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carbon nanotubes coated graphite surfaces for highly sensitive nitrite detection
Talanta 144 (2015) 1133–1138
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Polymer/carbon nanotubes coated graphite surfaces for highly sensitive nitrite detection Filiz Kuralay n, Mehmet Dumangöz, Selma Tunç Ordu University, Faculty of Arts and Sciences, Department of Chemistry, 52200 Ordu, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 6 April 2015 Received in revised form 27 July 2015 Accepted 31 July 2015 Available online 1 August 2015
This study describes the preparation of poly(vinylferrocenium)/multi-walled carbon nanotubes (PVF þ /MWCNTs) coated electrode and its use for sensitive electrochemical nitrite detection. PVF þ /MWCNTs composite coated disposable pencil graphite electrode (PVF þ /MWCNTs/PGE) was prepared by electropolymerization of poly(vinylferrocene) (PVF) in the presence of MWCNTs with one-step electropolymerization. Characterization of PVF þ /MWCNTs/PGE was carried out with scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Then electrochemical detection of nitrite was performed using differential pulse voltammetry (DPV). Nanocomposite coated electrode showed high sensitivity to nitrite with a detection limit of 0.1 μM. The electrode platform was successfully applied to a commercial mineral water sample. & 2015 Elsevier B.V. All rights reserved.
Keywords: Redox polymer Poly(vinylferrocenium) Multi-walled carbon nanotubes Nitrite
1. Introduction
NO2 2NO2 þe
(1)
Nitrite ion is an environmentally important molecule which is an intermediate species in the nitrogen cycle. It results from the oxidation of ammonia or from the reduction of nitrate [1–4]. It is commonly used as a preservative in food industry. Thus, its presence in food, water and soil is widespread [5–7]. Nitrite is classified as a hazardous species, since it is toxic. It combines with blood pigments to produce meta-hemoglobin which leads to oxygen depletion to the tissues. Nitrite ion can form carcinogenic N-nitrosamine products with secondary amines [8–10]. Hence, there is an urgent need for highly sensitive and accurate quantitative determination of nitrite. There has been great interest for nitrite detection using various methods including chromatography, spectrophotometry and electrochemistry. Among these techniques, electrochemical ones have been favorable because they provide low-cost, accurate, fast, sensitive and selective detection routes compared to the other methods [11–17]. Nitrite is an electroactive ion. There are two possible ways for the electrochemical detection of nitrite ions: oxidation or reduction of nitrite. Oxidation of nitrite is commonly used detection mechanism, since the reduction of nitrite suffers from interferences such as reduction of nitrate ions and molecular oxygen [18,19]. The oxidation of nitrite can be given by Eqs. (1) and (2) [20,21]
H2Oþ 2NO2-NO3 þ NO2 þ 2H þ
(2)
n
Corresponding author. Fax: þ 90 452 2339149. E-mail address: kuralay.fi[email protected] (F. Kuralay).
http://dx.doi.org/10.1016/j.talanta.2015.07.095 0039-9140/& 2015 Elsevier B.V. All rights reserved.
However, the oxidation of nitrite requires high potential using bare electrodes. Furthermore, sensitivity is usually very low [22– 24]. The redox reaction at bare electrodes commonly involves slow transfer kinetics. Santos et al. have used a glassy carbon electrode modified with alternated layers of iron(III) tetra-(N-methyl-4pyridyl)-porphyrin and copper tetrasulfonated phthalocyanine for nitrite determination by differential pulse voltammetry [6]. (4Ferrocenylethyne) phenylamine functionalized graphene oxide modified glassy carbon electrode have also been fabricated by Liu et al. [14]. Rocha et al. have reported anodic oxidation of nitrite at a glassy carbon electrode modified with molybdenum oxide layer [25]. In this work, we modify a disposable pencil graphite electrode (PGE) surface with a nanocomposite, poly(vinylferrocenium)/multi-walled carbon nanotubes (PVF þ /MWCNTs), in order to surpass the challenge. It is well known that electroactive polymers can provide great advantageous for building up systems having good electrical properties, high-effective surface area and good conductivity [26– 28]. There have been various attempts on electrochemical nitrite sensing. Electrocatalytic activity of nitrogen-containing carbon nanomaterials prepared by the carbonization of nanostructured polyaniline salts for the electrooxidation reaction of nitrite and ascorbic acid has been presented [29]. A voltametric method has been developed to determine nitrite by using carbon paste electrode modified with polyvinylimidazole [30]. Particularly, conducting polymer/nanomaterials composites are
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one of the important substrates in order to develop novel materials for electrochemical sensing applications. Gligor and Walcarius have reported the modification of a glassy carbon electrode with poly(Toluidine Blue O) and single-walled carbon nanotubes for the electrocatalytic oxidation of nitrite [31]. Guo et al. have fabricated a polyaniline and carbon nanotubes modified electrode for the reduction of nitrite [32]. An electrochemical sensor has been fabricated by electrodeposition of gold nanoparticles on poly(3-methylthiophene) modified glassy carbon electrode for the determination of nitrite and iodate [33]. The electrochemical solid-phase extraction (EC-SPE) and amperometric determination of nitrite with polypyrrole nanowire modified electrode have been reported by Tian et al. [34]. Electrodeposited conducting poly(Azure A) at a carbon nanotube modified glassy carbon electrode has been used for electrocatalytic reduction of nitrite [35]. A cytochrome c electrochemical biosensor constructed by immobilization of the biomolecule on a hybrid material consisting of the conducting polymer poly-(3-methylthiophene) and multi-walled carbon nanotubes for nitrite detection has been reported [36]. Zhang et al. have developed poly(ethylenedioxythiophene)–Au nanoparticles composite film toward nitrite oxidation [37]. Polydiphenylamine–Pt composite has been synthesized by electrochemical method on glassy carbon electrode by Unnikrishnan. The electrocatalytic activity of PDPA–Pt film modified electrode toward the oxidation of nitrite has been studied by cyclic voltammetry and amperometry [38]. In this work, the preparation of PVF þ /MWCNTs composite coated electrode in a simple one-step electropolymerization of poly(vinylferrocene) (PVF) in the presence of MWCNTs on a disposable pencil graphite electrode and the application of the prepared electrode in nitrite sensing were detailed. The composite electrode was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The oxidation of nitrite ion was investigated with differential pulse voltammetry (DPV). In particular, the oxidation of nitrite was found at less positive potentials proving the catalytical effect of PVF þ /MWCNTs coated electrode. The effect of possible interferents were examined using 10-fold concentrated solutions of ascorbic acid, uric acid, phosphate and sulfate. The analytical applicability of the electrode was demonstrated in a commercial mineral water sample with a high reliability. The results show that the coated electrode have improved electrochemical activity towards nitrite oxidation. To the best of our knowledge this is the first to determine nitrite ion using PVF þ /MWCNTs coated electrode. Such an electrode system provided nonenzymatic sensing platform having good electrocatalytic properties and sensitivity for nitrite detection with a fast response time. Furthermore, disposable pencil graphite electrode technology offers low-cost analysis.
2. Experimental 2.1. Chemicals Vinylferrocene (97%), multi-walled carbon nanotubes (MWCNTs) and sodium nitrite (NaNO2) were obtained from Sigma-Aldrich. Other chemicals were in analytical grade and supplied from Sigma and Merck. All solutions were prepared with ultra pure water and purged with nitrogen to eliminate oxygen. Nitrite solutions were prepared in 50 mM phosphate buffer solution at pH 7.4. 2.2. Apparatus The potential-controlled coulometry, cyclic voltammetry (CV)
and electrochemical impedance spectroscopy (EIS) were carried out with CH Instruments System; Model 660 C (USA). Differential pulse voltammetry was carried out with AUTOLAB-PGSTAT 204 analysis system supported with a NOVA software package (Metrohm, The Netherlands). Three electrode system consists of a pencil graphite working electrode (PGE), an Ag/AgCl reference electrode (BASi, Lafeyette, IN USA) and a Pt wire auxilary electrode (BASi, Lafeyette, IN USA) was used during the electrochemical measurements. The connector of the PGE (0.5 mm HB Tombow) was a Tombow pencil. Scanning electron microscopy (SEM) images of the electrodes were obtained using Zeiss Evo 50 EP-SEM (USA). 2.3. Synthesis of poly(vinylferrocene) Poly(vinylferrocene) (PVF) was polymerized from vinylferrocene using 2,2-Azo-bis(2-methylpropionitrile) (AIBN) as an initiator [39]. The polymerization was carried out at 70 °C for 20 h. PVF (1.0 mg mL 1)/MWCNTs (1.0 mg mL 1) suspension was prepared in methylene chloride/tetra-n-butylammonium perchlorate (TBAP) (0.1 M) solvent/supporting electrolyte system. No pretreatment was used for MWCNTs. The suspension was sonicated for 30 min before the experiments. The solution was kept under nitrogen atmosphere during the experiments. 2.4. Preparation of poly(vinylferrocenium)/multi-walled carbon nanotubes coated PGE electrode (PVF þ /MWCNTs/PGE) PVF þ /MWCNTs coated PGE was prepared via electrooxidation of PVF/MWCNTs at þ 0.7 V vs. Ag/AgCl using constant potential electrolysis. 1 cm of the pencil tip was dipped into the suspension. PVF þ coated PGE (PVF þ /PGE) was prepared via electrooxidation of PVF at þ0.7 V vs. Ag/AgCl. Different modification times of PVF þ /MWCNTs onto PGE were used (15–120 s). 2.5. Electrochemical characterization of PVF þ /MWCNTs/PGE Electrochemical characterization of PVF þ /MWCNTs/PGE was performed with CV and EIS. 2.5.1. Cyclic voltammetric characterization of PVF þ /MWCNTs/PGE Cyclic voltammograms (CVs) of uncoated PGE, PVF þ /PGE and PVF þ /MWCNTs/PGE were recorded in 50 mM phosphate buffer solution pH 7.4 containing 0.1 M NaClO4 between þ0.0 V and þ1.0 V vs. Ag/AgCl. 2.5.2. Impedance measurements EIS measurements were conducted at a DC voltage of 100 mV using 50 mM phosphate buffer solution pH 7.4 containing 0.1 M NaClO4. The frequency was varied over the range of 105–10 1 Hz. The perturbation voltage was 10 mV. 2.5.3. Voltammetric detection of nitrite Differential pulse voltamograms (DPVs) of PVF þ /MWCNTs/PGE were recorded at an initial potential of þ0.1 V and a final potential of þ1.2 V vs. Ag/AgCl with a pulse amplitude of 50 mV and a scan rate of 0.1 mV/s. Pulse duration was 50 ms. Commercial mineral water sample was used without any pretreatment.
3. Results and discussion Fig. 1A presents the schematic presentation of the study. In the first part of the study, we prepared a PVF þ /MWCNTs composite coated disposable pencil graphite surface by constant potential electrolysis and characterized the nanocomposite coated electrode with SEM, CV and EIS. Fig. 1B-a–f presents the SEM images of
F. Kuralay et al. / Talanta 144 (2015) 1133–1138
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Fig. 1. (A) Schematic presentation of the work. (B) SEM images of uncoated PGE (a and d), PVF þ coated PGE (b and d), PVF þ /MWCNTs coated PGE (c and f) at 20 μM and 200 nm, respectively.
uncoated PGE, PVF þ coated PGE and PVF þ /MWCNTs coated PGE. The images were given at low and high magnitude values. SEM image of the uncoated PGE show that the surface of the pencil graphite was rough (Fig. 1B-a and d). The morphology of the graphite surface changed after the modification of the PGE with PVF þ and PVF þ /MWCNTs composite. SEM images of PVF þ coated PGE at 2 different magnitude values are shown in Fig. 1B-b and e. Subsequently, SEM images of PVF þ /MWCNTs coated PGE at 2 different magnitude values are shown in Fig. 1B-c and f. SEM image of the PVF þ coated electrode shows the homogeneous distribution of the nanostructured polymer on the electrode (Fig. 1B-e). As shown in Fig. 1B-f, PVF þ /MWCNTs had a different surface morphologies, but still having a well-dispersed distribution of nanocomposite on the electrode. Cyclic voltammetric behaviors of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE were then investigated in order to perform the electrochemical characterization. Related voltammograms are given as Fig. 2A-a–c. In the cyclic voltammograms (CVs) of PVF þ /MWCNTs/PGE and PVF þ /PGE, there are well-defined oxidation and reduction peaks due to the ferrocenium/ferrocene centers of the polymer (Fig. 2A-a and Fig. 2A-b, respectively). The anodic peak was at about þ0.37 V vs. Ag/AgCl and the cathodic peak was at about þ 0.26 V vs. Ag/AgCl for the PVF þ /MWCNTs/PGE. The anodic peak was at about þ0.39 V vs. Ag/AgCl and the cathodic peak was at about þ0.24 V vs. Ag/AgCl/ for the PVF þ /PGE. In the presence of carbon nanotubes, oxidation/ reduction peak currents increased significantly indicating the fast electron transfer [40]. The anodic peak was at about þ0.39 V vs.
Fig. 2. (A) CVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE. (B) Nyquist diagrams of (a) PVF þ /MWCNTs/PGE and (b) PVF þ /PGE in 50 mM phosphate buffer (pH 7.4) containing 0.1 M NaClO4.
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Fig. 3. (A) DPVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE in 1 mM nitrite solution prepared in 50 mM phosphate buffer (pH 7.4). (B) CVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE in 1 mM nitrite solution prepared in 50 mM phosphate buffer (pH 7.4). (C) Effect of PVF þ /MWCNTs modification time (a) 15 s, (b) 30 s, (c) 45 s, (d) 60 s, and (e) 120 s (related DPVs).
Fig. 4. (A) DPVs of PVF þ /MWCNTs/PGE in (a) 1 μM, (b) 5 μM, (c) 10 μM, (d) 20 μM, (e) 50 μM, (f) 100 μM, (g) 150 μM, (h) 200 μM, (i) 250 mM, (j) 300 μM, and (k) 400 μM nitrite solution (inset: (a) DPV of uncoated PGE in 50 mM phosphate buffer, DPVs of PVF þ /MWCNTs/PGE in (a) 1 μM, (b) 5 μM, (c) 10 μM nitrite solution). (B) Calibration graph.
Ag/AgCl and the cathodic peak was at þ0.23 V vs. Ag/AgCl. The electroactive surface coverages (ΓEA) of the coated electrodes were calculated at a scan rate of 100 mV s 1 and found as 1.9( 70.4)*10 7 mol cm 2 for the PVF þ /MWCNTs coated electrode and 1.0( 70.2)*10 7 mol cm 2 for the PVF þ coated one
(n ¼3) [41]. Another electrochemical characterization method, electrochemical impedance spectroscopy, was applied in order to differentiate carbon nanotubes/polymer coated electrode and polymer coated electrode. Fig. 2B-a and b presents the electrochemical impedance spectra for PVF þ /MWCNTs/PGE and
F. Kuralay et al. / Talanta 144 (2015) 1133–1138
Fig. 5. DPVs of PVF þ /MWCNTs/PGE in mineral water containing (a) 100 μM, (b) 50 μM, (c) 10 μM, (d) 5 μM nitrite, and (e) DPV of uncoated PGE in 50 mM phosphate buffer. Table 1 Recovery studies with the proposed method. Added (μM)
Found (μM)
Recovery (%)
RSD (%) (n ¼3)
5 10 50 100
57 1 107 1 467 1 987 1
91 71 98 71 92 71 98 71
2.8 3.1 2.3 3.6
PVF þ /PGE, respectively. The average charge transfer resistance (Rct) values are 3.2( 7 0.1)*105 Ω and 3.9(70.1)*105 Ω for PVF þ /MWCNTs/PGE and PVF þ /PGE, respectively (n¼ 3). The presence of carbon nanotubes prompted the electron transfer between the electrode and the solution interface leading a smaller Rct. After the characterization of the modified surfaces and getting better electrochemical responses in the presence of carbon nanotubes, we tested the advantage of the nanocomposite modified electrode for nitrite detection. Fig. 3A-a–c presents the differential pulse voltammograms (DPVs) of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE in 1 mM nitrite solution prepared in pH 7.4 phosphate buffer. As seen in Fig. 3A-a, higher oxidation peak current for nitrite was obtained with PVF þ /MWCNTs coated PGE
1137
compared to the PVF þ coated PGE (Fig. 3A-b) and uncoated PGE (Fig. 3A-c). The oxidation peak current was 3-fold higher than the other two electrodes. The peak currents were almost the same with PVF þ coated PGE and uncoated PGE. However, the polymer coated one gave a less nitrite oxidation potential. The oxidation peak potentials were at about þ0.84 V vs. Ag/AgCl for the coated ones and the oxidation peak potential was at about 0.98 V vs. Ag/ AgCl for the uncoated PGE. Fig. 3B-a–c presents the CVs of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE in 1 mM nitrite solution prepared in pH 7.4 phosphate buffer. Differential pulse voltemmetry studies were correlated with cyclic voltammetry results. PVF þ /MWCNTs/PGE exhibited high oxidation peak current for nitrite at a lower potential (at about þ0.80 V vs. Ag/AgCl). These results confirm our characterization results explained above. Thus, further experiments were performed with the PVF þ /MWCNTs/PGE. DPVs of PVF þ /MWCNTs/PGE (Fig. 3A-a) and PVF þ /PGE (Fig. 3A-b) also show the oxidation peaks of the polymer at about þ 0.52 V vs. Ag/AgCl. Higher oxidation peak current was obtained in the presence of carbon nanotubes. Optimization of the nanocomposite modification time was performed between 15 s and 120 s in order to improve the sensitivity (Fig. 3B). DPVs were taken with PGEs coated with the nanocomposite using different modification time values. There was a gradual increase for the nitrite oxidation peak current up to 60 s of nanocomposite modification time (Fig. 3B-d). After 60 s there was a slight decrease in the oxidation peak current of nitrite (for 120 s, Fig. 3B-e). Hence, 60 s of immobilization time was chosen as the optimum condition. Next, we investigated the effect of concentration of nitrite on the response of the sensor using optimum experimental conditions; 60 s of nanocomposite modification at þ 0.7 V vs. Ag/AgCl. The concentration interval used in the study was 1–500 μM. The sensing platform developed in the study exhibited a good response having a linearity between 1 μM and 400 μΜ nitrite concentration (Fig. 4). After 400 μM the response of the electrode did not change appreciably. The relative standard deviations were calculated as 4.2% and 2.5% for three successive determinations at 1 μM and 200 μM nitrite, respectively. Limit of detection (LOD) was found as 0.1 μM for nitrite from the relationship LOD ¼3Sb/m, where Sb is the standard deviation of the blank and m is the slope of the calibration graph (n ¼3) [42]. This value was comparable with the reported studies [4,6,14,23,36,37]. Furthermore, the prepared electrode had a good analytical performance in terms of being low-cost, reliable, effortless and fast. And then we applied the nanocomposite modified electrode for
Fig. 6. DPVs of PVF þ /MWCNTs/PGE in (A) (a) 100 μM nitrite in the presence of ascorbic acid and (b) 100 μM nitrite. (B) (a) 100 μM nitrite in the presence of uric acid and (b) 100 μM nitrite.
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the determination of nitrite in commercial mineral water sample adding a certain amount of nitrite into the sample. No pretreatment was used in the sample. Fig. 5 shows the related DPVs obtained with PVF þ /MWCNTs coated PGE in mineral water solutions having 5, 10, 50 and 100 μM nitrite concentration. The coated electrode was successfully applied to the determination of nitrite in commercial mineral water sample. The recovery values were also calculated and given as Table 1. In addition, the interference effects of 10-fold excess amounts of ascorbic acid, uric acid, phosphate ion and sulfate ion were investigated and no interference effects of the given species were found for the determination of nitrite in the oxidation potential interval of nitrite obtained with this electrode system. Fig. 6A-a and b presents the DPVs of PVF þ /MWCNTs/PGE in 100 μM nitrite in the presence and absence of ascorbic acid, respectively. Fig. 6B-a and b presents the DPVs of PVF þ /MWCNTs/PGE in 100 μM nitrite in the presence and absence of uric acid, respectively. The electrode system was easily separated ascorbic acid (at about þ0.25 V), uric acid (at about þ0.35 V), polymer (at about 0.55 V) and nitrite (at about 0.80 V) oxidation peaks.
4. Conclusions In summary, a highly sensitive and selective sensor for nitrite was developed in the study using PVF þ /MWCNTs coated electrode. The preparation of the nanocomposite coated electrode was performed in a single and easy electropolymerization method. The nitrite oxidation was detected in a fast, low-cost and reliable way in the study. Detection limit was found as 0.1 μM for nitrite which is comparable with the reported studies [4,6,14,23,36,37]. We believe this sensing platform that combines disposable electrode technology and nanomaterials technology will be a good alternative for different biosensing applications.
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Polymer/carbon nanotubes coated graphite surfaces for highly sensitive nitrite detection Filiz Kuralay n, Mehmet Dumangöz, Selma Tunç Ordu University, Faculty of Arts and Sciences, Department of Chemistry, 52200 Ordu, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 6 April 2015 Received in revised form 27 July 2015 Accepted 31 July 2015 Available online 1 August 2015
This study describes the preparation of poly(vinylferrocenium)/multi-walled carbon nanotubes (PVF þ /MWCNTs) coated electrode and its use for sensitive electrochemical nitrite detection. PVF þ /MWCNTs composite coated disposable pencil graphite electrode (PVF þ /MWCNTs/PGE) was prepared by electropolymerization of poly(vinylferrocene) (PVF) in the presence of MWCNTs with one-step electropolymerization. Characterization of PVF þ /MWCNTs/PGE was carried out with scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Then electrochemical detection of nitrite was performed using differential pulse voltammetry (DPV). Nanocomposite coated electrode showed high sensitivity to nitrite with a detection limit of 0.1 μM. The electrode platform was successfully applied to a commercial mineral water sample. & 2015 Elsevier B.V. All rights reserved.
Keywords: Redox polymer Poly(vinylferrocenium) Multi-walled carbon nanotubes Nitrite
1. Introduction
NO2 2NO2 þe
(1)
Nitrite ion is an environmentally important molecule which is an intermediate species in the nitrogen cycle. It results from the oxidation of ammonia or from the reduction of nitrate [1–4]. It is commonly used as a preservative in food industry. Thus, its presence in food, water and soil is widespread [5–7]. Nitrite is classified as a hazardous species, since it is toxic. It combines with blood pigments to produce meta-hemoglobin which leads to oxygen depletion to the tissues. Nitrite ion can form carcinogenic N-nitrosamine products with secondary amines [8–10]. Hence, there is an urgent need for highly sensitive and accurate quantitative determination of nitrite. There has been great interest for nitrite detection using various methods including chromatography, spectrophotometry and electrochemistry. Among these techniques, electrochemical ones have been favorable because they provide low-cost, accurate, fast, sensitive and selective detection routes compared to the other methods [11–17]. Nitrite is an electroactive ion. There are two possible ways for the electrochemical detection of nitrite ions: oxidation or reduction of nitrite. Oxidation of nitrite is commonly used detection mechanism, since the reduction of nitrite suffers from interferences such as reduction of nitrate ions and molecular oxygen [18,19]. The oxidation of nitrite can be given by Eqs. (1) and (2) [20,21]
H2Oþ 2NO2-NO3 þ NO2 þ 2H þ
(2)
n
Corresponding author. Fax: þ 90 452 2339149. E-mail address: kuralay.fi[email protected] (F. Kuralay).
http://dx.doi.org/10.1016/j.talanta.2015.07.095 0039-9140/& 2015 Elsevier B.V. All rights reserved.
However, the oxidation of nitrite requires high potential using bare electrodes. Furthermore, sensitivity is usually very low [22– 24]. The redox reaction at bare electrodes commonly involves slow transfer kinetics. Santos et al. have used a glassy carbon electrode modified with alternated layers of iron(III) tetra-(N-methyl-4pyridyl)-porphyrin and copper tetrasulfonated phthalocyanine for nitrite determination by differential pulse voltammetry [6]. (4Ferrocenylethyne) phenylamine functionalized graphene oxide modified glassy carbon electrode have also been fabricated by Liu et al. [14]. Rocha et al. have reported anodic oxidation of nitrite at a glassy carbon electrode modified with molybdenum oxide layer [25]. In this work, we modify a disposable pencil graphite electrode (PGE) surface with a nanocomposite, poly(vinylferrocenium)/multi-walled carbon nanotubes (PVF þ /MWCNTs), in order to surpass the challenge. It is well known that electroactive polymers can provide great advantageous for building up systems having good electrical properties, high-effective surface area and good conductivity [26– 28]. There have been various attempts on electrochemical nitrite sensing. Electrocatalytic activity of nitrogen-containing carbon nanomaterials prepared by the carbonization of nanostructured polyaniline salts for the electrooxidation reaction of nitrite and ascorbic acid has been presented [29]. A voltametric method has been developed to determine nitrite by using carbon paste electrode modified with polyvinylimidazole [30]. Particularly, conducting polymer/nanomaterials composites are
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one of the important substrates in order to develop novel materials for electrochemical sensing applications. Gligor and Walcarius have reported the modification of a glassy carbon electrode with poly(Toluidine Blue O) and single-walled carbon nanotubes for the electrocatalytic oxidation of nitrite [31]. Guo et al. have fabricated a polyaniline and carbon nanotubes modified electrode for the reduction of nitrite [32]. An electrochemical sensor has been fabricated by electrodeposition of gold nanoparticles on poly(3-methylthiophene) modified glassy carbon electrode for the determination of nitrite and iodate [33]. The electrochemical solid-phase extraction (EC-SPE) and amperometric determination of nitrite with polypyrrole nanowire modified electrode have been reported by Tian et al. [34]. Electrodeposited conducting poly(Azure A) at a carbon nanotube modified glassy carbon electrode has been used for electrocatalytic reduction of nitrite [35]. A cytochrome c electrochemical biosensor constructed by immobilization of the biomolecule on a hybrid material consisting of the conducting polymer poly-(3-methylthiophene) and multi-walled carbon nanotubes for nitrite detection has been reported [36]. Zhang et al. have developed poly(ethylenedioxythiophene)–Au nanoparticles composite film toward nitrite oxidation [37]. Polydiphenylamine–Pt composite has been synthesized by electrochemical method on glassy carbon electrode by Unnikrishnan. The electrocatalytic activity of PDPA–Pt film modified electrode toward the oxidation of nitrite has been studied by cyclic voltammetry and amperometry [38]. In this work, the preparation of PVF þ /MWCNTs composite coated electrode in a simple one-step electropolymerization of poly(vinylferrocene) (PVF) in the presence of MWCNTs on a disposable pencil graphite electrode and the application of the prepared electrode in nitrite sensing were detailed. The composite electrode was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The oxidation of nitrite ion was investigated with differential pulse voltammetry (DPV). In particular, the oxidation of nitrite was found at less positive potentials proving the catalytical effect of PVF þ /MWCNTs coated electrode. The effect of possible interferents were examined using 10-fold concentrated solutions of ascorbic acid, uric acid, phosphate and sulfate. The analytical applicability of the electrode was demonstrated in a commercial mineral water sample with a high reliability. The results show that the coated electrode have improved electrochemical activity towards nitrite oxidation. To the best of our knowledge this is the first to determine nitrite ion using PVF þ /MWCNTs coated electrode. Such an electrode system provided nonenzymatic sensing platform having good electrocatalytic properties and sensitivity for nitrite detection with a fast response time. Furthermore, disposable pencil graphite electrode technology offers low-cost analysis.
2. Experimental 2.1. Chemicals Vinylferrocene (97%), multi-walled carbon nanotubes (MWCNTs) and sodium nitrite (NaNO2) were obtained from Sigma-Aldrich. Other chemicals were in analytical grade and supplied from Sigma and Merck. All solutions were prepared with ultra pure water and purged with nitrogen to eliminate oxygen. Nitrite solutions were prepared in 50 mM phosphate buffer solution at pH 7.4. 2.2. Apparatus The potential-controlled coulometry, cyclic voltammetry (CV)
and electrochemical impedance spectroscopy (EIS) were carried out with CH Instruments System; Model 660 C (USA). Differential pulse voltammetry was carried out with AUTOLAB-PGSTAT 204 analysis system supported with a NOVA software package (Metrohm, The Netherlands). Three electrode system consists of a pencil graphite working electrode (PGE), an Ag/AgCl reference electrode (BASi, Lafeyette, IN USA) and a Pt wire auxilary electrode (BASi, Lafeyette, IN USA) was used during the electrochemical measurements. The connector of the PGE (0.5 mm HB Tombow) was a Tombow pencil. Scanning electron microscopy (SEM) images of the electrodes were obtained using Zeiss Evo 50 EP-SEM (USA). 2.3. Synthesis of poly(vinylferrocene) Poly(vinylferrocene) (PVF) was polymerized from vinylferrocene using 2,2-Azo-bis(2-methylpropionitrile) (AIBN) as an initiator [39]. The polymerization was carried out at 70 °C for 20 h. PVF (1.0 mg mL 1)/MWCNTs (1.0 mg mL 1) suspension was prepared in methylene chloride/tetra-n-butylammonium perchlorate (TBAP) (0.1 M) solvent/supporting electrolyte system. No pretreatment was used for MWCNTs. The suspension was sonicated for 30 min before the experiments. The solution was kept under nitrogen atmosphere during the experiments. 2.4. Preparation of poly(vinylferrocenium)/multi-walled carbon nanotubes coated PGE electrode (PVF þ /MWCNTs/PGE) PVF þ /MWCNTs coated PGE was prepared via electrooxidation of PVF/MWCNTs at þ 0.7 V vs. Ag/AgCl using constant potential electrolysis. 1 cm of the pencil tip was dipped into the suspension. PVF þ coated PGE (PVF þ /PGE) was prepared via electrooxidation of PVF at þ0.7 V vs. Ag/AgCl. Different modification times of PVF þ /MWCNTs onto PGE were used (15–120 s). 2.5. Electrochemical characterization of PVF þ /MWCNTs/PGE Electrochemical characterization of PVF þ /MWCNTs/PGE was performed with CV and EIS. 2.5.1. Cyclic voltammetric characterization of PVF þ /MWCNTs/PGE Cyclic voltammograms (CVs) of uncoated PGE, PVF þ /PGE and PVF þ /MWCNTs/PGE were recorded in 50 mM phosphate buffer solution pH 7.4 containing 0.1 M NaClO4 between þ0.0 V and þ1.0 V vs. Ag/AgCl. 2.5.2. Impedance measurements EIS measurements were conducted at a DC voltage of 100 mV using 50 mM phosphate buffer solution pH 7.4 containing 0.1 M NaClO4. The frequency was varied over the range of 105–10 1 Hz. The perturbation voltage was 10 mV. 2.5.3. Voltammetric detection of nitrite Differential pulse voltamograms (DPVs) of PVF þ /MWCNTs/PGE were recorded at an initial potential of þ0.1 V and a final potential of þ1.2 V vs. Ag/AgCl with a pulse amplitude of 50 mV and a scan rate of 0.1 mV/s. Pulse duration was 50 ms. Commercial mineral water sample was used without any pretreatment.
3. Results and discussion Fig. 1A presents the schematic presentation of the study. In the first part of the study, we prepared a PVF þ /MWCNTs composite coated disposable pencil graphite surface by constant potential electrolysis and characterized the nanocomposite coated electrode with SEM, CV and EIS. Fig. 1B-a–f presents the SEM images of
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Fig. 1. (A) Schematic presentation of the work. (B) SEM images of uncoated PGE (a and d), PVF þ coated PGE (b and d), PVF þ /MWCNTs coated PGE (c and f) at 20 μM and 200 nm, respectively.
uncoated PGE, PVF þ coated PGE and PVF þ /MWCNTs coated PGE. The images were given at low and high magnitude values. SEM image of the uncoated PGE show that the surface of the pencil graphite was rough (Fig. 1B-a and d). The morphology of the graphite surface changed after the modification of the PGE with PVF þ and PVF þ /MWCNTs composite. SEM images of PVF þ coated PGE at 2 different magnitude values are shown in Fig. 1B-b and e. Subsequently, SEM images of PVF þ /MWCNTs coated PGE at 2 different magnitude values are shown in Fig. 1B-c and f. SEM image of the PVF þ coated electrode shows the homogeneous distribution of the nanostructured polymer on the electrode (Fig. 1B-e). As shown in Fig. 1B-f, PVF þ /MWCNTs had a different surface morphologies, but still having a well-dispersed distribution of nanocomposite on the electrode. Cyclic voltammetric behaviors of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE were then investigated in order to perform the electrochemical characterization. Related voltammograms are given as Fig. 2A-a–c. In the cyclic voltammograms (CVs) of PVF þ /MWCNTs/PGE and PVF þ /PGE, there are well-defined oxidation and reduction peaks due to the ferrocenium/ferrocene centers of the polymer (Fig. 2A-a and Fig. 2A-b, respectively). The anodic peak was at about þ0.37 V vs. Ag/AgCl and the cathodic peak was at about þ 0.26 V vs. Ag/AgCl for the PVF þ /MWCNTs/PGE. The anodic peak was at about þ0.39 V vs. Ag/AgCl and the cathodic peak was at about þ0.24 V vs. Ag/AgCl/ for the PVF þ /PGE. In the presence of carbon nanotubes, oxidation/ reduction peak currents increased significantly indicating the fast electron transfer [40]. The anodic peak was at about þ0.39 V vs.
Fig. 2. (A) CVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE. (B) Nyquist diagrams of (a) PVF þ /MWCNTs/PGE and (b) PVF þ /PGE in 50 mM phosphate buffer (pH 7.4) containing 0.1 M NaClO4.
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Fig. 3. (A) DPVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE in 1 mM nitrite solution prepared in 50 mM phosphate buffer (pH 7.4). (B) CVs of (a) PVF þ /MWCNTs/PGE, (b) PVF þ /PGE, and (c) uncoated PGE in 1 mM nitrite solution prepared in 50 mM phosphate buffer (pH 7.4). (C) Effect of PVF þ /MWCNTs modification time (a) 15 s, (b) 30 s, (c) 45 s, (d) 60 s, and (e) 120 s (related DPVs).
Fig. 4. (A) DPVs of PVF þ /MWCNTs/PGE in (a) 1 μM, (b) 5 μM, (c) 10 μM, (d) 20 μM, (e) 50 μM, (f) 100 μM, (g) 150 μM, (h) 200 μM, (i) 250 mM, (j) 300 μM, and (k) 400 μM nitrite solution (inset: (a) DPV of uncoated PGE in 50 mM phosphate buffer, DPVs of PVF þ /MWCNTs/PGE in (a) 1 μM, (b) 5 μM, (c) 10 μM nitrite solution). (B) Calibration graph.
Ag/AgCl and the cathodic peak was at þ0.23 V vs. Ag/AgCl. The electroactive surface coverages (ΓEA) of the coated electrodes were calculated at a scan rate of 100 mV s 1 and found as 1.9( 70.4)*10 7 mol cm 2 for the PVF þ /MWCNTs coated electrode and 1.0( 70.2)*10 7 mol cm 2 for the PVF þ coated one
(n ¼3) [41]. Another electrochemical characterization method, electrochemical impedance spectroscopy, was applied in order to differentiate carbon nanotubes/polymer coated electrode and polymer coated electrode. Fig. 2B-a and b presents the electrochemical impedance spectra for PVF þ /MWCNTs/PGE and
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Fig. 5. DPVs of PVF þ /MWCNTs/PGE in mineral water containing (a) 100 μM, (b) 50 μM, (c) 10 μM, (d) 5 μM nitrite, and (e) DPV of uncoated PGE in 50 mM phosphate buffer. Table 1 Recovery studies with the proposed method. Added (μM)
Found (μM)
Recovery (%)
RSD (%) (n ¼3)
5 10 50 100
57 1 107 1 467 1 987 1
91 71 98 71 92 71 98 71
2.8 3.1 2.3 3.6
PVF þ /PGE, respectively. The average charge transfer resistance (Rct) values are 3.2( 7 0.1)*105 Ω and 3.9(70.1)*105 Ω for PVF þ /MWCNTs/PGE and PVF þ /PGE, respectively (n¼ 3). The presence of carbon nanotubes prompted the electron transfer between the electrode and the solution interface leading a smaller Rct. After the characterization of the modified surfaces and getting better electrochemical responses in the presence of carbon nanotubes, we tested the advantage of the nanocomposite modified electrode for nitrite detection. Fig. 3A-a–c presents the differential pulse voltammograms (DPVs) of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE in 1 mM nitrite solution prepared in pH 7.4 phosphate buffer. As seen in Fig. 3A-a, higher oxidation peak current for nitrite was obtained with PVF þ /MWCNTs coated PGE
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compared to the PVF þ coated PGE (Fig. 3A-b) and uncoated PGE (Fig. 3A-c). The oxidation peak current was 3-fold higher than the other two electrodes. The peak currents were almost the same with PVF þ coated PGE and uncoated PGE. However, the polymer coated one gave a less nitrite oxidation potential. The oxidation peak potentials were at about þ0.84 V vs. Ag/AgCl for the coated ones and the oxidation peak potential was at about 0.98 V vs. Ag/ AgCl for the uncoated PGE. Fig. 3B-a–c presents the CVs of PVF þ /MWCNTs/PGE, PVF þ /PGE and uncoated PGE in 1 mM nitrite solution prepared in pH 7.4 phosphate buffer. Differential pulse voltemmetry studies were correlated with cyclic voltammetry results. PVF þ /MWCNTs/PGE exhibited high oxidation peak current for nitrite at a lower potential (at about þ0.80 V vs. Ag/AgCl). These results confirm our characterization results explained above. Thus, further experiments were performed with the PVF þ /MWCNTs/PGE. DPVs of PVF þ /MWCNTs/PGE (Fig. 3A-a) and PVF þ /PGE (Fig. 3A-b) also show the oxidation peaks of the polymer at about þ 0.52 V vs. Ag/AgCl. Higher oxidation peak current was obtained in the presence of carbon nanotubes. Optimization of the nanocomposite modification time was performed between 15 s and 120 s in order to improve the sensitivity (Fig. 3B). DPVs were taken with PGEs coated with the nanocomposite using different modification time values. There was a gradual increase for the nitrite oxidation peak current up to 60 s of nanocomposite modification time (Fig. 3B-d). After 60 s there was a slight decrease in the oxidation peak current of nitrite (for 120 s, Fig. 3B-e). Hence, 60 s of immobilization time was chosen as the optimum condition. Next, we investigated the effect of concentration of nitrite on the response of the sensor using optimum experimental conditions; 60 s of nanocomposite modification at þ 0.7 V vs. Ag/AgCl. The concentration interval used in the study was 1–500 μM. The sensing platform developed in the study exhibited a good response having a linearity between 1 μM and 400 μΜ nitrite concentration (Fig. 4). After 400 μM the response of the electrode did not change appreciably. The relative standard deviations were calculated as 4.2% and 2.5% for three successive determinations at 1 μM and 200 μM nitrite, respectively. Limit of detection (LOD) was found as 0.1 μM for nitrite from the relationship LOD ¼3Sb/m, where Sb is the standard deviation of the blank and m is the slope of the calibration graph (n ¼3) [42]. This value was comparable with the reported studies [4,6,14,23,36,37]. Furthermore, the prepared electrode had a good analytical performance in terms of being low-cost, reliable, effortless and fast. And then we applied the nanocomposite modified electrode for
Fig. 6. DPVs of PVF þ /MWCNTs/PGE in (A) (a) 100 μM nitrite in the presence of ascorbic acid and (b) 100 μM nitrite. (B) (a) 100 μM nitrite in the presence of uric acid and (b) 100 μM nitrite.
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the determination of nitrite in commercial mineral water sample adding a certain amount of nitrite into the sample. No pretreatment was used in the sample. Fig. 5 shows the related DPVs obtained with PVF þ /MWCNTs coated PGE in mineral water solutions having 5, 10, 50 and 100 μM nitrite concentration. The coated electrode was successfully applied to the determination of nitrite in commercial mineral water sample. The recovery values were also calculated and given as Table 1. In addition, the interference effects of 10-fold excess amounts of ascorbic acid, uric acid, phosphate ion and sulfate ion were investigated and no interference effects of the given species were found for the determination of nitrite in the oxidation potential interval of nitrite obtained with this electrode system. Fig. 6A-a and b presents the DPVs of PVF þ /MWCNTs/PGE in 100 μM nitrite in the presence and absence of ascorbic acid, respectively. Fig. 6B-a and b presents the DPVs of PVF þ /MWCNTs/PGE in 100 μM nitrite in the presence and absence of uric acid, respectively. The electrode system was easily separated ascorbic acid (at about þ0.25 V), uric acid (at about þ0.35 V), polymer (at about 0.55 V) and nitrite (at about 0.80 V) oxidation peaks.
4. Conclusions In summary, a highly sensitive and selective sensor for nitrite was developed in the study using PVF þ /MWCNTs coated electrode. The preparation of the nanocomposite coated electrode was performed in a single and easy electropolymerization method. The nitrite oxidation was detected in a fast, low-cost and reliable way in the study. Detection limit was found as 0.1 μM for nitrite which is comparable with the reported studies [4,6,14,23,36,37]. We believe this sensing platform that combines disposable electrode technology and nanomaterials technology will be a good alternative for different biosensing applications.
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