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Sensors and Actuators B 140 (2009) 51–57

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

etection of nitrite using poly(3,4-ethylenedioxythiophene) modified SPCEs

hia-Yu Lina, V.S. Vasanthaa, Kuo-Chuan Hoa,b,∗

Department of Chemical Engineering, National Taiwan University, Taipei 10617, TaiwanInstitute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

r t i c l e i n f o

rticle history:eceived 15 September 2008eceived in revised form 12 April 2009ccepted 24 April 2009vailable online 5 May 2009

eywords:

a b s t r a c t

The poly(3,4-ethylenedioxythiophene) (PEDOT)- and PEDOT/multi-wall carbon nanotubes-(PEDOT/MWCNTs) modified screen-printed carbon electrodes (SPCEs) were fabricated and theircatalytic properties towards nitrite were studied. Due to the electrostatic interaction between thenegatively-charged nitrite ions and the positively-charged PEDOT film, the operating potential for nitriteoxidation was shifted about 160 mV to negative side, compared to bare SPCE, as a PEDOT film wasdeposited on the SPCE. The diffusion coefficient obtained from RDE experiment is 2.05×10−5 cm2 s−1.

mperometic detectionulti-wall carbon nanotubes (MWCNTs)itriteoly(3,4-ethylenedioxythiophene)creen-printed carbon electrodes (SPCEs)

The electron transfer coefficient (˛) was increased from 0.515 to 0.615 as the sensing electrode waschanged from PEDOT-modified to PEDOT/MWCNTs-modified electrode. Therefore, PEDOT/MWCNTscomposite shows the superior catalytic property towards nitrite and the operating potential was furthershifted about 100 mV to the negative side. The sensitivity and limit of detection (LOD) for the PEDOT- andPEDOT/MWCNTs-modified SPCEs are about 100 mA cm−2 M−1, 1.72 �M and 140 mA cm−2 M−1, 0.96 �M,respectively. The possible interferences from several common ions were tested. The developed sensor

eterm

was also applied to the d

. Introduction

Nitrite is present ubiquitously in soils, waters, foods and physi-logical systems and has been reported as a human health-hazard.he excess uptake of nitrite would cause gastric cancer [1] and blueody [2]. Therefore, it is necessary to develop a reliable and sensitiveensor to detect nitrite in food, drinking water and environmentalamples.

Several techniques have been developed for nitrite determi-ation, including spectrophotometry [3], chemiluminescence [4],hromatography [5] and capillary electrophoresis [6]. However,hese determination methods usually have tedious detection pro-edures and therefore are time-consuming. Compared to theseethods, the electrochemical methods can provide cheaper, faster

nd real-time analysis and thus have attracted more atten-ion. The electrochemical oxidation of nitrite usually involves aarge overpotential at the surfaces of the bare electrodes, andherefore, the determination of nitrite tends to suffer inter-

erences from other more oxidizable compounds. To overcomehese problems, some electrochemically modified electrodesased on porphyrin [7], Pt nanoparticles [8], metallophthalocya-ine [9], and Nafion®/lead-ruthenate pyrochlore [10] have been

∗ Corresponding author at: Department of Chemical Engineering, National Taiwanniversity, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040.

E-mail address: kcho@ntu.edu.tw (K.-C. Ho).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.04.047

ination of nitrite concentration in tap water sample.© 2009 Elsevier B.V. All rights reserved.

explored to lower the operating potentials for nitrite oxida-tion.

Since the discovery of carbon nanotubes (CNTs), they haveattracted more and more attention due to their excellent elec-trical conductivity, chemical stability, high surface area and highmechanical strength [11]. Consequently, CNTs have been used asan electrode material or modifier to promote electron transferreactions between biomolecules and the underlying electrodes[12]. Recently, the electrocatalytic oxidation of nitrite on carbonnanotubes powder microelectrodes has been reported [13]. Onthe other hand, poly(3,4-ethylenedioxythiophene) (PEDOT) hasreceived significant amount of attention as an electrode materialin light emitting devices, electrochromic windows, polymer batter-ies, etc. [14–15]. In addition, the PEDOT-modified electrodes havealso been used for detection of dopamine [16], ascorbic acid [17],pesticides [18], and cysteine [19] recently.

Since the PEDOT film has been found to be in oxidized formwith high stability and conductivity at physiological pH [16], weselected the PEDOT film with the expectation of forming elec-trostatic interaction between PEDOT and nitrite ions. Besides, thecatalytic property and conductivity of PEDOT-modified electrodetowards nitrite can be enhanced by the application of MWC-

NTs undercoat layer before the PEDOT film formation. In thisstudy, PEDOT/MWCNTs-modified electrode has been fabricated byelectropolymerization of 3,4-ethylenedioxythiophene (EDOT) onMWCNTs-modified SPCE electrode by cyclic voltammetry. Cyclicvoltammetry technique was used to study the catalytic oxidation

5 d Actuators B 140 (2009) 51–57

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f nitrite on PEDOT/MWCNTs-modified electrode. The mechanismor the electrochemical oxidation of nitrite anion was studied bysing a rotating disk electrode. The effects of PEDOT film thickness,mount of MWCNTs and pH value of the electrolyte on the oxi-ation current response of PEDOT/MWCNTs/SPCEs to nitrite haveeen examined and discussed.

. Experimental

.1. Chemicals and instruments

(2-Hydroxypropyl)-�-cyclodextrin (H-�-C) and 3,4-thylenedioxythiophene (EDOT) were purchased from Aldrichnd used as received. 0.1 M solution of nitrite was preparedefore every experiments by direct dissolution of sodium nitriteRiedel-de Haën) in deionized water. Multiwall carbon nano-tubesMWCNTs) were purchased from Nanotech Port Co. (Taiwan) andhese MWCNTs were produced via the chemical vapor depositionCVD) method. The diameter and the length of the MWCNTs are0–50 nm and 3–5 �m, respectively. Other chemicals were ofnalytical grade and used without further purification. Deionizedater was used throughout the work.

Electrochemical measurements were carried out at a CHI 440lectrochemical workstation (CH Instruments, Inc., USA) with aonventional three-electrode system. A three-electrode type ofcreen printed carbon electrode (SPCE, Zensor R&D, Taiwan), witheometric area of 0.071 cm2 was used for the sensor preparation.he working electrode, reference electrode and counter electrodef the SPCE are carbon electrode, Ag/AgCl electrode and Pt wire,espectively. All electrochemical experiments were performed atoom temperature and all the potentials are reported vs. theg/AgCl. The rotating disk electrode (RDE) experiments were per-

ormed by a Pine Instruments Co. electrode with a Pt disk of 0.5 cmn diameter in conjunction with a CH Instruments CHI-660 poten-iostat that was connected to a model AFMSRX analytical rotator.

The microstructure of the modified electrode was examined byfield emission scanning electron microscopy (FESEM, JEOL JSM-700F, Japan).

.2. Preparation of PEDOT-modified electrode

The electropolymerization of EDOT, onto three-electrode typePCE, was carried out by cyclic voltammetric method in aqueousolution containing 0.01 M EDOT, 0.5 mM (2-hydroxypropyl)-�-yclodextrin and 0.1 M LiClO4 between 0 and 0.95 V at a scan ratef 50 mV/s for 1 to 4 cycles. After polymerization, the electrode wasreated with 0.1 PBS (pH 7.0) solution by repeated cycling in theotential range of 0.3–0.9 V at the scan rate of 25 mV/s to obtain atable background.

.3. Preparation of PEDOT/MWCNTs-modified electrode

A MWCNTs suspension was prepared by dispersing MWCNTs inimethylformamide with the aid of ultrasonic oscillation for severalours. Experimentally, it was found that MWCNTs can be dispersedell in DMF solvent as long as the concentration of MWCNTs is

qual to or below 2 mg/ml. Thus, we chose 2 mg/ml as the suitableoncentration for dispersion. After preparation, 2 �l of the MWCNTsuspension was drop-casted on the surface of the electrode and

he electrode was then dried at 60 ◦C. To optimize the amount of

WCNTs on the electrode, the above-mentioned procedure wasepeated for several times. After deposition of the MWCNTs, theEDOT film was then electrodeposited by the cyclic voltammetricethod mentioned in Section 2.2.

Fig. 1. Cyclic voltammograms of the bare SPCE and SPCE with electropolymerizationof the PEDOT film for 1, 2, 3, and 4 cycles in 0.1 M PBS (pH 7) solution containing1 mM nitrite. Scan rare: 25 mV/s.

2.4. Amperometric detection of nitrite

For detection of nitrite by using PEDOT-modified electrode, asuitable sensing potential in the limit current plateau region wasdetermined between 0.47 and 0.8 V by the linear sweep voltamme-try at a scan rate of 0.3 mV/s in the solution containing deaerated0.1 M PBS (pH 6.1) and 1 mM nitrite. This suitable sensing potentialwas determined as 0.7 V. The Current densities in the concentrationrange between 50 �M and 1.6 mM were collected and calibrationcurve for nitrite was constructed.

For detection of nitrite by using PEDOT/MWCNTs-modifiedelectrode, a suitable sensing potential in the same way as PEDOT-modified electrode, except that the scan rate and scan range wereset as 0.5 mV/s and 0.3–0.8 V, respectively. The suitable sensingpotential was determined as 0.6 V.

3. Results and discussions

3.1. Electrooxidation behavior of nitrite on the PEDOT-modifiedelectrode

Fig. 1 shows the cyclic voltammetric response of bare SPCE andPEDOT-modified SPCE in the presence of 1 mM NaNO2 at a scanrate of 25 mV/s. As it can be found that an increased oxidation cur-rent density was observed and the oxidation potential of nitriteshifted about 160 mV to the negative side as the PEDOT film wasdeposited on the SPCE. The results obtained can be attributed to theelectrostatic attraction between the oxidized PEDOT film and thenegatively-charged nitrite anions that resulted in preconcentrationof nitrite ions at the electrode surface/solution interface. To find theoptimal thickness of PEDOT, we controlled the thickness in terms ofthe cycle number during electropolymerization. As shown in Fig. 1,the oxidation potential of nitrite was shifted to more noble potentialas the cycle number is greater than 2. While the electrostatic inter-action was enhanced by increasing the thickness of PEDOT film, thediffusion barrier for nitrite increased simultaneously, and thereforethe advantageous effect of electrostatic interaction between PEDOTwas greatly reduced at greater film thickness. Therefore, the thick-

ness obtained at 2 cycles was found to be optimal thickness forelectrocatalytic oxidation of nitrite on PEDOT film.

The effect of solution pH on the current response to 1 mM nitritein 0.1 M PBS solution at pH 6.1, 7 and 8.1 was also examined. Asshown in Fig. 2, the peak potential shifted slightly to positive side

C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57 53

FfiS

apoo

2

2

siAtitwp

ncco

Fs2

Fig. 4. RDE voltammograms of the PEDOT-modified SPCE in 0.1 M PBS solution

ig. 2. Cyclic voltammograms of the SPCE with electropolymerization of the PEDOTlm for 2 cycles in 0.1 M PBS solution, with different pHs, containing 1 mM nitrite.can rare: 25 mV/s.

s the solution pH increased. According to Guidelli [20], the wholerocess for nitrite oxidation involves two steps, the electrochemicalxidation of nitrite into NO2 followed by rapid disproportionationf NO2 into NO2

− and NO3− as shown as Eqs. (1) and (2):

NO2− � 2NO2+2e− (1)

NO2+H2O → NO3−+NO2

−+2H+ (2)

The electro-oxidation of nitrite into NO2 is the rate-determiningtep which is proton independent, and therefore, the slight changen peak potential can be attributed to the nature of the PEDOT film.s the pH increased, the PEDOT film will become dedoped and thus

he electrostatic interaction will slightly decrease which resultingn slight increase in peak potential for nitrite oxidation. Althoughhe maximum oxidation current density can be obtained at pH 7.0,e chose solution pH of 6.1 for later experiments due to lower peakotential for nitrite oxidation.

Fig. 3 depicts the cyclic voltammograms for the oxidation ofitrite at the PEDOT-modified SPCE in the PBS solution (pH 6.1)ontaining 1 mM nitrite at various scan rates. Since the anodic peakurrent density of nitrite increases linearly with the square rootf the scan rate, the oxidation of nitrite at the PEDOT/SPCE was

ig. 3. Cyclic voltammograms of the PEDOT-modified SPCE in 0.1 M PBS (pH 6.1)olution containing 1 mM nitrite at various potential scan rates from 5, 10, 15, 20,5, 50, 75 and 100 mV/s. Inset: Jpa vs. scan rate.

(pH 6.1) containing 2.8 mM nitrite at different rotation rate ranging from 800 to1800 rpm. The inset shows a plot of the reciprocal of the limiting current density(Jl,a) vs. the reciprocal of (rotation rate)1/2. Electrode = Pt disk. Scan rate = 10 mV/s.

diffusion-controlled. In addition, the electron transfer coefficient(˛) can be obtained according to the Eq. (3) [21]:

(1 − ˛)n˛ = 47.7mVEp − Ep/2

(3)

where n� is the number of electron transfer involved in therate-determining step, all other parameters have their conven-tional meanings. The estimated value of (1 − ˛)n� was found to be0.97 ± 0.02. Since the electro-oxidation of nitrite into NO2, involv-ing 2 electrons transfer, is the rate-determining step [20], thereforethe values of n� and ˛ were estimated as 2 and 0.515, respectively.

3.2. Rotating disk voltammetry at the PEDOT-modified electrode

Fig. 4 portrays the electro-catalytic oxidation of nitrite at dif-ferent rotation speeds at the PEDOT/GC-modified rotating discelectrode in phosphate buffer solution (pH 6.1). The Kuotecky-Levich plot (the inset) that obtained for the voltammograms in the

Fig. 4 was found linear with square-root of different rotation rate.Besides, the plot did not pass through the origin suggesting that akinetic limitation is involved in the electron transfer reaction.

Fig. 5. SEM image of the surface of the PEDOT/MWCNTs-modified SPCE.

54 C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57

Fig. 6. (A) Cyclic voltammograms of PEDOT- and PEDOT/MWCNTs-modified SPCEin PBS solution (pH 6.1) containing 1 mM nitrite ions. Cycle number for electropoly-merization of PEDOT = 2. (B) Cyclic voltammograms of PEDOT/MWCNTs-modifiedSPCE with various layers of MWCNTs. Cycle number for electropolymerization ofPEDOT = 2. (C) Cyclic voltammograms of the MWCNTs-modified SPCE with elec-tropolymerization of the PEDOT film for 1, 2, 3, and 4 cycles in 0.1 M PBS (pH 6.1)solution containing 1 mM nitrite. Scan rare: 25 mV/s.

Fig. 7. Cyclic voltammograms of the PEDOT/MWCNTs-modified SPCE at differentpHs with PEDOT film electropolymerized for 2 cycles and 6 layers of MWCNTs in0.1 M PBS solution containing 1 mM nitrite. Scan rare: 25 mV/s.

The diffusion coefficient can then be calculated from the slopeof the Levich plot, shown as Eq. (4) [21]:

1j

= 1jK

+ 1jl,a

= 1jK

+ 10.62nFD2/3ω1/2�−1/6CO

(4)

where jK, jl,a, D, �, ω and CO are current density in the absenceof any mass-transfer effect, limiting current density, the diffusionco-efficient, the kinematic viscosity, the rotation speed and thebulk concentration of nitrite in the solution, respectively, and allother parameters have their conventional meanings. The numberof the electron transfer and concentration of nitrite used are 2 and2.8 mM, respectively. The value of diffusion coefficient is found to be2.05 × 10−5 cm2 s−1 which is close to that reported in the literature[22].

3.3. Electro-oxidation behavior of nitrite on thePEDOT/MWCNTs-modified electrode

Since the catalytic property of CNTs toward the electro-oxidation of nitrite has been proven previously [13], thePEDOT/MWCNTs electrode should have superior catalytic property

Fig. 8. Cyclic voltammograms of the PEDOT/MWCNTs-modified SPCE in 0.1 M PBS(pH 6.1) solution containing 1 mM nitrite at various potential scan rates from 6.25,12.5, 25, 50, and 100 mV/s. Inset: Jpa vs. scan rate.

C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57 55

Fig. 9. The amperometric current responses of (A) the PEDOT-modified SPCE, and (B)tsSc

fPdo

Table 2Effect of foreign ions on the amperometric detection of nitrite (1 mM).

Ions added Relative current responsea (%) at differentmolar ratios of ([nitrite]:[added ions])

1:1 1:10

SO32− 112 ± 3.7 400 ± 3

SO42− 97 ± 4.5 103 ± 2

NO3− 98.9 ± 5 100 ± 3.6

Cl− 99.8 ± 3.8 96.8 ± 2K+ 100 ± 3.6 97.8 ± 3.5Na+ 101 ± 1.6 94.6 ± 1.6Mg2+ 97 ± 4.5 103 ± 2

TA

T

CHTPCCHOP

he PEDOT/MWCNTs-modified SPCE to various nitrite concentrations in 0.1 M PBSolution (pH 6.1). The applied potentials for PEDOT- and PEDOT/MWCNTs-modifiedPCEs are 0.7 and 0.6 V vs. Ag/AgCl, respectively. The insets show the calibrationurves for the oxidation current density of nitrite vs. nitrite concentration.

or the electrochemical process of nitrite oxidation. To fabricatedEDOT/MWCNTs film on the SPCE, a layer of MWCNTs was firstrop-coated on the surface of SPCE before the electro-depositionf the PEDOT film (2 cycles). As shown in Fig. 5, the MWCNTs dis-

able 1partial list of literatures on the electrochemical nitrite sensing using carbon nanotubes.

ype of the electrode Performance

Sens.g (mA M−1 cm−2)

NTPMEa –3PMo12O40/PPyb/MWCNTs/Au 1214hionine/MWCNTs/Au –ANIc/MWCNTs/Au 719.2atalase/MWCNTs/GCEd 222.9hitosan/MWCNTs/GCEd –be/RTILsf/MWCNTs/GCEd 162.5s(bpy)3

2+/CNTPMEc 6800EDOT/MWCNTs/SPCEs 140

a Carbon nanotube powder microelectrode.b Polypyrrole.c Polyanaline.d Grassy carbon electrode.e Hemoglobin.f Room temperature ionic liquids.g Sensitivity.

a Relative response (%) = Initrite+added ions/Initrite, which were obtained from at leastthree repetitive experiments.

tribute uniformly and some of the MWCNTs were not covered bythe PEDOT film.

Fig. 6(A) shows the cyclic voltammetric response of PEDOT-modified SPCE and PEDOT/MWCNTs-modified SPCE in the presenceof 1 mM NaNO2 at a scan rate of 25 mV/s. It can be found thatalthough the current density response decreased, the oxidationpotential shifted to more negative side (∼100 mV) as a layer ofMWCNTs was deposited on the SPCE. Since the roughness of theMWCNTs/SPCE is much higher than that of the bare SPCE, the PEDOTcoverage would be lower on the MWCNTs/SPCE, if the same cyclenumber was applied to both SPCEs. As a consequence, the over-all pre-concentration effect caused by the electrostatic interactionwould be lower, thus resulting in a lower current response.

The amount of MWCNTs was expected to affect the currentresponse of PEDOT/MWCNTs SPCE to nitrite, and therefore theeffect of the amount of MWCNTs, controlled by repeating drop-coating procedure for sever times, was studied. As shown inFig. 6(B), the current response increased as the number of MWC-NTs layer was increased. However, as the number of MWCNTslayers exceeded 6 (with a thickness of ∼10 �m for the MWCNTslayer, as determined by the cross-sectional SEM image), the com-posite film became unstable and fall off during the experiment.Furthermore, the effect of the PEDOT layer thickness on the sensingperformance of the PEDOT/MWCNTs-modified SPCE was examinedafter optimizing the layer thickness of MWCNTs. The results areshown in Fig. 6(C), which indicates that the current response ofnitrite decreased as the deposition cycle number was greater than

2. Therefore, the optimal deposition cycle number for PEDOT andthe optimal layer number for MWCNTs were chosen to be 2 and 6,respectively.

Ref.

LOD (�M) Linear range (mM)

8 0.016–150 [13]1.0 0.005–34 [23]1.12 0.003–0.5 [24]1 0.005–15 [25]1.35 0.005–0.085 [26]0.1 0.0005–0.1 [27]0.81 0.004–32 [28]0.1 – [29]0.96 0.05–1 This work

56 C.-Y. Lin et al. / Sensors and Actu

Fig. 10. (A) The calibration curves, monitoring over a period of 7 weeks, for thePvf

Pnmofsfi

n(feo(

determined by reading the absorbance at 540 nm. The results

TT

S

TT

EDOT/MWCNTs-modified SPCEs obtained in 0.1 M PBS solution (pH 6.1) containingarious nitrite concentrations. (B) The time dependence of the normalized sensitivityor the PEDOT/MWCNTs-modified SPCEs. Applied potential: 0.6 V vs. Ag/AgCl.

The effect of solution pH on the current response of theEDOT/MWCNTs-modified SPCE was also examined in 1 mMitrite + 0.1 M PBS solution at pH 6.1, 7.0 and 8.1. Unlike PEDOT-odified SPCE, the PEDOT/MWCNTs-modified one did not show

bvious difference in current response for the solutions with dif-erent pHs, as seen in Fig. 7. Besides, the peak potential slightlyhifted to the positive side as the solution pH was increased, whichollows the same trend as that of the PEDOT-modified SPCE, as seenn Fig. 2.

Fig. 8 shows the cyclic voltammograms for the oxidation ofitrite at the PEDOT/MWCNTs-modified SPCE in the PBS solutionpH 6.1) containing 1 mM nitrite at various scan rates. It can be

ound that the anodic peak current density of nitrite increases lin-arly with the square root of the scan rate, and thus the oxidationf nitrite at the PEDOT/SPCE was diffusion-controlled. The value of1 − ˛)n� can also be extracted according the Eq. (3), and the esti-

able 3he determination of nitrite concentration in tap water sample by comparing the propose

ample/method Actual value (�M) Added (�

ap water/Griess method N. D.a 20ap water/electrochemicaldetection (This work) N. D. 20

a Not detectable.b Relative standard deviation.c Concentration found after adding 20 �M of nitrite/20 �M of nitrite.

ators B 140 (2009) 51–57

mated value of (1 − ˛)n� was found to be 0.77 ± 0.08. Therefore, thevalues ˛ was estimated as 0.615 which is higher than that obtainedby using PEDOT-modified SPCE, indicating that the electron trans-fer rate was enhanced by incorporating MWCNTs into the PEDOTfilm.

3.4. Amperometric detection of nitrite

The current densities of the PEDOT- and PEDOT/MWCNTs-modified SPCE as a function of the nitrite concentration with asampling time of 200 s at each concentration level were measuredand shown in Fig. 9(A) and (B), respectively. The current den-sity increases linearly with the increased nitrite concentration inboth modified electrodes. The sensitivity of the PEDOT/MWCNTs-modified SPCE is 140 mA cm−2 M−1, which is 1.4 times higher thanthat of the PEDOT-modified SPCE. The limit of detection (LOD),based on signal-to-noise ratio of 3, for PEDOT and PEDOT/MWCNTs-modified electrode are 1.72 �M and 0.96 �M, respectively. Table 1is a partial list of literatures on the electrochemical nitrite sens-ing using carbon nanotubes. It is noted that the limit of detectionobtained in this study based on the PEDOT/MWCNTs-modified SPCEis comparable with or lower than that obtained with other CNT-based modified electrodes [13,23–29]. Besides, the LOD for ourproposed method is below the maximum admissible level (∼3 �M)of nitrite in drinking water established by the European Commu-nity [30]. This implies that our proposed method is applicable forthe detection of nitrite in drinking water.

Fig. 10 shows the stability data for the PEDOT/MWCNTs-modified SPCEs. All modified electrodes used here were prepared atthe same time and stored in a desiccator with a relative humidity of40% at room temperature before use. The reproducibility of the pro-posed sensor, evaluated in terms of the relative standard deviation(n = 3), was determined to be 4.4% (data not shown). The stabilitydata for a typical PEDOT/MWCNTs-modified electrode were col-lected once every week or two weeks. A loss in the sensitivity of17% was observed after 49 days.

3.5. The interference studies

The interference effect of the foreign ions on the determinationof nitrite was examined by addition of various ions into the PBSsolution (pH 6.1) containing 1 mM nitrite and the results are sum-marized in Table 2. It can be found that most ions did not showany interference effect during the determination of nitrite by usingPEDOT/MWCNTs-modified electrode. However, 10-fold amount ofSO3

2− ion shows serious interference.Finally, the PEDOT/MWCNTs-modified SPCE was applied to

determine nitrite concentration in tap water sample. The Griessmethod was used for comparison. Equal volumes of 1X Griessreagent (Sigma–Aldrich) and tap water sample were mixed andincubated for 15 min. Thereafter, the concentration of nitrite was

obtained by both our method and the Griess method were sum-marized in Table 3. It can be found that the results obtained by ourmethod are in good agreement with those obtained by the Griessmethod. It should be noted that the original nitrite concentration

d method and the Griess method.

M) Value found after adding RSDb (%, n = 3) Recoveryc (%, n = 3)

21.2 2.3 10620.5 2.9 102.5

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C.-Y. Lin et al. / Sensors an

s undetectable by both methods, so we added 20 �M nitrite inape water sample and recorded the results as “value found afterdding”.

. Conclusion

The PEDOT- and PEDOT/MWCNTs-modified SPCEs were fab-icated and their catalytic properties towards nitrite werenvestigated. With modification of PEDOT and PEDOT/MWCNTs,he operating potential can be reduced about 160 and 260 mV,espectively, compared to that of bare SPCE electrode. The sensitiv-ty and limit of detection for PEDOT/MWCNTs-modified SPCE are40 mA cm−2 M−1 and 0.96 �M, respectively. The lower operatingotential and practical detection limit allows the PEDOT/MWCNTs-odified electrode for practical use.

cknowledgements

This work was sponsored by the National Research Council ofaiwan, the Republic of China, under grant number NSC 96-2220--006-015.

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Biographies

Chia-Yu Lin received his B.S. degree in Chemical Engineer-ing from National Cheng Kung University, Tainan, Taiwan,in 2003. He received his M.S. degree in Chemical Engi-neering from National Taiwan University, Taipei, Taiwanin 2005. Now, he is a Ph. D. student in second grade inChemical Engineering at National Taiwan University. Hisresearch interest mainly surrounds nanomaterials for gassensing applications.

V. S. Vasantha received B.S. in Chemistry in the year1985 at Madurai Kamaraj University and M.S. in Indus-trial Chemistry with specialization of electrochemistry inthe year 1988, at Alagappa University. She was awardedPh. D. in electrochemistry by Alagappa University whileworking as Senior Research Fellow at CECRI, in the year1994. She joined as a Research Associate and workedtill 2000 in conducting polymers modified electrodes atCECRI. She was engaged in teaching of Chemistry forthree years in the Junior college. She joined in NationalTaipei University of Technology as Post Doctoral Fellowin the year 2003 and the National Taiwan University in

the year 2005. Currently, Associate Professor at Alagappa University, Karaikudi inIndia.

Kuo-Chuan Ho received B.S. and M.S. degrees in Chem-ical Engineering from National Cheng Kung University,Tainan, Taiwan, in 1978 and 1980, respectively. In 1986,he received the Ph. D. degree in Chemical Engineering atthe University of Rochester. The same year he joined PPGIndustries, Inc., first as a Senior Research Engineer andthen, from 1990 until 1993, as a Research Project Engi-neer. He has worked on the electrochemical properties ofvarious electrode materials, with emphasis on improvingthe performances of sensor devices. Following a six-year

Associate Professor in the Chemical Engineering Department. In 1994, he moved tothe Department of Chemical Engineering at National Taiwan University. Currently,he is a Professor jointly appointed by the Department of Chemical Engineering andInstitute of Polymer Science and Engineering at National Taiwan University.