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• PAANI/MWNTs/GCE 5870 51 PAANI/GCE 11,197 PPy/MWCNT (10% w/w) 2 52 PPy/MWCNT (20% w/w) 0.9 PPy/MWCNT (30% w/w) 0.85 PMG/GCE 1300 This work PMG/MWCNT/GCE 494
• Chronoamperometric measurements Chronoamperometry, as well as other electrochemical methods, was employed for the investigation of electrode processes at chemically modified electrodes. 47 Figure 9A shows the current–time curves of PMG/MWCNT/GCE obtained by setting the working electrode potential at 170 mV versus Ag|AgCl|KCl (3M) for various concentrations of EP in phosphate buffered solutions (pH 7.0). The diffusion coefficient ( D app ) for oxidation of EP at the surface of the modified electrode can be estimated using Cottrell’s equation: 48 I = nF AD 1/2 app C b π −1/2 t −1/2 , (2) 3 2 4 6 t / s 0.43 0.53 0.63 0.73 t –1/2 / s –1/2 y = 619x – 0.194 R = 0.995 0.25 0.07 0.08 0.09 0.1 0.11 0.12 where D app and C b are the diffusion coefficient (cm 2 s −1 ) and the bulk concentration (mol cm −3 ) , respectively. At a mass transport limited rate condition, a plot of I vs. t −1/2 will be linear, and the value of D app can be calculated from the slope of this line. Figure 9B shows the obtained experimental plots for different concentrations of EP. The mean value of D app was found to be 5.7 × 10 −6 cm 2 s −1 using the slopes of the resulting straight lines plotted versus the EP concentrations (Figure 9C). Simultaneous determination of AA, EP, and UA at PMG/MWCNT/GCE Although the amount of peak separation in the obtained cyclic voltammograms by the modified electrode offered sufficient resolution for simultaneous determination of AA, EP, and UA, the presence of high background current in the CV method caused a decrease in peak clarity and current sensitivity spatially at low concentrations. Therefore, the electrooxidation processes of mixtures of AA, EP, and UA at the surface of PMG/MWCNT/GCE were investigated by the DPV method as a method with much higher current sensitivity and better resolution than CV. In addition, the charging current contribution to the background current, which is a limiting factor in analytical determination, is negligible in DPV mode. Figure 10 shows the DPVs for increasing concentrations of AA, EP, and UA in the ternary mixtures while concentration of the other 2 species remained constant. From Figure 10A, the peak current of AA increases linearly with increases in AA concentration from 0.4 to 100.0 µ M. The oxidation peak current of EP also increases linearly with increase in EP concentration from 0.1 to 100.0 µ M with a correlation coefficient of 0.9979, as shown in Figure 10B. Similarly, as shown in Figure 10C, the UA current increases in the linear range of 0.3–90.0 µ M by keeping the concentration of the other 2 compounds constant. Based on the above results, it is clear that the electrooxidation peaks for AA, EP, and UA oxidation at the PMG/MWCNT/GCE are well separated from each other although they coexist in the same solution, and the proposed sensor can be used for individual or simultaneous determination of AA, EP, and UA in mixture samples without any cross interferences. The resulted detection limits (3 σ) for AA, EP, and UA were 0.23, 0.0820, and 0.12 µ M, respectively. The obtained values for determination of AA, EP, and UA by the modified electrode are comparable with values reported by other research groups (Table 2). The stability of the electrode is also high; the current response decreased by about 2.4% in 5 days and 6% in 30 days. Table Comparison of results obtained by various modified electrodes in determination of AA, EP, and UA. Modified electrode Reference AA EP UA AA EP UA Caffeic acid/GCE 20–1000 0–80 0–300 0 0.20 0.60 53 MDWCNTPE 1 0.7–1200 25–750 0.216 8 54 FePc 2 -CPE 3 1–300 0.5 28 PMG/MWCNT/GCE 0.4–100.0 0.1–100.0 0.3–90.0 0.23 0.0820 0.12 This work 1 Modified double-walled carbon nanotube paste electrode. 2 Iron(II) phthalocyanines. 3 Carbon paste electrode.
• Real sample analysis Determination of AA and EP in injection samples One milliliter of epinephrine hydrochloride (specified content of EP is 1.00 mg L −1 ) and 0.1 mL of vitamin C (specified content of AA is 100 mg mL −1 ) injection solutions were diluted to 100 mL and 250 mL with water, respectively. Different volumes of diluted solutions were pipetted into each of a series of 20-mL volumetric flasks 20 –0.3 –0.1 0.1 0.3 0.5 0.7 0.9 (A) (B) (C) –0.2 0.2 0.4 0.6 0.8 1 50 100 I / µ A [EP] / µM a 4 –0.3 –0.1 0.1 0.3 0.5 20 40 60 80 100 I / µA [UA] / µM a Figure 10. A) Differential pulse voltammograms of fixed concentration of EP (30.0 µ M) and UA (3.5 µ M) and different concentrations of AA: (a) 0.4, (b) 1.0, (c) 1.6, (d) 2.0, (e) 3.0, (f) 4.0, (g) 5.0, (h) 10.0, (i) 15.0, (j) 20.0, (k) 30.0, (l) 40.0, (m) 0, (n) 60.0, (o) 70.0, (p) 80.0, and (q) 100.0 µ M; B) differential pulse voltammograms of fixed concentration of AA (0 µ M) and UA (7.0 µ M) and different concentrations of EP: (a) 0.1, (b) 1.2, (c) 2.0, (d) 4.0, (e) 15.0 (f) 20.0, (g) 0, (h) 65.0, (i) 80.0, and (j) 100.0 µ M; C) differential pulse voltammograms of fixed concentration of AA (0.15 µ M) and EP (0.1 µ M) and different concentrations of UA: (a) 0.3, (b) 1.2, (c) 3.0, (d) 6.0, (e) 9.0, (f) 12.0, (g) 20.0, (h) 0, (i) 55.0, (j) 75.0, and (k) 90.0 µ M at the PMG/MWCNT/GCE in 0.1 M phosphate buffer solution (pH 7.0). Amplitude: 0.02 V; pulse width: 0.05 s; pulse period: 0.2 s. Insets show corresponding calibration curves. and diluted to the mark with 0.1 M phosphate buffer (pH 7.0). An aliquot of 10 mL of this test solution was placed in the electrochemical cell. The DPV method was used for detection of AA and EP. The obtained results of AA and EP in the injections were 99.88 and 0.978 mg mL −1 , which corresponded well with the values that were given by injection characterizations. Each sample was analyzed 5 times and relative standard deviations (RSDs) obtained for AA and EP were 1.2% and 1.9%, respectively. Different standard concentrations of AA and EP were added to the diluted vitamin C and EP injections and the recoveries were between 98.3% and 6% for 5 measurements. Determination of UA in human urine samples The practical analytical utility of the modified electrode for determination of UA was investigated by its determination in normal urine human samples by DPV method. For the voltammetric measurements, the obtained urine human samples (obtained from Dr. Safiri Medical Diagnostic Laboratory, Babolsar, Iran) were diluted in 10 mL of phosphate buffer (pH 7.0). Each experiment was repeated in triplicate and recovery was estimated by standard addition method (n = 3). The obtained results are given in Table 3. Table Determination of UA in human urine samples. Sample UA added (µM) UA found (µM) Recovery (%) 1 0.0 24 (± 0.04) 0 30 (± 0.03) 2 2 0.0 14 (± 0.02) 0 11 (± 0.07) 85 3 0.0 86 (± 0.04) 0 08 (± 0.06) 98 4 0.0 42 (± 0.05) 0 38 (± 0.05) 85 Interference study The influence of various foreign species on the determination of 4.0 × 10 −5 M AA, 0 × 10 −5 M EP, and 0 × 10 −5 M UA was investigated. The tolerance limit was taken as the maximum concentration of the foreign substances, which caused an approximately ± 5% relative error in the determination. The tolerated concentration of foreign substances was 2.0 × 10 −2 M for Na + , Cl − , and K + ; 0 × 10 −1 M for Mg 2+ and Ca 2+ ; and 2 × 10 −2 M for L-lysine, glucose, L-asparagine, glycine, phenylalanine, N-acetyl-L-cysteine, glutathione, and L-cysteine. Dopamine (DA) was also tested as a potential interference; the results showed that DA exhibits a voltammetric signal at potential values close to that of EP oxidation. Therefore, DA can be considered as an interfering agent in the determination of EP in prepared modified electrode. Reproducibility of the PMG/MWCNT/GCE Reproducibility is one of the most important properties of the electrode. To ascertain the reproducibility of PMG/MWCNT/GCE, the oxidation peak current of 10 successive measurements of 30 µ M AA, 20 µ M EP, and 50 µ M UA in the mixture at the PMG/MWCNT/GCE was investigated. After each measurement the modified electrode was washed with 0.1 M PBS solution and measured for the same concentration. The RSD was calculated to be 3.4%, 2.6%, and 1.8% for AA, EP, and UA, respectively, indicating that the modified electrode is stable and does not suffer from surface fouling by oxidation products during the voltammetric measurements. Conclusions In the present work, a sensitive and selective electrochemical sensor has been developed using MWCNTs and PMG at a GCE electrode. The PMG/MWCNT/GCE resulted in catalytic effects toward the electrooxidation of AA, EP, and UA since it enhances the oxidation peak currents and lowers the oxidation overpotential. Therefore, the simple and applicable electrochemical sensor allowed the successful determination of AA, EP, and UA in pharmaceutical and biological preparations and proved that this method can be a good alternative and advantageous over the reported methods. In sum, this accurate, fast, and sensitive method could represent a useful tool for voltammetric determination of AA, EP, and UA. References Mahato, M.; Pal, P.; Tah, B.; Ghosh, M.; Talapatra, G. B. Colloids Surf. B 2011, 88, 141–149. Cao, Y. C. Nanomedicine 2008, 3, 467–469. Rosi, N. L.; Giljohann, D. A.; Thaxton, C. S.; Lytton-Jean, A. K. R.; Han, M. S.; Mirkin, C. A. Science 2006, 312, 1027–1030. Kaittanis, C.; Naser, S. A.; Perez, J. M. Nano Lett. 2006, 7, 380–383. Selvan, S. T.; Tan, T. T. Y.; Yi, D. K.; Jana, N. R. Langmuir 2009, 26, 11631–11641. Giljohann, D. A.; Mirkin, C. A. Nature 2009, 462, 461–464. Phillips, R. L.; Miranda, O. R.; You, C. C.; Rotello, V. M.; Bunz, U. H. F. Angew. Chem. Int. Ed. 2008, 47, 2590–2594. Bajaj, A.; Miranda, O. R.; Kim, I. B.; Phillips, R. L.; Jerry, D. J.; Bunz, U. H. F.; Rotello, V. M. Proc. Natl. Acad. Sci. 2009, 106, 10912–10916. Neely, A.; Perry, C.; Varisli, B.; Singh, A. K.; Arbneshi, T.; Senapati, D.; Kalluri, J. R.; Ray, P. C. ACS Nano 2009, 3, 2834–2840. Linse, S.; Cabaleiro-Lago, C.; Xue, W. F.; Lynch, I.; Lindman, S.; Thulin, E.; Radford, S. E.; Dawson, K. A. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 8691–8696. Wang, B.; Zhang, L.; Bae, S. C.; Granick, S. Proc. Natl. Acad. Sci. 2008, 105, 18171–18175. Vamvakaki, V.; Tsagaraki, K.; Chaniotakis, N. Anal. Chem. 2006, 78, 5538–5542. Han, D. S.; Lee, Y. J.; Kim, J. S.; Kim, E. Synth. Met. 2001, 117, 203–205. O’Regan, B.; Gr¨ atzel, M. Nature 1991, 353, 737–740. Umasankar, Y.; Prakash Periasamy, A.; Chen, S. M. Anal. Biochem. 2011, 411, 71–79. Reich, S.; Thomsen, C.; Maultzsch, J. Carbon Nanotubes: Basic Concepts and Physical Properties, Wiley–VCH, Weinheim, 2004. Liu, C.; Kuwahara, T.; Yamazaki, R.; Shimomura, M. Eur. Polym. J. 2007, 43, 3264–3276. Shahrokhian, S.; Asadian, E. J. Electroanal. Chem. 2009, 636, 40–46. Pihel, K.; Schroeder, T. J.; Wightman, R. M. Anal. Chem. 1994, 66, 4532–4537. Kalimuthu, P.; Abraham John, S. Anal. Chim. Acta 2009, 647, 97–103. Patric, G. L. An Introduction to Medical Chemistry, Oxford University Press, New York, 2005. Fotopoulou, M. A.; Ioannou, P. C. Anal. Chim. Acta 2002, 462, 179–185. Zhu, X.; Shaw, P. N.; Barrett, D. A. Anal. Chim. Acta 2003, 478, 259–269. Yao, H.; Sun, Y. Y.; Lin, X. H.; Chen, J. H.; Huang, L. Y. Luminescence 2006, 21, 112–117. Nagaraja, P.; Vasantha, R. A.; Sunitha, K. R. Talanta 2001, 55, 1039–1046. Wu, Y. Sens. Actuators B 2009, 137, 180–184. Salimi, A.; Banks, C. E.; Compton, R. G. Analyst 2004, 129, 225–228. Shahrokhian, S.; Ghalkhani, M.; Amini, M. K. Sens. Actuators B 2009, 137, 669–675. Manjunatha, H.; Nagaraju, D. H.; Suresh, G. S.; Venkatesha, T.V. Electroanalysis 2009, 21, 2198–2206.
• Behera, S.; Retna Raj, C. Biosens. Bioelectron. 2007, 23, 556–561.
• Ramesh, P.; Sampath, S. Electroanalysis 2004, 16, 866–869.
• Lupu, S.; Mucci, A.; Pigani, L.; Seeber, R.; Zanardi, C. Electroanalysis 2002, 14, 519–525.
• Salimi, A.; Pourbeyram, S. Talanta 2003, 60, 205–214.
• Ching, S.; Dudek, R. C.; Tabet, E. A. Langmuir 1994, 10, 1657–1659.
• Vasudevan, P.; Santhosh, M. N.; Tyagi, S. Transition Met. Chem. 1990, 15, 81–90.
• Morrin, A.; Wilbeer, F.; Ngamna, O.; Moulton, S. E.; Killard, A. J.; Wallace, G. G. Electrochem. Commun. 2005, 7, 317–322.
• Zen, J. M.; Kumar, A. S.; Chen, J. C. Anal. Chem. 2001, 73, 1169–1175.
• Ohnuki, Y.; Ohsaka, T.; Matsuda, H.; Oyama, N. J. Electroanal. Chem. 1983, 158, 55–68.
• Lin, L.; Chen, J.; Yao, H.; Chen, Y.; Zheng, Y.; Lin, X. Bioelectrochem. 2008, 73, 11–17.
• Milczarek, G.; Ciszewski, A. Electroanalysis 2004, 16, 1977–1983.
• Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253–1256.
• Ivanov, V.; Fonseca, A.; Nagy, J. B.; Lucas, A.; Lambin, P.; Bernaerts, D.; Zhang, X. B. Carbon 1995, 33, 1727– 17
• Kim, B.; Sigmund, W. M. Langmuir 2004, 20, 8239–8242.
• Raoof, J. B.; Ojani, R.; Amiri-Aref, M.; Baghayeri, M. Sens. Actuators B 2012, 166–167, 508–518.
• Raoof, J. B.; Ojani, R.; Baghayeri, M.; Amiri-Aref, M. Anal. Methods 2012, 4, 1579–1587.
• Wan, Q.; Wang, X.; Wang, X.; Yang, N. Polymer 2006, 47, 7684–7692.
• Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 2001.
• Raoof, J. B.; Ojani, R.; Baghayeri, M. Sens. Actuators B 2009, 143, 261–269.
• Harrison, J. A.; Khan, Z. A. J. Electroanal. Chem. 1970, 28, 131–138.
• Raoof, J. B.; Ojani, R.; Baghayeri, M. Anal. Methods 2011, 3, 2367–2373.
• Jiang, C.; Chen, H.; Yu, C.; Zhang, S.; Liu, B.; Kong, J. Electrochim. Acta 2009, 54, 1134–1140.
• Paul, S.; Lee, Y. S.; Choi, J. A.; Kang, Y. C.; Kim, D. W. Bull. Korean Chem. Soc. 2010, 31, 1228–1232.
• Ren, W.; Luo, H. Q.; Li, N. B. Biosens. Bioelectron. 2006, 21, 1086–1092.
• Beitollahi, H.; Ardakani, M. M.; Ganjipour, B.; Naeimi, H. Biosens. Bioelectron. 2008, 24, 362–368.