Highly sensitive amperometric sensor for carbamazepine determination based on electrochemically reduced graphene oxide–single-walled carbon nanotube composite film

Sensors and Actuators B 173 (2012) 274–280

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Highly sensitive amperometric sensor for carbamazepine determination based on electrochemically reduced graphene oxide–single-walled carbon nanotube composite film Binesh Unnikrishnan, Veerappan Mani, Shen-Ming Chen ∗ Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 2 May 2012 Received in revised form 26 June 2012 Accepted 30 June 2012 Available online 11 July 2012 Keywords: Carbamazepine Graphene–SWCNT composite Graphene oxide Electrochemical reduction Amperometry

a b s t r a c t We report a highly sensitive amperometric sensor for the determination of carbamazepine (CBZ) at an electrochemically reduced graphene oxide (ERGO) and single walled carbon nanotube (SWCNT) composite film modified glassy carbon electrode (GCE). The – stacking interactions between ERGO and SWCNT add good stability to the composite film which was confirmed by UV–visible and FTIR spectroscopy. Compared with SWCNT modified electrode, ERGO–SWCNT composite film modified electrode exhibits a 3.2 fold enhancement in peak current for CBZ oxidation. The proposed sensor detects CBZ as low as 29 nM and has a sensitivity of 5.1076 ␮A ␮M−1 cm−2 in the linear range of 50 nM to 3 ␮M. The practicality of this sensor has been evaluated by the determination of CBZ from Tegratol tablets showing good sensitivity and linearity. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbamazepine (CBZ) is an antiepileptic drug widely used in the treatment of nueralgia, seizure disorders, mental depression, etc. [1]. It has been accepted that CBZ is superior in efficacy to antiepileptic drugs such as gabapentin, phenobarbital, primidone, valproate and vigabatrin, though the mode of action is different. In some cases it has also been used in non epileptic treatment and it can act as a mood stabilizing agent [2]. However, CBZ can produce some adverse effects such as neurotoxicity which leads to blurred vision, dizziness, impaired task performance, hypersensitivity and leukopenia [3]. Cases of vertigo and nausea caused by a pharmacodynamic interaction of CBZ with lamotrigine have also been reported [4]. The neurotoxicity of CBZ is highly dependent on the dosage and notably, about 28% of the orally administered CBZ is discharged to the environment through feces [5,6]. Therefore its determination in biological samples, pharmaceutical preparations and environmental samples are of great importance. Though electrochemical methods have several advantages over other conventional analytical methods, only a few reports are available for electrochemical determination of CBZ. The great challenge in the electrochemical determination of CBZ is the electrode

∗ Corresponding author. Tel.: +886 2270 17147; fax: +886 2270 25238. E-mail address: [email protected] (S.-M. Chen). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.088

fouling due to the adsorption of the oxidized products on the electrode surface. Veiga et al. demonstrated that this imperative shortcoming could be overcome by treating the electrode in open circuit potential and rotating at a speed of 1000 rpm for 3 min between each run in voltammetric analysis [7]. Differential pulse adsorptive–stripping voltammetry using horseradish peroxidase modified screen printed carbon electrode displays good selectivity and silver nanoparticles modified electrode shows good sensitivity toward CBZ [8]. Atkins et al. achieved a limit of detection (LOD) of 3.89 × 10−6 M in acetonitrile electrolyte using differential pulse voltammetry (DPV) [9]. Though the afore-mentioned electrode materials displayed good performance toward CBZ sensing, the quest for novel materials is still in progress for achieving highly sensitive and selective determination. Recently, multiwalled carbon nanotube (MWCNT) modified electrode has been demonstrated as an excellent electrocatalyst for the oxidation of CBZ [10]. Graphene is a 2D material [11,12] with excellent electronic [13,14] and mechanical properties. It can be synthesized from graphite, an easily available material through an inexpensive route involving an oxidative treatment and formation of graphite oxide [15]. Graphite oxide can be exfoliated to produce graphene oxide (GO), which possess plenty of oxygen functionalities on its basal plane and edges [16]. These oxygen functionalities make it hydrophilic, thus it can form stable aqueous dispersions [17]. Besides it posses unoxidized aromatic regions which provide hydrophobicity and hence it can also be dispersed in non-aqueous

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solvents [18]. Owing to the various versatile characteristic features including the amphiphilic nature, GO has drawn considerable attention. The large surface area and the folded architecture of GO has promoted it as an attractive material for electrochemical sensor and biosensor applications [19]. Nevertheless, its insulating property limits its electrochemical applications. Therefore to improve the electronic properties of GO, it is reduced to graphene by chemical [20] or electrochemical methods [21]. However, the basal plane of graphene does not exhibit good electrochemical properties [22]. On the other hand, the rolled version of graphene, carbon nanotube (CNT) has excellent mechanical, electrical, catalytic and electrocatalytic properties [23]. Enormous amount of work has been done on the application of MWCNT and single walled carbon nanotubes (SWCNT) for electrochemical applications [24]. Pristine CNTs are highly hydrophobic and as a result it is impossible to prepare its stable aqueous dispersion [25]. Recent studies reveal that GO and CNTs can interact through – stacking to form a stable dispersion of GO–CNT composite [26,27]. The hydrophilic properties of GO in combination with the excellent electronic and antifouling properties of CNTs endorse this composite material for fascinating electrocatalytic applications. To further increase the conductivity of this composite, it was reduced via electrochemical route to prepare electrochemically reduced graphene oxide (ERGO)–SWCNT. Keeping the afore-mentioned advantages in mind, we attempt to employ the ERGO–SWCNT composite as a novel electrocatalyst for the electrocatalytic oxidation of CBZ. The developed GO–SWCNT composite based amperometric sensor displayed enhanced electrocatalytic activity, high sensitivity and low limit of detection toward CBZ.

2. Experimental 2.1. Apparatus The electrochemical measurements were carried out using CHI 611a work station with a conventional three electrode cell using BAS GCE as working electrode (area 0.0706 cm2 ), Ag|AgCl (sat. KCl) as reference electrode and Pt wire as counter electrode. Prior to each experiment, all the solutions were deoxygenated by passing prepurified N2 gas for 15 min. Amperometric (i–t curve) measurements were performed with analytical rotator AFMSRX (PINE instruments, USA). EIM6ex ZAHNER (Kroanch, Germany) was used for electrochemical impedance spectroscopy (EIS) studies. Surface morphological studies were carried out using Field emission scanning electron microscopic (FESEM) JSM-6500F. Fourier Transform Infra-red (FTIR) spectroscopy measurements were carried out using Perkin Elmer spectrum RXI. UV–visible absorption spectroscopy measurements were carried out using Hitachi U-3300 spectrophotometer.

2.2. Reagents and materials Graphite powder (<20 ␮m), SWCNTs (90%, OD: <2 nm, L: <20 ␮m), and carbamazepine (CBZ) were purchased from Sigma–Aldrich. All the chemicals used were of analytical grade and used without further purification. Tegretol tablets with 200 mg CBZ, from Novartis was acquired from a local pharmacy for real sample analysis. 0.1 M phosphate buffer solutions (PBS) were prepared using Na2 HPO4 and NaH2 PO4 . The pH was adjusted using dilute HCl or NaOH. Double distilled water (conductivity ≥ 18 M) was used for all the experiments. All the experimental solutions were deoxygenated with pre-purified N2 gas for 15 min prior to each experiment. The supporting electrolyte used for electrochemical studies was 0.1 M pH 5 phosphate buffer solution (PBS). A stock solution of CBZ (1 mM) was prepared in acetonitrile/water

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mixture (30:70) and stored in a refrigerator at 4 ◦ C when not in use. 2.3. Preparation of GO and fabrication of ERGO–SWCNT composite modified electrode Graphite oxide was synthesized by modified Hummer’s method as reported elsewhere [15,28]. The graphite oxide was then dispersed in deionized water (0.5 mg/mL) and exfoliated to GO by ultrasonication for 2 h [21]. It was then centrifuged at 4000 rpm for 30 min to remove the excess, unoxidized graphite and unexfoliated graphite oxide. The as-obtained GO solution is used as stock solution. To 10 mL of the GO solution, 5 mg pristine SWCNT was added and dispersed by ultrasonicsation for 2 h. A homogeneous solution of GO–SWCNT composite was thus obtained. The loosely bounded SWCNT and excess GO were removed by subjecting it to two centrifugation cycles (30 min each) at 8000 and 14,000 rpm, respectively [27]. Finally, the as-obtained composite material was washed with copious amounts of water and dried. 0.5 mg/mL of GO–SWCNT solution was prepared in water by ultrasonication and it was used for the fabrication of ERGO–SWCNT modified GCEs. Before each experiment, GCE was polished well with Buelher polishing cloth and 0.5 ␮m alumina slurry. The polished GCE surface was washed and ultrasonicated in water for 5 min to remove the adsorbed alumina particles and dried. 5 ␮L of the GO–SWCNT dispersion was drop casted onto clean GCE surface and dried at room temperature to get GO–SWCNT modified GCE (GCE/GO–SWCNT). The GCE/GO–SWCNT was then transferred to 0.1 M PBS solution (pH 5). The GO in the GO–SWCNT composite film was electrochemically reduced to ERGO by applying a constant potential of −1.3 V for 400 s (figure not shown) [21,26]. The electrochemical reduction pattern was in accordance with the reports of Qiu et al. [26] indicating the reduction of oxygen functionalities of GO. The obtained electrode is noted as GCE/ERGO–SWCNT. GO modified GCE (GCE/GO), ERGO modified GCE (GCE/ERGO) and SWCNT modified GCE (GCE/SWCNT) also have been prepared for comparison. 3. Results and discussions 3.1. Characterization of ERGO–SWCNT composite film Fig. 1(A) represents the FESEM image of the ERGO–SWCNT composite film. The surface morphology of the film reveals the well dispersed SWCNTs are adhered well to the ERGO sheets. The wide size distribution of ERGO sheets is assessed by the presence of numerous small ERGO platelets along with fewer larger sheets ranging over few nano meter to sub micro meter range. Several fine nanotube networks have also been distributed widely. The hydrophobic basal plane of ERGO [29] interacts with the sidewalls of SWCNT through – electronic interactions to form a stable composite film [30], which is further confirmed from the UV–visible spectral data provided in Fig. 1(B). The UV visible absorption spectra of GO and GO–SWCNT composite is given in Fig. 1B. GO exhibits two characteristic absorption peaks; a broad absorption peak at 230 nm and a shoulder peak at 280 nm. The peak at 230 nm is due to the –* transition of the C C bonds in the aromatic region of GO sheets and the latter one is due to the n–* transition in the C O bonds of the functional groups. In the absorption spectra of GO–SWCNT, the peak at the 230 nm is shifted to higher wavelength (247 nm), indicating a – stacking interaction between the aromatic basal plains of GO and the SWCNT [27,31]. Whereas the peak at 280 nm (due to n–* transition) does not shift, revealing the composite formation has not occurred via the functional groups.

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Fig. 1. (A) FESEM image of ERGO–SWCNT composite film, (B) UV–visible spectra of GO and GO–SWCNT composite.

The FTIR spectroscopy can provide information about the functional groups present in GO, ERGO and the composite (Fig. 2A). Curve (a) is the FTIR spectrum of GO. A broad peak at around 3413 cm−1 and a small dip at the 3200 cm−1 are due to O H stretching of the intercalated water molecules and O H coupling vibrations of the carboxylic acid group in the GO, respectively [21]. The C O vibrations of the carboxylic acid and carbonyl groups appear at 1730 cm−1 , while the C C (epoxy group) and C C vibration peaks of the unoxidized graphitic domains appear at 1073 and 1623 cm−1 respectively. However, the pristine SWCNT does not show any absorption peak (curve b) over the investigated wavelength range. When the GO forms composite with SWCNT, the characteristic peaks of GO still appears in the FTIR spectrum (curve c) [27]. Notably, when the GO–SWCNT composite is electrochemically reduced to ERGO–SWCNT, the intensity of the absorption peaks of oxygen functionalities are significantly reduced (curve d), indicating an efficient electrochemical reduction of GO. GCE surface modification with GO, ERGO, SWCNT or ERGO/SWCNT changes the double layer capacitance and interfacial electron transfer resistance of the electrode. Electrochemical impedance spectroscopy (EIS) can reveal the interfacial changes due to the surface modification of electrodes [32]. Fig. 2(B) demonstrates the EIS measurements of the modified electrodes represented as Nyquist plots. The semicircle appeared in the Nyquist plot indicates the parallel combination of electron transfer resistance (Ret ) and double layer capacitance (Cdl ) at the electrode surface resulting from electrode impedance [33], while the linear portion represents the diffusion limited process. It can be seen

Fig. 2. (A) FTIR spectra of (a) exfoliated GO, (b) SWCNT, (c) GO–SWCNT and (d) ERGO–SWCNT; (B) EIS recorded for modified GCEs in 5 mM Fe(CN)6 3− /Fe(CN)6 4− in pH 5. Applied AC voltage: 5 mV, frequency: 0.1 Hz to 100 kHz. Inset shows the EIS of GCE/GO and GCE/ERGO.

from Fig. 2(B) that the diameter of the semicircles obtained for various electrodes are different. The inset of Fig. 2(B) is the Nyquist plots of GO and ERGO modified electrodes. GO film exhibits a higher Ret value than ERGO film, indicating the significant decrease in Ret value upon the electrochemical reduction of GO to ERGO. In comparison with bare GCE, the Ret value of GCE/SWCNT decreased considerably, ascribed to the excellent conducting properties of the SWCNT. Compared with GO and GO–SWCNT, ERGO–SWCNT composite film shows the lowest Ret value, suggesting a very good interaction between GO and SWCNT as well the excellent conductance. EIS results also support the evidence for the formation of ERGO–SWCNT composite through – stacking interactions. 3.2. Electrochemical behavior of CBZ at various electrodes Cyclic voltammetric behavior of CBZ at GCE [34] reveals that CBZ exhibits two irreversible anodic peaks at 1.183 and 1.401 V, respectively, in pH 7.4 in the potential range between 0.8 and 1.7 V. The peak at 1.183 V is well defined and stable; hence, it could be well used for electroanalytical purposes. In this work, the cyclic voltammograms were recorded at various electrodes in pH 5 containing 10 ␮M CBZ over the potential range of 0.6–1.3 V (Fig. 3A). At bare GCE and GCE/ERGO feeble anodic peaks are observed for CBZ. Whereas compared with these electrodes, GCE/SWCNT exhibits an irreversible anodic peak at 1.14 V with a peak current (Ipa ) of 54 ␮A. Remarkably, GCE/ERGO–SWCNT exhibits a tremendous increase in sensitivity with Ipa of 173 ␮A (3.2 times than that of GCE). This result

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Fig. 4. (A) Cyclic voltammograms of 10 ␮M CBZ in N2 saturated pH 5 at different scan rates; (a) 25, (b) 50, (c) 75, (d) 125, (e) 150, (f) 175, (g) 225, (h) 250 and (i) 275 mV s−1 . Inset is the plot of log Ipa vs. log ␯, (B) plot of 1/2 vs. Ipa , (C) plot of log  vs. Epa . Fig. 3. (A) Cyclic voltammograms of 10 ␮M CBZ in N2 saturated PBS (pH 5) at (a) bare GCE, (b) GCE/ERGO, (c) GCE/SWCNT and (d) GCE/ERGO–SWCNT. Scan rate: 50 mV s−1 . (B) Cyclic voltammograms of 10 ␮M CBZ at GCE/ERGO–SWCNT in various pH aqueous solutions (N2 saturated). Scan rate: 50 mV s−1 .

reveals the high sensitivity of the composite film modified electrode toward CBZ oxidation. Although high sensitivity is achieved for CBZ by CV, the oxidized products are adsorbed at the electrode surface and decrease the sensitivity of the electrode after each run. Therefore, the removal of the products after each run is necessary in electroanalytical applications [7]. So, amperometric technique is a more efficient method compared to CV, in which the oxidized products are thrown away by the rotating electrode, and helps the analyte to diffuse easily to the electrode surface. Fig. 3(B) shows the cyclic voltammetric response of 10 ␮M CBZ at ERGO–SWCNT composite film in the pH range 1–11. With increase in pH the peak potential shifts toward lower potential and the peak disappears in pH 9 and above. Since the electrode shows maximum performance in pH 5, we conducted all the electrochemical and electroanalytical experiments in pH 5 PBS. The effect of scan rate () on the oxidation of CBZ has been studied by CV in the potential range, 0.7–1.3 V. Ipa increases with the

increase in scan rate. The inset of Fig. 4 shows the plot of log  vs. log Ipa . The dependence of Ipa on the scan rate can be represented as log Ipa = 0.564 log  + 2.642, R2 = 0.982. Where Ipa and  are measured in ␮A and Vs−1 respectively. The slope of 0.564 shows that the anodic process of CBZ taking place at the composite electrode is diffusion controlled [34,35]. The effect of scan rate on the Ip of CBZ oxidation at room temperature can be described by Randles–Sevcik equation as given in Eq. (1) [36]: Ip = 2.99 × 105 n[(1 − ˛)n˛ ]1/2 ACb D1/2 v1/2

(1)

where Ip is the forward peak current, n is the number of electrons exchanged per molecule, ˛ is the electron transfer coefficient, n˛ is the number of electrons involved in the rate-determining step, A is the area of the electrode, Cb is the bulk concentration of CBZ, D is the diffusion coefficient of CBZ and  is the scan rate. From the Eq. (1) and Fig. 4(B), it is clear that Ip increases linearly with 1/2 in the range of 25–275 mV s−1 . It can be described by the following linear regression equation: Ip (␮A) = 569.3 1/2 + 56.01, R2 = 0.978, suggesting that the process is controlled by diffusion. Also, the oxidation peak potential (Epa ) shifts to higher positive potentials with the increase in scan rate. A plot of Epa vs. log  presume a linear

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Table 1 Comparison of analytical parameters of various electrodes. Electrode

Method

pH

Linear range

Limit of detection

Ref.

ERGO–SWCNT GCE Graphite GCE/MWCNT GCE GCE/fullerene-C60 c Au/graphene–AuNp d BnMIM]PF6 /CPE

Amperometry DPV DPV b LSV DPV DPV LSV DPV

5 1 a ACN 6.89 ACN 7.2 ACN 6.8

50 nM–3 ␮M 5 × 10−6 –6 × 10−4 M 8.46 × 10−5 –8.46 × 10−4 M 0.13–1.60 ␮M – 90 nM–10 ␮M 5 × 10−6 –1 × 10−2 M 7.0 ␮M–0.7 mM

29 nM 1.8 ␮M 3.89 × 10−6 mol L−1 40 nM 4 ␮M 16.2 nM 3.03 × 10−6 M 0.98 ␮M

This work [34] [9] [7] [38] [39] [40] [41]

a b c d

Acetonitrile. Linear sweep voltammetry. AuNP is Au nanoparticles. 1-Benzyl-3-methylimidazole hexafluorophosphate modified carbon paste electrode.

relationship, and the regression equation is: (V) = 1.302 + 0.118 log  (Fig. 4C). This result Epa implies that CBZ electrocatalytic oxidation process is chemically irreversible. For the irreversible process, Epa can be described as given in Eq. (2) [36] Epa =



2.303 RT [n(1 − ˛)n˛ F]



log v + K

(2)

where R is the universal gas constant, T is the temperature in Kelvin and F is the Faraday constant. From the slope, (1 − ˛)n˛ has been

calculated to be 0.501. Assuming n˛ = 1 for one electron transfer process, the value of ˛ is obtained as 0.499 for CBZ. 3.3. Amperometric determination of CBZ at ERGO–SWCNTcomposite Fig. 5(A) represents the amperometric response of CBZ at ERGO–SWCNT modified rotating disc electrode (RDE) with surface area of 0.236 cm2 . The experiment was conducted in pH 5 PBS at an applied potential of 1.155 V with a rotation rate of 2500 rpm. CBZ solution was added at regular intervals of time (100 s). For every addition a quick response was observed and electrocatalytic oxidation of CBZ occurs at RDE in a totally mass transfer controlled condition. The inset of Fig. 5 shows the calibration plot (without background current correction). The linear regression equation for dependence of current with concentration of CBZ can be represented as Ip (␮A) = 1.2054 ␮M + 8.5032, R2 = 0.9968. The sensor shows a linear range of 50 nM to 3 ␮M. The sensitivity of the electrode is 5.1076 ␮A ␮M−1 cm−2 . The limit of detection (LOD) and the limit of quantification (LOQ) were calculated as 29 nM and 96 nM respectively. LOD has been calculated from the formula LOD = 3Sb /S and LOQ from LOQ = 10Sb /S, where, Sb is the standard deviation of blank signal and S is the sensitivity of the electrode [37]. A comparison of linear range and LOD of various electrodes reported earlier are given in Table 1. 3.4. Reproducibility and CBZ determination in pharmaceutical preparations To demonstrate the reproducibility of the sensor, relative standard deviation (RSD) of seven individual modified electrodes was determined by measuring the current response obtained in pH 5 containing 10 ␮M CBZ using CV. The RSD % value is obtained as 3.5%, indicating the good reproducibility of the developed sensor. The practical feasibility of the proposed method has been evaluated by determination of CBZ from commercially available Tegratol tablets. 20 tablets (200 mg CBZ/tablet) were powdered well in a mortar using pestle. Then, aliquots of the sample solutions (1 × 10−4 M CBZ) were prepared in pH 5 using calculated amounts of the powdered sample. Amperometry experiments were conducted under the same experimental conditions as mentioned in Section 3.3. The amperomtric response of CBZ in the real sample is presented in Fig. 5(B). The linear relationship between current Table 2 Determination of CBZ in pharmaceutical sample.

Fig. 5. (A) i–t amperometric response of CBZ at ERGO–SWCNT modified rotating disc glassy carbon electrode in N2 saturated pH 5, applied potential: 1.155 V. Rotation speed: 2500 rpm. Inset is the plot of Ipa vs. [CBZ]. B) Amperometric response of real sample with the same conditions as that of A and the inset is the plot of Ipa vs. [CBZ].

No.

Added (␮M)

Found (␮M)

Recovery (%)

1 2 3

0.3 0.6 0.9

0.28 0.57 0.866

94.80 95.1 96.3

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response and concentration of CBZ is given in the inset of Fig. 5(B). The sensor shows a good linear range of 99 nM–2.8 ␮M. The recovery of CBZ in real sample analysis has been calculated for three differernt concentrations and are presented in Table 2. CBZ has a recovery of 94.8–96.3%. 4. Conclusions We have demonstrated the ERGO–SWCNT composite film as a novel sensing platform for the amperometric determination of CBZ in pharmaceutical preparations. The as-synthesized composite have been characterized by FESEM, UV–visible, FTIR and EIS techniques. The developed sensor exhibits promising electrocatalytic activity toward CBZ oxidation with a low detection limit (as low as 29 nM). The high sensitivity, fast response and stable amperometric response reveal its scope for other sensor applications. Acknowledgments This project was supported by the National Science Council and the Ministry of Education of Taiwan (Republic of China). The authors like to thank Dr. Arun Prakash Periasamy for his help throughout this project. References [1] R.H. Mattson, Tricyclic anticonvulsants: efficacy in clinical trials, Epilepsy & Behavior 3 (2002) S9–S13. [2] A.B. Ettinger, C.E. Argoff, Use of antiepileptic drugs for nonepileptic conditions: psychiatric disorders and chronic pain, Neurotherapeutics 4 (2007) 75–83. [3] J.M. Pellock, Tricyclic anticonvulsants: safety and adverse effects, Epilepsy & Behavior 3 (2002) S14–S17. [4] H. Stefan, T.J. Feuerstein, Novel anticonvulsant drugs, Pharmacology & Therapeutics 113 (2007) 165–183. [5] Y. Zhang, S.U. Geissen, C. Gal, Carbamazepine and diclofenac: removal in wastewater treatment plants and occurrence in water bodies, Chemosphere 73 (2008) 1151–1161. [6] M.D.G. García, F.C. Canada, M.J. Culzonia, L.V. Candioti, G.G. Siano, H.C. Goicoechea, M.M. Galerab, Chemometric tools improving the determination of anti-inflammatory and antiepileptic drugs in river and wastewater by solid-phase microextraction and liquid chromatography diode array detection, Journal of Chromatography A 1216 (2009) 5489–5496. [7] A. Veigaa, A. Dordioa, A.J.P. Carvalho, D.M. Teixeira, J.G. Teixeira, Ultra-sensitive voltammetric sensor for trace analysis of carbamazepine, Analytica Chimica Acta 674 (2010) 182–189. [8] M.A.G. Garcia, O.D. Renedo, A.A. Lomillo, M.J.A. Martinez, Electrochemical methods of carbamazepine determination, Sensors Letters 7 (2009) 586–591. [9] S. Atkins, J.M. Sevilla, M. Blazquez, T. Pineda, J.G. Rodriguez, Electrochemical behaviour of carbamazepine in acetonitrile and dimethylformamide using glassy carbon electrodes and microelectrodes, Electroanalysis 22 (2010) 2961–2966. [10] E.L.K. Chng, M. Pumera, Nanographitic impurities are responsible for electrocatalytic activity of carbon nanotubes towards oxidation of carbamazepine, Electrochemistry Communications 13 (2011) 781–784. [11] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [12] A.K. Geim, K.S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183. [13] C.N.R. Rao, A.K. Sood, R. Voggu, K.S. Subrahmanyam, Some novel attributes of graphene, Journal of Physical Chemistry Letters 1 (2010) 572–580. [14] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-dimensional atomic crystals, Proceedings of the National Academy of Sciences of the United States of America 102 (2005) 10451–10453. [15] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, Journal of American Chemical Society 80 (1958) 1339. [16] H. He, J. Klinowski, M. Forster, A. Lerf, A new structural model for graphite oxide, Chemical Physics Letters 287 (1998) 53–56. [17] S. Stankovich, R.D. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff, Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate), Journal of Materials Chemistry 16 (2006) 155–158. [18] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chemical Society Reviews 39 (2010) 228. [19] L. Liu, D. Yu, C. Zeng, Z. Miao, L. Dai, Biocompatible graphene oxide-based glucose biosensors, Langmuir 26 (2010) 6158–6160. [20] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565.

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Biographies

Binesh Unnikrishnan received his B.S Degree in chemistry in 1999 from University of Calicut, Kerala, India. He received M.S Degree in chemistry in 2002 and M.Phil. Degree in chemistry in 2005 from Bharathiar University, Tamilnadu, India. Currently he is a fulltime Ph.D. student in the research group of Dr. Shen-Ming Chen at the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan. His research interests include electrocatalysis using composites of MWCNT, graphene, nanometals, nanometal oxides, study of the direct electrochemistry of redox enzymes and proteins for biosensor and biofuel cell applications.

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B. Unnikrishnan et al. / Sensors and Actuators B 173 (2012) 274–280

Veerappan Mani received his B.S Degree in chemistry from Periyar University, Tamilnadu, India, in 2007 and M.S Degree in chemistry from Bharathidasan University, Tamilnadu, India, in 2009. Currently he is a Ph.D. student in the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan. His research interest mainly focusing on the graphene based composite materials for electrochemical sensor and biofuel cell applications.

Dr. Shen-Ming Chen received his B.S. Degree in Chemistry in 1980 from National Kaohsiung Normal University, Taiwan. He received his M.S. Degree (1983) and Ph.D. Degree (1991) in Chemistry from National Taiwan University, Taiwan. He is currently a professor at the Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan. His current research interests include electroanalytical chemistry, bioelectrochemistry, fabrication of energy conservation and storage devices and nanomaterial synthesis for electrochemical applications. He has published more than 240 research articles in SCI journals.