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Electrochimica Acta 56 (2011) 10032– 10038

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

lassy carbon electrode modified with a bilayer of multi-walled carbon nanotubend polypyrrole doped with new coccine: Application to the sensitivelectrochemical determination of Sumatriptan

aeed Shahrokhiana,b,∗, Zahra Kamalzadeha, Reyhaneh-Sadat Saberia

Department of Chemistry, Sharif University of Technology, Tehran 11155-3516, IranInstitute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 23 April 2011eceived in revised form 22 August 2011ccepted 25 August 2011vailable online 7 September 2011

eywords:ulti-walled carbon nanotubes

olypyrrole

a b s t r a c t

A promising electrochemical sensor was developed based on a layer by layer process by electro-polymerization of pyrrole in the presence of new coccine (NC) as dopant anion on the surface of themulti-walled carbon nanotubes (MWCNTs) pre-coated glassy carbon electrode (GCE). The modifiedelectrode was used as a new and sensitive electrochemical sensor for voltammetric determination ofsumatriptan (SUM). The electrochemical behavior of SUM was investigated on the surface of the mod-ified electrode using linear sweep voltammetry (LSV). The results showed a remarkable increase (∼12times) in the anodic peak current of SUM in comparison to the bare GCE. The effect of experimentalvariables such as, drop size of the casted MWCNTs suspension, pH of the supporting electrolyte, accu-

ew coccineodified electrode

umatriptanoltammetry

mulation conditions and the number of cycles in the electro-polymerization process on the electroderesponse was investigated. Under the optimum conditions, the modified electrode showed a wide lineardynamic range of 0.02–10.0 �mol L−1 with a detection limit of 6 nmol L−1 for the voltammetric deter-mination of SUM. The prepared electrode showed high sensitivity, stability and good reproducibility inresponse to SUM. This sensor was successfully applied for the accurate determination of trace amountsof SUM in pharmaceutical and clinical preparations.

. Introduction

Carbon nanotubes (CNTs), discovered by lijima in 1991 [1], havettracted the attention of scientists in many research fields suchs chemistry, physics and material science, due to their extraor-inary structural, mechanical, chemical and electronic properties.NTs display high electrical conductivity, high surface area, goodechanical strength, high elasticity and tensile strength and excel-

ent thermal and chemical stability [2–5]. Therefore, CNTs areonsidered as potential nanomaterials for the development of var-ous sensor and biosensor devices [6–8]. Using of CNTs in theabrication of electrochemical sensors and biosensors, as electrode

odifiers, leads to obtaining higher sensitivities, lower limits ofetection, faster kinetics of electron transfer and decreased surfaceouling of the sensors [9].

Conducting polymers (CPs) are conjugated polymers, whichave similar electronic properties (magnetic, conductivity andptical) to metals, while retaining properties of conventional

∗ Corresponding author at: Department of Chemistry, Sharif University of Tech-ology, Tehran 11155-3516, Iran. Tel.: +98 21 66005718; fax: +98 21 66029165.

E-mail address: shahrokhian@sharif.edu (S. Shahrokhian).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.08.102

© 2011 Elsevier Ltd. All rights reserved.

organic polymers [10,11]. In the recent years, CPs with uniqueelectrochemical properties have aroused wide interests in the fabri-cation and development of electrochemical sensors and biosensors[11–16]. Many researchers have reviewed various applications ofCPs including chemical and biological sensors [10,17–22]. AmongCPs, polypyrrole (PPY) is one of the most extensively studied poly-mers and is considered to be the most promising material for thedevelopment of sensor devices [23,24]. This is because of its uniqueproperties such as, high conductivity, stability in ambient condi-tions, facile preparation and efficient polymerization at neutral pH,ease of formation from aqueous solutions, thickness controllabilityand good reversibility between its conducting and insulating states.

In the last years, composite materials based on the coupling ofCPs and CNTs with the aim of combining the properties of thesematerials, have been intensively studied in a large number of pub-lications [25,26]. These composite materials have been shown topossess complementary properties of the individual componentswith a synergistic effect [25]. The desirable merging of propertiesof CPs such as reproducibility, good stability, strong adherence,

high number of active sites and homogeneity in electrochemicaldeposition, together with high surface area and nanoporosity ofCNT films, leads to a superior performance of the resulting sensingdevices [27]. There are two electrochemical approaches which are

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ommonly used to prepare the composites of CNTs and CPs. Ones the electro-deposition of a polymer film on an electrode already

odified by the adsorbed CNT coatings [14–16,28]. The other is theolymerization of a monomer in the presence of negatively chargedNTs, acting as dopant [25,29,30].

Sumatriptan succinate (SUM) is a highly selective 5-ydroxytryptamine (5-HT) receptor agonist. It is a triptan drughich is effectively used in the treatment of migraine and clustereadache attacks [31]. There are two theories that explain theechanism of action of this agonist. One theory suggests that SUM

electively binds to serotonin type-1D receptors and activateshem, which results in vasoconstriction of extensively dilatedranial blood vessels and subsequent relief of headache. The otherheory suggests that SUM exerts its effect by inhibiting the releasef neurogenic inflammatory mediators [32]. SUM is administeredn several dosage forms including products for nasal, oral andectal delivery [33].

Several methods have been applied for the analytical deter-ination of SUM in pharmaceutical and clinical preparations

uch as, spectrophotometry [34,35], capillary electrophoresis [36],iquid chromatography–mass spectrometry (LC–MS) [37], liq-id chromatography–electrospray ionization-mass spectrometryLC–ESI-MS) [38], liquid chromatography–atmospheric pressurehemical ionization-mass spectrometry (LC–APCI-MS) [39], liq-id chromatography–tandem mass spectrometry (LC–MS/MS)40–42], high performance liquid chromatography (HPLC) withltraviolet (UV) detection [32,43], HPLC with fluorescence detec-ion [44], HPLC with electrochemical detection [45,46] andlectrochemical detection using voltammetric techniques [47,48].pectrophotometric and chromatographic methods usually haveome disadvantages such as, lengthy and tedious sample prepa-ation, expensive instrumentation, long analysis times and lowensitivity and selectivity. In comparison, electrochemical detec-ion methods using electrochemical sensors and biosensors havehown remarkable advantages such as, high sensitivity, inexpen-ive equipment and simple surface modification of electrodes. SUMs an electroactive compound that can be oxidized electrochemi-ally. To the best of our knowledge, only one work can be foundn the literature which has applied a modified electrode for theetermination of SUM [48].

In the present work, a nanostructured thin film (as a modi-er) is prepared on the surface of glassy carbon electrode (GCE)y coating it with a thin layer of multi-walled carbon nanotubesMWCNTs) and electro-deposition of pyrrole in the presence ofew coccine (NC) as dopant anion on the MWCNTs layer. Surfaceorphology of the modified electrode was investigated by scan-

ing electron microscopy (SEM) which showed a porous thin filmith high microscopic surface area on the electrode surface. An

ffective electrochemical method with nanomolar detection limitsing this modified electrode was described for the determinationf SUM. The results of voltammetric studies exhibit a considerablemprovement in the oxidation peak current of SUM. The prepared

odified electrode with important advantages such as, low detec-ion limit, high sensitivity and relatively wide linear dynamic rangeas successfully applied for voltammetric determination of SUM inharmaceutical preparations and human blood serum samples.

. Experimental

.1. Chemicals and reagents

SUM in the form of sumatriptan succinate (>99.0% purity) wasurchased from Natco India (Scheme S1, Supplementary Materials).ulti-walled carbon nanotubes (MWCNTs, purity > 95%) were pre-

ared from Nanostructured & Amorphous Materials (USA). NC was

Acta 56 (2011) 10032– 10038 10033

purchased from Acros Organics (France). Pyrrole and dimethylfor-mamide (DMF) were purchased from Merck. All other chemicalswere of analytical reagent grade from Merck. All aqueous solu-tions were prepared with doubly distilled deionized water. Stocksolutions of SUM were freshly prepared as required in appropri-ate buffer solution. In all electrochemical experiments, a stockBritton–Robinson (BR) buffer solution (containing 0.04 mol L−1 ofglacial acetic acid, orthophosphoric acid and boric acid) was usedas the supporting electrolyte. Buffer solutions of different pHs werethen prepared by the addition of 0.2 mol L−1 sodium hydroxide. Allsolutions were deoxygenated by purging with pure nitrogen gas(99.999% from Roham Gas Company, Iran) prior to each experiment.SUM tablets (50 mg, Hakim Pharmaceutical Co., Tehran, Iran) werepurchased from the local pharmacies. Fresh frozen plasma wasobtained from Iranian Blood Transfusion Organization. The serumsamples were prepared by adding 2% (v/v) of methanol, hard mix-ing, and diluting 10 times with 0.04 mol L−1 BR buffer solutions ofpH 7.4.

2.2. Apparatus

All electrochemical measurements were performed using anAutolab potentiostat/galvanostat model PGSTAT 101. An electro-chemical cell with a three-electrode configuration was used witha glassy carbon working electrode (unmodified and/or modified), asaturated Ag/AgCl reference electrode and a Pt wire counter elec-trode.

A digital pH/mV/Ion meter (Metrohm, pH Lab 827) was utilizedfor the pH measurements and preparation of the buffer solutions,which were used as supporting electrolyte in the voltammetricexperiments. The SEM experiments were performed on a HITACHIS4160 (Japan) scanning electron microscope.

2.3. Preparation of the modified electrode

Before modification, the GCE was polished with 0.05 �m alu-mina slurry on a polishing cloth, rinsed thoroughly with waterand sonicated in water for 5 min. An appropriate amount of pureMWCNTs was functionalized under concentrated nitric acid treat-ment process for 24 h in order to obtain more edge sites and betterdispersion of nanotubes. The MWCNTs suspension was preparedby dispersing 1 mg of the functionalized MWCNTs (CNT-COOH) in1 mL of DMF solvent under ultrasonic agitation for 30 min, giv-ing a black dispersion that is quite stable for 3–4 months. Then,a desired volume of the dispersed CNT was taken with a micro-syringe and after casting on the electrode surface, the electrode washeated in oven at 50 ◦C for 30 min. This electrode is recognized asCNT/GCE. The electro-polymerization of the PPY film on the surfaceof CNT/GCE was performed by potential cycling between −0.2 Vand +0.8 V (versus Ag/AgCl) at a scan rate of 50 mV s−1 for a totalof fifteen scans in an aqueous solution containing 5 mmol L−1 pyr-role and 2 mmol L−1 NC as dopant anion. This electrode, named asPPY/CNT/GCE, was used as the modified electrode in the electro-chemical investigations of SUM.

3. Results and discussion

3.1. SEM studies on the electrode surface

SEM was employed to investigate the surface morphology of theCNT/GCE and PPY/CNT/GCE as illustrated in Fig. 1A and B, respec-tively. From Fig. 1A it is clear that the GCE surface is coated with

a homogeneous layer of MWCNTs with the average diameter of70 nm. As can be seen in Fig. 1B, after electrochemical deposition ofNC-doped PPY on the surface of CNT/GCE, the surface morphologyis significantly changed. The SEM images show that MWCNTs act

10034 S. Shahrokhian et al. / Electrochimica Acta 56 (2011) 10032– 10038

f NC-d

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(caoc

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Fig. 1. SEM images of (A) CNT coated GCE, and (B) 15 cycles deposition o

s a nano-sized backbone for PPY polymerization and most MWC-Ts are densely covered with a very thin, porous and homogeneous

ayer of the polymer. The diameter of nanotubes is further increasedo around 109 nm after the electro-polymerization of PPY.

.2. Electrochemical deposition of PPY film

During PPY growth on the electrode surface, NC anionsdopants) participate in the polymerization to balance out positiveharges produced by the oxidation process. The chemical naturend size of the dopant counter ions can have significant influencen the morphology, thickness, stability and loading of the electro-hemically generated polymer film.

In the present work, a new counter ion, NC (Scheme S2, Supple-entary Materials) with three sulfonate groups was used as dopant

uring PPY electro-polymerization. NC is a red azo dye, which iseadily soluble in water. The results of the electrochemical poly-erization of PPY films on the GCE and CNT/GCE are shown in

ig. 2. Using NC as dopant caused to polymer growth with increas-ng current on sequential scans, which indicates an efficient andtable film deposition on the electrode surface. Thus, NC with an

ig. 2. Repetitive cyclic voltammograms for the PPY polymerization reaction on (A)NT/GCE, and (B) GCE in 5 mmol L−1 pyrrole + 2 mmol L−1 NC with 15 continuousyclic scanning at 50 mV s−1.

oped PPY film on the surface of GCE pre-coated with 10 �L of MWCNTs.

aromatic structure and effective �–� interactions with the polymerand also MWCNTs backbone, can successfully serve as an anionicdopant in the electro-deposition process and improved polymerloading on the surface of CNT/GCE. From Fig. 2, it is clear that thegrowth current of PPY on CNT/GCE was much greater than on thebare GCE. Also, it can be seen that the beginning potential of PPY filmgrowth on the CNT/GCE is about 0.5 V, while on the bare electrodethe growth begins at the potential of about 0.6 V. This differenceshows that carbon nanotube electrode has a very high real sur-face area due to its nanometer size. Moreover the surface of carbonnanotube electrode is more active than that of the flat GCE. Hence,lower nucleation energy is needed to initiate the polymerization.

The constructed electrode coated with an adhesive and porousfilm could be successfully used for the sensitive electrochemicaldetermination of SUM. In this work, 15 repetitive cycles in the rangeof −0.2 V to 0.8 V was selected for preparation of PPY film. Longerscans more than 15 cycles resulted in increasing of the thicknessof the polymer film and thus the capacitive background current isincreased, which leads to decrease of the electrochemical responseof SUM on the surface of PPY/CNT/GCE.

3.3. Electrochemical behavior of SUM on the surface of themodified electrode

The electrochemical behavior of SUM was studied by linearsweep voltammetry (LSV) technique at the surface of various elec-trodes. Fig. 3 shows voltammetric responses of 2 �mol L−1 SUM in0.04 mol L−1 BR buffer solution (pH 7.4) on the surface of bare GCE,CNT/GCE and PPY/CNT/GCE, recorded at scan rate of 100 mV s−1

after an accumulation time of 150 s at open circuit. As can be seen inFig. 3, SUM has a very weak voltammetric response with an oxida-tion peak current of 3.5 �A on the surface of bare GCE. However, incomparison to the bare electrode, a remarkable enhancement in thepeak current was observed by a factor of 5 (Ip,a = 17.6 �A) and 11.8(Ip,a = 41.4 �A) at the CNT/GCE and PPY/CNT/GCE, respectively. Theobserved increase in voltammetric response at the PPY/CNT/GCEcan be related to the increased active surface area of the elec-trode due to polymerization and strong adsorption of analyte onthe surface of the modified electrode.

During cyclic voltammetric studies of SUM, no cathodic peakwas observed in the reverse scan, which suggests a totally irre-versible behavior of the electrode process (data not shown).

3.4. Optimization of the amount of carbon nanotube on the

electrode surface

It is well known that in the case of thin film modified electrodes,the current responses toward the analyte species can be affected

S. Shahrokhian et al. / Electrochimica

E vs (Ag/AgCl)/V1.00.90.80.70.60.50.4

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Fig. 3. LSVs of 2 �mol L−1 of SUM on the surface of various electrodes; bare GCE(dotted line), CNT/GCE (dashed line) and PPY/CNT/GCE (solid line). Potential sweeprate was 100 mV s−1 and supporting electrolyte was 0.04 mol L−1 BR buffer solution(

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3.7. Effect of accumulation potential and time

FS

pH 7.4).

y the thickness of the modifier film. Thickness of the modifier filman influence the mass transfer mechanism via diffusion throughhe porous film [49–53]. Thus, the response of the electrode towardUM can be affected by the amount of MWCNTs on the electrodeurface. This can be controlled by using different volumes of a con-tant concentration of MWCNTs suspension (1 mg/mL) casted onhe GCE surface. As can be seen in Fig. 4, by increasing the vol-me of a MWCNTs suspension from 6 to 10 �L, the oxidation peakurrent for 2 �mol L−1 of SUM is increased. Further increase in theolume of suspension from 10 to 14 �L caused a gradual decrease inhe oxidation peak current of SUM while increasing the backgroundurrent. This can be attributed to sluggish mass transfer process ofUM, due to large film thickness of the deposited suspension on

he GCE. Therefore, a volume of 10 �L CNT was used as optimumolume for preparation of the modified electrode.

E vs (Ag/AgCl)/V1.00.90.80.70.60.50.4

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ig. 4. (A) LSVs of 2 �mol L−1 of SUM on the surface of GCE coated with various amountsupporting electrolyte was 0.04 mol L−1 BR buffer solution of pH 7.4 and sweep rate was

Acta 56 (2011) 10032– 10038 10035

3.5. Influence of pH on voltammetric responses

The effect of pH on the voltammetric response of 2 �mol L−1

SUM was studied at different pH values in the range of 2.0–10.0using 0.04 mol L−1 BR buffer as supporting electrolyte (Fig. 5A). Theresults showed that the anodic peak current increased by increas-ing pH from 3 to 7.4 (Fig. 5B). However, further increase in the pH ofthe buffer solution, decreased the peak height. Therefore, BR bufferwith pH 7.4 was selected as supporting electrolyte for obtainingbest sensitivity in all voltammetric determinations. It was foundthat the oxidation peak potential of SUM shifted negatively withincreasing pH, suggesting that H+ participates in the oxidation pro-cess. A good linear relationship was obtained between Ep,a and pHin the range of 3.0–9.0 (Fig. 5C). This relationship can be describedby the following equation:

Ep,a (mV) = −52.41 pH + 1111 (R2 = 0.9939) (1)

Regarding the slope value of −52.41 mV per pH unit, which isclose to the theoretical slope (−59 mV per pH unit), it can be con-cluded that equal numbers of electrons and protons are involved inthe electro-oxidation of SUM on the surface of the modified elec-trode.

3.6. Effect of potential sweep rate

In order to study the effect of potential sweep rate (�) on thepeak current (Ip,a), linear sweep voltammograms (LSVs) of thebuffered solutions containing 2 �mol L−1 SUM were recorded atdifferent potential sweep rates (25–450 mV s−1, Fig. S1, Supple-mentary Materials). The results indicated that the peak currentsvary linearly with the scan rate. The regression equation for thisrelationship is:

Ip,a = 0.048� − 0.874 (R2 = 0.9981; Ip,a/�A, �/mV s−1) (2)

This linear relationship confirms the adsorption-controlledprocess for the electro-oxidation of SUM on the surface ofPPY/CNT/GCE.

In the case of adsorption mechanisms, an accumulation step isusually a simple and effective way to increase the sensitivity of

Modifier amount / µL 16141210864

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of 1 mg/mL of MWCNTs suspension. (B) The plot of Ip,a versus amount of modifier.100 mV s−1.

10036 S. Shahrokhian et al. / Electrochimica Acta 56 (2011) 10032– 10038

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ig. 5. (A) LSVs of 2 �mol L SUM at PPY/CNT/GCE in various pHs (from 2.0 to 10xidation peak current (Ip,a) and (C) the oxidation peak potential (Ep,a) with pH solu

he determination. Accumulation time and corresponding poten-ial are two important parameters for this step, which can affecthe amount of adsorption of SUM at the surface of the modifiedlectrode. The influence of these parameters on the peak currentas investigated in 2 �mol L−1 SUM solution using LSV. As can

e seen in Fig. 6A, by increasing the accumulation time, the peakurrents significantly increased. Before reaching the equilibriumdsorption, the amount of the adsorbed analyte and therefore theesulted peak current can be increased by longer accumulationimes. The oxidation peak current increased until 150 s and thenecame nearly constant at higher accumulation times (Fig. 6B).o, 150 s was chosen as the optimum accumulation time. Theffect of accumulation potential was examined from −0.5 V to.5 V at 150 s accumulation time. It was found that accumulationotential has no influence on the oxidation peak current. Also, in

omparison to the open circuit conditions, it was observed thatccumulation potential had no obvious effect on the peak current.herefore, accumulation of SUM was performed under open cir-uit.

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ig. 6. (A) LSVs of 2 �mol L−1 SUM in 0.04 mol L−1 BR buffer solution (pH 7.4) at PPY-CNT/ccumulation time.

, 3.0, 4.0, 5.0, 6.0, 7.0, 7.4, 8.0, 9.0, 10.0) of buffer solution, dependence of (B) thescan rate 100 mV s−1.

3.8. Voltammetric determinations

The results of voltammetric studies for various concentrationsof SUM on the surface of PPY/CNT/GCE, obtained under the opti-mum experimental conditions, are shown in Fig. 7A. Using the plotof peak currents versus concentrations of SUM, two linear ranges of0.02–2.0 �mol L−1 and 2.0–10.0 �mol L−1 in 0.04 mol L−1 BR buffersolution (pH 7.4) were obtained (Fig. 7B). Linear regression equa-tions for the two regions are:

Ip,a (�A) = 20.719c/�mol L−1 + 0.2608 (R2 = 0.9969) (3)

Ip,a (�A) = 12.007c/�mol L−1 + 16.946 (R2 = 0.9972) (4)

A detection limit (S/N = 3) of 6 nmol L−1 was obtained for SUMusing the first range of the calibration plot.

To the best of our knowledge, two works can be found inthe literature which have applied voltammetric techniques fordetermination of SUM [47,48]. In one of these works, voltam-metric determination of SUM has been carried out at the bare

taccumulation /s

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S. Shahrokhian et al. / Electrochimica Acta 56 (2011) 10032– 10038 10037

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ig. 7. (A) LSVs for various concentrations of SUM in the range of (down to up), 0.alibration curve of peak current versus SUM concentration; scan rate 100 mV s−1.

CE in BR buffer solution of pH 5.0 [47]. A calibration sensitiv-ty of 1.75 �A mol−1 L and a detection limit of 0.5 �mol L−1 werebtained in this work. In another work, a pyrolytic graphite elec-rode modified with MWCNTs decorated with silver nanoparticlesas used for the voltammetric determination of SUM [48]. A sen-

itivity of 8.736 �A �mol−1 L and a detection limit of 40 nmol L−1

ere resulted in this work.In comparison to the previous works [47,48], the present

ork exhibits lower limit of detection and higher sensitivity20.719 �A �mol−1 L) for electrochemical determination of SUMnder human physiological conditions (pH 7.4). Thus, in this

ork we have presented a quite sensitive system with desirable

lectrochemical features by using an easy electrode modificationethod.

E vs (Ag/AgCl)/V1.00.90.80.70.60.50.4

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ig. 8. (A) LSVs for the addition of different concentrations of SUM (from down to up: 0.0ith 0.04 mol L−1 BR buffer solution (pH 7.4) at PPY/CNT/GCE; scan rate 100 mV s−1 and (

�mol L in 0.04 mol L BR buffer solution (pH 7.4) and (B) corresponding linear

3.9. Reproducibility and repeatability evaluations

The reproducibility of the method was evaluated by preparingsix modified electrodes at different days, with the same fabrica-tion procedure. These electrodes were used for the peak currentdetermination of a 2 �mol L−1 SUM solution, under the optimizedconditions. The relative standard deviation (RSD) for the betweenelectrode peak currents was calculated to be 4.1%. In order to studythe repeatability of the response of the modified electrode, peakcurrents of 10 replicate measurements were recorded at a fixedSUM concentration of 2 �mol L−1. The relative standard deviation

based on this replicates was 3.15%. As well, after storing the mod-ified electrode under ambient conditions for two weeks, the peakcurrent response of the electrode for a concentration of 2 �mol L−1

csum /μmol L-1

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y = 12.157x + 42.176 R² = 0.9975

B

, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 �mol L−1) in the real sample solution of SUM dilutedB) the plot of peak current versus added concentration of SUM.

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[50] G.P. Keeley, M.E.G. Lyons, Int. J. Electrochem. Sci. 4 (2009) 794.[51] L. Xiao, G.G. Wildgoose, R.G. Compton, Sens. Actuators B 138 (2009) 524.

0038 S. Shahrokhian et al. / Electroc

UM retained 95.6% of its initial response. This shows the longerm stability of the modified electrode. Therefore, in this work weave fabricated a modified electrode with an excellent repeatabilitynd reproducibility, which can be used for sensitive voltammetriceterminations of SUM.

.10. Analytical application

.10.1. Determination of SUM in pharmaceutical preparationsThe applicability of the proposed method was evaluated by

sing the modified electrode for the determination of the con-ent of SUM in tablet preparations as a real pharmaceutical sample.our tablets containing a labeled value of 50 mg SUM were accu-ately weighed and ground to a fine powder. A desired amount ofhe powdered sample was dissolved in 100 mL of BR buffer solu-ion with pH 7.4 to get a nominal concentration of 50 �mol L−1.hen, a 2.0 mL portion of the resulting solution was transferred to a0 mL volumetric flask and spiked with standard SUM solutions inhe range of 2.0–8.0 �mol L−1. The voltammetric response and cor-esponding calibration plot of peak currents versus concentrationre shown in Fig. 8A and B. The equation of the resulted standardddition-calibration plot was:

p,a (�A) = 12.157c/�mol L−1 + 42.176 (R2 = 0.9975) (5)

According to the obtained equation, it is concluded that theablet matrix does not show any interference with the electrochem-cal analysis of SUM. An amount of 51.47 mg with good accuracy of02.95% and a RSD of 3.8% (n = 4) was found for the analysis of tabletample with a labeled value of 50 mg. Moreover, for accuracy stud-es, recoveries were evaluated in the lower, middle and higher leveloncentrations of the spiked SUM to the pharmaceutical solutions.he recoveries were in the range from 96.60% to 106.17%.

.10.2. Determination of SUM in human blood serumFor preparation of serum samples, 2% (v/v) of methanol was

dded and thoroughly mixed. Then, the serum samples wereiluted 10 times with BR buffer solution (pH 7.4). The diluted serumamples were spiked with standard solutions of SUM in the rangef 0.06–2.0 �mol L−1. The modified electrode was applied for anal-sis of these samples. The results of recovery evaluations showedn average recovery of 97.21% for SUM added to the serum samples.

. Conclusions

In this study, a layer by layer process has been applied to pre-are a new, highly sensitive and effective electrochemical sensor.

new kind of dopant, NC, was successfully applied for the effi-ient polymerization of pyrrole on the CNT/GCE which resultedn formation of a highly stable and uniform modifier film on thelectrode surface. High microscopic surface area of the modifiedlectrode together with effective accumulation of SUM on the sur-ace of the electrode leads to a significant increase in the oxidationeak current of SUM. The modified electrode also has other remark-ble electrochemical advantages such as, high reproducibility, wideinear dynamic range and nanomolar detection limit. Therefore, thisensor has potential to be used for the successful determination ofUM in pharmaceutical and clinical preparations.

cknowledgements

The authors gratefully acknowledge the support of this work byhe Research Council and the Center of Excellence for Nanostruc-ures of the Sharif University of Technology, Tehran, Iran.

[

[

Acta 56 (2011) 10032– 10038

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2011.08.102.

References

[1] D.S. Iijima, Nature 354 (1991) 56.[2] B. Wu, Y. Kuang, X. Zhang, J. Chen, Nano Today 6 (2011) 75.[3] J. Li, E.K. Jessica, M.C. Alan, C. Hua, N.H. Tee, Y. Qi, F. Wendy, H. Jie, M. Meyyappan,

Electroanalysis 17 (2005) 15.[4] J. Wang, Electroanalysis 17 (2005) 7.[5] Y. Zhang, Y. Wen, Y. Liu, D. Li, J. Li, Electrochem. Commun. 6 (2004)

1180.[6] S. Shahrokhian, M. Ghalkhani, M. Adeli, M.K. Amini, Biosens. Bioelectron. 24

(2009) 3235.[7] J. Wang, S.B. Hocevar, B. Ogorevc, Electrochem. Commun. 6 (2004) 176.[8] K.A. Joshi, J. Tang, R. Haddon, J. Wang, W. Chen, A. Mulchandani, Electroanalysis

17 (2005) 54.[9] C.B. Jacobs, M.J. Peairs, B.J. Venton, Anal. Chim. Acta 662 (2010) 105.10] H. Peng, L. Zhang, C. Soeller, J. Travas-Sejdic, Biomaterials 30 (2009)

2132.11] J.C. Vidal, E. Garcia-Ruiz, J.R. Castillo, Microchim. Acta 143 (2003) 93.12] M.A. Rahman, P. Kumar, D.S. Park, Y.B. Shim, Sensors 8 (2008) 118.13] Y.C. Tsai, S.C. Li, S.W. Liao, Biosens. Bioelectron. 22 (2006) 495.14] P. Manisankar, P.L.A. Sundari, R. Sasikumar, S.P. Palaniappan, Talanta 76 (2008)

1022.15] M.D. Shirsat, C.O. Too, G.G. Wallace, Electroanalysis 20 (2008) 150.16] P. Manisankar, P.L.A. Sundari, R. Sasikumar, D.J. Roy, Electroanalysis 20 (2008)

2076.17] S. Nambiar, J.T.W. Yeow, Biosens. Bioelectron. 26 (2011) 1825.18] L. Xia, Z. Wei, M. Wan, J. Colloid Interface Sci. 341 (2010) 1.19] M. Ates, A.S. Sarac, Prog. Org. Coat. 66 (2009) 337.20] D.W. Hatchett, M. Josowicz, Chem. Rev. 108 (2008) 746.21] M. Trojanowicz, Microchim. Acta 143 (2003) 75.22] U. Lange, N.V. Roznyatovskaya, V.M. Mirsky, Anal. Chim. Acta 614 (2008) 1.23] P. Xu, X. Han, B. Zhang, N.H. Mack, S.H. Jeon, H.L. Wang, Polymer 50 (2009)

2624.24] S. Sen, A. Gok, H. Gülce, J. Appl. Polym. Sci. 106 (2007) 3852.25] V. Branzoi, L. Pilan, F. Branzoi, Electroanalysis 21 (2009) 557.26] V. Selvaraj, M. Alagar, K. Sathish Kumar, Appl. Catal. B: Environ. 75 (2007) 129.27] I. Tiwari, K.P. Singh, M. Singh, Russ. J. Gen. Chem. 79 (2009) 2685.28] S. Shahrokhian, E. Asadian, J. Electroanal. Chem. 636 (2009) 40.29] C. Peng, J. Jin, G.Z. Chen, Electrochim. Acta 53 (2007) 525.30] G. Han, J. Yuana, G. Shi, F. Wei, Thin Solid Films 474 (2005) 64.31] S. Villasmil-Sánchez, W. Drhimeur, S.C. Ospino, A.M. Rabasco Alvarez, M.L.

González-Rodríguez, Drug Dev. Ind. Pharm. 36 (2010) 666.32] R.J. Majithiya, J.B. Majithiya, M.L. Umrethia, P.K. Ghosh, R.S.R. Murthy, Ars.

Pharm. 47 (2006) 199.33] M.L. Christensen, R.K. Mottern, J.T. Jabbour, E. Fuseau, J. Clin. Pharmacol. 43

(2003) 721.34] A.B. Avadhanulu, J.S. Srinivas, Y. Anjaneyulu, Indian Drugs 33 (1996) 334.35] D.N. Tipre, P.R. Vavia, Indian Drugs 36 (1999) 501.36] K.D. Altria, S.D. Filbey, J. Liq. Chromatogr. 16 (1993) 2281.37] J. Oxford, M.S. Lant, J. Chromatogr. Biomed. Anal. 496 (1989) 137.38] B.D. Duléry, M.A. Petty, J. Schoun, M. David, N.D. Huebert, J. Pharm. Biomed.

Anal. 15 (1997) 1009.39] D.A. McLoughlin, T.V. Olah, J.D. Ellis, J.D. Gilbert, R.A. Halpin, J. Chromatogr. A

726 (1996) 115.40] A. Tan, P. Hang, J. Couture, S. Hussain, F. Vallée, J. Chromatogr. B 856 (2007) 9.41] K. Vishwanathan, M.G. Bartlett, J.T. Stewart, Rapid Commun. Mass Spectrom.

14 (2000) 168.42] K.N. Cheng, M.J. Redrup, A. Barrow, P.N. Williams, J. Pharm. Biomed. Anal. 17

(1998) 399.43] A. Femenía-Font, V. Merino, V. Rodilla, A. López-Castellano, J. Pharm. Biomed.

Anal. 37 (2005) 621.44] Z. Ge, E. Tessier, L. Neirinck, Z. Zhu, J. Chromatogr. B 806 (2004) 299.45] M. Dunne, P. Andrew, J. Pharm. Biomed. Anal. 14 (1996) 721.46] P.D. Andrew, H.L. Birch, D.A. Phillpot, J. Pharm. Sci. 82 (1993) 73.47] K. Sagar, J.M.F. Alvarez, C. Hua, M.R. Smyth, R. Munden, J. Pharm. Biomed. Anal.

10 (1992) 17.48] M. Ghalkhani, S. Shahrokhian, F. Ghorbani-Bidkorbeh, Talanta 80 (2009) 31.49] I. Streeter, G.G. Wildgoose, L. Shao, R.G. Compton, Sens. Actuators B 133 (2008)

462.

52] C.E. Banks, T.J. Davies, G.G. Wildgoose, R.G. Compton, Chem. Commun. (2005)829.

53] C.E. Banks, R.G. Compton, Analyst 131 (2006) 15.