Analytica Chimica Acta 852 (2014) 45–54

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Ultrasensitive and simultaneous detection of heavy metal ions based on three-dimensional graphene-carbon nanotubes hybrid electrode materials Hui Huang a , Ting Chen a , Xiuyu Liu b , Houyi Ma a, * a Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China b Shandong Academy of Sciences, Jinan 250114, China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T


Three-dimensional grapheneMWCNTs nanocomposites were prepared.
Graphene-MWCNTs based electrochemical sensor was used to detect heavy metal ions for the first time.
The proposed sensor was certified capable for real sample with satisfactory results.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 June 2014 Received in revised form 2 September 2014 Accepted 9 September 2014 Available online 16 September 2014

A green and facile method was developed to prepare a novel hybrid nanocomposite that consisted of one-dimensional multi-walled carbon nanotubes (MWCNTs) and two-dimensional graphene oxide (GO) sheets. The as-prepared three-dimensional GO–MWCNTs hybrid nanocomposites exhibit excellent water-solubility owing to the high hydrophilicity of GO components; meanwhile, a certain amount of MWCNTs loaded on the surface of GO sheets through p–p interaction seem to be “dissolved” in water. Moreover, the graphene(G)-MWCNTs nanocomposites with excellent conductivity were obtained conveniently by the direct electrochemical reduction of GO–MWCNTs nanocomposites. Seeing that there is a good synergistic effect between MWCNTs and graphene components in enhancing preconcentration efficiency of metal ions and accelerating electron transfer rate at G-MWCNTs/electrolyte interface, the G-MWCNTs nanocomposites possess fast, simultaneous and sensitive detection performance for trace amounts of heavy metal ions. The electrochemical results demonstrate that the G-MWCNTs nanocomposites can act as a kind of practical sensing material to simultaneously determine Pb2+ and Cd2+ ions in terms of anodic stripping voltammetry (ASV). The linear calibration plots for Pb2+ and Cd2+ ranged from 0.5 mg L1 to 30 mg L1. The detection limits were determined to be 0.2 mg L1 (S/N = 3) for Pb2+ and 0.1 mg L1 (S/N = 3) for Cd2+ in the case of a deposition time of 180 s. It is worth mentioning that the G-MWCNTs modified electrodes were successfully applied to the simultaneous detection of Cd2+ and Pb2+ ions in real electroplating effluent samples containing lots of surface active impurities, showing a good application prospect in the determination of trace amounts of heavy metals. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Graphene-carbon nanotubes hybrid Heavy metal ions Differential pulse anodic stripping voltammetry Detection

1. Introduction * Corresponding author. Tel.: +86 531 88364959; fax: +86 531 88564464. E-mail addresses: [email protected], [email protected] (H. Ma). http://dx.doi.org/10.1016/j.aca.2014.09.010 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

The environmental pollution caused by heavy metal ions has already become serious problems that influence the survival

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environment of human beings and other creatures. Therefore, it is essential to construct rapid, sensitive, simple, low-cost and portable monitoring devices, with the aim of achieving the on-site, real-time and on-line determination of trace amounts of heavy metals [1–3]. Anodic stripping voltammetry (ASV), as a simple and sensitive electroanalytical method, has been proved to be very effective for the analysis of trace concentrations of heavy metal ions in aqueous solutions [4–6]. Besides the advantages of high detection sensitivity, small sample consumption, simple operation, low cost and practical convenience, a distinct advantage of ASV over atomic absorption spectroscopy (AAS) method is its ability to detect several metal ions simultaneously. Seeing that the working electrode plays a critical role in carrying out the stripping analyses, an ideal working electrode should have multiple functions, including effective preconcentration, selective reaction surface, good repeatability and low background current over a wide potential range [7–9]. However, the commonly used hanging mercury drop electrodes (HMDE) [10] or thin mercury film electrodes (TMFE) [11] pose great risks to the environment and human health, much attention has been focused on how to construct high performance sensing electrodes made from environmental friendly “green materials”. In recent years, carbon-based nanostructural materials, especially graphene [12–15] and carbon nanotubes (CNTs) [16,17], have received increased interest in the electrochemical detection of heavy metals, due to their environmental safety, fast electron transfer rate, unique structural characteristics and physico-chemical properties. For example, Hwang et al. reported a bismuth-modified screen-printed CNTs electrode for the simultaneous detection of Zn2+, Cd2+, Pb2+ ions, with the linear range of 2–100 mg L1 [18]. Yu et al. developed a glucose and pyrene residues modified GO sensor with the detection limit of 0.1 nM, in which the glucose residue works as the adsorption sites for heavy metal ions and the pyrene residue non-covalently binds to the graphene surface by p bond [19]. In particular, Wang et al. reported that amination-based GO was applied to the determination of lead ions, with the detection limit down to 0.1 pM; however, toxic Hg was used in the enrichment step of heavy metal ions [20]. Although CNTs and the reduced graphene oxides (rGOs) have respective advantages, together they work better than either one does alone when they are used to design carbon-based sensing electrodes for detecting heavy metal ions. Thus, the combination of graphene and MWCNTs enables the resulting hybrid nanocomposites to act as an ideal electrode material for the fabrication of environmentally friendly sensing electrodes [21–23]. It is generally believed that graphene, a one-atom-thick layer of graphite, possesses excellent conductivity, high electron transfer rate and very large surface area [24,25]. However, these properties are strongly dependent on the preparation process of graphene sheets. In fact, for the rGO obtained by the reduction of GO sheets that are synthesized through the well-known Hummers method [26], its electrical conductivity and specific surface area are usually much lower than the theoretical expected values owing to the excessive defects originated from chemical oxidation process and the clustering caused by intensive p–p interaction. Instead, the main superiority of the as-prepared rGO is its strong hydrophilicity because some surface functional groups may be preserved after the reduction of GO, which enable rGO to strongly adsorb metal ions [27]. In contrast to the rGO, CNTs can be produced in large batch by floating catalyst chemical vapor deposition (FC-CVD), carbon arc methods, or laser evaporation [28,29]. The as-prepared CNTs own excellent electrical conductivity because the FC-CVD method avoids the excessive oxidation of carbon surfaces. However, CNTs primary deficiency is that there are only a few functional groups on its surface, such as  COOH,  CQO and  OH, making this kind of

carbon material highly hydrophobic. Consequently, unmodified CNTs are unable to chelate metal ions in aqueous solutions and can not work as good electrode materials for the ASV analyses [30]. As mentioned above, seeing that rGO or CNT has its own advantages and disadvantages, we tend to use two-dimensional (2D) GO nanosheets and one-dimensional (1D) MWCNTs to fabricate a type of G-MWCNTs hybrid nanocomposites with excellent conductivity and good water-solubility. In this way, the effective combination of two carbon nanostructures can not only make full use of respective advantages but also make up respective deficiencies. In this paper, we developed a simple and green method to fabricate three-dimensional (3D) G-MWCNTs nanocomposites. Thus, hydrophilic GO components may load CNTs through p–p interaction, making a certain amount of CNTs be “dissolved” in water, and the presence of CNTs components not only greatly enhances the conductivity of the nanocomposites but also suppresses to some degree the aggregation between GO nanosheets. Moreover, the hydrophilic hybrid nanocomposites are able to adsorb heavy metal ions from aqueous solution due to the rich chelating groups. These advantages make the G-MWCNTs nanocomposites be an ideal sensing nanomaterial suitable for the electrochemical detection or preconcentration of trace amounts of heavy metal ions, as illustrated by Scheme 1.

2. Experimental 2.1. Reagents Natural graphite powder (325 mesh) was purchased from Alfa Aesar. MWCNTs (purity: >95 wt.%; O.D.: 20–30 nm; length: 0.5–2 mm) produced by the CVD method were obtained from Chengdu Organic Chemicals Co. Ltd. (Chengdu, China). Nafion (5 wt.% in lower aliphatic alcohols) was obtained from Sigma–Aldrich and diluted to 1 wt.% by ethanol before use. Working solutions containing Zn2+, Cd2+, Pb2+, Cu2+ and Bi3+ ions were prepared by diluting high concentration standard stock solutions that were purchased from Aladdin (Shanghai, China). A 0.1 M HAc-NaAc buffer solution (pH 4.5) was prepared by mixing appropriate amounts of CH3COOH and CH3COONa and served as a supporting electrolyte during the analysis of metal ions. Ultrapure water (18 MV cm) was used throughout the experiments. All other chemicals used were of analytical reagent grade or better, and were used without further purification.

2.2. Apparatus Differential pulse anodic stripping voltammetry (DPASV) and cyclic voltammetry (CV) were performed in a three-electrode cell made of quartz beaker with a CHI750D electrochemical workstation (Chenhua Instruments, China). All potentials presented in this paper were measured with respect to the SCE. The solutions were deoxygenated by bubbling with high-purity nitrogen for at least 10 min prior to each experiment. Tapping-mode atomic force microscopy (AFM, NanoScope IIIA, Digital Instruments) was performed to examine the surface morphology on the Si/SiO2 substrate. Raman spectrum was obtained with an Ocean Optics QE65000 Raman system (laser wavelength 785 nm and laser focus size about 158 mm); all the spectra were baseline-corrected and calibrated with respect to the Raman mode of Si at 520.7 cm1. UV–vis adsorption spectroscopy and transmission electron microscopy (TEM) measurements were obtained using a Hitachi U-4100 spectrophotometer and a JEOL JEM-1011 TEM with an acceleration voltage of 100 kV, respectively.

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Scheme 1. Schematic diagram for the synthesis of GO–MWCNTs hybrid nanomaterials and their application as novel electrode materials which are applicable to the electrochemical detection of heavy metal ions by means of ASV method. Inset shows the digital photos for the GO, MWCNTs and GO–MWCNTs solutions, respectively.

2.3. Preparation of GO–MWCNTs nanocomposites Prior to use, MWCNTs were treated with mixed acids (volume ratio of 1:3 of concentrated HNO3 to concentrated H2SO4) for about 4 h followed by filtering, rinsing with water and drying in proper order. This process is known as the segmentation and carboxylation of MWCNTs [31]. 1.0 mg mL1 MWCNTs dispersion was prepared by ultrasonication for 1 h. Even so, the obtained dispersion was not stable since most MWCNTs were hydrophobic. GO nanosheets were obtained by 1 h ultrasonic exfoliation of graphite oxides, which were produced by the oxidative treatment of natural graphite based on the modified Hummer’s method [32,33]. The as-prepared homogeneous GO hydrosol (1.0 mg mL1) was brown in color. Afterwards, 10 mL of MWCNTs dispersion and 10 mL of GO hydrosol were mixed together and the resulting mixture was further treated with ultrasonication for 2 h. Excessive MWCNTs were removed by centrifugation at 5000 rpm and the unreacted GO was separated at 12,500 rpm. The obtained sediment was dried at 50  C in vacuum drying oven, forming the GO–MWCNTs hybrid composites that can be stably dispersed in aqueous phase for several months. 2.4. Fabrication of graphene-MWCNTs-Nafion composite film Ultrasonic agitation (1 h) was used to disperse the GO–MWCNTs nanocomposites into water to produce 0.5 mg mL1 GO–MWCNTs colloid. Before use, a bare glassy carbon electrode (GCE) was polished carefully with 1.0 mm, 0.3 mm and 0.05 mm alumina slurry, respectively, and washed ultrasonically with water, ethanol and ultrapure water in sequence. Then, the GCE was scanned between 0 and 2.0 V in 0.5 M H2SO4 at 100 mV s1 for

50 cycles to remove possible contaminations. Subsequently, an aliquot of 5 mL of the colloid was cast on the GCE surface, and then the solvent was evaporated under an infrared lamp. Finally, 5 mL of Nafion solution was deposited on the electrode to improve film stability and anions-resistant permselectivity. In this way, the GO/ MWCNTs/Nafion/GCE was thus obtained. For comparison, GO/GCE, MWCNTs/GCE electrodes were also prepared using the same procedures. 2.5. Procedure for electrochemical testing The electrochemical tests were performed in a quartz beaker containing a three-electrode setup at room temperature. Herein, the modified GCE (the diameter of 5 mm) worked as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum plate as counter electrode, with 60 mL of 0.1 M HAc–NaAc buffer solution (pH 4.5) as the electrolyte. For differential pulse anodic stripping voltammetry (DPASV) test, a modified electrode (GO/MWCNTs/Nafion/GCE) was immersed into the electrolyte containing Bi3+ (500 mg L1) and other metal ions under strong stirring to allow trace amounts of heavy metal ions to be preconcentrated at the modified GCE electrode. The deposition potential was chosen as 1.4 V and the preaccumulation time was 180 s. After the preconcentration, the potential was held at 1.4 V for 10 s under the static condition, followed by a positive-going DPV scan (with a step increment of 5 mV, amplitude of 50 mV, and pulse period of 0.2 s) from 1.4 V to 0.4 V. Prior to the next analysis, a preconditioning step (60 s at 0.4 V in stirred solution) was carried out in order to remove residual metal ions on the surface of working electrode. For the analysis of practical samples, 54 mL of electroplating effluent and

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6 mL of 1.0 M acetate buffer (pH 4.5) were fully mixed, and the resulting mixed solution served as the electrolytic solution. The electroplating effluent was filtered with 0.22 mm membrane before carrying out an electrochemical test. 3. Results and discussion 3.1. Characterizations of morphology and electronic structure of GO–MWCNTs nanocomposites GO can be dissolved in water, forming a brownish-yellow hydrosol with high stability, as the picture of the leftmost bottle in Scheme 1 shows. Due to the excellent hydrophilicity associated with rich carboxyl and hydroxyl groups, the GO hydrosol remained stable even after centrifuging at 12,000 rpm for 30 min. Instead, the pristine MWCNTs tended to form precipitates because of the high hydrophobicity and the strong Van der Waals force between MWCNTs, as evidenced by the second bottle on the left shown in Scheme 1. However, it is noteworthy that, after being fully mixed and further treated according to our proposed method, the as-obtained GO–MWCNTs hybrid nanocomposites displayed good hydrophilicity and could be stably stored in an aqueous solution for several months at room temperature, without resulting in any precipitation (see the rightmost bottle in Scheme 1). Moreover, the G-MWCNTs hybrid nanocomposites prepared via the direct electrochemical reduction of the GO–MWCNTs materials in aqueous solutions combine the conductivity of CNTs with the partial hydrophilicity of GO, and own the strong ability to catch metal ions through coordination reactions. Therefore, G-MWCNTs hybrid nanomaterials are expected to be promising sensing electrode materials for the ASV analyses, including DPASV, as shown in Scheme 1. The morphology and microstructure of as-prepared GO–MWCNTs nanocomposites were investigated using TEM and AFM. Fig. 1a shows that MWCNTs are dispersed well on the GO nanosheets, presenting in the form of individual fibers, with the length of 0.5–2 mm. Meanwhile, the GO sheets display large and unfolded planes. Even though the brownish black dispersion was centrifuged at 8000 rpm for 30 min, no obvious precipitate was observed. Besides, a very strong sonication did not cause the dispersion to separate into upper and lower phases, thereby demonstrating the good water-solubility and the strong stability of the present GO–MWCNTs nanocomposites. The AFM image shown in Fig. 1b shows that the GO film is nearly composed of a single layer of GO sheet, with the average thickness of 2 nm and the planar size of ~1 mm. It is believed that the combination of 2D GO sheets and 1D MWCNTs creates an interconnected framework, in which the aggregation tendency between the MWCNTs is effectively inhibited [34]. The presence of MWCNTs can greatly enhance the conductivity of the hybrid nanocomposites and also make the GO plane unfold, whereas, the GO components can give the hybrid an important property to capture metal ions in aqueous phase [35]. The electronic structure of carbon materials can be captured with Raman spectroscopy, which is a fast and convincing nondestructive tool to identify ordered and disordered crystal structure of carbon [36]. The Raman spectra of carbon materials have been reported to show the characteristic peaks at ~1350 cm1 (D band), 1580 cm1 (G band) and 2700 cm1 (2D band, also labelled as G0 band), respectively [36]. D band is the defect peak near 1350 cm1, which reflects the randomness of graphite layers, such as amorphous carbon or specific vibrations at the edges (e.g. oxides or CQC groups that appear only at the edges). G band is usually assigned to the characteristic band of E2g phonon of C sp2 atoms, representing the symmetry and degree of crystallization of carbon materials. And 2D band near 2700 cm1 originates from

two-phonon inelastic scattering. Fig. 2 shows the full (a) and local enlarged (b) Raman spectra of MWCNTs, GO and GO–MWCNTs hybrid samples, respectively. The Raman spectrum of MWCNTs exhibits a strong D band at 1321.0 cm1, a moderate G band at 1574.7 cm1, and a weak 2D band at 2648.6 cm1, respectively. The previous study shows that the D band of CNTs appears in the form of weak peak [37]. The apparent difference in the peak intensity of D band indicates the existence of more structural defects in the as-prepared MWCNTs, possibly caused by using intense ultrasonic waves, strong acid oxidization and the shorter MWCNTs. In order to obtain high performance electrode materials with the larger specific surface area, we purchased short-length MWCNTs and further shortened their length by means of a 4 h acid treating process. It is found that the shorter nanotubes tend to migrate in water and are easy to incorporate with GO. As for the Raman spectrum of GO, the intensity of D band is approximately equal to that of G band. Obviously, the G band of GO is much higher in intensity than G band of MWCNTs. However, as compared with MWCNTs, the spectrum of GO shows a very weak and broadened 2D peak, implying that there exist considerable defects in the resulting GO films. The results are in basic agreement with the previous report [35]. For GO–MWCNTs hybrid materials, although the strong D and G bands are observed, their peak positions are shifted to the lower frequency as compared to those corresponding to D and G bands of GO. It is observed more clearly from the enlarged D and G bands (Fig. 2b) that, the D band is red-shifted by 1.1 cm1 while the G band is red-shifted by 11.4 cm1, which should be due to the introduction of MWCNTs. In particular, it is interesting to point out that the 2D band of the GO–MWCNTs nanomaterials disappears. The significant changes in the intensity of 2D band reveal that: (i) new structural defects were created in the GO–MWCNTs nanocomposites, (ii) the combination of 1D CNTs with 2D graphene sheets generated 3D carbon networks to some extent. It is worth noting that, the ratios of the peak area of D to that of G band (ID/IG), in the order from low to high, are GO (ID/IG = 1.1), GO–MWCNTs (ID/IG = 1.2), and MWCNTs (ID/IG = 1.7), suggesting that the GO and MWCNTs co-exist in the composites and also confirming a fact that the GO–MWCNTs hybrid was synthesized successfully. The morphological characterizations and Raman spectra have demonstrated that GO has been successfully mixed with MWCNTs, but we want to understand how the two individual materials interact with each other. In order to have a better understanding of the problem, we measured UV–vis spectra of MWCNTs, GO and GO–MWCNTs suspensions since the UV–vis spectrum is very helpful for analyzing conjugated compounds and aromatic compounds [22]. It is seen from Fig. 3 that the UV–vis spectrum of GO exhibits two characteristic absorption peaks: a strong peak in the region between 220 nm and 230 nm is assigned to the p–p* transition of C C bonds, and a shoulder peak located around 300 nm is associated with the n–p* transition of carbonyl groups. But for the MWCNTs after being treated with strong acid, a broad and weak absorption peak at 225 nm related to p–p* transition is observed. According to the relevant report [23], the UV–vis spectrum of MWCNTs solution usually exhibits an unconspicuous absorption peak related to its insolubility. However, herein the weak absorption should originate from the small proportion of water-soluble MWCNTs since the hydrophilicity has been improved after acid treatment. Interestingly, except the peaks located in 220–230 nm and at 300 nm, a new absorption peak can be observed at 205 nm in the spectrum of GO–MWCNTs, which is ascribed to the p–p covalent attractions between the basal plane of GO and the nanowall of MWCNTs. In addition, the p–p* transition was red-shifted by about 10 nm. This result is in accordance with the result reported by Zhang’s group [23]. Thus, it is concluded that the interaction between MWCNTs and GO in the

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Fig. 1. (a) A typical low magnification TEM and (b) AFM images of GO–MWCNTs nanocomposites. Here the arrows point to the MWCNTs.

hybrid nanocomposites arises from the covalent interaction between p–conjugated aromatic rings of both GO and MWCNTs. 3.2. Electrochemical characterization of GO–MWCNTs nanocomposites GO–MWCNTs hybrid nanocomposites can be electrochemically reduced by carrying out repeated potential cycling in the potential range between 0 V and 1.7 V in the deoxygenized HAc–NaAc (pH 4.5) buffer solution [37]. In the first potential scan, a large reduction peak appeared at 1.5 V, as shown in Fig. 4a. Herein the onset potential for the electrochemical reduction was 0.45 V. It is obvious that the peak is attributed to the reduction of oxygencontaining functional groups, such as hydroxyl and epoxy groups produced during the synthetic process of GO via the Hummers method. The large cathodic peak no longer appeared since the second voltammetric cycle; meanwhile, the GO–MWCNTs nanocomposites could reach a stable state after three voltammetric cycles. For comparison, a control experiment was performed using a GO-modified GCE as the working electrode. As indicated in Fig. 4b, the onset potential for the electrochemical reduction of GO was 0.85 V, much more negative than that of the GO–MWCNTs nanocomposites. Besides, the complete reduction of GO needed more than 10 voltammetric cycles. These results clearly demonstrate that the good conductivity of CNTs is very beneficial to the reduction of GO. CNTs may act as conducting wires that connect

Fig. 2. (a) The comparison between Raman spectra of MWCNTs, GO and GO– MWCNTs. They are scaled to have the same height of the D peak at about 1350 cm1. (b) Magnification of D band and G band in the wavenumber range from 1200 cm1 to 1700 cm1.

Fig. 3. UV–vis absorption spectra of the as-prepared GO (dotted line), MWCNTs (dashed line) and GO–MWCNTs (solid line) dispersions.

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Fig. 4. The first 10 cyclic voltammograms (CVs) at the scan rate of 50 mV s1 for the electrochemical reduction of GO–MWCNTs modified GCE (a) and GO modified GCE (b) in 0.1 M HAc-NaAc buffer solution (pH 4.5) saturated with nitrogen gas. The initial potential is 0.0 V.

the different GO sheets and accelerate the electron transfer rate from one GO sheet to another GO sheet, thereby making the GO components in the hybrid nanocomposites be reduced more easily. The charge transfer rate at the electrode/solution interface can be evaluated well by CV [38]. Here we compared the chargetransfer behavior at the bare GCE and GO-, MWCNTs- and G-MWCNTs-functionalized GCEs in 0.1 M KCl solution using the [Fe(CN)6]3/4 couple as a redox probe. The recorded CVs are all shown in Fig. 5. The CVs of a bare GCE show a quasi-reversible redox process, with a peak potential difference (DEp) of 80 mV and a ratio of about 1:1 between anodic and cathodic peak currents, demonstrating that the GCE was a good reaction interface to the [Fe (CN)6]3/4 couple [13]. However, for the GO-functionalized GCE, both the anodic and cathodic peak currents were lowered considerably, indicating that the charge transfer rate became slow after the GCE was modified with GO nanomaterials. The reason is that the poor conductivity of GO slowed the charge-transfer rate at the modified electrode/solution interface. In contrast, the charge transfer rates at MWCNTs- and G-MWCNTs-functionalized GCEs

Fig. 5. Cyclic voltammograms (CVs) of modified and unmodified GCEs in 0.1 M KCl solution containing 5 mM K3Fe(CN)6 + 5 mM K4Fe(CN)6 at a scan rate of 50 mV s1.

had been speeded up in varying degrees, especially at the G-MWCNTs-functionalized GCE. MWCNTs are able to facilitate the electron transfer rate due to their high conductivity, without question. But the reason why the charge transfer rate at the G-MWCNTs-functionalized GCE is faster should be attributed to the synergistic effect between graphene and MWCNTs, since the introduction of CNTs provides a considerable number of electron transport channels for the [Fe(CN)6]3/4 redox couple. 3.3. Stripping behavior for heavy metal ions As mentioned above, the high conductivity and rich chelating groups of the G-MWCNTs make the functionalized GCE compelling for the electrochemical determination of heavy metal ions. To enhance the stripping performance of G-MWCNTs-GCE, a bismuth (Bi) film was in-situ formed on the surface of functionalized electrode. Bi, known as an environmentally friendly material, is used in a wide variety of cosmetics and medicines. In the earlier study carried out by Wang et al. the presence of Bi film was found to be beneficial to the acquirement of well-defined, single and sharp stripping peaks for heavy metal ions in contrast to the conventional Hg electrodes [9]. For this reason, here taking the electrochemical detection of Pb2+ and Cd2+ ions as a typical example, we firstly studied the influence of main experimental variables on the detection sensitivity for acquiring the best detection results. The influence of pHs of buffer solutions on the stripping peak currents was investigated in the pH range between 3.5 and 5.5 by DPASV. It is seen clearly from Fig. 6a that, the stripping peak currents were significantly influenced by the pH values of acetate buffer solutions. A maximum DPASV peak current was observed at about pH 4.5. The decrease of the currents at the pHs lower than 4.5 can be attributed to the protonation of the hydrophilic groups on the graphene plane, and the decrease at the pHs higher than 4.5 is due to the hydrolysis of Pb2+ and Cd2+ ions. Thus, pH 4.5 was chosen as the optimal pH for the analysis. The influence of the deposition potential on the stripping peak currents was studied in the potential region between 0.9 V and 1.5 V as shown in Fig. 6b. The more negative the preconcentration potential, the more easily Pb2+ and Cd2+ ions were reduced, thereby causing the obvious increase of the stripping peak currents.

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Fig. 6. The effects of solution pH (a), preconcentration potential (b), preconcentration time (c), and Bi3+ concentration (d) on the stripping currents of Cd2+ and Pb2+ ions at Bi/G/MWCNTs/GCE electrode in 0.1 M acetate buffer solution containing 15 mg L1 of Cd2+ and Pb2+ ions.

Fig. 7. DPASV stripping signals for various concentrations (0.5, 5.0, 7.0, 10, 15, 25, and 30 mg L1 from bottom to top) of Pb2+ and Cd2+ ions on the bismuth and Nafion films modified G/MWCNTs/GCE. Inset shows the calibration curves of Pb2+ and Cd2+ ions. Supporting electrolyte: 0.1 M acetate buffer (pH 4.5) containing 500 mg L1 Bi3 + ; deposition potential: 1.4 V; deposition time: 180 s; amplitude: 50 mV; increment potential: 5 mV; quiet time: 10 s.

However, when the preconcentration potential was more negative than 1.3 V, the stripping peak currents decreased because of the influence of hydrogen evolution reaction (HER). Considering that the stripping peak potential of Zn was located at 1.3 V, a more negative preconcentration potential (1.4 V) was selected in this paper. We also studied the effect of the deposition time, as shown in Fig. 6c. It is found that the stripping peak current increased with the increase of the deposition time. Here the choice of 180 s was a compromise between reducing practical measurement time and enhancing high sensitivity. Finally, we investigated the effect of Bi3+ concentration on the stripping response in the range of 0–700 mg L1, as shown in Fig. 6d. As for the influence of Bi3+ ion concentration on the stripping peak currents of Cd2+ and Pb2+ ions, in the case of using G-MWCNTs-GCE, the peak currents for both Cd2+ and Pb2+ ions increased rapidly upon increasing the Bi3+ concentration from 0 mg L1 to 500 mg L1 and then intended to be stable when the Bi3+ concentration was over 500 mg L1. In contrast, in the case of using bare GCE, the optimal Bi3+ concentration was between 100 and 400 mg L1. This difference in the Bi3+ ion concentration caused by using different types of working electrodes may be attributed to the different morphologies and the microstructures of the two electrodes. As compared with the clean and smooth surface of the bare GCE, the surface of G-MWCNTs-GCE is covered with the G-MWCNTs composite film, whose rough and hierarchical structure provides more active sites for the nucleation of the

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Table 1 A comparison of the different methods reported for the detection of Cd2+ and Pb2+. Analyte

Electrode

Modifier

Method

Deposition time (s)

Linear range (mg L1)

Detection limit (mg L1)

Refs.

Cd2+

GCE GCE Screen printed GCE GCE

G/Hg G/Bi MWCNTs/Bi Sn film G/MWCNTs/Bi

DPASV DPASV DPASV DPASV DPASV

500 120 300 180 180

0.2–15 1.5–30 2–100 10–110 0.5–30

0.005 0.02 0.7 1.1 0.1

[4] [6] [18] [41] This work

Pb2+

Screen printed GCE GCE GCE GCE

MWCNTs/Bi Amination of GO/Hg CN-Polymer Polyaniline/Bi G/MWCNTs/Bi

DPASV DPASV DPASV SWASV DPASV

300 180 60 120 180

2–100 0.00002–0.01 260–58730 5.0–30 0.5–30

1.3 0.00002 165 3.3 0.2

[18] [20] [42] [43] This work

SWASV, square wave ASV; CN-Polymer, cyano groups modified Poly(diphenylamine co-2-aminobenzonitrile).

Fig. 8. (a) DPASV stripping signals for Zn2+, Cd2+, Pb2+ and Cu2+ ions with different concentrations (0.5, 5.0, 7.0, 10, 15, 25 and 30 mg L1 from bottom to top) on bismuth and Nafion films modified G/MWCNTs/GCE. DPASV conditions are identical to those indicated in the caption of Fig. 6. (b) DPASV stripping signals for Zn2+, Cd2+, Pb2+ and Cu2+ ions with the same concentration (15 mg L1) on bismuth film modified bare GCE and G/MWCNTs/GCE.

bismuth. It is expected that, as the deposition time went on, Bi film would gradually become thick in a non-uniform growth manner since the newly formed Bi atoms tended to preferentially deposit on top of the nuclei already formed. In subsequent analytical tests, the concentration of Bi3+ ions used to form the Bi film was controlled at the level of 500 mg L1. Under the optimized conditions, the DPASV experiments about the simultaneous determination of Pb2+ and Cd2+ ions were performed in 0.1 M acetate buffer by using the G-MWCNTs hybrid modified GCE. The stripping peak currents for different concentrations of Pb2+ and Cd2+ ions, together with the calibration plots for the simultaneous determination, were illustrated in Fig. 7. The resulting calibration plot (see inset) for Pb2+ is observed to be linear over the range from 0.5 mg L1 to 30 mg L1. The equation of calibration curve is y = 0.1914x + 1.6709 (x: concentration/mg L1; y: current/mA) with the correlation coefficient of R = 0.978. The limit of detection is determined to be 0.2 mg L1 (S/N = 3) in the case of a deposition time of 180 s, which is lower than the recommended value of 10 mg L1 for Pb2+ in drinking water given by the World Health Organization (WHO). Similarly, the calibration plot for Cd2+ is linear in the concentration range from 0.5 mg L1 to 30 mg L1. The corresponding equation of calibration curve is y = 0.2358x + 2.0457 (x: concentration/mg L1,y: current/mA) with the correlation coefficient of R = 0.983. The limit of detection is 0.1 mg L1 (S/N = 3) in the case of a deposition time of 180 s, which is also lower than the guideline value of 3 mg L1 for Cd2+ in drinking water given by the WHO. Compared to the MWCNTs or graphene coated Bi film electrodes, the present analytical method

based on the G-MWCNTs functionalized GCE shows better sensitivity under our laboratory conditions. It should be noted that the baseline of the stripping peaks obtained using the functionalized electrode is higher than the previous values measured with the bare GCE, which should be attributed to the increase of the electrochemically active surface, together with the synergistic effect of MWCNTs and graphene [39]. A detailed comparison between the analytical performances of different modified electrodes was made (see Table 1). It is clear that the G/MWCNTs/Bi electrode shows the lower limit of detection than the MWCNTs/Bi electrode and the wider linear range than the G/Bi electrode for the detection of Cd2+ and Pb2+ [6,18]. Moreover, our analytical method is an environmentally friendly method, and the limits of detection for Cd2+ and Pb2+ ions are both much lower than the guideline values in drinking water given by the WHO. Although the detection limit of G/Hg electrode for Cd2+ ion is reported to be 0.005 mg L1 (S/N = 3), an extremely low value [4], the disadvantages of this analytical method include the long deposition time (500 s) and the toxicity of Hg to human beings. A significant advantage of the G-MWCNTs functionalized GCE over other modified electrodes is that several heavy metal ions can be determined simultaneously. We used the DPASV method to investigate the feasibility to detect simultaneously trace amounts of Zn2+, Cd2+, Pb2+ and Cu2+. Fig. 8a shows several groups of DPASV stripping signals measured in the working solution containing Zn2+, Cd2+, Pb2+ and Cu2+ ions. The four peaks corresponding to anodic stripping of Zn2+, Cd2+, Pb2+ and Cu2+ were well resolved and the corresponding peak currents increased with increasing the

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concentration of individual metal ions. Only the stripping peak of Zn2 was slightly distorted among four stripping peaks. If comparing the peak current signals of Cd2+ and Pb2+ shown in Fig. 7 and Fig. 8a, it is easy to find that the peak current signals of both Cd2+ and Pb2+ are larger under the coexistence of Cd2+ and Pb2+ ions than under the coexistence of Cd2+, Pb2+, Zn2+ and Cu2+ ions. This may be attributed to the interaction effect between different metals. In order to further show the outstanding advantages of the G-MWCNTs functionalized GCE in simultaneous determination of heavy metal ions, the DPASV curves for a bare GCE and a functionalized GCE measured in the same solution containing 15 mg L1 of Zn2+, Cd2+, Pb2+ and Cu2+ ions were also shown in Fig. 8b together. On the basis of DPASV results, only two heavy metal ions, Cd2+ and Pb2+, give very small stripping peaks when using the bare GCE as the working electrode. In contrast, four heavy metal ions can be detected simultaneously and accurately by means of the functionalized GCE, as demonstrated by the four well-defined stripping peaks. It should be pointed out that determination of Cu2+ by means of Bi-film modified electrode remains two problems: (i) the Bi-film has been already stripped before the appearance of the Cu peak [40], (ii) the co-deposition phenomenon of different metals resulting from the intrinsic nature of stripping voltammetry is commonly observed for most electrodes, including Bi-film electrodes and Hg electrodes. For instance, the co-deposition of Cu and Zn tends to form Cu-Zn alloy compounds during electrochemical reduction. The G-MWCNTs-GCE was also employed for the determination of Pb2+ and Cd2+ ions in practical samples from electroplating effluent. The water samples was filtered with a 0.22 mm membrane (Millipore) filter before use, and then added to 0.1 M acetate buffer (pH 4.5) containing 500 mg L1 Bi3+ ions. The average recovery for Pb2+ ions was (98.3  2.6)% and that for Cd2+ ions was (99.1  2.5)% (here 1.0 mg L1 standard Pb2+ or Cd2+ ions were added). The results are basically satisfactory, which confirms from one side the potential application value of the functionalized electrode in determining trace amounts of heavy metal ions.

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4. Conclusions In conclusion, novel 3D G-MWCNTs hybrid nanocomposites were fabricated via a simple and green method and their potential application in the simultaneous determination of several heavy metal ions were demonstrated. The G-MWCNTs hybrid electrode materials can be conveniently prepared by means of the direct electrochemical reduction of its precursors, GO–MWCNTs nanocomposites. As an ideal substitute for the poisonous HMDE and TMFE electrodes, the G-MWCNTs modified GCE electrode exhibits the high sensitivity for the electrochemical detection of trace amounts of Pb2+ and Cd2+ ions, with the lowest detection concentration of 0.5 mg L1 for Cd2+ and 0.5 mg L1 for Pb2+. In particular, Zn2+, Cd2+, Pb2+ and Cu2+ ions may be determined simultaneously by means of the novel modified electrode. The greatly enhanced detection sensitivity is due to the synergistic effect between MWCNTs and graphene components in improving preconcentration efficiency of metal ions and accelerating electron transfer rate at G-MWCNTs/electrolyte interface. Acknowledgments This work was supported by the National Research Foundation for the Doctoral Program of Higher Education of China (20120131110009), the National Science Foundation of China (21373129, 21175059), and the Natural Science Foundation of Shandong Province (ZR2010BQ029).

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