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DNA composite
Materials Science and Engineering C 29 (2009) 1093–1097
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Short communication
Studies on CNTs/DNA composite K.P.S.S. Hembram, G. Mohan Rao ⁎ Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 31 August 2008 Accepted 8 September 2008 Available online 22 September 2008
a b s t r a c t The electrical and optical properties of MWCNTs/DNA composite were studied. Electrical conductivity studies reveal that, the increase in CNTs concentration in DNA increases the conductivity. Fourier transformed Infrared (FTIR) spectrum shows that the CNTs are bonded to DNA covalently at the ends and defects sites and the wrapping of DNA on the CNTs is due to van der Waals force. © 2008 Elsevier B.V. All rights reserved.
Keywords: Experimental methods Materials and applications Carbon nanotubes DNA Composite Electron microscopy
1. Introduction Composite materials containing CNTs have currently drawn interest in devices, leading the nanotechnology. In biological sciences and engineering, CNTs/DNA composite has a special place not only in the field of biological sensors [1–3], but also in other applications like gene therapy [4,5]. DNA can also be used to assist the dispersion and separation of CNTs [6]. The functionalization of CNTs with DNA is the key factor for the formation of composite materials. With specific recognition of biosystems, it provides the biosensing role. Although the exact mechanism through which the CNTs interact with DNA is not known, the possible interactions are due to weak van der Waals interactions [7,8] and covalent bonding [9,10]. Fig. 1(a) shows the schematic diagram of functionalization of CNTs with DNA where (1) DNA wraps the CNTs and (2) DNA is attached to ends of CNTs, to form composite [11]. Fig. 1(b) shows the SEM image of functionalized CNTs with DNA which is the case in reality. Incase of composite not all the CNTs are parallel to each other, rather possess random orientation. In any sensor, conductivity is a function of different parameters like the constituent materials and that of different concentrations, humidity and temperature. The conducting species has to move through the CNTs and DNA with different conduction mechanisms, the sum of which gives rise to conductivity through the composite.
⁎ Corresponding author. E-mail address: [email protected] (G.M. Rao). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.09.021
DNA chains are flexible and have strong structural fluctuation, which crucially affects the conductivity. Different researchers have investigated the conductivity of the DNA differently [12–14] and have attributed to electron bombardment induced contaminations [15] and strong contact effects [16]. The accurate determination of conductivity value is important, which can provide the true mechanism of the transport properties. Meggers et al. have investigated through photochemical method that the energy of the hole is less when it stays on guanine than adenine, cytosine and thymine [17]. Keeping this concept they modeled that the charge transfer happens in DNA by super exchange or multistep hoping. The multistep hopping is supported by the study of Yu and Song [18]. Besides the fundamental mechanism, the conductivity also depends on the humidity and temperature [13,14]. In a mixture of CNTs generally 1/3 of the total possess the metallic part and the rest 2/3 constitute semiconducting part. In the semiconducting CNTs, the conductivity can be thought as hole doped conductivity [19] and hence a measurement of large hole density suggests that the CNTs are doped with accepters [20]. In metallic CNTs, the transport is dominated by Coulomb blockade [21,22]. Again electron–electron interaction in the CNTs forms as Luttinger liquid character. At higher temperature the Coulomb blockade is unimportant and electron–electron interaction and electro-phonon interaction come into picture and hence the weak localization [23,24] and strong localization [25,26] affect the transport properties. The overall transport mechanism is governed by diffusive [20,27], ballistic [28,29] or both the phenomena [30]. The semiconducting CNTs are more sensitive than their metallic counterpart [31]. The transport properties in the DNA and CNTs are individually complex and not yet understood fully and hence the transport
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Fig. 1. (a) Schematic diagram of the functionalization of DNA with CNT [11], (b) SEM image of functionalized CNTs with DNA.
properties of the composite are still a fertile research field. In this study we tried to understand the transport properties of the CNTs/DNA composite and in this paper the preparation and properties of CNTs/DNA composite have been described.
2. Experimental Salmon Testes DNA (Sigma Aldrich, D-1626) and Purified MWCNTs (prepared by pyrolysis of xylene) were taken as starting materials.
Fig. 2. SEM images of the CNTs/DNA composite of (a) D1, (b) D2, (c) D3, and (d) D4.
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3. Results and discussion 3.1. Scanning electron microscopy Fig. 2 shows the SEM images of the CNTs/DNA composite with different weight percentage of CNTs. It is clearly seen from the figures that, in the CNTs/DNA matrices, the CNTs concentration increases from Fig. 2(a) to (d). The CNTs are randomly oriented in the composite, with homogeneous distribution in all the cases. It can also be seen that as the CNT concentration increased, the connectivity among them also increased. 3.2. Electrical properties
Fig. 3. Complex impedance spectrum for the DNA and CNTs/DNA composite.
100 mg of DNA was taken in a test tube and 1 ml of de-ionized water was added to it. It was subjected to ultrasonication in a water bath at 80 °C for 10 min. Different weight percentage of CNTs was added to the DNA solution and again subjected to vortex, followed by ultrasonication until homogeneous black color fluid is obtained which is then dried to get the composite. The CNTs/DNA composite samples were given the code name for ease of discussion (D0 = DNA without CNTs, D1 = 27.28%, D2 = 36.97%, D3 = 46.05% and D4 = 54.83%, where D 1–4 = CNTs / (CNTs +DNA)%). Some amount of the above mentioned black color fluid is taken and put on alumina substrate and was kept inside a desiccator to dry. The dried samples were taken for microscopic study and electrical studies. For performing the complex impedance measurements, Agilant 4294A Impedance Analyzer having operating frequency range from 40 Hz to 110 MHz is used. DC conductivity studies have been carried out using Keithley 6487 model Pico ammeter/voltage source. Keithley Instruments supplied ExceLINX program is used for interfacing the meter and also for setting different bias voltage. For making electrodes silver paste was used and electrical contacts were taken with copper wires. The optical properties of the samples were studied using FTIR spectrophotometer (Bruker FTIR, TENSOR 27) in absorbance mode. 50 scans were collected with 4 cm− 1 resolution for each sample. As the spectra were taken in the solution form, water subtraction was carried out and considered to be achieved when there was a flat base line around 2200 cm− 1, where the water combination mode is located. This method yields a rough estimate of the subtraction scaling factor, but removes the spectral features of water in a satisfactory way [32].
3.2.1. Impedance spectrum analysis The electrical behavior of the system has been studied by complex impedance analyzer, over a wide range of frequencies. This technique enables us to separate the real and imaginary components of the electrical parameters and hence provides a true picture of the material properties [33]. Fig. 3 shows complex impedance spectrum of DNA and CNTs/DNA composites. The impedance spectrum is characterized by the appearance of semicircular arcs. The semicircular arcs in the pattern, the extent of their intercept on the real axis and their number in the spectrum provide very important information relating electrical behavior of the material under investigation. Such pattern can be modeled to a parallel combination of resistance and capacitance. The model semicircular arc and the equivalent circuit diagram are shown in Fig. 4(a) and (b) respectively. The decrease in arc radius of the semicircle gives an indication of the decrease in resistance, hence conductivity increases with the increase in CNTs concentration in DNA. The impedance value is typically more, without or with less concentration of CNTs and decreases gradually with increasing CNTs concentration as shown in Fig. 5. Although the resistance of DNA varies from condition to condition (K Ω to G Ω) [12,14,15] and that of CNT also (from (K Ω to M Ω) [20,21], the result that we obtained is of the order of K Ω, which resembles the order that is achieved by Guo et al. [34]. This result may be due to the increase in conducting path due to the increase in CNTs concentration. 3.2.2. AC conductivity Fig. 6 shows the variation of AC conductance as a function of frequency (40 Hz to 110 MHz) of DNA and CNTs/DNA composites. The magnitude of conductance in this frequency range has appreciably distinct values with a different concentration of CNTs in DNA. The typical features are the appearance of plateau region in the lower frequency region with a sharp increase in conductance at higher frequency region, showing a change in slope. The plateau region corresponds to the frequency-independent DC conductance and dispersion region corresponds to the frequencydependent part. The AC conductance pattern indicates a progressive
Fig. 4. (a) A model for the impedance plot. (b) A simple equivalent diagram.
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Fig. 5. Variation of resistance at different wt. percentages of CNTs.
rise in AC conductance with the increase in CNTs concentration having maximum enhancement in conductance at higher frequencies. A higher magnitude of conductance for the sample at higher frequencies region is in view of strong frequency dependence under these conditions. 3.2.3. DC conductivity The typical variation of DC conductivity of DNA and CNTs/DNA composites is shown in Fig. 7. The leakage current variation indicates an increase of conductivity with an increase in CNTs concentration. With higher CNT concentration, the conductivity is increased due to the increased conducting path through the network of CNTs. Similar behavior was observed in the impedance study and the ac conductivity study as discussed above. The electron micrographs shown in Fig. 2 also confirm the validity of this discussion. The variation in conductivity observed in this study is similar to that reported earlier by Kazukauskas et al. [35] and Atieh et al. [36] for CNT/polystyrene and CNT/polypyrrole composites respectively. The electrical conduction has been explained in terms of percolation threshold at low concentration of CNTs and on interconnected conducting network formed at higher concentration of CNTs. 3.2.4. Optical properties Fig. 8 shows the FTIR spectra of the composite samples in the range 800 cm− 1–1800 cm− 1. The peaks at 1088 cm− 1 PO2 (symmetric
Fig. 6. Variation of conductance as a function of frequency of DNA and CNTs/DNA composite.
Fig. 7. Current voltage characteristics of DNA and CNTs/DNA composite.
stretch) and 1226 cm− 1 (PO2 asymmetric stretch) are due to the phosphate of the DNA, whereas those at 1664 cm− 1 and 1700 cm− 1 are due to thymine and peak at 1575 cm− 1, 1600–1610 cm− 1, 1640– 1660 cm− 1 and 1690 cm− 1 are due to adenine. The peak at 1710 cm− 1 is due to guanine, 1294 cm− 1 and 1492 cm− 1 are due to cytosine, but most of the cases guanine and cytosine are strongly related as the peaks at 1374 cm− 1, 1425 cm− 1 and 1527 cm− 1 are both from guanine and cytosine [37]. It is reported that in a bundle of CNTs with same chirality and diameter, there are many infrared active modes over a range of frequencies. The prominent peaks of 865 cm− 1 and 1590 cm− 1 have been reported irrespective of different chirality and diameter [38]. However the peaks at 1230 cm− 1,1374 cm− 1,1510 cm− 1, and 1580 cm− 1 are due to arm chair CNTs and 1775 cm− 1 is due to the chiral nanotubes and zigzag tube also gives peak at 1510 cm− 1 [39]. The deviation of the spectra from that of pure DNA and CNTs, as shown in the spectra indicates the functionalization of CNTs with DNA. The shifting of absorption bands in the region of 1550–1800 cm− 1 is due to the in plane DNA vibrational frequencies. The absorption peaks in the range 1200–1450 cm− 1 are due to phosphate vibrational frequencies. The asymmetric stretch of the phosphate is more sensitive to the geometry of the molecule and favorable for functionalization and can be shifted from −5 to ±20 cm− 1 [35]. For example the phosphate being negative ion, tries to attach to the slight positive ends of the CNTs by
Fig. 8. FTIR spectra of DNA and CNTs/DNA composite.
K.P.S.S. Hembram, G.M. Rao / Materials Science and Engineering C 29 (2009) 1093–1097
covalent bond [1,9,10,40]. We also expect that the side wall of the CNTs also interacts via the pi electron with the other constituent of DNA via weak van der Waals interaction as the peaks of the guanine, cytosine, thymine and adenine are also a little bit shifted from its original peak position, but not like the phosphate peak shift [7,8,41]. 4. Conclusions Composites of CNTs along with Salmon Testes DNA were prepared with different concentrations of CNTs. Functionalization of CNTs was explained on the basis of the electrical and optical characteristics of the composite. There is a minimum threshold concentration of CNTs for changing the conductivity and also beyond certain concentration the conductivity is saturated. Fourier transformed Infrared (FTIR) spectrum shows that the CNTs are bonded to DNA covalently at the ends and defects sites and the wrapping of DNA on the CNTs is due to van der Waals force. Acknowledgement The authors thank the administration of Centre of nanoscience for extending the facilities. References [1] M. Guo, J. Chen, L. Nie, S. Yao, Electrochim. Acta. 49 (2004) 2637. [2] J. Li, Q. Liu, Y. Liu, S. Liu, S. Yao, Anal. Biochem. 346 (2005) 107. [3] G.A. Rivas, M.D. Rubianes, M.C. Rodriguez, N.F. Ferreyra, G.L. Luque, M.L. Pedano, S.A. Miscoria, C. Parrado, Talanta 74 (2007) 291. [4] M.P. Anglada, A. Felipe, F.J. Casado, Trends Pharcol. Sci. 19 (1998) 424. [5] R. Krajcik, A. Jung, A. Hirsch, W. Neuhuber, O. Zolk, Biochem. Biophys. Res. Comm. 369 (2008) 595. [6] M. Zheng, A. Jagota, E.D. Semke, B.A. Diner, R.S. Mclean, S.R. Lustig, R.E. Richardson, N.G. Tassi, Nat. Mater. 2 (2003) 338. [7] H. Gao, Y. Kong, D. Cui, C.S. Ozkan, Nano Lett. 3 (2003) 471. [8] S. Meng, W.L. Wang, P. Maragakis, E. Kaaxiras, Nano Lett. 7 (2007) 2312. [9] X. Guo, A.A. Gorodetsky, J. Hone, J.K. Barton, C. Nuckolls, Nat. Nanotechnol. 3 (2008) 163. [10] S. Roy, H. Vedala, A.D. Roy, D.H. Kim, M. Doud, K. Mathee, H.K. Shin, N. Shimamoto, V. Prasad, W. Choi, Nano Lett. 8 (2008) 26. [11] G.S. Pomales, C.R. Cabrera, J. Electroanal. Chem. 606 (2007) 47.
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[12] H.W. Fink, C. Schonenberger, Nature 398 (1999) 407. [13] D. Porath, A. Bezryadin, S. de Vries, C. Dekker, Nature 403 (2000) 635. [14] A.Y. Kasumov, D.V. Klinov, P.E. Roche, S. Gueron, H. Bouchiat, Appl. Phys. Lett. 84 (2004) 1007. [15] P.J. de Pablo, F.M. Herrero, J. Colchero, J.G. Herrero, P. Herrero, A.M. Baro, P. Ordejon, J.M. Soler, E. Artacho, Phys. Rev. Lett. 85 (2000) 4992. [16] P. Tran, B. Alavi, G. Gruner, Phys. Rev. Lett. 85 (2000) 1564. [17] E. Meggers, M.E.M. Beyerle, B. Giese, J. Am. Chem. Soc. 120 (1998) 12950. [18] Z.G. Yu, X. Song, Phys. Rev. Lett. 86 (2001) 6018. [19] R.D. Antonov, A.T. Johnson, Phys. Rev. Lett. 83 (1999) 3274. [20] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [21] A. Bezryadin, A.R.M. Versehueren, S.J. Tans, C. Dekker, Phys. Rev. Lett. 80 (1998) 4036. [22] D.H. Cobden, M. Bockrath, P. McEuen, A.G. Rinzler, R.E. Smalley, Phys. Rev., Lett. 81 (1998) 681. [23] L. Langer, V. Bayot, E. Grivei, J.P. Issi, J.P. Heremans, C.H. Olk, L. Stockman, C.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 76 (1996) 479. [24] G.T. Kim, E.S. Choi, D.C. Kim, D.S. Suh, Y.M. Park, K. Liu, G. Duesberg, S. Roth, Phys. Rev. B. 58 (1998) 16064. [25] T. Kostyrko, M. Bartkowiak, G.D. Mahan, Phys. Rev. B. 60 (1999) 10735. [26] S. Roche, G. Dresselhaus, M.S. Dresselhaus, R. Saito, Phys. Rev. B. 62 (2000) 16092. [27] J. Guo, M.A. Alam, Appl. Phys. Lett. 86 (2005) 023105. [28] P. Avouris, T. Hertel, R. Martel, T. Schmidt, H.R. Shea, R.E. Walkup, Appl. Surf. Sci. 141 (1999) 201. [29] H.J. Li, W.G. Lu, J.J. Li, X.D. Bai, C.Z. Gu, Phys. Rev. Lett. 95 (2005) 086601. [30] J. Wang, J.S. Wang, Appl. Phys. Lett. 88 (2006) 111909. [31] P.L. McEuen, M. Bockrath, D.H. Cobden, Y.G. Yoon, S.G. Louie, Phys. Rev., Lett. 83 (1999) 5098. [32] G.E. Walrafen, Water a Comprehensive Treatise, the Physics and Physical Chemistry of Water, vol. 1, Plenum, New York, 1972 Chapter 5. [33] J.R. MacDonald, Impedance Spectroscopy Emphasizing Solid materials and Systems, Wiley, NewYork, 1987 Chapter 4. [34] M. Guo, J. Chen, D. Liu, L. Nie, S. Yao, Biochemistry 62 (2004) 29. [35] V. Kazukauskas, V. Kalendra, C.W. Bumby, B.M. Ludbrook, A.B. Kaiser, Phys. Stat. Sol. (c) 5 (2008) 3172. [36] M.A. Atieh, Saheb Nouari, Mohammed Fettouhi, paper presented at IC2MS'08, Melaka, Malaysia, 5–7 August 2008. [37] S. Alex, P. Dupius, Inorg. Chim. Acta 157 (1989) 271. [38] R.A. Jishi, L. Venkataraman, M.S. Dresselhaus, G. Dresselhaus, Phys. Rev. B. 51 (1995) 11176. [39] O.P. Matyshevska, A.Y. Karlash, Y.V. Shtogun, A. Benilov, Y. Kirgizov, K.O. Gorchinskyy, E.V. Buzaneva, Y.I. Prylutskyy, P. Scharff, Mat. Sc. And Engg. C. 15 (2001) 249. [40] M. Hazani, R. Naaman, F. Hennrich, M.M. Kappes, Nano Lett. 3 (2003) 153. [41] Z. Guo, P.J. Sadler, S.C. Tsang, Adv. Mater. 10 (1998) 701.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Short communication
Studies on CNTs/DNA composite K.P.S.S. Hembram, G. Mohan Rao ⁎ Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 31 August 2008 Accepted 8 September 2008 Available online 22 September 2008
a b s t r a c t The electrical and optical properties of MWCNTs/DNA composite were studied. Electrical conductivity studies reveal that, the increase in CNTs concentration in DNA increases the conductivity. Fourier transformed Infrared (FTIR) spectrum shows that the CNTs are bonded to DNA covalently at the ends and defects sites and the wrapping of DNA on the CNTs is due to van der Waals force. © 2008 Elsevier B.V. All rights reserved.
Keywords: Experimental methods Materials and applications Carbon nanotubes DNA Composite Electron microscopy
1. Introduction Composite materials containing CNTs have currently drawn interest in devices, leading the nanotechnology. In biological sciences and engineering, CNTs/DNA composite has a special place not only in the field of biological sensors [1–3], but also in other applications like gene therapy [4,5]. DNA can also be used to assist the dispersion and separation of CNTs [6]. The functionalization of CNTs with DNA is the key factor for the formation of composite materials. With specific recognition of biosystems, it provides the biosensing role. Although the exact mechanism through which the CNTs interact with DNA is not known, the possible interactions are due to weak van der Waals interactions [7,8] and covalent bonding [9,10]. Fig. 1(a) shows the schematic diagram of functionalization of CNTs with DNA where (1) DNA wraps the CNTs and (2) DNA is attached to ends of CNTs, to form composite [11]. Fig. 1(b) shows the SEM image of functionalized CNTs with DNA which is the case in reality. Incase of composite not all the CNTs are parallel to each other, rather possess random orientation. In any sensor, conductivity is a function of different parameters like the constituent materials and that of different concentrations, humidity and temperature. The conducting species has to move through the CNTs and DNA with different conduction mechanisms, the sum of which gives rise to conductivity through the composite.
⁎ Corresponding author. E-mail address: [email protected] (G.M. Rao). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.09.021
DNA chains are flexible and have strong structural fluctuation, which crucially affects the conductivity. Different researchers have investigated the conductivity of the DNA differently [12–14] and have attributed to electron bombardment induced contaminations [15] and strong contact effects [16]. The accurate determination of conductivity value is important, which can provide the true mechanism of the transport properties. Meggers et al. have investigated through photochemical method that the energy of the hole is less when it stays on guanine than adenine, cytosine and thymine [17]. Keeping this concept they modeled that the charge transfer happens in DNA by super exchange or multistep hoping. The multistep hopping is supported by the study of Yu and Song [18]. Besides the fundamental mechanism, the conductivity also depends on the humidity and temperature [13,14]. In a mixture of CNTs generally 1/3 of the total possess the metallic part and the rest 2/3 constitute semiconducting part. In the semiconducting CNTs, the conductivity can be thought as hole doped conductivity [19] and hence a measurement of large hole density suggests that the CNTs are doped with accepters [20]. In metallic CNTs, the transport is dominated by Coulomb blockade [21,22]. Again electron–electron interaction in the CNTs forms as Luttinger liquid character. At higher temperature the Coulomb blockade is unimportant and electron–electron interaction and electro-phonon interaction come into picture and hence the weak localization [23,24] and strong localization [25,26] affect the transport properties. The overall transport mechanism is governed by diffusive [20,27], ballistic [28,29] or both the phenomena [30]. The semiconducting CNTs are more sensitive than their metallic counterpart [31]. The transport properties in the DNA and CNTs are individually complex and not yet understood fully and hence the transport
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Fig. 1. (a) Schematic diagram of the functionalization of DNA with CNT [11], (b) SEM image of functionalized CNTs with DNA.
properties of the composite are still a fertile research field. In this study we tried to understand the transport properties of the CNTs/DNA composite and in this paper the preparation and properties of CNTs/DNA composite have been described.
2. Experimental Salmon Testes DNA (Sigma Aldrich, D-1626) and Purified MWCNTs (prepared by pyrolysis of xylene) were taken as starting materials.
Fig. 2. SEM images of the CNTs/DNA composite of (a) D1, (b) D2, (c) D3, and (d) D4.
K.P.S.S. Hembram, G.M. Rao / Materials Science and Engineering C 29 (2009) 1093–1097
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3. Results and discussion 3.1. Scanning electron microscopy Fig. 2 shows the SEM images of the CNTs/DNA composite with different weight percentage of CNTs. It is clearly seen from the figures that, in the CNTs/DNA matrices, the CNTs concentration increases from Fig. 2(a) to (d). The CNTs are randomly oriented in the composite, with homogeneous distribution in all the cases. It can also be seen that as the CNT concentration increased, the connectivity among them also increased. 3.2. Electrical properties
Fig. 3. Complex impedance spectrum for the DNA and CNTs/DNA composite.
100 mg of DNA was taken in a test tube and 1 ml of de-ionized water was added to it. It was subjected to ultrasonication in a water bath at 80 °C for 10 min. Different weight percentage of CNTs was added to the DNA solution and again subjected to vortex, followed by ultrasonication until homogeneous black color fluid is obtained which is then dried to get the composite. The CNTs/DNA composite samples were given the code name for ease of discussion (D0 = DNA without CNTs, D1 = 27.28%, D2 = 36.97%, D3 = 46.05% and D4 = 54.83%, where D 1–4 = CNTs / (CNTs +DNA)%). Some amount of the above mentioned black color fluid is taken and put on alumina substrate and was kept inside a desiccator to dry. The dried samples were taken for microscopic study and electrical studies. For performing the complex impedance measurements, Agilant 4294A Impedance Analyzer having operating frequency range from 40 Hz to 110 MHz is used. DC conductivity studies have been carried out using Keithley 6487 model Pico ammeter/voltage source. Keithley Instruments supplied ExceLINX program is used for interfacing the meter and also for setting different bias voltage. For making electrodes silver paste was used and electrical contacts were taken with copper wires. The optical properties of the samples were studied using FTIR spectrophotometer (Bruker FTIR, TENSOR 27) in absorbance mode. 50 scans were collected with 4 cm− 1 resolution for each sample. As the spectra were taken in the solution form, water subtraction was carried out and considered to be achieved when there was a flat base line around 2200 cm− 1, where the water combination mode is located. This method yields a rough estimate of the subtraction scaling factor, but removes the spectral features of water in a satisfactory way [32].
3.2.1. Impedance spectrum analysis The electrical behavior of the system has been studied by complex impedance analyzer, over a wide range of frequencies. This technique enables us to separate the real and imaginary components of the electrical parameters and hence provides a true picture of the material properties [33]. Fig. 3 shows complex impedance spectrum of DNA and CNTs/DNA composites. The impedance spectrum is characterized by the appearance of semicircular arcs. The semicircular arcs in the pattern, the extent of their intercept on the real axis and their number in the spectrum provide very important information relating electrical behavior of the material under investigation. Such pattern can be modeled to a parallel combination of resistance and capacitance. The model semicircular arc and the equivalent circuit diagram are shown in Fig. 4(a) and (b) respectively. The decrease in arc radius of the semicircle gives an indication of the decrease in resistance, hence conductivity increases with the increase in CNTs concentration in DNA. The impedance value is typically more, without or with less concentration of CNTs and decreases gradually with increasing CNTs concentration as shown in Fig. 5. Although the resistance of DNA varies from condition to condition (K Ω to G Ω) [12,14,15] and that of CNT also (from (K Ω to M Ω) [20,21], the result that we obtained is of the order of K Ω, which resembles the order that is achieved by Guo et al. [34]. This result may be due to the increase in conducting path due to the increase in CNTs concentration. 3.2.2. AC conductivity Fig. 6 shows the variation of AC conductance as a function of frequency (40 Hz to 110 MHz) of DNA and CNTs/DNA composites. The magnitude of conductance in this frequency range has appreciably distinct values with a different concentration of CNTs in DNA. The typical features are the appearance of plateau region in the lower frequency region with a sharp increase in conductance at higher frequency region, showing a change in slope. The plateau region corresponds to the frequency-independent DC conductance and dispersion region corresponds to the frequencydependent part. The AC conductance pattern indicates a progressive
Fig. 4. (a) A model for the impedance plot. (b) A simple equivalent diagram.
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Fig. 5. Variation of resistance at different wt. percentages of CNTs.
rise in AC conductance with the increase in CNTs concentration having maximum enhancement in conductance at higher frequencies. A higher magnitude of conductance for the sample at higher frequencies region is in view of strong frequency dependence under these conditions. 3.2.3. DC conductivity The typical variation of DC conductivity of DNA and CNTs/DNA composites is shown in Fig. 7. The leakage current variation indicates an increase of conductivity with an increase in CNTs concentration. With higher CNT concentration, the conductivity is increased due to the increased conducting path through the network of CNTs. Similar behavior was observed in the impedance study and the ac conductivity study as discussed above. The electron micrographs shown in Fig. 2 also confirm the validity of this discussion. The variation in conductivity observed in this study is similar to that reported earlier by Kazukauskas et al. [35] and Atieh et al. [36] for CNT/polystyrene and CNT/polypyrrole composites respectively. The electrical conduction has been explained in terms of percolation threshold at low concentration of CNTs and on interconnected conducting network formed at higher concentration of CNTs. 3.2.4. Optical properties Fig. 8 shows the FTIR spectra of the composite samples in the range 800 cm− 1–1800 cm− 1. The peaks at 1088 cm− 1 PO2 (symmetric
Fig. 6. Variation of conductance as a function of frequency of DNA and CNTs/DNA composite.
Fig. 7. Current voltage characteristics of DNA and CNTs/DNA composite.
stretch) and 1226 cm− 1 (PO2 asymmetric stretch) are due to the phosphate of the DNA, whereas those at 1664 cm− 1 and 1700 cm− 1 are due to thymine and peak at 1575 cm− 1, 1600–1610 cm− 1, 1640– 1660 cm− 1 and 1690 cm− 1 are due to adenine. The peak at 1710 cm− 1 is due to guanine, 1294 cm− 1 and 1492 cm− 1 are due to cytosine, but most of the cases guanine and cytosine are strongly related as the peaks at 1374 cm− 1, 1425 cm− 1 and 1527 cm− 1 are both from guanine and cytosine [37]. It is reported that in a bundle of CNTs with same chirality and diameter, there are many infrared active modes over a range of frequencies. The prominent peaks of 865 cm− 1 and 1590 cm− 1 have been reported irrespective of different chirality and diameter [38]. However the peaks at 1230 cm− 1,1374 cm− 1,1510 cm− 1, and 1580 cm− 1 are due to arm chair CNTs and 1775 cm− 1 is due to the chiral nanotubes and zigzag tube also gives peak at 1510 cm− 1 [39]. The deviation of the spectra from that of pure DNA and CNTs, as shown in the spectra indicates the functionalization of CNTs with DNA. The shifting of absorption bands in the region of 1550–1800 cm− 1 is due to the in plane DNA vibrational frequencies. The absorption peaks in the range 1200–1450 cm− 1 are due to phosphate vibrational frequencies. The asymmetric stretch of the phosphate is more sensitive to the geometry of the molecule and favorable for functionalization and can be shifted from −5 to ±20 cm− 1 [35]. For example the phosphate being negative ion, tries to attach to the slight positive ends of the CNTs by
Fig. 8. FTIR spectra of DNA and CNTs/DNA composite.
K.P.S.S. Hembram, G.M. Rao / Materials Science and Engineering C 29 (2009) 1093–1097
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