs
0.964
9.61 x 109
43.79
2.0072
0.3 M
0.946
1.72 x 109
48.79
2.0111
0.6 M
0.947
1.22 x 109
49.27
2.0122
0.9 M
0.957
9.81 x 108
50.92
2.0122
FWHM (mT)
g - Factor
Valine concentration
Valine functionalized MWCNTs
30
Table 4 EDX elemental micro analysis (wt.%) of oxidized MWCNTs and valine functionalized MWCNTs
Normalized wt % Samples
C
O
N
97.67
2.33
-
94.58
2.37
3.05
Oxidized MWCNTs
Valine functionalized MWCNTs
31
32
Highlights The convenient and simple method for sidewall carboxylic acid functionalization of MWCNTs with valine was demonstrated. The enhanced XRD peak (002) intensity was observed for valine functionalized MWCNTs The blue shift was observed in the UV-Vis spectra of valine functionalized MWCNTs The EPR absorption spectral data were found to be best fit for the Gaussian lineshape. SEM images show that the increase in the diameter of the MWCNTs was observed for valine functionalized MWCNTs
33