- Email: [email protected]
Adsorptive stripping voltammetric detection of thorium on the multi-walled carbon nanotube modified screen printed electrode
Sensors and Actuators B 220 (2015) 1212–1216
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Adsorptive stripping voltammetric detection of thorium on the multi-walled carbon nanotube modified screen printed electrode Nasim Soltani a , Hedayat Haddadi a,c , Mehdi Asgari b,∗ , Nematollah Rajabzadeh b a b c
Department of Chemistry, Faculty of Sciences, Shahrekord University, Shahrekord, Iran NFCRI, NSTRI, Tehran, Iran Nanotechnology Center Research, Shahrekord University, 8818634141 Shahrekord, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2015 Received in revised form 5 June 2015 Accepted 12 June 2015 Available online 26 June 2015 Keywords: Thorium Oxine complex Screen printed electrode Multi-walled carbon nanotubes Stripping voltammetry
a b s t r a c t In this article, the application of a novel modified screen-printed electrode (SPE) to thorium detection is described. Multi-walled carbon nanotubes (MWCNTs) modified screen-printed electrodes present an interesting alternative for the determination of thorium (IV). This method can be described by square wave adsorptive stripping voltammetry of the thorium complex with oxine at a screen printed electrode. In comparison with the bare SPE, the modifications of MWCNTs were demonstrated providing more sensitive responses. The pH adjusted to 3.0, and adsorptive voltammetric peak regarding the adsorption of thorium–oxine in the complex occur at −0.3 V with adsorptive-preconcentration. The electrode displayed excellent linear behavior in the concentration range examined (0.5 × 10−9 to 2.2 × 10−8 M) with a limit of quantitation (LOQ) of 0.5 nM and limit of detection (LOD) by re-examining the data is 0.17 nM. The method has been successfully applied to the determination of trace Th(IV) in different water samples. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ions represent a major environmental problem and their detection and monitoring in waste water outlets, rivers, reservoirs or sources of drinking water is necessary. Thorium is a natural element that has a number of industrial (lantern mantles, welding operations, and preparation of refractories) and medical (thorotrast imaging agent) applications [1]. It has been reported that inhaling thorium dust causes an increased risk of developing lung and pancreas cancer. Bone cancer risk is also increased because thorium may be stored in the bones [2–9]. Many methods have already been developed for determination of thorium in real samples based on the chemical or physical principles [10]. The majority of thorium measurements were performed using ICP, atomic emission spectrometry [11], ICP-MS [12–14], ion selective electrode [15,16], IC [17], capillary zone electrophoresis [18], cation exchange resin [19], extraction [20], and ␥-ray spectrometry [21]. Polarography and voltammetry are particularly attractive due to the simplicity of the technique and the low cost instrumentation. However, because of the extremely negative reduction potentials of thorium, it cannot be determined by direct polarographic analysis [22]. However, indirect polarographic
procedures based on complexation with electroactive organic ligands such as cupferron, alizarin S, xylidyl blue I, alizarin complexion (ALC), etc. have been reported [23–26]. Various methods have been reported for the determination of thorium by polarographic complex adsorption wave with different reagents: alizarin red S (ARS), cupferron, xylidyl blue I and chrome azurol S [27–30]. Also the adsorptive voltammetric procedures for trace measurement of thorium at mercury electrode and CPE have been reported [31–34]. Screen printed technology is a rapid and cost reducing way to fabricate robust and solid electrodes. It offers several advantages, such as versatility of the design, reproducibility in the sensor preparation and low cost production [35,36], which permits the sensors to be disposed after a single use. The present article determines Th(IV) by square wave adsorptive stripping voltammetry (SWASV) using multi-walled carbon nanotubes and oxine modified screen-printed electrodes. On the basis of our literature review, this presents the first electrochemical detection system of Th(IV) with this type of nano-modified electrodes.
2. Experimental 2.1. Reagents and apparatus
∗ Corresponding author. Tel.: +98 21 88221117; fax: +98 21 88221116. E-mail address: [email protected] (M. Asgari). http://dx.doi.org/10.1016/j.snb.2015.06.106 0925-4005/© 2015 Elsevier B.V. All rights reserved.
A 1.00 × 10−2 M thorium standard stock solution was prepared by dissolving 0.294 g of Th(NO3 )4 ·6H2 O (Merck company) in 1.0 M
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
1213
Fig. 1. FE-SEM image of SPE/MWCNTs modified electrode.
HCl and diluting the solution to 50 ml with 1.0 M HCl. All solutions were prepared utilizing distilled water. MWCNTs with 95% purity (30–50 nm diameters and 3 m length) were obtained from Timesnano Co. Ltd. (China). The electrochemical experiments were carried out by means of an Autolab potentiostate/galvanostate 30(2) (ECO Chemie, Netherlands), equipped with a personal computer used for data storage and processing. A screen-printed carbon electrode (SPE) (3 mm in diameter) from Dropsens (Spain) was used as a planar three electrode based on a graphite working electrode, a carbon counter electrode and a silver pseudo-reference electrode. 2.2. Purification and activation of MWCNTs MWCNTs were purified and activated before use, as follows: the MWCNTs were first treated with reflux in a 1:3 (v/v) mixture of HNO3 (69%) and H2 SO4 (98%) at 85 ◦ C for 4 h. The product was centrifuged, washed with ethanol. Then the MWCNTs were dried under vacuum at 50 ◦ C overnight. 2.3. Modifications of screen-printed electrodes Before any modification, the SPEs were pretreated by applying a potential of −0.4 to +1.5 V in HCl 0.10 M using scan rate of 50 mV s−1 for 20 cycles. MWCNTs (10 mg) and 5 L of 5.0% nafion solution were dispersed in 5 mL solution containing 0.02 g oxine and 2-metyl ethanol utilizing ultrasonication for 1 h to get a homogenous suspension. 10 L of the suspension was cast onto the working section of screen printed electrode and dried at room temperature. Prior to the experiments a drop (50 l of total volume) of the sample solution was applied to the reaction region of modified screen printed electrode. 3. Results and discussion 3.1. Surface characterization The surface morphology of the SPE/MWCNTs was characterized by FE-SEM micrographs (Fig. 1). The structure of the composite showed a homogeneous film on the surface of SPE, which plays an important role in enhancing electro-conductivity of the film. Such an electrode exhibits an excellent stability, a broad surface coverage and a good contact among the SPE, oxine and the SWV of thorium by various electrochemical methods. MWCNTs have excellent mechanical strength with superior heat and electric conductivity. They also have high specific surface area,
Fig. 2. (A) Square wave voltammograms of 1.0 × 10−6 M thorium nitrate after 300 s of accumulation on the modified SPE/MWCNTs-oxine modified electrode (a) and the voltammograms of thorium on the SPE/MWCNTs modified electrode after 300 s accumulation time (b) and square wave voltammogram of thorium in the presence of oxine, without preconcentration on the SPE/MWCNTs-oxine modified electrode (c) with a pH of 3.0 solution. The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V. (B) Square wave voltammograms of 1.0 × 10−6 M thorium nitrate after 300 s of accumulation on the modified SPE/MWCNTs-oxine modified electrode in varied scan rates.
1214
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
Fig. 3. Effect of pH upon the stripping voltammetric response of 1.0 × 10−6 M thorium nitrate on the SPE/MWCNTs-oxine modified electrode. pHs were 1 (a), 2 (b), 3.1 (c), 4 (d), 4.5 (e), 5 (f) and 5.4 (g). The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V.
high crystallinity, and high length-to-diameter ratio. Presence of MWCNT on the surface of the electrode caused increase conductivity of electrode and decrease the charge transfer between electrode and electrolyte. 3.2. Voltammetric behavior of thorium on the SPE/MWCNTs-oxine Fig. 2A shows the square wave voltammetry of 1.0 × 10−6 M thorium between −0.8 and 0.2 V. Curve ‘a’ displays the square wave voltammograms of thorium after 300 s of adsorptive accumulation on the SPE/MWCNTs-oxine modified electrode. The voltammogram of thorium in the absence of oxine after 300 s adsorptive accumulation time on the modified SPE (curve ‘b’) is also shown. Curve ‘c’ shows the square wave voltammogram of thorium in the presence of oxine, without preconcentration on the modified SPE. A comparison of the voltammograms show that the height of thorium–oxine complex reduction peak depends on the duration of adsorptive preconcentration step and also on the presence of oxine, which reveals the adsorptive nature of the response. The experiment parameters were chose according to experiments. The results related to variation of scan rates were shown in Fig. 1B. 3.3. Optimization of operational parameters 3.3.1. Effect of pH The influence of pH on the stripping peak current is studied in the range 1–5.4 (Fig. 3). It is found that at a pH value higher than 3.5, the peak current decreases due to the precipitation of thorium hydroxides in solution, while at a pH lower than the mentioned range, the peak current falls sharply presumably due to the dissolution of the electrode surface in the strong acidic medium. Thus, most peak current and the most stability peak are in the range of pH 2–4. The most peak current (Ip) of the complex is in pH 3. 3.3.2. Effect time Under a fixed accumulation potential (in the open circuit potential), the stripping peak currents will improve as prolonging the accumulation time. As expected, the stripping peak currents increased in the first 300 s utilizing the potential and then leveled
Fig. 4. Effect of accumulation time upon the response to 1.0 × 10−6 M thorium nitrate. Times were 0 (a), 50 (b), 100 (c), 150 (d), 200 (e), 300 (f) and 340 (g) seconds. The pH 3.0 at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V.
off (Fig. 4). The curvature presumably indicates that a limiting value of the amount of accumulated thorium was achieved on the modified SPE surface when the accumulation time reached beyond 300 s. A further accumulation time increase did not cause saturation. Considering both sensitivity and working efficiency, an accumulation time of 300 s was employed. Although the sensitivity for lower concentrations was improved by increasing the accumulation time, the linear range was then diminished. 3.4. Calibration curve Under the optimum conditions, the adsorptive peak current of Th–Oxine yields well-defined concentration dependence from 0.5 × 10−9 to 2.2 × 10−8 M (Ip = 5.469 + 0.164 C, r2 = 0.998, Ip in nA, C in nM) as shown in Fig. 5. The lowest detectable concentration of thorium at 1000 s accumulation is estimated to be 0.50 × 10−9 M. The relative standard deviation of 10 tandem measurements of 2 × 10−6 M thorium was determined at one single modified electrode is 3.69%. After every measurement, the modified SPEs were remodified and the RSD of 2 × 10−6 M thorium was 4.267% (n = 8), suggesting that the CNT-oxine modified SPE exhibits excellent reproducibility when used for the determination of thorium. The modified electrode was also found to keep its activity after keeping in de-ionized water for at least 21 days. Table 1 compares the linear range, RSD and detection limit of the proposed carbon paste modified electrode with those of some of the best previously reported voltammetric and chemical sensors for thorium detection. 3.5. Interference The possible interference of some inorganic species was also tested. The results showed that the response for 2.0 × 10−6 M thorium was not affected by 300 g L−1 levels of uranium, zinc, aluminum, cadmium, bismuth, antimony, cobalt, nickel or vanadium. Common anions including chloride, chlorate and sulphate
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
1215
Table 1 Comparison of linear range, limit of detection and RSD of different methods reported for thorium determination. Method
RSD (%)
Detection limite (M)
Linear range (M)
Reference
Selective optode Polarography Stripping voltammetry Chronoamperometric stripping Stripping voltammetry This work
– 16 3.1 – 4.4 3.69
6 × 10−6 8 × l0−8 4 × l0−10 – 2 × 10−10 0.17 × 10−9
8.66 × 10−6 to 2.00 × 10−4 2 × 10−7 to 1 × 10−6 Up to 1.3 × 10−7 1 × 10−7 to 1 × 10−5 Up to 8 × 10−8 0.5 × 10−9 to 2.2 × 10−8
[37] [38] [31] [39] [33] –
Table 2 Determination of thorium in real waste water samples using the proposed method (n = 3). Sample
Experimental results (nM)
Waste water Waste water
10 ± 0.2 20 ± 0.3
Recovery (%) 98.7–101.4 98.8–103.6
that the amount of thorium in the sample was lower than detection limite of modified electrode. Three replicates were analyzed by using the same electrode. The significance of the developed method was also tested. The results thus obtained clearly revealed that the concentrations of thorium in the sample obtained by the proposed stripping voltammetry are in satisfactory agreement with those determined by ICP and the proposed stripping sensor has promising feasibility for trace-level analysis of thorium in complex samples. A waste water sample was collected from the research nuclear laboratory and it was assayed for thorium by the proposed method. The only treatment was supporting electrolyte addition to 0.1 M HCl (HCl concentration was adjusted directly in the voltammetric cell). Thorium was undetectable using square wave voltammetry, so the sample was fortified adding the appropriate amounts of thorium standard solutions to 10 and 20 nM. The concentrations of standards were determined utilizing ICP. Three replicates were analyzed using the same strip. The proposed method gave the recoveries summarized in Table 2. 4. Conclusion In conclusion, under the optimum conditions, trace levels of thorium can be determined rapidly with a simple approach using CNT-oxine modified screen printed electrode. Screen printed technology is a rapid and cost reducing way to fabricate robust and solid electrodes. This mercury-free thin film electrode can be employed to accumulate thorium. The adsorptive stripping peak current varies according to the concentration of thorium. Conflict of interest The authors declare that there are no conflicts of interest. Acknowledgements Fig. 5. Square wave stripping voltammograms for various concentrations of thorium nitrate from 0.5 × 10−9 to 2.2 × 10−8 M. The concentrations were 0.5 (a), 1 (b), 2 (c), 5 (d), 10 (e), 12 (f), 15 (g) and 22 (h) nM. The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V. Accumulation time was 300 s with a pH of 3.0 solution. (B) Calibration curve of thorium.
were tested at 0.002 M level. The results showed that none of them had an effect less than 5% in response to 1.0 × 10−8 M thorium. 3.6. Analytical application The applicability of the SPE/MWCNTs-oxine modified electrode for preconcentration of trace levels of thorium was tested using Tehran tape water sample. The results of this investigation show
The support of this work by research grants from the NSTRI and Shahrekord University Research Council is gratefully acknowledged. References [1] M. Corn (Ed.), Handbook of Hazardous Materials, Academic Press, San Diego, USA, 1993. [2] G.D. Muir (Ed.), Hazards in the Chemical Laboratory, The Chemical Society, London, UK, 1977. [3] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic: Principle of Structure and Reactivity, 4th ed., Harper Collins, New York, USA, 1993. [4] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth, UK, 1997. [5] X.G. Li, H. Feng, et al., Anal. Chem. 84 (2012) 134–140.
1216 [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
M.R. Huang, et al., Analyst 138 (2013) 3820–3829. M.R. Huang, et al., ACS Appl. Mater. Interfaces 6 (2014) 22096–22107. M.R. Huang, X.W. Rao, et al., Talanta 85 (2011) 1575–1584. M.R. Huang, Y.B. Ding, et al., ACS Comb. Sci. 15 (2014) 128–138. K. Mayer, H. Ottmar, G. Tamborini, I. Ray, H. Thiele, Radiat. Prot. Dosim. 97 (2001) 193–196. P. Shrivastav, S.K. Menon, Y.K. Agrawal, J. Radioanal. Nucl. Chem. 250 (2001) 459–464. S. Joannon, C. Pin, Spectrochim. Acta B 52 (1997) 1783–1789. P. Akhter, S.D. Orfi, H. Kawamura, N. Ahmad, M. Khaleeq-ur-Rahman, J. Environ. Radioact. 62 (2002) 123–127. M.A. White, A.M. Howe, P. Rosen, L. Holmes, Radiat. Prot. Dosim. 97 (2001) 101–114. H.A. Arida, M.A. Ahmed, A.M. El-Saied, Sensors 3 (2003) 424–437. S. Chandra, H. Agarwal, C. Kumar Singh, S. Kumar Sindhu, P. Kumar, Indian J. Chem. 44A (2005) 2060–2063. E.H. Borai, A.S. Mady, Appl. Radiat. Isot. 57 (2002) 463–469. B. Liu, L. Liu, J. Cheng, Talanta 47 (1998) 291–299. C. Lee, K.S. Chung, J. Korean Chem. Soc. 15 (1971) 5–9. N.E. El-Hefny, J.A. Daoud, J. Radioanal. Nucl. Chem. 261 (2004) 357–363. A.G. Pantelica, M.D. Magureanu, I. Margaritescu, E. Cincu, Radiat. Prot. Dosim. 97 (2001) 187–192. J.-N. Li, F.-Y. Yi, Z.-M. Jiang, J.-J. Fei, Microchim. Acta 143 (2003) 278–292. N.G. Donoso, M.A. Santa Ana, W.I. Chadwick, Anal. Chim. Acta 42 (1968) 109–118. K.Ł. Lenarczy, Z. Marczenko, M. Jarosz, Microchim. Acta 84 (1984) 485–490. Z.E. Zhao, X.H. Cai, P.B. Li, X.D. Yang, Talanta 33 (1986) 623–625. F.Y. Yi, J.N. Li, Y.W. Zhu, J.J. Fei, YankuangCeshi 20 (2001) 105–108. J.N. Li, Z.F. Zhao, FenxiHuaxue 12 (1984) 347–353. X.R. Yao, J.X. Zhou, C.F. Li, FenxiHuaxue 12 (1984) 671–678. Z.F. Zhao, X.H. Cai, P.B. Li, Talanta 33 (1986) 623–625. K.Q. Zheng, K. Liu, H.G. Liu, FenxiShiyanshi 10 (1991) 41–45. J. Wang, J.M. Zadeii, Anal. Chim. Acta 188 (1986) 187–194. Q.P. Chen, R. Wei, TongjiDaxueXuebao 15 (1987) 355–360.
[33] R. Setiadji, J. Wang, Talanta 40 (1993) 845–849. [34] L. Shumei, J. Li, X. Mao, P. Gao, Anal. Lett. 36 (2003) 1381–1392. [35] P. Fanjul-Bolado, D. Hernandez-Santos, P.J. Lamas-Ardisana, A. Martin-Pernia, A. Costa-Garcia, Electrochim. Acta 53 (2008) 3635–3642. [36] E. Jubete, O.A. Loaiza, E. Ochoteco, E. Pomposo, H. Grande, J. Rodriguez, J. Sens. 2009 (2009) 1–3. [37] A. Safavi, M. Sadeghi, Anal. Chim. Acta 567 (2006) 184–188. [38] G.A. Mazzocchin, S. Daniele, L.M. Moretto, Talanta 37 (1990) 317–324. [39] J. Adam, Talanta 29 (1982) 939–940.
Biographies Nasim Soltani is a master degree graduate in analytical chemistry. She received her MS degree from Shahrekord University, Shahrekord, Iran, in 2014. Her research interests are based on modified electrode and molecular electrochemistry together with its analytical applications in areas such as sensors and biosensors. Hedayat Haddadi is an Assistant Professor in analytical chemistry at Shahrekord University, Shahrekord, Iran. He received his PhD degree from Tarbiat Modares University, Iran, in 2010. His research interests are based on modified electrode, separation, NMR and spectroscopy. He has published over 20 scientific papers in international journals. Mehdi Asgari is an Assistance Professor in analytical chemistry at NSTR, Tehran, Iran. He received his PhD degree from Tarbiat Modares University, Iran, in 2012. His research interests are based on modified electrode and molecular electrochemistry as well as its analytical applications in areas such as sensors and biosensors. He has published over 30 scientific papers in international journals. Nematollah Rajabzadeh is a master degree graduate in physics. He received his MS degree from Payame Noor University, Tehran, Iran, in 2011. Her research interests center around nuclear physics and nuclear material accounting.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Adsorptive stripping voltammetric detection of thorium on the multi-walled carbon nanotube modified screen printed electrode Nasim Soltani a , Hedayat Haddadi a,c , Mehdi Asgari b,∗ , Nematollah Rajabzadeh b a b c
Department of Chemistry, Faculty of Sciences, Shahrekord University, Shahrekord, Iran NFCRI, NSTRI, Tehran, Iran Nanotechnology Center Research, Shahrekord University, 8818634141 Shahrekord, Iran
a r t i c l e
i n f o
Article history: Received 7 January 2015 Received in revised form 5 June 2015 Accepted 12 June 2015 Available online 26 June 2015 Keywords: Thorium Oxine complex Screen printed electrode Multi-walled carbon nanotubes Stripping voltammetry
a b s t r a c t In this article, the application of a novel modified screen-printed electrode (SPE) to thorium detection is described. Multi-walled carbon nanotubes (MWCNTs) modified screen-printed electrodes present an interesting alternative for the determination of thorium (IV). This method can be described by square wave adsorptive stripping voltammetry of the thorium complex with oxine at a screen printed electrode. In comparison with the bare SPE, the modifications of MWCNTs were demonstrated providing more sensitive responses. The pH adjusted to 3.0, and adsorptive voltammetric peak regarding the adsorption of thorium–oxine in the complex occur at −0.3 V with adsorptive-preconcentration. The electrode displayed excellent linear behavior in the concentration range examined (0.5 × 10−9 to 2.2 × 10−8 M) with a limit of quantitation (LOQ) of 0.5 nM and limit of detection (LOD) by re-examining the data is 0.17 nM. The method has been successfully applied to the determination of trace Th(IV) in different water samples. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ions represent a major environmental problem and their detection and monitoring in waste water outlets, rivers, reservoirs or sources of drinking water is necessary. Thorium is a natural element that has a number of industrial (lantern mantles, welding operations, and preparation of refractories) and medical (thorotrast imaging agent) applications [1]. It has been reported that inhaling thorium dust causes an increased risk of developing lung and pancreas cancer. Bone cancer risk is also increased because thorium may be stored in the bones [2–9]. Many methods have already been developed for determination of thorium in real samples based on the chemical or physical principles [10]. The majority of thorium measurements were performed using ICP, atomic emission spectrometry [11], ICP-MS [12–14], ion selective electrode [15,16], IC [17], capillary zone electrophoresis [18], cation exchange resin [19], extraction [20], and ␥-ray spectrometry [21]. Polarography and voltammetry are particularly attractive due to the simplicity of the technique and the low cost instrumentation. However, because of the extremely negative reduction potentials of thorium, it cannot be determined by direct polarographic analysis [22]. However, indirect polarographic
procedures based on complexation with electroactive organic ligands such as cupferron, alizarin S, xylidyl blue I, alizarin complexion (ALC), etc. have been reported [23–26]. Various methods have been reported for the determination of thorium by polarographic complex adsorption wave with different reagents: alizarin red S (ARS), cupferron, xylidyl blue I and chrome azurol S [27–30]. Also the adsorptive voltammetric procedures for trace measurement of thorium at mercury electrode and CPE have been reported [31–34]. Screen printed technology is a rapid and cost reducing way to fabricate robust and solid electrodes. It offers several advantages, such as versatility of the design, reproducibility in the sensor preparation and low cost production [35,36], which permits the sensors to be disposed after a single use. The present article determines Th(IV) by square wave adsorptive stripping voltammetry (SWASV) using multi-walled carbon nanotubes and oxine modified screen-printed electrodes. On the basis of our literature review, this presents the first electrochemical detection system of Th(IV) with this type of nano-modified electrodes.
2. Experimental 2.1. Reagents and apparatus
∗ Corresponding author. Tel.: +98 21 88221117; fax: +98 21 88221116. E-mail address: [email protected] (M. Asgari). http://dx.doi.org/10.1016/j.snb.2015.06.106 0925-4005/© 2015 Elsevier B.V. All rights reserved.
A 1.00 × 10−2 M thorium standard stock solution was prepared by dissolving 0.294 g of Th(NO3 )4 ·6H2 O (Merck company) in 1.0 M
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
1213
Fig. 1. FE-SEM image of SPE/MWCNTs modified electrode.
HCl and diluting the solution to 50 ml with 1.0 M HCl. All solutions were prepared utilizing distilled water. MWCNTs with 95% purity (30–50 nm diameters and 3 m length) were obtained from Timesnano Co. Ltd. (China). The electrochemical experiments were carried out by means of an Autolab potentiostate/galvanostate 30(2) (ECO Chemie, Netherlands), equipped with a personal computer used for data storage and processing. A screen-printed carbon electrode (SPE) (3 mm in diameter) from Dropsens (Spain) was used as a planar three electrode based on a graphite working electrode, a carbon counter electrode and a silver pseudo-reference electrode. 2.2. Purification and activation of MWCNTs MWCNTs were purified and activated before use, as follows: the MWCNTs were first treated with reflux in a 1:3 (v/v) mixture of HNO3 (69%) and H2 SO4 (98%) at 85 ◦ C for 4 h. The product was centrifuged, washed with ethanol. Then the MWCNTs were dried under vacuum at 50 ◦ C overnight. 2.3. Modifications of screen-printed electrodes Before any modification, the SPEs were pretreated by applying a potential of −0.4 to +1.5 V in HCl 0.10 M using scan rate of 50 mV s−1 for 20 cycles. MWCNTs (10 mg) and 5 L of 5.0% nafion solution were dispersed in 5 mL solution containing 0.02 g oxine and 2-metyl ethanol utilizing ultrasonication for 1 h to get a homogenous suspension. 10 L of the suspension was cast onto the working section of screen printed electrode and dried at room temperature. Prior to the experiments a drop (50 l of total volume) of the sample solution was applied to the reaction region of modified screen printed electrode. 3. Results and discussion 3.1. Surface characterization The surface morphology of the SPE/MWCNTs was characterized by FE-SEM micrographs (Fig. 1). The structure of the composite showed a homogeneous film on the surface of SPE, which plays an important role in enhancing electro-conductivity of the film. Such an electrode exhibits an excellent stability, a broad surface coverage and a good contact among the SPE, oxine and the SWV of thorium by various electrochemical methods. MWCNTs have excellent mechanical strength with superior heat and electric conductivity. They also have high specific surface area,
Fig. 2. (A) Square wave voltammograms of 1.0 × 10−6 M thorium nitrate after 300 s of accumulation on the modified SPE/MWCNTs-oxine modified electrode (a) and the voltammograms of thorium on the SPE/MWCNTs modified electrode after 300 s accumulation time (b) and square wave voltammogram of thorium in the presence of oxine, without preconcentration on the SPE/MWCNTs-oxine modified electrode (c) with a pH of 3.0 solution. The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V. (B) Square wave voltammograms of 1.0 × 10−6 M thorium nitrate after 300 s of accumulation on the modified SPE/MWCNTs-oxine modified electrode in varied scan rates.
1214
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
Fig. 3. Effect of pH upon the stripping voltammetric response of 1.0 × 10−6 M thorium nitrate on the SPE/MWCNTs-oxine modified electrode. pHs were 1 (a), 2 (b), 3.1 (c), 4 (d), 4.5 (e), 5 (f) and 5.4 (g). The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V.
high crystallinity, and high length-to-diameter ratio. Presence of MWCNT on the surface of the electrode caused increase conductivity of electrode and decrease the charge transfer between electrode and electrolyte. 3.2. Voltammetric behavior of thorium on the SPE/MWCNTs-oxine Fig. 2A shows the square wave voltammetry of 1.0 × 10−6 M thorium between −0.8 and 0.2 V. Curve ‘a’ displays the square wave voltammograms of thorium after 300 s of adsorptive accumulation on the SPE/MWCNTs-oxine modified electrode. The voltammogram of thorium in the absence of oxine after 300 s adsorptive accumulation time on the modified SPE (curve ‘b’) is also shown. Curve ‘c’ shows the square wave voltammogram of thorium in the presence of oxine, without preconcentration on the modified SPE. A comparison of the voltammograms show that the height of thorium–oxine complex reduction peak depends on the duration of adsorptive preconcentration step and also on the presence of oxine, which reveals the adsorptive nature of the response. The experiment parameters were chose according to experiments. The results related to variation of scan rates were shown in Fig. 1B. 3.3. Optimization of operational parameters 3.3.1. Effect of pH The influence of pH on the stripping peak current is studied in the range 1–5.4 (Fig. 3). It is found that at a pH value higher than 3.5, the peak current decreases due to the precipitation of thorium hydroxides in solution, while at a pH lower than the mentioned range, the peak current falls sharply presumably due to the dissolution of the electrode surface in the strong acidic medium. Thus, most peak current and the most stability peak are in the range of pH 2–4. The most peak current (Ip) of the complex is in pH 3. 3.3.2. Effect time Under a fixed accumulation potential (in the open circuit potential), the stripping peak currents will improve as prolonging the accumulation time. As expected, the stripping peak currents increased in the first 300 s utilizing the potential and then leveled
Fig. 4. Effect of accumulation time upon the response to 1.0 × 10−6 M thorium nitrate. Times were 0 (a), 50 (b), 100 (c), 150 (d), 200 (e), 300 (f) and 340 (g) seconds. The pH 3.0 at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V.
off (Fig. 4). The curvature presumably indicates that a limiting value of the amount of accumulated thorium was achieved on the modified SPE surface when the accumulation time reached beyond 300 s. A further accumulation time increase did not cause saturation. Considering both sensitivity and working efficiency, an accumulation time of 300 s was employed. Although the sensitivity for lower concentrations was improved by increasing the accumulation time, the linear range was then diminished. 3.4. Calibration curve Under the optimum conditions, the adsorptive peak current of Th–Oxine yields well-defined concentration dependence from 0.5 × 10−9 to 2.2 × 10−8 M (Ip = 5.469 + 0.164 C, r2 = 0.998, Ip in nA, C in nM) as shown in Fig. 5. The lowest detectable concentration of thorium at 1000 s accumulation is estimated to be 0.50 × 10−9 M. The relative standard deviation of 10 tandem measurements of 2 × 10−6 M thorium was determined at one single modified electrode is 3.69%. After every measurement, the modified SPEs were remodified and the RSD of 2 × 10−6 M thorium was 4.267% (n = 8), suggesting that the CNT-oxine modified SPE exhibits excellent reproducibility when used for the determination of thorium. The modified electrode was also found to keep its activity after keeping in de-ionized water for at least 21 days. Table 1 compares the linear range, RSD and detection limit of the proposed carbon paste modified electrode with those of some of the best previously reported voltammetric and chemical sensors for thorium detection. 3.5. Interference The possible interference of some inorganic species was also tested. The results showed that the response for 2.0 × 10−6 M thorium was not affected by 300 g L−1 levels of uranium, zinc, aluminum, cadmium, bismuth, antimony, cobalt, nickel or vanadium. Common anions including chloride, chlorate and sulphate
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
1215
Table 1 Comparison of linear range, limit of detection and RSD of different methods reported for thorium determination. Method
RSD (%)
Detection limite (M)
Linear range (M)
Reference
Selective optode Polarography Stripping voltammetry Chronoamperometric stripping Stripping voltammetry This work
– 16 3.1 – 4.4 3.69
6 × 10−6 8 × l0−8 4 × l0−10 – 2 × 10−10 0.17 × 10−9
8.66 × 10−6 to 2.00 × 10−4 2 × 10−7 to 1 × 10−6 Up to 1.3 × 10−7 1 × 10−7 to 1 × 10−5 Up to 8 × 10−8 0.5 × 10−9 to 2.2 × 10−8
[37] [38] [31] [39] [33] –
Table 2 Determination of thorium in real waste water samples using the proposed method (n = 3). Sample
Experimental results (nM)
Waste water Waste water
10 ± 0.2 20 ± 0.3
Recovery (%) 98.7–101.4 98.8–103.6
that the amount of thorium in the sample was lower than detection limite of modified electrode. Three replicates were analyzed by using the same electrode. The significance of the developed method was also tested. The results thus obtained clearly revealed that the concentrations of thorium in the sample obtained by the proposed stripping voltammetry are in satisfactory agreement with those determined by ICP and the proposed stripping sensor has promising feasibility for trace-level analysis of thorium in complex samples. A waste water sample was collected from the research nuclear laboratory and it was assayed for thorium by the proposed method. The only treatment was supporting electrolyte addition to 0.1 M HCl (HCl concentration was adjusted directly in the voltammetric cell). Thorium was undetectable using square wave voltammetry, so the sample was fortified adding the appropriate amounts of thorium standard solutions to 10 and 20 nM. The concentrations of standards were determined utilizing ICP. Three replicates were analyzed using the same strip. The proposed method gave the recoveries summarized in Table 2. 4. Conclusion In conclusion, under the optimum conditions, trace levels of thorium can be determined rapidly with a simple approach using CNT-oxine modified screen printed electrode. Screen printed technology is a rapid and cost reducing way to fabricate robust and solid electrodes. This mercury-free thin film electrode can be employed to accumulate thorium. The adsorptive stripping peak current varies according to the concentration of thorium. Conflict of interest The authors declare that there are no conflicts of interest. Acknowledgements Fig. 5. Square wave stripping voltammograms for various concentrations of thorium nitrate from 0.5 × 10−9 to 2.2 × 10−8 M. The concentrations were 0.5 (a), 1 (b), 2 (c), 5 (d), 10 (e), 12 (f), 15 (g) and 22 (h) nM. The square wave voltammograms were recorded at scan rate of 0.05 V s−1 and pulse amplitude of 0.05 V. Accumulation time was 300 s with a pH of 3.0 solution. (B) Calibration curve of thorium.
were tested at 0.002 M level. The results showed that none of them had an effect less than 5% in response to 1.0 × 10−8 M thorium. 3.6. Analytical application The applicability of the SPE/MWCNTs-oxine modified electrode for preconcentration of trace levels of thorium was tested using Tehran tape water sample. The results of this investigation show
The support of this work by research grants from the NSTRI and Shahrekord University Research Council is gratefully acknowledged. References [1] M. Corn (Ed.), Handbook of Hazardous Materials, Academic Press, San Diego, USA, 1993. [2] G.D. Muir (Ed.), Hazards in the Chemical Laboratory, The Chemical Society, London, UK, 1977. [3] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic: Principle of Structure and Reactivity, 4th ed., Harper Collins, New York, USA, 1993. [4] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth, UK, 1997. [5] X.G. Li, H. Feng, et al., Anal. Chem. 84 (2012) 134–140.
1216 [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
N. Soltani et al. / Sensors and Actuators B 220 (2015) 1212–1216
M.R. Huang, et al., Analyst 138 (2013) 3820–3829. M.R. Huang, et al., ACS Appl. Mater. Interfaces 6 (2014) 22096–22107. M.R. Huang, X.W. Rao, et al., Talanta 85 (2011) 1575–1584. M.R. Huang, Y.B. Ding, et al., ACS Comb. Sci. 15 (2014) 128–138. K. Mayer, H. Ottmar, G. Tamborini, I. Ray, H. Thiele, Radiat. Prot. Dosim. 97 (2001) 193–196. P. Shrivastav, S.K. Menon, Y.K. Agrawal, J. Radioanal. Nucl. Chem. 250 (2001) 459–464. S. Joannon, C. Pin, Spectrochim. Acta B 52 (1997) 1783–1789. P. Akhter, S.D. Orfi, H. Kawamura, N. Ahmad, M. Khaleeq-ur-Rahman, J. Environ. Radioact. 62 (2002) 123–127. M.A. White, A.M. Howe, P. Rosen, L. Holmes, Radiat. Prot. Dosim. 97 (2001) 101–114. H.A. Arida, M.A. Ahmed, A.M. El-Saied, Sensors 3 (2003) 424–437. S. Chandra, H. Agarwal, C. Kumar Singh, S. Kumar Sindhu, P. Kumar, Indian J. Chem. 44A (2005) 2060–2063. E.H. Borai, A.S. Mady, Appl. Radiat. Isot. 57 (2002) 463–469. B. Liu, L. Liu, J. Cheng, Talanta 47 (1998) 291–299. C. Lee, K.S. Chung, J. Korean Chem. Soc. 15 (1971) 5–9. N.E. El-Hefny, J.A. Daoud, J. Radioanal. Nucl. Chem. 261 (2004) 357–363. A.G. Pantelica, M.D. Magureanu, I. Margaritescu, E. Cincu, Radiat. Prot. Dosim. 97 (2001) 187–192. J.-N. Li, F.-Y. Yi, Z.-M. Jiang, J.-J. Fei, Microchim. Acta 143 (2003) 278–292. N.G. Donoso, M.A. Santa Ana, W.I. Chadwick, Anal. Chim. Acta 42 (1968) 109–118. K.Ł. Lenarczy, Z. Marczenko, M. Jarosz, Microchim. Acta 84 (1984) 485–490. Z.E. Zhao, X.H. Cai, P.B. Li, X.D. Yang, Talanta 33 (1986) 623–625. F.Y. Yi, J.N. Li, Y.W. Zhu, J.J. Fei, YankuangCeshi 20 (2001) 105–108. J.N. Li, Z.F. Zhao, FenxiHuaxue 12 (1984) 347–353. X.R. Yao, J.X. Zhou, C.F. Li, FenxiHuaxue 12 (1984) 671–678. Z.F. Zhao, X.H. Cai, P.B. Li, Talanta 33 (1986) 623–625. K.Q. Zheng, K. Liu, H.G. Liu, FenxiShiyanshi 10 (1991) 41–45. J. Wang, J.M. Zadeii, Anal. Chim. Acta 188 (1986) 187–194. Q.P. Chen, R. Wei, TongjiDaxueXuebao 15 (1987) 355–360.
[33] R. Setiadji, J. Wang, Talanta 40 (1993) 845–849. [34] L. Shumei, J. Li, X. Mao, P. Gao, Anal. Lett. 36 (2003) 1381–1392. [35] P. Fanjul-Bolado, D. Hernandez-Santos, P.J. Lamas-Ardisana, A. Martin-Pernia, A. Costa-Garcia, Electrochim. Acta 53 (2008) 3635–3642. [36] E. Jubete, O.A. Loaiza, E. Ochoteco, E. Pomposo, H. Grande, J. Rodriguez, J. Sens. 2009 (2009) 1–3. [37] A. Safavi, M. Sadeghi, Anal. Chim. Acta 567 (2006) 184–188. [38] G.A. Mazzocchin, S. Daniele, L.M. Moretto, Talanta 37 (1990) 317–324. [39] J. Adam, Talanta 29 (1982) 939–940.
Biographies Nasim Soltani is a master degree graduate in analytical chemistry. She received her MS degree from Shahrekord University, Shahrekord, Iran, in 2014. Her research interests are based on modified electrode and molecular electrochemistry together with its analytical applications in areas such as sensors and biosensors. Hedayat Haddadi is an Assistant Professor in analytical chemistry at Shahrekord University, Shahrekord, Iran. He received his PhD degree from Tarbiat Modares University, Iran, in 2010. His research interests are based on modified electrode, separation, NMR and spectroscopy. He has published over 20 scientific papers in international journals. Mehdi Asgari is an Assistance Professor in analytical chemistry at NSTR, Tehran, Iran. He received his PhD degree from Tarbiat Modares University, Iran, in 2012. His research interests are based on modified electrode and molecular electrochemistry as well as its analytical applications in areas such as sensors and biosensors. He has published over 30 scientific papers in international journals. Nematollah Rajabzadeh is a master degree graduate in physics. He received his MS degree from Payame Noor University, Tehran, Iran, in 2011. Her research interests center around nuclear physics and nuclear material accounting.