DOI: 10.1002/asia.201500579
Full Paper
Nanoparticles
Nitrite Oxidation with Copper–Cobalt Nanoparticles on Carbon Nanotubes Doped Conducting Polymer PEDOT Composite** Junjie Wang, Guiyun Xu, Wei Wang, Shenghao Xu, and Xiliang Luo*[a] Abstract: Copper–cobalt bimetal nanoparticles (Cu¢Co) have been electrochemically prepared on glassy carbon electrodes (GCEs), which were electrodeposited with conducting polymer nanocomposites of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with carbon nanotubes (CNTs). Owing to their good conductivity, high mechanical strength, and large surface area, the PEDOT/CNTs composites offered excellent substrates for the electrochemical deposition of Cu¢Co nanoparticles. As a result of their nanostructure and the synergic effect between Cu and Co, the Cu¢Co/PEDOT/
Introduction Nitrite is ubiquitous within the environment and food, and it interacts with amines to generate carcinogenic nitrosamines.[1] The presence of excess nitrite in vegetables, drinking water, and food products is a serious threat to human health,[2] as excess nitrite promotes the irreversible oxidation of hemoglobin and hinders the blood from transporting oxygen. Therefore, accurate determination of nitrite is of vital importance for food safety and human health. During the past few years, several detection techniques have been utilized for the determination of nitrite such as spectrophotometry,[3] chemiluminescence,[4] capillary electrophoresis,[5] and chromatography.[6] However, these analytical methods usually suffer from some disadvantages including the interference of coexisting cationic or anionic species, expensive equipment, tedious detection procedures, and long assay times. Recently, because of their rapid response, cost effectiveness, sensitivity, and simple use, electroanalytical methods[7–13] have attracted much attention for the detection of nitrite. Until now, many kinds of electrochemical nitrite sensors have been fabricated based on chemically modified electrodes.[14–18] Furthermore, the interest in
[a] J. Wang, Dr. G. Xu, W. Wang, Dr. S. Xu, Prof. X. Luo Key Laboratory of Sensor Analysis of Tumor Marker Ministry of Education, College of Chemistry and Molecular Engineering Qingdao University of Science and Technology Qingdao 266042 (China) E-mail: [email protected] [**] PEDOT = poly(3,4-ethylenedioxythiophene). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500579. Chem. Asian J. 2015, 10, 1892 – 1897
CNTs composites exhibited significantly enhanced catalytic activity towards the electrochemical oxidation of nitrite. Under optimized conditions, the nanocomposite-modified electrodes had a fast response time within 2 s and a linear range from 0.5 to 430 mm for the detection of nitrite, with a detection limit of 60 nm. Moreover, the Cu¢Co/PEDOT/ CNTs composites were highly stable, and the prepared nitrite sensors could retain more than 96 % of their initial response after 30 days.
modified electrodes based on nanocomposites for nitrite detection has been the subject of several previous reports.[19, 20] Carbon nanotubes (CNTs), have dominated material science research since their discovery in 1991, owing to the outstanding physicochemical properties.[21] CNTs have attracted increasing interest for potential applications in sensors due to their electronic, chemical, and thermal properties.[22] With large specific surface area, wide electrochemical window, and high electrical conductivity, CNTs can provide a good support for metal nanomaterials deposition on electrodes[23] and have become one of the most promising materials in electrochemical sensors. Meanwhile, the relatively large specific surface area of CNTs provides reaction active sites for the electrocatalytic oxidation or reduction. Therefore, CNTs can favorably catalyze the electrochemical oxidation of nitrite.[43] As another kind of widely used material, conducting polymers have also been used extensively in the development of sensors because of their interesting electrical, optical, and electrocatalytic properties. Poly(3,4-ethylenedioxythiophene) (PEDOT), a type of conducting polymer, has been considered as the most promising one because its well-defined chemical structure offers outstanding stability and conductivity.[24, 25] Previous work showed that the electrical properties of electrodes coated with PEDOT[26, 27] can be greatly improved. CNTs doped with PEDOT can significantly increase the mechanical properties of the polymer, and improve the electrical properties by facilitating the charge-transfer process between these two components, and thus catalyze related reactions.[28] Electrodes modified with metal or metal oxide nanostructures have attracted significant interest.[29, 30] Many metal or metal oxide materials, including Cu,[31, 32] Au,[33, 34] Cu2O,[35] CoO,[36] Co,[37] and especially bimetal ones,[38] have been used to develop unique chemically modi-
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Full Paper fied electrodes. Metal nanoparticles with a large specific surface area have also been proved to be of high catalytic efficiency.[31] Compared with monometal, bimetal-nanomaterialmodified electrodes have many excellent properties (such as good selectivity, remarkable catalytic activity, and high stability) as a result of the synergistic effect of the two metals. These advantages make the bimetal nanoparticles an ideal material in electrochemical sensors.[39–42] Previous studies have indicated that electrodes modified with bimetal nanoparticles exhibited high sensitivity and low applied potential in electrochemical detection, and Cu¢Co bimetal have attracted extensive attention as economical electrocatalysts.[31] In this study, a simple method has been developed to deposit Cu¢Co bimetal nanoparticles onto PEDOT/CNTs modified glassy carbon electrode (Cu¢Co/PEDOT/CNTs/GCE), through the electrochemical deposition of PEDOT/CNTs and Cu¢Co on the electrode surface in sequence. Thanks to the excellent properties of the PEDOT/CNTs nanocomposite and Cu¢Co bimetal nanoparticles, the Cu¢Co/PEDOT/CNTs/GCE had a large specific surface area and high catalytic activity towards the electrooxidation of nitrite, thereby allowing highly sensitive, stable, and fast sensing of nitrite.
Results and Discussion 2.1. Characterization of the Prepared Nanocomposites SEM images of PEDOT/CNTs (A), Co/PEDOT/CNTs (B), Cu/ PEDOT/CNTs (C), Cu-Co/PEDOT/CNTs (D), and Cu¢Co (E) nanomaterials electrodeposited on electrode surfaces are shown in Figure 1. Clearly, the PEDOT/CNTs film (Figure 1 A) showed a network-like microstructure possessing a porous structure in the nanoscale with an enlarged surface area, which can facilitate the further deposition of metal nanomaterials. Figure 1 B and 1 C showed the microstructure of the Co/PEDOT/CNTs and Cu/PEDOT/CNTs composites, respectively. The conducting polymer/carbon nanotube composites were almost fully covered
Figure 2. Nyquist plots of the EIS for the bare GCE (a), the PEDOT/CNTs/GCE (b), and the Cu¢Co/PEDOT/CNTs/GCE (c).
by the deposited metal nanomaterials, with the former showing a slightly flat surface and the latter showing a particulate microstructure. The deposition of Cu¢Co bimetal nanoparticles on PEDOT/CNTs led to the formation of a coralloid-like structure (Figure 1 D). The nucleation of Cu¢Co might be accelerated in the coexistence of bimetal and accordingly resulted in the formation of the coralloid microstructure[44–46] of Cu¢Co/ PEDOT/CNTs. It was noticeable that Cu¢Co directly deposited on the GCE was amorphous and irregular (Figure 1 E). Obviously, the PEDOT/CNTs provided a large surface area for the deposition of the Cu¢Co bimetal nanoparticles and assisted the formation of the coralloid-like microstructure. The energy dispersive X-ray spectroscopy (EDX) of Cu¢Co/PEDOT/CNTs (Figure 1 F) was performed to investigate the components of the Cu¢Co/PEDOT/CNTs on the GCE, and it clearly indicated that both Cu and Co existed. The inserted table in Figure 1 F demonstrated that the molar ratio of the two components was equal to the initial precursor solution. Figure 2 shows the Nyquist plots of different modified electrodes. The semicircle portion of the plot corresponded to the charge transfer process, with the diameter of the semicircle equivalent to the charge transfer resistance (Rct),[47] while the linear portion reflected the diffusion limited process at the electrode interface. With the electrochemical deposition of PEDOT/ CNTs on the GCE, the obtained PEDOT/CNTs/GCE showed a much lower Rct than the bare GCE and the Rct was further decreased after the deposition of Cu¢Co on the PEDOT/CNTs/GCE. This was consistent with the fact that the electrodeposited nanocomposite film was conductive, and it could increase the effective surface area of the modified electrode.
Figure 1. SEM images of (A) PEDOT/CNTs, (B) Co/PEDOT/CNTs, (C) Cu/PEDOT/CNTs, (D) Cu¢Co/PEDOT/CNTs, and (E) Cu¢Co electrodeposited on electrode surfaces. (F) EDX spectra of Cu¢Co/PEDOT/CNTs. Chem. Asian J. 2015, 10, 1892 – 1897
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Full Paper 2.2. Electrocatalytic Oxidation of Nitrite at the Cu¢Co/ PEDOT/CNTs/GCE From the above characterization, it was concluded that the Cu¢Co/PEDOT/CNTs nanocomposite exhibited a large surface area and good conductivity. As large surface area can increase the contact area between the electrode and nitrite, and high conductivity can facilitate electron transfer, it was expected that the Cu¢Co/PEDOT/CNTs/GCE may possess enhanced properties for the electrochemical oxidation of nitrite. To optimize the electrolyte for the electrochemical oxidation of nitrite, 0.1 m NaOH, 0.1 m H2SO4, and 0.1 m PBS of pH 7.4 were tested. As shown in Figure S1 (see the Supporting Information), the most significant response to nitrite was obtained in 0.1 m H2SO4, and therefore it was selected as the detection electrolyte. Figure 3 shows the cyclic voltammograms of the bare GCE (curves a and b) and the Cu¢Co/PEDOT/CNTs/GCE (curves c and d) in 0.1 m H2SO4 in the absence and presence of 5.0 mm
Figure 3. CVs of bare GCE (curves a and b) and Cu¢Co/PEDOT/CNTs/GCE (curves c and d) in 0.1 m H2SO4 in the presence and absence of 5.0 mm nitrite, scan rate: 100 mV s¢1.
nitrite in the potential range from 0.5 to 1.2 V at a scan rate of 100 mV s¢1. The bare GCE showed no obvious anodic peak after the addition of nitrite, while a significant anodic peak located at around 0.9 V was observed at the Cu¢Co/PEDOT/ CNTs/GCE. The oxidation peak potential of the modified electrode is about 105 mV lower than that of the bare GCE (1.05 V), indicating that the Cu¢Co/PEDOT/CNTs nanocomposite possess high electrocatalytic activity towards the oxidation of nitrite. Figure 4 shows the CVs obtained for 5.0 mm nitrite in 0.1 m H2SO4 at different electrodes at a scan rate of 100 mV s¢1. A remarkable larger peak current was obtained at the Cu¢Co/ PEDOT/CNTs/GCE (145.7 mA), which is much higher than that of the Cu¢Co/GCE (73.69 mA), the PEDOT/CNTs/GCE (80.81 mA), the Co/PEDOT/CNTs/GCE (98.41 mA), and the Cu/PEDOT/CNTs/ GCE (90.67 mA). This enhancement in sensor response might be attributed to the following two reasons: (i) the Cu¢Co/ PEDOT/CNTs/GCE provided a larger specific surface area than other modified electrodes, and (ii) Cu¢Co nanoparticles on the surface of PEDOT/CNTs modified electrode could cause a synerChem. Asian J. 2015, 10, 1892 – 1897
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Figure 4. CVs of different electrodes in 0.1 m H2SO4 in the presence of 5.0 mm nitrite at 100 mV s¢1: (a) Cu¢Co/GCE, (b) PEDOT/CNTs/GCE, (c) Co/ PEDOT/CNTs/GCE, (d) Cu/PEDOT/CNTs/GCE, and (e) Cu¢Co/PEDOT/CNTs/GCE.
gic enhancement effect for nitrite oxidation. This synergic enhancement effect could be related to the unique structure of the Cu¢Co bimetal material (Co prefers to form a thin layer on the PEDOT/CNT substrate, Figure 1 B, and Cu prefers to form particles, Figure 1 C), which combined the properties of two components and exhibited higher activity than the single material. These data indicated that the Cu¢Co/PEDOT/CNTs/GCE had good catalytic activity toward nitrite compared to other modified electrodes. As Cu¢Co bimetal nanoparticles on the surface of electrodes modified with PEDOT/CNTs can cause a synergic enhancement effect for nitrite oxidation, we investigated the influence of different molar ratios of Cu and Co, that is, Cu2 + and Co2 + in the solution for the electrodeposition of Cu¢Co particles (Figure 5). Clearly, for the studied different ratio of Cu2 + /Co2 + , the highest sensor response was obtained when the ratio was 1:1 (curve c). Therefore, the Cu2 + /Co2 + ratio of 1:1 was selected for the preparation of the Cu¢Co/PEDOT/CNTs/GCE. 2.3. Amperometric Determination of Nitrite In this study, the amperometric current-time (I-t curve) technique was used for the detection of nitrite, and the effect of the applied potential on the sensor response was studied with
Figure 5. CVs of 5.0 mm nitrite at the Cu¢Co/PEDOT/CNTs/GCE prepared with a different molar ratio of Cu2 + and Co2 + in the electrodeposition solution: (a) Cu2 + /Co2 + = 2:1, (b) Cu2 + /Co2 + = 1:2, and (c) Cu2 + /Co2 + = 1:1.
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Full Paper different potentials ranging from 0.8 to 1.0 V (see Figure S2 in the Supporting Information). Clearly, the response current increases with an increase in the potential from 0.8 to 0.9 V and then becomes approximately stable. Therefore, the following amperometric measurements were carried out at 0.90 V. Amperometric measurements were carried out at 0.90 V at the Cu¢Co/PEDOT/CNTs/GCE by successive injection of nitrite (Figure 6) into a stirring solution of 0.1 m H2SO4. The sensor exhibited a clear response to nitrite across a very wide concentration range. The linear range of the nitrite detection was from 0.5 to 430 mm (R2 = 0.9980, inset A of Figure 6) with a detection limit of about 60 nm (S/N = 3), which was much lower Figure 7. Amperometric response of nitrite and the interference of glucose, sodium benzoate, and potassium sorbate.
2.4. Reproducibility and Stability of the Cu¢Co/PEDOT/CNTs/ GCE
Figure 6. Amperometric response of Cu¢Co/PEDOT/CNTs/GCE toward the successive additions of nitrite into a stirring solution of H2SO4 (0.1 m). The working potential was 0.90 V, and the nitrite concentrations added were 0.5, 1.0, 2.0, 4.0, 6.0, 6.0, 6.0, 6.0, 8.0, 8.0, 10.0, 10.0, 12.0, 15.0, 15.0, 20.0, 25.0, 25.0, 30.0, 40.0, 50.0, 60.0, and 70.0 mm in sequence. Insets: (A) the linear calibration curve of the nitrite sensor, (B) response time of the nitrite, and (C) magnified portion of the amperometric response curve of the nitrite sensor.
The stability and reproducibility of the as-prepared Cu¢Co/ PEDOT/CNTs/GCE were also investigated. It was found that the relative standard deviation (RSD) of the current response toward 20 mm nitrite was 2.85 % for five repetitive experiments with the same Cu¢Co/PEDOT/CNTs/GCE. The current response decreased about 1.21 % for the first 10 days and 3.8 % current loss after 30 days storage at room temperature. These results suggested that the nitrite sensor has high repetitive detection stability and high desirable storage stability. To investigate the reproducibility of the Cu¢Co/PEDOT/CNTs/GCE, the CVs of 5 mm nitrite in a 0.1 m H2SO4 solution were recorded with five independently fabricated electrodes. A 3.38 % RSD was obtained, thus indicating the excellent reproducibility of the sensor. 2.5. Determination of Nitrite in Sausages
¢1
than the maximum level (3 mg L ) of nitrite permitted in drinking water by the World Health Organization.[48] The inset B of Figure 6 showed the response time of the Cu¢Co/PEDOT/ CNTs/GCE to nitrite in the stirred solutions. The oxidation current reached a steady-state value and achieved 95 % of the steady state current within 2 s. The results indicated that Cu¢ Co/PEDOT/CNTs/GCE possessed a satisfactory catalytic activity and sensitivity towards the oxidation of nitrite. Because glucose, sodium benzoate, and potassium sorbate commonly coexist with nitrite in food samples, it is important to investigate their interference with nitrite sensing. As shown in Figure 7, a typical sensor showed a clear response towards the addition of 10 mm nitrite, while the successive addition of 100 mm glucose, 100 mm sodium benzoate, and 100 mm potassium sorbate gave no significant responses. Moreover, further addition of nitrite (20 mm) continued to elicit a response indicating the assay approach had an excellent selectivity toward nitrite. The effect of possibly coexisting ascorbic acid in the assay of nitrite in real samples such as sausages has also been investigated (see Figure S3, see the Supporting Information), and there was no significant interference on the sensor response. Chem. Asian J. 2015, 10, 1892 – 1897
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The practical feasibility of the proposed sensor was demonstrated by the determination of nitrite in different kinds of sausages. A standard addition method was adopted to find the recoveries and the respective results are shown Table 1. The added and found values showed good recoveries from 95.60 % to 102.80 %; this suggested that this method could be successfully applied for the detection of nitrite in real samples.
Conclusions An electrochemical nitrite sensor was developed through the electrodeposition of Cu¢Co bimetal nanoparticles onto the
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Table 1. Results of the determination of nitrite in different samples of sausages (n = 5). Sample Content [mM] Added [mM] Found [mM] Recovery [%] RSD [%] No. 1 2 3
4.25 4.02 3.89
5.00 5.00 5.00
4.78 4.82 5.14
95.60 96.40 102.8
3.12 3.04 2.79
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Full Paper surface of the PEDOT/CNTs nanocomposite, which possesses large surface area and good conductivity. Compared to single Cu or Co nanoparticles, the Cu¢Co bimetal nanoparticles exhibited a synergic enhancement effect for the electrochemical catalytic oxidation of nitrite. Based on the enhanced electrochemical catalytic activity of the Cu¢Co/PEDOT/CNTs composite, a nitrite sensor with excellent performance in sensitivity, selectivity, and reproducibility was obtained. The linear range of the fabricated nitrite sensor was from 0.5 to 430 mm (R2 = 0.9980) with a detection limit of 60 nm (S/N = 3). These advantages, in addition to the simple electrochemical preparation process, make the nitrite sensor potentially suitable for real sample assays.
Scheme 1. Schematic illustration of the preparation of the Cu¢Co/PEDOT/ CNTs/GCE.
Experimental Section Materials Multi-walled CNTs with the length of 10–30 mm and diameter of 20–30 nm were purchased from Nanjing Xian Feng Nanomaterials Technology Co., Ltd. 3,4-ethylenedioxythiophene (EDOT), glucose, sodium benzoate, potassium sorbate, Na2SO4, CuSO4, and CoCl2 of analytical grade were obtained from Aladdin Reagents (Shanghai, China). Millipore water from a Milli-Q water purifying system was used throughout all experiments.
Apparatus The electrochemical experiments, including electrochemical impedance spectroscopy, amperometric I-t curve, and cyclic voltammetry were carried out with the CHI660E electrochemical work station (CH Instruments, Shanghai, China), using a conventional three-electrode system with the platinum wire as the counter electrode, Ag/ AgCl (3 m KCl) as the reference electrode and the GCE or modified GCE as the working electrode (diameter 3.0 mm). The surface morphologies and microstructures of the modified electrodes were examined using a field emission scanning electron microscopy (SEM) instrument (JSM-7500 F, Hitachi High-Technology Co., Japan), with an acceleration voltage of 5.0 kV.
Preparation of the Cu¢Co/PEDOT/CNTs/GCE The GCE was polished, washed, and eletrochemically oxidized in a phosphate buffered saline solution (PBS) according to a previous report.[49] CNTs doped PEDOT was first deposited onto the pretreated GCE in a solution containing 2 mg mL¢1 CNTs and 0.02 m PEDOT by amperometric I-t curve (1.0 V, 300 s) to obtain the PEDOT/CNTs/ GCE. Then the Cu¢Co/PEDOT/CNTs/GCE was prepared by electrodepositing Cu¢Co onto the surface of PEDOT/CNTs/GCE in the solution containing 50 mm Na2SO4, 5 mm CuSO4, and 5 mm CoCl2 at ¢0.95 V for 100 s (Scheme 1). For comparison, the Cu¢Co/GCE, Cu/ PEDOT/CNTs/GCE, and Co/PEDOT/CNTs/GCE were also prepared with the same procedure as described above.
Acknowledgements This research was supported by the National Natural Science Foundation of China (21275087, 21422504), the Natural Science Foundation of Shandong Province of China (ZR2012M008, JQ201406), and the Taishan Scholar Program of Shandong Province, China. Chem. Asian J. 2015, 10, 1892 – 1897
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Keywords: Cu¢Co bimetal nanoparticles · electrocatalysis · nanotubes · oxidation · poly(3,4-ethylenedioxythiophene)
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Manuscript received: June 4, 2015 Accepted Article published: July 16, 2015 Final Article published: August 6, 2015
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Nanoparticles
Nitrite Oxidation with Copper–Cobalt Nanoparticles on Carbon Nanotubes Doped Conducting Polymer PEDOT Composite** Junjie Wang, Guiyun Xu, Wei Wang, Shenghao Xu, and Xiliang Luo*[a] Abstract: Copper–cobalt bimetal nanoparticles (Cu¢Co) have been electrochemically prepared on glassy carbon electrodes (GCEs), which were electrodeposited with conducting polymer nanocomposites of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with carbon nanotubes (CNTs). Owing to their good conductivity, high mechanical strength, and large surface area, the PEDOT/CNTs composites offered excellent substrates for the electrochemical deposition of Cu¢Co nanoparticles. As a result of their nanostructure and the synergic effect between Cu and Co, the Cu¢Co/PEDOT/
Introduction Nitrite is ubiquitous within the environment and food, and it interacts with amines to generate carcinogenic nitrosamines.[1] The presence of excess nitrite in vegetables, drinking water, and food products is a serious threat to human health,[2] as excess nitrite promotes the irreversible oxidation of hemoglobin and hinders the blood from transporting oxygen. Therefore, accurate determination of nitrite is of vital importance for food safety and human health. During the past few years, several detection techniques have been utilized for the determination of nitrite such as spectrophotometry,[3] chemiluminescence,[4] capillary electrophoresis,[5] and chromatography.[6] However, these analytical methods usually suffer from some disadvantages including the interference of coexisting cationic or anionic species, expensive equipment, tedious detection procedures, and long assay times. Recently, because of their rapid response, cost effectiveness, sensitivity, and simple use, electroanalytical methods[7–13] have attracted much attention for the detection of nitrite. Until now, many kinds of electrochemical nitrite sensors have been fabricated based on chemically modified electrodes.[14–18] Furthermore, the interest in
[a] J. Wang, Dr. G. Xu, W. Wang, Dr. S. Xu, Prof. X. Luo Key Laboratory of Sensor Analysis of Tumor Marker Ministry of Education, College of Chemistry and Molecular Engineering Qingdao University of Science and Technology Qingdao 266042 (China) E-mail: [email protected] [**] PEDOT = poly(3,4-ethylenedioxythiophene). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500579. Chem. Asian J. 2015, 10, 1892 – 1897
CNTs composites exhibited significantly enhanced catalytic activity towards the electrochemical oxidation of nitrite. Under optimized conditions, the nanocomposite-modified electrodes had a fast response time within 2 s and a linear range from 0.5 to 430 mm for the detection of nitrite, with a detection limit of 60 nm. Moreover, the Cu¢Co/PEDOT/ CNTs composites were highly stable, and the prepared nitrite sensors could retain more than 96 % of their initial response after 30 days.
modified electrodes based on nanocomposites for nitrite detection has been the subject of several previous reports.[19, 20] Carbon nanotubes (CNTs), have dominated material science research since their discovery in 1991, owing to the outstanding physicochemical properties.[21] CNTs have attracted increasing interest for potential applications in sensors due to their electronic, chemical, and thermal properties.[22] With large specific surface area, wide electrochemical window, and high electrical conductivity, CNTs can provide a good support for metal nanomaterials deposition on electrodes[23] and have become one of the most promising materials in electrochemical sensors. Meanwhile, the relatively large specific surface area of CNTs provides reaction active sites for the electrocatalytic oxidation or reduction. Therefore, CNTs can favorably catalyze the electrochemical oxidation of nitrite.[43] As another kind of widely used material, conducting polymers have also been used extensively in the development of sensors because of their interesting electrical, optical, and electrocatalytic properties. Poly(3,4-ethylenedioxythiophene) (PEDOT), a type of conducting polymer, has been considered as the most promising one because its well-defined chemical structure offers outstanding stability and conductivity.[24, 25] Previous work showed that the electrical properties of electrodes coated with PEDOT[26, 27] can be greatly improved. CNTs doped with PEDOT can significantly increase the mechanical properties of the polymer, and improve the electrical properties by facilitating the charge-transfer process between these two components, and thus catalyze related reactions.[28] Electrodes modified with metal or metal oxide nanostructures have attracted significant interest.[29, 30] Many metal or metal oxide materials, including Cu,[31, 32] Au,[33, 34] Cu2O,[35] CoO,[36] Co,[37] and especially bimetal ones,[38] have been used to develop unique chemically modi-
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Full Paper fied electrodes. Metal nanoparticles with a large specific surface area have also been proved to be of high catalytic efficiency.[31] Compared with monometal, bimetal-nanomaterialmodified electrodes have many excellent properties (such as good selectivity, remarkable catalytic activity, and high stability) as a result of the synergistic effect of the two metals. These advantages make the bimetal nanoparticles an ideal material in electrochemical sensors.[39–42] Previous studies have indicated that electrodes modified with bimetal nanoparticles exhibited high sensitivity and low applied potential in electrochemical detection, and Cu¢Co bimetal have attracted extensive attention as economical electrocatalysts.[31] In this study, a simple method has been developed to deposit Cu¢Co bimetal nanoparticles onto PEDOT/CNTs modified glassy carbon electrode (Cu¢Co/PEDOT/CNTs/GCE), through the electrochemical deposition of PEDOT/CNTs and Cu¢Co on the electrode surface in sequence. Thanks to the excellent properties of the PEDOT/CNTs nanocomposite and Cu¢Co bimetal nanoparticles, the Cu¢Co/PEDOT/CNTs/GCE had a large specific surface area and high catalytic activity towards the electrooxidation of nitrite, thereby allowing highly sensitive, stable, and fast sensing of nitrite.
Results and Discussion 2.1. Characterization of the Prepared Nanocomposites SEM images of PEDOT/CNTs (A), Co/PEDOT/CNTs (B), Cu/ PEDOT/CNTs (C), Cu-Co/PEDOT/CNTs (D), and Cu¢Co (E) nanomaterials electrodeposited on electrode surfaces are shown in Figure 1. Clearly, the PEDOT/CNTs film (Figure 1 A) showed a network-like microstructure possessing a porous structure in the nanoscale with an enlarged surface area, which can facilitate the further deposition of metal nanomaterials. Figure 1 B and 1 C showed the microstructure of the Co/PEDOT/CNTs and Cu/PEDOT/CNTs composites, respectively. The conducting polymer/carbon nanotube composites were almost fully covered
Figure 2. Nyquist plots of the EIS for the bare GCE (a), the PEDOT/CNTs/GCE (b), and the Cu¢Co/PEDOT/CNTs/GCE (c).
by the deposited metal nanomaterials, with the former showing a slightly flat surface and the latter showing a particulate microstructure. The deposition of Cu¢Co bimetal nanoparticles on PEDOT/CNTs led to the formation of a coralloid-like structure (Figure 1 D). The nucleation of Cu¢Co might be accelerated in the coexistence of bimetal and accordingly resulted in the formation of the coralloid microstructure[44–46] of Cu¢Co/ PEDOT/CNTs. It was noticeable that Cu¢Co directly deposited on the GCE was amorphous and irregular (Figure 1 E). Obviously, the PEDOT/CNTs provided a large surface area for the deposition of the Cu¢Co bimetal nanoparticles and assisted the formation of the coralloid-like microstructure. The energy dispersive X-ray spectroscopy (EDX) of Cu¢Co/PEDOT/CNTs (Figure 1 F) was performed to investigate the components of the Cu¢Co/PEDOT/CNTs on the GCE, and it clearly indicated that both Cu and Co existed. The inserted table in Figure 1 F demonstrated that the molar ratio of the two components was equal to the initial precursor solution. Figure 2 shows the Nyquist plots of different modified electrodes. The semicircle portion of the plot corresponded to the charge transfer process, with the diameter of the semicircle equivalent to the charge transfer resistance (Rct),[47] while the linear portion reflected the diffusion limited process at the electrode interface. With the electrochemical deposition of PEDOT/ CNTs on the GCE, the obtained PEDOT/CNTs/GCE showed a much lower Rct than the bare GCE and the Rct was further decreased after the deposition of Cu¢Co on the PEDOT/CNTs/GCE. This was consistent with the fact that the electrodeposited nanocomposite film was conductive, and it could increase the effective surface area of the modified electrode.
Figure 1. SEM images of (A) PEDOT/CNTs, (B) Co/PEDOT/CNTs, (C) Cu/PEDOT/CNTs, (D) Cu¢Co/PEDOT/CNTs, and (E) Cu¢Co electrodeposited on electrode surfaces. (F) EDX spectra of Cu¢Co/PEDOT/CNTs. Chem. Asian J. 2015, 10, 1892 – 1897
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Full Paper 2.2. Electrocatalytic Oxidation of Nitrite at the Cu¢Co/ PEDOT/CNTs/GCE From the above characterization, it was concluded that the Cu¢Co/PEDOT/CNTs nanocomposite exhibited a large surface area and good conductivity. As large surface area can increase the contact area between the electrode and nitrite, and high conductivity can facilitate electron transfer, it was expected that the Cu¢Co/PEDOT/CNTs/GCE may possess enhanced properties for the electrochemical oxidation of nitrite. To optimize the electrolyte for the electrochemical oxidation of nitrite, 0.1 m NaOH, 0.1 m H2SO4, and 0.1 m PBS of pH 7.4 were tested. As shown in Figure S1 (see the Supporting Information), the most significant response to nitrite was obtained in 0.1 m H2SO4, and therefore it was selected as the detection electrolyte. Figure 3 shows the cyclic voltammograms of the bare GCE (curves a and b) and the Cu¢Co/PEDOT/CNTs/GCE (curves c and d) in 0.1 m H2SO4 in the absence and presence of 5.0 mm
Figure 3. CVs of bare GCE (curves a and b) and Cu¢Co/PEDOT/CNTs/GCE (curves c and d) in 0.1 m H2SO4 in the presence and absence of 5.0 mm nitrite, scan rate: 100 mV s¢1.
nitrite in the potential range from 0.5 to 1.2 V at a scan rate of 100 mV s¢1. The bare GCE showed no obvious anodic peak after the addition of nitrite, while a significant anodic peak located at around 0.9 V was observed at the Cu¢Co/PEDOT/ CNTs/GCE. The oxidation peak potential of the modified electrode is about 105 mV lower than that of the bare GCE (1.05 V), indicating that the Cu¢Co/PEDOT/CNTs nanocomposite possess high electrocatalytic activity towards the oxidation of nitrite. Figure 4 shows the CVs obtained for 5.0 mm nitrite in 0.1 m H2SO4 at different electrodes at a scan rate of 100 mV s¢1. A remarkable larger peak current was obtained at the Cu¢Co/ PEDOT/CNTs/GCE (145.7 mA), which is much higher than that of the Cu¢Co/GCE (73.69 mA), the PEDOT/CNTs/GCE (80.81 mA), the Co/PEDOT/CNTs/GCE (98.41 mA), and the Cu/PEDOT/CNTs/ GCE (90.67 mA). This enhancement in sensor response might be attributed to the following two reasons: (i) the Cu¢Co/ PEDOT/CNTs/GCE provided a larger specific surface area than other modified electrodes, and (ii) Cu¢Co nanoparticles on the surface of PEDOT/CNTs modified electrode could cause a synerChem. Asian J. 2015, 10, 1892 – 1897
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Figure 4. CVs of different electrodes in 0.1 m H2SO4 in the presence of 5.0 mm nitrite at 100 mV s¢1: (a) Cu¢Co/GCE, (b) PEDOT/CNTs/GCE, (c) Co/ PEDOT/CNTs/GCE, (d) Cu/PEDOT/CNTs/GCE, and (e) Cu¢Co/PEDOT/CNTs/GCE.
gic enhancement effect for nitrite oxidation. This synergic enhancement effect could be related to the unique structure of the Cu¢Co bimetal material (Co prefers to form a thin layer on the PEDOT/CNT substrate, Figure 1 B, and Cu prefers to form particles, Figure 1 C), which combined the properties of two components and exhibited higher activity than the single material. These data indicated that the Cu¢Co/PEDOT/CNTs/GCE had good catalytic activity toward nitrite compared to other modified electrodes. As Cu¢Co bimetal nanoparticles on the surface of electrodes modified with PEDOT/CNTs can cause a synergic enhancement effect for nitrite oxidation, we investigated the influence of different molar ratios of Cu and Co, that is, Cu2 + and Co2 + in the solution for the electrodeposition of Cu¢Co particles (Figure 5). Clearly, for the studied different ratio of Cu2 + /Co2 + , the highest sensor response was obtained when the ratio was 1:1 (curve c). Therefore, the Cu2 + /Co2 + ratio of 1:1 was selected for the preparation of the Cu¢Co/PEDOT/CNTs/GCE. 2.3. Amperometric Determination of Nitrite In this study, the amperometric current-time (I-t curve) technique was used for the detection of nitrite, and the effect of the applied potential on the sensor response was studied with
Figure 5. CVs of 5.0 mm nitrite at the Cu¢Co/PEDOT/CNTs/GCE prepared with a different molar ratio of Cu2 + and Co2 + in the electrodeposition solution: (a) Cu2 + /Co2 + = 2:1, (b) Cu2 + /Co2 + = 1:2, and (c) Cu2 + /Co2 + = 1:1.
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Full Paper different potentials ranging from 0.8 to 1.0 V (see Figure S2 in the Supporting Information). Clearly, the response current increases with an increase in the potential from 0.8 to 0.9 V and then becomes approximately stable. Therefore, the following amperometric measurements were carried out at 0.90 V. Amperometric measurements were carried out at 0.90 V at the Cu¢Co/PEDOT/CNTs/GCE by successive injection of nitrite (Figure 6) into a stirring solution of 0.1 m H2SO4. The sensor exhibited a clear response to nitrite across a very wide concentration range. The linear range of the nitrite detection was from 0.5 to 430 mm (R2 = 0.9980, inset A of Figure 6) with a detection limit of about 60 nm (S/N = 3), which was much lower Figure 7. Amperometric response of nitrite and the interference of glucose, sodium benzoate, and potassium sorbate.
2.4. Reproducibility and Stability of the Cu¢Co/PEDOT/CNTs/ GCE
Figure 6. Amperometric response of Cu¢Co/PEDOT/CNTs/GCE toward the successive additions of nitrite into a stirring solution of H2SO4 (0.1 m). The working potential was 0.90 V, and the nitrite concentrations added were 0.5, 1.0, 2.0, 4.0, 6.0, 6.0, 6.0, 6.0, 8.0, 8.0, 10.0, 10.0, 12.0, 15.0, 15.0, 20.0, 25.0, 25.0, 30.0, 40.0, 50.0, 60.0, and 70.0 mm in sequence. Insets: (A) the linear calibration curve of the nitrite sensor, (B) response time of the nitrite, and (C) magnified portion of the amperometric response curve of the nitrite sensor.
The stability and reproducibility of the as-prepared Cu¢Co/ PEDOT/CNTs/GCE were also investigated. It was found that the relative standard deviation (RSD) of the current response toward 20 mm nitrite was 2.85 % for five repetitive experiments with the same Cu¢Co/PEDOT/CNTs/GCE. The current response decreased about 1.21 % for the first 10 days and 3.8 % current loss after 30 days storage at room temperature. These results suggested that the nitrite sensor has high repetitive detection stability and high desirable storage stability. To investigate the reproducibility of the Cu¢Co/PEDOT/CNTs/GCE, the CVs of 5 mm nitrite in a 0.1 m H2SO4 solution were recorded with five independently fabricated electrodes. A 3.38 % RSD was obtained, thus indicating the excellent reproducibility of the sensor. 2.5. Determination of Nitrite in Sausages
¢1
than the maximum level (3 mg L ) of nitrite permitted in drinking water by the World Health Organization.[48] The inset B of Figure 6 showed the response time of the Cu¢Co/PEDOT/ CNTs/GCE to nitrite in the stirred solutions. The oxidation current reached a steady-state value and achieved 95 % of the steady state current within 2 s. The results indicated that Cu¢ Co/PEDOT/CNTs/GCE possessed a satisfactory catalytic activity and sensitivity towards the oxidation of nitrite. Because glucose, sodium benzoate, and potassium sorbate commonly coexist with nitrite in food samples, it is important to investigate their interference with nitrite sensing. As shown in Figure 7, a typical sensor showed a clear response towards the addition of 10 mm nitrite, while the successive addition of 100 mm glucose, 100 mm sodium benzoate, and 100 mm potassium sorbate gave no significant responses. Moreover, further addition of nitrite (20 mm) continued to elicit a response indicating the assay approach had an excellent selectivity toward nitrite. The effect of possibly coexisting ascorbic acid in the assay of nitrite in real samples such as sausages has also been investigated (see Figure S3, see the Supporting Information), and there was no significant interference on the sensor response. Chem. Asian J. 2015, 10, 1892 – 1897
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The practical feasibility of the proposed sensor was demonstrated by the determination of nitrite in different kinds of sausages. A standard addition method was adopted to find the recoveries and the respective results are shown Table 1. The added and found values showed good recoveries from 95.60 % to 102.80 %; this suggested that this method could be successfully applied for the detection of nitrite in real samples.
Conclusions An electrochemical nitrite sensor was developed through the electrodeposition of Cu¢Co bimetal nanoparticles onto the
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Table 1. Results of the determination of nitrite in different samples of sausages (n = 5). Sample Content [mM] Added [mM] Found [mM] Recovery [%] RSD [%] No. 1 2 3
4.25 4.02 3.89
5.00 5.00 5.00
4.78 4.82 5.14
95.60 96.40 102.8
3.12 3.04 2.79
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Full Paper surface of the PEDOT/CNTs nanocomposite, which possesses large surface area and good conductivity. Compared to single Cu or Co nanoparticles, the Cu¢Co bimetal nanoparticles exhibited a synergic enhancement effect for the electrochemical catalytic oxidation of nitrite. Based on the enhanced electrochemical catalytic activity of the Cu¢Co/PEDOT/CNTs composite, a nitrite sensor with excellent performance in sensitivity, selectivity, and reproducibility was obtained. The linear range of the fabricated nitrite sensor was from 0.5 to 430 mm (R2 = 0.9980) with a detection limit of 60 nm (S/N = 3). These advantages, in addition to the simple electrochemical preparation process, make the nitrite sensor potentially suitable for real sample assays.
Scheme 1. Schematic illustration of the preparation of the Cu¢Co/PEDOT/ CNTs/GCE.
Experimental Section Materials Multi-walled CNTs with the length of 10–30 mm and diameter of 20–30 nm were purchased from Nanjing Xian Feng Nanomaterials Technology Co., Ltd. 3,4-ethylenedioxythiophene (EDOT), glucose, sodium benzoate, potassium sorbate, Na2SO4, CuSO4, and CoCl2 of analytical grade were obtained from Aladdin Reagents (Shanghai, China). Millipore water from a Milli-Q water purifying system was used throughout all experiments.
Apparatus The electrochemical experiments, including electrochemical impedance spectroscopy, amperometric I-t curve, and cyclic voltammetry were carried out with the CHI660E electrochemical work station (CH Instruments, Shanghai, China), using a conventional three-electrode system with the platinum wire as the counter electrode, Ag/ AgCl (3 m KCl) as the reference electrode and the GCE or modified GCE as the working electrode (diameter 3.0 mm). The surface morphologies and microstructures of the modified electrodes were examined using a field emission scanning electron microscopy (SEM) instrument (JSM-7500 F, Hitachi High-Technology Co., Japan), with an acceleration voltage of 5.0 kV.
Preparation of the Cu¢Co/PEDOT/CNTs/GCE The GCE was polished, washed, and eletrochemically oxidized in a phosphate buffered saline solution (PBS) according to a previous report.[49] CNTs doped PEDOT was first deposited onto the pretreated GCE in a solution containing 2 mg mL¢1 CNTs and 0.02 m PEDOT by amperometric I-t curve (1.0 V, 300 s) to obtain the PEDOT/CNTs/ GCE. Then the Cu¢Co/PEDOT/CNTs/GCE was prepared by electrodepositing Cu¢Co onto the surface of PEDOT/CNTs/GCE in the solution containing 50 mm Na2SO4, 5 mm CuSO4, and 5 mm CoCl2 at ¢0.95 V for 100 s (Scheme 1). For comparison, the Cu¢Co/GCE, Cu/ PEDOT/CNTs/GCE, and Co/PEDOT/CNTs/GCE were also prepared with the same procedure as described above.
Acknowledgements This research was supported by the National Natural Science Foundation of China (21275087, 21422504), the Natural Science Foundation of Shandong Province of China (ZR2012M008, JQ201406), and the Taishan Scholar Program of Shandong Province, China. Chem. Asian J. 2015, 10, 1892 – 1897
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Keywords: Cu¢Co bimetal nanoparticles · electrocatalysis · nanotubes · oxidation · poly(3,4-ethylenedioxythiophene)
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Manuscript received: June 4, 2015 Accepted Article published: July 16, 2015 Final Article published: August 6, 2015
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