[(4-Clph)4B]2 Nanoislands Directly Assembled on an ITO Electrode

Nov 14, 2008 - Department of Chemistry, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, PR China. ReceiVed September 25, 2008. ReVised ...
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Langmuir 2009, 25, 1253-1258

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[Ru(dpp)3][(4-Clph)4B]2 Nanoislands Directly Assembled on an ITO Electrode Surface and Its Electrogenerated Chemiluminescence Ying Chen,† Jianfei Mao,† Chunhua Liu,† Hongyan Yuan,‡ Dan Xiao,*,†,‡ and Martin M. F. Choi*,§ Colleges of Chemistry and Chemical Engineering, Sichuan UniVersity, Chengdu 610065, PR China, and Department of Chemistry, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, PR China ReceiVed September 25, 2008. ReVised Manuscript ReceiVed NoVember 14, 2008 In this work, solid-state tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) ditetrakis(4-chlorophenyl)borate ([Ru(dpp)3][(4-Clph)4B]2) nanoislands are assembled spontaneously and simultaneously on an indium-doped tin oxide (ITO) glass electrode surface via a facile dewetting procedure. The fabrication process is very simple and also amenable to mass production. The as-prepared ruthenium complex nanoislands exhibit useful properties. The electrode is more electrochemically active and can produce strong, stable, reproducible solid-state electrochemiluminescence (ECL) signals using oxalate as the coreactant. The self-assembled nanoislands exhibit semiconductor-like broad, red-shift ECL spectrum. More importantly, they extend the application of the ruthenium complex ECL system from the usual alkaline to acidic conditions. The pH turn-off behavior of the ECL is observed for the first time and can serve as an ultrasensitive pH sensor around physiological pH 7.0. The solid-state [Ru(dpp)3][(4-Clph)4B]2 ECL signal is efficiently inhibited by phenol even at a very low concentration (i.e., 20 nM), thus providing the potential for the determination of phenolic compounds in practical applications.

Introduction Electrochemiluminescence (ECL) involves the generation of species at electrode surfaces that then undergo electron-transfer reactions to form excited states and emit light.1 Since pristine detailed studies of ECL2 and the ECL from tris(2,2′-bipyridyl) ruthenium(II) (Ru(bpy)32+) were reported,3 interest in ECL has increased in the past several decades because of its versatility, simple optical setup, low background noise, and good temporal and spatial control.4 Ruthenium (Ru) complexes are one of the most important metal-organic complexes, which have been extensively studied and widely used in ECL to perform analytical applications5 such as oxygen detection,6 flow injection analysis,7 high-performance liquid chromatography (HPLC),8 capillary electrophoresis (CE),4a and so forth, for their high sensitivity and stability. To reduce the consumption of expensive reagents and make the sensors reusable and more operable, considerable effort have been made to immobilize Ru complexes on solid * Corresponding authors. E-mail: [email protected], mfchoi@ hkbu.edu.hk. † College of Chemistry, Sichuan University ‡ College of Chemical Engineering, Sichuan University § Hong Kong Baptist University (1) (a) Richter, M. M. Chem. ReV. 2004, 104, 3003–3036. (b) Miao, W. J. Chem. ReV. 2008, 130, 2506–2553. (2) (a) Hercules, D. M. Science 1964, 145, 808–809. (b) Visco, R. E.; Chandross, E. A. J. Am. Chem. Soc. 1964, 86, 5350–5351. (3) Tokel, N. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862–2863. (4) (a) Yin, X. B.; Wang, E. Anal. Chim. Acta 2005, 535, 113–120. (b) Sun, X.; Du, Y.; Dong, S.; Wang, E. Anal. Chem. 2005, 77, 8166–8169. (c) Zhang, L.; Dong, S. Anal. Chem. 2006, 78, 5119–5123. (d) Maus, R. G.; Wightman, M. Anal. Chem. 2001, 73, 3993–3998. (5) Yin, X. B.; Dong, S. J.; Wang, E. K. Trends Anal. Chem. 2004, 23, 432– 441. (6) Bukowski, R. M.; Ciriminna, R.; Pagliaro, M.; Bright, F. V. Anal. Chem. 2005, 77, 2670–2672. (7) (a) Lin, F.; Pang, Y. Q.; Lin, X. Q.; Cui, H. Talanta 2003, 59, 627–636. (b) Pang, Y. Q.; Cui, H.; Zheng, H. S.; Wan, G.-H.; Liu, L.-J.; Yu, X.-F. Luminescence 2005, 20, 8–15. (8) (a) Li, M. J.; Chu, B. W. K.; Yam, V. W. W. Chem.sEur. J. 2006, 12, 3528–3537. (b) Morita, H.; Konishi, M. Anal. Chem. 2003, 75, 940–946.

substrates, taking into account their advantage in cost effectiveness as compared to the solution-phase Ru complex ECL system.9-13 Advances in nanotechnology have enabled the fabrication of nanoscale sensors for the controlled assembly of nanobuilding blocks as a prerequisite for the realization of nanosensors. Silica nanoparticles,4c,14 Au nanoparticles,4b and carbon nanotubes15 have been fabricated as 3-D ordered arrays on the surface of an electrode. The {SiO2/Ru-(bpy)32+}n multilayer films have been shown to have high sensitivity and good stability.14a Zhang and Dong employed an RuDS-CHIT composite film as a solid substrate that could exhibit good sensitivity and stability.4c The as-prepared Ru-AuNP-modified ITO electrode is quite stable and hence holds great promise for a detection system in CE.4b Wei et al. recently assembled biomolecules-functionalized [Ru(bpy)3]2+-doped silica nanoparticles by the Langmuir-Blodgett method, which could be used as stable, efficient ECL tag materials.16 Bard et al. recently prepared tris(2,2′-bipyridyl)Ru(II) derivative nanobelts by a simple reprecipitation method and investigated its ECL.17 However, the pretreatment of the electrodes or the fabrication methods of the modified electrodes of these reported methods are relatively complicated. In addition, Ru(bpy)32+ is hydrophilic and dissolves easily in aqueous solutions, making the common problem of leaching even more serious. Furthermore, the transfer of electron and coreactant (9) Bucur, C. B.; Schlenoff, J. B. Anal. Chem. 2006, 78, 2360–2365. (10) (a) Cui, H.; Zhao, X. Y.; Lin, X. Q. Luminescence 2003, 18, 199–202. (b) Ding, S. N.; Xu, J. J.; Chen, H. Y. Electrophoresis 2005, 26, 1737–1744. (c) Su, X. D.; Zhuang, Z. J.; Lv, B. Q.; Xiao, D. Sens. Lett. 2007, 5, 578–583. (11) (a) Zhang, X.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566–5569. (b) Miller, C. J.; Mcmord, P.; Bard, A. J. Langmuir 1991, 7, 2781–2787. (12) (a) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195–201. (b) Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 384, 57–66. (13) Sun, X.; Du, Y.; Zhang, L.; Dong, S.; Wang, E. Anal. Chem. 2007, 79, 2588–2592. (14) (a) Guo, Z.; Shen, Y.; Wang, M.; Zhao, F.; Dong, S. Anal. Chem. 2004, 76, 184–191. (b) Wang, L.; Yang, C. Y.; Tan, W. H. Nano Lett. 2005, 5, 37–43. (15) Guo, H.; Dong, S. Anal. Chem. 2004, 76, 2683–2688. (16) Wei, H.; Liu, J. F.; Zhou, L. L.; Li, J.; Jiang, X.; Kang, J. Z.; Yang, X. R.; Dong, S. J.; Wang, E. K. Chem.sEur. J. 2008, 14, 3687–3693. (17) Yu, J. G.; Fan, F. F.; Pan, S. L.; Lynch, V. M.; Omer, K. M.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 7196–7197.

10.1021/la803151x CCC: $40.75  2009 American Chemical Society Published on Web 12/17/2008

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Scheme 1. Structural Formula of [Ru(dpp)3][(4-Clph)4B]2

molecules may be hindered when using nonconductive material as an assisting matrix,16 and the amount of the Ru complex would be limited by the small composition proportion.4c Herein, we immobilized solid-state [Ru(dpp)3][(4-Clph)4B]2 (Scheme 1) nanoislands in an orderly manner on an ITO electrode via a facile dewetting method. To the best of our knowledge, this is a first report on the direct immobilization of pure solid-state Ru complex nanoislands spontaneously and simultaneously on an electrode surface. The as-prepared Ru complex nanoislands exhibit useful properties. The self-assembled nanoislands exhibit a semiconductor-like broad, red-shifted ECL spectrum compared to its photoluminescence (PL) spectrum. The modified electrode is more electrochemically active and can produce strong, stable, reproducible ECL signals using oxalate as the coreactant. In addition, they extend the application of the ruthenium complex ECL system from the usual alkaline to acidic conditions. The pH turn-off behavior of the ECL was observed and can serve as an ultrasensitive pH sensor around physiological pH 7.0. Compared with the common Ru(bpy)32+ ECL system, this [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode exhibits several advantages, including better electrochemical activity, low consumption of expensive reagent, ease of fabrication, good stability, and mass-production viability. The ECL signal of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified electrodes can be efficiently inhibited by phenol even at a very low concentration (i.e., 20 nM) and has been successfully used as a sensitive sensor for determining phenols in aqueous solution.

Experimental Section Materials. Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride was supplied by Alfa Aesar China (Tianjin, China,). Potassium tetrakis(4-chlorophenyl)borate was obtained from Fluka Chemicals (Buchs, Switzerland). Catechol, hydroquinone, p-nitrophenol, phenol, potassium chloride, potassium ferrocyanide, potassium ferricyanide, resorcinol, sodium oxalate, and tripropylamine were purchased from Chengdu Chemicals (Sichuan, China). All chemicals were of analytical purity and used as received without further purification. All solutions were prepared with doubly distilled (DD) water. Instrumentation. The ECL signals were detected by a model MIP-E CE-ECL analyzer system (Xi’an Remax Electronic HighTech Ltd., Xi’an, China). Cyclic voltammetric analyses were carried out on an Autolab electrochemical analyzer system (ECO-Chemie, Utrecht, The Netherlands). Electrochemical measurements were performed in a conventional three-electrode system. The bare or modified ITO glasses (10 × 10 mm2) were employed as a working electrode, an Ag/AgCl (saturated KCl) electrode was used as the reference electrode, and a 0.2-mm-diameter platinum wire was used as the counter electrode. Fluorescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan). The morphology of the surface of [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO electrode was investigated with a Seiko SII SPA400 atomic force microscope (Tokyo, Japan) and a Hitachi S-4800

Figure 1. (a) 2-D and (b) 3-D AFM images and (c) 3-D SEM image of the surface of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode.

scanning electron microscope (Tokyo, Japan). The mass spectrum was determined with a Shimadzu GCMS-QP1000(A) spectrometer (Tokyo, Japan). Synthesis and Characterization of [Ru(dpp)3][(4-Clph)4B]2. [Ru(dpp)3][(4-Clph)4B]2 was synthesized using the procedure described in the literature.23 The molecular weight and structure of [Ru(dpp)3][(4-Clph)4B]2 were fully verified by mass spectrometry as shown in Figure S1 (Supporting Information). Preparation of [Ru(dpp)3][(4-Clph)4B]2 Nanoislands on the ITO Electrode. ITO glasses were sonicated in soap solution, acetone, and doubly distilled water in turn. After being air dried at room temperature, the ITO glasses were immersed in a desired amount of [Ru(dpp)3][(4-Clph)4B]2 in acetone (0.50 mg/mL) for 2 min and were withdrawn from the solution at a rate of 0.2 cm/s. They were then kept at room temperature to let the solvent evaporate. Thereafter, the center interfacial region of the ITO electrode was sealed with nonconducting silicon rubber so as to maintain the exposed square electrode area (10 × 10 mm2) at a constant value before ECL experiments. The fabrication process is very simple and also amenable to mass production.

Results and Discussion Characterization of the [Ru(dpp)3][(4-Clph)4B]2 Nanoisland-Modified ITO Electrode. The surface morphology of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Figure 1 depicts the (a) 2-D and (b) 3-D AFM images of [Ru(dpp)3][(4-Clph)4B]2 nanoislands and (c) 3-D SEM image on an ITO electrode. Nanoislands of [Ru(dpp)3][(4-Clph)4B]2 that are 40-80 nm tall and 150 ( 70 nm in diameter were formed on the ITO electrode surface. In essence, these AFM and SEM images corroborate that [Ru(dpp)3][(4Clph)4B]2 nanoislands have been successfully immobilized on the ITO surface.

[Ru(dpp)3][(4-Clph)4B]2 Nanoislands

Dewetting is a process by which an initially uniform liquid film on a solid substrate breaks up into droplets.18,19 When an ITO glass electrode was immersed in the desired amount of [Ru(dpp)3][(4-Clph)4B]2 in acetone solution (0.50 mg/mL) and withdrawn from the solution, a liquid film was formed on the ITO surface. Subsequently, as the solvent evaporates, the [Ru(dpp)3][(4-Clph)4B]2 solution was condensed, and the film became thinner. When the thickness was lower than a certain value and the [Ru(dpp)3][(4-Clph)4B]2 concentration reached saturation, the dewetting process occurred. The heterogeneous nucleation of [Ru(dpp)3][(4-Clph)4B]2 occurred on the ITO surface, possibly because of the electrostatic attraction between the surface-adsorbed [Ru(dpp)3]2+ and the surface-exposed negatively charged oxide on the ITO glass. The nuclei increased the contact angle, and then the liquid film shrunk to form drops centered at the nucleation site. The original uniform liquid film broke up into drops of liquid.19 [Ru(dpp)3][(4-Clph)4B]2 molecules homogeneously grew on the primary nuclei via a self-assembly process as the solvent evaporated.17,18 Finally, orderly [Ru(dpp)3][(4-Clph)4B]2 nanoisland arrays were firmly attached to the ITO surface. In our present study, solid-state [Ru(dpp)3][(4-Clph)4B]2 ({[Ru(dpp)3][(4-Clph)4B]2}s) can be adsorbed on the surface of ITO glass and exhibits excellent ECL behavior. Our other studies suggested that this ruthenium(II) complex can also be adsorbed on many kinds of electrode surfaces besides ITO glass. The fabrication methods for [Ru(dpp)3][(4-Clph)4B]2-immobilized graphite, glassy carbon, and carbon fiber are very similar to those for the ITO-modified electrode. We found that all of the modified electrodes exhibited good ECL phenomenon in 0.10 M Na2C2O4 solution (pH 6.7), and the ECL curves of these [Ru(dpp)3][(4-Clph)4B]2-modified electrodes are shown in Figures S2-S4 (Supporting Information). Compared with other electrodes, the ITO glass can be cut into various shapes and sizes and has excellent optical transparence. As such, it can be easily coupled with photon detectors such as photomultiplier tubes and photodiode array detectors for other applications including capillary electrophoresis and high-performance liquid chromatography. For these attributes, ITO glass was chosen as the working electrode in our further study. Cyclic voltammetric analyses of bare and [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrodes in 0.10 M KCl solution containing 10 mM K4Fe(CN)6/K3Fe(CN)6 are displayed in Figure 2. A well-shaped cyclic voltammogram with a peakto-peak separation of 151 mV was observed at the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode. This peak potential separation is even 30 mV smaller than for the bare ITO electrode, demonstrating that [Ru(dpp)3][(4-Clph)4B]2 nanoislands provide good conductivity for the electron transfer of the ferrocyanide/ ferricyanide couple. Furthermore, the ratio of oxidative to reductive peak currents is equal to 1 for both electrodes. The cyclic voltammograms illustrate that [Fe(CN)6]4-/[Fe(CN)6]3undergoes a more rapid reversible redox reaction process on the modified electrode as compared to that on the bare ITO electrode. Surface-adsorbed [Ru(dpp)3][(4-Clph)4B]2 nanoislands do not hinder the electron-transfer kinetics of the [Fe(CN)6]4-/ [Fe(CN)6]3- redox couple at all. This is quite different from many other kinds of modified ITO electrodes because they (18) (a) Tomioka, A.; Kinoshita, S.; Fujimoto, A. Thin Solid Films 2008, 516, ´ . P. D.; Lazzaroni, R.; Amabilino, 2363–2370. (b) Iavicoli, P.; Linares, M.; Pino, A D. B. Superlattices Microstruct. 2008, 44, 556–562. (19) (a) Cavallini, M.; Biscarini, F.; Massi, M.; Farran-Morales, A.; Leigh, D. A.; Zerbetto, F. Nano Lett. 2002, 2, 635–639. (b) Bischof, J.; Scherer, D.; Herminghaus, S.; Leiderer, P. Phys. ReV. Lett. 1996, 19, 1536–1539. (c) Herminghaus, S.; Brochard, F. C. R. Phys. 2006, 7, 1073–1081. (d) Kargupta, K.; Konnur, R.; Sharma, A. Langmuir 2000, 16, 10243–10253.

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Figure 2. Cyclic voltammograms of the (a) bare and (b) [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode in 0.10 M KCl solution containing 10 mM K4Fe(CN)6/K3Fe(CN)6 at a scan rate of 50 mV/s.

Figure 3. (a) ECL intensity-potential and (b) cyclic voltammograms of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode in 0.001 M Na2C2O4 solution (pH 6.7) at a scan rate of 200 mV/s.

generally slow down the electron transfer of the [Fe(CN)6]4-/ [Fe(CN)6]3-couple.20 [Ru(dpp)3][(4-Clph)4B]2 is very hydrophobic and does not dissolve in aqueous medium but dissolves only in organic solvents; this behavior is different from that of many other Ru complexes such as [Ru(bpy)3]Cl2. As such, compared to other Ru complexmodified electrodes, our resulting solid-state [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode should possess high electrical activity, a longer lifetime, and better stability (vide infra). ECL Behavior and Mechanisms of the [Ru(dpp)3][(4Clph)4B]2 Nanoisland-Modified ITO Electrode. Figure 3 shows the corresponding ECL intensity-potential curve and cyclic voltammogram of the [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO electrode in 0.001 M Na2C2O4 solution at a scan rate of 200 mV/s. The characteristic redox peak and a strong ECL peak of [Ru(dpp)3] [(4-Clph)4B]2 were obtained at about (20) (a) Li, L. S.; Wang, R.; Fitzsimmons, M.; Li, D. Q. J. Phys. Chem. B 2000, 104, 11195–11201. (b) Yu, A.; Liang, Z.; Cho, J.; Caruso, F. Nano Lett. 2003, 3, 1203–1207.

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Scheme 2. ECL Mechanism of the [Ru(dpp)3][(4-Clph)4B]2 Nanoisland-Modified ITO Electrodes

+1.35 V. The onset of luminescence occurred near +1.0 V, and then the ECL intensity rose steeply until it reached a maximum at +1.35 V. These observations are similar to the ECL behavior of Ru(bpy)32+, of which the characteristic redox wave appears at near +1.1 V.14a By contrast, no obvious ECL peak or characteristic redox peak was found in the absence of Na2C2O4 after the second potential scan cycle as shown in Figures S5 and S6 (Supporting Information). Analogous to the Ru(bpy)32+ ECL system,1 the ECL mechanisms of the solid-state [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO electrode can be formulated in Scheme 2. Under the positive potential scan, solid-state [Ru(dpp)3][(4-Clph)4B]2 and C2O42- are oxidized to {[Ru(dpp)3][(4-Clph)4B]2}s+ and C2O4 · -, respectively, on the electrode. Subsequently, C2O4 · decomposes to CO2 · - and CO2. Then CO2 · - in situ transfers an electron to {[Ru(dpp)3][(4-Clph)4B]2}s+ to form the emissive excited state of {[Ru(dpp)3][(4-Clph)4B]2}s*. Lastly, the excited state of {[Ru(dpp)3][(4-Clph)4B]2}s* decays quickly back to the ground state {[Ru(dpp)3][(4-Clph)4B]2}s with the simultaneous release of photon energy. The ECL deep-red light could be observed easily with the naked eye in the dark. Figure 4 shows the ECL spectrum of the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode at a scan potential of 1.35 V. The ECL intensity of the constant potential at 1.35 V versus time is displayed in Figure S7 (Supporting Information). The PL spectrum of the [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO glass displays strong emission with a peak at 600 nm when excited at 470 nm as shown in the inset of Figure 4. The emission peak for the ECL spectrum is at ∼650 nm, which is bathochromatically shifted 50 nm and is broader than its PL spectrum. This phenomenon illustrates that the electronic excited state of [Ru(dpp)3]2+* from the ECL process is different from that resulting from the PL mechanism. The strong interaction

Figure 4. ECL emission spectrum of [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO glass. The inset is the PL spectrum of the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode excited at 470 nm.

Figure 5. Corresponding ECL intensity-time curve of the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode under continuous CV for 10 cycles in 0.10 M Na2C2O4 solution (pH 6.7) at a scan rate of 200 mV/s. The potential scan range was -1.0 V to +1.8 V.

between the solid-state [Ru(dpp)3][(4-Clph)4B]2 molecules in the nanoislands and the coreactants in the solution may be responsible for the broadening, red-shift effect that is similar to the surface-state ECL of the semiconducter.21 Control experiments showed no PL and ECL occurred when a bare ITO glass was used under the same experimental conditions. The as-prepared electrodes showed excellent stability. Figure 5 demonstrates that solid-state [Ru(dpp)3][(4-Clph)4B]2 was stably adhered onto the ITO electrode. After storage in doubly distilled water, the ECL intensity changed only slightly when it was used repeatedly over a 30 day period or even longer. This observation indicates that solid-state [Ru(dpp)3][(4-Clph)4B]2 nanoislands can still be strongly fastened onto the ITO electrode surface even after prolong use and can possibly be used as a reproducible detector for CE and HPLC applications. Effect of Scan Rate and pH on ECL. The effect of the scan rate on the ECL intensity of the electrodes was investigated. Figure 6 shows the peak ECL intensity at different scan rates (20-500 mV/s). The ECL intensity increases sharply with the increase in scan rate over the range of 20-100 mV/s and then increases slowly at 100-500 mV/s. Because the electrochemical oxidation-reduction reactions are very fast, changes in the ECL intensity in response to the scan rate will largely depend on the diffusion rate of C2O42- to the electrode. The higher the scan rate, the more easily the reactions between {[Ru(dpp)3][(4Clph)4B]2}s and C2O42- can be obtained. However, the diffusion rate of C2O42- will remain almost constant at a very high scan rate;10,12 in other words, ECL will be under diffusion control. Because ECL of the Ru complex is commonly sensitive to reaction pH, it is essential to scrutinize the effect of pH on the ECL intensity of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode. Figure 7 displays the pH effect on the ECL intensity of the [Ru(dpp)3][(4-Clph)4B]2-modified ITO electrode. The solid-state luminophor ECL system maintains a strong ECL under acidic conditions, and the ECL signal of the modified ITO electrode shows almost no change from pH 3.5 to 7.0 but drops dramatically from 7.0 to 7.5 and almost vanishes from 8.5 to 10.0. This is much different from the commonly used Ru(bpy)32+ ECL system. The maximum ECL emission of [Ru(bpy)3]2+ is normally observed under slightly basic conditions and almost (21) (a) Myung, N.; Ding, Z. F.; Bard, A. J. Nano Lett. 2002, 2, 1315–1319. (b) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 1053–1055. (c) Myung, N.; Lu, X. M.; Johnston, K. P.; Bard, A. J. Nano Lett. 2004, 4, 183–185.

[Ru(dpp)3][(4-Clph)4B]2 Nanoislands

Figure 6. Effect of scan rate on the ECL intensity of the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode in 0.10 M Na2C2O4 solution (pH 6.7).

Figure 7. Effect of pH on the ECL intensity of the [Ru(dpp)3][(4Clph)4B]2 nanoisland-modified ITO electrode in 0.10 M Na2C2O4 solution with pH from 3.5 to 10.0. The potential scan rate was 200 mV/s, and the potential scan range was -1.0 to +1.8 V.

disappears under highly acidic conditions.7,14a Besides, the ECL behavior of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode exhibits better stability and sensitivity with coreactant C2O42- than with commonly used tripropylamine (TPrA). The solid-state [Ru(dpp)3][(4-Clph)4B]2 nanoisland ECL system can provide an alterative pH environment for some analytes in lowpH solutions. The pH turn-off behavior of the ECL can serve as an ultrasensitive pH sensor around pH 7.0, which is approximately physiological pH. Inhibition of ECL by Phenols. In ECL analysis, numerous analytes are detected because of their inhibition effect on ECL signals. For example, McCall and Richer observed the inhibition effect of phenols on Ru(bpy)32+ ECL signals.22 In this work, the ECL intensity of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode was also inhibited by phenol, implying that phenol can act as an inhibitor in ECL analysis. Figure 8 displays the ECL intensity of the electrode in a 0.10 M Na2C2O4 solution (pH 6.7) with various concentrations of phenol at a scan rate of 200 (22) (a) McCall, J.; Alecander, C.; Richter, M. M. Anal. Chem. 1999, 71, 2523–2527. (b) McCall, J.; Richter, M. M. Analyst 2000, 125, 545–548. (23) Klimant, I.; Wolfbeis, O. S. Anal. Chem. 1995, 67, 3160–3166.

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Figure 8. ECL response of the [Ru(dpp)3][(4-Clph)4B]2 nanoislandmodified ITO electrode on exposure to various concentrations of phenol in 0.10 M Na2C2O4 solution (pH 6.7). The concentrations of phenol were (a) 0.0, (b) 2.0 × 10-8, (c) 2.0 × 10-7, and (d) 2.0 × 10-5 M. The potential scan rate was 200 mV/s, and the potential scan range was -1.0 to +1.8 V.

mV/s. Similar to the Ru(bpy)32+-tripropylamine (TPrA) system, the ECL inhibition mechanisms may be due to an energy transfer between freshly electrogenerated [Ru(dpp)3]2+* and phenol. It was also proposed that the CO2 · - might interact with the phenolic compounds before reacting with [Ru(dpp)3]3+.22 To our surprise, even a very low concentration (2.0 × 10-8 M) of phenol can inhibit the ECL intensity. As a result, it is possible to determine phenol by two decades of concentration lower than that of the aqueous-phase ECL method.24 In addition, the detection ranges of phenol spans in at least 4 orders of magnitude (Figure 8). This may lead to the further development of an ultrasensitive method for the determination of the phenolic compounds. Although the {[Ru(dpp)3][(4-Clph)4B]2}s-TPrA system can also exhibit the ECL phenomenon, its stability and ECL intensity are inferior to those of the {[Ru(dpp)3][(4-Clph)4B]2}s-C2O42- system. Furthermore, the ECL signal in TPrA can also be inhibited by phenols, but its response time to low concentrations of phenol is longer than that in C2O42- as shown in Figure S8 (Supporting Information). In brief, our {[Ru(dpp)3][(4-Clph)4B]2}s-C2O42system has a higher sensitivity to phenol detection, which is better than in a previous ECL study.21 Figure 9 depicts the inhibition effect of some phenolic compounds on the {[Ru(dpp)3][(4-Clph)4B]2}s-C2O42- system. Different types of phenols displayed different inhibition efficiencies. Among the investigated phenolic compounds, phenol had the highest inhibition efficiency. Resorcinol exhibited the smallest magnitude of quenching in the three derivatives of phenol, and the ECL intensity showed little response to low concentrations of p-nitrophenol because the ECL quenching mechanism of these systems might be due to energy transfer between freshly electrogenerated {[Ru(dpp)3][(4-Clph)4B]2}s* and phenol. Besides, the quenching may also be related to the electrostatic attraction between {[Ru(dpp)3][(4-Clph)4B]2}s+ and these species. It is possible that the steric effect of substituents hinders the formation of oxidation products that are responsible for quenching the {[Ru(dpp)3][(4-Clph)4B]2}s* excited state.22 Among these phenolic compounds, p-nitrophenol is relatively unable to donate an electron because of the presence of a strong electron(24) (a) Cui, H.; Li, F.; Shi, M. J.; Pang, Y. Q.; Lin, X. Q. Electroanalysis 2005, 17, 589–598. (b) Zheng, H.; Zu, Y. J. Phys. Chem. B 2005, 109, 16047– 16051. (c) Ala-Kleme, T.; Kulmala, S.; Jiang, Q. Luminescence 2006, 21, 118– 125.

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electrode directly has been proposed for the first time. The nanoisland-modified ITO electrode has many merits such as low consumption of expensive reagents, ease of fabrication, massproduction viability, better electrochemical activity, and longer lifetime. The ECL shows excellent reproducibility, stability, and sensitivity response to phenol in oxalate solution. Besides, the as-prepared modified electrode extends the use of the Ru-complex ECL system from the usual alkaline to acidic conditions. Therefore, because the ITO glass is optically transparent and can be molded into any kind of shape based on individual requirements, it can be fabricated as a flexible detector for CE or HPLC applications. The ECL signal is so sensitively inhibited by phenol that the solid-state [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO glass electrode can be employed as a sensor for determining phenols in practical applications. Figure 9. Effect of different phenolic species on the ECL intensity of the [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrode in a 0.10 M Na2C2O4 solution (pH 6.7).

withdrawing nitro group; as a result, its quenching efficiency is lower. Resorcinol exhibiting a smaller magnitude of quenching may also be related to the oxidation products because quenching is believed to occur via energy transfer from {[Ru(dpp)3][(4Clph)4B]2}s* to benzoquinone.22 Moreover, it is noteworthy to observe that the ECL intensity changes only slightly when it is in a nitrogen- or oxygen-saturated aqueous solution; therefore, the determination results will not be affected by the oxygen content in solution in practical applications. This is also a salient advantage of our proposed method.

Conclusions In summary, a facile dewetting strategy to fabricate the solidstate [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO glass

Acknowledgment. We express our sincere thanks to Mr. Liu Juntao for recording the SEM images. Financial support from the National Science Foundation of China (nos. 20570542 and 20770505) is gratefully acknowledged. Supporting Information Available: Mass spectrum, ECL intensity-time curve of [Ru(dpp)3][(4-Clph)4B]2-immobilized graphite, glassy carbon, and carbon fiber electrodes under continuous CVs in a 0.1 M Na2C2O4 solution (pH 6.7) with a scan rate of 200 mV/s, ECL intensity-time curve of [Ru(dpp)3][(4-Clph)4B]2 nanoisland-modified ITO electrodes in the absence of Na2C2O4 for 3 circles, ECL intensity of constant potential at 1.35 V versus time, and ECL of the [Ru(dpp)3][(4Clph)4B]2-modified ITO electrode in 0.2 M PBS-TPrA (pH 6.7) on exposure to various concentrations of phenol. This material is available free of charge via the Internet at http://pubs.acs.org. LA803151X