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Application of multi-walled carbon nanotubes modified carbon ionic liquid electrode for electrocatalytic oxidation of dopamine
Colloids and Surfaces B: Biointerfaces 88 (2011) 402–406
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Application of multi-walled carbon nanotubes modified carbon ionic liquid electrode for electrocatalytic oxidation of dopamine Yonghong Li a , Xinsheng Liu b , Xiaoying Liu a , Nannan Mai a , Yuandong Li a , Wanzhi Wei a,∗ , Qingyun Cai a a b
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Hunan, Changsha 410082, PR China Hunan Nonferrous Xiangxiang Fluoride Chemical Co. Ltd., Hunan, Xiangxiang 411400, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 6 July 2011 Accepted 6 July 2011 Available online 20 July 2011 Keywords: Carbon ionic liquid electrode Multi-walled carbon nanotubes Square wave voltammetry Dopamine Selective detection
a b s t r a c t A simple, sensitive, and reliable method based on a multi-walled carbon nanotubes (MWNTs) modified carbon ionic liquid electrode (CILE) has been successfully developed for determination of dopamine (DA) in the presence of ascorbic acid (AA). The acid-treated MWNTs with carboxylic acid functional groups could promote the electron-transfer reaction of DA and inhibit the voltammetric response of AA. Due to the good performance of the ionic liquid, the electrochemical response of DA on the MWNTs/CILE was better than that of other MWNTs modified electrodes. Under the optimum conditions a linear calibration plot was obtained in the range 5.0 × 10−8 to 2.0 × 10−4 mol L−1 and the detection limit was 1.0 × 10−8 mol L−1 .
1. Introduction Recently, the determination of dopamine (DA) has attracted considerable interest because it plays an important role in the function of the central nervous, renal, hormonal and cardiovascular systems [1]. However, there are many interfering compounds in real biological matrixes. Ascorbic acid (AA) is a main interferent [2–4], because the oxidation potentials of DA and AA are rather similar at conventional electrodes. Therefore, a major problem in DA determination is the solution between DA and coexisting species. During the past few years, a new type of carbon paste electrode, which is named as carbon ionic liquid electrode (CILE), has been developed widely. The CILE is prepared by using conductive ionic liquids (ILs) as pasting binders in place of nonconductive organic binders. Its behavior in electrochemistry is better than other kinds of carbon materials, such as glassy carbon electrode (GCE), and conventional carbon paste electrode (CPE) [5]. This type of electrode shows some advantages such as high conductivity, provision of fast electron transfer, good stability and antifouling properties [6]. Sun et al. [7] had reported that an ionic liquid modified carbon paste electrode (IL-CPE) showed enhanced electrochemical response and strong analytical activity towards the electrochemical oxidation of
∗ Corresponding author. Tel.: +86 731 88821960; fax: +86 731 88821967. E-mail addresses: [email protected] (W. Wei), [email protected] (Q. Cai). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.07.021
© 2011 Elsevier B.V. All rights reserved.
dopamine (DA). Safavi et al. [8] had made used of CILE for simultaneous determination of dopamine, ascorbic acid, and uric acid. The results showed that these three biomolecules could be well separated at the CILE, and could not interfere with each other. In order to improve the selectivity and sensitivity of the sensor, various materials are utilized to modify the electrode. Carbon nanotubes (CNTs) have attracted great attentions due to the unique structures and good electrical, mechanical and chemical properties [9–11]. Recently, many CNTs modified electrodes have been widely applied for the determination of DA in the presence of AA, with satisfactory results. For example, in Britto’s work the multi-walled carbon nanotubes (MWNTs) were cast on glassy carbon electrode (GCE) to form carbon nanotubes modified electrodes and the sensor had been successfully used in the oxidation of dopamine (DA) [12]. Later, Zhang et al. [13] used layer-by-layer (LBL) method to assemble MWNTs on GCE, The assembled MWNT multilayer films were studied with respect to the electrocatalytic activity towards ascorbic acid (AA) and dopamine (DA) and were further applied for selective determination of DA in the presence of AA. Zhang et al. [14] used the acid-treated MWNTs to modify the gold electrode, which offered obvious improvements in voltammetric sensitivity and selectivity towards the determination of dopamine (DA) in the presence of high concentration of AA. Under the chosen conditions, the peak currents were correspondent linearly to the concentrations of DA in the range of 5 × 10−7 to 4 × 10−4 mol L−1 with a limit of detection of 2 × 10−7 mol L−1 . The reported results demonstrate that acid-treated MWNTs is a superior electrode material to detect
Y. Li et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 402–406
403
dopamine selectively, so the MWNTs modified electrode displays higher selectivity in voltammetric measurements of DA from its interferences (AA and UA) in their mixture solution. In this paper, the acid-treated MWNTs were modified on the CILE (MWNTs/CILE) for the selective determination of DA by using square-wave voltammetry techniques (SWVs). The SWVs exhibits the advantages of large speed and high sensitivity. The sensitivity is generally larger than the one of differential pulse voltammetry (DPV), in which the reverse current is not measured. A comparison between SWV and DPV for both, reversible and irreversible system, shows that, for analogous experimental conditions, the peak currents obtained by SWV are about 2–4 times larger than those ones obtained by DPV [15]. In addition, it is easier to implement and is less time consuming [16]. The high sensitivity of this method allows low detection limits and good precision in the detection of DA. The effective but simple method proposed in this paper may have the potential for the practical analysis of DA. 2. Experimental 2.1. Apparatus and chemicals The electrochemical measurements were carried out with CHI660A electrochemical workstation (CH Instruments, Shanghai Chenhua Instruments Corporation, China). A conventional three electrode system was employed comprising a multi-walled carbon nanotubes modified carbon ionic liquid electrode (MWNTs/CILE) (3.0 mm in diameter) as working electrode, a platinum wire as counter electrode, and a Ag/AgCl (3 M KCl) electrode as reference electrode. All potentials were reported with respect to the reference electrode. A magnetic stirrer (Model 79-1) was used to stir the testing solution during the pre-concentration step. All the experiments were carried out at room temperature. Scanning electron micrographs were obtained with a Hitachi S-4800 scanning electron microscope (Hitachi Ltd., Tokyo, Japan). The ionic liquid N-octylpyridium hexafluorophosphate (OPPF6 ) was purchased from Shanghai Chengjie Chemical Co., Ltd. (Shanghai, China), graphite powder (extra pure) were obtained from Shanghai Chemical Reagents (Shanghai, China). Shortened multiwalled carbon nanotubes (MWNTs) with a diameter of about 10–30 nm (95% purity) were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). Dopamine (DA) and ascorbic acid (AA) were purchased from Sigma–Aldrich. 0.1 M phosphate buffer solutions (PBS) of various pH values were used as the supporting electrolytes. All other chemicals were of analytical grade, and used without further purification. Double-distilled water was used throughout. 2.2. Pretreatment of MWNTs Pretreatment of MWNTs was progressed according to the previous literature [17]. Firstly, MWNTs were heated in air at 600 ◦ C for 2 h, and then soaked in 6 M HCl solution for 24 h and centrifuged. The precipitate was rinsed with deionized water and dried in air. The MWNTs were chemically functionalized by ultrasonic agitation in a mixture of sulfuric acid and nitric acid (3:1) for 10 h. Then MWNTs were washed with deionized water (until pH 7.0 was obtained) and separated by centrifuging three times and then dried. 2.3. Electrode preparation Carbon ionic liquid electrode (CILE) was prepared by mixing the graphite powder and OPPF6 with a ratio of 50/50 (w/w) and then a portion of the resulting paste was packed firmly into the cavity (3 mm in diameter) of a Teflon holder. The electrode was then heated simply by using a hair drier for 2 min. It was then left to
Fig. 1. SEM images of (A) CILE and (B) MWNTs/CILE.
cool to room temperature. The electric contact was established via a stainless steel handle. A new surface was obtained by smoothing the electrode onto a weighing paper. Multi-walled carbon nanotubes modified carbon ionic liquid electrode (MWNTs/CILE) was prepared in the following procedure. First, 1 mg MWNTs was dispersed with the aid of ultrasonic agitation in 1 mL double-distilled water to give a 1 mg mL−1 black suspension. Then, 5 L of 1 mg mL−1 MWNTs was dropped on the surface of CILE and dried under an infrared lamp. This modified electrode is denoted as MWNTs/CILE. 2.4. Analytical procedure The sensor was immersed in 0.1 mol L−1 PBS (pH 7.0) containing a known concentration of DA for 120 s accumulation at open circuit potential with stirring. The square wave voltammograms (SWVs) were then recorded from −0.2 to 0.4 V, experiment parameters: 4 mV step height, 20 mV pulse height, 2 Hz frequency, and 10 s quiet time. 3. Results and discussion 3.1. Surface morphologies of different electrodes The morphologies of CILE and MWNTs/CILE were characterized by SEM. As shown in Fig. 1A, an uniform surface topography and no separated carbon layers could be observed at the surface of the CILE. The uniformity of the surface showed the good adherence of OPPF6 to graphite due to its high viscosity. After the MWNTs were cast on the CILE, it could be seen that the MWNTs were distributed homogeneously on the surface of CILE with special three-dimensional
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Fig. 2. Cyclic voltammograms of 5 mM Fe(CN)6 3−/4− containing 0.1 M KCl at (a) CILE (b) MWNTs/CILE, v = 50 mV s−1 .
structure (shown in Fig. 1B), suggesting that the MWNTs were successfully immobilized on the surface of CILE. 3.2. Electrochemical characterization of MWNTs/CILE and CILE Electrochemical behavior of different modified electrodes in a 5.0 mmol L−1 [Fe(CN)6 ]3-/4− and 0.1 mol L−1 KCl solution was investigated by cyclic voltammetry and the results are shown in Fig. 2. A pair of quasi-reversible redox peaks was separately observed on CILE and MWNTs/CILE, but the redox peak currents were increased and the peak-to-peak potential separation (Ep ) was decreased on the MWNTs/CILE (curve b) compared with those on the CILE (a). The results indicated that the MWNTs immobiled on the surface of the CILE could promote the electron transfer rates and further increase current response, due to their good electronic conductivity and a large surface area. 3.3. Electrochemical behavior of DA and AA at the modified electrode The electrochemical behavior of DA and AA at CILE and MWNTs/CILE was explored by SWVs in 0.1 M PBS at pH 7.0. As shown in Fig. 3, the peak current of DA at the MWNTs/CILE was about 11-fold higher than that at the bare CILE, but the peak current of AA only changed a little. It demonstrated that MWNTs/CILE had a selective catalytic activity to DA comparing with the bare CILE. The surface of the MWNTs-modified electrode brings negative charge
Fig. 4. Cyclic voltammograms obtained at different scan rates from the MWNTs/CILE in 0.1 mol L−1 PBS (pH 7.0) containing 10 M DA. Scan rates from inner to outer (mV s−1 ): 20, 50, 100, 150, 200, 250, 300.
because of the MWNTs with carboxylic acid groups on the surface of the CILE. In neutral aqueous solution or biological environments, DA exits as cations while AA exits as anions [18]. DA with positive charges can be attracted to the surface of the modified electrode while AA with negative charges would be prevented to get close to the surface. Therefore, the MWNTs as modifier can improve the selectivity of the sensor and promote the electron transfer rates. In addition, the peak potential separation between AA and DA at the MWNTs/CILE was 230 mV, which was increased about 20 mV compared with bare CILE. The MWNTs/CILE showed very remarkable resolution between the voltammetric peaks of AA and DA, and it could readily detect DA without interference from AA. 3.4. Optimization of conditions 3.4.1. Scan rate In order to evaluate the influence of adsorption processes on the electrochemical oxidation of DA at the MWNTs/CILE, the effect of the scan rate on the voltammetric signals was studied from 20 to 300 mV s−1 (shown in Fig. 4). The results showed that the anodic (ipa ) and cathodic (ipc ) peak currents were proportional to the scan rate. The linear regression equation were ipa (A) = 0.0609 v (mV s−1 ) + 1.9168 (r2 = 0.9978) and ipc (A) = −0.0535 v (mV s−1 ) − 0.3556 (r2 = 0.9996), suggesting that the electrode reaction is an adsorption-controlled process. 3.4.2. Effect of pH In order to investigate the mechanism of electrochemical reactions of DA on the MWNTs/CILE, the effect of pH on the peak potential and current was investigated by SWVs in the solution containing 20 M DA in the range of 5.0–9.0. As shown in Fig. 5A, the peak potentials shifted towards negative potential with increasing pH of the solution. The relationship of Ep and pH could be described by the following equation: Ep (V) = 0.5336 − 0.0472 pH (r2 = 0.9972), which showed that redox of DA underwent an equal proton-electron process, which was consistent with that reported in literatures [19]. On the other hand, from Fig. 5B increasing the pH from 5.0 to 7.0 would lead to a sharp increase of peak current, but the peak current decreased with further increase of pH. Therefore, PBS 7.0 was chosen as supporting electrolyte and it was close to the pH value of physiological condition.
Fig. 3. SWVs of 200 M AA and 20 M DA in 0.1 M PBS (pH 7.0) at the (a) CILE, (b) MWNTs/CILE.
3.4.3. Effect of accumulation time Because the oxidization of DA on the surface of the modified electrode is a typical adsorption-controlled process, the accumu-
Y. Li et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 402–406
Fig. 5. Effects of pH on the (A) the potential and (B) the peak current of 20 M DA at the MWNTs/CILE.
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Fig. 7. (A) SWVs at MWNTs/CILE in 0.1 M pH 7.0 PBS containing 5 × 10−4 M AA in the presence different concentrations of DA (from bottom to upper): 5.0 × 10−8 , 5.0 × 10−7 , 5.0 × 10−6 , 1.0 × 10−5 , 5.0 × 10−5 , 1.0 × 10−4 , 2 × 10−4 M. (B) The plot of current versus concentration of DA.
the selected conditions, the oxidative peak currents increased linearly with the concentrations of DA in the range of 5.0 × 10−8 to 2.0 × 10−4 mol L−1 with a correlation coefficient of 0.9937 (shown in Fig. 7B). The linear regression equation was expressed as Ip (A) = 0.8514 + 0.2819c (M). When the concentration of DA was higher than 2.0 × 10−4 mol L−1 , the working curve tended to level off. When the signal-to-noise ratio (SNR) was 3, the detection limit was 1.0 × 10−8 mol L−1 . The proposed MWNTs/CILE for DA determination was compared with other CNTs modified electrodes and the results are listed in Table 1. It can be seen that this method can provide a comparable
Fig. 6. Effect of accumulation time on the peak current of 10 M DA at the MWNTs/CILE.
lation time will remarkably influence the current response of DA. Thus, the effect of accumulation time in 10 M DA solution on peak current at the MWNTs/CILE was investigated. It was clear from Fig. 6 that the peak current of DA was almost constant after 120 s, indicating an accumulation time of 120 s was sufficient to achieve DA saturation of the electrode. An accumulation time of 120 s was therefore adopted in subsequent experiments. 3.5. Calibration curve Fig. 7A shows the SWVs of an MWNTs modified electrode to the variation of DA concentration in the presence of 0.5 mM AA. Under
Table 1 Comparison of different modified electrodes for DA determination. Electrodes
Method
Linear range (M)
Detection limit (M)
Reference
MWNTs/Au EBNBHCNPE PA-SWNTs/Pt MWNTs/SPE MWNTs/GCE MWNTs/CILE
It DPV DPV ASV DPV SWV
0.5–400 0.1–900 0.2–10 0.05–1 0.5–100 0.05–200
0.2 0.087 0.08 0.015 0.2 0.01
[14] [20] [21] [22] [23] This work
Au, gold electrode; MWNTs, multi-walled carbon nanotube; EBNBHCNPE, 2,2 -[1,2ethanediylbis(nitriloethylidyne)]-bis-hydroquinone modified carbon nanotube paste electrode; It, amperometric current–time response curve; DPV, differential pulse voltammetry; Pt, platinum electrode; SWNTs, single-walled carbon nanotube; PA, phytic acid; SPE, screen-printed electrodes; GCE, glassy carbon electrode; CILE, carbon ionic liquid electrode; ASV, adsorptive stripping voltammetry; SWV, square wave voltammetry.
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interfering substances in human blood serum, such as proteins and glucose did not interfere with the determination of DA. The recovery and RSD were acceptable, indicating that the proposed methods could be efficiently used for the determination of DA in practical applications. 4. Conclusion
Fig. 8. Cyclic voltammograms of 200 M AA, 50 M DA and 20 M UA in 0.1 M PBS (pH 7.0) at the MWNTs/CILE. Table 2 Experimental results for the determination of DA in human blood serum. Samples
DA spiking (M)
DA found (M)
Recovery (%)
RSD (%)
1 2 3
1.0 5.0 10.0
1.02 4.89 9.65
102 97.8 96.5
2.2 1.9 1.8
linear range and detection limit by SWVs with a simple electrode preparation procedure. 3.6. Reproducibility, stability and selectivity of the MWNTs/CILE In order to examine the reproducibility of the modified electrode, repeated cyclic voltammetric experiment was run in 10 M DA in 0.1 M pH 7.0 PBS. The relative standard deviation (RSD) was about 3.1% after 50 successive CVs, indicating that the proposed sensor had an excellent reproducibility. After measurement, the modified electrode was cleaned with voltammetric cycles in pH 7.0 PBS to eliminate the adsorption and stored in air. The modified electrode retained 94% of its initial response after 2 weeks. These results showed that the MWNTs/CILE was stable and the voltammetric studies could be replicated for the sensitive determination of DA. Ascorbic acid (AA) and uric acid (UA) are the main coexisting substance in real samples, so the electrochemical response of DA in the presence of AA and UA on the MWNTs/CILE is investigated. Fig. 8 shows cyclic voltammograms of 200 M AA, 50 M DA and 20 M UA in 0.1 M PBS (pH 7.0) at the MWNTs/CILE. Three independent oxidation peaks at −0.002 V, 0.172 V and 0.292 V were observed at the MWNTs/CILE, indicating that their oxidation took place independently at the MWNTs/CILE. Therefore, the modified electrode was suitable for the selective detection of DA. 3.7. Analytical applications In order to examine the reliability of the proposed sensor in practical applications, it was applied for the determination of DA in human blood serum. The results are shown in Table 2. Some other
In summary, the acid-treated MWNTs could been stably assembled on carbon ionic liquid electrode, and the modified electrode showed large peak separations between DA, AA and UA by using CV or SWV. What’s more, a remarkable enhancement in the current response of DA was observed at the modified electrode in neutral pH. Therefore, selective and sensitive detection of dopamine could be achieved at the modified electrode. Under the chosen conditions, the peak currents were correspondent linearly to the concentrations of DA in the range of 5.0 × 10−8 to 2.0 × 10−4 mol L−1 with a limit of detection of 1 × 10−8 mol L−1 . What’s more, the sensor exhibited good reproducibility and stability. All these properties are beneficial for fabricating a simple, sensitive and selective biosensor for DA. Acknowledgements The financial support from National Outstanding Youth Foundations of China, National Science Foundation of China (50725825) and Special Research Found for the Doctoral Program of Higher Education of China (20060532006) is acknowledged. References [1] P. Damier, E.C. Hirsch, Y. Agid, A.M. Graybiel, Brain 122 (1999) 1437. [2] P. Cappella, B. Ghasemzadeh, K. Mithchell, R.N. Adams, Electroanalysis 2 (1990) 175. [3] R.M. Wightman, L.J. May, A.C. Michael, Anal. Chem. 60 (1988) 769A. [4] G. Mihaela, W.M.A. Damien, Electrochim. Acta 49 (2004) 4743. [5] A. Safavi, N. Maleki, F. Tajab adi, E. Farjami, Electrochem. Commun. 9 (2007) 1963. [6] N. Maleki, A. Safavi, F. Tajabadi, Anal. Chem. 78 (2006) 3820. [7] W. Sun, M.X. Yang, K. Jiao, Anal. Bioanal. Chem. 389 (2007) 1283. [8] A. Safavi, N. Maleki, O. Moradlou, F. Tajabadi, Anal. Biochem. 359 (2006) 224. [9] K. Balasubramanian, M. Burghard, Anal. Bioanal. Chem. 385 (2006) 452. [10] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [11] M. Musameh, J. Wang, A. Merkoci, Y.H. Lin, Electrochem. Commun. 4 (2002) 743. [12] P.J. Britto, K.S.V. Santhanam, P.M. Ajayan, Bioelectrochem. Bioenerg. 41 (1996) 121. [13] M.N. Zhang, K.P. Gong, H.W. Zhang, L.Q. Mao, Biosens. Bioelectron. 20 (2005) 1270. [14] P. Zhang, F.H. Wu, G.C. Zhao, X.W. Wei, Bioelectrochemistry 67 (2005) 109. [15] P. Qiu, Y.N. Ni, Chin. Chem. Lett. 19 (2008) 1337. [16] J.Y. Tilquin, J. Glibert, P. Claes, Electrochim. Acta 38 (1993) 479. [17] B. Kim, W.M. Sigmund, Langmuir 20 (2004) 8239. [18] F. Malem, D. Mandler, Anal. Chem. 65 (1993) 37. [19] C.R. Raj, T. Ohsaka, J. Electroanal. Chem. 496 (2001) 44. [20] M. Mazloum-Ardakani, H. Beitollahi, B. Ganjipour, H. Naeimi, M. Nejati, Bioelectrochemistry 75 (2009) 1. [21] S. Jo, H. Jeong, S.R. Bae, S. Jeon, J. Microchem. 88 (2008) 1. [22] M. Moreno, A.S. Arribas, E. Bermejo, M. Chicharro, A. Zapardiel, M.C. Rodríguez, Y. Jalit, G.A. Rivas, Talanta 80 (2010) 2149. [23] L.Y. Jiang, C.Y. Liu, L.P. Jiang, Z. Peng, G.H. Lu, Anal. Sci. 20 (2004) 1055.
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Application of multi-walled carbon nanotubes modified carbon ionic liquid electrode for electrocatalytic oxidation of dopamine Yonghong Li a , Xinsheng Liu b , Xiaoying Liu a , Nannan Mai a , Yuandong Li a , Wanzhi Wei a,∗ , Qingyun Cai a a b
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Hunan, Changsha 410082, PR China Hunan Nonferrous Xiangxiang Fluoride Chemical Co. Ltd., Hunan, Xiangxiang 411400, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 6 July 2011 Accepted 6 July 2011 Available online 20 July 2011 Keywords: Carbon ionic liquid electrode Multi-walled carbon nanotubes Square wave voltammetry Dopamine Selective detection
a b s t r a c t A simple, sensitive, and reliable method based on a multi-walled carbon nanotubes (MWNTs) modified carbon ionic liquid electrode (CILE) has been successfully developed for determination of dopamine (DA) in the presence of ascorbic acid (AA). The acid-treated MWNTs with carboxylic acid functional groups could promote the electron-transfer reaction of DA and inhibit the voltammetric response of AA. Due to the good performance of the ionic liquid, the electrochemical response of DA on the MWNTs/CILE was better than that of other MWNTs modified electrodes. Under the optimum conditions a linear calibration plot was obtained in the range 5.0 × 10−8 to 2.0 × 10−4 mol L−1 and the detection limit was 1.0 × 10−8 mol L−1 .
1. Introduction Recently, the determination of dopamine (DA) has attracted considerable interest because it plays an important role in the function of the central nervous, renal, hormonal and cardiovascular systems [1]. However, there are many interfering compounds in real biological matrixes. Ascorbic acid (AA) is a main interferent [2–4], because the oxidation potentials of DA and AA are rather similar at conventional electrodes. Therefore, a major problem in DA determination is the solution between DA and coexisting species. During the past few years, a new type of carbon paste electrode, which is named as carbon ionic liquid electrode (CILE), has been developed widely. The CILE is prepared by using conductive ionic liquids (ILs) as pasting binders in place of nonconductive organic binders. Its behavior in electrochemistry is better than other kinds of carbon materials, such as glassy carbon electrode (GCE), and conventional carbon paste electrode (CPE) [5]. This type of electrode shows some advantages such as high conductivity, provision of fast electron transfer, good stability and antifouling properties [6]. Sun et al. [7] had reported that an ionic liquid modified carbon paste electrode (IL-CPE) showed enhanced electrochemical response and strong analytical activity towards the electrochemical oxidation of
∗ Corresponding author. Tel.: +86 731 88821960; fax: +86 731 88821967. E-mail addresses: [email protected] (W. Wei), [email protected] (Q. Cai). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.07.021
© 2011 Elsevier B.V. All rights reserved.
dopamine (DA). Safavi et al. [8] had made used of CILE for simultaneous determination of dopamine, ascorbic acid, and uric acid. The results showed that these three biomolecules could be well separated at the CILE, and could not interfere with each other. In order to improve the selectivity and sensitivity of the sensor, various materials are utilized to modify the electrode. Carbon nanotubes (CNTs) have attracted great attentions due to the unique structures and good electrical, mechanical and chemical properties [9–11]. Recently, many CNTs modified electrodes have been widely applied for the determination of DA in the presence of AA, with satisfactory results. For example, in Britto’s work the multi-walled carbon nanotubes (MWNTs) were cast on glassy carbon electrode (GCE) to form carbon nanotubes modified electrodes and the sensor had been successfully used in the oxidation of dopamine (DA) [12]. Later, Zhang et al. [13] used layer-by-layer (LBL) method to assemble MWNTs on GCE, The assembled MWNT multilayer films were studied with respect to the electrocatalytic activity towards ascorbic acid (AA) and dopamine (DA) and were further applied for selective determination of DA in the presence of AA. Zhang et al. [14] used the acid-treated MWNTs to modify the gold electrode, which offered obvious improvements in voltammetric sensitivity and selectivity towards the determination of dopamine (DA) in the presence of high concentration of AA. Under the chosen conditions, the peak currents were correspondent linearly to the concentrations of DA in the range of 5 × 10−7 to 4 × 10−4 mol L−1 with a limit of detection of 2 × 10−7 mol L−1 . The reported results demonstrate that acid-treated MWNTs is a superior electrode material to detect
Y. Li et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 402–406
403
dopamine selectively, so the MWNTs modified electrode displays higher selectivity in voltammetric measurements of DA from its interferences (AA and UA) in their mixture solution. In this paper, the acid-treated MWNTs were modified on the CILE (MWNTs/CILE) for the selective determination of DA by using square-wave voltammetry techniques (SWVs). The SWVs exhibits the advantages of large speed and high sensitivity. The sensitivity is generally larger than the one of differential pulse voltammetry (DPV), in which the reverse current is not measured. A comparison between SWV and DPV for both, reversible and irreversible system, shows that, for analogous experimental conditions, the peak currents obtained by SWV are about 2–4 times larger than those ones obtained by DPV [15]. In addition, it is easier to implement and is less time consuming [16]. The high sensitivity of this method allows low detection limits and good precision in the detection of DA. The effective but simple method proposed in this paper may have the potential for the practical analysis of DA. 2. Experimental 2.1. Apparatus and chemicals The electrochemical measurements were carried out with CHI660A electrochemical workstation (CH Instruments, Shanghai Chenhua Instruments Corporation, China). A conventional three electrode system was employed comprising a multi-walled carbon nanotubes modified carbon ionic liquid electrode (MWNTs/CILE) (3.0 mm in diameter) as working electrode, a platinum wire as counter electrode, and a Ag/AgCl (3 M KCl) electrode as reference electrode. All potentials were reported with respect to the reference electrode. A magnetic stirrer (Model 79-1) was used to stir the testing solution during the pre-concentration step. All the experiments were carried out at room temperature. Scanning electron micrographs were obtained with a Hitachi S-4800 scanning electron microscope (Hitachi Ltd., Tokyo, Japan). The ionic liquid N-octylpyridium hexafluorophosphate (OPPF6 ) was purchased from Shanghai Chengjie Chemical Co., Ltd. (Shanghai, China), graphite powder (extra pure) were obtained from Shanghai Chemical Reagents (Shanghai, China). Shortened multiwalled carbon nanotubes (MWNTs) with a diameter of about 10–30 nm (95% purity) were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). Dopamine (DA) and ascorbic acid (AA) were purchased from Sigma–Aldrich. 0.1 M phosphate buffer solutions (PBS) of various pH values were used as the supporting electrolytes. All other chemicals were of analytical grade, and used without further purification. Double-distilled water was used throughout. 2.2. Pretreatment of MWNTs Pretreatment of MWNTs was progressed according to the previous literature [17]. Firstly, MWNTs were heated in air at 600 ◦ C for 2 h, and then soaked in 6 M HCl solution for 24 h and centrifuged. The precipitate was rinsed with deionized water and dried in air. The MWNTs were chemically functionalized by ultrasonic agitation in a mixture of sulfuric acid and nitric acid (3:1) for 10 h. Then MWNTs were washed with deionized water (until pH 7.0 was obtained) and separated by centrifuging three times and then dried. 2.3. Electrode preparation Carbon ionic liquid electrode (CILE) was prepared by mixing the graphite powder and OPPF6 with a ratio of 50/50 (w/w) and then a portion of the resulting paste was packed firmly into the cavity (3 mm in diameter) of a Teflon holder. The electrode was then heated simply by using a hair drier for 2 min. It was then left to
Fig. 1. SEM images of (A) CILE and (B) MWNTs/CILE.
cool to room temperature. The electric contact was established via a stainless steel handle. A new surface was obtained by smoothing the electrode onto a weighing paper. Multi-walled carbon nanotubes modified carbon ionic liquid electrode (MWNTs/CILE) was prepared in the following procedure. First, 1 mg MWNTs was dispersed with the aid of ultrasonic agitation in 1 mL double-distilled water to give a 1 mg mL−1 black suspension. Then, 5 L of 1 mg mL−1 MWNTs was dropped on the surface of CILE and dried under an infrared lamp. This modified electrode is denoted as MWNTs/CILE. 2.4. Analytical procedure The sensor was immersed in 0.1 mol L−1 PBS (pH 7.0) containing a known concentration of DA for 120 s accumulation at open circuit potential with stirring. The square wave voltammograms (SWVs) were then recorded from −0.2 to 0.4 V, experiment parameters: 4 mV step height, 20 mV pulse height, 2 Hz frequency, and 10 s quiet time. 3. Results and discussion 3.1. Surface morphologies of different electrodes The morphologies of CILE and MWNTs/CILE were characterized by SEM. As shown in Fig. 1A, an uniform surface topography and no separated carbon layers could be observed at the surface of the CILE. The uniformity of the surface showed the good adherence of OPPF6 to graphite due to its high viscosity. After the MWNTs were cast on the CILE, it could be seen that the MWNTs were distributed homogeneously on the surface of CILE with special three-dimensional
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Fig. 2. Cyclic voltammograms of 5 mM Fe(CN)6 3−/4− containing 0.1 M KCl at (a) CILE (b) MWNTs/CILE, v = 50 mV s−1 .
structure (shown in Fig. 1B), suggesting that the MWNTs were successfully immobilized on the surface of CILE. 3.2. Electrochemical characterization of MWNTs/CILE and CILE Electrochemical behavior of different modified electrodes in a 5.0 mmol L−1 [Fe(CN)6 ]3-/4− and 0.1 mol L−1 KCl solution was investigated by cyclic voltammetry and the results are shown in Fig. 2. A pair of quasi-reversible redox peaks was separately observed on CILE and MWNTs/CILE, but the redox peak currents were increased and the peak-to-peak potential separation (Ep ) was decreased on the MWNTs/CILE (curve b) compared with those on the CILE (a). The results indicated that the MWNTs immobiled on the surface of the CILE could promote the electron transfer rates and further increase current response, due to their good electronic conductivity and a large surface area. 3.3. Electrochemical behavior of DA and AA at the modified electrode The electrochemical behavior of DA and AA at CILE and MWNTs/CILE was explored by SWVs in 0.1 M PBS at pH 7.0. As shown in Fig. 3, the peak current of DA at the MWNTs/CILE was about 11-fold higher than that at the bare CILE, but the peak current of AA only changed a little. It demonstrated that MWNTs/CILE had a selective catalytic activity to DA comparing with the bare CILE. The surface of the MWNTs-modified electrode brings negative charge
Fig. 4. Cyclic voltammograms obtained at different scan rates from the MWNTs/CILE in 0.1 mol L−1 PBS (pH 7.0) containing 10 M DA. Scan rates from inner to outer (mV s−1 ): 20, 50, 100, 150, 200, 250, 300.
because of the MWNTs with carboxylic acid groups on the surface of the CILE. In neutral aqueous solution or biological environments, DA exits as cations while AA exits as anions [18]. DA with positive charges can be attracted to the surface of the modified electrode while AA with negative charges would be prevented to get close to the surface. Therefore, the MWNTs as modifier can improve the selectivity of the sensor and promote the electron transfer rates. In addition, the peak potential separation between AA and DA at the MWNTs/CILE was 230 mV, which was increased about 20 mV compared with bare CILE. The MWNTs/CILE showed very remarkable resolution between the voltammetric peaks of AA and DA, and it could readily detect DA without interference from AA. 3.4. Optimization of conditions 3.4.1. Scan rate In order to evaluate the influence of adsorption processes on the electrochemical oxidation of DA at the MWNTs/CILE, the effect of the scan rate on the voltammetric signals was studied from 20 to 300 mV s−1 (shown in Fig. 4). The results showed that the anodic (ipa ) and cathodic (ipc ) peak currents were proportional to the scan rate. The linear regression equation were ipa (A) = 0.0609 v (mV s−1 ) + 1.9168 (r2 = 0.9978) and ipc (A) = −0.0535 v (mV s−1 ) − 0.3556 (r2 = 0.9996), suggesting that the electrode reaction is an adsorption-controlled process. 3.4.2. Effect of pH In order to investigate the mechanism of electrochemical reactions of DA on the MWNTs/CILE, the effect of pH on the peak potential and current was investigated by SWVs in the solution containing 20 M DA in the range of 5.0–9.0. As shown in Fig. 5A, the peak potentials shifted towards negative potential with increasing pH of the solution. The relationship of Ep and pH could be described by the following equation: Ep (V) = 0.5336 − 0.0472 pH (r2 = 0.9972), which showed that redox of DA underwent an equal proton-electron process, which was consistent with that reported in literatures [19]. On the other hand, from Fig. 5B increasing the pH from 5.0 to 7.0 would lead to a sharp increase of peak current, but the peak current decreased with further increase of pH. Therefore, PBS 7.0 was chosen as supporting electrolyte and it was close to the pH value of physiological condition.
Fig. 3. SWVs of 200 M AA and 20 M DA in 0.1 M PBS (pH 7.0) at the (a) CILE, (b) MWNTs/CILE.
3.4.3. Effect of accumulation time Because the oxidization of DA on the surface of the modified electrode is a typical adsorption-controlled process, the accumu-
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Fig. 5. Effects of pH on the (A) the potential and (B) the peak current of 20 M DA at the MWNTs/CILE.
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Fig. 7. (A) SWVs at MWNTs/CILE in 0.1 M pH 7.0 PBS containing 5 × 10−4 M AA in the presence different concentrations of DA (from bottom to upper): 5.0 × 10−8 , 5.0 × 10−7 , 5.0 × 10−6 , 1.0 × 10−5 , 5.0 × 10−5 , 1.0 × 10−4 , 2 × 10−4 M. (B) The plot of current versus concentration of DA.
the selected conditions, the oxidative peak currents increased linearly with the concentrations of DA in the range of 5.0 × 10−8 to 2.0 × 10−4 mol L−1 with a correlation coefficient of 0.9937 (shown in Fig. 7B). The linear regression equation was expressed as Ip (A) = 0.8514 + 0.2819c (M). When the concentration of DA was higher than 2.0 × 10−4 mol L−1 , the working curve tended to level off. When the signal-to-noise ratio (SNR) was 3, the detection limit was 1.0 × 10−8 mol L−1 . The proposed MWNTs/CILE for DA determination was compared with other CNTs modified electrodes and the results are listed in Table 1. It can be seen that this method can provide a comparable
Fig. 6. Effect of accumulation time on the peak current of 10 M DA at the MWNTs/CILE.
lation time will remarkably influence the current response of DA. Thus, the effect of accumulation time in 10 M DA solution on peak current at the MWNTs/CILE was investigated. It was clear from Fig. 6 that the peak current of DA was almost constant after 120 s, indicating an accumulation time of 120 s was sufficient to achieve DA saturation of the electrode. An accumulation time of 120 s was therefore adopted in subsequent experiments. 3.5. Calibration curve Fig. 7A shows the SWVs of an MWNTs modified electrode to the variation of DA concentration in the presence of 0.5 mM AA. Under
Table 1 Comparison of different modified electrodes for DA determination. Electrodes
Method
Linear range (M)
Detection limit (M)
Reference
MWNTs/Au EBNBHCNPE PA-SWNTs/Pt MWNTs/SPE MWNTs/GCE MWNTs/CILE
It DPV DPV ASV DPV SWV
0.5–400 0.1–900 0.2–10 0.05–1 0.5–100 0.05–200
0.2 0.087 0.08 0.015 0.2 0.01
[14] [20] [21] [22] [23] This work
Au, gold electrode; MWNTs, multi-walled carbon nanotube; EBNBHCNPE, 2,2 -[1,2ethanediylbis(nitriloethylidyne)]-bis-hydroquinone modified carbon nanotube paste electrode; It, amperometric current–time response curve; DPV, differential pulse voltammetry; Pt, platinum electrode; SWNTs, single-walled carbon nanotube; PA, phytic acid; SPE, screen-printed electrodes; GCE, glassy carbon electrode; CILE, carbon ionic liquid electrode; ASV, adsorptive stripping voltammetry; SWV, square wave voltammetry.
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interfering substances in human blood serum, such as proteins and glucose did not interfere with the determination of DA. The recovery and RSD were acceptable, indicating that the proposed methods could be efficiently used for the determination of DA in practical applications. 4. Conclusion
Fig. 8. Cyclic voltammograms of 200 M AA, 50 M DA and 20 M UA in 0.1 M PBS (pH 7.0) at the MWNTs/CILE. Table 2 Experimental results for the determination of DA in human blood serum. Samples
DA spiking (M)
DA found (M)
Recovery (%)
RSD (%)
1 2 3
1.0 5.0 10.0
1.02 4.89 9.65
102 97.8 96.5
2.2 1.9 1.8
linear range and detection limit by SWVs with a simple electrode preparation procedure. 3.6. Reproducibility, stability and selectivity of the MWNTs/CILE In order to examine the reproducibility of the modified electrode, repeated cyclic voltammetric experiment was run in 10 M DA in 0.1 M pH 7.0 PBS. The relative standard deviation (RSD) was about 3.1% after 50 successive CVs, indicating that the proposed sensor had an excellent reproducibility. After measurement, the modified electrode was cleaned with voltammetric cycles in pH 7.0 PBS to eliminate the adsorption and stored in air. The modified electrode retained 94% of its initial response after 2 weeks. These results showed that the MWNTs/CILE was stable and the voltammetric studies could be replicated for the sensitive determination of DA. Ascorbic acid (AA) and uric acid (UA) are the main coexisting substance in real samples, so the electrochemical response of DA in the presence of AA and UA on the MWNTs/CILE is investigated. Fig. 8 shows cyclic voltammograms of 200 M AA, 50 M DA and 20 M UA in 0.1 M PBS (pH 7.0) at the MWNTs/CILE. Three independent oxidation peaks at −0.002 V, 0.172 V and 0.292 V were observed at the MWNTs/CILE, indicating that their oxidation took place independently at the MWNTs/CILE. Therefore, the modified electrode was suitable for the selective detection of DA. 3.7. Analytical applications In order to examine the reliability of the proposed sensor in practical applications, it was applied for the determination of DA in human blood serum. The results are shown in Table 2. Some other
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