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Comparative study of graphene nanosheet- and multiwall carbon nanotube-based electrochemical sensor for the sensitive detection of cadmium Lidong Wu, Xiaochen Fu, Huan Liu, Jincheng Li, Yi Song * Chinese Academy of Fishery Sciences, Beijing 100141, 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

 A nanocomposite based on nanographene and Nafion is used as a platform for cadmium detection.  The performance of the nanographene-based sensor was compared with that of MWCNT.  It indicated that the nanographenebased sensor possessed significant advantages over MWCNT.  The nanographene-based sensor proved to be a reliable tool for rapid detection of cadmium.

Schematic diagram of nanographene-based sensor detection of cadmium ions by stripping analysis.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 June 2014 Received in revised form 7 August 2014 Accepted 11 August 2014 Available online xxx

A novel nanocomposite was obtained through the controlled surface modification of graphene nanosheets (nanographene) with Nafion by ultrasonic oscillation. The composite was used as an ultrasensitive platform for the detection of cadmium ions (Cd2+) by differential pulse anodic stripping voltammetry (DPASV) analysis. The performance of the nanographene-based sensor was systematically compared with that of a multiwall carbon nanotube (MWCNT)-modified sensor. The results indicate that the nanographene-based sensor exhibits significant advantages over the MWCNT-based sensor in terms of repeatability, sensitivity and limit of detection (LOD). The nanographene-based sensor displayed superior analytical performance over a linear range of Cd2+ concentrations from 0.25 mg L1 to 5 mg L1, with a LOD of 3.5 ng L1. This sensor was also used to systematically screen for 6 types of chemicals, including sodium salts, magnesium salts and zinc salts. It was observed that the sensor could successfully differentiate cadmium ions from interferents (magnesium salts, zinc salts, etc.). The nanographene-based sensor was also demonstrated to be a promising and reliable tool for the rapid detection of cadmium existing in tap water and for the rapid on-site analysis of critical pollution levels of cadmium. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Nanographene Multiwall carbon nanotubes Rapid detection Cadmium

1. Introduction

* Corresponding author. Tel.: +86 10 68690715; fax: +86 10 68690715. E-mail address: [email protected] (Y. Song).

Cadmium is an extremely toxic metal that usually exists in industrial workplaces (electroplating, metallurgy, batteries, etc.) [1]. The element poses severe harm to the environment and can

http://dx.doi.org/10.1016/j.aca.2014.08.021 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

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cause profound biochemical and neurological changes in the body [2–4]. Cadmium exposure may cause flu-like symptoms, renal tubular dysfunction and bone degeneration [5]. As a highly toxic and dangerous environmental pollutant, cadmium has caused several major pollution incidents [6,7]. In the late 1960s, the amount of environmental cadmium caused an epidemic of bone disease (itai–itai disease) in Japan [8,9] and later in Taiwan [10]. Furthermore, the United Nations Environment Programme (UNEP) proposed 12 types of priority hazardous substances that pose a global threat, with cadmium ranked first due to its high toxicity [11,12]. The International Agency for Research on Cancer (IARC) has classified cadmium as a carcinogenic chemical toward humans [13]. Thus, there is a crucial need to develop a new method that can rapidly detect cadmium in a sensitive manner to enable the timely remediation of unexpected accidents. Cadmium is typically analyzed using prominent methods such as atomic absorption [14,15], ion chromatography [16,17] and inductively coupled plasma-mass spectrometry (ICP-MS) [18,19]. These spectrometric methods offer good precision and resolution, but they are expensive and time-consuming, involve complex operation steps and are not suitable for on-site detection [20–22]. As an ideal, alternative method for cadmium determination, electrochemical sensing is one of the best techniques due to its cost-effectiveness, speed, portability, ease of operation and reliability [23–25]. In particular, differential pulse anodic stripping voltammetry (DPASV) is a sensitive electrochemical method for the analysis of trace cadmium ions [26,27]. Currently, mercury electrodes have been intensively researched, providing improvements in the speed, portability, selectivity and cost of detecting trace cadmium by DPASV [28–30]. Based on metal complexation with pyrogallol red (PR) and subsequent adsorptive deposition on a mercury-coated glassy carbon electrode coated with Nafion, Nagles et al. developed a biosensor for the detection of cadmium by the adsorptive stripping voltammetric (AdSV) method, achieving detection limits of 0.01 mg L1 [31]. A series of studies devoted to the use of a mercury electrode for cadmium detection have been reported [32,33]. Owing to low conductivity, the response signals of these mercury electrodes have been reported to be relatively poor [34,35]. As is well known, sensing materials play a vital role in the development of metal analysis techniques [36]. Superior sensing materials represent the core technology used in electroanalytical techniques for improving limits of detection (LOD), sensitivity, etc. As a novel carbon nanomaterial, graphene possesses excellent chemical and thermal stability, high mechanical strength, high specific surface area and good electrical conductivity, thus holding good prospects for application in metal ion analysis [30,37]. Pure graphene is hydrophobic and tends to agglomerate in aqueous solutions, which restricts the material’s application as a sensing platform. Compared with hydrophobic graphene, hydrophilic nanographene is prepared by the simple ball milling of graphite, which produces abundant hydroxyl and carboxyl (approximately 8.4%) groups on the defect sites and edges of graphite [38]. The resulting material disperses well in aqueous solutions. In addition, the hydroxyl and carboxyl functional groups are favorable for cadmium ion enrichment via electrostatic interactions. A novel nanocomposite film based on Nafion and nanographene (NGP–Nafion) was obtained for the ultrasensitive detection of cadmium ions. The performance of the nanographene-based electrochemical sensor was systematically compared with that of a multiwall carbon nanotube (MWCNT)-based sensor. The results indicate that the NGP-based sensor exhibits superior repeatability, sensitivity and LOD compared with the MWCNT-based sensor. This distinct electrochemical performance of the NGP-based sensor was

mainly attributed to the sensor’s low capacity, high electrical conductivity and large specific surface area. 2. Experimental 2.1. Materials and solutions Cadmium chloride and other chemicals used in this study were of analytical reagent grade and were purchased from Sigma Chemical Ltd. (USA). Millipore Milli-Q water (18 MV cm) was used throughout all experiments. Unless indicated otherwise, 100 mmol L1 acetate buffer (pH 5.0) was used as the electrolyte in all electrochemical experiments. 2.2. Apparatus Cyclic voltammetry (CV) and DPASV were carried out on glassy carbon electrodes (GC) using a CHI 660 Electrochemical Workstation (CHI Instruments Inc., Shanghai, China). The threeelectrode system employed consisted of a NGP–Nafion modified GC electrode as the working electrode, a Ag/AgCl as the reference electrode, and a platinum wire as the auxiliary electrode. Transmission electron microscopy (TEM) images were obtained using a JEM-2000EX TEM (JEOL, Japan) operated at an accelerating voltage of 300 kV. Raman spectra were obtained using a HORIBA Jobin Yvon LABRAM HR 800 confocal Raman spectrometer (Jobin Yvon, France). Nitrogen adsorption–desorption isotherms were obtained using a Micromeritics ASAP 2010 apparatus (Micromeritics, USA) at 196  C, and specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The concentration of cadmium ions was determined by atomic absorption spectrometry (AAS, Jena, Germany). 2.3. Preparation of hydrophilic nanographene and preparation of modified electrode Hydrophilic nanographene was prepared by the ball-milling method detailed in our previous report [38]. In accordance with this procedure, 2.0 g graphite powder and 60 g steel balls (diameter, 1–1.3 cm) were placed into a hardened steel vial and purged with high-purity argon (99.999%) for 20 min before the vials were sealed. Ball milling was thenperformed at 450 rpm for 20 h to yield graphene nanosheets. The surface of the prepared GC electrode was polished with alumina powder (0.05 mm) and washed ultrasonically with Milli-Q water and ethanol, successively. Then, the cleaned surface of the GC electrode was dried with a stream of high-purity nitrogen. Five microliters of a 0.8 mg mL1 graphene solution was mixed with 5 mL of Nafion (5 wt%) and 10 mL Milli-Q water via ultrasonication. Then, an aliquot of 5 mL of the mixture was coated on the GC electrode to obtain a NGP–Nafion/GC electrode. Other GC electrodes were prepared by a similar procedure. A mixture containing 0.2 mg mL1 MWCNTs and 1.25% Nafion was used to prepare a MWCNT–Nafion/GC electrode, and a solution containing 1.25% Nafion was used to prepare a Nafion/GC electrode. 2.4. Detection of cadmium by the NGP–Nafion/GC electrode Mercury was plated onto the NGP–Nafion/GC electrode by the electrodeposition of 10 mg L1 Hg2+ for 120 s under stirring. After a preconcentration step, the stirring was stopped for 10 s, and the DPASV was recorded. Measurements were performed in 8 mL of 100 mmol L1 acetate buffer (pH 5.0) under stirring by DPASV at room temperature with the successive addition of cadmium.

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3. Results and discussion 3.1. Physical characterization of nanographene and MWCNTs Fig. S1 shows representative TEM images of the nanographene sheets and MWCNTs. As shown in Fig. S1a, high-quality, singlelayer and few-layer nanographene were obtained using our previously reported method [38]. As shown in Fig. S1b, the outer diameter of the MWCNTs was 10–20 nm, the inner diameter was 2–5 nm, and the length was on the order of micrometers [39]. Specific surface area is one of the most important characteristics of nanomaterials; indeed, a high effective surface area allows for high GC electrode performance. The specific surface areas of the carbon nanomaterials fabricated in this study were estimated by the BET method. The specific surface areas of the nanographene and MWCNTs were determined to be 905 m2 g1 and 77.6 m2 g1, respectively [39,40]. An electrode material with high electrical conductivity is favorable for the construction of electrochemical sensors. The electrical conductivity of carbon nanomaterials is associated with the materials’ graphitization degree, which can be characterized by Raman spectroscopy. Fig. S2 shows the Raman spectra of the nanographene (a) and MWCNTs (b). As shown in Fig. S2, two peaks at approximately 1600 cm1 and 1345 cm1 were observed. The peak at 1600 cm1 is associated with the E2g vibration mode of graphite layers (G band), and the peak at 1345 cm1 is ascribed to the vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite (D band) [41]. The D/G intensity ratio (ID/IG) decreased with an increase in graphitization degree. The ID/IG ratios of the nanographene and MWCNTs were 1.58 and 1.05, respectively, which indicated that the graphitization degree and electrical conductivity of the MWCNTs were greater than those of the nanographene. 3.2. Electrochemical properties of nanographene-modified GC electrode Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement technique that can be used to monitor the formation of an adsorption layer on GC electrodes and the corresponding electron transfer reaction. The immobilization process of the NGP–Nafion/GC electrode is shown in Fig. 1. Fig. 1 shows the CV of the bare GC, Nafion/GC and NGP–Nafion/GC electrodes in a 2 mmol L1 K3[Fe(CN)6]/K4[Fe(CN)6] solution containing 0.1 mol L1 KCl. The highest CV on the bare GC electrode indicates the fastest electron transfer rate between the bare

Fig. 2. DPASVs of bare GC electrode (red line), Nafion/GC electrode (black line) and NGP–Nafion/GC (blue line) electrode at a cadmium ion concentration of 10 mg L1 with 10 mg L1 Hg2+ electrodeposited for 120 s under stirring in 100 mmol L1 acetate buffer (pH 5.0).

electrode and [Fe(CN)6]3/4. The CV of the Nafion/GC electrode was lower than that of the bare GC electrode, indicating that a Nafion film had formed on the surface of the GC electrode. After the nanographene was combined with Nafion to modify the GC electrode, the CV increased. The improved CV signal may be attributed to the introduction of nanographene, which played a role in promoting the direct electron transfer between [Fe(CN)6]3/4 and the surface of the electrode. This finding also indicated that nanographene and Nafion films had formed on the surface of the GC electrode. 3.3. Electrochemical properties of the nanographene-modified GC electrode As shown in Fig. 2, stripping voltammograms of cadmium ions at the bare GC electrode, Nafion/GC electrode and NGP–Nafion/GC electrode were obtained between 0.7 and 0.45 V in 100 mmol L1 acetate buffer (pH 5.0) with 10 mg L1 Hg2+ by electrodepositing for 120 s under stirring. A small stripping peak of cadmium ions was observed at the bare GC electrode. Under the same conditions, a higher stripping peak of cadmium ions was obtained at the Nafion/GC electrode. After the introduction of nanographene, the stripping peak of cadmium ions became

4

Current (µA)

Current (µA)

6 5

3 2 1 0

4

1 2 3 4 5 Concentration (µg L-1)

3 2 1 -0.76

Fig. 1. Cyclic voltammograms of the bare GC (red line), Nafion/GC (black line) and NGP–Nafion/GC (blue line) electrodes in 2 mmol L1 [Fe(CN)6]3/4 at a scan rate of 100 mV s1.

-0.72

-0.68 -0.64 Potential (V)

-0.60

Fig. 3. DPASVs obtained for different concentrations of Cd2+ using a mercurycoated NGP–Nafion electrode; from bottom to top, 0.25, 0.5, 0.75, 2.5, 5.0 mg L1. Inset: calibration curves of Cd2+ at different concentrations.

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Table 1 Analysis of cadmium by various reported sensing methods. Electrode

Detection limit (ng L1)

Deposition time (s)

Background noise (mA)

Reference

NGP–Nafion/GC (mercury film) MWCNT–Nafion/GC (mercury film) Graphene oxide–Nafion/GC (mercury film) Ionic liquid–Nafion/GC (mercury film) Graphene–ionic liquid/screen-printed electrode (bismuth film) Graphene-poly(p-aminobenzene sulfonic acid)/GC (stannum film)

3.5 25 5 130 80 50

120 120 500 60 120 300

1.41 3.72 3 – – –

Present work Present work [30] [35] [44] [45]

superior to that observed for the Nafion/GC electrode. This finding could be attributed to the improvement of the electron transfer rate and the specific surface area, which may have facilitated the effective deposition of cadmium ions from solution to the electrode surface. The increased stripping peak also indicated that nanographene had been effectively immobilized on the GC electrode. 3.4. Comparative study of nanographene and MWCNT-modified GC electrodes DPASV is a very useful and sensitive electrochemical method for the detection of cadmium ions. Fig. 3 illustrates a series of stripping voltammograms for cadmium ions over the concentration range of 0.25–5.0 mg L1 in 100 mmol L1 acetate buffer (pH 5.0) with 10 mg L1 Hg2+ electrodepositing over a period of 120 s. As shown in the inset of Fig. 3, the stripping current peaks increased linearly with the increase in the cadmium ion concentration, and the correlation coefficients reached a value of 0.99. In addition, the performances of the MWCNT–Nafion/GC electrode was also comparatively and systemically studied. The MWCNT–Nafion/GC electrode displayed a linear relationship with the cadmium concentration over the range of 10–250 mg L1. As one of the most important performance characteristics of sensors, the LOD is a very useful parameter for evaluating detection capability. The LOD of the NGP–Nafion/GC electrode was 3.5 ng L1 at a signal-to-noise ratio of 3, superior to that of the MWCNT–Nafion/GC electrode (25 ng L1). This result indicated that the LOD of the NGP–Nafion/GC electrode was much lower than the standards required for detecting drinking water quality in China (GB 5749-2006, Cd 0.005 mg L1). Compared with the LOD obtained using a previously reported method [44,45] (Table 1), the LOD of the NGP–Nafion/GC electrode (3.5 ng L1 after a 120 s preconcentration period) was better than the LOD of the ionic liquid–Nafion/GC (130 ng L1, electrodeposition of Hg2+ for 60 s) [35], graphene oxide–Nafion/GC (5 ng L1, electrodeposition of

Hg2+ for 500 s) [30] and graphene-poly(p-aminobenzene sulfonic acid)/GC (50 ng L1, electrodeposition of Sn2+ for 300 s) [45] electrodes. Thus, this sensor could be used as a reliable and ultrasensitive tool for the rapid detection of cadmium in the environment. Reproducibility is another attractive feature of the NGP–Nafion/ GC electrode. A series of 8 repeated measurements for the detection of 12.5 mg L1 cadmium ions yielded highly reproducible stripping peaks with a relative standard deviation (RSD) of 0.29% (Fig. 4). The inter-electrode RSD of the MWCNT–Nafion/GC electrode was 12%, demonstrating that the repeatability of the NGP–Nafion/GC electrode was better than that of the MWCNT–Nafion/GC electrode. In addition, the background signals of NGP–Nafion/GC and MWCNT–Nafion/GC electrodes were 1.41 and 3.72 mA, respectively. Based on the above-described comparison, the NGP–Nafion/GC electrode demonstrated an excellent LOD, good reproducibility and low background noise. The good sensing performance of the NGP–Nafion/GC electrode may be attributed to the following factors: the large specific surface area of nanographene could greatly increase the active surface area of the GC electrode for cadmium ion adsorption (Scheme 1a); the high electric conductivity of nanographene could improve the electron transfer rate between the surface of the GC electrode and cadmium ions in the bulk solution; and the archetypal 2D nanostructure with a single atomic layer of carbon could provide a very broad space for the easy transfer of cadmium ions (diameter 0.316 nm) [42]. Due to these features, the NGP–Nafion/GC electrode exhibited a low LOD, low background noise and good reproducibility. The small specific surface area of the MWCNTs resulted in a poor LOD. As shown in Scheme 1b, due to the close interlayer distance between MWCNTs (0.34 nm, slightly greater than the diameter of cadmium ions (0.316 nm)) [43], cadmium ions were adsorbed continuously, causing the MWCNTs to become enriched with cadmium ions. This adsorption ultimately resulted in the poor reproducibility of the

45

Current (μA)

40 35 30 25 20

0

1

2

3

4

5

6

7

8

9

Number (n) Fig. 4. The stability of 8 repeated measurements of 12.5 mg L1 Cd2+ in 0.1 mol L1 acetate buffer (pH 5.0) containing 10 mg L1 Hg2+.

Scheme 1. Schematic diagram of cadmium ion absorption on (a) nanographene and (b) MWCNT.

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MWCNT–Nafion/GC electrode. Thus, the NGP–Nafion/GC electrode shows greater potential for use as a “pre-alarm” tool than the MWCNT–Nafion/GC electrode. 3.5. Real sample detection using the NGP–Nafion/GC electrode The NGP–Nafion/GC electrode was used to detect cadmium ions in a sewage sample by standard addition method. The tested sample, without enrichment, was directly injected into the test container and detected by the NGP–Nafion/GC electrode. A standard solution of cadmium ions was injected into the test container using a procedure similar to that for the injection of the sewage sample. The standard addition method indicated that the concentration of cadmium ions in the sample was 4.4 mg L1. The detection result obtained using the NGP–Nafion/GC electrode was validated by AAS. The concentration of cadmium ions (4.9 mg L1) detected by AAS correlated well with the value (4.4 mg L1) measured by the electrochemical method. The electrochemical method also showed relative error of approximately 10%. Potential coexisting interferents, such as zinc ions, nickel ions, manganese ions, potassium ions, aluminum ions and magnesium ions, were also systematically evaluated using the NGP–Nafion/GC electrode. The results showed that these chemical species did not produce any interfering signals. Therefore, the proposed sensor proved to be a promising tool for the rapid detection of cadmium ions existing in water samples. 4. Conclusion In summary, a NGP–Nafion/GC electrode was successfully constructed for cadmium ion detection, and the analytical performance of the electrode was systematically and comparatively studied in parallel with a MWCNT-based electrode. The results show that nanographene provided several significant advantages over MWCNTs in achieving lower background noise, a better LOD and better reproducibility, which can be attributed to the larger specific surface area, unique nanosheet structure and high electrical conductivity of the former. The NGP–Nafion/GC electrode proved to be a promising and reliable tool for the rapid detection of cadmium on site. The electrode also holds great prospect for application in the rapid on-site analysis of critical pollution levels of cadmium. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21307161) and the Special Scientific Research Funds for Central Non-profit Institutes, Chinese Academy of Fishery Sciences (No. 2013C006). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.08.021. References [1] A. Vanek, L. Boruvka, O. Drabek, M. Mihaljevic, M. Komarek, Mobility of lead, zinc and cadmium in alluvial soils heavily polluted by smelting industry, Plant Soil. Environ. 51 (2005) 316–321. [2] C. Wang, J.F. Ji, Z.F. Yang, L.X. Chen, P. Browne, R.L. Yu, Effects of soil properties on the transfer of cadmium from soil to wheat in the Yangtze River Delta region, China—a typical industry-agriculture transition area, Biol. Trace Elem. Res. 148 (2012) 264–274. [3] M.M. Brzoska, M. Galazyn-Sidorczuk, I. Dzwilewska, Ethanol consumption modifies the body turnover of cadmium: a study in a rat model of human exposure, J. Appl. Toxicol. 33 (2013) 784–798.

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Please cite this article in press as: L. Wu, et al., Comparative study of graphene nanosheet- and multiwall carbon nanotube-based electrochemical sensor for the sensitive detection of cadmium, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.08.021

Comparative study of graphene nanosheet- and multiwall carbon nanotube-based electrochemical sensor for the sensitive detection of cadmium.

A novel nanocomposite was obtained through the controlled surface modification of graphene nanosheets (nanographene) with Nafion by ultrasonic oscilla...
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