Amperometric nitrite sensor based on a glassy carbon electrode modified with multi-walled carbon nanotubes and poly(toluidine blue) Juan Dai, Dongli Deng, Yali Yuan, Jinzhong Zhang, Fei Deng & Shuang He
Microchimica Acta Analytical Sciences Based on Micro- and Nanomaterials ISSN 0026-3672 Volume 183 Number 5 Microchim Acta (2016) 183:1553-1561 DOI 10.1007/s00604-016-1773-z
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Author's personal copy Microchim Acta (2016) 183:1553–1561 DOI 10.1007/s00604-016-1773-z
ORIGINAL PAPER
Amperometric nitrite sensor based on a glassy carbon electrode modified with multi-walled carbon nanotubes and poly(toluidine blue) Juan Dai 1 & Dongli Deng 1,2 & Yali Yuan 3 & Jinzhong Zhang 1,4 & Fei Deng 1 & Shuang He 1
Received: 15 October 2015 / Accepted: 1 February 2016 / Published online: 24 February 2016 # Springer-Verlag Wien 2016
Abstract An amperometric nitrite sensor modified with m u l t i - w a l l e d c a r b o n n a n o t u b e s ( M W C N Ts ) a n d poly(toluidine blue) (PTB) on glassy carbon electrode was constructed. The surface morphology of the composite- modified electrode was characterized by scanning electron microscopy, and the electrochemical response behavior and electrocatalytic oxidation mechanism of nitrite were investigated by cyclic voltammetry. The high surface-to-volume ratio of MWCNTs and PTB brings the electrochemical sensing unit and nitrite in full contact. This renders the electrochemical response extremely sensitive to nitrite. Under the optimal measurement conditions and a working voltage of 0.73 V (vs. SCE), a linear relationship is obtained between the oxidation peak current and nitrite concentration in the range of 39 nM–1.1 mM, and the limit of detection is lowered to 19 nM (at an S/ N ratio of 3). The sensor was successfully applied to the determination of nitrite in greenhouse soils.
* Jinzhong Zhang
[email protected]
1
Key Laboratory of Eco-environments in Three Gorges Reservoir Region, Ministry of Education, College of Resources and Environment, Southwest University, Chongqing 400715, China
2
Faculty of Chemical and Pharmaceutical Engineering, Chongqing Industry Polytechnic College, Chongqing 401120, China
3
College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
4
Chongqing Key Laboratory of Agricultural Resources and Environment, Chongqing 400716, China
Keywords Cyclic voltammetry . Scanning electron microscopy . Amperometry . Electrocatalytic oxidation . Electropolymerization . Nanocomposite . Greenhouse soil
Introduction Nitrite is one of the intermediate products of nitrogen cycling, and widely exists in soil, natural water and food, which is closely related to the life process. Nitrite can enter human body through the food chain, which may not only cause oxygen poisoning, but also interact with amines to form carcinogenic nitrosamines [1]. Owing to the potential toxicity of nitrite anion, it is of great significance to detect trace nitrite for environmental protection and public health. Some analytical techniques have been used to determine nitrite, such as spectrometry (UV–vis, chemiluminescence, fluorescence, infrared spectrum, Raman) [2–5], chromatography (particularly gas chromatography) [6], capillary electrophoresis [7, 8], and electrochemical methods (voltammetry, potentiometry) [9–11]. However, most of them have disadvantages of time-consuming, tedious pretreatment procedures, and low sensitivity. Among them, electrochemical sensing techniques are favorable for nitrite determination with the advantages of rapid response, high sensitivity, low cost, and simple use [12]. Although nitrite is electroactive on bare glassy carbon electrode (GCE) [10], its oxidation requires undesirably high overvoltage [13, 14], and the application of bare electrode is limited because several species may poison the electrode surface and reduce sensitivity and selectivity [15]. Therefore, it is necessary to construct stable electrochemical sensor by modifying suitable material on carbon electrode surface to improve the electrocatalytic ability, and further improve the sensitivity and selectivity for nitrite determination.
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Carbon nanotubes (CNTs) have been widely used to prepare electrochemical sensor based on their good electrical conductivity, high electrocatalytic activity, and strong adsorption ability [16–18]. Some electrochemical methods tried to modify CNTs on GCE surface to reduce the overvoltage for nitrite detection [19, 20]. However, these researches obtained narrow linear ranges with two or three orders of magnitude, and the electrochemical determination was prone to suffer interference by other readily oxidizable compounds [19, 20]. It is necessary to further extend linear range and lower detection limit for nitrite detection in actual environmental samples. The composite film electrodes combined nanophase materials with organic dyes have become a hotspot in the field of chemical modified electrodes. As a phenothiazine dye with electrocatalytic activity, toluidine blue has been used as a modifier to construct chemical and biological sensors for the determination of NADH, glucose, and multidrug resistance gene [21–23]. To the best of our knowledge, no report can be found on the preparation of the electrochemical sensor based on the composite film of multi-walled carbon nanotubes (MWCNTs) and poly(toluidine blue) (PTB) for nitrite determination. The objectives of this work were to prepare an amperometric sensor through electropolymerization of toluidine blue on MWCNTs modified GCE, examine the electrocatalytic oxidation behavior and mechanism of nitrite on the composite modified electrode, and establish highly sensitive and selective determination method. Compared with other electrochemical sensors modified with carbon nanomaterials, this PTBMWCNTs composite film based sensor displayed an excellent linear response range with the limit of detection of 19 nM, and was successfully applied to the determination of nitrite in greenhouse soils.
Materials and methods Regents and materials All chemicals were of analytical grade without further purification. Toluidine blue was purchased from Aldrich (http:// www.sigmaaldrich.com). NaNO 2 , Na 2 HPO 4 · 12H 2 O, NaH2PO4, KCl and NaCl were purchased from Chengdu Kelong Chemical Company (http://www.cdkelong.com). Double-distilled deionized water was used to prepare all solutions. NaNO2 was dissolved in pure water to prepare a stock solution (0.1 M) and stored at 4 °C. Phosphate buffer containing 0.1 M KCl, 0.1 M Na2HPO4 · 12H2O and 0.1 M NaH2PO4 was prepared, and the pH values were adjusted by using 1 M HCl and 1 M NaOH. Multi-walled carbon nanotubes (MWCNTs) with purity of over 95 %, tube diameter of 8–15 nm and length of 0.5–2 μm were purchased from Nanjing XFNANO Materials Tech Co., Ltd. (http://www.xfnano.com).
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Instruments All electrochemical measurements were performed on a CHI 660E electrochemical measurement system (Shanghai Chenhua Instrument Company, http://www.chinstr.com/) controlled by a personal computer, a three-electrode system was employed with a PTB-MWCNTs modified glassy carbon electrode (GCE) as working electrode, a platinum wire electrode as auxiliary electrode, and a saturated calomel electrode (SCE) as reference electrode. The surface morphology of the modified electrodes was characterized by using a JSM6510LV scanning electron microscope (Japan Electron Optics Laboratory Co., Ltd., http://www.jeol.co.jp). Preparation of the modified electrodes The GCE (3 mm in diameter, Tianjin Aidahengsheng Tech Co., Ltd., http://www.tjaida.cn) was polished with 0.3 μm and then 0.05 μm alumina slurries, and successively cleaned with sonication in ethanol and double-distilled deionized water for 5 min to remove any adhesive substances on the electrode surface [24]. The MWCNTs (5 mg) were ultrasonically dispersed in 5 mL of double-distilled deionized water for 10 h, and then a stable suspension was obtained. For better dispersion, the suspension was ultrasonically dispersed for 5 min before modification. The water-dispersed MWCNTs (8 μL, 1 mg mL−1) was taken and coated onto the clean surface of GCE, dried at room temperature, and then immersed in 0.1 M phosphate buffer (pH 6.8) containing 0.5 mM toluidine blue, and electropolymerized by cyclic voltammetry between −0.8 and 1.1 V at 50 mV s−1 for 25 cycles to construct the composite film based sensor (PTB-MWCNTs/GCE). The modified electrode was rinsed with double-distilled deionized water for several minutes to remove the residual toluidine blue on the electrode surface, and then stored in 0.1 M phosphate buffer (pH 6.8) at 4 °C, and rinsed with double-distilled deionized water prior to use. The MWCNTs/GCE and PTB/GCE were also prepared. The MWCNTs/GCE was fabricated through coating 8 μL of the water-dispersed MWCNTs (1 mg mL−1) onto GCE surface, and then dried at room temperature. The PTB/GCE was obtained by scanning bare GCE in 0.1 M phosphate buffer (pH 6.8) containing 0.5 mM toluidine blue with cyclic voltammetry between −0.8 and 1.1 V at 50 mV s−1 for 25 cycles. Electrochemical behaviors of nitrite on the composite modified electrode The cyclic voltammograms were obtained in 0.1 M phosphate buffer (pH 5.0) containing 1 mM nitrite on the PTBMWCNTs/GCE at scan rates of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 and 150 mV s−1, and the electrochemical behaviors of nitrite on the composite modified electrode were
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examined through the relationships between the oxidation peak current, peak potential and scan rate. Analytical procedures Prior to nitrite detection, the composite modified electrode should be immersed in the supporting electrolyte (0.1 M phosphate buffer, pH 5.0) to sweep until a stable current baseline appears. The oxidation peak currents of nitrite at different concentrations were measured in 10 mL of 0.1 M phosphate buffer (pH 5.0) with gentle stir using magnetic stirrer. The measurement parameters of current-time (i-t) curve were as follows: operational potential, 0.73 V; hold time, 20 s; injection interval, 0.01 s; total time, 600 s. All measurements were performed at room temperature. According to the standard for environmental quality monitoring of agricultural soils (NY/T 395–2000), the greenhouse soil samples (0–20 cm profile) were collected from the experimental farm of Southwest University, and extracted with double-distilled deionized water, filtered, and adjusted pH to 5.0 with the supporting electrolyte for nitrite determination.
Results and discussion Electropolymerization of toluidine blue on the surface of MWCNTs modified electrode and its spectroscopic evidence The electropolymerization of toluidine blue on the surface of bare GCE and MWCNTs/GCE was performed by cyclic voltammetry, and the obtained cyclic voltammograms are shown in Fig. 1. It can be seen that the cyclic voltammograms of the electropolymerization of toluidine blue on the surface of the two electrodes are very similar. If the potential scan is confined to the range from −0.8 to 0.6 V, only a pair of peaks at about −0.25 V (couple I) is observed. However, when sweep positively over 0.8 V, the oxidation current increases obviously, which indicates that toluidine blue is initially polymerized
30 25 20 15 10 5 0 -5 -10 -15 -1.0
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Fig. 1 Cyclic voltammograms of 0.5 mM toluidine blue on bare glassy carbon electrode (a) and multi-walled carbon nanotubes/ glassy carbon electrode (b) in 0.1 M phosphate buffer (pH 6.8) at scan rate of 50 mV s−1 for 25 cycles
on the electrode surface. A pair of new redox peaks appears at −0.07 V (couple II) after the second scan. Based on Ref. [25], couple II is ascribed to the redox of the resulting polymerized toluidine blue, while couple I ascribed to the mono-type redox peaks (monomer toluidine blue). With the further scans, the redox peak current of couple II and the anodic current of couple I increase steeply, while the cathodic current of couple I decrease gradually until reaching stable status. When the electrode is rinsed with double-distilled deionized water and transferred into the deaerated phosphate buffer (pH 6.8) without toluidine blue, couple II still maintains the similar trend, whereas couple I almost disappears. These phenomena indicate that PTB film is deposited on the electrode surface [26]. An olive-drab film can be clearly observed on the electrode surface, which is an evidence of the existence of PTB film. Compared Fig. 1a with Fig. 1b, the oxidation peak current of monomer toluidine blue or polymerized toluidine blue on MWCNTs modified electrode increase about 2-fold than that on bare GCE. This phenomenon may be caused by the adsorption of more toluidine blue on MWCNTs owing to its large hydrophobic specific surface area and π-conjugated structure. The surface morphology of MWCNTs film and PTBMWCNTs composite film was investigated by scanning electron microscope (SEM), and the images are shown in Fig. 2. It can be clearly seen that MWCNTs possess small bundles and single nanotubes with special three-dimensional network structure (Fig. 2a), and almost uniformly distribute on GCE surface. In Fig. 2b, PTB-MWCNTs surface becomes much coarse than MWCNTs surface, indicating that a large number of polymerized toluidine blue have directly attached on the tube wall of MWCNTs, and the PTB-MWCNTs composite film forms successfully. Compared with single-walled carbon nanotubes (SWCNTs) formed by single layer graphite, the MWCNTs are formed by the volume set of multilayer graphite. This PTB-MWCNTs composite film can be used as electrochemical sensing unit to fully contact nitrite, and improve the signal-to-noise ratio and the sensitivity for nitrite determination.
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Electrocatalytic oxidation of nitrite on the composite modified electrode Electrochemical response of nitrite on different electrodes The electrochemical behaviors of nitrite on bare GCE, MWCNTs/GCE, PTB/GCE and PTB-MWCNTs/GCE were investigated by cyclic voltammetry, and the obtained cyclic voltammograms are shown in Fig. 3. It can be seen that only background current is observed on bare GCE in phosphate buffer without nitrite, and the peak current increase a little after MWCNTs (Fig. 3b) and PTB (Fig. 3c) modified on GCE. However, irreversible oxidation peaks appear on the four electrodes in the presence of 1 mM nitrite, and only a small and wide peak (0.828 V, 13.07 μA) can be observed on bare GCE (Fig. 3a), and the oxidation peak current reaches 31.86 μA on MWCNTs/GCE (Fig. 3b),
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and the peak potential shifts to 0.771 V. The peak current only increases to 18.20 μA on PTB/GCE (Fig. 3c), and the peak potential shifts to 0.892 V. It should be noted that a welldefined oxidation peak of nitrite with a peak current of 47.69 μA is obtained at 0.778 V on PTB-MWCNTs/GCE (Fig. 3d). The largest peak current and the highest current density (681.3 μA⋅cm−2) at a relatively low peak potential suggest that the PTB-MWCNTs composite film has excellent electrocatalytic activity for nitrite oxidation. This is because MWCNTs have small three-dimensional porous structure, which makes MWCNTs easily to be wetted by water, and thus a better electrode-solution interface can be formed to promote electron transfer. One the other hand, the porous structure of MWCNTs can increase the specific surface area largely, which allows more nitrite anion to be strongly adsorbed on the electrode surface. Moreover, the crosslinked PTB on MWCNTs possesses quinonyl and hydroxyl
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groups, the protonated PTB (PTBH+) may attract the negatively charged nitrite anion on the electrode surface. The synergetic function of MWCNTs and PTB contributes to great enhancement of the oxidation peak current of nitrite on PTB-MWCNTs/GCE.
PTBHþ NO2 − þ H2 O→PTBHþ NO3 − þ 2Hþ þ 2e− ð3Þ
Electrochemical oxidation mechanism of nitrite on poly(toluidine blue)-MWCNTs composite film based sensor The electrochemical oxidation mechanism of nitrite on PTBMWCNTs composite film based sensor was investigated by cyclic voltammetry. The results showed that only an oxidation peak appears in the cyclic voltammogram for each scan, which suggests that the oxidation of nitrite is an irreversible process on the composite modified electrode. The effect of scan rate (v) on the electrochemical oxidation of nitrite was examined, and a good linear relationship was obtained between the oxidation peak current (ip) and the square root of scan rate (v1/2) in the range of 10–150 mV s−1. The linear equation can be expressed as i p = 2.583v 1 / 2 + 9.380 (R2 = 0.993, n = 11), which indicates a diffusion-controlled electrochemical oxidation process of nitrite on the composite modified electrode. The relationship between the oxidation peak potential (Ep) and scan rate was also examined. It can be found that Ep increased with the increase of the scan rate in the range of 10–150 mV s−1, and a good linear relationship was obtained between Ep and the natural logarithm of the scan rate (lnv) with an equation of E p = 0.022lnv + 0.690 (R2 = 0.991, n = 11). According to the theory of Laviron and Roulier [27], the peak potential should be linearly related to lnv as follows:
Preparation conditions of the composite modified electrode and measurement conditions of nitrite In order to obtain good response for nitrite on the composite modified electrode, the preparation conditions of the electrode and measurement conditions of nitrite were also examined. Generally, the film thickness of MWCNTs can be controlled by the modified volume of the water-dispersed MWCNTs, while that of PTB controlled by electropolymerization cycles. The results indicated that the appropriate volume of the MWCNTs suspension coated on GCE surface was 7–10 μL, and the peak current of 1 mM nitrite on MWCNTs modified electrode reached the maximum (31.57 μA) when the coated v o l u m e w a s 8 μ L . S i m i l a r l y, t h e a p p r o p r i a t e electropolymerization cycle of toluidine blue on MWCNTs modified electrode was 20–30, and the peak current of 1 mM nitrite on the composite modified electrode reached maximum (46.72 μA) when the electropolymerization cycle is 25. This is because that the number of active site on the composite modified electrode is dependent on MWCNTs amount, while the electrocatalytic activity of PTB on its thickness. If the composite film is too thick, the electron transfer will be hindered between solution and electrode, and the electrocatalytic activity of PTB to nitrite will decrease, and thus lead to the increase of background current and the decrease of peak current. We also found that the appropriate pH range of the substrate solution was 4–6, and the peak current of 1 mM nitrite on the composite modified electrode reached maximum (46.76 μA) when the substrate solution pH was 5. These results indicated that weak acid medium is more favorable for the
ð1Þ
where Ep is peak potential; E0’ is formal potential; k0 is standard rate constant; α is electron transfer coefficient; n is electron transfer number; v is scan rate; F is Faraday constant; T is thermodynamic temperature. For an irreversible electrochemical process, α is considered as 0.5. Based on Eq. 1, we can calculate the electron transfer number as 2.3 ≈ 2. In an acidic solution, PTB is positively charged due to the protonation of amine group, and donated as PTBH+. PTBH+ can attract negatively charged nitrite anion, resulting in better diffusion and enrichment of nitrite onto the electrode surface, which can promote the electrochemical oxidation of nitrite. Furthermore, the modified MWCNTs on GCE can effectively enlarge the specific surface area to produce more electroactive sites. Although the detailed catalytic oxidation mechanism of nitrite may be very complex, the possible electrode reactions can be expressed as follows: PTBHþ þ NO2 − →PTBHþ NO2 −
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Microchim Acta (2016) 183:1553–1561 Comparison of the detection parameters for nitrite obtained with various carbon nanomaterials modified glassy carbon electrode
Modified material
Linear range
Limit of detection Sensitivity (μA⋅μM−1) Ref.
Carbon nanotubes
2–10 μM 20 μM–1 mM 0.5 μM
–
[19]
Poly(azure A)-SWCNTs
3.0 μM–4.5 mM
1.0 μM
–
[20]
Thionin-MWCNTs Hemoglobin-MWCNTs-gold nanoparticle DNA functionalized SWNTs-Cu2+ complex
6 μM–15 mM 3.6 μM–3.0 mM 30 nM–2.6 mM
4 μM 0.96 μM 30 nM
0.002 0.002 –
[28] [29] [30]
MWCNTs-poly(amidoamine)-chitosan Poly(4-vinylpyridine)/MWCNTs/phosphotungstic acid
0.2–80 μM 1.2–17.5 μM
0.03 μM 0.2 μM
– –
[31] [32]
Reduced graphene oxide-sulfydryl functionalized MWCNTs Cobalt phthalocyanine functionalized MWCNTs
75 μM–6.06 mM 96 nM–0.34 μM
25 μM 62 nM
0.010 0.030
[33] [34]
n-Octylpyridinum hexafluorophosphate/SWCNTs
1.0 μM–12.0 mM
0.1 μM
0.080
[35]
Iron nanoparticles decorated graphene-MWCNTs Poly(5-amino-1,3,4-thiadiazole-2-thiol)/acid functionalized MWCNTs Graphene-polypyrrole-chitosan
0.1 μM–1.68 mM 10 nM–1 μM
75.6 nM 0.2 nM
0.697 0.892
[36] [37]
0.5–722 μM
0.1 μM
0.04
Cobalt nanoparticles/poly(3,4-ethylenedioxythiophene)/graphene Au nanoparticles-sulfonated graphene Ag nanoparticles-graphene oxide Graphite-supported Pd nanoparticles Poly(toluidine blue)-MWCNTs
0.5–240 μM 10 μM–3.96 mM 10–180 μM 0.3–50.7 μM 39 nM–1.1 mM
0.15 μM 0.2 μM 2.1 μM 71 nM 19 nM
0.976 0.045 – 0.29 0.072
electrocatalytic oxidation of nitrite. So 0.1 M phosphate buffer (pH 5.0) was used as the substrate solution for nitrite determination. Amperometric detection of nitrite Figure 4 shows the amperometric i-t curve of successive injection with nitrite at different concentrations. The results Table 2 Selectivity for nitrite determination by using the composite modified electrode
[38] [39] [40] [41] [42] This work
indicated that the amperometric response of the added nitrite is very fast with good reproducibility. The calibration curve of the oxidation peak current vs. nitrite concentration was obtained. There is an excellent linear relationship between the peak current and nitrite concentration in the range of 39 nM– 1.1 mM with a linear equation of i p = 0.0724c + 0.0137 (R2 = 0.9998, n = 23), and the limit of detection is obtained as 19 nM (S/N = 3).
Interfering ion
Concentration (mM)
Selectivity coefficient
Response change (%)
Na+ K+
0.4 0.4
5.0 × 10−3 5.0 × 10−3
−3.89 4.91
Ca2+
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.2 0.2 0.2
1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 2.7 × 10−5 2.7 × 10−5 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 1.0 × 10−4 5.0 × 10−3 5.0 × 10−3 1.4 × 10−4 1.0 × 10−2 1.0 × 10−2
−4.17 −4.45 4.37 2.39 3.41 −4.25 3.71 3.35 −4.12 −3.04 −4.90 3.60 −3.17 2.94 3.03
2+
Mg Cu2+ Al3+ Fe3+ Zn2+ Cd2+ Pb2+ Hg2+ SO42− NO3− Cl− CO32− I− Br−
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Table 3 Nitrite determination in four greenhouse soil samples by electrochemical and spectral analysis (μg g−1, n = 3) Electrochemical analysisa
Spectral analysisa
Kim greenhouse
0.68 ± 0.01
0.70 ± 0.11
Cotton greenhouse Maize greenhouse
0.63 ± 0.04 0.99 ± 0.02
0.63 ± 0.14 1.02 ± 0.16
Orange greenhouse
0.86 ± 0.01
0.86 ± 0.13
Soil sample
a
Average value ± standard deviation
Table 1 shows the detection parameters for nitrite obtained with various carbon nanomaterials modified GCE. It can be seen that PTB-MWCNTs/GCE has wide linear range with five orders of magnitude and very low detection limit, and the analytical performance is even better than many previous researches. Validation of method Stability and reproducibility are two important indices to evaluate the sensor performance. For the amperometric measurement, the sensor maintained 94 % of its initial response when stored in air for 7 days, and 79 % when stored in 0.1 M phosphate buffer (pH 6.8) at 4 °C for 80 day. For successive measurement of 2 μM nitrite, the current response was very close with a relative standard deviation (RSD, n = 9) of 3.8 % using the same sensor, and with a RSD of 4.1 % using five sensors. These data indicated that the PTB-MWCNTs composite film based sensor has good stability and reproducibility for nitrite determination. Moreover, the modified electrode is very easy to regenerate through rinsing with double-distilled deionized water for several minutes after nitrite determination. Selectivity coefficient can be used to evaluate the response ability of the electrode to different ions, the lower the selectivity coefficient, the higher the electrode selectivity to the measured ion. When the measured ion and interfering ion coexist in the same solution, the selectivity coefficient (Kij) can be calculated by the following equation. Ki j ¼
ai a j Z i =Z j
ð5Þ
where i is the measured ion, j is the interfering ion; ai and aj are the activities of the measured ion and interfering ion, respectively, and can be replaced by their concentrations in dilute solution; zi and zj are the charge numbers of the measured ion and interfering ion, respectively. The possible interference of some common cations and anions for nitrite detection was examined, and the obtained results are shown in Table 2. It can be seen that the selectivity coefficients of nitrite to Na+, K+, Ca2+, Mg2+, Cu2+, Al3+, Fe3+, Zn2+, Cd2+, Pb2+, Hg2+, SO42−, NO3−, Cl− and CO32−
are in the range of 2.7 × 10−5–5.0 × 10−3, while those to I− and Br− are 1.0 × 10−2. The changes in current response on the composite modified electrode keep within 5 % in the presence of 200-fold Na+, K+, Ca2+, Mg2+, Cu2+, Al3+, Fe3+, Zn2+, Cd2+, Pb2+, Hg2+, SO42−, NO3− and Cl−, and 100-fold CO32 − − , I and Br− in 2 μM nitrite solution, indicating that they have no obvious interference for nitrite detection. Under weak acid medium (pH 5.0), the hydrogen bonds may form between nitrite and hydroxyl groups of toluidine blue, which contributes to high selectivity of the composite modified electrode for nitrite. The nitrite contents in four greenhouse soil samples were determined by electrochemical and spectral analysis (Griess method), and the obtained results are shown in Table 3. It can be seen that the measured results by electrochemical analysis are in good agreement with those by spectral analysis, and the relative standard deviations of the former are lower than those of the latter. These results suggested that the sensor is promising to detect nitrite in environmental samples.
Conclusions Combined with the advantages of multi-walled carbon nanotubes and toluidine blue, an amperometric nitrite sensor based on the PTB-MWCNTs composite film has been constructed, which displayed an excellent performance in the linear range of 39 nM–1.1 mM with the detection limit of 19 nM. The composite modified electrode also showed good response during continuously use and different storage conditions with good stability and reproducibility, high sensitivity and selectivity, and is promising to apply to detect nitrite in environmental samples. Acknowledgments This work was supported by the National High Technology Research and Development Program (863 Program) of China (2012AA101405). Compliance with ethical standards Competing interest All authors declare that no competing interests exist.
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