Electrochemiluminescent Reaction between Ru(bpy)32+ and Oxygen

Nov 3, 2009 - E-mail: [email protected] (Y.C.); [email protected] (G.C.). ... Hiroshi Tokue , Kenichi Oyaizu , Takashi Sukegawa , and Hiroyuki Nishid...
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J. Phys. Chem. C 2009, 113, 20316–20321

Electrochemiluminescent Reaction between Ru(bpy)32+ and Oxygen in Nafion Film Liyan Zheng, Yuwu Chi,* Qingqing Shu, Yongqiang Dong, Lan Zhang, and Guonan Chen* Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, and Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian, 350108, China, and Fujian ProVincial Key Laboratory of Analysis and Detection for Food Safety, Fuzhou UniVersity, Fuzhou, Fujian, 350108, China ReceiVed: March 12, 2009; ReVised Manuscript ReceiVed: October 10, 2009

For years, dissolved oxygen (O2) has been regarded as a quenching species in Ru(bpy)32+ electrochemiluminescent (ECL) systems; however, in the present study, O2 was found to act as a coreactant for Ru(bpy)32+ ECL in Nafion film, resulting in a strong ECL light emission. ECL experiments were carried out at a Ru(bpy)32+/ Nafion film-modified glassy carbon electrode (GCE) immersing in air-saturated phosphate buffer solution (pH 7.4). Scanning in the potential range of +1.5 to -1.0 V resulted in three luminescent processes, including two potential-dependent luminescence peaks (ECL-1 and ECL-2), and one potential-independent persistent luminescence emission (CL-P). Therein, ECL-2 occurring at potential less than -0.5 V was demonstrated to be a new chemiluminescent reactions between O2 and Ru(bpy)32+. In ECL-2, Nafion film plays important roles in stabilizing Ru(bpy)33+ and O2•- radical essentially for producing the new ECL. Unlike previously reported ECL processes, ECL-2 peak potential is dependent on the reduction potential of the coreactant (i.e., O2) rather than the redox potentials of the luminopore, Ru(bpy)32+, which would provide a useful way to probe O2, O2•- radical, and their stabilities in electrochemical reactions. Introduction Immobilization of electrochemiluminescence (ECL) materials, such as Ru(bpy)32+,1-12 luminol,13,14 and quantum dots (nanocrystals),15-18 at electrode surfaces has received tremendous attention for its possible applications in developing highly sensitive and selective ECL sensors,5-10,16-19 and new types of display devices.11,12 The immobilization of these ECL materials has been carried out by using several effective methods, such as ion exchanging,1-6,10 doping,9,11 polymerization,7,8 covalent binding,13,14 and self-assembling methods.15-19 Thereinto, the most popular method for immobilization is based on an ion exchanging technology, which immobilizes the ECL material, normally Ru(bpy)32+, by using Nafion cationic exchange polymer. By this method, very stable immobilization of Ru(bpy)32+ can be obtained while keeping most of its ECL activity.1-3 Although, much work has been done on the immobilization of ECL materials and its applications in ECL chemical sensors, not so much attention has been paid to revealing ECL mechanism in the thin solid-state layer immobilized on the electrode. Since first ECL phenomenon of Ru(bpy)32+ in Nafion film was reported by Bard and co-workers,1 some important ECL mechanisms in Nafion, such as coreactant ECL,2 electron and mass transfer of Ru(bpy)32+ in Nafion film,3 and ECL quenching effect,4 have been studied. However, there still remain several unsolved problems, such as How do species change during the potential scan? How do the species and their intermediates distribute in the solid-state thin layer? Why can some ECL phenomena be observed in solid phases but not in solution phases for a given ECL system? and so on. Apparently, the resolutions of these problems are very important for more clearly understanding ECL mechanisms in Nafion polymers or other types of solid-state ECL based on various immobilizations, for controlling ECL species in * Corresponding author. E-mail: [email protected] (Y.C.); gnchen@ fzu.edu.cn (G.C.).

solid-state layers, and for finding new ECL systems in solid phases, which might be quite different from those in solution phases. Recently, we observed some new ECL processes for Ru(bpy)32+ immobilized in Nafion film at a glassy carbon electrode (GCE) in the presence of dissolved oxygen. These new ECL phenomena could not be observed for free Ru(bpy)32+ in the same aqueous solution containing dissolved oxygen. In this paper, ECL behaviors of Ru(bpy)32+ in Nafion film in the presence of O2 were investigated. On this basis, new ECL mechanisms, especially concentration profiles of species during the potential scan, the function of Nafion film in the new ECL processes, and a new type of “reductive-reductive” coreactant (O2) were discussed in detail. Experimental Section Chemicals and Solutions. Tris(2,2-bipyridine)ruthenium(II) chloride hexahydrate (Ru(bpy)3Cl2.6H2O) and Nafion 117 solution (5%, D20 ) 0.87 g mL-1) were purchased from SigmaAldrich and Fluka, respectively. Pure O2 gas (99.99%) provided by local gas station (Xinhang Gas, Fuzhou) was used to prepare O2-saturated solutions. The concentrations of dissolved O2 in solutions were evaluated from Ostwald’s solubility coefficient for a given partial pressure of O2.20 Apparatus. ECL and electrochemical measurements were carried out on an ECL detection system (MPI-E, Remex Electronic Instrument Lt. Co., Xi’an, China), equipped with a three-electrode ECL cell. ECL spectra were measured by placing cutoff filters of 535, 555, 575, 620, 640, 680, and 705 nm (provided by Beijing Institute of Biophysics, Academia Sinica, China) before the photomultiplier tube (PMT) window and detecting the intensities of ECL passing through these filters respectively under same experimental conditions.21 The working electrode was a Ru(bpy)32+/Nafion film-coated GCE (Ru(bpy)32+/ Nafion/GCE), which was prepared as follow:1 3.0 µL Nafion 117 solution was dropped on a GCE electrode (with diameter

10.1021/jp902239j CCC: $40.75  2009 American Chemical Society Published on Web 11/03/2009

ECL Reaction between Ru(bpy)32+ and O2 in Nafion Film

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Ru(bpy)32+ - e- f Ru(bpy)33+

(1)

Ru(bpy)33+ + OH- f Ru(bpy)32+* + 1/2H2O + 1/4O2 (2) Ru(bpy)32+* f Ru(bpy)32+ + hν

Figure 1. ECL (A) and electrochemical (B) responses obtained for Ru(bpy)32+/Nafion/GCE in air-saturated (I) and N2-saturated (II) PBS (0.1 mol L-1, pH7.4). Potential scan rate was 100 mV s-1.

of 4.0 mm), and kept dry at room temperature. The thickness of hydrated Nafion film was calculated to be 6.58 µm from the weight of Nafion contained in the Nafion solution (3.0 µL, 5% Nafion 117, D20 ) 0.87 g mL-1), the area covered by Nafion film (4.0 mm in diameter), and the hydrated Nafion density of 1.58 g cm-3.22 Other thickness measurements of Nafion film were obtained by changing the volume or concentration of Nafion solution added to the electrode surface. After being coated with the Nafion film, the GCE was immersed into a 1 × 10-3 mol L-1 Ru(bpy)32+ solution for 8 h to obtain a stable Ru(bpy)32+/Nafion/GCE-modified working electrode. The reference electrode was a Ag/AgCl (3 mol L-1 KCl) electrode, and the counter electrode was a Pt wire electrode. To avoid possible ECL interference, the Pt wire electrode was immersed in a glass jacket containing 1 mol L-1 phosphate and separated from the test solution by a porous glass tip. Results and Discussion Three luminescent processes, i.e., ECL-1, CL-P, and ECL-2 were observed during scanning Ru(bpy)32+/Nafion/GCE in 7.4 phosphate buffered saline (PBS) solution over a potential range from -1.0 to +1.5 V (Figure 1). These ECL phenomena can also be observed when using other buffer solutions, such as Borate and Britton-Robinson buffers (data not shown). Detailed descriptions of these ECL processes and discussion on their reaction mechanisms were given as follows. ECL-1 Process. Figure 1 shows ECL (Figure 1A) and electrochemical (Figure 1B) responses of Ru(bpy)32+/Nafion/ GCE in pH 7.4 phosphate solution during cycling the potential of the working electrode between -1.0 and +1.5 V. First, when the potential was scanned anodically from -1.0 V, no ECL happened at the potential domain between -1.0 and +1.0 V. Until the potential reached higher than 1.0 V, where Ru(bpy)32+ could be oxidized to Ru(bpy)33+ (see Figure 1B), a weak ECL emission, i.e., ECL-1 (see curve I in Figure 1A) with maximum emission wavelength at 620 nm was observed, indicating that the ECL was from Ru(bpy)32+* light emission.21,23-25 ECL-1 can be assigned to the anodic ECL reactions between the low concentration of OH- in PBS and the high concentration of Ru(bpy)32+ accumulated in Nafion film:26

(3)

CL-P Process. After anodic polarization, the working electrode was reversely scanned from +1.5 to -1.0 V. A persistent light emission (CL-P) was observed after ECL-1 over a wide potential window (from +1.5 to -0.5 V). CL-P could not be observed for Ru(bpy)32+ in homogeneous solution, which might result from the chemiluminescent reaction between residual Ru(bpy)33+ and OH- in Nafion film. As the coreactant of CL-P, OH- anions could keep their concentration (c.a. 2.5 × 10-7 mol L-1) unchanged during the chemiluminescent reaction with Ru(bpy)33+ for the pH7.4 PBS was used. As the luminopore of CL-P, Ru(bpy)33+ was electrogenerated from Ru(bpy)32+ and immobilized in the Nafion layer due to its strong electrostatic interaction with the Nafion ionic exchange polymer; hence the diffusion of these species in the film was significantly decreased,2 and their leak into PBS phase was also negligible during experiments. The low diffusion velocity of Ru(bpy)33+ in the film results in partial Ru(bpy)33+ molecules being rereduced to Ru(bpy)32+ upon applying cathodically shifted potential, while the other Ru(bpy)33+ molecules remained in the Nafion film. Apparently, the residual Ru(bpy)33+ in the film reacted with OH- to give rise to the persistent ECL. Unusually, a gradual increase in luminescence intensity of CL-P was observed over the anodic potential range from +1.5 to -0.5 V. This unusual phenomenon can be explained by a “Excited-State Quenching” mechanism,2,4 i.e., some of Ru(bpy)32+* will be quenched by Ru(bpy)33+. During the potential scanning from +1.5 to -0.5 V, an excess amount of Ru(bpy)33+ molecules that quench Ru(bpy)32+* are reduced to Ru(bpy)32+ and thus the chemiluminescence CL-P is increased gradually. A further experiment showed that a constant concentration of OH-, i.e., the use of buffer solution, was necessary for the formation of gradually increasing CL-P, since CL-P was found to decrease over the anodic potential range from +1.5 to -1.0 V when using unbuffered Na2SO4 solution (pH7.4, adjusted by NaOH) degassed with nitrogen (data not shown). ECL-2 Process. When the potential was further shifted to less than -0.5 V, a strong ECL emission, i.e., ECL-2 (see curve I in Figure 1A) with maximum emission wavelength at 620 nm was observed for the Ru(bpy)32+/Nafion/GCE PBS system in the presence of dissolved oxygen (air-saturated PBS). To the best of our knowledge, ECL-2 has never been reported for a Ru(bpy)32+ ECL system in homogeneous aqueous solution. Our experimental data showed that ECL-2 resulted from the chemiluminescent reaction between residual Ru(bpy)33+ and an electrogenerated reducing mediate, O2•-, from dissolved oxygen. 1. ECL-2 Related to O2•- Radical. The occurrence of ECL-2 was highly dependent on the electrochemical reduction and the existence of oxygen in the solution. The comparison of curve I of Figure 1A and that of Figure 1B in negative potential range (0 to -1.0 V) showed that ECL-2 appeared to accompany the electrochemical reduction of oxygen. This indicates that the reductive product of oxygen rather than oxygen itself reacts with Ru(bpy)33+ in the Nafion film to produce ECL-2. Curve II in Figure 1A showed that ECL-2 disappeared after removing oxygen from the solution by nitrogen, suggesting that ECL-2 intensity is dependent on oxygen concentration. Detailed

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Figure 2. ECL (A) responses obtained for Ru(bpy)32+/Nafion film exposed to dissolved O2 at various concentrations: (a) 0.263; (b) 0.223; (c) 0.173; (d) 0.136; (e) 0.080; (f) 0.051; (g) 0.030; (h) 0.017 mmol L-1. Potential scan rate was 100 mV s-1. pH 7.4, 0.1 mol L-1 PBS was used. Inset graph (B) is the calibration curve.

dependence of ECL intensity of ECL-2 on the concentration of oxygen was investigated and recorded in Figure 2. It is evident that ECL peak height is decreased with decreasing oxygen concentration (see a-h in Figure 2A), and there is a good linear relationship between ECL intensity (IECL) and the concentration of oxygen (CO2) in the range of 0 to 0.3 mmol L-1 (see Figure 2B). Consequently, it can be followed from these data that oxygen is involved in the ECL-2 reaction. In general, several reactive oxygen species (ROS), such as O2•- (HO2•), H2O2 and • OH, may be produced by the electro-reduction of oxygen.27 To test which species might be essential for ECL-2, H2O2 was added to N2-saturated PBS (in the absence of O2). The experimental result showed that ECL-2 did not occur in the presence of H2O2, indicating that O2•- or HO2• (electrochemically generated from O2) rather than other ROS (e.g., H2O2 and its reduction intermediates, •OH) reduces Ru(bpy)33+ into Ru(bpy)32+* and subsequently produces ECL-2. An additional experimental result showed that ECL-2 was pH-dependent, i.e., the higher the pH, the stronger the ECL intensity (data not shown). This can lead to a conclusion that O2•- (the conjugate base) rather other HO2• (the conjugate acid) acts as the highly reducing species in ECL-2. In this ECL process, Nafion film might play an important role in stabilizing O2•-, since Nafion, as well-known, contains both the hydrophilic sulfonic group and the hydrophobic fluorocarbon backbone. The latter provide a hydrophobic environment in Nafion film,3,4,28 which makes the electrogenerated O2•- stable until its reaction with Ru(bpy)33+. 2. ECL-2 Related to Ru(bpy)33+. ECL-2 was also highly dependent on Ru(bpy)33+. This was confirmed by the following two comparison experiments. In both experiments, ECL was recorded for the Ru(bpy)32+/Nafion/GCE working electrode in the potential range of -1.0 to 0.2 V; however, in one experiment, a pre-electrolysis at 1.4 V (to produce Ru(bpy)33+ in the Nafion film) was performed for 60 s before recording ECL in the potential range of -1.0 to 0.2 V, while in the other experiment (the control experiment), no pre-electrolysis was carried out before the ECL measurement. The experimental results are shown in Figure 3. In the case without pre-electrolysis at 1.4 V, i.e., without Ru(bpy)33+ in the Nafion film, no ECL-2 was observed (see curve a in Figure 3). In contrast, in the case with pre-electrolysis at 1.4 V, where Ru(bpy)33+ was produced from Ru(bpy)32+ in the Nafion film, obvious ECL-2 peaks were observed (see curve b in Figure 3). Therefore, these experimental results suggest that Ru(bpy)33+ is essential for ECL-2. Additionally, it is evident that the Nafion film play an important role in the formation and storage of Ru(bpy)33+ at the electrode surface.

Zheng et al.

Figure 3. ECL responses obtained for Ru(bpy)32+/Nafion/GCE in PBS (0.1 mol L-1, pH7.4) in the potential range from +0.2 to -1.0 V without (a) and with (b) pre-electrolysis at +1.4 V for 2 min. Potential scan rate was 100 mV s-1.

3. Concentration Profiles of Species in Nafion Film. Figure 3b also showed that ECL-2 intensity was changed with cycling the potential of the working electrode in the range from -1.0 to 0.2 V. The ECL-2 intensity changes underwent two different steps. At the first step, ECL-2 peak intensities were kept almost constant in the first two potential cycles, i.e., 48 s (peaks I and II), while at the second step, ECL-2 peak intensities were decreased significantly with time (see peaks III-VIII). Apparently, the decrease of ECL-2 intensity resulted from the depletion of ECL species, i.e., Ru(bpy)33+ and the coreactant O2. In the present ECL system, the concentration of dissolved O2 might be kept constant from cycle to cycle of the potential scan, since depleted O2 in the Nafion film could be completely restored by the diffusion of dissolved O2 from contacting bulk solution under a relatively low scan rate (100 mV s-1 was used in present study).29 Therefore, the change of ECL-2 intensity mainly resulted from the change of Ru(bpy)33+ concentration profile in the Nafion film. Apparently, in the applied potential range mentioned above (-1.0 to 0.2 V), which was much lower than the redox potential of the Ru(bpy)33+/Ru(bpy)32+ couple (c.a. +1.1 V vs Ag/AgCl), the reduction of Ru(bpy)33+ at the electrode should be very fast and diffusion-controlled. It has been previously reported that, at shorter electrolysis time, the Ru(bpy)33+ concentration profile in Nafion film can be described by the semi-infinite planar diffusion equation:3,30

CRu3+(x, t) ) C*Ru3+ erf

[

x 2(DRu3+t)1/2

]

(4)

where CRu3+(x, t) is the concentration of Ru(bpy)33+ at distance x at time t, C*Ru3+ and DRu3+ are, respectively, the initial concentration and diffusion coefficient of Ru(bpy)33+ in the Nafion film. It should be noted here that the leakage of Ru(II) and Ru(III) from the Nafion film into bulk solution can be neglected during experimental measurements because of the strong electrostatic interaction between the Nafion polymer and Ru species. However, at longer electrolysis time, the diffusionlayer thickness (δ) approaches the film thickness (l), and a transition to thin-layer electrolysis behavior occurs. In this case, the Ru(bpy)33+ concentration profile in Nafion film can be depicted by eq 5:30,31

CRu3+(x, t) )

(

)

4C*Ru3+ -π2DRu3+t πx sin exp π 2l 4l2

(5)

ECL Reaction between Ru(bpy)32+ and O2 in Nafion Film

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where l is the thickness of the Nafion film. Some typical concentration profiles, i.e., CRu3+(x, t)/C*Ru3+ ∼ x curves, for different electrolysis time were plotted and shown in Figure 4 to fit the experimental result shown in Figure 3. It is evident that, in the case of shorter electrolysis time (50 s), the Ru(bpy)33+ concentration profile (dashed lines) changes with time more significantly for thin-layer electrolysis. In this case, Ru(bpy)33+ is depleted in both the inner and outer layers of the Nafion film, causing a relatively fast decrease in ECL-2 intensity with time (peaks III-VIII in Figure 3). According to Figures 3 and 4, it can be concluded that the diffusion-layer thickness (δ) approached the film thickness (6.58 µm, see Experimental Section) within 50 s. Herein, the apparent diffusion coefficient of Ru(bpy)33+, Dapp, in Nafion film can be readily calculated without knowing the initial concentration of Ru(bpy)33+, C*Ru3+, in the film by eq 6:30

δ ) 4√Dappt

(6)

The value of Dapp calculated by this method is 5.4 × 10-10 cm2 s-1, which is consistent with that previously reported.3 The effect of the thickness of Nafion film on the ECL-2 intensity was investigated. It was found that both too thin and too thick Nafion films did not benefit the ECL response. The ECL-2 intensity was obviously decreased when using Nafion films with thickness less than 3 µm, which could be explained by the fact that thin-layer electrolysis, i.e., over depletion of Ru(bpy)33+, happened during cathodic polariza-

Figure 5. Schematic diagram for the reaction mechanism of the Ru(bpy)32+/Nafion/GCE|O2 ECL system (ECL-2) in aqueous solution.

tion. Using thicker Nafion film resulted in higher ECL intensity; however, Nafion film would easily crack, and significant ECL noise was found when the thickness of Nafion film was higher than 11 µm. A stable and sensitive ECL-2 response could be obtained when the thickness of Nafion film was in the range of 4-9 µm. 4. ECL Mechanism. On the basis of above experimental results, the processes of the new ECL (ECL-2) in the Nafion film was described in Figure 5, and the corresponding mechanism was proposed as follows:

Ru(bpy)32+/Nafion/GCE - e- f Ru(bpy)33+/Nafion/GCE (7) O2 + e- f O2•-

(8a)

O2•- + H2O f HO2• + OH-

(8b)

Ru(bpy)33+ /Nafion/GCE + O2•- f Ru(bpy)32+*/Nafion/GCE + O2 (9) Ru(bpy)32+* /Nafion/GCE f Ru(bpy)32+ /Nafion/GCE + hν (10)

Figure 4. Concentration profiles of Ru(bpy)33+ and O2•- in Nafion film coated on a GCE during scanning potential (at 100 mV s-1) between +0.2 and -1.0 V. Ru(bpy)33+ concentration profiles at shorter (solid lines) and longer (dashed lines) electrolysis times were plotted according to eqs 5 and 6, respectively. O2•- concentration profiles were calculated with eq 5 by assuming 10 s electrolysis of O2 (i.e., O2 is reduced at maxium rate between -0.5 and -1.0 V in each potential cycle). In calculation, the values of the diffusion coefficient of Ru(bpy)33+ and O2 were assumed to be 7.0 × 10-10 cm2 s-13 and 1.0 × 10-5 cm2 s-1,29a respectively.

Apparently, besides an oxidation process (eq 7), the mechanism of this new ECL includes two coupled reductive processes, i.e., the electrochemical reduction of O2 to a strongly reducing intermediate, O2•- (see eq 8a), and the chemical reduction of Ru(bpy)33+ by the strongly reducing O2•- to produce the excited state luminopore, Ru(bpy)32+*, intheNafionfilm(eq9).Herein,wecallO2 a“reductive-reductive” coreactant for its ability to form highly reducing agent upon electrochemical reduction. It is obvious that this “reductivereductive” coreactant is a new type of coreactant for it is different from the well-known “oxidative-reductive” coreactants, such as C2O42-,32 and “reductive-oxidative” coreactant, S2O82-.24 Compared with previously reported ECL systems, the new ECL systems have one significant advantage: ECL peak potential is dependent on the coreactants (e.g., O2) rather than the luminopore (e.g., Ru(bpy)32+), in other words, ECL measurements for the coreactants are potentialresolved. It is envisioned that this potential-resolved ECL

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would improve the selectivity of ECL measurement and have promising applications in ECL sensors. Presently, the three luminescent processes, i.e., ECL-1, CLP, and ECL-2, observed during scanning Ru(bpy)32+/Nafion/ GCE in PBS can be experimentally assigned to chemiluminescent reactions between electrogenerated Ru(bpy)33+ and OH(eqs 1-3), between residual Ru(bpy)33+ and OH- (eqs 2-3), and between residual Ru(bpy)33+ and electrogenerated O2•- (eqs 7-10), respectively. However, there is a puzzle of the contradiction between the relatively low energy available by the ECL reactions and the relatively high energy required for forming the excited-state luminopore, i.e., Ru(bpy)32+*, normally 2.1 eV.33 It has been known that the standard redox potentials (versus normal hydrogen electrode (NHE)) of E0(O2/OH-) and E0(O2/O2•-) are +0.815 and -0.33 V, respectively.34 This implies that the reducing activity of O2•- is much stronger than that of OH-; thus the chemiluminescent reaction between Ru(bpy)33+ and O2•- to produce Ru(bpy)32+* (eq 9) seems reasonable if eq 2 is valid. When the ROS, i.e., OH-, and O2•reduce Ru(bpy)33+ into Ru(bpy)32+ (E0(Ru(bpy)33+/Ru(bpy)32+) ) 1.26 V vs NHE),33b they should provide energy of 0.41 and 1.59 eV, respectively, for eqs 2 and 9, according to the following equation:

Zheng et al. charged Ru(bpy)33+/Ru(bpy)32+ and the negatively charged Nafion polymer result in immobilization and slow diffusion of Ru(bpy)33+/Ru(bpy)32+ species in Nafion film. The immobilization prevents the leakage of electrogenerated Ru(bpy)33+ into homogeneous solution while the slow diffusion of species in Nafion film allows the establishment of the concentration gradient of Ru(bpy)33+ in the Nafion film. Evidently, both of these two effects may prolong the lifetime of Ru(bpy)33+ during the cathodic potential scan and lead to the observation of the new ECL. It is envisioned that the present work on the new ECL in Nafion film would be helpful for discovering and explaining other types of solid-state ECL systems, since similar effects of immobilization and low diffusion would exist there. Acknowledgment. This study was supported by National Natural Science Foundation of China (20775014, 20735002, 20675016), the Key Project of Chinese Ministry of Education (207052), the Natural Science Funds for Distinguished Young Scholar of Fujian Province, China (2009J06003), National Hightech R&D Project of China (2008AA06A406), National Basic Research Program of China (2010CB732403). References and Notes

-∆G ) F(E0(Ru(bpy)33+ /Ru(bpy)32+) - E0′(O2 /ROS) (11) Apparently, both of the energy available by the reactions of Ru(bpy)33+ with OH- (0.41 eV) and with O2•- (1.59 eV) are lower than 2.1 eV required for producing Ru(bpy)32+*. This means that ECL emission might not occur via the reactions between Ru(bpy)33+ and the ROS, especially OH-, which conflicts with the fact that chemiluminescent reactions between Ru(bpy)33+ and OH- have been widely observed.26 Actually, a similar puzzle of ECL energy has been mentioned in some of the literature;26a,e however, to date, no satisfied ECL mechanism has been proposed for the chemiluminescent reactions between Ru(bpy)33+ and OH- or ROS in terms of energy. Probably, real mechanisms of chemiluminescent reactions of Ru(bpy)33+ with OH- or ROS are much more complicated than those have been proposed,26 for example, the formation of Ru(bpy)32+* is associated with charge transfer transitions between a d-orbital on the ruthenium and a π*-antibonding orbital on the ligand,35 thus reactions on the ligand, e.g. nucleophilic attack by hydroxide on the bound bipyridine ring,36 addition of one electron in the ligand rather than the metal centered orbital for reducing Ru(bpy)32+ into Ru(bpy)3+,37 might have some effects on the d-π* charge tranfer processes as well as energy involved; however, these kinds of effects have been omitted in estimating the energy change in the ECL reaction (eq 11). Further studies focused on revealing the mechanism of ECL reactions between Ru(bpy)32+ and O2 (or OH-) in terms of energy are going on in our lab. Conclusions ECL reactions of Ru(bpy)32+ with dissolved O2 in Nafion film include a new ECL process (ECL-2) that can not be observed in homogeneous solution. Anodically generated Ru(bpy)33+ in Nafion film reacts with cathodically formed O2•- species from dissolved oxygen to produce the strong light emission, ECL-2. The Nafion film may play a key role in stabilizing the species, Ru(bpy)33+ and O2•- essential for ECL-2. The strong electrostatic interaction between positively

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