J. Phys. Chem. C 2008, 112, 16663–16667
16663
Electrogenerated Chemiluminescence of the Tris(2,2′-bipyridine)ruthenium(II)/ Tri-n-propylamine (TPrA) System: Crucial Role of the Long Lifetime of TPrA•+ Cation Radicals Suggested by Electrode Surface Effects Zuofeng Chen† and Yanbing Zu*,†,‡ Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, China, and Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, Singapore 138669 ReceiVed: April 3, 2008; ReVised Manuscript ReceiVed: August 19, 2008
We describe the effects of electrochemical pretreatment of glassy carbon electrodes (GCEs) on the electrogenerated chemiluminescence (ECL) of the Ru(bpy)32+ (bpy ) 2,2′-bipyridine)/tri-n-propylamine (TPrA) system. It has been found that the oxidation of TPrA at the anodically pretreated GCEs became more facile, while the intensity of ECL could be greatly suppressed. The contravention between the electrochemical reaction kinetics and the light emission intensity of the coreactant ECL system was reported for the first time and has been attributed to the rapid deprotonation of TPrA•+ cation radicals by the aid of oxygen-containing surface species formed on the GCE during the pretreatment. Because the lifetime of TPrA•+ was reduced, the products of the deprotonation reaction, TPrA• free radicals, would be more subject to oxidative consumption on the electrode subsequently, leading to weaker ECL signals. When the ECL was produced mainly following a catalytic route, however, the electrode surface effect was much less significant. This study suggests that the long lifetime of TPrA•+ cation radicals may be crucial for the intense light emission, which allows a sufficient amount of highly reducing intermediate radicals, TPrA•, to participate in the ECL process within a relatively thick reaction layer before being destroyed by the electrode. Ru(bpy)33+ + TPrA• f Ru(bpy)32+* + P
Introduction The electrogenerated chemiluminescence (ECL) of Ru(bpy)32+ (bpy ) 2,2′-bipyridine) with aliphatic amines as coreactants was discovered about two decades ago.1 Due to its high ECL efficiency, the Ru(bpy)32+/tri-n-propylamine (TPrA) system is most commonly employed in immunoassay and DNA analysis, with Ru(bpy)32+ or its derivatives as ECL labels.2 The commercial success of the coreactant ECL system has greatly invigorated the mechanistic study.3 Although the direct experimental evidence is scarce, it is generally believed that the oxidation of TPrA in a phosphate buffer solution leads to the formation of TPrA radicals, including the cationic and free ones, as described below:
TPrAH+ + HPO42- a TPrA + H2PO4+ (pKa ≈ 10.4) (1) TPrA - e f TPrA•+ •+
•
(2) +
TPrA f TPrA + H
(3)
•
TPrA - e f P
(4)
where TPrA•+ ) (CH3CH2CH2)3N•+, TPrA• ) (CH3CH2CH2)2NC• HCH2CH3, and P ) (CH3CH2CH2)2N+ d CHCH2CH3. The radical species play key roles in producing the excited state of Ru(bpy)32+:
Ru(bpy)32+ + TPrA• f Ru(bpy)3+ + P
(5)
+ Ru(bpy)3+ f Ru(bpy)32+*
(6)
•+
TPrA
+ TPrA
* To whom correspondence should be addressed. Phone: +65 68247190. Fax: +65 6478 9085. E-mail:
[email protected]. † The University of Hong Kong. ‡ Institute of Bioengineering and Nanotechnology.
(7)
To achieve higher ECL intensity, an efficient way is to make TPrA oxidation more facile on the electrode. These mainly comprise studies of different approaches to the modification of Au and Pt electrodes.3b,g,h,4 It has been revealed that a more hydrophobic electrode surface may result in faster kinetics of TPrA oxidation and, therefore, more intense ECL emission.4 Compared to Au and Pt, a freshly polished glassy carbon electrode (GCE) can oxidize TPrA more efficiently.3b However, the GCE surface is difficult to regenerate in situ, making the electrode not suitable for a flow-injection ECL detection system. In the literature, many pretreatment approaches were reported to activate the GCE and improve the reproducibility.5-8 A commonly used method is oxidizing the GCE surface anodically, sometimes followed by a reduction step at a negative potential.5 Because proton-coupled electron-transfer reactions might be facilitated by the surface species formed during the pretreatment, the oxidation of a number of biomolecules was made more facile.5 To the best of our knowledge, there has been little literature discussion regarding the effect of these steps on ECL reactions. In this paper, we demonstrate how the oxidation of TPrA and the ECL signal can be affected upon the electrochemical pretreatment of the GCE. It has been found that the pretreated electrodes could be made more active for the oxidation of TPrA; however, the ECL signal was significantly suppressed. A possible mechanism has been proposed for the electrode surface effects which involves reactions between TPrA•+ cation radicals and the oxygen-containing surface species. This study may help to achieve a better understanding of the complicated reaction mechanism of the coreactant ECL system and may have consequences for its applications.
10.1021/jp802873e CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
16664 J. Phys. Chem. C, Vol. 112, No. 42, 2008
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Figure 1. SEM images of a freshly polished GCE (a) and a GCE pretreated in the potential range from -0.2 to +1.8 V for 10 cycles (b).
TABLE 1: Results of EDS Elemental Analysis (%) of GCEs
Experimental Section Chemicals. Tris(2,2′-bipyridyl)ruthenium(II) dichloride hexahydrate (Ru(bpy)3Cl2 · 6H2O, minimum 98%) and TPrA (98%) were purchased from Sigma-Aldrich. Fe(CN)63-/4- were BDH chemicals, and Co(bpy)33+ was prepared by following a procedure in the literature.9 Other chemicals were analytical reagent grade and used as received. All solutions were prepared with deionized water (Milli-Q, Millipore). The pH of the phosphate-buffered solution (PBS; 0.15 M) containing TPrA was adjusted with concentrated NaOH or phosphoric acid. Apparatus. Scanning electron microscopy (SEM) images and energy-dispersive spectra for elemental analysis were obtained with a Leo 1530 FEG. Cyclic voltammetry (CV) was performed with the model 760B electrochemical workstation (Chenhua Instruments, Shanghai). The three-electrode system consisted of a working electrode, a coiled Pt wire counter electrode, and a saturated calomel electrode (SCE). The ECL signal was measured with a photomultiplier tube (PMT; Hamamatsu R928) installed under the electrochemical cell. A voltage of -800 V was supplied to the PMT with the Sciencetech PMH-02 instrument (Sciencetech Inc., Hamilton, Ontario, Canada). Procedures. GCEs of 3 mm diameter were polished mechanically with an alumina slurry from 1.0 to 0.05 µm, followed by sonication in distilled water for 2 min to remove debris, and were thoroughly rinsed with Milli-Q water. The electrochemical pretreatment of the GCEs was conducted by repeated scanning over different potential ranges in 0.15 M PBS (pH 7.5) with a scan rate of 500 mV/s.8 To eliminate the influence of oxygen,10 solutions for the ECL study were deaerated by bubbling highpurity (99.995%) N2, and a constant flow of N2 was maintained over the solution during the measurements. The potentials reported are referred to the SCE. All the experiments were performed at room temperature. Results and Discussion Characterization of the Electrochemically Pretreated GCEs. The GCE surface layer formed via the electrochemical pretreatment, the so-called electrochemical graphite-like oxide,11 has been exhaustively studied by means of a broad spectrum of techniques: electrochemical techniques,12 X-ray photoelectron spectroscopy (XPS)/Auger electron spectroscopy (AES),13 SEM,14 scanning tunneling microscopy (STM)/atomic force microscopy (AFM),15 FTIR,16 Raman spectroscopy,17 and ellipsometry.18 It is generally accepted that the GCE surface oxide is a highly porous, hydrated, and insulating 3-D film that grows at highly positive potentials and can become partially conducting upon reduction at a negative potential. In the present study, the surface topography of freshly polished and electrochemically pretreated GCEs was first examined by SEM. Figure 1 shows that the freshly polished surface was relatively smooth, with some randomly oriented polishing scratches, and virtually devoid of alumina particles,
element
freshly polished GCE
GCE after pretreatment 1a
GCE after pretreatment 2b
C O
96.9 3.1
94.5 5.5
91.5 8.5
a CV from -0.2 to +1.8 V, 10 cycles. b CV from -0.2 to +2.4 V, 10 cycles.
while the GCE surface after pretreatment at a positive potential became rougher. The surface roughening of pretreated GCEs has been attributed to the surface etching at the highly anodic potentials (i.e., carbon consumption due to CO/CO2 evolution, formation of soluble humic acids or salts, and physical loss of carbon particles).11a,15b,19 In fact, a transparent film which refracted light and caused a color change from black to metallic green, and finally to blue, could be observed along with the pretreatment. The energy-dispersive spectroscopy (EDS) elemental analysis revealed that the oxygen content on the GCE surface increased significantly after the pretreatment (Table 1), which is consistent with the results obtained previously by XPS/ AES.13d The oxygen-containing surface species usually involve phenolic, quinoidal, and carboxylic functionalities.13,20 The benchmark anionic and cationic redox couples Fe(CN)63-/4- and Co(bpy)33+/2+ were selected to probe the change of GCE electrochemical properties. Figure 2a shows that the electrode pretreatment led to an obvious decrease of the redox current and an increase of the redox peak separation in the Fe(CN)63-/4- system. This could be attributed to the electrostatic repulsion between the negatively charged oxygencontaining species on the electrode surface and the redox couple. Previous studies also revealed the slower reaction kinetics of Fe(CN)63-/4- on oxidized carbon fiber microcylinder electrodes.21 Voltammograms of Co(bpy)33+/2+ displayed different features at the pretreated GCEs. As shown in Figure 2b, the amplitude of the cathodic wave was enhanced. It has been found that the interaction between Co(bpy)33+ and the pretreated GCE surface was quite strong. Clear redox waves appeared in a blank buffer after the oxidized electrode was exposed to the solution containing the cobalt species for a while. The process might involve chemisorption, electrostatic uptake, hydrophobic interaction, and ion exchange, etc.7 A previous report showed similar behavior of Ru(NH3)63+ and Co(NH3)63+ at oxidized carbon fiber microcylinder electrodes.21 ECL at the Electrochemically Pretreated GCEs. A Ru(bpy)32+ ECL with TPrA as the coreactant can be produced via several routes.1-3 In the presence of a trace amount of Ru(bpy)32+ and a high concentration of TPrA, the direct oxidation of TPrA at the electrode plays a predominant role in generating ECL intermediate radicals, i.e., TPrA•+ and TPrA•. In this case, the cyclic voltammogram displayed a broad oxidation wave of TPrA at a freshly polished GCE, as shown in Figure 3. During the positive potential scan, two ECL peaks appeared at 0.95 and 1.15 V, resulting from the formation of
ECL of the Ru(bpy)32+/TPrA System
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16665
Figure 2. CV curves of 1 mM Fe(CN)63-/4- (a) and 1 mM Co(bpy)33+ (b) in 0.15 M PBS (pH 7.5) at a freshly polished GCE (solid line) and the GCE pretreated in the potential range from -0.2 to +1.8 V for 10 cycles (dashed line). Scan rate 100 mV/s.
Figure 3. CV-ECL curves of 1 µM Ru(bpy)32+ and 10 mM TPrA in 0.15 M PBS (pH 7.5) at a freshly polished GCE (solid line) and the GCE pretreated in the potential range of -0.2 to +1.8 V for 10 cycles (dashed line). Scan rate 100 mV/s.
excited-state Ru(bpy)32+* via reactions 6 and 7, respectively.1-3 At the pretreated GCE, the oxidation current of TPrA increased and the peak potential shifted negatively. However, along with the more facile TPrA oxidation, the ECL emission was greatly suppressed. Typically, no ECL signal could be observed after the GCE was pretreated by a continuous scan of 10 cycles in the potential range from -0.2 to +2.4 V. Figure 4 shows the values of the peak potential and current of TPrA oxidation as well as the corresponding ECL intensity at the GCEs pretreated under different conditions. The oxidation of TPrA was obviously improved following the growth of the surface oxides. The most active GCE surface was obtained upon cycling in the potential range of -0.2 to +2.4 V 10 times. More intensive pretreatment led to a positive shift of the TPrA oxidation potential and a slight drop of the oxidation current, probably because the GCE surface oxide layer became less conductive. In contrast, the ECL emission intensity dropped rapidly with the growth of the electrode surface oxide layer. The oxidation of TPrA and the ECL emission at the pretreated GCE were also examined in solutions of different pH values, as shown in Figure 5. Generally, the increase of pH in the range of 5-8 facilitated the reaction and led to more intense light emission. The effect on TPrA oxidation was less pronounced at higher pH. Previous studies indicated that the pH effect mainly results from the deprotonation reaction of TPrAH+ prior to its oxidation.1b,3h This should also be the case when the oxidized GCEs are used. At different pH values, the electrochemical pretreatment exhibited similar influences on TPrA oxidation and the ECL signal, i.e., promoting the oxidation and suppressing the ECL. The negative shift of the TPrA anodic wave at the oxidized GCEs suggests a faster reaction kinetics. The electrocatalysis behavior of GCE surface oxides has been observed for a number
of proton-coupled electron-transfer reactions, such as the oxidation of ascorbate and hydroquinone.5,13d,22 The pretreatment protocols engaged in this study also resulted in similar effects for these reactions (see the Supporting Information, Figures S1 and S2). The oxidation of TPrA involves two deprotonation steps, reactions 1 and 3, which are the preceding and following steps of the electron-transfer reaction 2, respectively. The electrode surface oxides might also play roles in these reactions. As the rate constant of reaction 1 (k1) in a phosphate solution is very large (∼108.2 M-1 s-1),23 a further increase of the reaction kinetics could not lead to an obvious shift of the TPrA oxidation wave along the potential axis.24 However, previous studies indicated that reaction 3 might proceed much slower than expected, and the rate constant (k3) has been estimated to be ∼540 s-1.3f,23b Therefore, we believe that the oxygen-containing functional groups on the oxidized GCE surface could serve as proton acceptors and accelerate the oxidation of TPrA by facilitating the deprotonation of TPrA•+, as illustrated in Scheme 1. Since reaction 3 is a chemical step following the first electron-transfer process, the increase of its reaction rate would shift the oxidation peak potential negatively.24 Digital simulation of the reactions also revealed the effects of k1 and k3 on the anodic wave potential (see the Supporting Information, Figures S3 and S4). It was somewhat surprising that the more facile TPrA oxidation led to much less efficient ECL emission. As the ratelimiting step of the ECL process of the Ru(bpy)32+/TPrA system might be the deprotonation of the amine radical cation, i.e., reaction 3, it has been suggested that emission can be improved by conditions that promote the deprotonation reaction.23b However, the current work shows that when the deprotonation of TPrA•+ was facilitated by the electrode surface oxides, the ECL intensity was greatly suppressed. Because the second electron-transfer reaction in the ECL process (reaction 4) would consume the highly reducing species, TPrA• free radicals, it competed with reactions 5-7 that led to the formation of the excited states. We believe that the faster deprotonation might significantly reduce the opportunity for TPrA•+ to escape from the electrode surface. Consequently, the product of the deprotonation reaction, TPrA•, would be more readily destroyed by the electrode before participating in the ECL reactions, leading to a decrease of emission intensity. It has been determined experimentally that the diffusion distance of TPrA•+ is ∼6 µm, corresponding to a half-life of ∼0.2 ms.3f The long lifetime of TPrA•+ could be of great significance in producing intense ECL signals. If the deprotonation rate of TPrA•+ is obviously increased, as that observed in this study, the ECL reaction layer will become very thin and very close to the electrode surface, resulting in a weaker emission.
16666 J. Phys. Chem. C, Vol. 112, No. 42, 2008
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Figure 4. Effects of the electrode pretreatment on the peak potential (Ep) and peak current (Ip) of TPrA oxidation as well as the maximum ECL intensity (Iecl). In (a) and (c), the pretreatment cycles were conducted in the potential range from -0.2 to +2.4 V. In (b) and (d), the pretreatment scans were conducted for 10 cycles. ECL solution: 0.15 M PBS (pH 7.5) containing 1 µM Ru(bpy)32+ and 10 mM TPrA. Scan rate 100 mV/s.
Figure 5. Variation of the TPrA oxidation peak potential (a) and peak current (b) and maximum ECL intensity (c) with pH at a freshly polished GCE (0) and GCEs pretreated by 10-cycle scans in the potential range from -0.2 to +1.8 V (O) or from -0.2 to +2.4 V (∆). ECL solution: 0.15 M PBS (with different pH values) containing 1 µM Ru(bpy)32+ and 10 mM TPrA. Scan rate 100 mV/s.
SCHEME 1: Oxidation of TPrA at an Electrochemically Pretreated GCE
Another possible influence of the surface species on the ECL emission might result from their interactions with the excited state, Ru(bpy)32+*. It is known that phenolic species in solution are able to quench the ECL of Ru(bpy)32+.25 However, if the ECL reactions occurred in a relatively thick layer around the electrode (e.g., ∼6 µm normally),3f it is very unlikely that the phenolic and quinoidal functionalities on the electrode surface could affect the emission considerably by destroying Ru(bpy)32+*. When the lifetime of the TPrA cation radical is significantly reduced, as discussed above, all the reactive species in the ECL reactions, including TPrA free radicals and Ru(bpy)32+*, would be confined within a thin layer around the electrode, and then the interactions between Ru(bpy)32+* and the surface species might contribute to the ECL drop. However, our previous study indicated that the phenolic compounds can efficiently quench the coreactant ECL mainly by consuming TPrA free radicals, while their reactions with Ru(bpy)32+* play
a less important role.25a On the basis of these results, we believe that the weak ECL emission at the oxidized GCEs should be predominantly attributed to the reduction of the lifetime of TPrA cation radicals and the larger consumption of TPrA free radicals on the electrodes. For comparison, ECL was also produced in a solution containing relatively concentrated Ru(bpy)32+ (1 mM) and dilute TPrA (0.1 mM). Under this condition, the ECL reaction proceeded mainly via the catalytic pathway; i.e., TPrA was catalytically oxidized by electrogenerated Ru(bpy)33+. Figure 6 shows the typical CV-ECL curves obtained at the freshly polished and electrochemically pretreated GCEs. A bit enhanced oxidation wave was observed upon electrochemical pretreatment, which could be attributed to Ru(bpy)32+ adsorption at the oxidized GCE. Along with Ru(bpy)32+ oxidation, ECL was produced. In contrast to the cases where the direct coreactant oxidation route was predominant, the influence of the electrode pretreatment on the ECL signal produced via the catalytic pathway was not obvious. As indicated by the slow decay of the ECL signal in the reverse (negative) potential scan, the lifetime of Ru(bpy)33+ was quite long. Therefore, the catalytic oxidation of TPrA by Ru(bpy)33+ could occur within a relatively thick layer around the electrode, and the surface oxides exerted much less influence on the ECL reactions. Conclusions TPrA oxidation could proceed faster at an anodically pretreated GCE. The oxygen-containing surface species may be
ECL of the Ru(bpy)32+/TPrA System
Figure 6. CV-ECL curves of 1 mM Ru(bpy)32+ and 0.1 mM TPrA in 0.15 M PBS (pH 7.5) at a freshly polished GCE (solid line) and a GCE pretreated in the potential range of -0.2 to +1.8 V for 10 cycles (dashed line). Scan rate 100 mV/s.
involved in the deprotonation reaction of TPrA•+ cation radicals and accelerate the reaction rate. Under the conditions where the direct coreactant oxidation played a predominant role in producing ECL, the light emission signal was significantly suppressed. It is believed that the lifetime of TPrA•+ was reduced because of its rapid deprotonation on the electrode surface, and the reaction product, TPrA•, would be more readily oxidized by the electrode. As these intermediate radicals play crucial roles in the ECL process, their lifetime scales determine the light emission intensity. The experimental results shown here indicated that the long lifetime of the cation radicals was important for the coreactant system to produce an intense ECL signal. Acknowledgment. We thank Professor V. W. W. Yam for helpful discussions. This work has been supported by the University Development Fund on Molecular Functional Materials of The University of Hong Kong and CERG grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (HKU 7061/04P and HKU 7059/ 05P). Z.-F.C. acknowledges the receipt of a postgraduate studentship from The University of Hong Kong. Supporting Information Available: Effects of the electrochemical pretreatment of GCEs on other proton-coupled redox reactions and digital simulation results showing the influence of the deprotonation reaction rate of the TPrA cation radical on the TPrA oxidation peak potential. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865. (b) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127. (2) (a) Bard, A. J. Electrogenerated Chemiluminescence; Marcel Dekker: New York, 2004. (b) Xu, X-H. N.; Zu, Y. In New Frontiers in UltrasensitiVe Bioanalysis: AdVanced Analytical Chemistry Applications in Nanobiotechnology, Single Molecule Detection, and Single Cell Analysis;
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JP802873E
