Journal of Electroanalytical Chemistry 751 (2015) 30–36

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

A sensitive electrochemical nitrate sensor based on polypyrrole coatedpalladium nanoclusters

http://dx.doi.org/10.1016/j.jelechem.2015.05.0261572-6657/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Department of Chemistry, University of Malaya, KualaLumpur 50603, Malaysia.

E-mail address: M_R_mahmoudian@yahoo.com (M.R. Mahmoudian).

M.R. Mahmoudian a,b,⇑, Y. Alias a, W.J. Basirun a, Pei MengWoi a, Farid Jamali-Sheini c, M. Sookhakian d,M. Silakhori e

a Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysiab Department of Chemistry, ShahidSherafat, University of Farhangian, 15916 Tehran, Iranc Department of Physics, Ahwaz Branch, Islamic Azad University, Ahwaz, Irand Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysiae Department of Mechanical Engineering and Advanced Material Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 21 December 2014Received in revised form 18 May 2015Accepted 19 May 2015Available online 19 May 2015

Keywords:Composite materialsNanostructuresElectrical propertiesNitrateSensor

a b s t r a c t

This study examines the synthesis and characterization of polypyrrole coated palladium nanocluster (PdNCs-PPy) composites and their application in nitrate detection at pH = 7.0. The synthesis was performedvia the direct reduction of an aqueous solution of Pd (CH3COO)2 in the presence of pyrrole monomers inNaOH. X-ray diffraction and field emission scanning electron microscopy observations showed that thePd cations were completely reduced to Pd with the formation of Pd NCs-PPy cluster morphology, respec-tively. Electrochemical properties of the Pd NCs-PPy electrode were studied by differential pulse voltam-metry (DPV), cyclic voltammetry and electrochemical impedance spectroscopy. From the DPV results, theestimated limit of detection, limit of quantification (S/N = 3) for the two linear segments (lower andhigher concentration of nitrate) are 0.7444, 2.4815 and 0.4535, 1.5117 lM, respectively. The sensitivityof these two linear segments is 0.124 and 0.204 lA lM�1 cm�2, respectively. Results from the repro-ducibility experiments show that the Pd NCs-PPy is feasible for the quantitative determination of certainconcentration ranges of nitrate.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Polypyrrole (PPy), an electroactive polymer, has wide applica-tions in various fields of research due to its excellent properties[1]. Recent investigations have shown that PPy or its compositeswith metal oxides and metals such as silver and palladium are use-ful for the fabrication of different type of devices, due to their goodconductivity and reductive ability [2–7]. On the other hand, recentresults have confirmed the effects of size and morphology of thenanocomposites on the catalytic properties [8,9]. It is clear thatthe catalyst efficiency can be increased with the increase of thesurface area. Therefore, based on this important factor, the mostimportant research is focused on the synthesis of electroactivepolymer in the presence of noble metal or metal oxide nanoparti-cles [10,11]. On the other hand, the cost and the type of metal ormetal oxide nanoparticles are other important factors which mustbe considered.

Recently, it is clearly recognized that nitrate pollution of watersources is a huge problem for the society. Several techniques havebeen proposed for the efficient removal of nitrate, but such appli-cations are limited by their expensive cost [12–16]. On the otherhand several methods such as spectrophotometry, chromatogra-phy and electrochemistry have been used for nitrate reduction orsensing [17]. Among these different methods, electrochemistryhas numerous advantages such as low cost, simplicity, high sensi-tivity and selectivity. A viable method for the detection of nitrate isbased on its electroreduction in the presence of metallic orbimetallic catalyst [18–21,17]. A noble metal such as palladiumor its alloy can be used as a catalyst for the reduction and sensingof nitrate. Recent reports have shown that palladium and bimetal-lic catalyst such as copper–palladium, or other metal catalysts canbe used for nitrate electroreduction in acidic [22–24] and basic[25,26] media, although the majority of ground and drinkingwaters are close to neutral pH. Therefore, the focus on the typeof catalysts for the sensing or electroreduction of nitrate withoutany pretreatment is an interesting area of research. Most of thereports on nitrate electroreduction or sensing in neutral mediagave very small ranges of nitrate concentrations [23,27]. In this

M.R. Mahmoudian et al. / Journal of Electroanalytical Chemistry 751 (2015) 30–36 31

work, we report a facile synthetic method for the fabrication ofpolypyrrole coated palladium nanocluster (Pd NCs-PPy) compos-ites via a direct reduction of palladium cations in the presence ofpyrrole monomers. The performance of these new nanocompositestoward nitrate sensing at pH = 7.0 is investigated.

2. Experimental methods

2.1. Synthesis of polypyrrole coated palladium nanocluster (Pd NCs-PPy) composites

Palladium (II) acetate (98%) was procured from Sigma-Aldrich(St. Louis, MO, USA). The pure pyrrole monomer (reagent grade,98%, Malaysia) was stored in the dark, prior to synthesis. In thissynthesis process, 1 mL of 0.1 M (CH3COO)2Pd was added to30 mL, 7 M NaOH solution in a reaction vessel; the reaction wasallowed to occur at room temperature with continuous stirring at500 rpm with a mechanical stirrer. After 20 min, 0.5 mL pyrrolemonomer was added and the color of the solution turned fromlight grey to black. The reaction between the pyrrole monomerand Pd2+ occurred for 30 min. This was followed by the solvother-mal reduction process with the addition of 0.01 mL hydrazinemonohydrate into the reaction mixture and rising the temperatureto 60 �C at a rate of 1.5 �C min�1. This process was continued foranother 60 min until the completion of the reaction. In the finalstep, the reaction mixture was centrifuged at 4000 rpm for10 min to separate the Pd NCs-PPy from the solution, followedby drying in a vacuum oven at 60 �C for 12 h.

2.2. Electrode preparation

The synthesized Pd NCs-PPy (1 mg) was dispersed in DMF(1 ml) with ultra-sonication for 1 h to obtain a homogenous sus-pension. Then, 5 lL of the homogenous suspension was droppedon the surface of a polished glassy carbon electrode (GCE) anddried at room temperature. The current density was calculatedbased on the active area of GCE (0.07 cm2) which was covered withthe synthesized Pd NCs-PPy.

Fig. 1. (a) FESEM images of the Pd NCs-PPy and (b) a higher magnification imageof (a).

2.3. Apparatus and characterizations

The morphology of the Pd NCs-PPy was studied using fieldemission scanning electron microscopy (FESEM, Quanta 200F).The sample was dropped on the ITO surface and dried at room tem-perature before the FESEM characterization. X-ray diffraction(Siemens D5000) with Cu Ka radiation measurements was usedto analyze the structure and phase composition of the preparedPd NCs-PPy. The FT-IR spectrum was obtained using a Spectrum400 (FT-IR/FT-FIR spectrometer). Electrochemical impedance spec-troscopy (EIS) measurements were performed at 0 V potential in1 mM Fe(CN)6

3�/4� (1:1) solution with 0.1 M KCl supporting elec-trolyte (Potassium ferricyanide K3Fe(CN)6, 99.2%, Sigma-Aldrichand potassium ferrocyanide K4Fe(CN)6�3H2O, Fischer Scientific).The impedance spectra were obtained over a frequency range of100 kHz–0.1 Hz, with an acquisition of 10 points per decade andsignal amplitude of 5 mV around the open circuit potential. Theanalysis of the impedance spectra was done by fitting theexperimental results to equivalent circuits using the non-linearleast-square fitting procedure. A potentiostat/galvanostat fromAutolab, PGSTAT-302N (Ecochemie, Netherlands), controlled by aUSB IF030 (MetrohmAutolab) interface card with the FRA.EXEsoftware (version: 409.007, distributor: Metrohm Malaysia)installed in a PC was used to perform these experiments.

3. Results and discussions

The Pd NCs-PPy nanocluster morphology is confirmed by theFESEM images (Fig. 1a–b). The images show Pd nanoparticleswhich are bundled together with the PPy. This structure is clearlyconfirmed by a higher magnification of the FESEM image in Fig. 1b.The FESEM results show available surface area of the Pd NCs-PPywhich can be utilized as a catalyst for nitrate reduction.

The XRD pattern of Pd NCs-PPy is shown in Fig. 2. The intensityof the (111), (200) and (220) peaks in the diffractogram arerelated to the Pd-PPy (Ref. code: 00-001-1201 and JCPDS cardno. 05-0681). A broad amorphous diffraction peak between2h = 5–25� in the XRD diagram of the Pd NCs-PPy is attributed tothe scattering from the bare polymer chains at the interplanarspacing [28,29]. The XRD result clearly confirms that Pd2+ was con-verted to Pd element in the presence of pyrrole monomers duringthe synthesis of Pd NCs-PPy. Two conclusions can be drawn fromthe XRD and FESEM results of the Pd NCs-PPy. First, the pyrrolemonomers are able to reduce Pd2+ to Pd, which alone undergoesoxidative polymerization to PPy, and second, the polymerizedPPy encapsulates the surface of Pd nanoparticles forming a nucleusfor further growth, via the Ostwald ripening process [30–32],which acts as a barrier against the corrosive NaOH environment.In addition, the surface energy and capping effect are consideredas the major driving force for the nanoparticle formation [33].

The FTIR spectra of Pd NCs-PPy are shown in Fig. 3. The peak at3454 cm�1 is attributed to the N–H bond, while the peak at1682 cm�1 is attributed to the C–N–C bond or the C@O group,

Fig. 2. XRD patterns of the Pd NCs-PPy.

Fig. 4. EIS of: (1) bare GCE, (2) Pd NCs-PPy/GCE and (3) PPy/GCE in a 0.1 M KClsolution containing 1 mM Fe(CN)6

3�/4�(1:1). The parameters obtained from: (1)GCE/electrolyte, (2) Pd NCs-PPy/GCE/electrolyte and (3) PPy/GCE/electrolyte;equivalent circuit models where the chi-squared (v2) is minimized at 10�4.

32 M.R. Mahmoudian et al. / Journal of Electroanalytical Chemistry 751 (2015) 30–36

which suggests that an over-oxidation of the pyrrole monomershad taken place during the PPy polymerization. The over-oxidationof PPy occurs through the apparition of C–OH and C@O functionalgroups in the polymer backbone, as well as the formation of CO2 atsufficiently positive potentials or in the presence of a strong oxi-dizer [34]. The strong band 3027 cm�1 is related to the aliphaticC–H vibration. On the other hand, the peaks at 1174 and1059 cm�1 are due to the C–N and C–H groups in the compositespectrum, respectively. In addition, the peak at 1436 cm�1 is char-acteristic of a typical PPy ring vibration. The FTIR spectrum clearlyconfirms that the pyrrole monomers are polymerized to PPy in thepresence of Pd2+cations.

The interfacial properties of the GCE electrodes modified withPd NCs-PPy were studied by EIS. Fig. 4 shows the Nyquist plotsof the bare GCE (1), Pd NCs-PPy/GCE (2), PPy/GCE (3) and theirequivalents circuits in 1 mM Fe(CN)6

3�/4� (1:1) with 0.1 M KCl sup-porting electrolyte. A comparison between the Nyquist plots of thebare GCE (1) and Pd NCs-PPy GCE (2) clearly confirms the effect ofPd NCs-PPy on the charge transfer resistance. The decrease of thecharge transfer resistance can be related to the conductivity ofPPy and Pd metal. It is notable that the semicircle diameter inthe Nyquist plot of PPy/GCE is smaller than Pd NCs-PPy, which isattributed to the over-oxidation of polymerized PPy in the pres-ence of Pd2+. Therefore, it can be concluded that the existence ofPd nanoparticles in the PPy could not increase the conductivityof the over-oxidized PPy. The electrochemical parameters obtainedby simulation of the EIS results of the bare GCE, Pd NCs-PPy/GCE,PPy/GCE are given in Table 1. A significant decrease of the Rct

(charge transfer resistance) can be seen from the bare GCE to PdNCs-PPy/GCE. This resistance can be attributed to the primaryresistance against ion mobility which occurs from the solution tothe coating layer. The Nyquist plots of Pd NCs-PPy/GCE andPPy/GCE shows a ‘‘depressed semi-circle’’ with the center of the

Fig. 3. FT-IR of Pd NCs-PPy.

semi-circle below the Zre axis, which is due to the deviation fromthe double-layer capacitance. Instead of a pure capacitor, aconstant phase element (CPE) was introduced in the fittingprocedure to obtain a good agreement between the simulatedand experimental data. The impedance of CPE is defined asZCPE = Q�1 (jx)�n; where Q is the combination of properties relatedto the surface and electroactive species independent of frequency;‘‘n’’ is related to the slope of the log Z vs. log f in the Bode diagram.The decrease of ‘‘n’’ from unity is due to the porous and the inho-mogeneous surface. The increase of the CPE and decrease of ‘‘n’’ ofthe Pd NCs-PPy/GCE compared to PPy/GCE (Table 1) confirms ahighly available surface area of the Pd NCs-PPy/GCE. The Nyquistplot of PPy/GCE shows a straight line (Warburg impedance) withan angle of 45� to the Zre axis in the mid-frequency region. TheWarburg impedance is due to a diffusion controlled process acrossthe deposited layer.

The cyclic voltammograms of Pd NCs-PPy/GCE (1) and bare GCE(3) in the presence of 10 mM nitrate in 0.1 M phosphate buffersolution at pH 7 are shown in (Fig. 5). In addition, (Fig. 5) showsthe cyclic voltammogram of Pd NCs-PPy/GCE (2) in the absenceof 10 mM nitrate. The sweep potential was from �1.2 to 0.2 V witha scan rate of 50 mV s�1. The cathodic peak between �0.6 and �1 Vin the cyclic voltammogram of Pd NCs-PPy/GCE (1) in the presenceof 10 mM nitrate is due to the nitrate reduction [35]. With theincrease of potential more negative than �1V, hydrogen evolutionreaction, which is induced by proton adsorption, can be observed[35]. At the positive scan, in the cyclic voltammogram of Pd NCs-PPy/GCE (1) the peak between �0.4 and 0 V in the presence of10 mM nitrate is probably related to the electrooxidation of nitriteformed by the electrochemical reduction of nitrate [36]. The cyclic

Table 1Electrochemical parameters obtained by simulation of the EIS results of the Pd NCs-PPy/GCE in 1 mM Fe(CN)6

3�/4� (1:1) with 0.1 M KCl supporting electrolyte.

Electrode R1 (Rs) (Ohm cm2) R2 (Rct) (kOhm cm2) C (lF) Q1 Y (mOhm�1 sn cm�2) n1 W Y (mS s1/2 cm�2)

GCE 8.21 7.263 2.375 – – –Pd NCs-PPy 8.23 2.724 – 0.1836 0.6186 –PPy/GCE 8.34 0.1357 – 0.0548 0.7017 0.0263

Fig. 5. The cyclic voltammograms of Pd NCs-PPy/GCE (1) and bare GCE (3) in thepresence of 10 mM nitrate in 0.1 M phosphate buffer solution at pH 7 and Pd NCs-PPy/GCE (2) in the absence of 10 mM nitrate with a scan rate of 50 mV s�1.

Fig. 6. CVs of Pd NCs-PPy/GCE in 0.1 M phosphate buffer solution at pH 7, atdifferent scan rates.

M.R. Mahmoudian et al. / Journal of Electroanalytical Chemistry 751 (2015) 30–36 33

voltammogram of Pd NCs-PPy/GCE (2) in the absence of 10 mMnitrate in 0.1 M phosphate buffer solution at pH 7 shows an oxida-tion peak at �0.4 to 0.1 and a reduction peak at �0.6 to �0.8 whichcan be related to the oxidation and reduction of PPy [37,38]. Thisobservation clearly confirms the sensitivity of Pd NCs-PPy towardthe detection of nitrate. It is notable that the electrode perfor-mance depends on the liberation and regeneration of adsorbednitrogenated compounds on the electrode surface that otherwisewould cause its poisoning that it is reported earlier [24,27].Other researchers believe that the mechanism of nitrate reductionis influenced by the pH and nitrate concentration [17]. In an acidicsolution (pH lower than 6) and low concentration of nitrate (lowerthan 10�5 mol L�1), the nascent hydrogens form on the catalystsurface and react with the nitrate, which is subsequently adsorbedon the electrode surface to form nitrite. Other researcher showsthat the nitrates occupy the catalyst surface at higher nitrate con-centration (more than 10�5 mol L�1), therefore the percentage ofnascent hydrogens on the electrode surface decreases [39].Therefore, the best strategy to solve this limitation is to increasethe catalyst surface area for the nitrate reduction at pH 7, with anitrate concentration more than 10�5mol L�1.The cathodic currentshows that the Pd NCs-PPy/GCE is capable for the reduction ofnitrate at pH 7.

Fig. 6 shows the CVs of Pd NCs-PPy/GCE in 0.1 M phosphatebuffer solution at pH 7, at different scan rates. The linear rise ofthe cathodic peak currents with scan rates shows that theelectrode reaction is a surface controlled process (inset of Fig. 6)[40]. The linear regression equation is expressed as Ipc =�0.0878x(mV s�1)� 1.2553 (R2 = 0.9867).

A typical amperometric response of the Pd NCs-PPy/GCE to con-secutive nitrate concentration changes was studied with differen-tial pulse voltammetry (DPV) in 0.1 M phosphate buffer solutionat pH 7. Fig. 7(a) shows the dependence of the cathodic peak cur-rent (Ipc) on the nitrate concentration. The sensor was calibratedthree times and the standard deviations were calculated. The cali-bration curve for the Pd NCs-PPy/GCE shows two linear segments:the first linear segment increases from 100 to 800 lM with alinear regression equation of Ipc = 0.124 (lA lM�1 cm�2) + 3.953

(R2 = 0.995) (Fig. 7(b)), and the second linear segment increasesup to 1400 lM with a linear regression equation of Ipc = 0.204(lA lM�1 cm�2) + 70.84 (R2 = 0.987) (Fig. 7(c)). The limit ofdetection (LOD) and the limit of quantification (LOQ) of the PdNCs-PPy/GCE were calculated from the following equations [41]:

LOD ¼ 3SB=b ð1Þ

LOQ ¼ 10SB=b ð2Þ

where SB is the standard deviation of the blank solution and b is theslope of the analytical curve, as shown in Fig. 7((b) and (c)). Theestimated LOD, LOQ (S/N = 3) for the two linear segments (lowand high concentration of nitrate) are 0.7444, 2.4815 and 0.4535,1.5117 lM, respectively. In addition, the sensitivity for these twolinear segments is 0.124 and 0.204 lA lM�1 cm�2, respectively.

For analytical applications, a recovery experiment was con-ducted to examine the viability of the Pd NCs-PPy/GCE. Three dif-ferent concentrations of nitrate (100, 500 and 1000 lmol L�1)were added into 0.1 M phosphate buffer solution at pH 7. The aver-age recoveries obtained from four replicate experiments (n = 4) arelisted in Table 2. The recovery percentage and relative standarddeviation (RSD) in Table 2 demonstrate the feasibility of the pro-posed method for the quantitative detection of certain concentra-tion ranges of nitrate.

Table 3 indicates that the LOD of nitrate for Pd NCs-PPy/GCE islower compared to Gold electrode modified by AgNP [41], andPPy/Ag composite matrix on a glassy carbon electrode [47] andthe present result is comparable to silver disk electrode and silver,graphite powder and methacrylate resin composite electrode [42]and copper nanostructure [44]. In this method, a low amount of(CH3COO)2Pd was used for the fabrication of the Pd NCs-PPy/GCE,therefore a lower cost of fabrication for the detection of nitrate isthe one of the main advantage of this work.

The analytical application of the Pd NCs-PPy/GCE electrode wastested with ground water in university Malaya and pipe water

Fig. 7. (a) The DPV curves of different concentration of nitrate in 0.1 M phosphate buffer solution (100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300 and1400 lM, at pH 7). (b) The calibration curve at low concentration range and (c) the calibration curve at high concentration range. Error bars represent ± standard deviation.

Table 2The detection of nitrate concentration in test samples (results based on four replicatedeterminations per sample).

Sample Add (lM) Found (lM) RSD (%) (n = 4) Recovery (%)

1 100 97.758 5.6381 97.75882 500 496.964 3.8045 99.39293 1000 996.553 3.5247 99.6553

Table 3A summary and comparison of the estimated LOD values from the present work with pre

Electrode construction Sample medium and

Pyrolytic graphite electrode 0.1 M NaH2PO4 (pHCdCl2 + 50 lM CuCl2

Gold electrode modified by AgNP Synthetic sea waterSilver disk electrode and silver, graphite powder and

methacrylate resin composite electrode0.1 M NaOH

PPy-PSS-MWNT-PEI-CuNP Nanocomposite 0.10 M Na2SO4

Copper nanostructures –Nitrate-doped polypyrrole (PPy) films on a glassy carbon Acetonitrile solution

lithium nitrate andSilver electrode 0.01 M NaOHPPy/Ag composite matrix on a glassy carbon electrode 0.10 M Na2SO4

Pd NCs-PPy 0.1 M PBS solution,

34 M.R. Mahmoudian et al. / Journal of Electroanalytical Chemistry 751 (2015) 30–36

from our lab, with the spiking 100 and 500 lmol L�1 of nitrate. Theaverage recoveries from four replicate experiments (n = 4) arelisted in Table 4. This result clearly confirms that the pipe wateris free from nitrate while the ground water consists of some nitrateconcentration. The existence of nitrate anion shows that the

vious reports.

pH Linear calibration range(mol L�1)

LOD (lmol L�1) Reference

4.5) + 10 lM 1 � 10�6–1 � 10�3 – [17]

= [NaCl], pH = 5 10 � 10�6–10 � 10�3 10 [40]4.8 � 10�9–4.8 � 10�7 and5.6 � 10�9–8.0 � 10�7

1.2 � 10�2 and7.2 � 10�1

[42]

0–5 � 10�3 30 [43]1 � 10�6–3.5 � 10�5 0.59 [44]

containing15% plasticizer

0.1–7.4 � 10�5 47 [45]

1 � 10�3–1 � 10�2 1.7 � 103 [46]0–1 � 10�2 5 [47]

pH = 7 1 � 10�4–8 � 10�4 0.7444 This work9 � 10�4–1.4 � 10�3 0.4535

Table 4Analytical recovery of NO3

� concentration from nitrate spiked water samples.

Water sample Add (lM) Found (lM) Recovery (%)

Ground 100 102.651 102.651water 500 504.261 100.852Pipe 100 98.231 497.258water in lab 500 98.231 99.451

M.R. Mahmoudian et al. / Journal of Electroanalytical Chemistry 751 (2015) 30–36 35

utilization of nitrate fertilizer in the university compound affectsthe concentration of nitrate in the ground water.

4. Conclusions

The Pd NCs-PPy were synthesized by direct reduction of palla-dium cations in the presence of pyrrole monomers in an aqueoussolution of Pd (CH3COO)2 and NaOH. The performance of thesenew materials toward nitrate sensing at pH = 7.0 was investigated.The FESEM results showed nanocluster morphology of the synthe-sized Pd NCs-PPy. The XRD result confirmed that the Pd cationswere completely reduced to Pd. The EIS observation clearly con-firmed the sensitivity of the Pd NCs-PPy toward nitrate detection.From the DPV results, the estimated limit of detection, limit ofquantification (S/N = 3) for the two linear segments (lower andhigher concentration of nitrate) are 0.7444, 2.4815 and 0.4535,1.5117 lM, respectively. The sensitivity of these two linear seg-ments is 0.124 and 0.204 lA lM�1 cm�2, respectively.

Acknowledgments

The authors wish to thank MojdehYeganeh for valuable discus-sion. This research was supported by High Impact Research MoEGrant UM.C/625/1/HIR/MoE/SC/04 from the Ministry of EducationMalaysia, UMRG Program RP012A-14SUS, PRGS Grant PR002-2014A, FP033 2013A, GC001C-14SBS and University MalayaCentre for Ionic Liquids (UMCiL).

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