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Electrochemical synthesis of gold nanoparticles decorated flower-like graphene for high sensitivity detection of nitrite
Journal of Colloid and Interface Science 488 (2017) 135–141
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Electrochemical synthesis of gold nanoparticles decorated flower-like graphene for high sensitivity detection of nitrite Cui’e Zou a, Beibei Yang a, Duan Bin a, Jin Wang a, Shumin Li a, Ping Yang a, Caiqin Wang a, Yukihide Shiraishi b, Yukou Du a,⇑ a b
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China Tokyo University of Science Yamaguchi, SanyoOnoda-shi, Yamaguchi 756-0884, Japan
g r a p h i c a l a b s t r a c t The spherical Au nanoparticles/3D flower-like graphene was prepared by a facile and low-cost electrochemical method and used for sensitive determination of nitrite.
a r t i c l e
i n f o
Article history: Received 20 September 2016 Revised 24 October 2016 Accepted 29 October 2016 Available online 31 October 2016 Keywords: Au nanoparticles Graphene Electrodeposition NaNO2
a b s t r a c t In this paper, the spherical Au nanoparticles and 3D flower-like structure graphene were successively deposited on glassy carbon electrode (GCE) (Au/f-GE/GCE) via a facile and two-step electrodeposition method for the detection of nitrite ions (NaNO2). The morphology and composition elements were confirmed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction measurements (XRD). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the electrochemical behaviors of NaNO2 on the as-prepared electrode. Compared to f-GE/GCE and Au/GCE, Au/f-GE/GCE showed a sharp and obvious oxidation peak at 0.78 V. The oxidation peak current of NaNO2 was linearly proportional to its concentration in the range from 0.125 to 20375.98 lM, with a detection limit of 0.01 lM (at S/N = 3). Furthermore, the experiment results also showed that the as-prepared electrode exhibited excellent reproducibility and long-term stability, as well as good recovery when applied to the determination of NaNO2 in pickled pork samples. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.jcis.2016.10.088 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nitrite ion (NO 2 ), an inorganic compound, has been widely exploited in our daily life as an additive in foods and a corrosion inhibitor, and recognized as an alarming pollutant to the environment and human health [1,2]. An excessive level of nitrite
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in the body of human, not only can lead to the irreversible oxidation of hemoglobin to methemoglobin, but also can react with dietary components to form a nitrosamine, resulting in cancer and hypertension [3–5]. The World Health Organization has reported that the fatal dose of nitrite ingestion is between 8.7 lM and 28.3 lM [6,7]. As a consequence, it is necessary to develop reliable methods for the detection and monitoring of nitrite. Up to now, many techniques have been developed to determine NO 2 levels, such as spectrophotometry [8,9], chemiluminescence [10,11], high-performance liquid chromatography [12] and electrochemical methods [13–16]. Among them, the electrochemical methods are proven to be powerful tools due to its fast procedure, low cost, low detection limit and high accuracy. Unfortunately, the electrochemical oxidation of nitrite at the traditional electrodes suffers large overpotentials and the electrodes tend to be poisoned by the products generated during the electrochemical process. To resolve these problems, one strategy is to use various noble metal nanostructures in the construction of the working electrode to enhance the sensitivity for the detection of nitrite due to their excellent catalysis, unique dimensions and high effective surface area [17–19]. In particular, Au nanoparticles have been extensively explored for the determination of nitrite because of its excellent conductivity, unique structure, well electrocatalytic ability and biocompatibility. For instance, Jiang et al. used coupled graphene and Au nanoparticles to fabricate the electrochemical biosensor for detecting nitrite [20]. Li and his co-workers prepared the sensor of gold nanoparticles (AuNPs) and sulfonated graphene, which displayed electrocatalytic activity in the detection of nitrite [21]. It is worth noting that the Au based electrodes could be modified with some other supporting materials, such as transition metal oxides, conducting polymer and graphene, to achieve great selectivity and sensitivity for the detection of the nitrite [20–25]. On another front, as a potentially excellent electrode supporting material in electrochemical application such as supercapacitors, electrocatalysis and sensors, graphene (GE) has been attracted particular attentions owing to its peculiar properties including large specific surface area, unique electronic properties, excellent physicochemical properties, high chemical and thermal stability [26–28]. However, such surface expansion is still inherently limited by the two-dimensional (2D) nature of the planar electrodes. Some attempts have been made to construct three-dimensional (3D) electrodes. Dong et al. synthesized 3D graphene/Cobalt oxide electrode for high-performance supercapacitor and non-enzymatic glucose detection by the chemical vapor deposition [29]. Yue et al. have developed a supercapacitor with the 3D flower-like graphene by the electrochemical method [30]. Therefore, fabricating a novel sensor modified with Au nanoparticles (AuNPs) and 3D flower-like graphene for detecting nitrite should be elaborately considered and designed. Herein, a facile and efficient electrochemical approach was been used to prepare 3D flower-like graphene (f-GE). And then the AuNPs were reduced on the graphene surface to form Au/f-GE by potentiostatic deposition. With large specific surface area, highly conductive pathways, and well-defined flowers structure, this new 3D Au/f-GE electrode architecture holds a great promise for electrochemical sensing nitrite. This work might exploit the opportunities for developing novel 3D electrochemical sensors with excellent sensitivity and reproducibility.
2. Experimental 2.1. Materials and reagents Graphite powder, disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), cupric sulphate anhy-
drous (CuSO4), chloroauric acid (HAuCl4), glucose, potassium chloride (KCl) and disodium chloride (NaCl) were purchased from Sinopharm Chemicals Reagent Co., Ltd. Ascorbic acid (AA), uric acid (UA) and dopamine (DA) were obtained from Acros Organics. All chemicals were of analytical reagent grade and used as received. 0.1 M phosphate buffer solution (PBS) with pH 7.0 prepared from NaH2PO4 and Na2HPO4 was chosen for the electrolyte solution in the present experiments. Double distilled water was used throughout the experimental process.
2.2. Apparatus Scanning electron microscopy (SEM) (S-4700, Hitachi High Technologies Corporation, Japan) was used to characterize the morphologies and the energy-dispersive X-ray analyzer (EDX) of the obtained materials. X-ray diffraction (XRD) measurements were carried out to analyze the sample structure using an X’Pert PRO multiple crystals (powder) X-ray diffractometer (PANalytical Company, Holland). All the electrochemical experiments were performed in a conventional three-electrode system at room temperature using a CHI 760E potentiostat/galvanostat (CH Instrumental Co. Ltd, China). The glassy carbon electrodes (GCEs, 3 mm diameter) modified by the as-prepared nanocomposites were used as working electrodes, a Pt wire and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. All of the measurements were carried out at room temperature.
2.3. Preparation of 3D Au/f-GE/GCE Prior to use, the bare GCEs were firstly polished with alumina powder (0.3 and 0.05 lm) to obtain mirror-like surfaces, and then sonicated with absolute ethanol and double water for about 5 min, respectively. Subsequently, the GCEs were rinsed thoroughly with double water and dried for the following experiment use. Graphene oxide (GO) solution was prepared by the modified Hummers’ method [31]. The fabrication of f-GE/GCE was as follows: Firstly, 10 lL GO ink (0.5 mg mL1) was sprayed onto the GCE followed by electrochemical reduction to reduced graphene oxide (GE) in a Na-PBS (0.1 M, pH = 4.1) solution at a constant potential of 0.9 V. Secondly, copper nanoparticles were deposited thereon in 5.0 mM CuSO4 solution at a constant potential of 0.4 V with a charge of 1.0 102 C to form the Cu/GE. And then, another 10 lL GO ink was added on the above obtained surface that was the same as the above strategy to fabricate a sandwich construction of GE/Cu/GE. Finally, 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 1000 s and simultaneously flower-like graphene formed from the surface of the underlying GE layer on the electrode. Here, the Cu nanoparticles were used as the template, when a potential higher than the oxidation potential of Cu, an oxidation reaction (Cu ? Cu2+ + 2e) occurred on the copper particles. As the reaction progressed, the GE sheets which were coated on the copper particles would transform and turned to stand up from the surface of the substrate. The rose GE sheets interconnected with each other, and as a result, the 3D flower-like GE structures were formed. The Au/f-GE/GCE was synthesized by the obtained immersed into a 3.0 mM HAuCl4/0.5 M H3PO4 solution at a constant potential of 0.2 V with a charge of 5.0 103 C. The amount of Au was calculated to be about 3.4 lg according to the integrated charge of Au electro-deposition (assuming a 100% current efficiency). The overall synthetic procedure for f-GE was illustrated in Scheme 1. For comparison, f-GE/GCE and Au/GCE were also fabricated by the similar method.
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Scheme 1. Schematic illustration for fabricating 3D Au/f-GE/GCE.
Fig. 1. SEM (A, B and C) images of Au/GCE and f-GE/GCE. Inset: histogram of the size of Au nanoparticles.
3. Results and discussion 3.1. Characterizations of Au/f-GE/GCE Firstly, Au/GCE was obtained easily using a potentiostatic electrodeposition method by immersing GCE into a 3.0 mM HAuCl4/0.5 M H3PO4 solution at a constant potential of 0.2 V with a charge of 5.0 103 C. As Fig. 1A depicted, Au nanoparticles were mainly consisted of spherical nanostructures having uniform size, which were well dispersed on the GCE surface without obvious agglomerations. From the histogram shown insert Fig. 1A, we can calculate that the average size of Au nanoparticle is approximately 90.94 nm. The spherical nanostructures of AuNPs may possess the larger surface area and the more active sites, which leads to the enhancement of catalytic performances toward the oxidation of NaNO2. The flower-like graphene was facilely synthesized using an electrochemical approach. As the SEM images of f-GE/GCE in Fig. 1B and C described, GE with 3D blooming flower-like structure was dispersed on the substrate with few regions of exposed GE among the flowers and the diameter of each flower was about 5–10 lm. The formation of f-GE in our experiment was related to the removal process of the copper particles. When a potential of 0.1 V was added, an oxidation reaction (Cu ? Cu2+ + 2e) occurred. With the amount of Cu2+ continuously transporting across the exterior GE layer through the interspace between the two GE nanosheets, the rose GE sheets interconnected with each other and the blooming flower-like structure was then formed. Finally, the Au/f-GE/GCE was obtained by potentiostatic electrodeposition of Au nanoparticles on f-GE/GCE. As Fig. 2A and B depicted, each of the as-synthesized AuNPs were uniformly dispersed on the surface of f-GE. The flower-like structure of GE supporting with Au nanoparticles provided a largest specific surface area for the significant improvement in electrochemical properties [32]. Energy-dispersive X-ray spectrum for the synthesized
Au/f-GE (EDX) data shown in Fig. 2C revealed the presence of C, O and Au elements which further proved the formation of Au/fGE/GCE. The crystal structures of the as-prepared Au/ITO, f-GE/ITO and Au/f-GE/ITO were further analyzed by X-ray diffraction (XRD). As displayed in Fig. 3b, a broad diffraction peak at 23° corresponding to the C (0 0 2) diffraction of GE nanosheets indicated that GO was completely reduced by potentiostatic reduction [26,30]. The peaks at 38.2°, 44.4°, 64.5° and 78.1° were attributed to the reflections from the planes of Au (1 1 1), Au (2 0 0), Au (2 2 0) and Au (311) (JCPDS 4-0784), respectively, as shown in Fig. 3c [5,33]. The diffraction peaks of ITO was shown in Fig. 3d. After electrodeposition of AuNPs on the surface of f-GE, the Au/f-GE/ITO (Fig. 3a) possessed all the peaks observed in the XRD patterns of Au/ITO and f-GE/ ITO exhibiting the successful hybridization of the spherical AuNPs and flower-like graphene. 3.2. Electrochemical behavior on the modified electrodes Fig. 4A showed the cyclic voltammograms (CVs) of different electrodes (Au/f-GE/GCE, f-GE/GCE, Au/GCE) in 5.0 mM K3Fe(CN)6 containing 1 M KCl solution at a scan rate of 50 mV s1. As displayed in Fig. 4A, the Au/f-GE/GCE exhibited an increase in the anodic peak current (65.74 lA) compared to f-GE/GCE (35.67 lA) and Au/GCE (55.37 lA). In addition, the effective surface area of the modified electrodes was estimated by cyclic voltammetry according to the Randles–Sevcik equation Eq. (1) [30]
Ip ¼ 2:69 105 n3=2 Aeff D1=2 t1=2 C
ð1Þ
where Ip is the anodic peak current, n is the number of electrons transferred, Aeff is ECSA of the electrode, D refers to the diffusion coefficient of the molecule (equal to (6.70 ± 0.02) 106 cm2 s1), C corresponds to the bulk concentration of the probe (mol cm3),
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Fig. 2. SEM (A and B) images and EDX analysis (C) of Au/f-GE/GCE.
calculated to be 0.084, 0.046 and 0.071 cm2, respectively, demonstrating that combination of AuNPs and f-GE significantly improves the electrochemical active surface area of the Au/f-GE/GCE and facilitates the electron transfer process. The electrochemical behavior of the three modified electrodes in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 was examined via CVs with a potential range from 0.2 to +1.0 V at a scan rate of 50 mV s1 shown in Fig. 4B. Compared to those oxidation currents obtained at the f-GE/GCE (111.8 lA) and Au/GCE (123.8 lA), the Au/f-GE/GCE modified electrode exhibites a remarkably larger peak current (161.5 lA). Moreover, it is noted that at Au/f-GE/GCE, a prominent peak appeared at 0.78 V. It indicates that the heterogeneous electron transfer rate at the electrode is fast, and thus greatly facilitate the oxidation of nitrite. 3.3. Electrochemical parameters of NaNO2 at the Au/f-GE/GCE
Fig. 3. XRD patterns of Au/f-GE/ITO (a), f-GE/ITO (b), Au/ITO (c) and ITO (d).
and ʋ is the scan rate (V s1). From the slope of the Ip ʋ1/2 relation, the ECSAs of the Au/f-GE/GCE, f-GE/GCE and Au/GCE were
As we know, investigating the effect of scan rate on the oxidation peak potential and peak current can evaluate the kinetics of electrode reaction. Fig. 5A represented the CVs of Au/f-GE/GCE in 0.1 M PBS (pH = 7.0) solution with 5 mM NaNO2 at different scan rates (20–200 mV s1). It can be found that the anodic peak currents increase linearly with the scan rate and the calibration
Fig. 4. CVs of Au/f-GE/GCE, f-GE/GCE and Au/GCE electrodes recorded in 5 mM K3Fe(CN)6 + 1 M KCl solution (A), 0.1 M PBS (pH = 7.0) containing 5 mM NaNO2 (B). Scan rate: 50 mV s1.
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Fig. 5. CVs of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 at scan rates from 20 to 200 mV s1 (A), the plots of anodic peak currents to the scan rates (B), anodic peak potentials versus lg ʋ (C).
equation is Ipa (lA) = 0.6105ʋ (mV s1) + 113.66 (R2 = 0.988) (Fig. 5B). This property indicates that the electron transfer for NaNO2 at Au/f-GE/GCE is controlled by an adsorption process [31,34]. In addition, it can be seen that the redox peak potentials shift slightly along with the increase of the scan rate and the linear regression equation of Epa versus the logarithm of the scan rates shown in Fig. 5C are expressed as Epa = 0.04897 lg ʋ + 0.6937 (R2 = 0.989). The electron transfer number (n) are calculated according to the following Laviron’s equation [34]:
Epa
RT ¼ E þ 2:30 lg v ð1 aÞnF 00
ð2Þ
where a is the electron transfer coefficient, n is the number of elec0 tron transferred, E0 is the formal potential, ʋ is the scan rate. R, T and F have their conventional meanings. The n values is calculated to be 2.4, when a is taken 0.5. It means that about two electrons are involved in the irreversible reaction of NaNO2, which is in agreement with Eq. (3).
NO2 þ H2 O ! NO3 þ 2Hþ þ 2e
ð3Þ
The solution pH is an important factor which influences the electrochemical reaction. Fig. 6A described the effect of pH on the CVs response to 5 mM NaNO2 investigating in 0.1 M PBS solution
with the pH range of 4.0–8.0 at a scan rate of 50 mV s1. It can be clearly observed in Fig. 6B that the largest anodic peak current is obtained at the pH of 7.0. As Brylev et al. reported at low pH value, the generating of N2O may cause the peak current to decrease [35]. Furthermore, the oxidation peaks of NaNO2 shift very negatively along with the increasing pH. Therefore, the pH 7.0 was selected as the optimum pH value in our experiments.
3.4. Determination of NaNO2 on the Au/f-GE/GCE Differential pulse voltammetry (DPV) commonly has the higher sensitivity than cyclic voltammograms, accordingly, the DPV technology was carried out for the quantitative detection of NaNO2 on the as-prepared Au/f-GE/GCE. As shown in Fig. 7A, under the optimal conditions, the oxidation peak current of NaNO2 increased with its concentration in the range from 0.125 to 20375.98 lM with two linear ranges. As Fig. 7B described, the two linear regression equations were expressed as Ipa (lA) = 0.03573 C (lM) + 99.32, R2 = 0.994 (C = 0.125–269.71 lM) and Ipa (lA) = 0.0114 C (lM) + 221.29, R2 = 0.990(C = 269.71–20375.98 lM). The detection limit (LOD) is found to be 0.01 lM (S/N = 3), which is lower than those modified electrodes listed in Table 1. The obtained electrode showed wider linear range and even lower
Fig. 6. CVs of the Au/f-GE/GCE electrode in 0.1 M PBS solution containing 5 mM NaNO2 with the pH ranging from 4.0 to 8.0, scan rate: 50 mV s1 (A). The anodic peak current against pH (B).
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Fig. 7. DPV curves of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing different concentrations of NaNO2 from 0.125 to 20375.98 lM (A). The relation between oxidation current versus the concentration of NaNO2 (B).
Table 1 Comparison of different modified electrodes for nitrite determination.
Table 2 Determination of nitrite at various concentrations in pickled pork.
Modified electrode
Linear range (lM)
LOD (lM)
Reference
AgNP/GCE Cu-NDs/RGO/GCE K-modified graphene/GCE CR-GO/GCE Pd/SWCNT/GCE Pd/RGO/GCE AuNPs/CP/GCE AuNPs/SG/GCE Co(II) MTpAP/cit-AuNPs/GCE Au/f-GE/GCE
10–1000 1.25–13000 0.5–7800 8.9–167 2–1230 1–1000 1–100 10–3960 0.5–4700 0.125–20375.98
1.2 0.4 0.2 1 0.25 0.23 0.093 0.2 0.5 0.01
[2] [3] [6] [15] [16] [17] [18] [21] [24] This work
SWCNT: single-walled carbon nanotube; CP: carbon paper; SG: sulfonated graphene; Co(II) MTpAP: meso-tetra(para-aminophenyl)porphyrinatocobalt(II); AuNPs: gold nanoparticles.
detection limit, indicating that the obtained electrode has an excellent electrochemical activity for the oxidation of NaNO2.
Samples
Content (mg kg1)
Added (mg kg1)
Found (mg kg1)
Recovery (%)
R.S.D. (%) (n = 5)
1 2 3
15.6 15.6 15.6
10 20 30
25.8 34.3 47.2
100.8 96.3 103.5
3.2 2.5 4.7
addition of various species into 0.1 M PBS solution (pH 7.0) containing 5 mM NaNO2. As Fig. 8B showed, most of the species, such as NaCl and KCl in a 100-fold concentration, glucose, AA, UA and DA in a 10-fold concentration had a little interference (lower than 5%) toward the determination of NaNO2. All above results suggest that the Au/f-GE/GCE has a good reproducibility and stability and well anti-interference ability for the determination of NaNO2.
3.6. Determination of NaNO2 in real samples 3.5. Stability, reproducibility, interference studies of the Au/f-GE/GCE The reproducibility of our fabricated electrode was investigated by CVs with more than five different electrodes in the same 0.1 M PBS solution (pH 7.0) containing 5 mM NaNO2 solution. It was found that the relative standard deviation (RSD) of the oxidation current was about 2.7%. The stability of the modified electrode was also studied in this work by CVs. From Fig. 8A, we can observe that the oxidation current response of the electrode still remained up to 96.03% after 100 successive assays. Possible interference for the detection of NaNO2 on Au/f-GE/GCE was investigated by
To illustrate the feasibility and application potential of the electrode, the Au/f-GE/GCE was applied to determine NaNO2 from pickled pork using the standard addition technique. The collected pickled pork was disposed (Firstly 12.5 mL borax saturated solution was added under a boiling water bath for 15 min, then 2.5 mL of 30% ZnSO4 solution was used to precipitate protein. After cooled down, the resulting mixture was diluted to 50 mL) and the spiked NaNO2 concentrations were 10, 20 and 30 mg kg1, respectively. As shown in Table 2, the recoveries are 100.8%, 96.3% and 103.5%, correspondingly. The good recovery is obtained suggesting the practical applicability of the proposed method.
Fig. 8. The 1st and 100th CVs of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 at a scan rate of 50 mV s1 (A). Results of the interference study on the response of 100-fold KCl and NaCl, 10-fold glucose, AA, UA and DA (B).
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4. Conclusions In summary, the spherical Au nanoparticles/3D flower-like structure graphene was prepared by a facile and low-cost electrochemical method. The flower-like and spherical hybrid material with large surface area represented an excellent electrochemical performance in the oxidation of NaNO2. Moreover, the prepared electrode also displayed excellent reproducibility, nice stability and remarkable anti-interference ability. What is more, when it was applied to determine NaNO2 in pickled pork, the novel Au/fGE also obtained a good recovery. Compared with other methods for the determination of NaNO2, the electrochemical method demonstrated herein could be simple, inexpensive and ‘‘green” method suitable for practical applicability. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] J.E. Stuff, E.T. Goh, S.L. Barrera, M.L. Bondy, M.R. Forman, J. Food Compos. Anal. 22S (2009) S42–S47. [2] Z.F. Wang, F. Liao, T.T. Guo, S.W. Yang, C.M. Zeng, J. Electroanal. Chem. 664 (2012) 135–138. [3] D. Zhang, Y.X. Fang, Z.Y. Miao, M. Ma, X. Du, S. Takahashi, J.-I. Anzai, Q. Chen, Electrochim. Acta 107 (2013) 656–663. [4] R. Yue, Q. Lu, Y.K. Zhou, Biosens. Bioelectron. 26 (2011) 4436–4441. [5] S.-S. Li, Y.-Y. Hu, A.-J. Wang, X.X. Weng, J.-R. Chen, J.-J. Feng, Sens. Actuat., B 208 (2015) 468–474. [6] X.-R. Li, F.-Y. Kong, J. Liu, T.-M. Liang, J.-J. Xu, H.-Y. Chen, Adv. Funct. Mater. 22 (2012) 1981–1988. [7] S.S.M. Silva, M.L. Henrique, Electroanalysis 10 (1998) 1200–1203. [8] N. Pourreza, M.R. Fat’hi, A. Hatami, Microchem. J. 104 (2012) 22–25.
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Electrochemical synthesis of gold nanoparticles decorated flower-like graphene for high sensitivity detection of nitrite Cui’e Zou a, Beibei Yang a, Duan Bin a, Jin Wang a, Shumin Li a, Ping Yang a, Caiqin Wang a, Yukihide Shiraishi b, Yukou Du a,⇑ a b
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China Tokyo University of Science Yamaguchi, SanyoOnoda-shi, Yamaguchi 756-0884, Japan
g r a p h i c a l a b s t r a c t The spherical Au nanoparticles/3D flower-like graphene was prepared by a facile and low-cost electrochemical method and used for sensitive determination of nitrite.
a r t i c l e
i n f o
Article history: Received 20 September 2016 Revised 24 October 2016 Accepted 29 October 2016 Available online 31 October 2016 Keywords: Au nanoparticles Graphene Electrodeposition NaNO2
a b s t r a c t In this paper, the spherical Au nanoparticles and 3D flower-like structure graphene were successively deposited on glassy carbon electrode (GCE) (Au/f-GE/GCE) via a facile and two-step electrodeposition method for the detection of nitrite ions (NaNO2). The morphology and composition elements were confirmed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction measurements (XRD). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the electrochemical behaviors of NaNO2 on the as-prepared electrode. Compared to f-GE/GCE and Au/GCE, Au/f-GE/GCE showed a sharp and obvious oxidation peak at 0.78 V. The oxidation peak current of NaNO2 was linearly proportional to its concentration in the range from 0.125 to 20375.98 lM, with a detection limit of 0.01 lM (at S/N = 3). Furthermore, the experiment results also showed that the as-prepared electrode exhibited excellent reproducibility and long-term stability, as well as good recovery when applied to the determination of NaNO2 in pickled pork samples. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.jcis.2016.10.088 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nitrite ion (NO 2 ), an inorganic compound, has been widely exploited in our daily life as an additive in foods and a corrosion inhibitor, and recognized as an alarming pollutant to the environment and human health [1,2]. An excessive level of nitrite
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in the body of human, not only can lead to the irreversible oxidation of hemoglobin to methemoglobin, but also can react with dietary components to form a nitrosamine, resulting in cancer and hypertension [3–5]. The World Health Organization has reported that the fatal dose of nitrite ingestion is between 8.7 lM and 28.3 lM [6,7]. As a consequence, it is necessary to develop reliable methods for the detection and monitoring of nitrite. Up to now, many techniques have been developed to determine NO 2 levels, such as spectrophotometry [8,9], chemiluminescence [10,11], high-performance liquid chromatography [12] and electrochemical methods [13–16]. Among them, the electrochemical methods are proven to be powerful tools due to its fast procedure, low cost, low detection limit and high accuracy. Unfortunately, the electrochemical oxidation of nitrite at the traditional electrodes suffers large overpotentials and the electrodes tend to be poisoned by the products generated during the electrochemical process. To resolve these problems, one strategy is to use various noble metal nanostructures in the construction of the working electrode to enhance the sensitivity for the detection of nitrite due to their excellent catalysis, unique dimensions and high effective surface area [17–19]. In particular, Au nanoparticles have been extensively explored for the determination of nitrite because of its excellent conductivity, unique structure, well electrocatalytic ability and biocompatibility. For instance, Jiang et al. used coupled graphene and Au nanoparticles to fabricate the electrochemical biosensor for detecting nitrite [20]. Li and his co-workers prepared the sensor of gold nanoparticles (AuNPs) and sulfonated graphene, which displayed electrocatalytic activity in the detection of nitrite [21]. It is worth noting that the Au based electrodes could be modified with some other supporting materials, such as transition metal oxides, conducting polymer and graphene, to achieve great selectivity and sensitivity for the detection of the nitrite [20–25]. On another front, as a potentially excellent electrode supporting material in electrochemical application such as supercapacitors, electrocatalysis and sensors, graphene (GE) has been attracted particular attentions owing to its peculiar properties including large specific surface area, unique electronic properties, excellent physicochemical properties, high chemical and thermal stability [26–28]. However, such surface expansion is still inherently limited by the two-dimensional (2D) nature of the planar electrodes. Some attempts have been made to construct three-dimensional (3D) electrodes. Dong et al. synthesized 3D graphene/Cobalt oxide electrode for high-performance supercapacitor and non-enzymatic glucose detection by the chemical vapor deposition [29]. Yue et al. have developed a supercapacitor with the 3D flower-like graphene by the electrochemical method [30]. Therefore, fabricating a novel sensor modified with Au nanoparticles (AuNPs) and 3D flower-like graphene for detecting nitrite should be elaborately considered and designed. Herein, a facile and efficient electrochemical approach was been used to prepare 3D flower-like graphene (f-GE). And then the AuNPs were reduced on the graphene surface to form Au/f-GE by potentiostatic deposition. With large specific surface area, highly conductive pathways, and well-defined flowers structure, this new 3D Au/f-GE electrode architecture holds a great promise for electrochemical sensing nitrite. This work might exploit the opportunities for developing novel 3D electrochemical sensors with excellent sensitivity and reproducibility.
2. Experimental 2.1. Materials and reagents Graphite powder, disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), cupric sulphate anhy-
drous (CuSO4), chloroauric acid (HAuCl4), glucose, potassium chloride (KCl) and disodium chloride (NaCl) were purchased from Sinopharm Chemicals Reagent Co., Ltd. Ascorbic acid (AA), uric acid (UA) and dopamine (DA) were obtained from Acros Organics. All chemicals were of analytical reagent grade and used as received. 0.1 M phosphate buffer solution (PBS) with pH 7.0 prepared from NaH2PO4 and Na2HPO4 was chosen for the electrolyte solution in the present experiments. Double distilled water was used throughout the experimental process.
2.2. Apparatus Scanning electron microscopy (SEM) (S-4700, Hitachi High Technologies Corporation, Japan) was used to characterize the morphologies and the energy-dispersive X-ray analyzer (EDX) of the obtained materials. X-ray diffraction (XRD) measurements were carried out to analyze the sample structure using an X’Pert PRO multiple crystals (powder) X-ray diffractometer (PANalytical Company, Holland). All the electrochemical experiments were performed in a conventional three-electrode system at room temperature using a CHI 760E potentiostat/galvanostat (CH Instrumental Co. Ltd, China). The glassy carbon electrodes (GCEs, 3 mm diameter) modified by the as-prepared nanocomposites were used as working electrodes, a Pt wire and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. All of the measurements were carried out at room temperature.
2.3. Preparation of 3D Au/f-GE/GCE Prior to use, the bare GCEs were firstly polished with alumina powder (0.3 and 0.05 lm) to obtain mirror-like surfaces, and then sonicated with absolute ethanol and double water for about 5 min, respectively. Subsequently, the GCEs were rinsed thoroughly with double water and dried for the following experiment use. Graphene oxide (GO) solution was prepared by the modified Hummers’ method [31]. The fabrication of f-GE/GCE was as follows: Firstly, 10 lL GO ink (0.5 mg mL1) was sprayed onto the GCE followed by electrochemical reduction to reduced graphene oxide (GE) in a Na-PBS (0.1 M, pH = 4.1) solution at a constant potential of 0.9 V. Secondly, copper nanoparticles were deposited thereon in 5.0 mM CuSO4 solution at a constant potential of 0.4 V with a charge of 1.0 102 C to form the Cu/GE. And then, another 10 lL GO ink was added on the above obtained surface that was the same as the above strategy to fabricate a sandwich construction of GE/Cu/GE. Finally, 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 1000 s and simultaneously flower-like graphene formed from the surface of the underlying GE layer on the electrode. Here, the Cu nanoparticles were used as the template, when a potential higher than the oxidation potential of Cu, an oxidation reaction (Cu ? Cu2+ + 2e) occurred on the copper particles. As the reaction progressed, the GE sheets which were coated on the copper particles would transform and turned to stand up from the surface of the substrate. The rose GE sheets interconnected with each other, and as a result, the 3D flower-like GE structures were formed. The Au/f-GE/GCE was synthesized by the obtained immersed into a 3.0 mM HAuCl4/0.5 M H3PO4 solution at a constant potential of 0.2 V with a charge of 5.0 103 C. The amount of Au was calculated to be about 3.4 lg according to the integrated charge of Au electro-deposition (assuming a 100% current efficiency). The overall synthetic procedure for f-GE was illustrated in Scheme 1. For comparison, f-GE/GCE and Au/GCE were also fabricated by the similar method.
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Scheme 1. Schematic illustration for fabricating 3D Au/f-GE/GCE.
Fig. 1. SEM (A, B and C) images of Au/GCE and f-GE/GCE. Inset: histogram of the size of Au nanoparticles.
3. Results and discussion 3.1. Characterizations of Au/f-GE/GCE Firstly, Au/GCE was obtained easily using a potentiostatic electrodeposition method by immersing GCE into a 3.0 mM HAuCl4/0.5 M H3PO4 solution at a constant potential of 0.2 V with a charge of 5.0 103 C. As Fig. 1A depicted, Au nanoparticles were mainly consisted of spherical nanostructures having uniform size, which were well dispersed on the GCE surface without obvious agglomerations. From the histogram shown insert Fig. 1A, we can calculate that the average size of Au nanoparticle is approximately 90.94 nm. The spherical nanostructures of AuNPs may possess the larger surface area and the more active sites, which leads to the enhancement of catalytic performances toward the oxidation of NaNO2. The flower-like graphene was facilely synthesized using an electrochemical approach. As the SEM images of f-GE/GCE in Fig. 1B and C described, GE with 3D blooming flower-like structure was dispersed on the substrate with few regions of exposed GE among the flowers and the diameter of each flower was about 5–10 lm. The formation of f-GE in our experiment was related to the removal process of the copper particles. When a potential of 0.1 V was added, an oxidation reaction (Cu ? Cu2+ + 2e) occurred. With the amount of Cu2+ continuously transporting across the exterior GE layer through the interspace between the two GE nanosheets, the rose GE sheets interconnected with each other and the blooming flower-like structure was then formed. Finally, the Au/f-GE/GCE was obtained by potentiostatic electrodeposition of Au nanoparticles on f-GE/GCE. As Fig. 2A and B depicted, each of the as-synthesized AuNPs were uniformly dispersed on the surface of f-GE. The flower-like structure of GE supporting with Au nanoparticles provided a largest specific surface area for the significant improvement in electrochemical properties [32]. Energy-dispersive X-ray spectrum for the synthesized
Au/f-GE (EDX) data shown in Fig. 2C revealed the presence of C, O and Au elements which further proved the formation of Au/fGE/GCE. The crystal structures of the as-prepared Au/ITO, f-GE/ITO and Au/f-GE/ITO were further analyzed by X-ray diffraction (XRD). As displayed in Fig. 3b, a broad diffraction peak at 23° corresponding to the C (0 0 2) diffraction of GE nanosheets indicated that GO was completely reduced by potentiostatic reduction [26,30]. The peaks at 38.2°, 44.4°, 64.5° and 78.1° were attributed to the reflections from the planes of Au (1 1 1), Au (2 0 0), Au (2 2 0) and Au (311) (JCPDS 4-0784), respectively, as shown in Fig. 3c [5,33]. The diffraction peaks of ITO was shown in Fig. 3d. After electrodeposition of AuNPs on the surface of f-GE, the Au/f-GE/ITO (Fig. 3a) possessed all the peaks observed in the XRD patterns of Au/ITO and f-GE/ ITO exhibiting the successful hybridization of the spherical AuNPs and flower-like graphene. 3.2. Electrochemical behavior on the modified electrodes Fig. 4A showed the cyclic voltammograms (CVs) of different electrodes (Au/f-GE/GCE, f-GE/GCE, Au/GCE) in 5.0 mM K3Fe(CN)6 containing 1 M KCl solution at a scan rate of 50 mV s1. As displayed in Fig. 4A, the Au/f-GE/GCE exhibited an increase in the anodic peak current (65.74 lA) compared to f-GE/GCE (35.67 lA) and Au/GCE (55.37 lA). In addition, the effective surface area of the modified electrodes was estimated by cyclic voltammetry according to the Randles–Sevcik equation Eq. (1) [30]
Ip ¼ 2:69 105 n3=2 Aeff D1=2 t1=2 C
ð1Þ
where Ip is the anodic peak current, n is the number of electrons transferred, Aeff is ECSA of the electrode, D refers to the diffusion coefficient of the molecule (equal to (6.70 ± 0.02) 106 cm2 s1), C corresponds to the bulk concentration of the probe (mol cm3),
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Fig. 2. SEM (A and B) images and EDX analysis (C) of Au/f-GE/GCE.
calculated to be 0.084, 0.046 and 0.071 cm2, respectively, demonstrating that combination of AuNPs and f-GE significantly improves the electrochemical active surface area of the Au/f-GE/GCE and facilitates the electron transfer process. The electrochemical behavior of the three modified electrodes in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 was examined via CVs with a potential range from 0.2 to +1.0 V at a scan rate of 50 mV s1 shown in Fig. 4B. Compared to those oxidation currents obtained at the f-GE/GCE (111.8 lA) and Au/GCE (123.8 lA), the Au/f-GE/GCE modified electrode exhibites a remarkably larger peak current (161.5 lA). Moreover, it is noted that at Au/f-GE/GCE, a prominent peak appeared at 0.78 V. It indicates that the heterogeneous electron transfer rate at the electrode is fast, and thus greatly facilitate the oxidation of nitrite. 3.3. Electrochemical parameters of NaNO2 at the Au/f-GE/GCE
Fig. 3. XRD patterns of Au/f-GE/ITO (a), f-GE/ITO (b), Au/ITO (c) and ITO (d).
and ʋ is the scan rate (V s1). From the slope of the Ip ʋ1/2 relation, the ECSAs of the Au/f-GE/GCE, f-GE/GCE and Au/GCE were
As we know, investigating the effect of scan rate on the oxidation peak potential and peak current can evaluate the kinetics of electrode reaction. Fig. 5A represented the CVs of Au/f-GE/GCE in 0.1 M PBS (pH = 7.0) solution with 5 mM NaNO2 at different scan rates (20–200 mV s1). It can be found that the anodic peak currents increase linearly with the scan rate and the calibration
Fig. 4. CVs of Au/f-GE/GCE, f-GE/GCE and Au/GCE electrodes recorded in 5 mM K3Fe(CN)6 + 1 M KCl solution (A), 0.1 M PBS (pH = 7.0) containing 5 mM NaNO2 (B). Scan rate: 50 mV s1.
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Fig. 5. CVs of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 at scan rates from 20 to 200 mV s1 (A), the plots of anodic peak currents to the scan rates (B), anodic peak potentials versus lg ʋ (C).
equation is Ipa (lA) = 0.6105ʋ (mV s1) + 113.66 (R2 = 0.988) (Fig. 5B). This property indicates that the electron transfer for NaNO2 at Au/f-GE/GCE is controlled by an adsorption process [31,34]. In addition, it can be seen that the redox peak potentials shift slightly along with the increase of the scan rate and the linear regression equation of Epa versus the logarithm of the scan rates shown in Fig. 5C are expressed as Epa = 0.04897 lg ʋ + 0.6937 (R2 = 0.989). The electron transfer number (n) are calculated according to the following Laviron’s equation [34]:
Epa
RT ¼ E þ 2:30 lg v ð1 aÞnF 00
ð2Þ
where a is the electron transfer coefficient, n is the number of elec0 tron transferred, E0 is the formal potential, ʋ is the scan rate. R, T and F have their conventional meanings. The n values is calculated to be 2.4, when a is taken 0.5. It means that about two electrons are involved in the irreversible reaction of NaNO2, which is in agreement with Eq. (3).
NO2 þ H2 O ! NO3 þ 2Hþ þ 2e
ð3Þ
The solution pH is an important factor which influences the electrochemical reaction. Fig. 6A described the effect of pH on the CVs response to 5 mM NaNO2 investigating in 0.1 M PBS solution
with the pH range of 4.0–8.0 at a scan rate of 50 mV s1. It can be clearly observed in Fig. 6B that the largest anodic peak current is obtained at the pH of 7.0. As Brylev et al. reported at low pH value, the generating of N2O may cause the peak current to decrease [35]. Furthermore, the oxidation peaks of NaNO2 shift very negatively along with the increasing pH. Therefore, the pH 7.0 was selected as the optimum pH value in our experiments.
3.4. Determination of NaNO2 on the Au/f-GE/GCE Differential pulse voltammetry (DPV) commonly has the higher sensitivity than cyclic voltammograms, accordingly, the DPV technology was carried out for the quantitative detection of NaNO2 on the as-prepared Au/f-GE/GCE. As shown in Fig. 7A, under the optimal conditions, the oxidation peak current of NaNO2 increased with its concentration in the range from 0.125 to 20375.98 lM with two linear ranges. As Fig. 7B described, the two linear regression equations were expressed as Ipa (lA) = 0.03573 C (lM) + 99.32, R2 = 0.994 (C = 0.125–269.71 lM) and Ipa (lA) = 0.0114 C (lM) + 221.29, R2 = 0.990(C = 269.71–20375.98 lM). The detection limit (LOD) is found to be 0.01 lM (S/N = 3), which is lower than those modified electrodes listed in Table 1. The obtained electrode showed wider linear range and even lower
Fig. 6. CVs of the Au/f-GE/GCE electrode in 0.1 M PBS solution containing 5 mM NaNO2 with the pH ranging from 4.0 to 8.0, scan rate: 50 mV s1 (A). The anodic peak current against pH (B).
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Fig. 7. DPV curves of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing different concentrations of NaNO2 from 0.125 to 20375.98 lM (A). The relation between oxidation current versus the concentration of NaNO2 (B).
Table 1 Comparison of different modified electrodes for nitrite determination.
Table 2 Determination of nitrite at various concentrations in pickled pork.
Modified electrode
Linear range (lM)
LOD (lM)
Reference
AgNP/GCE Cu-NDs/RGO/GCE K-modified graphene/GCE CR-GO/GCE Pd/SWCNT/GCE Pd/RGO/GCE AuNPs/CP/GCE AuNPs/SG/GCE Co(II) MTpAP/cit-AuNPs/GCE Au/f-GE/GCE
10–1000 1.25–13000 0.5–7800 8.9–167 2–1230 1–1000 1–100 10–3960 0.5–4700 0.125–20375.98
1.2 0.4 0.2 1 0.25 0.23 0.093 0.2 0.5 0.01
[2] [3] [6] [15] [16] [17] [18] [21] [24] This work
SWCNT: single-walled carbon nanotube; CP: carbon paper; SG: sulfonated graphene; Co(II) MTpAP: meso-tetra(para-aminophenyl)porphyrinatocobalt(II); AuNPs: gold nanoparticles.
detection limit, indicating that the obtained electrode has an excellent electrochemical activity for the oxidation of NaNO2.
Samples
Content (mg kg1)
Added (mg kg1)
Found (mg kg1)
Recovery (%)
R.S.D. (%) (n = 5)
1 2 3
15.6 15.6 15.6
10 20 30
25.8 34.3 47.2
100.8 96.3 103.5
3.2 2.5 4.7
addition of various species into 0.1 M PBS solution (pH 7.0) containing 5 mM NaNO2. As Fig. 8B showed, most of the species, such as NaCl and KCl in a 100-fold concentration, glucose, AA, UA and DA in a 10-fold concentration had a little interference (lower than 5%) toward the determination of NaNO2. All above results suggest that the Au/f-GE/GCE has a good reproducibility and stability and well anti-interference ability for the determination of NaNO2.
3.6. Determination of NaNO2 in real samples 3.5. Stability, reproducibility, interference studies of the Au/f-GE/GCE The reproducibility of our fabricated electrode was investigated by CVs with more than five different electrodes in the same 0.1 M PBS solution (pH 7.0) containing 5 mM NaNO2 solution. It was found that the relative standard deviation (RSD) of the oxidation current was about 2.7%. The stability of the modified electrode was also studied in this work by CVs. From Fig. 8A, we can observe that the oxidation current response of the electrode still remained up to 96.03% after 100 successive assays. Possible interference for the detection of NaNO2 on Au/f-GE/GCE was investigated by
To illustrate the feasibility and application potential of the electrode, the Au/f-GE/GCE was applied to determine NaNO2 from pickled pork using the standard addition technique. The collected pickled pork was disposed (Firstly 12.5 mL borax saturated solution was added under a boiling water bath for 15 min, then 2.5 mL of 30% ZnSO4 solution was used to precipitate protein. After cooled down, the resulting mixture was diluted to 50 mL) and the spiked NaNO2 concentrations were 10, 20 and 30 mg kg1, respectively. As shown in Table 2, the recoveries are 100.8%, 96.3% and 103.5%, correspondingly. The good recovery is obtained suggesting the practical applicability of the proposed method.
Fig. 8. The 1st and 100th CVs of the Au/f-GE/GCE electrode in 0.1 M PBS (pH = 7.0) solution containing 5 mM NaNO2 at a scan rate of 50 mV s1 (A). Results of the interference study on the response of 100-fold KCl and NaCl, 10-fold glucose, AA, UA and DA (B).
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4. Conclusions In summary, the spherical Au nanoparticles/3D flower-like structure graphene was prepared by a facile and low-cost electrochemical method. The flower-like and spherical hybrid material with large surface area represented an excellent electrochemical performance in the oxidation of NaNO2. Moreover, the prepared electrode also displayed excellent reproducibility, nice stability and remarkable anti-interference ability. What is more, when it was applied to determine NaNO2 in pickled pork, the novel Au/fGE also obtained a good recovery. Compared with other methods for the determination of NaNO2, the electrochemical method demonstrated herein could be simple, inexpensive and ‘‘green” method suitable for practical applicability. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] J.E. Stuff, E.T. Goh, S.L. Barrera, M.L. Bondy, M.R. Forman, J. Food Compos. Anal. 22S (2009) S42–S47. [2] Z.F. Wang, F. Liao, T.T. Guo, S.W. Yang, C.M. Zeng, J. Electroanal. Chem. 664 (2012) 135–138. [3] D. Zhang, Y.X. Fang, Z.Y. Miao, M. Ma, X. Du, S. Takahashi, J.-I. Anzai, Q. Chen, Electrochim. Acta 107 (2013) 656–663. [4] R. Yue, Q. Lu, Y.K. Zhou, Biosens. Bioelectron. 26 (2011) 4436–4441. [5] S.-S. Li, Y.-Y. Hu, A.-J. Wang, X.X. Weng, J.-R. Chen, J.-J. Feng, Sens. Actuat., B 208 (2015) 468–474. [6] X.-R. Li, F.-Y. Kong, J. Liu, T.-M. Liang, J.-J. Xu, H.-Y. Chen, Adv. Funct. Mater. 22 (2012) 1981–1988. [7] S.S.M. Silva, M.L. Henrique, Electroanalysis 10 (1998) 1200–1203. [8] N. Pourreza, M.R. Fat’hi, A. Hatami, Microchem. J. 104 (2012) 22–25.
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