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High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film
Analytica Chimica Acta 649 (2009) 196–201
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film Jing Li a,b , Shaojun Guo a,b , Yueming Zhai a,b , Erkang Wang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
a r t i c l e
i n f o
Article history: Received 19 May 2009 Received in revised form 12 July 2009 Accepted 13 July 2009 Available online 19 July 2009 Keywords: Nafion-graphene Nanosheets Lead Cadmium Bismuth
a b s t r a c t Graphene nanosheets, dispersed in Nafion (Nafion-G) solution, were used in combination with in situ plated bismuth film electrode for fabricating the enhanced electrochemical sensing platform to determine the lead (Pb2+ ) and cadmium (Cd2+ ) by differential pulse anodic stripping voltammetry (DPASV). The electrochemical properties of the composite film modified glassy carbon electrode were investigated. It is found that the prepared Nafion-G composite film not only exhibited improved sensitivity for the metal ion detections, but also alleviated the interferences due to the synergistic effect of graphene nanosheets and Nafion. The linear calibration curves ranged from 0.5 g L−1 to 50 g L−1 for Pb2+ and 1.5 g L−1 to 30 g L−1 for Cd2+ , respectively. The detection limits (S/N = 3) were estimated to be around 0.02 g L−1 for Pb2+ and Cd2+ . The practical application of the proposed method was verified in the water sample determination. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As is well known, lead and cadmium pose severe risks to human health with toxic effects on living organism. For example, the toxicity of lead in humans mainly comes from its detrimental mimicking action by occupying the calcium binding sites on numerous calcium-dependent proteins in cells (such as, calmodulin and enzyme protein kinase C), thus resulting in the corresponding impairment of physiological functions [1,2]. Accordingly, exploring the sensitive, rapid and simple analytical method for precise monitoring of Pb2+ and Cd2+ is urgently needed. The usual methods adopted for the concentration assessments of the metal ions are mainly focused on the use of atomic absorption or inductively coupled plasma (ICP) atomic emission spectroscopy, ICP-mass spectrometry and electrochemical (EC) techniques. However, spectroscopy methods are somewhat cumbersome and not suitable for the in situ measurement due to the ponderous and complicated instruments. On the contrary, EC techniques have attracted growing interests due to high-sensitivity, portability and low cost. Among all the EC methods, electrochemical stripping voltammetric analysis provides a powerful tool for the determination of metal ions [3], which possess high-sensitivity for the metal analysis due to
∗ Corresponding author at: State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun Jilin, 130022, China. Tel.: +86 431 85262003; fax: +86 431 85689711. E-mail address: [email protected] (E. Wang). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.07.030
the built-in preconcentration step and is very suitable for on-site and in situ analysis [4,5]. Most commonly, mercury based electrodes including hanging drop mercury electrode and mercury film electrodes were adopted because of excellent reproducibility and high-sensitivity [6]. However, due to the dramatic toxicity of mercury, numerous attempts have been made to replace it with new mercury-free and reliable electrode. Recently, the bismuth film electrode (BFE) has drawn the increasing attentions in the field of the stripping technique due to the remarkably low toxicity and the ability to form alloy with many metals as well as its wide potential window, which has proved to be equal to or even superior to that of MFEs [7–9]. On the other hand, in order to alleviate interferences and improve the sensitivity of the sensing interface, some strategies have also been exploited for the stripping analysis, such as, the chemical modified electrode (CME), heated electrodes, microwaved electrodes, and insonated electrodes [10–12]. Among these, CME capable of accumulating target analytes from dilute aqueous solution has been developed as the fascinating and effective way for the anodic stripping voltammertry (ASV) determination of heavy metal ions. Prominent examples include functionalized mesoporous silica electrode [13], carbon nanotubes (CNT) modified electrode [14,15], ordered mesoporous carbon [16], acetylene black paste electrode [17], nanocrystalline diamond thin-film electrode [18], and thickfilm modified graphite-containing electrode [19]. Although all the above material showed improved stripping signals, especially the CNT modified electrode prepared by Jin’s group [15], new materials are still needed to develop highly sensitive and antifouling metal ions sensing platform.
J. Li et al. / Analytica Chimica Acta 649 (2009) 196–201
Graphene, a single atomic plane of graphite packed into a dense honeycomb crystal structure, was novel and fascinating carbon material since experimentally produced in 2004 [20]. Much research effort has been made to explore its fascinating applications in electroanalytical chemistry or electrochemistry as novel electrode material for various purposes due to the excellent electrical and mechanical properties [21–25]. For instance, Li’s group [21,22] has demonstrated the use of the graphene nanosheets to develop a high-performance electrochemical sensor for dopamine and electrocatalytic oxidation of methanol. Worden et al. [24,25] has also fabricated an excellent glucose biosensor based on the exfoliated graphite nanoplatelets as a viable and inexpensive filler alternative to CNT. Recently, our group [26] developed a Cd2+ sensing platform with the MFE based on the graphene nanosheets. Although, the sensing platform showed ultra-sensitivity for the detection of Cd2+ , its wide use was limited due to the toxicity of the mercury. In this work, graphene nanosheets were used in combination with in situ BFE to fabricate a sensitive and mercury-free electrochemical platform for the analysis of the Pb2+ and Cd2+ . Herein, Nafion not only acts as an effective solubilizing agent for graphene nanosheets (Nafion-G), but also as an antifouling coating to reduce the influence of the surface-active macromolecules. This electrochemical sensing interface exhibited excellent stripping performance for the analysis of Pb2+ and Cd2+ combining the advantageous of the graphene nanosheets (higher electrical conductivity, huge specific area) together with the unique features of the in situ plating BFE. 2. Experimental 2.1. Reagents Nafion (5 wt.% in low aliphatic alcohols), was purchased from Aldrich (Milwaukee, WI), and then diluted to 1 wt.% Nafion with 2-propanol. Stock solutions of Pb2+ and Cd2+ were prepared by diluting the corresponding standard stock solutions prepared with CdCl2 and Pb(NO3 )2 , respectively. Bi(NO3 )3 was used for the BFE by diluting the corresponding standard stock solution. A 0.1 M acetate buffer (pH 4.5) prepared by mixing appropriate amounts of CH3 COOH and CH3 COONa, was served as to prepare solution of the supporting electrolyte. Unless otherwise stated, all solutions were prepared with double-distilled water. All chemicals employed in this work were of analytical reagent grade and used as received. 2.2. Apparatus Differential pulse ASV (DPASV) was performed in a conventional three-electrode cell with a CHI 842B electrochemical workstation (CH Instruments, Shanghai, China). The modified glass carbon (GC, 3 mm in diameter) electrode with the Nafion-graphene was served as the working electrode. An Ag/AgCl (saturated. KCl) electrode and a platinum foil were used as the reference electrode and auxiliary electrode, respectively. Electrochemical impedance spectroscopy (EIS) was measured with an Autolab/PG30 electrochemical analyzer system (ECO Chemie B.V., Netherlands) in a grounded Faraday cage at ambient condition. The dc potential was 0.24 V vs. Ag/AgCl (saturated KCl), and the conductivity was determined by ac impedance in the frequency range between 0.01 Hz and 100,000 Hz with a perturbation signal of 5 mV. All electrochemical experiments were carried out in a one-compartment electrochemical cell. Atomic force microscopy (AFM) measurement was performed on the mica with a SPI3800N to characterize the graphene nanosheets. 2.3. Preparation of the graphene solution Firstly, the graphite oxide was synthesized from natural graphite powder according to the literature with a little modification [27,28].
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Then, exfoliation of graphite oxide to graphene oxide was achieved by ultrasonication of the dispersion for 40 min (1000 W, 20% amplitude) according to the reference with a slight modification [29]. Finally, a homogeneous aqueous dispersion (0.5 mg mL−1 ) was obtained. In a typical procedure for chemical conversion of graphene oxide to graphene, the resulting homogeneous dispersion (100 mL) was mixed with 70 L of hydrazine solution (50 wt.% in water) and 0.7 mL of ammonia solution (28 wt.% in water). After being vigorously shaken or stirred for a few minutes, the solution was stirred for 1 h at the temperature of 95 ◦ C. 2.4. Fabrication of the Nafion-G composite film Ultrasonic agitation for ca.30 min was used to disperse the graphene nanosheets into 100 L 1 wt.% Nafion-isopropyl-alcohol to give 0.25 mg mL−1 Nafion-G suspension. Prior to the surface modification, the bare GC electrode was polished carefully with 1.0 m, 0.3 m and 0.05 m alumina powder, respectively, and rinsed with deionized water, followed by sonicated in acetone and doubly distilled water successively and dried under nitrogen. Then, an aliquot of 5 L of the mixture was coated on the electrode, and then the solvent was evaporated under an infrared lamp to obtain the Nafion-G composite film modified GC electrode. 2.5. Procedure for DPASV analysis The three electrodes were immersed into the electrochemical cell, containing 5 mL 0.1 M acetate buffer (pH 4.5), Bi3+ and the target metals ions. The Nafion-G modified GC electrode with bismuth film was plated in situ by spiking the sample with the required concentration of Bi3+ and simultaneously depositing Bi and the target metals on the surface of the electrode at −1.2 V under stirring for 120 s. Following the preconcentration step, the stirring was stopped and after 10 s, the voltammogram was recorded by applying a positive-going differential pulse voltammetry scan (with a step increment of 5 mV, amplitude of 80 mV, and pulse period of 0.2 s). The scan was terminated at 0 V. Prior to the next cycle, a preconditioning step (60 s at 0.3 V, with solution stirring) was used to remove the target metals and Bi. The lake water was obtained in Changchun and was filtered through a 0.22 m membrane (Millipore). For the DPASV analysis, 9 mL of lake water and 1 mL of 1.0 M acetate buffer (pH 4.5) were mixed. 3. Results and discussion 3.1. Morphologic characterization of the graphene nanosheets The structure and morphology of the resulting graphene deposited on the mica were characterized by AFM. The results (Fig. 1) found that the graphene sheets were almost single-layer. And the average thickness of single-layer graphene sheets was <1 nm. This unique nanostructure may be favourable as potential material for the electrochemical determination of Pb2+ and Cd2+ . 3.2. Electrochemical characterization of the Nafion-G composite film Electrochemical experiments were used to characterize the Nafion-G composite film. Different cyclic voltammograms (CVs) at bare, Nafion, and Nafion-G modified GC electrodes in a freshly prepared solution containing 1.0 mM K3 Fe(CN)6 in the presence of 1.0 M KCl are shown in Fig. 2. A well-shaped CV with a peak-to-peak separation of 77 mV was observed at the bare GC electrode (solid line). After being modified with Nafion (dashed line), the anodic and cathodic peaks almost disappeared similar to that reported in previous work [30], demonstrating that the negative charged
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J. Li et al. / Analytica Chimica Acta 649 (2009) 196–201
Fig. 2. CVs for bare (solid line), Nafion (dashed line), Nafion-G (dotted line) modified GC electrodes in 1.0 mM K3 Fe(CN)6 in 1.0 M KCl, scan rate, 50 mV s−1 .
Fig. 1. AFM image of graphene on freshly cleaved mica.
skeleton of Nafion acted as the blocking layer for the diffusion of Fe(CN)6 3− into the film and hindered the electron and mass transfer. However, at the Nafion-G modified GC electrode (dotted line), the peak current increases obviously compared with that observed at Nafion modified GC electrode, indicating that the introduction of the graphene provided the conducting bridges for the electrontransfer of Fe(CN)6 3− . Fig. 3(A) shows the DPASV analytical characteristics of different film coated GC electrodes, e.g., Nafion (dotted line), Nafion-G (solid line) by in situ plated BFE for Pb2+ and Cd2+ determination. The stripping voltammograms were obtained in a solution containing 20 g L−1 each of Pb2+ , Cd2+ , 1 mg L−1 Bi3+ in 0.1 M acetate buffer (pH 4.5) without deaeration. The sharper and higher peak current for the target metal ions were obtained at the Nafion-G
modified electrode. Compared with the Nafion coated electrode, signals were improved about 79%. While under the same condition, the carbon nanotubes/Nafion modified electrode showed ca. 69% improvement. It is indicated that the graphene show the similar behaviour to the CNTs. As to the signal enhancement, it is can be attributed to two factors: (i) the change of the morphologies and the structure, and (ii) the interfacial electron-transfer properties. In order to verify, the scanning electron microscopy (SEM) and ESI were carried out. From the results of the SEM (Fig. 3B), it could be seen that the morphologies and the structure of pure Nafion and Nafion-G were distinctly different. The pure Nafion showed a flat and homogeneous film and the Bi3+ was firstly exchanged into the Nafion film and become nuclei during the electrodeposition procedure. However, when the electrode was coated with the Nafion-G composite film, a rough and stratified structure was formed, which endowed the more effective active area for the nucleation of the bismuth due to the unique structure. As the deposition time increased, bismuth nanofilm was expected to be thick in a non-uniform way as it grows preferentially on top of already deposited nuclei [31]. ESI,
Fig. 3. (A) DPASVs for 20 g L−1 each of Cd2+ and Pb2+ on an in situ plated Nafion-BFE, Nafion-G-BFE in solution containing 1 mg L−1 Bi3+ , (B) SEM image of Bi film deposited on the Nafion (a) and Nafion-G (b) modified GCE. Supporting electrolyte: 0.1 M acetate buffer (pH 4.5); deposition potential: −1.2 V; deposition time: 120 s; amplitude: 80 mV; increment potential: 5 mV; quiet time: 10 s.
J. Li et al. / Analytica Chimica Acta 649 (2009) 196–201
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Fig. 4. Effect of the accumulation potential (A) and deposition time (B) on the stripping peak current of 20 g L−1 each of Cd2+ and Pb2+ on an in situ plated Nafion-G-BFE modified electrode in solution containing 1 mg L−1 Bi3+ . Other conditions are identical to Fig. 3.
a powerful technique, was used to obtain the information on the impedance changes of the electrode surface. Compared with that of the pure Nafion modified electrode, the electron-transfer resistances of the Nafion-G sensing platform was reduced greatly (not shown), which indicated that the interfusion of the graphene into the Nafion film greatly reduced electron-transfer-limited process due to the high conductivity resulting from the extensive conjugated sp2 carbon network [32]. 3.3. The effect of the experimental variables The effect of the deposition potential on the peak current of Cd2+ and Pb2+ after 120 s accumulation was studied in the potential range from −1.3 V to −0.9 V and the obtained results are shown in Fig. 4(A). When the accumulation potential shifts from −0.9 V to −1.1 V, the stripping peak currents increased. As the accumulation potential became more negative, the peak current reduced more completely. The different trends observed for Cd2+ and Pb2+ may be attributed to the different standard potentials. To obtain the good sensitivity for both Cd2+ and Pb2+ , −1.2 V was used as the optimal accumulation potential for the subsequent experiment. The sensitivity of the proposed method was undoubtedly improved by the accumulation time. As depicted in Fig. 4(B), with the increase of the deposition time, the response of the stripping peak currents of 20 g L−1 Cd2+ and Pb2+ increased linearly with the preconcentration time due to the increased amount of analytes on the Nafion-G modified electrode. However, the plot tended to be curved with diminished slope value up to 120 s because of the rapid surface saturation, which lowers the upper detection limit. Therefore, 120 s was chosen as the optimal accumulation time. The concentration of bismuth controlled the thickness of the bismuth film, which had a profound influence upon the DPASV response. The effect of the bismuth ion concentration on the stripping peak currents of Cd2+ and Pb2+ was studied (Fig. 5). The heights of peaks Cd2+ and Pb2+ increased rapidly upon raising the bismuth concentration from 0 mg L−1 to 0.4 mg L−1 and then decrease slightly above to the 0.4 mg L−1 . It indicated too high concentrations of bismuth ions (>0.4 mg L−1 ) led to the thicker film, which hindered the mass transfer of metal ions during the stripping step. Therefore, the optimized bismuth concentration was chosen as 0.4 mg L−1 to get the good sensitivity.
achieved in 0.1 M acetate buffer containing 0.4 mg L−1 Bi3+ solutions by increasing Cd2+ and Pb2+ concentrations from 0.5 g L−1 to 50 g L−1 . The DPASV response for different concentrations of Cd2+ and Pb2+ were illustrated in Fig. 6. The resulting calibration plots are linear over the range from 0.5 g L−1 to 50 g L−1 and 1.5 g L−1 to 30 g L−1 for Pb2+ and Cd2+ with a preconcentration time of 120 s, respectively. The calibration curves and correlation coefficients are y = 0.95x + 1.67, R = 0.993 for Pb2+ , y = 1.13x − 0.67, R = 0.991 for Cd2+ , respectively (x: concentration/g L−1 , y: current/A). Based on three times the background noise, the limits of detection were 0.02 g L−1 for Cd2+ and Pb2+ with a deposition time of 5 min, which are more sensitive than those of Nafion film modified bismuth electrode (0.1 g L−1 for Cd2+ and Pb2+ [33]), and ordered mesoporous carbon coated GCE (0.84 g L−1 for Pb2+ [16]). When compared with the Nafion/CNT coated bismuth film electrode (25 ng L−1 for Pb2+ and 40 ng L−1 for Cd2+ [15]), the present method shows the comparable sensitivity. 3.5. Application As is known, the surface-active macromolecules have a profound effect on the stripping responses of BFEs [34] and the fouling effect of surfactants can be circumvented by the use of the protective
3.4. Analytical performance Calibration plots for the simultaneous determination of Cd2+ and Pb2+ on the Nafion-G modified based on the in situ plated BFE were
Fig. 5. Effect of the bismuth concentration on the stripping peak current of 20 g L−1 each of Cd2+ and Pb2+ on Nafion-G modified electrode. Other conditions are identical to Fig. 3.
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Fig. 6. Striping voltammograms for the different concentrations of Cd2+ and Pb2+ on an in situ plated Nafion-G-BFE in solution containing 0.4 mg L−1 Bi3+ . From bottom to top, 0.5 g L−1 , 1.5 g L−1 , 4 g L−1 , 6 g L−1 , 10 g L−1 , 20 g L−1 , 30 g L−1 , and 40–50 g L−1 . (Inset) the calibration curve of Cd2+ and Pb2+ , respectively. Other conditions are identical to Fig. 3.
Fig. 7. Effect of different type of surfactants on the stripping responses for 20 g L−1 Cd (A) and Pb (B) on an in situ plated Nafion-G-BFE in solution containing 0.4 mg L−1 Bi3+ . Other conditions are identical to Fig. 3.
coating. In order to verify the practical applications of the Nafion-G modified electrode, the influence of different type of surfactants on the stripping voltammetric response was investigated. The peak currents of metal ions before (I0 ) and after adding different concentrations of surfactants (Ip ) were recorded in Fig. 7. It is obvious in most cases the Cd2+ stripping peak current was influenced drastically compared with the Pb2+ by the three surfactants. In addition, Triton X-100 produced the severe interference upon the current signal among these surfactants, which is consistent with the previous report [35]. However, as for the sodium dodecyl sulfate (SDS), it led to the least effect on the current signal. While for the cationic surfactant, cetyltrimethylammonium bromide (CTAB), it gave moderate decrease in the current signal. In addition, the effect of the surfactants on the different film coated GCE was compared. As depicted in Table 1, the Nafion-G-BFE presented the better antifouling ability to surfactants compared with that of the Nafion-BFE, especially for CTAB. The reason for this may be attributed to the different nature and structure of the bismuth film electrode [34]. All these results indicate that the electrode modified with the Nafion-G composite film can be used in the practical application with low concentration of surfactant. The as-prepared electrode was finally applied to the determination of Cd2+ and Pb2+ in the real samples. The concentration of Cd2+ and Pb2+ in the lake water sample was 0.47 g L−1 and 0.37 g L−1 using the standard addition method, respectively. With
Table 1 The different effects of surfactant on the different modified electrodes containing 12 mg L−1 Triton X-100, CTAB and SDS. Metal ion
Electrode
Ip /Io /% Triton X-100
CTAB
SDS
Cd(II)
Nafion-BFE Nafion-G-BFE
25 35
30 89
88 105
Pb(II)
Nafion-BFE Nafion-G-BFE
23 45
48 95
95 101
the confidence level (P = 95%), the confidence interval for the Pb2+ and Cd2+ in the lake water were calculated to be 0.47 ± 0.05 g L−1 and 0.37 ± 0.09 g L−1 , respectively. The data obtained by ICP-MS was 0.52 ± 0.03 g L−1 for Pb2+ and 0.45 ± 0.06 g L−1 for Cd2+ . The two methods provide consistent results. Therefore, the as-prepared electrode could act as an effective method for the determination of Cd2+ and Pb2+ in real samples. 4. Conclusions A new and highly enhanced sensing platform based on the Nafion-graphene composite film was established for the simultaneous determination of Cd2+ and Pb2+ by anodic stripping
J. Li et al. / Analytica Chimica Acta 649 (2009) 196–201
voltammetry. The composite film combining the advantages of graphene (e.g., strong adsorptive capability and huge specific surface area due to the nanosized graphene sheet and nanoscale thickness of these sheets and good conductivity) and the cationic exchange capacity of Nafion enhanced the sensitivity for metal ion assays compared with that of Nafion coated electrodes. Furthermore, the Nafion-G-BFE acted as the better protecting coating to alleviate the fouling effect of surfactants. The practical analytical application of the Nafion-G modified electrode was assessed by measurement of the local lake water sample and the result was consistent with the results obtained by ICP-MS. Acknowledgements This work is supported by 863 Project 2007AA061501, 973 Project 2009CB930100 as well as the support of 2006BAE03B08 from MOST and the Innovation Method fund of China (2008IM040100). References [1] J.A. Lewis, S.M. Cohen, Inorg. Chem. 43 (2004) 6534. [2] B.M. Bridgewater, G. Parkin, J. Am. Chem. Soc. 122 (2000) 7140. [3] J. Wang, Stripping Analysis: Principles, Instrumentation and Applications, VCH, Deerfield Beach, FL, 1985. [4] H.P. Wu, Anal. Chem. 68 (1996) 1639. [5] G.S. Sanna, M.I. Pilo, P.C. Piu, A. Tapparo, R. Seeber, Anal. Chim. Acta 415 (2000) 165. [6] A. Economou, P.R. Fielden, Analyst 128 (2003) 205. [7] J. Wang, J. Lu, S.B. Hocoeevar, P.A.M. Farias, B. Ogorevc, Anal. Chem. 72 (2000) 3218. [8] J. Wang, J. Lu, S.B. Hocoeevar, B. Ogorevc, Electroanalysis 13 (2001) 13.
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High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film Jing Li a,b , Shaojun Guo a,b , Yueming Zhai a,b , Erkang Wang a,∗ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
a r t i c l e
i n f o
Article history: Received 19 May 2009 Received in revised form 12 July 2009 Accepted 13 July 2009 Available online 19 July 2009 Keywords: Nafion-graphene Nanosheets Lead Cadmium Bismuth
a b s t r a c t Graphene nanosheets, dispersed in Nafion (Nafion-G) solution, were used in combination with in situ plated bismuth film electrode for fabricating the enhanced electrochemical sensing platform to determine the lead (Pb2+ ) and cadmium (Cd2+ ) by differential pulse anodic stripping voltammetry (DPASV). The electrochemical properties of the composite film modified glassy carbon electrode were investigated. It is found that the prepared Nafion-G composite film not only exhibited improved sensitivity for the metal ion detections, but also alleviated the interferences due to the synergistic effect of graphene nanosheets and Nafion. The linear calibration curves ranged from 0.5 g L−1 to 50 g L−1 for Pb2+ and 1.5 g L−1 to 30 g L−1 for Cd2+ , respectively. The detection limits (S/N = 3) were estimated to be around 0.02 g L−1 for Pb2+ and Cd2+ . The practical application of the proposed method was verified in the water sample determination. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As is well known, lead and cadmium pose severe risks to human health with toxic effects on living organism. For example, the toxicity of lead in humans mainly comes from its detrimental mimicking action by occupying the calcium binding sites on numerous calcium-dependent proteins in cells (such as, calmodulin and enzyme protein kinase C), thus resulting in the corresponding impairment of physiological functions [1,2]. Accordingly, exploring the sensitive, rapid and simple analytical method for precise monitoring of Pb2+ and Cd2+ is urgently needed. The usual methods adopted for the concentration assessments of the metal ions are mainly focused on the use of atomic absorption or inductively coupled plasma (ICP) atomic emission spectroscopy, ICP-mass spectrometry and electrochemical (EC) techniques. However, spectroscopy methods are somewhat cumbersome and not suitable for the in situ measurement due to the ponderous and complicated instruments. On the contrary, EC techniques have attracted growing interests due to high-sensitivity, portability and low cost. Among all the EC methods, electrochemical stripping voltammetric analysis provides a powerful tool for the determination of metal ions [3], which possess high-sensitivity for the metal analysis due to
∗ Corresponding author at: State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun Jilin, 130022, China. Tel.: +86 431 85262003; fax: +86 431 85689711. E-mail address: [email protected] (E. Wang). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.07.030
the built-in preconcentration step and is very suitable for on-site and in situ analysis [4,5]. Most commonly, mercury based electrodes including hanging drop mercury electrode and mercury film electrodes were adopted because of excellent reproducibility and high-sensitivity [6]. However, due to the dramatic toxicity of mercury, numerous attempts have been made to replace it with new mercury-free and reliable electrode. Recently, the bismuth film electrode (BFE) has drawn the increasing attentions in the field of the stripping technique due to the remarkably low toxicity and the ability to form alloy with many metals as well as its wide potential window, which has proved to be equal to or even superior to that of MFEs [7–9]. On the other hand, in order to alleviate interferences and improve the sensitivity of the sensing interface, some strategies have also been exploited for the stripping analysis, such as, the chemical modified electrode (CME), heated electrodes, microwaved electrodes, and insonated electrodes [10–12]. Among these, CME capable of accumulating target analytes from dilute aqueous solution has been developed as the fascinating and effective way for the anodic stripping voltammertry (ASV) determination of heavy metal ions. Prominent examples include functionalized mesoporous silica electrode [13], carbon nanotubes (CNT) modified electrode [14,15], ordered mesoporous carbon [16], acetylene black paste electrode [17], nanocrystalline diamond thin-film electrode [18], and thickfilm modified graphite-containing electrode [19]. Although all the above material showed improved stripping signals, especially the CNT modified electrode prepared by Jin’s group [15], new materials are still needed to develop highly sensitive and antifouling metal ions sensing platform.
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Graphene, a single atomic plane of graphite packed into a dense honeycomb crystal structure, was novel and fascinating carbon material since experimentally produced in 2004 [20]. Much research effort has been made to explore its fascinating applications in electroanalytical chemistry or electrochemistry as novel electrode material for various purposes due to the excellent electrical and mechanical properties [21–25]. For instance, Li’s group [21,22] has demonstrated the use of the graphene nanosheets to develop a high-performance electrochemical sensor for dopamine and electrocatalytic oxidation of methanol. Worden et al. [24,25] has also fabricated an excellent glucose biosensor based on the exfoliated graphite nanoplatelets as a viable and inexpensive filler alternative to CNT. Recently, our group [26] developed a Cd2+ sensing platform with the MFE based on the graphene nanosheets. Although, the sensing platform showed ultra-sensitivity for the detection of Cd2+ , its wide use was limited due to the toxicity of the mercury. In this work, graphene nanosheets were used in combination with in situ BFE to fabricate a sensitive and mercury-free electrochemical platform for the analysis of the Pb2+ and Cd2+ . Herein, Nafion not only acts as an effective solubilizing agent for graphene nanosheets (Nafion-G), but also as an antifouling coating to reduce the influence of the surface-active macromolecules. This electrochemical sensing interface exhibited excellent stripping performance for the analysis of Pb2+ and Cd2+ combining the advantageous of the graphene nanosheets (higher electrical conductivity, huge specific area) together with the unique features of the in situ plating BFE. 2. Experimental 2.1. Reagents Nafion (5 wt.% in low aliphatic alcohols), was purchased from Aldrich (Milwaukee, WI), and then diluted to 1 wt.% Nafion with 2-propanol. Stock solutions of Pb2+ and Cd2+ were prepared by diluting the corresponding standard stock solutions prepared with CdCl2 and Pb(NO3 )2 , respectively. Bi(NO3 )3 was used for the BFE by diluting the corresponding standard stock solution. A 0.1 M acetate buffer (pH 4.5) prepared by mixing appropriate amounts of CH3 COOH and CH3 COONa, was served as to prepare solution of the supporting electrolyte. Unless otherwise stated, all solutions were prepared with double-distilled water. All chemicals employed in this work were of analytical reagent grade and used as received. 2.2. Apparatus Differential pulse ASV (DPASV) was performed in a conventional three-electrode cell with a CHI 842B electrochemical workstation (CH Instruments, Shanghai, China). The modified glass carbon (GC, 3 mm in diameter) electrode with the Nafion-graphene was served as the working electrode. An Ag/AgCl (saturated. KCl) electrode and a platinum foil were used as the reference electrode and auxiliary electrode, respectively. Electrochemical impedance spectroscopy (EIS) was measured with an Autolab/PG30 electrochemical analyzer system (ECO Chemie B.V., Netherlands) in a grounded Faraday cage at ambient condition. The dc potential was 0.24 V vs. Ag/AgCl (saturated KCl), and the conductivity was determined by ac impedance in the frequency range between 0.01 Hz and 100,000 Hz with a perturbation signal of 5 mV. All electrochemical experiments were carried out in a one-compartment electrochemical cell. Atomic force microscopy (AFM) measurement was performed on the mica with a SPI3800N to characterize the graphene nanosheets. 2.3. Preparation of the graphene solution Firstly, the graphite oxide was synthesized from natural graphite powder according to the literature with a little modification [27,28].
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Then, exfoliation of graphite oxide to graphene oxide was achieved by ultrasonication of the dispersion for 40 min (1000 W, 20% amplitude) according to the reference with a slight modification [29]. Finally, a homogeneous aqueous dispersion (0.5 mg mL−1 ) was obtained. In a typical procedure for chemical conversion of graphene oxide to graphene, the resulting homogeneous dispersion (100 mL) was mixed with 70 L of hydrazine solution (50 wt.% in water) and 0.7 mL of ammonia solution (28 wt.% in water). After being vigorously shaken or stirred for a few minutes, the solution was stirred for 1 h at the temperature of 95 ◦ C. 2.4. Fabrication of the Nafion-G composite film Ultrasonic agitation for ca.30 min was used to disperse the graphene nanosheets into 100 L 1 wt.% Nafion-isopropyl-alcohol to give 0.25 mg mL−1 Nafion-G suspension. Prior to the surface modification, the bare GC electrode was polished carefully with 1.0 m, 0.3 m and 0.05 m alumina powder, respectively, and rinsed with deionized water, followed by sonicated in acetone and doubly distilled water successively and dried under nitrogen. Then, an aliquot of 5 L of the mixture was coated on the electrode, and then the solvent was evaporated under an infrared lamp to obtain the Nafion-G composite film modified GC electrode. 2.5. Procedure for DPASV analysis The three electrodes were immersed into the electrochemical cell, containing 5 mL 0.1 M acetate buffer (pH 4.5), Bi3+ and the target metals ions. The Nafion-G modified GC electrode with bismuth film was plated in situ by spiking the sample with the required concentration of Bi3+ and simultaneously depositing Bi and the target metals on the surface of the electrode at −1.2 V under stirring for 120 s. Following the preconcentration step, the stirring was stopped and after 10 s, the voltammogram was recorded by applying a positive-going differential pulse voltammetry scan (with a step increment of 5 mV, amplitude of 80 mV, and pulse period of 0.2 s). The scan was terminated at 0 V. Prior to the next cycle, a preconditioning step (60 s at 0.3 V, with solution stirring) was used to remove the target metals and Bi. The lake water was obtained in Changchun and was filtered through a 0.22 m membrane (Millipore). For the DPASV analysis, 9 mL of lake water and 1 mL of 1.0 M acetate buffer (pH 4.5) were mixed. 3. Results and discussion 3.1. Morphologic characterization of the graphene nanosheets The structure and morphology of the resulting graphene deposited on the mica were characterized by AFM. The results (Fig. 1) found that the graphene sheets were almost single-layer. And the average thickness of single-layer graphene sheets was <1 nm. This unique nanostructure may be favourable as potential material for the electrochemical determination of Pb2+ and Cd2+ . 3.2. Electrochemical characterization of the Nafion-G composite film Electrochemical experiments were used to characterize the Nafion-G composite film. Different cyclic voltammograms (CVs) at bare, Nafion, and Nafion-G modified GC electrodes in a freshly prepared solution containing 1.0 mM K3 Fe(CN)6 in the presence of 1.0 M KCl are shown in Fig. 2. A well-shaped CV with a peak-to-peak separation of 77 mV was observed at the bare GC electrode (solid line). After being modified with Nafion (dashed line), the anodic and cathodic peaks almost disappeared similar to that reported in previous work [30], demonstrating that the negative charged
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Fig. 2. CVs for bare (solid line), Nafion (dashed line), Nafion-G (dotted line) modified GC electrodes in 1.0 mM K3 Fe(CN)6 in 1.0 M KCl, scan rate, 50 mV s−1 .
Fig. 1. AFM image of graphene on freshly cleaved mica.
skeleton of Nafion acted as the blocking layer for the diffusion of Fe(CN)6 3− into the film and hindered the electron and mass transfer. However, at the Nafion-G modified GC electrode (dotted line), the peak current increases obviously compared with that observed at Nafion modified GC electrode, indicating that the introduction of the graphene provided the conducting bridges for the electrontransfer of Fe(CN)6 3− . Fig. 3(A) shows the DPASV analytical characteristics of different film coated GC electrodes, e.g., Nafion (dotted line), Nafion-G (solid line) by in situ plated BFE for Pb2+ and Cd2+ determination. The stripping voltammograms were obtained in a solution containing 20 g L−1 each of Pb2+ , Cd2+ , 1 mg L−1 Bi3+ in 0.1 M acetate buffer (pH 4.5) without deaeration. The sharper and higher peak current for the target metal ions were obtained at the Nafion-G
modified electrode. Compared with the Nafion coated electrode, signals were improved about 79%. While under the same condition, the carbon nanotubes/Nafion modified electrode showed ca. 69% improvement. It is indicated that the graphene show the similar behaviour to the CNTs. As to the signal enhancement, it is can be attributed to two factors: (i) the change of the morphologies and the structure, and (ii) the interfacial electron-transfer properties. In order to verify, the scanning electron microscopy (SEM) and ESI were carried out. From the results of the SEM (Fig. 3B), it could be seen that the morphologies and the structure of pure Nafion and Nafion-G were distinctly different. The pure Nafion showed a flat and homogeneous film and the Bi3+ was firstly exchanged into the Nafion film and become nuclei during the electrodeposition procedure. However, when the electrode was coated with the Nafion-G composite film, a rough and stratified structure was formed, which endowed the more effective active area for the nucleation of the bismuth due to the unique structure. As the deposition time increased, bismuth nanofilm was expected to be thick in a non-uniform way as it grows preferentially on top of already deposited nuclei [31]. ESI,
Fig. 3. (A) DPASVs for 20 g L−1 each of Cd2+ and Pb2+ on an in situ plated Nafion-BFE, Nafion-G-BFE in solution containing 1 mg L−1 Bi3+ , (B) SEM image of Bi film deposited on the Nafion (a) and Nafion-G (b) modified GCE. Supporting electrolyte: 0.1 M acetate buffer (pH 4.5); deposition potential: −1.2 V; deposition time: 120 s; amplitude: 80 mV; increment potential: 5 mV; quiet time: 10 s.
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Fig. 4. Effect of the accumulation potential (A) and deposition time (B) on the stripping peak current of 20 g L−1 each of Cd2+ and Pb2+ on an in situ plated Nafion-G-BFE modified electrode in solution containing 1 mg L−1 Bi3+ . Other conditions are identical to Fig. 3.
a powerful technique, was used to obtain the information on the impedance changes of the electrode surface. Compared with that of the pure Nafion modified electrode, the electron-transfer resistances of the Nafion-G sensing platform was reduced greatly (not shown), which indicated that the interfusion of the graphene into the Nafion film greatly reduced electron-transfer-limited process due to the high conductivity resulting from the extensive conjugated sp2 carbon network [32]. 3.3. The effect of the experimental variables The effect of the deposition potential on the peak current of Cd2+ and Pb2+ after 120 s accumulation was studied in the potential range from −1.3 V to −0.9 V and the obtained results are shown in Fig. 4(A). When the accumulation potential shifts from −0.9 V to −1.1 V, the stripping peak currents increased. As the accumulation potential became more negative, the peak current reduced more completely. The different trends observed for Cd2+ and Pb2+ may be attributed to the different standard potentials. To obtain the good sensitivity for both Cd2+ and Pb2+ , −1.2 V was used as the optimal accumulation potential for the subsequent experiment. The sensitivity of the proposed method was undoubtedly improved by the accumulation time. As depicted in Fig. 4(B), with the increase of the deposition time, the response of the stripping peak currents of 20 g L−1 Cd2+ and Pb2+ increased linearly with the preconcentration time due to the increased amount of analytes on the Nafion-G modified electrode. However, the plot tended to be curved with diminished slope value up to 120 s because of the rapid surface saturation, which lowers the upper detection limit. Therefore, 120 s was chosen as the optimal accumulation time. The concentration of bismuth controlled the thickness of the bismuth film, which had a profound influence upon the DPASV response. The effect of the bismuth ion concentration on the stripping peak currents of Cd2+ and Pb2+ was studied (Fig. 5). The heights of peaks Cd2+ and Pb2+ increased rapidly upon raising the bismuth concentration from 0 mg L−1 to 0.4 mg L−1 and then decrease slightly above to the 0.4 mg L−1 . It indicated too high concentrations of bismuth ions (>0.4 mg L−1 ) led to the thicker film, which hindered the mass transfer of metal ions during the stripping step. Therefore, the optimized bismuth concentration was chosen as 0.4 mg L−1 to get the good sensitivity.
achieved in 0.1 M acetate buffer containing 0.4 mg L−1 Bi3+ solutions by increasing Cd2+ and Pb2+ concentrations from 0.5 g L−1 to 50 g L−1 . The DPASV response for different concentrations of Cd2+ and Pb2+ were illustrated in Fig. 6. The resulting calibration plots are linear over the range from 0.5 g L−1 to 50 g L−1 and 1.5 g L−1 to 30 g L−1 for Pb2+ and Cd2+ with a preconcentration time of 120 s, respectively. The calibration curves and correlation coefficients are y = 0.95x + 1.67, R = 0.993 for Pb2+ , y = 1.13x − 0.67, R = 0.991 for Cd2+ , respectively (x: concentration/g L−1 , y: current/A). Based on three times the background noise, the limits of detection were 0.02 g L−1 for Cd2+ and Pb2+ with a deposition time of 5 min, which are more sensitive than those of Nafion film modified bismuth electrode (0.1 g L−1 for Cd2+ and Pb2+ [33]), and ordered mesoporous carbon coated GCE (0.84 g L−1 for Pb2+ [16]). When compared with the Nafion/CNT coated bismuth film electrode (25 ng L−1 for Pb2+ and 40 ng L−1 for Cd2+ [15]), the present method shows the comparable sensitivity. 3.5. Application As is known, the surface-active macromolecules have a profound effect on the stripping responses of BFEs [34] and the fouling effect of surfactants can be circumvented by the use of the protective
3.4. Analytical performance Calibration plots for the simultaneous determination of Cd2+ and Pb2+ on the Nafion-G modified based on the in situ plated BFE were
Fig. 5. Effect of the bismuth concentration on the stripping peak current of 20 g L−1 each of Cd2+ and Pb2+ on Nafion-G modified electrode. Other conditions are identical to Fig. 3.
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Fig. 6. Striping voltammograms for the different concentrations of Cd2+ and Pb2+ on an in situ plated Nafion-G-BFE in solution containing 0.4 mg L−1 Bi3+ . From bottom to top, 0.5 g L−1 , 1.5 g L−1 , 4 g L−1 , 6 g L−1 , 10 g L−1 , 20 g L−1 , 30 g L−1 , and 40–50 g L−1 . (Inset) the calibration curve of Cd2+ and Pb2+ , respectively. Other conditions are identical to Fig. 3.
Fig. 7. Effect of different type of surfactants on the stripping responses for 20 g L−1 Cd (A) and Pb (B) on an in situ plated Nafion-G-BFE in solution containing 0.4 mg L−1 Bi3+ . Other conditions are identical to Fig. 3.
coating. In order to verify the practical applications of the Nafion-G modified electrode, the influence of different type of surfactants on the stripping voltammetric response was investigated. The peak currents of metal ions before (I0 ) and after adding different concentrations of surfactants (Ip ) were recorded in Fig. 7. It is obvious in most cases the Cd2+ stripping peak current was influenced drastically compared with the Pb2+ by the three surfactants. In addition, Triton X-100 produced the severe interference upon the current signal among these surfactants, which is consistent with the previous report [35]. However, as for the sodium dodecyl sulfate (SDS), it led to the least effect on the current signal. While for the cationic surfactant, cetyltrimethylammonium bromide (CTAB), it gave moderate decrease in the current signal. In addition, the effect of the surfactants on the different film coated GCE was compared. As depicted in Table 1, the Nafion-G-BFE presented the better antifouling ability to surfactants compared with that of the Nafion-BFE, especially for CTAB. The reason for this may be attributed to the different nature and structure of the bismuth film electrode [34]. All these results indicate that the electrode modified with the Nafion-G composite film can be used in the practical application with low concentration of surfactant. The as-prepared electrode was finally applied to the determination of Cd2+ and Pb2+ in the real samples. The concentration of Cd2+ and Pb2+ in the lake water sample was 0.47 g L−1 and 0.37 g L−1 using the standard addition method, respectively. With
Table 1 The different effects of surfactant on the different modified electrodes containing 12 mg L−1 Triton X-100, CTAB and SDS. Metal ion
Electrode
Ip /Io /% Triton X-100
CTAB
SDS
Cd(II)
Nafion-BFE Nafion-G-BFE
25 35
30 89
88 105
Pb(II)
Nafion-BFE Nafion-G-BFE
23 45
48 95
95 101
the confidence level (P = 95%), the confidence interval for the Pb2+ and Cd2+ in the lake water were calculated to be 0.47 ± 0.05 g L−1 and 0.37 ± 0.09 g L−1 , respectively. The data obtained by ICP-MS was 0.52 ± 0.03 g L−1 for Pb2+ and 0.45 ± 0.06 g L−1 for Cd2+ . The two methods provide consistent results. Therefore, the as-prepared electrode could act as an effective method for the determination of Cd2+ and Pb2+ in real samples. 4. Conclusions A new and highly enhanced sensing platform based on the Nafion-graphene composite film was established for the simultaneous determination of Cd2+ and Pb2+ by anodic stripping
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voltammetry. The composite film combining the advantages of graphene (e.g., strong adsorptive capability and huge specific surface area due to the nanosized graphene sheet and nanoscale thickness of these sheets and good conductivity) and the cationic exchange capacity of Nafion enhanced the sensitivity for metal ion assays compared with that of Nafion coated electrodes. Furthermore, the Nafion-G-BFE acted as the better protecting coating to alleviate the fouling effect of surfactants. The practical analytical application of the Nafion-G modified electrode was assessed by measurement of the local lake water sample and the result was consistent with the results obtained by ICP-MS. Acknowledgements This work is supported by 863 Project 2007AA061501, 973 Project 2009CB930100 as well as the support of 2006BAE03B08 from MOST and the Innovation Method fund of China (2008IM040100). References [1] J.A. Lewis, S.M. Cohen, Inorg. Chem. 43 (2004) 6534. [2] B.M. Bridgewater, G. Parkin, J. Am. Chem. Soc. 122 (2000) 7140. [3] J. Wang, Stripping Analysis: Principles, Instrumentation and Applications, VCH, Deerfield Beach, FL, 1985. [4] H.P. Wu, Anal. Chem. 68 (1996) 1639. [5] G.S. Sanna, M.I. Pilo, P.C. Piu, A. Tapparo, R. Seeber, Anal. Chim. Acta 415 (2000) 165. [6] A. Economou, P.R. Fielden, Analyst 128 (2003) 205. [7] J. Wang, J. Lu, S.B. Hocoeevar, P.A.M. Farias, B. Ogorevc, Anal. Chem. 72 (2000) 3218. [8] J. Wang, J. Lu, S.B. Hocoeevar, B. Ogorevc, Electroanalysis 13 (2001) 13.
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