- Email: [email protected]
ionic liquid: Determination of some catecholamines
Electrochemistry Communications 42 (2014) 9–12
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
High performance electrochemical sensor based on fullerene-functionalized carbon nanotubes/ionic liquid: Determination of some catecholamines Mohammad Mazloum-Ardakani ⁎, Alireza Khoshroo 1 Department of Chemistry, Faculty of Science, Yazd University, Yazd 89195-741, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2014 Received in revised form 29 January 2014 Accepted 29 January 2014 Available online 12 February 2014 Keywords: Fullerene-functionalized carbon nanotubes Ionic liquid Catecholamine
a b s t r a c t The combination of fullerene (C60)-functionalized multiwalled carbon nanotubes (CNTs) and ionic liquid (IL) that yields a nanostructured modified electrode formed a novel kind of structurally uniform and electrocatalytic activity material. The modified electrode was characterized by different methods. It was found that the nanocomposite film of C60-CNT/IL on glassy carbon electrode exhibits excellent electrocatalytic activity towards catecholamines, including norepinephrine (NE), isoprenaline (IP) and dopamine (DA) oxidation, resulting in a marked lowering in the peak potential and considerable improvement of the peak current as compared to the electrochemical activity at the bare glassy carbon electrode. The improvement of electrochemical response to catecholamine oxidation was found at modified electrode, revealing the synergetic effect of C60-CNT and IL. Furthermore, the catecholamines were successfully used for the determination of catecholamines in real samples. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In electrochemistry, particularly in the area of electroanalysis, electrocatalysis offers reductions in overpotential and increase in the magnitude of voltammetric peak heights, allowing low detection limits to be more achievable [1]. Since the first finding of C60, the electrochemistry of fullerenes or their derivatives (including carbon nanotubes) has been one of the most intensely studied aspects of fullerene chemistry [2] and fullerene has previously been applied to electrocatalysis [3–6]. C60 appears to be attractive to researchers for modifying electrode surfaces because it is chemically stable, metallic impurity free and relatively simple to implement and gives rise to reproducible electrocatalytic responses [7,8]. The reports that C60 modified electrodes confer electrocatalysis have not been widely accepted [9]. However, recent work by Compton [10] has shown that the observed electrocatalysis in some cases was caused by either the small amount of graphite impurity in C60 sample, or the oxygenated species formed on the surface of glassy carbon electrodes by “electrode pretreatment”, while electrocatalysis mediated by C60 is only likely “where C60 itself becomes oxidized or reduced” [10]. However the area remains contentious. Carbon nanotubes (CNTs) have been deemed as excellent electrocatalysts for a large variety of compounds [11,12]. Strong ⁎ Corresponding author. Tel.: +98 3518211670; fax: +98 3518210644. E-mail address: [email protected] (M. Mazloum-Ardakani). 1 Tel.: +98 3518211670; fax: +98 3518210644.
http://dx.doi.org/10.1016/j.elecom.2014.01.026 1388-2481/© 2014 Elsevier B.V. All rights reserved.
promises were given, raising high expectations [13]. Due to rich electrochemistry of fullerene, electrochemistry of fullerene peapods (C60@CNT and C70@CNT) was investigated since synthesis of fullerene peapods was reported. Carbon nanotubes have also been applied to this purpose because of their remarkable electrocatalytic properties [14]. As an excellent electron acceptor, C60 could be dispersed in nanomaterials, such as carbon nanotubes, to facilitate electron transfer. Recent investigations revealed that C60-functionalized multiwalled carbon nanotube (MWCNT) films were more effective in facilitating the direct electron transfer of hemoglobin than MWCNT films [15]. Zhu et al. successfully constructed a C60-functionalized MWCNT film to achieve a dopamine detection limit of 0.03 μM, compared to 0.15 μM at a MWCNT modified electrode [16]. Wael et al. developed a C60-functionalized MWCNT film for the electrocatalytic determination of the vinclozolin [17]. To the best of our knowledge, no study has been published so far reporting the electroanalytical applications of C60-functionalized CNT and ionic liquid composites (C60-CNT/IL). Here in continuation to our studies concerning the preparation of modified electrodes [14,18–20], we developed a C60-CNT and ionic liquid composite film as a new modifier for the electrocatalytic determination of catecholamines in human serum and urine samples. The results showed that the composite film of C60-CNT/IL on a glassy carbon (GC) electrode is more sensitive for the detection of catecholamines compared to bare GC electrode and CNT/ GC electrode. The improvement electrochemical response of catecholamines was found at C60-CNT/IL/GC, revealing the synergetic effect of C60-CNT and IL.
10
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
2. Experimental
C60 (MWCNTs/C60 = 2:1, mass ratio) with a total amount of about 1 mg were dispersed in 10 mL toluene in an ultrasound bath for 30 min to give a 0.1 mg mL−1 suspension. A volume of 10 μL of the suspension was applied directly on a GC electrode surface and the solvent was allowed to evaporate at room temperature (10 min). This C60-MWCNT film electrode was subjected to potential scanning in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate between 0.0 and −2.0 V (vs Ag/AgCl) until reversible multistep electron-transfer reaction was obtained [21]. The resulting C60-CNT film electrode was then washed with acetonitrile several times to remove the electrolytes and dried at room temperature. Then, 50 μL of IL was dispersed in 0.3 mL 1% chitosan solution in 1 M of acetic acid. After the mixture was sonicated for 30 min, 3.0 μL was applied to the surface using a microsyringe and this was allowed to dry in a stream of hot air (15 min).
2.1. Apparatus and chemicals Electrochemical measurements were performed by an Autolab potentiostat/galvanostat (PGSTAT-302 N, Eco Chemie, Netherlands). A three-electrode system was used, where a GC electrode or a modified GC electrode (2 mm diameter) served as the working electrode, a platinum wire as the counter electrode and an Ag/AgCl/KCl (3.0 M) electrode as the reference electrode. The morphology of nanomaterial was characterized using scanning electron microscopy (SEM, Hitachi S-4160) and transmission electron microscopy (TEM) (Philips EM208). For impedance measurements, a frequency range of 100 kHz to 0.1 Hz was employed. The AC voltage amplitude and DC voltage were 5 mV and 150 mV, respectively. Norepinephrine, isoprenaline, dopamine, 1-butyl-3-methylimidazolium tetrafluoroborate and fullerene-C60 (99.5%) were reagentgrade from Sigma Aldrich. Phosphate salt, sodium hydroxide, solvents and reagents were of pro-analysis grade from Merck (Darmstadt, Germany). Multiwalled carbon nanotubes (purity more than 95%) were purchased from Plasma Chem (Germany).
3. Results and discussion 3.1. Characterization of the C60-CNT/IL composites In order to understand the performance of the C60-CNT/IL composites, we investigated modified electrodes by TEM, SEM, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), respectively. The response of a modified electrode is related to its physical morphology. Fig. 1A and B shows the TEM and SEM micrographs of C60CNTs/GC electrode and C60-CNTs/IL/GC electrode, respectively. Fig. 1A clearly shows nanoclusters of C60 and CNTs that resulted from the
2.2. Preparation of the electrode Modified electrodes were prepared by a simple casting method. Prior to the surface coating, the GC electrode was polished on a polishing cloth with 0.05 μm alumina powder. Purified MWCNTs and
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Fig. 1. (A) TEM image of C60-CNT, (B) SEM image of C60-CNT/IL nanocomposite, (C) Nyquist plots of bare GC electrode (a), CNT/GC (b), C60-CNT/GC (c), C60-CNT/IL/GC, (D) 1st cycle and 50th cycle of C60-CNT/IL/GC in 1 mM Fe(CN)3+/4+ containing 0.1 M KCl.
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
formation of a nanocomposite film. CNTs are discernible after the C60CNTs were applied on the GC electrode; when IL was introduced to the C60-CNT composite, the film becomes more uniform and even, most likely because of the binding and blanketing effect of IL (Fig. 1B). We also added the resulting C60-CNT composites into toluene and subjected the suspension to ultrasonication for several hours, but the toluene solution did not turn purple, a characteristic color of C60. These results indicate that C60 was strongly attached to CNTs. Electrochemical activity of a C60-CNT/IL/GC electrode was examined using EIS and the results were then compared to those obtained at a CNT/GC electrode, a C60-CNT/GC electrode and a bare GC electrode. Fig. 1C shows the Nyquist plots for bare GC electrode (a), CNT/GC (b), C60-CNT/GC (c) and C60-CNT/IL/GC (d) in 0.1 M KCl containing 1 mM [Fe(CN)6]3 − and 1 mM [Fe(CN)6]4 −. The charge transfer resistance (Rct) is evaluated from the diameter of the semicircle in the high frequency region of the Nyquist plot. It could be seen that on the C60CNT/IL/GC (curve d, Rct = 1.8 ± 0.03 kΩ) the value of Rct is smaller than that of C60-CNT/GC (curve c, Rct = 2.5 ± 0.03 kΩ) and CNT/GC (curve b, Rct = 3.8 ± 0.03 kΩ), suggesting the synergistic effect of C60-CNT and IL on the modified electrode effectively enhanced the conductivity of the modified electrode and made it easier for the electron transfer from the electrode to the film. Because the interfacial potential between the C60-CNT/IL composite modified electrode and the aqueous solution will cause an offset in the redox potential scale, the modified electrode was characterized using CV in 0.1 M KCl containing 1 mM [Fe(CN)6]3 − and 1 mM [Fe(CN)6]4− solutions to detect whether the interfacial potential exists or has an obvious effect on the electrochemical response, the 1st and 50th cycles of the CVs are shown in Fig. 1D. It was found that the C60CNT/IL/GC exhibits a pair of well-defined redox peaks. It was also found that after 50 cycles of cycling, the peak separation remains unchanged and the redox peak currents only varied no more than 2%, which revealed good stability and reproducibility of the modified electrode. 3.2. Electrocatalytic oxidation of catecholamines at a C60-CNT/IL/GC The comparison of oxidation towards catecholamines at different electrodes is shown in Fig. 2. As can be seen, the anodic peak potential for norepinephrine (NE), isoprenaline (IP) and dopamine (DA) 10
oxidation at the bare GC electrode are about 400, 420 and 340 mV, respectively, while the corresponding potential at C60-CNT/IL/GC electrode are 230, 210 and 200 mV, respectively (curves a and d in Fig. 2). The comparison of the oxidation of catecholamines at C60-CNT/IL/GC (curve d) and C60-CNT/GC (curve c) shows an enhancement of the anodic peak current (71%, 70% and 73% for NE, IP and DA, respectively), which indicated that the presence of ILs could enhance the peak currents. The advantages of C60-CNT/IL/GC had been elucidated with higher conductivity, fast electron transfer rate, good anti-fouling properties and inherent catalytic ability of ILs [22,23]. To investigate the roles of C60 on CNT surface in the electrochemical oxidation of catecholamines, the cyclic response of catecholamines at the C60-CNT/GC (curve c) and CNT/GC (curve b) electrodes were recorded. The results indicated that the presence of C60 on CNT surface exhibited 58%, 48% and 55% improvement on the oxidation peak current for NE, IP and DA, respectively. The higher electrocatalytic activity of the C60-CNT film on the electrochemistry of catecholamines than that of the CNTs film is presumably caused by a more favorable electrochemical environment for catecholamines on the electrode [21]. The close interaction between C60 and CNTs in the C60-CNT composite allows the C60 to accumulate more electrons in C60-CNT composite than in CNTs without C60. Moreover, because of the fine dispersion, nanometer size, large accessible surface per volume, and low resistance, the C60-CNT composite offers a favorite microenvironment for electron transfer between catecholamines and the underlying electrode, which results in the improvement on the electrochemical response. Compton et al. showed that pure C60 film modified electrodes offer no electrocatalysis effect [10]. In the case of the C60 film, the modification of the electrode surface serves to act as inert particles, resulting in a partially blocked electrode surface [1]. Compton and co-workers have revisited this problem, and have argued that the apparent electrocatalytic detection of several analytes at C60 modified electrodes was simply due to the redistribution of amorphous of C60 on the electrode surface that changes the porosity or the accessibility of the underlying substrate to the electrolyte and any redox active species in it. We suggest that this situation may be similar in the case of C60-CNT, where the electrocatalysis in CNTs is well known [13,24]. Now the CNT surface is best described as porous materials, the smaller blocks allow more material to diffuse around them on the timescale of the voltammetric experiment and undergo electrolysis at the exposed electrode surface. We claim that the porous C60-CNT film also
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Fig. 2. CVs recorded in 0.1 M phosphate buffer solution (pH 7.0) and in +0.30 mM of (A) NE, (B) IP and (C) DA at the surface of GC (a), CNT/GC (b), C60-CNT/GC (c) and C60-CNT/IL/GC (D). Scan rate 20 mV s−1.
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
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Fig. 3. DPVs of C60-CNT/IL/GC in 0.1 M phosphate buffer solution (pH 7.0) containing different concentrations of (A) NE, numbers 1–20 correspond to 0.07–750.0 μM; (B) IP, numbers 1–18 correspond to 0.1–700.0 μM; (C) DA, numbers 1–20 correspond to 0.06–800.0 μM; insets: plot of the peak currents as a function of catecholamine concentration.
exhibits some degree of thin-layer behavior [25]. Thus the change in the porosity of this electrode, combined with possible thin-layer effects can be used to explain the observed improvement on the electrochemical response in electroanalysis of catecholamines. The improvement of electrochemical response of catecholamines was found at C60-CNT/IL/ GC, revealing the synergetic effect of C60-CNT and IL. 3.3. Calibration plot and limit of detection for catecholamines DPV was used to obtain the linear concentration range and detection limit of catecholamines at the C60-CNT/IL/GC electrode (Fig. 3). The results show that the dependence of electrocatalytic peak currents of catecholamine oxidation on their concentrations at the surface of the C60CNT/IL/GC consists of two linear segments with different slopes; the linear response were: NE, a slope of 0.650 μA μM−1 for the first linear segment (0.07–30.0 μM) and a slope of 0.103 μA μM−1 for the second linear segment (30.0–750.0 μM); IP, a slope of 0.550 μA μM−1 for the first linear segment (0.1–25.0 μM) and a slope of 0.098 μA μM−1 for the second linear segment (25.0–700.0 μM); DA, a slope of 0.777 μA μM−1 for the first linear segment (0.06–25.0 μM) and a slope of 0.101 μA μM−1 for the second linear segment (25.0–800.0 μM). The decrease in sensitivity (slope) of the second linear segment is likely due to kinetic limitation [26]. The detection limits (3σ) were 18 ± 2 nM for NE, 22 ± 2 nM for IP and 15 ± 2 nM for DA. The proposed method in real-life sample analysis was also investigated by direct analysis of catecholamines in serum samples and urine samples. No observable catecholamine content was observed in these samples and hence known amount of catecholamines was spiked in real-life samples to carry out the recovery study. Values of 19.6, 20.2 and 19.7 nM were determined for the spiked concentration of 20 nM of NE, IP and DA, respectively, which accounts for recovery of 98%, 101% and 98.5%.
nanocomposite. Here, we investigated the construction and the use of C60-CNT/IL modified electrode in the voltammetry methods. The results show that the determination of catecholamines by using C60-CNT/IL/GC is simpler and more sensitive than the use of other methods. Acknowledgments The authors wish to thank the Yazd University Research Council, IUT Research Council and Excellence in Sensors for financial support of this research.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
4. Conclusions It is worth noticing that the C60-CNT/IL nanocomposite is ideally suited for implementation in electrochemical applications. This work puts forward an approach for the fabrication of a novel nanostructured modified electrode by the use of C60-CNT/IL nanocomposite. As far as we know, it is the first time that IL has been used to fabricate the C60-CNT/IL
[23] [24] [25] [26]
C.E. Banks, T.J. Davies, G.G. Wildgoose, R.G. Compton, Chem. Commun. (2005) 829. D. Deutsch, J. Tarábek, M. Krause, P. Janda, L. Dunsch, Carbon 42 (2004) 1137. W. Zhilei, L. Zaijun, S. Xiulan, F. Yinjun, L. Junkang, Biosens. Bioelectron. 25 (2010) 1434. S.S. Kalanur, S. Jaldappagari, S. Balakrishnan, Electrochim. Acta 56 (2011) 5295. X. Miao, L. Liu, S. Wang, H. Lin, B. Sun, J. Hu, M. Li, Electrochem. Commun. 12 (2010) 90. W. Liu, X. Gao, Electrochem. Commun. 10 (2008) 1377. W.T. Tan, A.M. Bond, S.W. Ngooi, E.B. Lim, J.K. Goh, Anal. Chim. Acta. 491 (2003) 181. L. Xiao, G.G. Wildgoose, A. Crossley, R.G. Compton, Sensors Actuators B Chem. 138 (2009) 397. L. Xiao, G.G. Wildgoose, R.G. Compton, Sensors Actuators B Chem. 138 (2009) 524. R.T. Kachoosangi, C.E. Banks, R.G. Compton, Anal. Chim. Acta. 566 (2006) 1. C.E. Banks, R.G. Compton, Analyst 131 (2006) 15. C.E. Banks, R.R. Moore, T.J. Davies, R.G. Compton, Chem. Commun. 16 (2004) 1804. M. Pumera, Chem. Rec. 12 (2012) 201. M. Mazloum-Ardakani, A. Khoshroo, Electrochim. Acta 103 (2013) 77. H. Zhang, L. Fan, S. Yang, Chem. Eur. J. 12 (2006) 7161. H. Zhu, W. Wu, H. Zhang, L. Fan, S. Yang, Electroanalysis 21 (2009) 2660. J.A. Rather, K. De Wael, Sensors Actuators B Chem. 171 (2012) 907. M. Mazloum-Ardakani, A. Khoshroo, Anal. Chim. Acta. 798 (2013) 25. M. Mazloum-Ardakani, Z. Taleat, A. Khoshroo, H. Beitollahi, H. Dehghani, Biosens. Bioelectron. 35 (2012) 75. M. Mazloum-Ardakani, L. Hosseinzadeh, A. Khoshroo, H. Naeimi, M. Moradian, Electroanalysis (2013), http://dx.doi.org/10.1002/elan.201300401. H. Zhang, L. Fan, Y. Fang, S. Yang, Chem. Phys. Lett. 413 (2005) 346. M.M. Musameh, R.T. Kachoosangi, L. Xiao, A. Russell, R.G. Compton, Biosens. Bioelectron. 24 (2008) 87. M. Mazloum-Ardakani, A. Khoshroo, J. Electroanal. Chem. 717–718 (2014) 17. R.T. Kachoosangi, M.M. Musameh, I. Abu-Yousef, J.M. Yousef, S.M. Kanan, L. Xiao, S.G. Davies, A. Russell, R.G. Compton, Anal. Chem. 81 (2008) 435. I. Streeter, G.G. Wildgoose, L. Shao, R.G. Compton, Sensors Actuators B Chem. 133 (2008) 462. A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed. Wiley, 2000.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
High performance electrochemical sensor based on fullerene-functionalized carbon nanotubes/ionic liquid: Determination of some catecholamines Mohammad Mazloum-Ardakani ⁎, Alireza Khoshroo 1 Department of Chemistry, Faculty of Science, Yazd University, Yazd 89195-741, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2014 Received in revised form 29 January 2014 Accepted 29 January 2014 Available online 12 February 2014 Keywords: Fullerene-functionalized carbon nanotubes Ionic liquid Catecholamine
a b s t r a c t The combination of fullerene (C60)-functionalized multiwalled carbon nanotubes (CNTs) and ionic liquid (IL) that yields a nanostructured modified electrode formed a novel kind of structurally uniform and electrocatalytic activity material. The modified electrode was characterized by different methods. It was found that the nanocomposite film of C60-CNT/IL on glassy carbon electrode exhibits excellent electrocatalytic activity towards catecholamines, including norepinephrine (NE), isoprenaline (IP) and dopamine (DA) oxidation, resulting in a marked lowering in the peak potential and considerable improvement of the peak current as compared to the electrochemical activity at the bare glassy carbon electrode. The improvement of electrochemical response to catecholamine oxidation was found at modified electrode, revealing the synergetic effect of C60-CNT and IL. Furthermore, the catecholamines were successfully used for the determination of catecholamines in real samples. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In electrochemistry, particularly in the area of electroanalysis, electrocatalysis offers reductions in overpotential and increase in the magnitude of voltammetric peak heights, allowing low detection limits to be more achievable [1]. Since the first finding of C60, the electrochemistry of fullerenes or their derivatives (including carbon nanotubes) has been one of the most intensely studied aspects of fullerene chemistry [2] and fullerene has previously been applied to electrocatalysis [3–6]. C60 appears to be attractive to researchers for modifying electrode surfaces because it is chemically stable, metallic impurity free and relatively simple to implement and gives rise to reproducible electrocatalytic responses [7,8]. The reports that C60 modified electrodes confer electrocatalysis have not been widely accepted [9]. However, recent work by Compton [10] has shown that the observed electrocatalysis in some cases was caused by either the small amount of graphite impurity in C60 sample, or the oxygenated species formed on the surface of glassy carbon electrodes by “electrode pretreatment”, while electrocatalysis mediated by C60 is only likely “where C60 itself becomes oxidized or reduced” [10]. However the area remains contentious. Carbon nanotubes (CNTs) have been deemed as excellent electrocatalysts for a large variety of compounds [11,12]. Strong ⁎ Corresponding author. Tel.: +98 3518211670; fax: +98 3518210644. E-mail address: [email protected] (M. Mazloum-Ardakani). 1 Tel.: +98 3518211670; fax: +98 3518210644.
http://dx.doi.org/10.1016/j.elecom.2014.01.026 1388-2481/© 2014 Elsevier B.V. All rights reserved.
promises were given, raising high expectations [13]. Due to rich electrochemistry of fullerene, electrochemistry of fullerene peapods (C60@CNT and C70@CNT) was investigated since synthesis of fullerene peapods was reported. Carbon nanotubes have also been applied to this purpose because of their remarkable electrocatalytic properties [14]. As an excellent electron acceptor, C60 could be dispersed in nanomaterials, such as carbon nanotubes, to facilitate electron transfer. Recent investigations revealed that C60-functionalized multiwalled carbon nanotube (MWCNT) films were more effective in facilitating the direct electron transfer of hemoglobin than MWCNT films [15]. Zhu et al. successfully constructed a C60-functionalized MWCNT film to achieve a dopamine detection limit of 0.03 μM, compared to 0.15 μM at a MWCNT modified electrode [16]. Wael et al. developed a C60-functionalized MWCNT film for the electrocatalytic determination of the vinclozolin [17]. To the best of our knowledge, no study has been published so far reporting the electroanalytical applications of C60-functionalized CNT and ionic liquid composites (C60-CNT/IL). Here in continuation to our studies concerning the preparation of modified electrodes [14,18–20], we developed a C60-CNT and ionic liquid composite film as a new modifier for the electrocatalytic determination of catecholamines in human serum and urine samples. The results showed that the composite film of C60-CNT/IL on a glassy carbon (GC) electrode is more sensitive for the detection of catecholamines compared to bare GC electrode and CNT/ GC electrode. The improvement electrochemical response of catecholamines was found at C60-CNT/IL/GC, revealing the synergetic effect of C60-CNT and IL.
10
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
2. Experimental
C60 (MWCNTs/C60 = 2:1, mass ratio) with a total amount of about 1 mg were dispersed in 10 mL toluene in an ultrasound bath for 30 min to give a 0.1 mg mL−1 suspension. A volume of 10 μL of the suspension was applied directly on a GC electrode surface and the solvent was allowed to evaporate at room temperature (10 min). This C60-MWCNT film electrode was subjected to potential scanning in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate between 0.0 and −2.0 V (vs Ag/AgCl) until reversible multistep electron-transfer reaction was obtained [21]. The resulting C60-CNT film electrode was then washed with acetonitrile several times to remove the electrolytes and dried at room temperature. Then, 50 μL of IL was dispersed in 0.3 mL 1% chitosan solution in 1 M of acetic acid. After the mixture was sonicated for 30 min, 3.0 μL was applied to the surface using a microsyringe and this was allowed to dry in a stream of hot air (15 min).
2.1. Apparatus and chemicals Electrochemical measurements were performed by an Autolab potentiostat/galvanostat (PGSTAT-302 N, Eco Chemie, Netherlands). A three-electrode system was used, where a GC electrode or a modified GC electrode (2 mm diameter) served as the working electrode, a platinum wire as the counter electrode and an Ag/AgCl/KCl (3.0 M) electrode as the reference electrode. The morphology of nanomaterial was characterized using scanning electron microscopy (SEM, Hitachi S-4160) and transmission electron microscopy (TEM) (Philips EM208). For impedance measurements, a frequency range of 100 kHz to 0.1 Hz was employed. The AC voltage amplitude and DC voltage were 5 mV and 150 mV, respectively. Norepinephrine, isoprenaline, dopamine, 1-butyl-3-methylimidazolium tetrafluoroborate and fullerene-C60 (99.5%) were reagentgrade from Sigma Aldrich. Phosphate salt, sodium hydroxide, solvents and reagents were of pro-analysis grade from Merck (Darmstadt, Germany). Multiwalled carbon nanotubes (purity more than 95%) were purchased from Plasma Chem (Germany).
3. Results and discussion 3.1. Characterization of the C60-CNT/IL composites In order to understand the performance of the C60-CNT/IL composites, we investigated modified electrodes by TEM, SEM, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), respectively. The response of a modified electrode is related to its physical morphology. Fig. 1A and B shows the TEM and SEM micrographs of C60CNTs/GC electrode and C60-CNTs/IL/GC electrode, respectively. Fig. 1A clearly shows nanoclusters of C60 and CNTs that resulted from the
2.2. Preparation of the electrode Modified electrodes were prepared by a simple casting method. Prior to the surface coating, the GC electrode was polished on a polishing cloth with 0.05 μm alumina powder. Purified MWCNTs and
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Fig. 1. (A) TEM image of C60-CNT, (B) SEM image of C60-CNT/IL nanocomposite, (C) Nyquist plots of bare GC electrode (a), CNT/GC (b), C60-CNT/GC (c), C60-CNT/IL/GC, (D) 1st cycle and 50th cycle of C60-CNT/IL/GC in 1 mM Fe(CN)3+/4+ containing 0.1 M KCl.
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
formation of a nanocomposite film. CNTs are discernible after the C60CNTs were applied on the GC electrode; when IL was introduced to the C60-CNT composite, the film becomes more uniform and even, most likely because of the binding and blanketing effect of IL (Fig. 1B). We also added the resulting C60-CNT composites into toluene and subjected the suspension to ultrasonication for several hours, but the toluene solution did not turn purple, a characteristic color of C60. These results indicate that C60 was strongly attached to CNTs. Electrochemical activity of a C60-CNT/IL/GC electrode was examined using EIS and the results were then compared to those obtained at a CNT/GC electrode, a C60-CNT/GC electrode and a bare GC electrode. Fig. 1C shows the Nyquist plots for bare GC electrode (a), CNT/GC (b), C60-CNT/GC (c) and C60-CNT/IL/GC (d) in 0.1 M KCl containing 1 mM [Fe(CN)6]3 − and 1 mM [Fe(CN)6]4 −. The charge transfer resistance (Rct) is evaluated from the diameter of the semicircle in the high frequency region of the Nyquist plot. It could be seen that on the C60CNT/IL/GC (curve d, Rct = 1.8 ± 0.03 kΩ) the value of Rct is smaller than that of C60-CNT/GC (curve c, Rct = 2.5 ± 0.03 kΩ) and CNT/GC (curve b, Rct = 3.8 ± 0.03 kΩ), suggesting the synergistic effect of C60-CNT and IL on the modified electrode effectively enhanced the conductivity of the modified electrode and made it easier for the electron transfer from the electrode to the film. Because the interfacial potential between the C60-CNT/IL composite modified electrode and the aqueous solution will cause an offset in the redox potential scale, the modified electrode was characterized using CV in 0.1 M KCl containing 1 mM [Fe(CN)6]3 − and 1 mM [Fe(CN)6]4− solutions to detect whether the interfacial potential exists or has an obvious effect on the electrochemical response, the 1st and 50th cycles of the CVs are shown in Fig. 1D. It was found that the C60CNT/IL/GC exhibits a pair of well-defined redox peaks. It was also found that after 50 cycles of cycling, the peak separation remains unchanged and the redox peak currents only varied no more than 2%, which revealed good stability and reproducibility of the modified electrode. 3.2. Electrocatalytic oxidation of catecholamines at a C60-CNT/IL/GC The comparison of oxidation towards catecholamines at different electrodes is shown in Fig. 2. As can be seen, the anodic peak potential for norepinephrine (NE), isoprenaline (IP) and dopamine (DA) 10
oxidation at the bare GC electrode are about 400, 420 and 340 mV, respectively, while the corresponding potential at C60-CNT/IL/GC electrode are 230, 210 and 200 mV, respectively (curves a and d in Fig. 2). The comparison of the oxidation of catecholamines at C60-CNT/IL/GC (curve d) and C60-CNT/GC (curve c) shows an enhancement of the anodic peak current (71%, 70% and 73% for NE, IP and DA, respectively), which indicated that the presence of ILs could enhance the peak currents. The advantages of C60-CNT/IL/GC had been elucidated with higher conductivity, fast electron transfer rate, good anti-fouling properties and inherent catalytic ability of ILs [22,23]. To investigate the roles of C60 on CNT surface in the electrochemical oxidation of catecholamines, the cyclic response of catecholamines at the C60-CNT/GC (curve c) and CNT/GC (curve b) electrodes were recorded. The results indicated that the presence of C60 on CNT surface exhibited 58%, 48% and 55% improvement on the oxidation peak current for NE, IP and DA, respectively. The higher electrocatalytic activity of the C60-CNT film on the electrochemistry of catecholamines than that of the CNTs film is presumably caused by a more favorable electrochemical environment for catecholamines on the electrode [21]. The close interaction between C60 and CNTs in the C60-CNT composite allows the C60 to accumulate more electrons in C60-CNT composite than in CNTs without C60. Moreover, because of the fine dispersion, nanometer size, large accessible surface per volume, and low resistance, the C60-CNT composite offers a favorite microenvironment for electron transfer between catecholamines and the underlying electrode, which results in the improvement on the electrochemical response. Compton et al. showed that pure C60 film modified electrodes offer no electrocatalysis effect [10]. In the case of the C60 film, the modification of the electrode surface serves to act as inert particles, resulting in a partially blocked electrode surface [1]. Compton and co-workers have revisited this problem, and have argued that the apparent electrocatalytic detection of several analytes at C60 modified electrodes was simply due to the redistribution of amorphous of C60 on the electrode surface that changes the porosity or the accessibility of the underlying substrate to the electrolyte and any redox active species in it. We suggest that this situation may be similar in the case of C60-CNT, where the electrocatalysis in CNTs is well known [13,24]. Now the CNT surface is best described as porous materials, the smaller blocks allow more material to diffuse around them on the timescale of the voltammetric experiment and undergo electrolysis at the exposed electrode surface. We claim that the porous C60-CNT film also
10 C60-CNT/IL/GC
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Fig. 2. CVs recorded in 0.1 M phosphate buffer solution (pH 7.0) and in +0.30 mM of (A) NE, (B) IP and (C) DA at the surface of GC (a), CNT/GC (b), C60-CNT/GC (c) and C60-CNT/IL/GC (D). Scan rate 20 mV s−1.
M. Mazloum-Ardakani, A. Khoshroo / Electrochemistry Communications 42 (2014) 9–12
100
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8
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Fig. 3. DPVs of C60-CNT/IL/GC in 0.1 M phosphate buffer solution (pH 7.0) containing different concentrations of (A) NE, numbers 1–20 correspond to 0.07–750.0 μM; (B) IP, numbers 1–18 correspond to 0.1–700.0 μM; (C) DA, numbers 1–20 correspond to 0.06–800.0 μM; insets: plot of the peak currents as a function of catecholamine concentration.
exhibits some degree of thin-layer behavior [25]. Thus the change in the porosity of this electrode, combined with possible thin-layer effects can be used to explain the observed improvement on the electrochemical response in electroanalysis of catecholamines. The improvement of electrochemical response of catecholamines was found at C60-CNT/IL/ GC, revealing the synergetic effect of C60-CNT and IL. 3.3. Calibration plot and limit of detection for catecholamines DPV was used to obtain the linear concentration range and detection limit of catecholamines at the C60-CNT/IL/GC electrode (Fig. 3). The results show that the dependence of electrocatalytic peak currents of catecholamine oxidation on their concentrations at the surface of the C60CNT/IL/GC consists of two linear segments with different slopes; the linear response were: NE, a slope of 0.650 μA μM−1 for the first linear segment (0.07–30.0 μM) and a slope of 0.103 μA μM−1 for the second linear segment (30.0–750.0 μM); IP, a slope of 0.550 μA μM−1 for the first linear segment (0.1–25.0 μM) and a slope of 0.098 μA μM−1 for the second linear segment (25.0–700.0 μM); DA, a slope of 0.777 μA μM−1 for the first linear segment (0.06–25.0 μM) and a slope of 0.101 μA μM−1 for the second linear segment (25.0–800.0 μM). The decrease in sensitivity (slope) of the second linear segment is likely due to kinetic limitation [26]. The detection limits (3σ) were 18 ± 2 nM for NE, 22 ± 2 nM for IP and 15 ± 2 nM for DA. The proposed method in real-life sample analysis was also investigated by direct analysis of catecholamines in serum samples and urine samples. No observable catecholamine content was observed in these samples and hence known amount of catecholamines was spiked in real-life samples to carry out the recovery study. Values of 19.6, 20.2 and 19.7 nM were determined for the spiked concentration of 20 nM of NE, IP and DA, respectively, which accounts for recovery of 98%, 101% and 98.5%.
nanocomposite. Here, we investigated the construction and the use of C60-CNT/IL modified electrode in the voltammetry methods. The results show that the determination of catecholamines by using C60-CNT/IL/GC is simpler and more sensitive than the use of other methods. Acknowledgments The authors wish to thank the Yazd University Research Council, IUT Research Council and Excellence in Sensors for financial support of this research.
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4. Conclusions It is worth noticing that the C60-CNT/IL nanocomposite is ideally suited for implementation in electrochemical applications. This work puts forward an approach for the fabrication of a novel nanostructured modified electrode by the use of C60-CNT/IL nanocomposite. As far as we know, it is the first time that IL has been used to fabricate the C60-CNT/IL
[23] [24] [25] [26]
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