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Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012
A facile fabrication of copper particle-decorated novel graphene flower composites for enhanced detecting of nitrite Huiwen Wang, Caiqin Wang, Beibei Yang, Chunyang Zhai, Duan Bin, Ke Zhang, Ping Yang and Yukou Du*
DOI: 10.1039/x0xx00000x www.rsc.org/
We described a simple electrochemical preparation method of a novel three dimensional (3D) graphene material, porous flower-like reduced graphene oxide (f-RGO) nanosheets, which was explored as the support for the Cu particles on glassy carbon electrode (Cu/f-RGO/GCE) for detecting nitrite. In morphology, scanning electron microscopy (SEM) demonstrates the 3D porous structure of f-RGO enlarges the surface area of electrode and promotes more Cu particles depositing on the surface of fRGO with homogeneous dispersion. In cyclic voltammetry (CV), a well-defined voltammetric peak along with the remarkable reduction current indicates excellent electrocatalytic activity of the Cu/f-RGO/GCE for NaNO 2 reduction compared with other corresponding electrodes. The effects of pH value and detection potential on the current responses of Cu/f-RGO/GCE towards nitrite were optimized to obtain the maximal sensitivity. In the optimal experimental conditions, Cu/f-RGO/GCE displays the wide detection range from 0.15 μM to 10500 μM and the low limit of detection of 0.06 μM (S/N=3) with fast response time 2 s for detecting NaNO 2 through amperometric method. Furthermore, the presence of K+, Na+ , Cl −, NH4+, NO 3 -, SO 42− and ascorbic acid show a negligible effect on the current response of nitrite determination suggesting Cu/f-RGO/GCE have the high selectivity for detecting nitrite even in the presence of high concentration of interferes. Moreover, the real sample determination experiment indicated practical feasibility of the obtained sensor. The prepared sensor for determination of NaNO 2 exhibited wide liner range, low detection limit, good reproducibility, nice stability and remarkable antiinterference ability. In this paper, not only did the Cu/f-RGO/GCE show high performance for determination of nitrite, but also it was simple to prepare, user-friendly and cost-effective.
1. Introduction Nitrite, an important reagent in industry, is widely used as a preservative 1, food additive 2 and fertilizer 3, but hazardous to human health. In human body, nitrite can oxidize hemoglobin to methemoglobin irreversibly, which reduces capacity to transport oxygen of blood 4. Furthermore, it reacts with dietary compounds to form carcinogenic nitrosamines in human digestive system 5. It is known that the consumption of 0.3-0.5 g nitrite can lead to poisoning or even death. As a result, quantitative analysis of nitrite in living environment and food products is extremely essential for human health. Up to now, 6 Several techniques such as chromatography , 7 8 spectrophotometry , capillary electrophoresis and electrochemical techniques 4 have been applied in nitrite detection. The traditional methods including spectrophotometry and chromatography are reliable but time-consuming, complicated and expensive, which are not universally
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satisfactory. Compared to these methods, the electrochemical technique is considered superior because it can provide high sensitive, simple, cheap and real-time analysis. To improve the performance of the electrodes for nitrite detection, many suitable materials have been explored to modify electrodes, such as metal nanoparticles, including gold 9, platinum 10, copper 11 and silver 12 and various kinds of supporting materials, including carbon nanotube 13, conducting polymer 14 and graphene 15. The modified electrodes with outstanding electrochemical properties can improve performance of sensors in nitrite determination. Among the supporting materials, it should be noted that, graphene (reduced graphene oxide, denoted as RGO), an ideal two-dimensional layered material, has recently attracted a great attention in electrochemistry because of its unique mechanical properties, large specific surface area, rapid heterogeneous electron transfer, high electrical conductivity and large amount of edge-plan-like defects 16. Graphene and graphene-based
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PAPER materials hold technological promise in the areas of fuel cells 17 , capacitors 18, actuators 19, sensors 20 and so on. The electrodes modified with graphene-based composites for the detection of nitrite also have been reported in recent years 15. What is more, three-dimensional (3D) porous graphene has been also considered as a promising catalyst support material nowadays because 3D graphene not only can overcome the problem of a strong tendency to agglomerate about RGO sheets due to the enormous van der Waals force, but also has more unique physical and chemical properties 21. Therefore, fabricating a novel sensor modified with the composites of suitable materials and 3D porous RGO for detecting nitrite can be considered. Various techniques have been adopted to synthesize 3D porous RGO, such as chemical vapor deposition 22, hydrogel, aerogel techniques 23 and electrochemical method 24. In this work, we prepare 3D flower-like RGO (f-RGO) nanosheets on glassy carbon electrode (GCE) via electrochemical strategy which is facile and cost-effective compared with other preparation methods. And then, Cu particles are deposited on the surface of f-RGO (Cu/f-RGO) by potentiostatic electrodeposition. Compared with Cu/GCE and Cu/RGO/GCE, the prepared modified electrode (Cu/f-RGO/GCE) exhibits high electrocatalytic activities towards the reduction of nitrite and provides excellent property in quantitative detection of nitrite through the electrochemical reduction method. In addition, facile and simple preparation processes and non-noble metal included in the electrode could reduce the preparation costs. This work presents a new attempt to use the 3D porous f-RGO nanosheets as support in electrochemical determination field, and simple and cost-effective processes of preparation and determination indicate application potential of the sensor in the future.
Co., Ltd., China). GCE with a surface area of 0.07 cm2 was used as the working electrode. A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. All the experiments were performed at 25 ºC. 2.2 Electrode preparation Graphene oxide (GO) solution was prepared by the previously reported method 25. First, 10 µL 0.5mg mL-1 GO solution ink was dropped on the surface of GCE followed by electrochemical reduction to RGO layer in a Na-PBS (0.1 M, pH = 4.1) solution at a constant potential of -0.9 V. Second, copper particles were electrodeposited on the surface of formed RGO layer by reduction of 5.0 mM CuSO4 solution at a constant potential of -0.4 V with a charge of 1.0 × 10-2 C. And then another GO ink (10uL) was again coated the obtained electrode (Cu/RGO/GCE) followed by the above reduction strategy and a sandwich construction was fabricated (RGO/Cu/RGO/GCE). Subsequently, the Cu particles were electrolyzed at a constant potential of 0.1 V in the Na-PBS (0.1 M, pH = 4.1) solution for 1000s. As the reaction progressed, with the Cu2+ ions continuously produced and transporting across the exterior RGO layer, the structure of the RGO sheets coated on the Cu particles would transform and turn to stand up from the surface of the substrate. The raised RGO sheets interconnected with each other, and then the porous f-RGO could be obtained 26. Finally, the f-RGO/GCE was immersed into 0.01M CuSO4 solution containing 0.1 M Na2SO4 and Cu particles were electrodeposited at -0.4 V with a charge of 2.0 × 10-2 C, and Cu/f-RGO/GCE could be obtained. For comparison, Cu/GCE, RGO/GCE and Cu/RGO/GCE were also fabricated in the same manner.
3. Results and discussion 2. Materials and methods 2.1 Chemicals and apparatus Graphene oxide, sodium nitrite, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, sodium sulphate and cupric sulphate anhydrous were purchased from Sinopharm Chemicals Reagent Co., Ltd. All the chemicals were analytical grade. Doubly distilled water was used throughout the experiments. Freshly prepared 0.1M phosphate buffer solution (PBS, pH 2.0), consisting of NaH2PO4 and H3PO4, was used as the electrolyte. All experimental solutions were deoxygenated by high-purity nitrogen and continuous nitrogen was blown on the surface of solutions gently. The scanning electrode microscopy (SEM) (S-4700, Hitachi High Technologies Corporation, Japan) was used to characterize the morphologies of the obtained electrodes. All the electrochemical experiments were carried out in a conventional three-electrode system using a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument
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3.1 Characterizations of Cu/f-RGO/GCE. Fig.1 is the SEM images of f-RGO and Cu/f-RGO on GCE at different resolutions. As shown in Fig.1 (A-B), several welldefined 3D structured RGO flowers are dispersed on the substrate with few regions of exposed RGO substrate among the flowers. The diameter of each flower can be estimated to be about 5 µm. The RGO-sheet petals are interconnected and composed the whole RGO flowers with typical porous architecture. In Fig.1 (C), some wrinkles also can be seen on the surface of thin RGO petals. It is believed that the porous structure of RGO flower and the ripples on the RGO petals increase the surface area of electrode. The characteristics of fRGO in morphology indicate it is a good candidate as the support material to modify electrode. In comparison with the smooth surface of the pure f-RGO shown in Fig.1 (A-C), the obvious rough surface of Cu/f-RGO in Fig.1 (D-F) is exhibited due to the present of uniform Cu particles on the surface of each RGO petal. In Fig.1 (F), Cu particles are dispersed on the walls of the f-RGO pores so the surface of petals becomes grainy and the thickness increases. From the SEM images, the
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PAPER DOI: 10.1039/C4AN01924E
diameter of Cu particles can be estimated to be 70-100 nm. In morphology, it is believed that the f-RGO nanosheets increase the surface area of electrode and support more electro-deposited Cu particles to disperse on electrode.
electrodes. Ignore the peaks originating from the ITO substrate, a broadened peak is observed at about 2θ=24.1° arising from the reduction of graphene oxide (002) for RGO, f-RGO and Cu/f-RGO. The broadness of this peak is presumably attributed of the multilayer character of graphene sheets and the structural defects induced by electrochemical reduction 26. The similar XRD patterns of RGO and f-RGO demonstrate that f-RGOs are constitute of RGO sheets. In addition, three new diffraction peaks at 2θ= 43.7°, 50.9° and 74.1° corresponding to Miller indices (1 1 1), (2 0 0) and (2 2 0), can be indexed to facecentered cubic Cu crystals, which is in good accordance with reported results 27.
Fig.1 SEM images of f-RGO (A-C) and Cu/f-RGO (D-F)
Fig.2 shows the cyclic voltammograms of GCE and RGO/GCE and f-RGO/GCE in 5 mM K3[Fe(CN)6] + K4[Fe(CN)6] containing 1 M KCl solution. We can calculated the electrochemical active surface area (ECSA) of the fRGO/GCE on the basis of the charge associated with the oxidation and reduction peak currents in CV curve, which can be also an important criterion to evaluate the electrochemical performance of porous materials. For comparison, the ECSA of the bare GCE and RGO/GCE were also calculated based on the Randles-Sevcik equation, assuming mass transport only by the diffusion process 26,
Fig.2 CVs of the bare GC (a), RGO/GCE (b), and f-RGO/GCE (C) in 5 mM K3[Fe(CN)6] + K4[Fe(CN)6] aqueous solution containing 1 M KCl at scan rate of 50 mV s-1.
IP = 2.69×105AD1/2nγ1/2C where IP refers to the anodic peak current, A is ECSA of the electrode, n is the number of electrons participating in the redox reaction, D is the diffusion coefficient of the molecule (equal to (6.70 ± 0.02) × 10-6 cm2 s-1), C is the concentration of the probe molecule in the solution (mol cm-3), and γ is the scan rate (V s1). From the slope of the IP-γ1/2 relation, the ECSA of the bare GCE, RGO/GCE, f-RGO/GCE were calculated a 0.028, 0.038 and 0.046 cm2, respectively. The result demonstrates that the porous structure of RGO flowers increase the electrochemical active surface area of electrode. The RGO, f-RGO and Cu/f-RGO formed on ITO were used for XRD characterization. Fig.3 shows the XRD patterns of the bare ITO, RGO/ITO, f-RGO/ITO and Cu/f-RGO/ITO
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Fig.3 XRD patterns of ITO (a), RGO/ITO (b), f-RGO/ITO (c) and Cu/f-RGO/ITO (d).
3.2 Cyclic voltammetry responses of Cu/f-RGO/GCE for nitrite reduction The electrocatalytic reduction of nitrite on the corresponding modified electrodes was investigated via cyclic voltammetry (CV). In Fig.4 (A), it is noted that the reduction current of Cu/f-
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PAPER RGO/GCE increases significantly after adding 5 mM NaNO2 in pH = 2 PBS solution and the reduction peak current appeared at -0.6 V, which suggests that electrocatalytic reduction of nitrite occurred on the modified electrode. It has been reported that nitrite can be electro-reduced to ammonia in the presence of copper-contained catalyst. The electrocatalytic reduction mechanism can be described as below 27:
Cu (0) + NO2−+ H+→ Cu (oxidized) + NH3 Cu (oxidized) +e-→ Cu (0) And in the highly acidic condition, NO is produced due to the disproportionation reaction of nitrous acid, and the Cu nanoparticles also show electrocatalytic activity to the reduction of NO. The reaction follows:
3HNO2→ H++ NO3−+ 2NO + H2O Cu (0) + NO + H+→ Cu (oxidized) + NH3 Cu (oxidized) +e-→ Cu (0) Fig.4(B) shows the cyclic voltammograms of GCE (a), RGO/GCE (b), f-RGO/GCE (c) Cu/GCE (d), Cu/RGO/GCE (e), and Cu/f-RGO/GCE (f) in 0.1 M pH= 2.0 PBS solution containing 5 mM NaNO2. No obvious electrochemical responses are observed on GCE, RGO/GCE and f-RGO/GCE which demonstrates RGO does not has ability of catalytic reduction for NaNO2. In contrast, the obvious cathodic peaks appear at -0.6 V on the curves of Cu/GCE, Cu/RGO/GCE and Cu/f-RGO/GCE. It demonstrates that Cu particles have the catalytic ability for reduction of nitrite. Among them, Cu/fRGO/GCE displays the highest peak current, which indicates Cu/f-RGO/GCE possesses the highest ability for electrocatalytic reduction of nitrite. It is because that the 3D structure of f-RGO increased the surface area and accelerated the electron transfer of electrode and the more Cu particle dispersed on f-RGO.
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Fig.4 (A) Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution without (a) and with (b) 5 mM NaNO2. Scan rate: 50 mV/s. (B) Cyclic voltammograms of GCE (a), RGO/GCE (b), f-RGO/GCE (c) Cu/GCE (d), Cu/RGO/GCE (e), and Cu/f-RGO/GCE (f) in 0.1 M pH= 2.0 PBS solution with 5mM NaNO2. Scan rate: 50 mV/s.
3.3 Effect of scan rate. Cyclic voltammetric response of nitrite at different scan rates was also investigated. Fig.5 shows the CVs of Cu/f-RGO/GCE in pH= 2.0 PBS with 5 mM NaNO2 at different scan rates. It is seen that the reduction peak current gradually increases with the increase of scan rates (10-100 mV s-1) and the reduction peak potential of nitrite is observed to shift to more negative potentials. As shown in the inset of Fig.5, a linear relationship is established between the cathodic peak current and the scan rate. The linear equation is I (µA) = -1.15υ (mV/s) -159.81, R² = 0.991, which indicates that the reaction is a surface-controlled process.
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0.5 V, -0.6 V and -0.7 V) were chosen for study. Fig.7 shows that amperometric responses of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution at different potentials with a successive addition of 0.5 mM NaNO2 every time. The amperometric currents increased after step-by-step adding the NaNO2 in the three potentials respectively. The maximum amperometric response appears at -0.6 V compared with the other two potentials. Thus, -0.6 V was selected as the optimal detection potential in subsequent amperometric detecting nitrite.
Fig.5 Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution containing 5mM NaNO2 at different scan rates of 10, 20, 30, 40, 50, 60, 70, 80, -1 90 and 100 mV s . Inset: plot of reduction peak current versus scan rate.
3.4 Optimization of parameters of the determination The pH values of supporting electrolyte influenced the results of experiments effectively, and the pH value could be controlled through adjusting the ratio of NaH2PO4/H3PO4 concentrations in PBS. Fig.6 shows the CVs of Cu/f-RGO/GCE in different pH (1.0, 1.5, 2.0, 2.5, and 3.0) solutions with 5 mM NaNO2. Peak current increases with pH in the range of 1.0-2.0 and when pH is higher than 2.0, a decrease in current is observed. The maximum cathodic peak current appears at pH=2.0. Small peak current at low pH (< 2.0) may be attributed to the instability of Cu particles in highly acidic condition, while at high pH (> 2.0), nitrous acid was difficult to be produced which adversely affected catalytic reaction. Therefore, pH=2.0 supporting electrolyte was selected in the subsequent experiments.
Fig.7 Amperometric responses of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution at different potentials (-0.5V, -0.6V and -0.7V) with a successive addition of 0.5 mM NaNO2.
3.5 Amperometric determination of nitrite The amperometric responses of the Cu/f-RGO/GCE were illustrated in Fig.8. In order to carry out this experiment, NaNO2 with different concentrations were added in 25 mL 0.1 M pH= 2 PBS solution with stirring. Fig.7 depicts the optimal amperometric response for reduction of NaNO2 at the potential of -0.6 V. In Fig.8 (A), it can be seen that there are fast and stable amperometric current increasing after step-by-step adding the NaNO2 with certain concentration. The reduction current rapidly increases to attain 95% steady-state value within 2 s after addition of NaNO2. Fig.8 (B) shows that the correlation between the reduction current and concentration of NaNO2 derives from Fig.8 (A). The corresponding linear function is expressed as I (µA) = -0.08162 CNaNO2 (µM) - 13.54 (R = 0.992). The liner range of NaNO2 determination is 0.15 µM to 10500 µM, and the limit of detection (LOD) is observed at 0.06 µM (S/N=3). Various kinds of sensors for comparison were summarized in Table 1 with the line ranges and LODs. The results confirm that Cu/f-RGO/GCE possesses wide detection range and a low detection limit toward nitrite determination.
Fig.6 Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M PBS solutions of pH 1.0 (a), 1.5 (b), 2.0 (c), 2.5(d) and 3.0 (e).
The effect of detection potential on the amperometric response for detecting nitrite also should be studied. Based on the previous experiments, three different detection potentials (-
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3.6 Reproducibility, stability, interference and real sample analysis of Cu/f-RGO/GCE for nitrite determination In the reproducibility tests, Cu/f-RGO/GCE was evaluated by detecting NaNO2 via CV in 0.1M pH=2.0 PBS for six times in the same condition. It was found that the cathodic peak current response is almost constant, and the relative standard deviation (RSD) was 3.3%, which indicated that the Cu/f-RGO/GCE possesses excellent reproducibility. The stability of Cu/fRGO/GCE was also investigated. The designed sensor was stored for 15 days at room temperature before the experiment. The peak current intensity of NaNO2 decreased 4.1%, demonstrating a nice storage stability of the electrode. Possible interference for detecting nitrite with a Cu/fRGO/GCE was also examined. 25 mM KCl, NaCl, Na2SO4, ascorbic acid (AA) and NaNO3 were added into 0.1 M pH=2.0 PBS solution for detecting 0.5 mM nitrite. (The concentrations of interfering substances were 50 times higher than that of NaNO3). None of these substances in high concentration had a serious influence in the detection of nitrite, which was shown in Fig.9 It indicates that the Cu/f-RGO/GCE for detecting nitrite has an acceptable degree of tolerance to common ions that can cause interference. To evaluate the applicability of Cu/fRGO/GCE in real samples, nitrite was detected in river water and tap water by standard addition method (Table 2). The recoveries range between 99.5% and 103.2%, which demonstrates that the sensor has the potential to be used to detecting nitrite in real samples.
Fig.8 (A) Amperometric responses of Cu/f-RGO/GCE after the successive additions of NaNO2 in 0.1 M pH= 2.0 PBS solution at -0.6V. Inset: The amplification of the i-t curve at low concentration region. (B) The liner calibration curve for NaNO2 from 0.15 μM to 10500 μm.
Table 1 The analytical performances of different modified electrodes for the determination of nitrite. Detection limit (µ mol L-1) 0.4
Linear range (µ mol L-1) 4-1440
0.4
1.25-13000
27
1
3-1000
28
0.25
1-380
29
1.65
5-6750
30
AuNPs/SG/GCE f
0.2
10-3960
31
Cu/f-RGO/GCE
0.06
0.15-10500
This work
Electrodes Ag-PAMAM/GCE
a
Cu-NDs/RGO/GCE b PATP-PtNPs/Au
c
GR-CS/AuNPs/GCE d CDP-GS-MWCNTs/GCE
a
e
Reference 12
Fig.9 Interference test of the Cu/f-RGO/GCE upon successive additions: 0.5 mM NaNO2 and 25mM other interferents in 0.1 M pH= 2.0 PBS solution. Applied potential: -0.6 V.
Ag-polyamidoamine modified glassy carbon electrode
b
Copper nanodendrites-reduced graphene oxide modified glassy carbon electrode
c
Pt nanoparticles-poly(2-aminothiophenol) modified Au electrode
d
Graphene -chitosan/Au nanoparticles modified glassy carbon electrode
e
Cyclodextrin-cyclodextrin prepolymer-graphene sheets-multiwall carbon nanotubes modified glassy carbon electrode
Table 2 Determination of nitrite in real samples. Samples Tap water River water
1 2 1 2
Original (µM) 2.21 2.37
Added (µM) 20.00 50.00 20.00 50.00
Found (µM) 20.34 51.61 22.83 52.09
Recovery (%) 101.7 103.2 102.8 99.5
Five replicate measurements were made on each sample. f
Au nanoparticles-sulfonated graphene modified glassy carbon electrode
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4. Conclusions In this paper, a facile and low-cost electrochemical method was developed for the preparation of 3D flower-like RGO nanosheets on GCE which was then employed as the support material for electro-deposition of Cu particles to fabricate a sensor for nitrite determination. The prepared Cu/f-RGO/GCE exhibited excellent electrocatalytic capability for the reduction of nitrite compared with Cu/GCE and Cu/ RGO/GCE in CV curves. While in the determination of nitrite the prepared sensor possessed a wide liner range (0.15-10500 µM), a low detection limit (0.06 µM), excellent reproducibility, nice stability and remarkable anti-interference ability. These results demonstrated the special electrochemical characters of flower-like RGO nanosheets. This work provides a simple and cost-effective approach which is expected to have promising application prospects.
Acknowledgment The authors are grateful for the financial support from the National Natural Science Foundation of China (grant nos. 51373111, 51073114 and 20933007), the Opening Project of Xinjiang Key Laboratory of Electronic Information Materials and Devices (XJYS0901-2010-01), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Academic Award for Young Graduate Scholar of Soochow University.
Notes and References College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P R China * Corresponding author: Tel: 86-512-65880089, Fax: 86-512-65880089; E-mail: [email protected] (Y. Du).
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DOI: 10.1039/C4AN01924E
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Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012
A facile fabrication of copper particle-decorated novel graphene flower composites for enhanced detecting of nitrite Huiwen Wang, Caiqin Wang, Beibei Yang, Chunyang Zhai, Duan Bin, Ke Zhang, Ping Yang and Yukou Du*
DOI: 10.1039/x0xx00000x www.rsc.org/
We described a simple electrochemical preparation method of a novel three dimensional (3D) graphene material, porous flower-like reduced graphene oxide (f-RGO) nanosheets, which was explored as the support for the Cu particles on glassy carbon electrode (Cu/f-RGO/GCE) for detecting nitrite. In morphology, scanning electron microscopy (SEM) demonstrates the 3D porous structure of f-RGO enlarges the surface area of electrode and promotes more Cu particles depositing on the surface of fRGO with homogeneous dispersion. In cyclic voltammetry (CV), a well-defined voltammetric peak along with the remarkable reduction current indicates excellent electrocatalytic activity of the Cu/f-RGO/GCE for NaNO 2 reduction compared with other corresponding electrodes. The effects of pH value and detection potential on the current responses of Cu/f-RGO/GCE towards nitrite were optimized to obtain the maximal sensitivity. In the optimal experimental conditions, Cu/f-RGO/GCE displays the wide detection range from 0.15 μM to 10500 μM and the low limit of detection of 0.06 μM (S/N=3) with fast response time 2 s for detecting NaNO 2 through amperometric method. Furthermore, the presence of K+, Na+ , Cl −, NH4+, NO 3 -, SO 42− and ascorbic acid show a negligible effect on the current response of nitrite determination suggesting Cu/f-RGO/GCE have the high selectivity for detecting nitrite even in the presence of high concentration of interferes. Moreover, the real sample determination experiment indicated practical feasibility of the obtained sensor. The prepared sensor for determination of NaNO 2 exhibited wide liner range, low detection limit, good reproducibility, nice stability and remarkable antiinterference ability. In this paper, not only did the Cu/f-RGO/GCE show high performance for determination of nitrite, but also it was simple to prepare, user-friendly and cost-effective.
1. Introduction Nitrite, an important reagent in industry, is widely used as a preservative 1, food additive 2 and fertilizer 3, but hazardous to human health. In human body, nitrite can oxidize hemoglobin to methemoglobin irreversibly, which reduces capacity to transport oxygen of blood 4. Furthermore, it reacts with dietary compounds to form carcinogenic nitrosamines in human digestive system 5. It is known that the consumption of 0.3-0.5 g nitrite can lead to poisoning or even death. As a result, quantitative analysis of nitrite in living environment and food products is extremely essential for human health. Up to now, 6 Several techniques such as chromatography , 7 8 spectrophotometry , capillary electrophoresis and electrochemical techniques 4 have been applied in nitrite detection. The traditional methods including spectrophotometry and chromatography are reliable but time-consuming, complicated and expensive, which are not universally
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satisfactory. Compared to these methods, the electrochemical technique is considered superior because it can provide high sensitive, simple, cheap and real-time analysis. To improve the performance of the electrodes for nitrite detection, many suitable materials have been explored to modify electrodes, such as metal nanoparticles, including gold 9, platinum 10, copper 11 and silver 12 and various kinds of supporting materials, including carbon nanotube 13, conducting polymer 14 and graphene 15. The modified electrodes with outstanding electrochemical properties can improve performance of sensors in nitrite determination. Among the supporting materials, it should be noted that, graphene (reduced graphene oxide, denoted as RGO), an ideal two-dimensional layered material, has recently attracted a great attention in electrochemistry because of its unique mechanical properties, large specific surface area, rapid heterogeneous electron transfer, high electrical conductivity and large amount of edge-plan-like defects 16. Graphene and graphene-based
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PAPER materials hold technological promise in the areas of fuel cells 17 , capacitors 18, actuators 19, sensors 20 and so on. The electrodes modified with graphene-based composites for the detection of nitrite also have been reported in recent years 15. What is more, three-dimensional (3D) porous graphene has been also considered as a promising catalyst support material nowadays because 3D graphene not only can overcome the problem of a strong tendency to agglomerate about RGO sheets due to the enormous van der Waals force, but also has more unique physical and chemical properties 21. Therefore, fabricating a novel sensor modified with the composites of suitable materials and 3D porous RGO for detecting nitrite can be considered. Various techniques have been adopted to synthesize 3D porous RGO, such as chemical vapor deposition 22, hydrogel, aerogel techniques 23 and electrochemical method 24. In this work, we prepare 3D flower-like RGO (f-RGO) nanosheets on glassy carbon electrode (GCE) via electrochemical strategy which is facile and cost-effective compared with other preparation methods. And then, Cu particles are deposited on the surface of f-RGO (Cu/f-RGO) by potentiostatic electrodeposition. Compared with Cu/GCE and Cu/RGO/GCE, the prepared modified electrode (Cu/f-RGO/GCE) exhibits high electrocatalytic activities towards the reduction of nitrite and provides excellent property in quantitative detection of nitrite through the electrochemical reduction method. In addition, facile and simple preparation processes and non-noble metal included in the electrode could reduce the preparation costs. This work presents a new attempt to use the 3D porous f-RGO nanosheets as support in electrochemical determination field, and simple and cost-effective processes of preparation and determination indicate application potential of the sensor in the future.
Co., Ltd., China). GCE with a surface area of 0.07 cm2 was used as the working electrode. A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. All the experiments were performed at 25 ºC. 2.2 Electrode preparation Graphene oxide (GO) solution was prepared by the previously reported method 25. First, 10 µL 0.5mg mL-1 GO solution ink was dropped on the surface of GCE followed by electrochemical reduction to RGO layer in a Na-PBS (0.1 M, pH = 4.1) solution at a constant potential of -0.9 V. Second, copper particles were electrodeposited on the surface of formed RGO layer by reduction of 5.0 mM CuSO4 solution at a constant potential of -0.4 V with a charge of 1.0 × 10-2 C. And then another GO ink (10uL) was again coated the obtained electrode (Cu/RGO/GCE) followed by the above reduction strategy and a sandwich construction was fabricated (RGO/Cu/RGO/GCE). Subsequently, the Cu particles were electrolyzed at a constant potential of 0.1 V in the Na-PBS (0.1 M, pH = 4.1) solution for 1000s. As the reaction progressed, with the Cu2+ ions continuously produced and transporting across the exterior RGO layer, the structure of the RGO sheets coated on the Cu particles would transform and turn to stand up from the surface of the substrate. The raised RGO sheets interconnected with each other, and then the porous f-RGO could be obtained 26. Finally, the f-RGO/GCE was immersed into 0.01M CuSO4 solution containing 0.1 M Na2SO4 and Cu particles were electrodeposited at -0.4 V with a charge of 2.0 × 10-2 C, and Cu/f-RGO/GCE could be obtained. For comparison, Cu/GCE, RGO/GCE and Cu/RGO/GCE were also fabricated in the same manner.
3. Results and discussion 2. Materials and methods 2.1 Chemicals and apparatus Graphene oxide, sodium nitrite, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, sodium sulphate and cupric sulphate anhydrous were purchased from Sinopharm Chemicals Reagent Co., Ltd. All the chemicals were analytical grade. Doubly distilled water was used throughout the experiments. Freshly prepared 0.1M phosphate buffer solution (PBS, pH 2.0), consisting of NaH2PO4 and H3PO4, was used as the electrolyte. All experimental solutions were deoxygenated by high-purity nitrogen and continuous nitrogen was blown on the surface of solutions gently. The scanning electrode microscopy (SEM) (S-4700, Hitachi High Technologies Corporation, Japan) was used to characterize the morphologies of the obtained electrodes. All the electrochemical experiments were carried out in a conventional three-electrode system using a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument
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3.1 Characterizations of Cu/f-RGO/GCE. Fig.1 is the SEM images of f-RGO and Cu/f-RGO on GCE at different resolutions. As shown in Fig.1 (A-B), several welldefined 3D structured RGO flowers are dispersed on the substrate with few regions of exposed RGO substrate among the flowers. The diameter of each flower can be estimated to be about 5 µm. The RGO-sheet petals are interconnected and composed the whole RGO flowers with typical porous architecture. In Fig.1 (C), some wrinkles also can be seen on the surface of thin RGO petals. It is believed that the porous structure of RGO flower and the ripples on the RGO petals increase the surface area of electrode. The characteristics of fRGO in morphology indicate it is a good candidate as the support material to modify electrode. In comparison with the smooth surface of the pure f-RGO shown in Fig.1 (A-C), the obvious rough surface of Cu/f-RGO in Fig.1 (D-F) is exhibited due to the present of uniform Cu particles on the surface of each RGO petal. In Fig.1 (F), Cu particles are dispersed on the walls of the f-RGO pores so the surface of petals becomes grainy and the thickness increases. From the SEM images, the
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diameter of Cu particles can be estimated to be 70-100 nm. In morphology, it is believed that the f-RGO nanosheets increase the surface area of electrode and support more electro-deposited Cu particles to disperse on electrode.
electrodes. Ignore the peaks originating from the ITO substrate, a broadened peak is observed at about 2θ=24.1° arising from the reduction of graphene oxide (002) for RGO, f-RGO and Cu/f-RGO. The broadness of this peak is presumably attributed of the multilayer character of graphene sheets and the structural defects induced by electrochemical reduction 26. The similar XRD patterns of RGO and f-RGO demonstrate that f-RGOs are constitute of RGO sheets. In addition, three new diffraction peaks at 2θ= 43.7°, 50.9° and 74.1° corresponding to Miller indices (1 1 1), (2 0 0) and (2 2 0), can be indexed to facecentered cubic Cu crystals, which is in good accordance with reported results 27.
Fig.1 SEM images of f-RGO (A-C) and Cu/f-RGO (D-F)
Fig.2 shows the cyclic voltammograms of GCE and RGO/GCE and f-RGO/GCE in 5 mM K3[Fe(CN)6] + K4[Fe(CN)6] containing 1 M KCl solution. We can calculated the electrochemical active surface area (ECSA) of the fRGO/GCE on the basis of the charge associated with the oxidation and reduction peak currents in CV curve, which can be also an important criterion to evaluate the electrochemical performance of porous materials. For comparison, the ECSA of the bare GCE and RGO/GCE were also calculated based on the Randles-Sevcik equation, assuming mass transport only by the diffusion process 26,
Fig.2 CVs of the bare GC (a), RGO/GCE (b), and f-RGO/GCE (C) in 5 mM K3[Fe(CN)6] + K4[Fe(CN)6] aqueous solution containing 1 M KCl at scan rate of 50 mV s-1.
IP = 2.69×105AD1/2nγ1/2C where IP refers to the anodic peak current, A is ECSA of the electrode, n is the number of electrons participating in the redox reaction, D is the diffusion coefficient of the molecule (equal to (6.70 ± 0.02) × 10-6 cm2 s-1), C is the concentration of the probe molecule in the solution (mol cm-3), and γ is the scan rate (V s1). From the slope of the IP-γ1/2 relation, the ECSA of the bare GCE, RGO/GCE, f-RGO/GCE were calculated a 0.028, 0.038 and 0.046 cm2, respectively. The result demonstrates that the porous structure of RGO flowers increase the electrochemical active surface area of electrode. The RGO, f-RGO and Cu/f-RGO formed on ITO were used for XRD characterization. Fig.3 shows the XRD patterns of the bare ITO, RGO/ITO, f-RGO/ITO and Cu/f-RGO/ITO
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Fig.3 XRD patterns of ITO (a), RGO/ITO (b), f-RGO/ITO (c) and Cu/f-RGO/ITO (d).
3.2 Cyclic voltammetry responses of Cu/f-RGO/GCE for nitrite reduction The electrocatalytic reduction of nitrite on the corresponding modified electrodes was investigated via cyclic voltammetry (CV). In Fig.4 (A), it is noted that the reduction current of Cu/f-
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PAPER RGO/GCE increases significantly after adding 5 mM NaNO2 in pH = 2 PBS solution and the reduction peak current appeared at -0.6 V, which suggests that electrocatalytic reduction of nitrite occurred on the modified electrode. It has been reported that nitrite can be electro-reduced to ammonia in the presence of copper-contained catalyst. The electrocatalytic reduction mechanism can be described as below 27:
Cu (0) + NO2−+ H+→ Cu (oxidized) + NH3 Cu (oxidized) +e-→ Cu (0) And in the highly acidic condition, NO is produced due to the disproportionation reaction of nitrous acid, and the Cu nanoparticles also show electrocatalytic activity to the reduction of NO. The reaction follows:
3HNO2→ H++ NO3−+ 2NO + H2O Cu (0) + NO + H+→ Cu (oxidized) + NH3 Cu (oxidized) +e-→ Cu (0) Fig.4(B) shows the cyclic voltammograms of GCE (a), RGO/GCE (b), f-RGO/GCE (c) Cu/GCE (d), Cu/RGO/GCE (e), and Cu/f-RGO/GCE (f) in 0.1 M pH= 2.0 PBS solution containing 5 mM NaNO2. No obvious electrochemical responses are observed on GCE, RGO/GCE and f-RGO/GCE which demonstrates RGO does not has ability of catalytic reduction for NaNO2. In contrast, the obvious cathodic peaks appear at -0.6 V on the curves of Cu/GCE, Cu/RGO/GCE and Cu/f-RGO/GCE. It demonstrates that Cu particles have the catalytic ability for reduction of nitrite. Among them, Cu/fRGO/GCE displays the highest peak current, which indicates Cu/f-RGO/GCE possesses the highest ability for electrocatalytic reduction of nitrite. It is because that the 3D structure of f-RGO increased the surface area and accelerated the electron transfer of electrode and the more Cu particle dispersed on f-RGO.
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Fig.4 (A) Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution without (a) and with (b) 5 mM NaNO2. Scan rate: 50 mV/s. (B) Cyclic voltammograms of GCE (a), RGO/GCE (b), f-RGO/GCE (c) Cu/GCE (d), Cu/RGO/GCE (e), and Cu/f-RGO/GCE (f) in 0.1 M pH= 2.0 PBS solution with 5mM NaNO2. Scan rate: 50 mV/s.
3.3 Effect of scan rate. Cyclic voltammetric response of nitrite at different scan rates was also investigated. Fig.5 shows the CVs of Cu/f-RGO/GCE in pH= 2.0 PBS with 5 mM NaNO2 at different scan rates. It is seen that the reduction peak current gradually increases with the increase of scan rates (10-100 mV s-1) and the reduction peak potential of nitrite is observed to shift to more negative potentials. As shown in the inset of Fig.5, a linear relationship is established between the cathodic peak current and the scan rate. The linear equation is I (µA) = -1.15υ (mV/s) -159.81, R² = 0.991, which indicates that the reaction is a surface-controlled process.
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0.5 V, -0.6 V and -0.7 V) were chosen for study. Fig.7 shows that amperometric responses of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution at different potentials with a successive addition of 0.5 mM NaNO2 every time. The amperometric currents increased after step-by-step adding the NaNO2 in the three potentials respectively. The maximum amperometric response appears at -0.6 V compared with the other two potentials. Thus, -0.6 V was selected as the optimal detection potential in subsequent amperometric detecting nitrite.
Fig.5 Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution containing 5mM NaNO2 at different scan rates of 10, 20, 30, 40, 50, 60, 70, 80, -1 90 and 100 mV s . Inset: plot of reduction peak current versus scan rate.
3.4 Optimization of parameters of the determination The pH values of supporting electrolyte influenced the results of experiments effectively, and the pH value could be controlled through adjusting the ratio of NaH2PO4/H3PO4 concentrations in PBS. Fig.6 shows the CVs of Cu/f-RGO/GCE in different pH (1.0, 1.5, 2.0, 2.5, and 3.0) solutions with 5 mM NaNO2. Peak current increases with pH in the range of 1.0-2.0 and when pH is higher than 2.0, a decrease in current is observed. The maximum cathodic peak current appears at pH=2.0. Small peak current at low pH (< 2.0) may be attributed to the instability of Cu particles in highly acidic condition, while at high pH (> 2.0), nitrous acid was difficult to be produced which adversely affected catalytic reaction. Therefore, pH=2.0 supporting electrolyte was selected in the subsequent experiments.
Fig.7 Amperometric responses of Cu/f-RGO/GCE in 0.1 M pH= 2.0 PBS solution at different potentials (-0.5V, -0.6V and -0.7V) with a successive addition of 0.5 mM NaNO2.
3.5 Amperometric determination of nitrite The amperometric responses of the Cu/f-RGO/GCE were illustrated in Fig.8. In order to carry out this experiment, NaNO2 with different concentrations were added in 25 mL 0.1 M pH= 2 PBS solution with stirring. Fig.7 depicts the optimal amperometric response for reduction of NaNO2 at the potential of -0.6 V. In Fig.8 (A), it can be seen that there are fast and stable amperometric current increasing after step-by-step adding the NaNO2 with certain concentration. The reduction current rapidly increases to attain 95% steady-state value within 2 s after addition of NaNO2. Fig.8 (B) shows that the correlation between the reduction current and concentration of NaNO2 derives from Fig.8 (A). The corresponding linear function is expressed as I (µA) = -0.08162 CNaNO2 (µM) - 13.54 (R = 0.992). The liner range of NaNO2 determination is 0.15 µM to 10500 µM, and the limit of detection (LOD) is observed at 0.06 µM (S/N=3). Various kinds of sensors for comparison were summarized in Table 1 with the line ranges and LODs. The results confirm that Cu/f-RGO/GCE possesses wide detection range and a low detection limit toward nitrite determination.
Fig.6 Cyclic voltammograms of Cu/f-RGO/GCE in 0.1 M PBS solutions of pH 1.0 (a), 1.5 (b), 2.0 (c), 2.5(d) and 3.0 (e).
The effect of detection potential on the amperometric response for detecting nitrite also should be studied. Based on the previous experiments, three different detection potentials (-
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3.6 Reproducibility, stability, interference and real sample analysis of Cu/f-RGO/GCE for nitrite determination In the reproducibility tests, Cu/f-RGO/GCE was evaluated by detecting NaNO2 via CV in 0.1M pH=2.0 PBS for six times in the same condition. It was found that the cathodic peak current response is almost constant, and the relative standard deviation (RSD) was 3.3%, which indicated that the Cu/f-RGO/GCE possesses excellent reproducibility. The stability of Cu/fRGO/GCE was also investigated. The designed sensor was stored for 15 days at room temperature before the experiment. The peak current intensity of NaNO2 decreased 4.1%, demonstrating a nice storage stability of the electrode. Possible interference for detecting nitrite with a Cu/fRGO/GCE was also examined. 25 mM KCl, NaCl, Na2SO4, ascorbic acid (AA) and NaNO3 were added into 0.1 M pH=2.0 PBS solution for detecting 0.5 mM nitrite. (The concentrations of interfering substances were 50 times higher than that of NaNO3). None of these substances in high concentration had a serious influence in the detection of nitrite, which was shown in Fig.9 It indicates that the Cu/f-RGO/GCE for detecting nitrite has an acceptable degree of tolerance to common ions that can cause interference. To evaluate the applicability of Cu/fRGO/GCE in real samples, nitrite was detected in river water and tap water by standard addition method (Table 2). The recoveries range between 99.5% and 103.2%, which demonstrates that the sensor has the potential to be used to detecting nitrite in real samples.
Fig.8 (A) Amperometric responses of Cu/f-RGO/GCE after the successive additions of NaNO2 in 0.1 M pH= 2.0 PBS solution at -0.6V. Inset: The amplification of the i-t curve at low concentration region. (B) The liner calibration curve for NaNO2 from 0.15 μM to 10500 μm.
Table 1 The analytical performances of different modified electrodes for the determination of nitrite. Detection limit (µ mol L-1) 0.4
Linear range (µ mol L-1) 4-1440
0.4
1.25-13000
27
1
3-1000
28
0.25
1-380
29
1.65
5-6750
30
AuNPs/SG/GCE f
0.2
10-3960
31
Cu/f-RGO/GCE
0.06
0.15-10500
This work
Electrodes Ag-PAMAM/GCE
a
Cu-NDs/RGO/GCE b PATP-PtNPs/Au
c
GR-CS/AuNPs/GCE d CDP-GS-MWCNTs/GCE
a
e
Reference 12
Fig.9 Interference test of the Cu/f-RGO/GCE upon successive additions: 0.5 mM NaNO2 and 25mM other interferents in 0.1 M pH= 2.0 PBS solution. Applied potential: -0.6 V.
Ag-polyamidoamine modified glassy carbon electrode
b
Copper nanodendrites-reduced graphene oxide modified glassy carbon electrode
c
Pt nanoparticles-poly(2-aminothiophenol) modified Au electrode
d
Graphene -chitosan/Au nanoparticles modified glassy carbon electrode
e
Cyclodextrin-cyclodextrin prepolymer-graphene sheets-multiwall carbon nanotubes modified glassy carbon electrode
Table 2 Determination of nitrite in real samples. Samples Tap water River water
1 2 1 2
Original (µM) 2.21 2.37
Added (µM) 20.00 50.00 20.00 50.00
Found (µM) 20.34 51.61 22.83 52.09
Recovery (%) 101.7 103.2 102.8 99.5
Five replicate measurements were made on each sample. f
Au nanoparticles-sulfonated graphene modified glassy carbon electrode
6 | Analyst, 2014, 00, 1-3
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4. Conclusions In this paper, a facile and low-cost electrochemical method was developed for the preparation of 3D flower-like RGO nanosheets on GCE which was then employed as the support material for electro-deposition of Cu particles to fabricate a sensor for nitrite determination. The prepared Cu/f-RGO/GCE exhibited excellent electrocatalytic capability for the reduction of nitrite compared with Cu/GCE and Cu/ RGO/GCE in CV curves. While in the determination of nitrite the prepared sensor possessed a wide liner range (0.15-10500 µM), a low detection limit (0.06 µM), excellent reproducibility, nice stability and remarkable anti-interference ability. These results demonstrated the special electrochemical characters of flower-like RGO nanosheets. This work provides a simple and cost-effective approach which is expected to have promising application prospects.
Acknowledgment The authors are grateful for the financial support from the National Natural Science Foundation of China (grant nos. 51373111, 51073114 and 20933007), the Opening Project of Xinjiang Key Laboratory of Electronic Information Materials and Devices (XJYS0901-2010-01), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Academic Award for Young Graduate Scholar of Soochow University.
Notes and References College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P R China * Corresponding author: Tel: 86-512-65880089, Fax: 86-512-65880089; E-mail: [email protected] (Y. Du).
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DOI: 10.1039/C4AN01924E
