- Email: [email protected]
Tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence (ECL) enhanced by rutin on platinum electrode
Chinese Chemical Letters 18 (2007) 561–564 www.elsevier.com/locate/cclet
Tris(2,20 -bipyridyl)ruthenium(II) electrochemiluminescence (ECL) enhanced by rutin on platinum electrode Da Xu, Zhong Lan Gao, Na Li *, Ke An Li Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received 23 November 2006
Abstract Ru(bpy)32+ electrochemiluminescence (ECL) was applied to determination of rutin. ECL intensity of Ru(bpy)32+could be enhanced in the presence of rutin in basic solution on platinum electrode. At pH 9.9, light emission intensity was found to be linear with rutin in the range of 1–50 mmol/L. # 2007 Na Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Ru(bpy)32+; Electrochemiluminescence; Rutin; Platinum electrode
Since its innovation in 1980s, electrochemiluminescence (also called electrogenerated chemiluminescence, ECL) played a distinguished role amongst detection methods of analytical chemistry, excelling in sensibility and versatility [1]. Among all the ECL reagents, tris(bipyridyl)ruthenium(II) chloride (Ru(bpy)32+) has been most widely studied, because it has the advantages to produce light in an aqueous medium and does not need to deoxygen a prior to test [2]. Flavonoids, together with other nutraceutical molecules are extensively studied in chemoprevention of diseases, such as cancer, aging and cardiovascular disease, because these compounds showed antioxidant activity by reducing the incidence of DNA scission and lipid peroxidation [3–6]. Up to now, HPLC with UV–vis, fluorescence detectors [7– 9] and electrochemical approaches [10,11] were used to determine antioxidant capability of flavonoids. The anodic cyclic voltammetry peak signal on glassy-carbon electrode was almost unanimously adopted in electrochemical quantification of antioxidant capability of flavonoids. For determination of flavonoids by cyclic voltammetry, glassycarbon electrode often needs to be modified to get better response, because the adsorption ability of flavonoids to the electrode surface is very poor. One of the modification method is by carbon nanotubes [12]. It is even more difficult to obtain CV response on platinum electrode. However, enhanced electrochemiluminescence intensity was observed on platinum electrode, which could be advantageous for determination of rutin as well as helpful with understanding ECL mechanism. To the best of our knowledge, there are few studies using ECL emission for antioxidant activity determination as well as quantification of rutin. In this work, we for the first time explored the ECL of Ru(bpy)32+–rutin (chemical structure, see Fig. 1A) system using platinum electrode as the working electrode in a three-electrode system. The ECL
* Corresponding author. E-mail address: [email protected] (N. Li). 1001-8417/$ – see front matter # 2007 Na Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.03.010
562
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564
Fig. 1. (A) Molecular structure of rutin and (B) pH effect of ECL.
signal at negative potential was used in quantification of flavonoids. We have established an ECL method for determination of quercetin based on anodic light emission on glassy carbon electrode [13]. 1. Experimental Tris(2,20 -bipyridyl)ruthenium(II) chloride hexahydrate were purchased from Aldrich Chemical Co. Phosphoric acid (85%) and glacial acetic acid were purchased from Beijing Chemicals Co. Rutin, boric acid, sodium hydroxide and sodium chloride were purchased from Chemical Regents (Beijing, China). Double-distilled water (DDW) was used throughout the study. The electrochemiluminescence was performed on MPI-B multi-channel data-analysis system (Xi’an Remax Electronic Science Tech. Co. Ltd.) using three-electrode configuration. A platinum electrode (Ø 0.3 mm) was used as the working electrode. A Ag/AgCl (1 mol/L KCl) was used as the reference electrode. A platinum wire was used as the auxiliary electrode. Rutin and Ru(bpy)32+ were dissolved in DDW as the stock solution. The working solution was prepared daily by diluting the stock solution with Britton-Robinson buffer. 2. Results and discussion Influence of cyclovoltammetry scan range on electrochemiluminescence behaviour of Ru(bpy)32+–rutin system was studied. Strong steady ECL peaks (Fig. 2A) was observed during negative potential scan. The ECL peak intensity of positive potential scan (Fig. 2B) was about one tenth that of negative potential scan. The light emission was also obtained with potential in the range of 1.35 V to 1.35 V (Fig. 2C). Light emission intensity did not change when scan was performed in only negative potential range or in full potential range. The same phenomenon was observed for that of positive potential scan. However, electrochemical process during negative potential scan seemed to produce higher luminescence efficiency in Ru(bpy)32+–rutin system. pH effect on the ECL behavior of solutions with and without analyte was investigated. Enhancement of Ru(bpy)32+ ECL occurred at around 1.3 V (versus Ag/AgCl (1 mol/L KCl)) and the extent of enhancement changed with pH. From Fig. 1B can be seen that ECL intensity for Ru(bpy)32+ solution without rutin remained below 20 with pH less than 9, and increased to around 100 in pH of 9–12. For Ru(bpy)32+ solution with rutin, markedly enhanced ECL was observed as pH was greater than 8 and the emission intensity kept increasing as pH increased. When pH was greater than 10.4, ECL intensity began to increase dramatically. Corresponding cyclic voltammograms of Ru(bpy)32+–rutin solution at three different pH values are given in Fig. 3. The small reductive peak at around 0.7 V can be seen at pH 6.8 (Fig. 3A). It became smaller at pH 8.6 (Fig. 3B), and could not be observed at pH 11.6 (Fig. 3C). Because rutin has very poor solubility in acidic solution, ECL emission behavior at pH lower than 6.37 was not investigated. Subsequent study was carried out at pH 9.9 (Britton–Robinson formula), in order to get reasonably good ECL enhancement with low background signal. The enhanced ECL intensity (DI) was obtained by subtracting the ECL intensity of reagent blank (in the absence of rutin) from the total ECL (in the presence of both Ru(bpy)32+ and rutin). Slope of standard curves increased as concentration of Ru(bpy)32+ increased. For [Ru(bpy)32+] = 200 mmol/L, the DI was linear to concentration of rutin in
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564
563
Fig. 2. Cyclic voltammogram and corresponding ECL responses. (A) Scan range 1.35 V to 0.0 V, (B) scan range 0.0–1.35 V and (C) scan range 1.35 V to 1.35 V, all vs. Ag/AgCl (1 mol/L KCl). [Ru(bpy)32+] = 1.0 104 mol/L; [rutin] = 2.0 105 mol/L; scan rate 50 mV/s; PMT 800 V.
Fig. 3. Cyclic voltammograms of pH effect experiment. (A) pH 6.8, (B) pH 8.6 and (C) pH 11.6. For all, [Ru(bpy)32+] = 1.0 104 mol/L; [rutin] = 5.0 105 mol/L; scan range 0.0 V to 1.35 V (vs. Ag/AgCl (1 mol/L KCl)); scan rate 50 mV/s.
the range of 1 106 mol/L to 5 105 mol/L as the linear equation of DI = 4.18 107c + 2.97 102 (R2 = 0.99) with the detection limit of 0.2 mmol/L. 3. Conclusion Direct determination of rutin by ECL technique based on Ru(bpy)32+ was established at negative potential on platinum electrode. The pH dependency of ECL emission suggested that the role of the OH in the ECL emission is crucial. Efforts are put on improving the sensitivity. Study on the negative potential electrochemical process and luminescence mechanism with Ru(bpy)32+–rutin system on platinum electrode are also underway. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20475004) and Instrumental Analysis Fund of Peking University. References [1] R.D. Gerardi, N.W. Barnett, S.W. Lewis, Anal. Chim. Acta 378 (1–3) (1999) 1.
564 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564 A.W. Knight, Trac-Trends Anal. Chem. 18 (1) (1999) 47. K.E. Heim, A.R. Tagliaferro, D.J. Bobilya, J. Nutr. Biochem. 13 (10) (2002) 572. G. Williamson, C. Manach, Am. J. Clin. Nutr. 81 (1) (2005) 243S. S.U. Lule, W.S. Xia, Food Rev. Int. 21 (4) (2005) 367. C. Rice-Evans, Curr. Med. Chem. 8 (7) (2001) 797. U. Justesen, P. Knuthsen, T. Leth, J. Chromatogr. A 799 (1/2) (1998) 101. P. Hollman, J. Trijp, M. Buysman, Anal. Chem. 68 (19) (1996) 3511. H. Merken, G. Beecher, J. Agric. Food Chem. 48 (3) (2000) 577. B. Yang, et al. Abstr. Papers Am. Chem. Soc. 229 (2005) U54. M.E. Ghica, A.M.O. Brett, Electroanalysis 17 (4) (2005) 313. J.L. He, Y. Yang, X. Yang, Y.L. Liu, Z.H. Liu, G.L. Shen, R.Q. Yu, Sens. Actuators B 114 (2006) 94. R. Lei, H.W. Liu, N. Li, K.A. Li, Chin. Chem. Lett. 17 (11) (2006) 1499.
Tris(2,20 -bipyridyl)ruthenium(II) electrochemiluminescence (ECL) enhanced by rutin on platinum electrode Da Xu, Zhong Lan Gao, Na Li *, Ke An Li Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received 23 November 2006
Abstract Ru(bpy)32+ electrochemiluminescence (ECL) was applied to determination of rutin. ECL intensity of Ru(bpy)32+could be enhanced in the presence of rutin in basic solution on platinum electrode. At pH 9.9, light emission intensity was found to be linear with rutin in the range of 1–50 mmol/L. # 2007 Na Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Ru(bpy)32+; Electrochemiluminescence; Rutin; Platinum electrode
Since its innovation in 1980s, electrochemiluminescence (also called electrogenerated chemiluminescence, ECL) played a distinguished role amongst detection methods of analytical chemistry, excelling in sensibility and versatility [1]. Among all the ECL reagents, tris(bipyridyl)ruthenium(II) chloride (Ru(bpy)32+) has been most widely studied, because it has the advantages to produce light in an aqueous medium and does not need to deoxygen a prior to test [2]. Flavonoids, together with other nutraceutical molecules are extensively studied in chemoprevention of diseases, such as cancer, aging and cardiovascular disease, because these compounds showed antioxidant activity by reducing the incidence of DNA scission and lipid peroxidation [3–6]. Up to now, HPLC with UV–vis, fluorescence detectors [7– 9] and electrochemical approaches [10,11] were used to determine antioxidant capability of flavonoids. The anodic cyclic voltammetry peak signal on glassy-carbon electrode was almost unanimously adopted in electrochemical quantification of antioxidant capability of flavonoids. For determination of flavonoids by cyclic voltammetry, glassycarbon electrode often needs to be modified to get better response, because the adsorption ability of flavonoids to the electrode surface is very poor. One of the modification method is by carbon nanotubes [12]. It is even more difficult to obtain CV response on platinum electrode. However, enhanced electrochemiluminescence intensity was observed on platinum electrode, which could be advantageous for determination of rutin as well as helpful with understanding ECL mechanism. To the best of our knowledge, there are few studies using ECL emission for antioxidant activity determination as well as quantification of rutin. In this work, we for the first time explored the ECL of Ru(bpy)32+–rutin (chemical structure, see Fig. 1A) system using platinum electrode as the working electrode in a three-electrode system. The ECL
* Corresponding author. E-mail address: [email protected] (N. Li). 1001-8417/$ – see front matter # 2007 Na Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.03.010
562
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564
Fig. 1. (A) Molecular structure of rutin and (B) pH effect of ECL.
signal at negative potential was used in quantification of flavonoids. We have established an ECL method for determination of quercetin based on anodic light emission on glassy carbon electrode [13]. 1. Experimental Tris(2,20 -bipyridyl)ruthenium(II) chloride hexahydrate were purchased from Aldrich Chemical Co. Phosphoric acid (85%) and glacial acetic acid were purchased from Beijing Chemicals Co. Rutin, boric acid, sodium hydroxide and sodium chloride were purchased from Chemical Regents (Beijing, China). Double-distilled water (DDW) was used throughout the study. The electrochemiluminescence was performed on MPI-B multi-channel data-analysis system (Xi’an Remax Electronic Science Tech. Co. Ltd.) using three-electrode configuration. A platinum electrode (Ø 0.3 mm) was used as the working electrode. A Ag/AgCl (1 mol/L KCl) was used as the reference electrode. A platinum wire was used as the auxiliary electrode. Rutin and Ru(bpy)32+ were dissolved in DDW as the stock solution. The working solution was prepared daily by diluting the stock solution with Britton-Robinson buffer. 2. Results and discussion Influence of cyclovoltammetry scan range on electrochemiluminescence behaviour of Ru(bpy)32+–rutin system was studied. Strong steady ECL peaks (Fig. 2A) was observed during negative potential scan. The ECL peak intensity of positive potential scan (Fig. 2B) was about one tenth that of negative potential scan. The light emission was also obtained with potential in the range of 1.35 V to 1.35 V (Fig. 2C). Light emission intensity did not change when scan was performed in only negative potential range or in full potential range. The same phenomenon was observed for that of positive potential scan. However, electrochemical process during negative potential scan seemed to produce higher luminescence efficiency in Ru(bpy)32+–rutin system. pH effect on the ECL behavior of solutions with and without analyte was investigated. Enhancement of Ru(bpy)32+ ECL occurred at around 1.3 V (versus Ag/AgCl (1 mol/L KCl)) and the extent of enhancement changed with pH. From Fig. 1B can be seen that ECL intensity for Ru(bpy)32+ solution without rutin remained below 20 with pH less than 9, and increased to around 100 in pH of 9–12. For Ru(bpy)32+ solution with rutin, markedly enhanced ECL was observed as pH was greater than 8 and the emission intensity kept increasing as pH increased. When pH was greater than 10.4, ECL intensity began to increase dramatically. Corresponding cyclic voltammograms of Ru(bpy)32+–rutin solution at three different pH values are given in Fig. 3. The small reductive peak at around 0.7 V can be seen at pH 6.8 (Fig. 3A). It became smaller at pH 8.6 (Fig. 3B), and could not be observed at pH 11.6 (Fig. 3C). Because rutin has very poor solubility in acidic solution, ECL emission behavior at pH lower than 6.37 was not investigated. Subsequent study was carried out at pH 9.9 (Britton–Robinson formula), in order to get reasonably good ECL enhancement with low background signal. The enhanced ECL intensity (DI) was obtained by subtracting the ECL intensity of reagent blank (in the absence of rutin) from the total ECL (in the presence of both Ru(bpy)32+ and rutin). Slope of standard curves increased as concentration of Ru(bpy)32+ increased. For [Ru(bpy)32+] = 200 mmol/L, the DI was linear to concentration of rutin in
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564
563
Fig. 2. Cyclic voltammogram and corresponding ECL responses. (A) Scan range 1.35 V to 0.0 V, (B) scan range 0.0–1.35 V and (C) scan range 1.35 V to 1.35 V, all vs. Ag/AgCl (1 mol/L KCl). [Ru(bpy)32+] = 1.0 104 mol/L; [rutin] = 2.0 105 mol/L; scan rate 50 mV/s; PMT 800 V.
Fig. 3. Cyclic voltammograms of pH effect experiment. (A) pH 6.8, (B) pH 8.6 and (C) pH 11.6. For all, [Ru(bpy)32+] = 1.0 104 mol/L; [rutin] = 5.0 105 mol/L; scan range 0.0 V to 1.35 V (vs. Ag/AgCl (1 mol/L KCl)); scan rate 50 mV/s.
the range of 1 106 mol/L to 5 105 mol/L as the linear equation of DI = 4.18 107c + 2.97 102 (R2 = 0.99) with the detection limit of 0.2 mmol/L. 3. Conclusion Direct determination of rutin by ECL technique based on Ru(bpy)32+ was established at negative potential on platinum electrode. The pH dependency of ECL emission suggested that the role of the OH in the ECL emission is crucial. Efforts are put on improving the sensitivity. Study on the negative potential electrochemical process and luminescence mechanism with Ru(bpy)32+–rutin system on platinum electrode are also underway. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20475004) and Instrumental Analysis Fund of Peking University. References [1] R.D. Gerardi, N.W. Barnett, S.W. Lewis, Anal. Chim. Acta 378 (1–3) (1999) 1.
564 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
D. Xu et al. / Chinese Chemical Letters 18 (2007) 561–564 A.W. Knight, Trac-Trends Anal. Chem. 18 (1) (1999) 47. K.E. Heim, A.R. Tagliaferro, D.J. Bobilya, J. Nutr. Biochem. 13 (10) (2002) 572. G. Williamson, C. Manach, Am. J. Clin. Nutr. 81 (1) (2005) 243S. S.U. Lule, W.S. Xia, Food Rev. Int. 21 (4) (2005) 367. C. Rice-Evans, Curr. Med. Chem. 8 (7) (2001) 797. U. Justesen, P. Knuthsen, T. Leth, J. Chromatogr. A 799 (1/2) (1998) 101. P. Hollman, J. Trijp, M. Buysman, Anal. Chem. 68 (19) (1996) 3511. H. Merken, G. Beecher, J. Agric. Food Chem. 48 (3) (2000) 577. B. Yang, et al. Abstr. Papers Am. Chem. Soc. 229 (2005) U54. M.E. Ghica, A.M.O. Brett, Electroanalysis 17 (4) (2005) 313. J.L. He, Y. Yang, X. Yang, Y.L. Liu, Z.H. Liu, G.L. Shen, R.Q. Yu, Sens. Actuators B 114 (2006) 94. R. Lei, H.W. Liu, N. Li, K.A. Li, Chin. Chem. Lett. 17 (11) (2006) 1499.