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Enhanced electrochemiluminescent of lucigenin at an electrically heated cylindrical microelectrode
Electrochemistry Communications 9 (2007) 269–274 www.elsevier.com/locate/elecom
Enhanced electrochemiluminescent of lucigenin at an electrically heated cylindrical microelectrode Zhenyu Lin, Jianjun Sun, Jinhua Chen, Liang Guo, Guonan Chen
*
The Ministry of Education, Key Laboratory of Analysis and Detection Technology for Food Safety, Fuzhou University, Fuzhou, Fujian 350002, China Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China Received 12 August 2006; received in revised form 4 September 2006; accepted 8 September 2006 Available online 19 October 2006
Abstract In this paper, an ECL detection system equipped with an electrically heating controlled cylindrical microelectrode (HME) was used to study the ECL behavior of lucigenin. The ECL intensity of lucigenin would be increased at elevated electrode temperature but the noise had not been increased. It was found that ECL intensity at higher temperature of electrode surface (80 C) was more than two magnitudes stronger than that at the room temperature (22 C). The detection limit for ECL of lucigenin on a HME is much lower than that on an electrode without heating, based on which, it is possible to establish a more sensitive method for measurement of ECL by using a HME. The heating of electrode has been used to renew the electrode, which avoid the tedious work for refreshing the electrode surface. The reproducibility of lucigenin ECL system at HME is satisfactory. 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemiluminescence; Heating microelectrode; Lucigenin
1. Introduction Electrochemiluminescence (ECL), also known as electrogenerated chemiluminescence, is a detection technique based on electrolytically generated CL. In the generally accepted reaction mechanism of ECL, both cation and anion radicals are electrolytically generated on the electrode surfaces and subsequently undergo an annihilation reaction to emit light. The analytical methods based on ECL have the advantages of high sensitivity, high selectivity, good reproducibility and being controlled easily. Therefore, ECL has become an important detection method in analytical chemistry in recent years [1–3]. ECL usually occurs at the surface of electrode or in the solution adjacent to the electrode, therefore adjusting the position of electrode or changing the material of electrode can * Corresponding author. Address: Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China. Tel.: +86 591 87893315; fax: +86 591 83713866. E-mail address: [email protected] (G. Chen).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.004
always improve the behavior of ECL. Wallace and Bard had reported the effect of temperature on the ECL efficiency of RuðbpyÞ2þ in acetonitrile, however, they only 3 considered the effect of temperature of bulk solution not the temperature of electrode surface [4]. In fact, the ECL occurred at the surface of electrode is not only related to the diffusion of the luminescent compound at the electrode, but also related to the convection of luminescent compounds near the electrode surface, while convection is greatly affected by temperature. ECL investigations at elevated temperatures can be performed on a directly heated electrodes, the most promising features of this way is that only a thin solution layer becomes hot and molecules outside this layer are not exposed to high temperature when the electrode is heated. This approach is particularly advantageous for studying the ECL behavior of volatile substances and dissolved gases at increased temperature. Moreover, many electrochemical reactions are inert at room temperature, while take place at appreciable rates when the temperature is increased [5]. Therefore, investigation of the effect of temperature at electrode surface is very
270
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
significant in improving the sensitivity, selectivity, stability and reproducibility of ECL. The main problem for investigating the effect of temperature of electrode surface on ECL reaction is to design and establish an electrode system, which will leave the temperature of bulk solution unchanged when the temperature of the electrode surface is increased. Heating controlled microelectrode is probably a useful tool to solve this problem. However, no attention, except our previous work [6], has been paid to the effect of temperature of electrode surface on ECL. The CL of lucigenin (N,N 0 -dimethyl-9-9 0 -biacridinium dication) has been known since 1935 [7] and the ECL of lucigenin has been studied in non-aqueous and aqueous solutions [8–13]. Tamamushi and Akiyama [8] observed light emission during electrolysis of lucigenin in aqueous alkaline solutions at a platinum cathode. Haapakka and Kankare’s study confirmed Tamamushi and Akiyama’s result [9]. Legg and Hercules [10] investigated this electrolysis system under the same conditions and observed no light emission, but by replacing the platinum cathode with a mercury pool cathode and by using an unbuffered solution at pH 7.0, the light emission was observed. They thought that the light emission at the platinum electrode was not possible, because oxygen was directly reduced to hydroxide ions under these conditions, where as in neutral solution at a mercury cathode, oxygen was reduced to hydrogen peroxide. Cui et al. [11] studied the ECL of lucigenin at glassy carbon electrode and found two pathways for the ECL of lucigenin. Till now, only few papers on ECL of lucigenin in aqueous media have been reported and few analytical applications involving ECL of lucigenin have been proposed, one of the reasons is that it is very hard to get the reproducible results [11–13]. If the electrode surface could be renewed easily, the application of lucigenin ECL would be extensive greatly. Gru¨ndler had introduced a hot-wire electrode and applied it in many electrochemical applications [14– 22,5,23–26]. Such studies rely on the use of very fast temperature change and a well-controlled temperature at the surface and in a thin solution layer with the constant thickness. The main advantage of this technique is that the temperature of the microelectrodes can affect the electrochemical reaction at the surface of electrode, but keeping the temperature of the bulk solution unchanged, and the pollutants on the electrode surface can also be cleaned up easily by heating. So it is possible to get a reproducible result. This technique is based on a symmetric electrode arrangement, complicated manual fabrication and special equipment for generating alternating heating current are needed. Based on this technique, we developed a new ECL detection system equipped with an electrically heating controlled microelectrode. This system has been 2þ used to investigate the ECL behavior of RuðbpyÞ3 and 2þ 2 RuðbpyÞ3 –C2 O4 under heating, and found that the detec2þ 2 tion limit for ECL of RuðbpyÞ2þ 3 and RuðbpyÞ3 –C2 O4 on a heated cylindrical microelectrode was much lower than that on an electrode without heating [6].
In this paper, the ECL detection system equipped with an electrically controlled heating cylindrical microelectrode was used to investigate the ECL behavior of lucigenin at different electrode surface temperature. It was found that ECL intensity at higher temperature of electrode surface (80 C) was more than two magnitudes stronger than that at the room temperature (22 C), and the tedious procedure for clean-up the surface of electrode could be avoided, which undoubtedly indicated that this new controllable approach for the temperature of electrode surface would have great potential application to future ECL studies. 2. Experimental 2.1. Reagents and apparatus Lucigenin was obtained from Fluka and used without further purification. All the other chemicals were of analytical grade. The concentrations of H2O2 is 30%, HCl is 36.5%. Water used was obtained by the purification of distilled water with a Millipore Milli-Q system. A laboratory-built ECL detection system equipped with an electrically heating controlled cylindrical microelectrode (HME) was used in the experiment, and the detail description of this system has been reported previously [6]. Two platinum wires (25 lm in diameter and 6 mm in long, Goodfellow Cambridge, UK) worked as the working electrode. The 100 kHz alternating current (AC) generated from a function generator was used to heat the microelectrode. Changing the output of the function generator could change the temperature of the HME. The ECL detection system consisted with a BPCL UltraWeak Luminescence Analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) and a CHI 660a electrochemical system (CH Instruments, USA). An electrochemical cell with three-electrode system was used, the cell has an optically flat bottom; the HME was used as the working electrode, a platinum wire as counter electrode and Ag/AgCl (saturated with KCl) as reference electrode. The working electrode was aligned parallel to the cell bottom. In order to prevent any light emission from the counter electrode, the counter electrode was immerged in a glass tube with porous glass filter in the end. And the blank solution without lucigenin was filled in the tube. The reaction cell was placed directly at the top of the PMT and was fixed in a dark detection chamber. A computer was used for data treatment. Preparation of the heated cylindrical microelectrode has been described in our previous work [6]. 2.2. Temperature calibration The temperature of electrode surface was determined by using a chronopotentiometric method, which has been described in the literature [5]. In brief, two similarly HME were mounted in two separate cells each contain solutions of 0.005 mol/L K3[Fe(CN)6] and 0.005 mol/L
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
K4[Fe(CN)6] in 1 mol/L KCl, and the two cells were connected electrically through a salt bridge filled with the same solution. Then the temperature coefficient of the electrode potential for isothermal was determined firstly, and the relationship between the electrode potential and the heating voltage (V) (the AC voltage from the function generator) was determined. Then the relationship between the heating voltage and the temperature of electrode surface (Te) can be deduced. 2.3. Procedure The HME was cleaned with ethanol, rinsed with water and finally scanned with cyclic voltammetry in 0.1 mol/L aqueous KCl solution between 0.2 and 0.5 V at room temperature for several times before each measurement. The ECL cell was washed with 0.2 mol/L nitric acid and water before use. KCl solution and lucigenin were added successively to a 10 mL volumetric flask and diluted with water to required volume. Five millilitre of this solution was transferred to the ECL cell. Then the working electrode was heated and differential pulse voltammetry was preformed to get the ECL signal. 3. Results and discussion 3.1. Control and measurement of the temperature at surface of HME In order to examine the electrochemical and ECL signals acquired with heated microelectrodes under different temperature, it is important to develop a convenient method to control and measure the temperature of the electrode surface. Direct measurement of temperature using commercially available sensing elements is not practically feasible. In this experiment, the temperature was determined according to the method described in the literature [5]. The relationship between the heating voltage (V) and the temperature of electrode surface (Te) is shown in Fig. 1, which agrees with Gru¨ndler’s report [5].
3.2. The ECL behavior of lucigenin on the conventional Pt electrode Since the results of many previous studies were conflicted, the ECL behavior of lucigenin on the conventional platinum electrode was studied first. When an applied potential was scanned at a platinum electrode from 0 to 0.8 V, an ECL peak of lucigenin could be observed at about 0.6 V. This phenomenon indicated that the light emission was probably occurred at the surface of platinum electrode, which confirmed Haapakka and Tamamushi’s conclusion [9]. When the solution had been deaerated, the ECL signal disappeared, based on which we could conclude that the ECL emission of lucigenin was directly related to the dissolved oxygen in neutral solution. The ECL peak of lucigenin was mainly obtained from the reaction of reduced lucigenin with hydrogen peroxide electrogenerated at the surface of the electrode. Linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) were used to examine the effects of electrochemical scanning mode on this ECL system. The result showed that, when DPV was used, a stable ECL intensity could be obtained. So DPV mode was selected for the subsequent investigation. To establish the optimal conditions, the luminescent intensity was measured as a function of pulse amplitude, pulse width and pulse period. The selected optimum parameters for DPV were showed in Table 1. The typical ECL curve of lucigenin was shown in Fig. 2.
Table 1 Differential pulse voltammetry parameters Init E (V)
Final E (V)
Incr E (V)
Amplitude (V)
Pulse width (s)
Sampling width (s)
Pulse period (s)
0.0
0.8
0.004
0.05
0.05
0.0167
0.2
1.0x10 -5
1200
Te/ o C
ECL intensity
800
Current/A
8.0x10 -6
1000
90 80 70 60 50 40 30 20 10 0
271
B
a
6.0x10-6 4.0x10-6
a' 2.0x10-6
600
b'
0.0 0.0
-0.2
-0.4
-0.6
-0.8
Potential/V
400 200
A b
0 0.0
-0.2
-0.4
-0.6
-0.8
Potential/V
0
2
4
6 Voltage/V
8
10
12
Fig. 1. The relationship between heating voltage (the AC voltage from the function generator) and Te.
Fig. 2. (A) Typical ECL curves of lucigenin on Platinum electrode in 0.1 mol/L KCl solution under DPV scanning; (B) DPV curves lucigenin on Platinum electrode in 0.1 mol/L KCl solution. (a, a 0 ) solution saturated with air and (b, b 0 ) solution had been deaerated. Concentration of lucigenin: 1.0 · 104 mol/L. The reference electrode was Ag/AgCl (saturated with KCl) electrode. The DPV parameters were shown in Table 1.
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Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
3.3. Electrochemical and ECL behavior of lucigenin solution saturated with air
4000
4000 3500
3500
a
3000
B
2500
Current/A
8.0x10
-6
7.0x10
-6
6.0x10
-6
5.0x10
-6
4.0x10
-6
3.0x10
-6
2.0x10
-6
1.0x10
-6
A
a b
B
c
2000
3000
1500 1000
ECL Intensity/a.u.
Since the mechanism of lucigenin ECL has been known to be related to the reduction of oxygen dissolved in the solution, the electrochemical reduction of oxygen on Pt electrode, like most electrode reactions, is temperature dependent, so the temperature of the electrode will affected the ECL of lucigenin. Fig. 3 shows the DPV curves of the 1.0 · 104 mol/L lucigenin solution at different Te. There are two obviously reduction peak in the curve. Peak A comes from the reduction of oxygen, peak B comes from the reduction of lucigenin. The peak currents are increased with the increasing of the electrode temperature. The convection seems to be responsible for these increments. When the microelectrode is heated, the temperature of the electrode surface increases quickly, a higher temperature at the electrode surface will speed up the diffusion and induce convection. The convection is favorable for transport of electroactive substances to the electrode, unlike non-convective isothermal systems, where the concentration gradient is the only driving force for mass transfer. And mass transfer is greatly improved at the electrode surface, this leads to a larger current being observed. The peak potentials for the reduction of oxygen and lucigenin are shifted positively respectively with the increasing of electrode surface temperature, which indicates that higher temperature also causes a decrease in overvoltage (which can be seen from the spot lines in the Fig. 3). Fig. 4 shows the ECL of 1.0 · 105 mol/L lucigenin at different electrode temperature. It can be seen from the figure that the ECL intensity increase with the increasing of electrode temperature. At room temperature (22 C), the ECL of the studied system is very weak, but when the temperature is risen to 80 C, the ECL intensity is 140 times as
2500
g
500 0 20
30
40
2000
50
60
70
80
Temperature
1500 1000
A
500 0 0.0
-0.2
-0.4
-0.6
-0.8
Potential /v
Fig. 4. (A) The ECL intensity-potential curves of 1.0 · 105 mol/L lucigenin at different Te. (B) The ECL intensity at different temperature. (a) 80 C; (b) 76 C; (c) 67 C; (d) 59 C; (e) 44 C; (f) 28 C and (g) 22 C. The other conditions were the same as in Fig. 3.
that at room temperature, and the preferred potential for occurrence of ECL have a little shift toward positive direction with increasing of temperature, which also indicates that the higher temperature is favorable to decrease the overvoltage. It was reported that trace H2O2 would greatly increase the ECL of lucigenin at platinum electrode, and the enhancement of intensity increased with the increasing of the concentration of H2O2 [3]. It had also been reported that mass transfer for H2O2 would increase at elevated temperature [27]. At elevated electrode temperature, more H2O2 had been produced, together with the increase of mass transfer caused by temperature. So the ECL intensity of the system was enhanced greatly at elevated electrode temperature. The most obvious application of HME is based on its enhancement for the analytical signals of the analytes, however, an increase in signal is beneficial only if there is no significant increase in the noise level. The effect of temperature on the background noise level in this ECL system had been examined, the results indicated that no significant increase in noise could be observed when the electrode was heated. But when the temperature was elevated to 85 C, an increase of noise was observed because of the noticeable formation of air bubbles at the surface. So in this experiment, the highest temperature of the electrode was set at 80 C. 3.4. Reproducibility
0.2
0.0
-0.2
-0.4
-0.6
-0.8
Potential/v 4
Fig. 3. DPV curves of 1.0 · 10 mol/L lucigenin at different Te in absence of oxygen. (a) 44 C; (b) 28 C and (c) 22 C. The working electrodes were 25 lm in diameter and 6 mm in long. The reference electrode was Ag/AgCl (saturated with KCl) electrode. The DPV parameters were shown in Table 1.
The big problem for the application of an ECL system is the tedious procedure for cleaning-up the poisoned electrode surface to get the reproducible result. Though the ECL of lucigenin is of interest because of its possible applications in determination of trace metal and some organic compounds, till now there were only few reports related to the analytical application of ECL of lucigenin. The main
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
reason for this is the low solubility of lucigenin’s reduction product, which can precipitate onto the platinum electrode surface. In order to solve this problem, Cui et al. [11] used glassy carbon electrode to study the ECL of lucigenin because the possible precipitation of the reduction product could be cleaned up more easily than that at platinum electrode. Okajima and Ohsaka [28] modified the electrodes with self-assembled monolayers and studied the ECL of lucigenin in the solutions with surfactants. All these have somewhat improved the reproducible of the ECL. It was reported that elevated electrode temperature could minimize surface fouling during voltammetric and amperometric measurement of NADH on platinum [27]. Heating the electrode could also be useful for regeneration of electrode surface. In our ECL detection system, it was found that when the HME was heated in the air, the temperature of the electrode surface could be as high as several hundreds degree, which would make the fouling to be easily decomposed. Under such treatment, the surface of electrode can be renewed to avoid the tedious procedure for cleaning-up the electrode. Fig. 5 shows the reproducibility of lucigenin ECL at the HME, the relative standard deviation for intensity of the ECL is 3.4% (n = 5). Which indicated that the electrode could be renewed by heating, which confirmed Baranski’s proposal, who proposed to use electrode heating for cleaning [29]. 3.5. Linear response range and detection limit of lucigenin at HME It was found that the ECL intensity (IECL) has linear relationship with the concentrations of lucigenin. Eq. (1)
300
100 50
ty/a.u. ECLIntensi
150
0 5 4 0.0
3 -0.1
-0 .2
-0 .3
Pote
ntia
-0.4
l/v
2 -0 .5
-0.6
-0.7
shows the linearity relationship between ECL intensity(I) and concentration of lucigenin(C) at the electrode without heating, i.e. at room temperature (22 C) I=a:u: ¼ 3:89 106 C=mol=L þ 23:2;
R ¼ 0:9989
ð1Þ
The linear range was found to be in the range of 1.0 · 105–3.0 · 104 mol/L, the detection limit was found to be 7.0 · 106 mol/L (defined as the concentration that gave rise to a signal-to-noise ratio of 3) when Te was 22 C, however, the detection limit was found to be 2.0 · 108 mol/L at 80 C and the linear range was found to be 1.0 · 107–4.0 · 106 mol/L, and the regression equation was shown in Eq. (2) I=a:u: ¼ 6:44 108 C=mol=L þ 23:4;
R ¼ 0:9993
ð2Þ
Which indicates that the detection limit can be improved greatly at higher Te. 4. Conclusions We have demonstrated the dramatic effect of electrode surface temperature on the ECL intensity of lucigenin, and the noise of this system has not been affected by the temperature in the studied range. The detection limit for ECL of lucigenin on a heated cylindrical microelectrode is much lower than that on an electrode without heating. Heating of the electrode has also been applied to clean the electrode successfully, which is especially important to the lucigenin ECL system. This technique offers not only new methods to explore ECL behavior of lucigenin but also the possibility of extending the application of ECL system at HME. Many other possible application fields of ECL at HME are being studied in our laboratory, for example, the temperature of HME will affect enzyme’s activity; based on which a sensitive biosensor is possible to be prepared. Acknowledgements
250 200
273
1 -0.8
Fig. 5. The reproductibility of ECL of 1.0 · 105 mol/L lucigenin when Te is 35 C. Other conditions are the same as in Fig. 3.
This project was financially supported by the National Nature Science Foundation of China (20575011) and the Science Foundation of State Education Department (20040386002). References [1] G.N. Chen, R.E. Lin, Z.F. Zhao, J.P. Duan, L. Zhang, Anal. Chim. Acta 341 (1997) 251. [2] A.W. Knight, Trends Anal. Chem. 18 (1999) 47. [3] G.N. Chen, L. Zhang, R.E. Lin, Z.C. Yang, J.P. Duan, H.Q. Chen, D.B. Hibbert, Talanta 50 (2000) 1275. [4] W.L. Willace, A.J. Bard, J. Phys. Chem. 83 (1979) 1350. [5] T. Zerihun, P. Gru¨ndler, J. Electroanal. Chem. 404 (1996) 243. [6] Z. Lin, J. Sun, L. Guo, G.N. Chen, Anal. Chim. Acta 564 (2006) 226. [7] K. Gleu, W. Petsch, Angew. Chem. 48 (1935) 57. [8] B. Tamamushi, H. Akiyama, Trans. Faraday Soc. 35 (1939) 491. [9] K.E. Haapakka, J.J. Kankare, Anal. Chim. Acta 130 (1981) 415. [10] K.D. Legg, D.M. Hercules, J. Am. Chem. Soc. 91 (1969) 1902.
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[16] [17] [18] [19]
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274 Y.G. Sun, H. Cui, X.Q. Lin, J. Luminesc. 92 (2001) 205. Y.Y. Su, J. Wang, G.N. Chen, Talanta 65 (2005) 531. H.L. Qi, C.X. Zhang, J. Luminesc. 19 (2004) 21. P. Gru¨ndler, T. Zerihun, A. Kirbs, H. Grabow, Anal. Chim. Acta 305 (1995) 232. J. Wang, P. Gru¨ndler, G.U. Flechsig, M. Jasinski, J.M. Lu, J.Y. Wang, Z.Q. Zhao, B.M. Tian, Anal. Chim. Acta 396 (1999) 33. O. Korbut, P. Gru¨ndler, J. Electroanal. Chem. 506 (2001) 143. P. Gru¨ndler, A. Kirbs, T. Zerihum, Analyst 121 (1996) 805. J. Wang, P. Gru¨ndler, G.U. Flechsig, M. Jasinski, G. Rivas, E. Sahlin, J.L.L. Paz, Anal. Chem. 72 (2000) 3752. T. Voss, P. Gru¨ndler, C.M.A. Brett, O.A.M. Brett, J. Pharmac. Biomedi. Anal. 19 (1999) 127.
[20] A. Beckmann, A. Schneider, P. Gru¨ndler, Electrochem. Comm. 1 (1999) 46. [21] M. Jasinski, A. Kirbs, M. Schmehl, P. Gru¨ndler, Electrochem. Comm. 1 (1999) 26. [22] A. Beckmann, B.A. Coles, R.G. Compton, P. Gru¨ndler, F. Marken, A. Neudick, J. Phys. Chem. B 104 (2000) 764. [23] P. Gru¨ndler, Fresenius J. Anal.Chem. 362 (1998) 180. [24] P. Gru¨ndler, G.U. Flechsig, Electrochim. Acta 43 (1998) 3451. [25] T. Zerihun, P. Gru¨ndler, J. Electroanal. Chem. 415 (1996) 85. [26] J. Wang, P. Gru¨ndler, J. Electroanal. Chem. 540 (2003) 153. [27] C. Lau, G.U. Flechsig, P. Gru¨ndler, J. Wang, Anal. Chim. Acta 554 (2005) 74. [28] T. Okajima, T. Ohsaka, J. Electroanal. Chem. 534 (2002) 181. [29] S.A. Baranski, Anal. Chem. 74 (2002) 1294.
Enhanced electrochemiluminescent of lucigenin at an electrically heated cylindrical microelectrode Zhenyu Lin, Jianjun Sun, Jinhua Chen, Liang Guo, Guonan Chen
*
The Ministry of Education, Key Laboratory of Analysis and Detection Technology for Food Safety, Fuzhou University, Fuzhou, Fujian 350002, China Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China Received 12 August 2006; received in revised form 4 September 2006; accepted 8 September 2006 Available online 19 October 2006
Abstract In this paper, an ECL detection system equipped with an electrically heating controlled cylindrical microelectrode (HME) was used to study the ECL behavior of lucigenin. The ECL intensity of lucigenin would be increased at elevated electrode temperature but the noise had not been increased. It was found that ECL intensity at higher temperature of electrode surface (80 C) was more than two magnitudes stronger than that at the room temperature (22 C). The detection limit for ECL of lucigenin on a HME is much lower than that on an electrode without heating, based on which, it is possible to establish a more sensitive method for measurement of ECL by using a HME. The heating of electrode has been used to renew the electrode, which avoid the tedious work for refreshing the electrode surface. The reproducibility of lucigenin ECL system at HME is satisfactory. 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemiluminescence; Heating microelectrode; Lucigenin
1. Introduction Electrochemiluminescence (ECL), also known as electrogenerated chemiluminescence, is a detection technique based on electrolytically generated CL. In the generally accepted reaction mechanism of ECL, both cation and anion radicals are electrolytically generated on the electrode surfaces and subsequently undergo an annihilation reaction to emit light. The analytical methods based on ECL have the advantages of high sensitivity, high selectivity, good reproducibility and being controlled easily. Therefore, ECL has become an important detection method in analytical chemistry in recent years [1–3]. ECL usually occurs at the surface of electrode or in the solution adjacent to the electrode, therefore adjusting the position of electrode or changing the material of electrode can * Corresponding author. Address: Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China. Tel.: +86 591 87893315; fax: +86 591 83713866. E-mail address: [email protected] (G. Chen).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.004
always improve the behavior of ECL. Wallace and Bard had reported the effect of temperature on the ECL efficiency of RuðbpyÞ2þ in acetonitrile, however, they only 3 considered the effect of temperature of bulk solution not the temperature of electrode surface [4]. In fact, the ECL occurred at the surface of electrode is not only related to the diffusion of the luminescent compound at the electrode, but also related to the convection of luminescent compounds near the electrode surface, while convection is greatly affected by temperature. ECL investigations at elevated temperatures can be performed on a directly heated electrodes, the most promising features of this way is that only a thin solution layer becomes hot and molecules outside this layer are not exposed to high temperature when the electrode is heated. This approach is particularly advantageous for studying the ECL behavior of volatile substances and dissolved gases at increased temperature. Moreover, many electrochemical reactions are inert at room temperature, while take place at appreciable rates when the temperature is increased [5]. Therefore, investigation of the effect of temperature at electrode surface is very
270
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
significant in improving the sensitivity, selectivity, stability and reproducibility of ECL. The main problem for investigating the effect of temperature of electrode surface on ECL reaction is to design and establish an electrode system, which will leave the temperature of bulk solution unchanged when the temperature of the electrode surface is increased. Heating controlled microelectrode is probably a useful tool to solve this problem. However, no attention, except our previous work [6], has been paid to the effect of temperature of electrode surface on ECL. The CL of lucigenin (N,N 0 -dimethyl-9-9 0 -biacridinium dication) has been known since 1935 [7] and the ECL of lucigenin has been studied in non-aqueous and aqueous solutions [8–13]. Tamamushi and Akiyama [8] observed light emission during electrolysis of lucigenin in aqueous alkaline solutions at a platinum cathode. Haapakka and Kankare’s study confirmed Tamamushi and Akiyama’s result [9]. Legg and Hercules [10] investigated this electrolysis system under the same conditions and observed no light emission, but by replacing the platinum cathode with a mercury pool cathode and by using an unbuffered solution at pH 7.0, the light emission was observed. They thought that the light emission at the platinum electrode was not possible, because oxygen was directly reduced to hydroxide ions under these conditions, where as in neutral solution at a mercury cathode, oxygen was reduced to hydrogen peroxide. Cui et al. [11] studied the ECL of lucigenin at glassy carbon electrode and found two pathways for the ECL of lucigenin. Till now, only few papers on ECL of lucigenin in aqueous media have been reported and few analytical applications involving ECL of lucigenin have been proposed, one of the reasons is that it is very hard to get the reproducible results [11–13]. If the electrode surface could be renewed easily, the application of lucigenin ECL would be extensive greatly. Gru¨ndler had introduced a hot-wire electrode and applied it in many electrochemical applications [14– 22,5,23–26]. Such studies rely on the use of very fast temperature change and a well-controlled temperature at the surface and in a thin solution layer with the constant thickness. The main advantage of this technique is that the temperature of the microelectrodes can affect the electrochemical reaction at the surface of electrode, but keeping the temperature of the bulk solution unchanged, and the pollutants on the electrode surface can also be cleaned up easily by heating. So it is possible to get a reproducible result. This technique is based on a symmetric electrode arrangement, complicated manual fabrication and special equipment for generating alternating heating current are needed. Based on this technique, we developed a new ECL detection system equipped with an electrically heating controlled microelectrode. This system has been 2þ used to investigate the ECL behavior of RuðbpyÞ3 and 2þ 2 RuðbpyÞ3 –C2 O4 under heating, and found that the detec2þ 2 tion limit for ECL of RuðbpyÞ2þ 3 and RuðbpyÞ3 –C2 O4 on a heated cylindrical microelectrode was much lower than that on an electrode without heating [6].
In this paper, the ECL detection system equipped with an electrically controlled heating cylindrical microelectrode was used to investigate the ECL behavior of lucigenin at different electrode surface temperature. It was found that ECL intensity at higher temperature of electrode surface (80 C) was more than two magnitudes stronger than that at the room temperature (22 C), and the tedious procedure for clean-up the surface of electrode could be avoided, which undoubtedly indicated that this new controllable approach for the temperature of electrode surface would have great potential application to future ECL studies. 2. Experimental 2.1. Reagents and apparatus Lucigenin was obtained from Fluka and used without further purification. All the other chemicals were of analytical grade. The concentrations of H2O2 is 30%, HCl is 36.5%. Water used was obtained by the purification of distilled water with a Millipore Milli-Q system. A laboratory-built ECL detection system equipped with an electrically heating controlled cylindrical microelectrode (HME) was used in the experiment, and the detail description of this system has been reported previously [6]. Two platinum wires (25 lm in diameter and 6 mm in long, Goodfellow Cambridge, UK) worked as the working electrode. The 100 kHz alternating current (AC) generated from a function generator was used to heat the microelectrode. Changing the output of the function generator could change the temperature of the HME. The ECL detection system consisted with a BPCL UltraWeak Luminescence Analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) and a CHI 660a electrochemical system (CH Instruments, USA). An electrochemical cell with three-electrode system was used, the cell has an optically flat bottom; the HME was used as the working electrode, a platinum wire as counter electrode and Ag/AgCl (saturated with KCl) as reference electrode. The working electrode was aligned parallel to the cell bottom. In order to prevent any light emission from the counter electrode, the counter electrode was immerged in a glass tube with porous glass filter in the end. And the blank solution without lucigenin was filled in the tube. The reaction cell was placed directly at the top of the PMT and was fixed in a dark detection chamber. A computer was used for data treatment. Preparation of the heated cylindrical microelectrode has been described in our previous work [6]. 2.2. Temperature calibration The temperature of electrode surface was determined by using a chronopotentiometric method, which has been described in the literature [5]. In brief, two similarly HME were mounted in two separate cells each contain solutions of 0.005 mol/L K3[Fe(CN)6] and 0.005 mol/L
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
K4[Fe(CN)6] in 1 mol/L KCl, and the two cells were connected electrically through a salt bridge filled with the same solution. Then the temperature coefficient of the electrode potential for isothermal was determined firstly, and the relationship between the electrode potential and the heating voltage (V) (the AC voltage from the function generator) was determined. Then the relationship between the heating voltage and the temperature of electrode surface (Te) can be deduced. 2.3. Procedure The HME was cleaned with ethanol, rinsed with water and finally scanned with cyclic voltammetry in 0.1 mol/L aqueous KCl solution between 0.2 and 0.5 V at room temperature for several times before each measurement. The ECL cell was washed with 0.2 mol/L nitric acid and water before use. KCl solution and lucigenin were added successively to a 10 mL volumetric flask and diluted with water to required volume. Five millilitre of this solution was transferred to the ECL cell. Then the working electrode was heated and differential pulse voltammetry was preformed to get the ECL signal. 3. Results and discussion 3.1. Control and measurement of the temperature at surface of HME In order to examine the electrochemical and ECL signals acquired with heated microelectrodes under different temperature, it is important to develop a convenient method to control and measure the temperature of the electrode surface. Direct measurement of temperature using commercially available sensing elements is not practically feasible. In this experiment, the temperature was determined according to the method described in the literature [5]. The relationship between the heating voltage (V) and the temperature of electrode surface (Te) is shown in Fig. 1, which agrees with Gru¨ndler’s report [5].
3.2. The ECL behavior of lucigenin on the conventional Pt electrode Since the results of many previous studies were conflicted, the ECL behavior of lucigenin on the conventional platinum electrode was studied first. When an applied potential was scanned at a platinum electrode from 0 to 0.8 V, an ECL peak of lucigenin could be observed at about 0.6 V. This phenomenon indicated that the light emission was probably occurred at the surface of platinum electrode, which confirmed Haapakka and Tamamushi’s conclusion [9]. When the solution had been deaerated, the ECL signal disappeared, based on which we could conclude that the ECL emission of lucigenin was directly related to the dissolved oxygen in neutral solution. The ECL peak of lucigenin was mainly obtained from the reaction of reduced lucigenin with hydrogen peroxide electrogenerated at the surface of the electrode. Linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) were used to examine the effects of electrochemical scanning mode on this ECL system. The result showed that, when DPV was used, a stable ECL intensity could be obtained. So DPV mode was selected for the subsequent investigation. To establish the optimal conditions, the luminescent intensity was measured as a function of pulse amplitude, pulse width and pulse period. The selected optimum parameters for DPV were showed in Table 1. The typical ECL curve of lucigenin was shown in Fig. 2.
Table 1 Differential pulse voltammetry parameters Init E (V)
Final E (V)
Incr E (V)
Amplitude (V)
Pulse width (s)
Sampling width (s)
Pulse period (s)
0.0
0.8
0.004
0.05
0.05
0.0167
0.2
1.0x10 -5
1200
Te/ o C
ECL intensity
800
Current/A
8.0x10 -6
1000
90 80 70 60 50 40 30 20 10 0
271
B
a
6.0x10-6 4.0x10-6
a' 2.0x10-6
600
b'
0.0 0.0
-0.2
-0.4
-0.6
-0.8
Potential/V
400 200
A b
0 0.0
-0.2
-0.4
-0.6
-0.8
Potential/V
0
2
4
6 Voltage/V
8
10
12
Fig. 1. The relationship between heating voltage (the AC voltage from the function generator) and Te.
Fig. 2. (A) Typical ECL curves of lucigenin on Platinum electrode in 0.1 mol/L KCl solution under DPV scanning; (B) DPV curves lucigenin on Platinum electrode in 0.1 mol/L KCl solution. (a, a 0 ) solution saturated with air and (b, b 0 ) solution had been deaerated. Concentration of lucigenin: 1.0 · 104 mol/L. The reference electrode was Ag/AgCl (saturated with KCl) electrode. The DPV parameters were shown in Table 1.
272
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
3.3. Electrochemical and ECL behavior of lucigenin solution saturated with air
4000
4000 3500
3500
a
3000
B
2500
Current/A
8.0x10
-6
7.0x10
-6
6.0x10
-6
5.0x10
-6
4.0x10
-6
3.0x10
-6
2.0x10
-6
1.0x10
-6
A
a b
B
c
2000
3000
1500 1000
ECL Intensity/a.u.
Since the mechanism of lucigenin ECL has been known to be related to the reduction of oxygen dissolved in the solution, the electrochemical reduction of oxygen on Pt electrode, like most electrode reactions, is temperature dependent, so the temperature of the electrode will affected the ECL of lucigenin. Fig. 3 shows the DPV curves of the 1.0 · 104 mol/L lucigenin solution at different Te. There are two obviously reduction peak in the curve. Peak A comes from the reduction of oxygen, peak B comes from the reduction of lucigenin. The peak currents are increased with the increasing of the electrode temperature. The convection seems to be responsible for these increments. When the microelectrode is heated, the temperature of the electrode surface increases quickly, a higher temperature at the electrode surface will speed up the diffusion and induce convection. The convection is favorable for transport of electroactive substances to the electrode, unlike non-convective isothermal systems, where the concentration gradient is the only driving force for mass transfer. And mass transfer is greatly improved at the electrode surface, this leads to a larger current being observed. The peak potentials for the reduction of oxygen and lucigenin are shifted positively respectively with the increasing of electrode surface temperature, which indicates that higher temperature also causes a decrease in overvoltage (which can be seen from the spot lines in the Fig. 3). Fig. 4 shows the ECL of 1.0 · 105 mol/L lucigenin at different electrode temperature. It can be seen from the figure that the ECL intensity increase with the increasing of electrode temperature. At room temperature (22 C), the ECL of the studied system is very weak, but when the temperature is risen to 80 C, the ECL intensity is 140 times as
2500
g
500 0 20
30
40
2000
50
60
70
80
Temperature
1500 1000
A
500 0 0.0
-0.2
-0.4
-0.6
-0.8
Potential /v
Fig. 4. (A) The ECL intensity-potential curves of 1.0 · 105 mol/L lucigenin at different Te. (B) The ECL intensity at different temperature. (a) 80 C; (b) 76 C; (c) 67 C; (d) 59 C; (e) 44 C; (f) 28 C and (g) 22 C. The other conditions were the same as in Fig. 3.
that at room temperature, and the preferred potential for occurrence of ECL have a little shift toward positive direction with increasing of temperature, which also indicates that the higher temperature is favorable to decrease the overvoltage. It was reported that trace H2O2 would greatly increase the ECL of lucigenin at platinum electrode, and the enhancement of intensity increased with the increasing of the concentration of H2O2 [3]. It had also been reported that mass transfer for H2O2 would increase at elevated temperature [27]. At elevated electrode temperature, more H2O2 had been produced, together with the increase of mass transfer caused by temperature. So the ECL intensity of the system was enhanced greatly at elevated electrode temperature. The most obvious application of HME is based on its enhancement for the analytical signals of the analytes, however, an increase in signal is beneficial only if there is no significant increase in the noise level. The effect of temperature on the background noise level in this ECL system had been examined, the results indicated that no significant increase in noise could be observed when the electrode was heated. But when the temperature was elevated to 85 C, an increase of noise was observed because of the noticeable formation of air bubbles at the surface. So in this experiment, the highest temperature of the electrode was set at 80 C. 3.4. Reproducibility
0.2
0.0
-0.2
-0.4
-0.6
-0.8
Potential/v 4
Fig. 3. DPV curves of 1.0 · 10 mol/L lucigenin at different Te in absence of oxygen. (a) 44 C; (b) 28 C and (c) 22 C. The working electrodes were 25 lm in diameter and 6 mm in long. The reference electrode was Ag/AgCl (saturated with KCl) electrode. The DPV parameters were shown in Table 1.
The big problem for the application of an ECL system is the tedious procedure for cleaning-up the poisoned electrode surface to get the reproducible result. Though the ECL of lucigenin is of interest because of its possible applications in determination of trace metal and some organic compounds, till now there were only few reports related to the analytical application of ECL of lucigenin. The main
Z. Lin et al. / Electrochemistry Communications 9 (2007) 269–274
reason for this is the low solubility of lucigenin’s reduction product, which can precipitate onto the platinum electrode surface. In order to solve this problem, Cui et al. [11] used glassy carbon electrode to study the ECL of lucigenin because the possible precipitation of the reduction product could be cleaned up more easily than that at platinum electrode. Okajima and Ohsaka [28] modified the electrodes with self-assembled monolayers and studied the ECL of lucigenin in the solutions with surfactants. All these have somewhat improved the reproducible of the ECL. It was reported that elevated electrode temperature could minimize surface fouling during voltammetric and amperometric measurement of NADH on platinum [27]. Heating the electrode could also be useful for regeneration of electrode surface. In our ECL detection system, it was found that when the HME was heated in the air, the temperature of the electrode surface could be as high as several hundreds degree, which would make the fouling to be easily decomposed. Under such treatment, the surface of electrode can be renewed to avoid the tedious procedure for cleaning-up the electrode. Fig. 5 shows the reproducibility of lucigenin ECL at the HME, the relative standard deviation for intensity of the ECL is 3.4% (n = 5). Which indicated that the electrode could be renewed by heating, which confirmed Baranski’s proposal, who proposed to use electrode heating for cleaning [29]. 3.5. Linear response range and detection limit of lucigenin at HME It was found that the ECL intensity (IECL) has linear relationship with the concentrations of lucigenin. Eq. (1)
300
100 50
ty/a.u. ECLIntensi
150
0 5 4 0.0
3 -0.1
-0 .2
-0 .3
Pote
ntia
-0.4
l/v
2 -0 .5
-0.6
-0.7
shows the linearity relationship between ECL intensity(I) and concentration of lucigenin(C) at the electrode without heating, i.e. at room temperature (22 C) I=a:u: ¼ 3:89 106 C=mol=L þ 23:2;
R ¼ 0:9989
ð1Þ
The linear range was found to be in the range of 1.0 · 105–3.0 · 104 mol/L, the detection limit was found to be 7.0 · 106 mol/L (defined as the concentration that gave rise to a signal-to-noise ratio of 3) when Te was 22 C, however, the detection limit was found to be 2.0 · 108 mol/L at 80 C and the linear range was found to be 1.0 · 107–4.0 · 106 mol/L, and the regression equation was shown in Eq. (2) I=a:u: ¼ 6:44 108 C=mol=L þ 23:4;
R ¼ 0:9993
ð2Þ
Which indicates that the detection limit can be improved greatly at higher Te. 4. Conclusions We have demonstrated the dramatic effect of electrode surface temperature on the ECL intensity of lucigenin, and the noise of this system has not been affected by the temperature in the studied range. The detection limit for ECL of lucigenin on a heated cylindrical microelectrode is much lower than that on an electrode without heating. Heating of the electrode has also been applied to clean the electrode successfully, which is especially important to the lucigenin ECL system. This technique offers not only new methods to explore ECL behavior of lucigenin but also the possibility of extending the application of ECL system at HME. Many other possible application fields of ECL at HME are being studied in our laboratory, for example, the temperature of HME will affect enzyme’s activity; based on which a sensitive biosensor is possible to be prepared. Acknowledgements
250 200
273
1 -0.8
Fig. 5. The reproductibility of ECL of 1.0 · 105 mol/L lucigenin when Te is 35 C. Other conditions are the same as in Fig. 3.
This project was financially supported by the National Nature Science Foundation of China (20575011) and the Science Foundation of State Education Department (20040386002). References [1] G.N. Chen, R.E. Lin, Z.F. Zhao, J.P. Duan, L. Zhang, Anal. Chim. Acta 341 (1997) 251. [2] A.W. Knight, Trends Anal. Chem. 18 (1999) 47. [3] G.N. Chen, L. Zhang, R.E. Lin, Z.C. Yang, J.P. Duan, H.Q. Chen, D.B. Hibbert, Talanta 50 (2000) 1275. [4] W.L. Willace, A.J. Bard, J. Phys. Chem. 83 (1979) 1350. [5] T. Zerihun, P. Gru¨ndler, J. Electroanal. Chem. 404 (1996) 243. [6] Z. Lin, J. Sun, L. Guo, G.N. Chen, Anal. Chim. Acta 564 (2006) 226. [7] K. Gleu, W. Petsch, Angew. Chem. 48 (1935) 57. [8] B. Tamamushi, H. Akiyama, Trans. Faraday Soc. 35 (1939) 491. [9] K.E. Haapakka, J.J. Kankare, Anal. Chim. Acta 130 (1981) 415. [10] K.D. Legg, D.M. Hercules, J. Am. Chem. Soc. 91 (1969) 1902.
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