Comparative Studies on Electrogenerated Chemiluminescence of

Dec 9, 2006 - When the potential was stepped from 0 to 1.0 V, BrO- was generated ...... Leeuwen , Jacques Buffle , Jérôme F. L. Duval , and Raewyn M...
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Langmuir 2007, 23, 523-529

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Comparative Studies on Electrogenerated Chemiluminescence of Luminol on Gold Nanoparticle Modified Electrodes Yong-Ping Dong, Hua Cui,* and Yang Xu Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed June 14, 2006. In Final Form: October 8, 2006 Comparative studies on the electrogenerated chemiluminescence (ECL) behavior of luminol on various electrodes modified with gold nanoparticles of different size were carried out in neutral solution by conventional cyclic voltammetry (CV). The results demonstrated that the gold nanoparticle modified electrodes could generate strong luminol ECL in neutral pH conditions. The catalytic performance of gold nanoparticle modified electrodes on luminol ECL depended not only on the gold nanoparticles but also on the substrate. Gold electrode and glassy carbon electrode were the most suitable substrates for the self-assembly of gold nanoparticles. Moreover, the gold nanoparticle modified gold and glassy carbon electrode had satisfying stability and reproducibility and did not need tedious pretreatment of electrode surface before each measurement. It was also found that luminol ECL behavior depended on the size of gold nanoparticles. The most intense ECL signals were obtained on a 16-nm-diameter gold nanoparticle modified electrode. The modified electrode prepared by the self-assembly method exhibited much better catalytic effect on luminol ECL than that prepared by the electrically deposited method. The ECL behavior of luminol on a gold nanoparticle self-assembled gold electrode was also investigated by other transient-state electrochemical techniques, such as chronoamperometry, differential pulse voltammetry, normal pulse voltammetry, and square wave voltammetry. The strongest ECL intensity was obtained under square wave voltammetric condition.

Introduction Electrogenerated chemiluminescence (ECL) is a special form of chemiluminescence (CL) in which light emission is generated by electrolysis. In recent years, ECL has become an important and valuable detection method in analytical chemistry. The ECL behavior of luminol on various conventional electrodes such as glassy carbon, paraffin-impregnated graphite, gold, and platinum electrodes has been documented.1-5 Luminol ECL on these electrodes often suffered from several problems, such as electrode fouling, poor analytical reproducibility, and weak ECL signals in neutral aqueous solution. Previous work has revealed that the behavior of ECL is strongly affected by many factors, such as the applied potential, electrode material, and surface state of the electrode. Therefore, it is expected that ECL-based detection can be improved through optimization of the composition and surface structure of electrode. Recently, gold nanoparticles have been applied to CL and ECL and exhibit unique electrochemical, catalytic, and chemical properties.6-12 In our group, it was found that on a 16-nm gold nanoparticle self-assembled gold electrode, two anodic and one cathodic ECL peaks of luminol in neutral aqueous solution were * Author to whom correspondence should be addressed. Tel: +86-5513606645; fax: +86-551-3600730; e-mail address: [email protected]. (1) Cui, H.; Zou, G. Z.; Lin, X. Q. Anal. Chem. 2003, 75, 324-331. (2) Sun, Y. G.; Cui, H.; Lin, X. Q. Chin. J. Anal. Chem. 1999, 27, 497-503. (3) Cui, H.; Zhang, Z. F.; Zou, G. Z.; Lin, X. Q. J. Electroanal. Chem. 2004, 566, 305-311. (4) Haapakka, K. Anal. Chim. Acta 1982, 141, 263-268. (5) Haapakka, K.; Kankare, J. J. Anal. Chim. Acta 1982, 138, 263-275. (6) Cui, H.; Zhang, Z. F.; Shi, M. J. J. Phys. Chem. B 2005, 109, 3099-3103. (7) Cui, H.; Zhang, Z. F.; Shi, M. J.; Xu, Y.; Wu, Y. L. Anal. Chem. 2005, 77, 6402-6406. (8) Zhang, Z. F.; Cui, H.; Lai, C. Z.; Liu, L. J. Anal. Chem. 2005, 77, 33243329. (9) Cui, H.; Xu, Y.; Zhang, Z. F. Anal. Chem. 2004, 76, 4002-4010. (10) Cui, H.; Dong, Y. P. J. Electroanal. Chem. 2006, 595, 37-46. (11) Yin, X. B.; Qi, B.; Sun, X. P.; Yang, X. R.; Wang, E. K. Anal. Chem. 2005, 77, 3525-3530. (12) Sun, X. P.; Du, Y.; Dong, S. J.; Wang, E. K. Anal. Chem. 2005, 77, 8166-8169.

greatly enhanced and a new cathodic ECL peak was initiated compared with a bare gold electrode, whereas, in alkaline solution, two anodic ECL peaks much stronger than those on a bare gold electrode were obtained.9 In addition, two greatly enhanced cathodic ECL peaks of lucigenin were also observed on such a modified electrode.10 These results suggest that a gold nanoparticle self-assembled electrode may be of great application potential in electrochemistry and electroanalytical chemistry. It was reported that the catalytic performance of nanoparticles depended markedly on the particle size, the nature of the support, and the method of preparation.13-16 However, the effect of such factors by use of gold nanoparticle modified electrode on luminol ECL has not been studied. Therefore, in this work, comparative studies of luminol ECL in neutral aqueous solution were carried out on various gold nanoparticle modified electrodes by different electrochemical techniques for fundamental interests and practical applications. The effects of the substrates such as glassy carbon, paraffin-impregnated graphite, and gold electrodes; the diameter of gold nanoparticles such as 6, 16, 25, 38, 68, and 87 nm; and the method for the preparation of modified electrode on luminol ECL were examined in detail. The surface state of gold nanoparticle modified electrodes was characterized with scanning electron microscopy (SEM). The electrochemical techniques involved in this study included chronoamperometry, differential pulse voltammetry (DPV), normal pulse voltammetry (NPV), and square wave voltammetry (SWV). Herein, we report the results of the comparative studies. Experimental Section Chemicals and Solutions. Luminol was obtained from Merck (Germany). A 1.0 × 10-2 mol/L stock solution of luminol was (13) Haruta, M. Catal. Today 1997, 36, 153-166. (14) Grunwaldt, J. D.; Kiener, C.; Wogerbauer, C.; Baiker, A. J. Catal. 1999, 181, 223-232. (15) Boccuzzi, F.; Chiorino, A.; Manzola, M. Mater. Sci. Eng., C 2001, 15, 215-217. (16) El-Deab, M. S.; Ohsaka, T. Electrochem. Commun. 2002, 4, 288-292.

10.1021/la0617107 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006

524 Langmuir, Vol. 23, No. 2, 2007 prepared by dissolving luminol in 0.1 mol/L sodium hydroxide solutions. Working solution of luminol was prepared by diluting the stock solution with phosphate buffer solutions (PBS, 0.1 mol/L, pH, 7). HAuCl4‚4H2O (48% w/w) was obtained from Shanghai Reagent (China). A 1.0 g/L HAuCl4 stock solution was prepared by dissolving 1 g HAuCl4 in 1 L redistilled water and was stored at 4 °C. All other reagents were of analytical grade, and redistilled water was used throughout. Preparation of Gold Nanoparticles. All glassware used in the following procedures were cleaned in a bath of freshly prepared 3:1 (v/v) HNO3 + HCl, were rinsed thoroughly in redistilled water, and were dried in air. HAuCl4 and trisodium citrate solutions needed to be filtered through a 0.22-µm microporous membrane filter prior to use. Prepared solutions were stored in brown glass bottles at 4 °C. Colloidal gold nanoparticles of 6-nm diameter were synthesized by the hydroborate reduction method,17 while colloidal gold nanoparticles of 16-, 25-, 38-, 68-, and 87-nm diameters were synthesized by the citrate reduction method.18 The resulting gold nanoparticles were characterized by transmission electron microscopy (TEM) (Hitachi H-800, Japan) and UV-vis spectra (Shimadzu UV-2401 PC spectrophotometer, Japan). Statistical analysis of TEM data revealed that the average diameter of the gold colloid was about 6.0 ( 3.5, 16 ( 1, 25 ( 2, 38 ( 3, 68 ( 5, and 87 ( 11 nm, respectively. Fabrication of Gold Nanoparticle Modified Electrode. (A) SelfAssembly Method. Paraffin-impregnated graphite electrode (PIGE) and glassy carbon electrode (GCE) were polished with abrasive paper, were rinsed with ethanol and redistilled water to remove the trace remainder, and were cleaned in an ultrasonic bath. The cleaned electrodes were preoxidized at 1.2 V (vs SCE (standard calomel electrode)) in 0.5 mol/L sulfuric acid for 20 min to activate the electrodes. Gold electrode was cycled between 0∼1.5 V (vs SCE) in 0.5 mol/L sulfuric acid at a scan rate of 100 mV/s. This potential cycling was continued until reproducible voltammograms for gold oxide formation/reduction were obtained, indicating that a clean surface of gold electrode was obtained. Then, gold electrode was again rinsed with redistilled water and was cleaned in an ultrasonic bath. These cleaned electrodes were first immersed in 0.1 mol/L cysteine aqueous solution for 2 h at room temperature in darkness to form cysteine monolayer. The cysteine modified electrode was rinsed thoroughly with redistilled water and was soaked in redistilled water for 12 h to remove physically adsorbed cysteine. Then, it was dipped into the colloidal gold solution for 24 h at 4 °C. Finally, the gold nanoparticle self-assembled electrode was dipped into redistilled water for conservation at 4 °C, and the surface state of gold nanoparticle self-assembled electrode was characterized by scan electron microscopy (SEM) (JEOL JSM-6700F, Japan). (B) Electrically Deposited Method. A bare electrode was pretreated following the method mentioned above. Then, the bare electrode was soaked in colloidal gold solution and was maintained at 1.5 V for 20 min with an SCE as the reference electrode.19 The electrode obtained in this method was immersed in redistilled water for conservation at 4 °C, and the surface state of gold nanoparticle modified electrode was characterized by SEM (JEOL JSM-6700F, Japan). Electrochemical and ECL Measurements. ECL and electrochemical (EC) measurements were performed by a homemade ECLEC system, including a model CHI832 electrochemical working station (Chenhua Inc., China, i.e., the Chinese Distributor of CH Instruments Inc., Austin, United States), a H-type electrochemical cell (homemade), a model 1P21 photomultiplier tube (PMT) (Binsong Electronic Co., China), a model GD-1 luminometer (Ruimai Electronic Science Co., China), and a computer. The H-type ECL cell was constructed as described previously.1 A gold foil with 6.8 × 7.0 mm2, or a paraffin-impregnated graphite electrode with 28.3 mm2, or a glassy carbon slice with 5.5 × 6.0 mm2 served as the working electrode, a platinum wire served as the counter electrode, (17) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154-1157. (18) Frens, G. Nat. Phys. Sci. 1973, 241, 20-22. (19) Jin, B. K.; Zhang, H. Anal. Lett. 2002, 35, 1907-1918.

Dong et al. and a silver wire served as the quasi-reference electrode (AgQRE); ∆E ) EAg/Ag+ - ESCE in different solutions was measured for potential calibrations. To the working compartment and the auxiliary compartment of the ECL cell, a 2.0-mL portion of the sample solution and the blank solution without luminol was filled in, respectively. When the potential was applied to the working electrode, ECL signal was generated. The curves of ECL intensity versus applied potential (IECL-E) and the curves of current versus applied potential (i-E) were recorded simultaneously. AC Impedance Measurements. A model CHI 760B electrochemical working station (Chenhua Inc., China) was used to record electrochemical impedance spectroscopy (EIS). The ac impedance measurements were carried out at amplitude of 5 mV in the frequency range from 10 kHz to 0.01 Hz. Complex impedance plots were obtained at 0.21 V (vs SCE) with [Fe(CN)6]3-/[Fe(CN)6]4- pair acting as a probe.

Results and Discussion Effect of Substrate. Under the same experimental conditions, cyclic voltammograms (CVs) and IECL-E curves of luminol on 16-nm gold nanoparticle self-assembled glassy carbon electrode (GCE), gold electrode (GE), paraffin-impregnated graphite electrode (PIGE), and correspondingly bare electrodes are shown in Figure 1. It was found that three bare electrodes exhibited different cyclic voltammetric behavior under the same experimental conditions, corresponding to each electrode material, respectively. When gold nanoparticles were modified on the surface of bare electrodes, three modified electrodes exhibited the cyclic voltammetric properties similar to that of bare gold electrode only with a difference in intensity of redox peaks. Therefore, we could conclude that the electrochemical properties of gold nanoparticle modified electrodes were mainly due to the gold nanoparticles self-assembled on the surface of electrodes. The explanation of all cyclic voltammetric peaks has been described previously.9 It was found from IECL-E curves that luminol ECL intensity on bare electrodes was quite weak, whereas it was significantly enhanced on gold nanoparticle modified electrodes in neutral solution. The ECL intensity on gold nanoparticle modified glassy carbon and gold electrodes was enhanced about 2 orders of magnitude compared with that on correspondingly bare electrodes, and ECL signals on gold nanoparticle modified paraffinimpregnated graphite electrode were enhanced only about 5-fold compared with that on bare PIGE. The number of ECL peaks was the same on the three modified electrodes. The results indicate that gold nanoparticles play a predominant role on luminol ECL. As the ECL intensity was not the same on the different electrodes, it is deduced that the ECL signal has a close relationship with the substrates. It is well-known that electrochemical impedance spectroscopy (EIS) is an effective tool for studying the interface properties of surface-modified electrodes. In electrochemical impedance spectroscopy, the semicircle diameters of EIS equal the electrontransfer resistance which controls the electron-transfer kinetics at electrode interface. EIS of gold nanoparticle modified electrodes and bare electrodes were recorded as shown in Figure 2. The results show that the semicircle diameters of EIS of gold nanoparticle modified electrodes were smaller than those of bare electrodes. Moreover, the semicircle diameters of EIS of gold nanoparticle modified electrodes increase in the following order: GE < GCE < PIGE, which was in good agreement with the order that the ECL intensity of luminol on gold nanoparticle modified electrodes decreased. Therefore, the enhancing effect of gold nanoparticle modified electrodes on luminol ECL is probably because gold nanoparticles could catalyze electron-

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Figure 1. Cyclic voltammograms (CVs) and IECL/E curves of luminol ECL on gold nanoparticle self-assembled electrodes and bare electrodes (A and B, gold electrode; C and D, glassy carbon electrode; E and F, paraffin-impregnated graphite electrode). NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L; scan rate, 40 mV/s; all high voltages applied to the PMT were maintained at -550 V.

Figure 2. Electrochemical impedance spectroscopy (EIS) on modified electrodes and bare electrodes. The insets show EIS at high modulation frequency. K3Fe(CN)6, 1 mM; K4Fe(CN)6, 1 mM; PBS, 0.1 mol/L, pH ) 7.

transfer process occurring on the surface of modified electrodes. The substrate can significantly influence the electron-transfer resistance, leading to a difference in ECL intensity. According to the experimental results, the strongest luminol ECL was obtained on gold nanoparticle modified gold electrode and glassy carbon electrode, therefore, GE and GCE were chosen as the optimal substrates to self-assemble gold nanoparticles, and most of the conditional experiments were carried out on gold nanoparticle self-assembled gold and glassy carbon electrodes in neutral aqueous solutions. Effect of Adsorption Time. The catalytic activity of gold nanoparticle self-assembled electrode was also relevant to the adsorption time in cysteine solution and colloidal gold solution. When the adsorption time in cysteine solution was longer than 2 h, the amount of the adsorbed cysteine on the electrode was

too large and, subsequently, the aggregation of gold nanoparticles was observed. As a result, the intensity of luminol ECL on the modified electrode decreased. When the cysteine-modified gold electrode was soaked in colloidal gold solution for more than 24 h, no extra enhancing effect on luminol ECL was observed compared with that when the cysteine-modified gold electrode was soaked in colloidal gold solution for 24 h, which meant that 24 h was long enough for constructing gold nanoparticles monolayer on the surface of bare electrode. Effect of Diameters of Gold Nanoparticles. It is well-known that the rate of heterogeneous catalysis increases with the available active surface area of the catalyst.20,21 In our group, it was found that the chemiluminescence intensity varied with the size of gold nanoparticles in gold nanoparticles involved in liquid chemiluminescence reactions. For example, the most intensive CL for KIO4-NaOH-Na2CO3 system was obtained with 68-nm gold nanoparticles, while the most intensive CL signals for luminolH2O2 system was obtained with 38-nm gold nanoparticles.6,8 Therefore, it is necessary to study the size effect of gold nanoparticles on luminol ECL. The 6-, 16-, 25-, 38-, 68-, and 87-nm-diameter Au colloids were used to modify the electrode, and cysteine was employed as a link reagent. As shown in Figure 3, the enhancing effect on luminol ECL increased with a decrease in size of gold nanoparticles and reached a maximum on 16-nm gold nanoparticle modified electrode; then, the enhancing effect of gold nanoparticles decreased with the further decrease in diameter of gold nanoparticles. For the particle sizes larger than 16 nm in diameter, the active surface areas of the gold nanoparticles decreased with an increase in particle size, and the catalytic efficiency of the gold nanoparticles decreased accordingly. During the experiment of the self-assembly of 6-nm gold nanoparticles on gold electrode, the aggregation of gold nanoparticles could be observed. For (20) Sharma, R. K.; Sharma, P.; Maitra, A. J. Colloid Interface Sci. 2003, 265, 134-140. (21) Henglein, A. J. Phys. Chem. 1993, 97, 5457-5471.

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Figure 5. SEM images of a bare gold electrode (A) and a 16-nm gold nanoparticle self-assembled gold electrode (B).

Figure 3. Effect of the diameters of gold nanoparticles on luminol ECL. NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L; scan rate, 40 mV/s.

Figure 4. SEM images of a bare glassy carbon electrode (A) and a 16-nm gold nanoparticle self-assembled glassy carbon electrode (B).

6-nm gold nanoparticles, the available active surface area increased dramatically compared with other sizes of gold nanoparticles, which made 6-nm gold nanoparticles easier to react with other reagents during the self-assembly. Therefore, the aggregation of 6-nm gold nanoparticles occurred, and the catalytic effect of 6-nm gold nanoparticle modified electrode on luminol ECL decreased. According to the experimental results, 16-nm gold nanoparticles were the optimal size of gold nanoparticles for the modification of electrode. Surface State of Gold Nanoparticle Self-Assembled Electrodes. The surface state of gold nanoparticle self-assembled GE and GCE was characterized by SEM. The SEM images of 16-nm gold nanoparticle self-assembled GCE and GE, as well as bare GCE and GE, are shown in Figure 4 and Figure 5, respectively. According to SEM images, gold nanoparticles were randomly bound to the surface, which was probably because the repulsive forces that kept colloidal Au apart in solution prohibited a close packing of particles on the surface. Moreover, the average diameters of gold nanoparticles self-assembled on GE and GCE were 20 ( 2 and 21 ( 1 nm, respectively, which were comparable with the corresponding gold nanoparticle sizes. Additionally,

during the procedure of focusing, gold nanoparticles might be melted by high-energy electron beam, which led to the enlargement of some nanoparticles. Stability of Gold Nanoparticle Self-Assembled Electrodes. The stability of gold nanoparticle self-assembled gold electrode and glassy carbon electrode was tested for the response of both electrochemistry and ECL (Figure 6). The results showed that 16-nm gold nanoparticle modified gold electrode and glassy carbon electrode were very stable. The validity of a gold nanoparticle modified gold electrode and a gold nanoparticle modified glassy carbon electrode could reach as long as 30 days and more than 2 weeks, respectively, in the condition that it was stored in redistilled water at 4 °C after each measurement. Cysteine was a commonly used self-assembly reagent in the preparation of modified electrode. Self-assembly of cysteine on gold substrate was previously studied by XPS (X-ray photoelectron spectroscopy), quartz crystal microbalance (QCM), cyclic voltammetry, and electrochemical impedance spectroscopy.22 The results showed that cysteine was adsorbed onto the gold surface through a sulfur-gold covalent interaction with the -COO- and -NH3+ extending freely in the ambient solutions. Therefore, in this case, the negatively charged gold nanoparticles could be adsorbed on the cysteine SAMs on the surface of gold electrode through the electrostatic interaction between gold nanoparticles and -NH3+, which was proven to be roughly as strong as the sulfur-gold covalent interaction.23 As a result, gold nanoparticle monolayer had remarkable stability on gold electrode. When glassy carbon electrode was pretreated by electrochemical oxidation, some oxygen-containing groups, such as carbonyl group, hydroxyl group, and carboxyl group, could be generated on the surface of the electrode.24 These groups could react with -NH3+ of cysteine to form SAMs on the surface of glassy carbon electrode by virtue of electrostatic interaction;25 then, -SH of cysteine could react with gold nanoparticles to form a monolayer by virtue of sulfur-gold covalent interaction.26 Therefore, the gold nanoparticle monolayer formed on gold electrode and glass carbon electrode was relatively stable for the (22) Li, Q.; Hong, G.; Wang, Y.; Luo, G.; Ma, J. Electroanalysis 2001, 13, 1342-1346. (23) Shipway, A. N.; Lahav, M.; Willner, I. AdV. Mater. 2000, 12, 993-998. (24) Laser, D.; Ariel, M. J. Electroanal. Chem. 1974, 52, 291-303. (25) Kuhn, A.; Mano, N.; Vidal, C. J. Electroanal. Chem. 1999, 462, 187194. (26) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772.

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Figure 6. Stability of 16-nm gold nanoparticle modified gold electrode and glassy carbon electrode. CVs (A, C) and IECL-E curves (B, D) of luminol ECL. NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L; scan rate, 40 mV/s.

Figure 7. CVs (A, C) and IECL-E (B, D) curves of luminol ECL on 16-nm gold nanoparticle modified gold electrode and glassy carbon electrode prepared by the self-assembly method (dash line) and the electrically deposited method (solid line). NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L; scan rate, 40 mV/s.

existing chemical bond and electrostatic interaction. However, the electrostatic interaction of oxygen-containing groups with -NH3+ was not as stable as that of the sulfur-gold covalent interaction.26 As a result, the stability of gold nanoparticle modified gold electrode was better than that of gold nanoparticle modified glassy carbon electrode. As the contamination of the electrode surface was a serious problem for bare GE and GCE and the pretreatment of the electrode surface must be carefully carried out before each measurement to obtain reproducible ECL, the excellent reproducibility of ECL achieved on gold nanoparticle self-assembled gold electrode and glassy carbon electrode without any pretreatment made these modified electrodes be of great application potential in ECL detection. Effect of the Fabrication Method of Modified Electrodes. Gold nanoparticle modified GCE and GE were also prepared by

electrically deposited method according to Jin and Zhang.19 A 16-nm-diameter Au colloid was chosen in this case for a comparison. The effect of the modified electrodes prepared by the self-assembly method and by the electrically deposited method on luminol ECL is shown in Figure 7. The enhancing effect of the modified electrode prepared by the self-assembly method on luminol ECL was much better than that of the modified electrode prepared by the electrically deposited method. The SEM images of gold nanoparticle modified GCE and GE (Figure 8) prepared by electrically deposited method demonstrated that the diameter of gold nanoparticles increased to micrometer size during the electrically deposited process. It was indicated in Figure 3 that the catalytic effect of gold nanoparticles on luminol ECL decreased with an increase in the diameter of gold nanoparticles. Therefore, the gold nanoparticle modified electrode prepared by

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Figure 8. SEM images of gold nanoparticle modified gold electrode (A) and glassy carbon electrode (B) prepared by electrochemically deposited method.

Figure 9. i-t (A) and IECL-t (B) curves of luminol ECL under chronoamperometric condition on a gold nanoparticle self-assembled gold electrode. NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L.

electrically deposited method exhibited almost no catalytic effect on luminol ECL for the extremely large gold particles formed on the electrode surface. ECL under Other Electrochemical Techniques. Cyclic voltammetry used above is only one of the transient-state electrochemical techniques. Herein, luminol ECL on a gold nanoparticle self-assembled gold electrode was studied under other transient-state electrochemical techniques, such as chronoamperometry, normal pulse voltammetry (NPV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV). Chronoamperometry and ECL behavior. Chronoamperometric currents (i-t) and their corresponding ECL (IECL-t) on a gold nanoparticle modified gold electrode are shown in Figure 9. The potentials were stepped from 0 to 0.50, 0.65, and 1.0 V, where the peak value of ECL-1 and ECL-2 was reached under cyclic voltammetry condition. The electrode currents were almost the same when the potential was stepped from 0 to 0.50 and 0.65 V, which was the initial potential and the peak potential, respectively, where luminol anion was oxidized to luminol anion radical, and increased greatly when the potential was stepped from 0 to 1.0 V, where Br- began to be oxidized to BrO-. The intensity of ECL reached its maximum when the potential was stepped from 0 to 0.65 V and decreased when the potential was stepped from 0 to 1.0 V. It was reported in our previous work

Dong et al.

that ECL-1 is related to luminol anion radicals and dissolved oxygen, while ECL-2 is related to luminol anion radicals and electrogenerated BrO-.9 When the potential was stepped to 0.65 V, luminol was oxidized to luminol anion radicals, which could react with dissolved O2 in the solution to generate ECL on a gold nanoparticle modified gold electrode. The gold nanoparticles assembled on the electrode could catalyze the electro-oxidization of luminol, and thus the strong ECL was obtained when the potential was stepped to 0.65 V in neutral aqueous solution. When the potential was stepped from 0 to 1.0 V, BrO- was generated on the surface of the electrode through the oxidation of Br-, which could react preferentially with luminol anion radicals on the surface of the electrode but not with the dissolved O2 in the solution. Therefore, the ECL intensity yielded by the reaction of luminol anion radicals with BrO- at 1.0 V was weaker than that generated by the reaction of luminol anion radicals with dissolved O2 at 0.65 V. Differential Pulse Voltammetry (DPV), Normal Pulse Voltammetry (NPV), Square Wave Voltammetry (SWV), and ECL Behavior. The behavior of luminol ECL under CV, NPV, DPV, and SWV conditions is shown in Figure 10. The cathodic ECL peaks observed under CV condition could not be obtained under other electrochemical techniques. Therefore, the potential sweep range applied in these techniques was between 0 and 1.6 V. The applied potentials of these techniques are shown in the insets in Figure 10. Two anodic ECL peaks were also obtained under NPV, DPV, and SWV techniques in neutral luminol solution, which were completely comparable with those under CV condition, only with a slight difference in the peak potentials of ECL peaks. Therefore, the mechanism for every anodic ECL peaks was similar to that under CV condition. Under normal pulse voltammetry condition, when the pulse potentials in each pulse were returned to 0 V, ECL signal disappeared immediately in each pulse; however, the emission did not disappear immediately when the potentials were stepped to more than 1.0 V. This may be because a great deal of Br- was oxidized to BrOat the higher potential and the residual BrO- continued to react with luminol anion radicals, leading to a weaker light emission and a shift of peak potential of ECL-2 to positive value. The ECL intensity increased in the following order: DPV < CV < NPV < SWV. The strongest ECL intensity was obtained under SWV condition, which was about severalfolds stronger than that obtained by other electrochemical techniques. This may be due to its efficient elimination of capacitance current.27 Therefore, square wave voltammetric technique might be the optimal electrochemical technique for the sensitive detection of biologically important compounds in neutral condition.

Conclusion The influence factors of the catalytic performance of gold nanoparticle modified electrodes on luminol ECL were studied in neutral condition. Electrochemical characters of modified electrodes were mainly due to the gold nanoparticles selfassembled on the surface of electrodes, while the intensities of ECL related not only to the gold nanoparticles but also to the substrates. Gold nanoparticle modified electrode prepared by the self-assembly method exhibited better catalytic effect on luminol ECL than that prepared by the electrically deposited method. Gold electrode and glassy carbon electrode were proven to be the most suitable substrates for the self-assembly of gold nanoparticles. A 16-nm gold nanoparticle self-assembled gold electrode had the maximal catalytic effect on luminol ECL than (27) Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S. L.; Natan, M. J. Anal. Chem. 1997, 69, 471-477.

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Figure 10. IECL-E curves of luminol on a gold nanoparticle self-assembled gold electrode under CV (A), NPV (B), DPV (C), and SWV (D) techniques. The insets are the time dependence of potential under different electrochemical techniques. NaBr, 0.1 mol/L; pH, 7.0; PBS, 0.1 mol/L; luminol, 1 × 10-4 mol/L. Experimental conditions: (B) initial E ) 0 V, final E ) 1.6 V, increase E ) 0.01 V, pulse width ) 0.1 s, pulse period ) 0.2 s. (C) Initial E ) 0 V, final E ) 1.6 V, increase E ) 0.01 V, amplitude ) 0.02 V, pulse width ) 0.1 s, pulse period ) 0.2 s. (D) Initial E ) 0 V, final E ) 1.6 V, increase E ) 0.01 V, amplitude ) 0.02 V, frequency ) 10 Hz.

other sizes of gold nanoparticle self-assembled gold electrodes. Gold nanoparticle self-assembled electrodes exhibited outstanding reactivity and stability and could avoid any tedious pretreatment before each measurement. Two anodic luminol ECL peaks could also be obtained on a gold nanoparticle self-assembled gold electrode in neutral pH condition by using different electrochemical transient techniques, and the strongest ECL intensity was obtained under SWV condition. These results will be helpful for expanding the application of luminol ECL on chemically modified electrodes and will have great potential for the sensitive

detection of biologically important compounds in physiological pH because of the excellent biocompatible property of gold nanoparticles. Acknowledgment. The support of this research by the National Natural Science Foundation of P. R. China (Grant Nos. 20375037) and by the Overseas Outstanding Young Scientists Program of Chinese Academy of Sciences is gratefully acknowledged. LA0617107