Electrogenerated Chemiluminescence of the Tris(2,2′-bipyridine

J. Phys. Chem. C 2009, 113, 21877–21882

21877

Electrogenerated Chemiluminescence of the Tris(2,2′-bipyridine)ruthenium(II)/Tertiary Amine Systems: Effects of Electrode Surface Hydrophobicity on the Low-Oxidation-Potential Emission 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: August 21, 2009; ReVised Manuscript ReceiVed: October 20, 2009

Previous studies revealed an electrogenerated chemiluminescence (ECL) of the [Ru(bpy)3]2+ (bpy ) 2,2′bipyridine)/tri-n-propylamine (TPrA) system at electrode potentials well before the oxidation of [Ru(bpy)3]2+. The low-oxidation-potential (LOP) ECL signals can be produced efficiently at bare glassy carbon and fluorosurfactant (i.e., Zonyl FSO or FSN) modified gold electrodes. Herein, we compare the LOP ECL behavior of [Ru(bpy)3]2+ with different tertiary amines as the coreactants. The amines studied include tri-n-ethylamine (TEtA), TPrA, and tri-n-butylamine (TBuA). At the FSO-modified gold electrode, the electro-oxidation of the amines becomes more facile as the FSO adsorption layer is more compact; however, the LOP ECL signals increase first and then decrease. The results suggest that the hydrophobicity of the electrode surface plays an important role in determining the ECL behavior. The drop of the emission intensity at the electrodes with a high coverage of FSO may result from the lifetime shortening of the amine cation radicals in a less polar reaction layer. An unexpected weak LOP ECL signal was found for the [Ru(bpy)3]2+/TBuA system, which is attributed to the extremely short lifetime of TBuA cation radicals at the hydrophobic electrode. This work provides more evidence of the effects of electrode surface hydrophobicity on the coreactant ECL systems. Introduction Fundamentals and applications of electrogenerated chemiluminescence (ECL) have been intensively studied over the past few decades. For immunoassays, the ECL system of [Ru(bpy)3]2+ (bpy ) 2,2′-bipyridine)/tri-n-propylamine (TPrA) is widely used, where the ruthenium chelate serves as a tag for biomolecule labeling and TPrA is the coreactant used to produce oxidizingandreducingreactiveradicalsuponitselectro-oxidation.1-4 Although the oxidative-reduction mechanism has been wellaccepted, reaction details affecting the emission intensity are still not completely understood. A number of competitive pathways involving both heterogeneous and homogeneous reactions that may occur in the ECL process of the system are shown in Table 1.5-12 In the experiments where glassy carbon (GC) or gold are used as the working electrodes, two ECL waves appear when the electrode potential is swept positively. The first emission signal generated at a potential below 1.0 V versus SCE, referred to as the low-oxidation-potential (LOP) ECL herein, is believed to follow a reaction pathway of eq 3 f eq 4 f eq 6 f eq 7 f eq 12 f eq 13, where no oxidation of [Ru(bpy)3]2+ is required. As the electrode potential becomes more positive, [Ru(bpy)3]2+ oxidation will be involved and a stronger emission signal is produced. Usually, the LOP ECL signal is less efficient as compared to the conventional one at higher potentials. However, the LOP ECL route may contribute significantly to the total emission signal in the ECL immunoassay employing magnetic microbeads for [Ru(bpy)3]2+-tagged species immobilization and collection.13-16 More detailed study on this ECL process is needed. * Corresponding author. Tel.: +65 68247190. Fax: +65 6478 9085. E-mail:[email protected]. † The University of Hong Kong. ‡ Institute of Bioengineering and Nanotechnology.

Recently, we found that strong LOP ECL signals of the [Ru(bpy)3]2+/TPrA system could be produced at gold electrodes modified with nonionic fluorosurfactant species (i.e., Zonyl FSN or FSO).17-21 General features of the LOP ECL have been described.18 Because [Ru(bpy)3]2+ excited state is generated via the intermediacy of the TPrA cation radical (TPrA•+), the lifetime of TPrA•+ might be of extreme importance in the LOP ECL reactions. Direct evidence for the existence of TPrA•+ in neutral aqueous solution was obtained by Miao et al. with flow cell electron spin resonance (ESR) experiment at room temperature, and the half-life of TPrA•+ was estimated to be within the millisecond time scale (∼0.2 ms).13 Besides TPrA, a wide range of amine compounds may serve as the coreactants for [Ru(bpy)3]2+ ECL reaction,22-28 among which tertiary amines are usually more efficient than secondary and primary amines. In the present paper, the electrochemical behavior and ECL characteristics of [Ru(bpy)3]2+/tertiary amine [including tri-n-ethylamine (TEtA), TPrA, and tri-n-butylamine (TBuA)] systems were examined in aqueous solutions at bare GC and FSO-modified gold electrodes. The modification of the gold electrode with the fluorosurfactant facilitated the oxidation of the tertiary amines, and strong LOP ECL signals were observed in the cases of TEtA and TPrA. Surprisingly, TBuA was found to be much less efficient in generating the LOP ECL, although its electro-oxidation proceeded more rapidly at the FSO-modified gold electrode. The results suggest the substantial effects of electrode surface hydrophobicity on the lifetime of the cation radicals of the tertiary amines. Experimental Section Chemicals. Tris(2,2′-bipyridyl)ruthenium(II) dichloride hexahydrate ([Ru(bpy)3]Cl2 · 6H2O, min 98%); tertiary amines (TAs), including TEtA (99.5%), TPrA (98%), and TBuA (98.5%);

10.1021/jp908072v  2009 American Chemical Society Published on Web 11/17/2009

21878

J. Phys. Chem. C, Vol. 113, No. 52, 2009

Chen and Zu

TABLE 1: Reactions Involved in the ECL Process of the [Ru(bpy)3]2+/TPrA Systema reaction

a

ref for E°

[Ru(bpy)3]2+ - e- f [Ru(bpy)3]3+

(E° ∼ 1.02 V vs SCE)

(1)

13

[Ru(bpy)3]2+ - e- f [Ru(bpy)3]3+

(E° ∼ -1.57 V vs SCE)

(2)

13

ΤΡrΑΗ+ + ΗΡΟ42- f ΤΡrΑ + Η2ΡΟ4-

(3)

ΤΡrΑ - e- f ΤΡrΑ•+

(4)

(E° ∼ 0.88V vs SCE)

[Ru(bpy)3]3+ + ΤΡrΑ f [Ru(bpy)3]2+ + ΤΡrΑ•+

(5)

ΤΡrΑ•+ + ΤΡrΑ• + Η+

(6)

[Ru(bpy)3]2+ + ΤΡrΑ• f [Ru(bpy)3]+ + Ρ

(7)

ΤΡrΑ•+ + ΤΡrΑ• f ΤΡrΑ + Ρ

(8)

ΤΡrΑ• - e- f Ρ

(Ε° ∼ -1.7 V vs SCE)

(9)

[Ru(bpy)3]3+ + [Ru(bpy)3]+ f [Ru(bpy)3]2+* + [Ru(bpy)3]2+

(10)

[Ru(bpy)3]3+ + ΤΡrΑ• f [Ru(bpy)3]2+* + Ρ

(11)

[Ru(bpy)3]+ + ΤΡrΑ•+ f [Ru(bpy)3]2+* + ΤΡrΑ

(12)

[Ru(bpy)3]2+* f [Ru(bpy)3]2+ + hυ

(13)

8

29

TPrA•+ ) (CH3CH2CH2)3N•+, TPrA• ) (CH3CH2CH2)2NC•HCH2CH3, P ) (CH3CH2CH2)2N+dCHCH2CH3.

Zonyl FSO-100 [F(CF2CF2)1-7CH2CH2O(CH2CH2O)0-15H]; 1-hexanethiol; and 6-mercapto-1-hexanol were purchased from Sigma-Aldrich. Other chemicals were analytical reagent graded and used as received. All solutions were prepared with Milli-Q ultrapure water (>18 MΩ), except that 1-hexanethiol and 6-mercapto-1-hexanol were dissolved in 25% (v/v) acetonitrile/ H2O mixture. The pH of the phosphate-buffered saline (PBS) solution containing TAs was adjusted with concentrated NaOH or phosphoric acid. Apparatus. Cyclic voltammetry (CV) was performed with the model 600A electrochemical workstation (CH Instruments, Austin, TX). The three-electrode system consisted of a working electrode, a coiled Pt wire counter electrode, and a saturated calomel reference electrode (SCE) separated from the working cell by a salt bridge. 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. CV was performed with a scan rate of 100 mV/ s. GC and gold electrodes of 2-mm-diameter were polished with 0.05-µm alumina slurry to obtain a mirror surface and then were sonicated and thoroughly rinsed with Milli-Q water. Before each experiment, the gold working electrode was subjected to repeated scanning in the potential ranges from -0.5 to 1.4 V in 0.15 M PBS (pH 7.5) until reproducible voltammograms were obtained. The modification of the gold electrode with FSO-100 (FSO-Au) was conducted by immersing the electrochemically cleaned gold electrode in 5 wt % FSO aqueous solution for 5 min, followed by thoroughly rinsing with Milli-Q water. In some experiments, the FSO-Au electrodes were further treated by dipping in 2 mM 1-hexanethiol or 6-mercapto-1-hexanol solution for a certain time. In order to eliminate the influence of oxygen,19 electrolyte solutions were deaerated by bubbling with high purity (99.995%) N2, and a constant flow of N2 was maintained over the solution during the measurements. Reported values for TA oxidation current and ECL intensity were based upon the average of at least three scans with a relative standard

ECL of the Ru(bpy)32+/Tertiary Amine Systems

Figure 1. CV-ECL curves for 20 mM TEtA (red), 10 mM TPrA (green), and 10 mM TBuA (blue) in the presence of 1 µM [Ru(bpy)3]2+ at the GC electrode (a), the bare gold electrode (b), or the FSO-Au electrode (c). Buffer solution was 0.15 M PBS (pH 7.5). Scan rate was 100 mV/s.

deviation (RSD) of (5%. All potentials reported in this paper were referred to the SCE. All experiments were performed at room temperature. Results and Discussion Compared with primary and secondary amines, tertiary amines are easier to oxidize because of their lower ionization potentials originating from electrons in the nitrogen orbitals.22 Within the group of tertiary amines, the ionization potential decreases in magnitude when the length of the alkyl chain increases.22 The electron-donating effects of the alkyl chains influence the electrochemical behavior of the amines as well as the emission intensity as they react with [Ru(bpy)3]3+.22-24 Figure 1 shows the typical CV and the corresponding ECL curves obtained at GC and gold electrodes for three [Ru(bpy)3]2+ ECL systems, where TEtA, TPrA, and TBuA serve as the coreactants, respectively (note that, because tri-n-methylamine is much more difficult to oxidize electrochemically and tertiary amines with alkyl chains longer than that of TBuA are difficult to dissolve in aqueous solutions with a reasonable concentration, these species are not addressed in the current study). At the GC electrode (Figure 1a), the amines started to be oxidized as

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21879 the electrode potential was scanned positively beyond 0.8 V. The standard oxidation potential of TPrA has been reported to be ∼0.8-0.9 V vs SCE.8 It is obvious that the oxidation kinetics of the tertiary amines becomes faster as the length of the alkyl chain is longer. Note that more concentrated TEtA was used in our experiments to make the signals more evident. The CV profiles of the tertiary amines at the bare gold electrode are more complicated (Figure 1b), mainly because of the formation of the electrode surface oxides. First anodic peak appeared in the potential range from ∼0.1 to 0.45 V, which might result from the oxidation of the amines catalyzed by some oxygen-containing surface species.6,30 At potentials more positive than 0.7 V, the gold surface was oxidized and the amine oxidation was severely inhibited. When the gold electrode was premodified with FSO (Figure 1c), the growth of the electrode surface oxides was retarded (see Supporting Information, Figure S1), leading to the suppression of the first anodic waves and a great enhancement of the oxidation of the amines in the potential range from ∼0.7 to 1.0 V. The anodic peak potentials for the oxidation of TBuA, TPrA, and TEtA are 0.78, 0.88, and 0.96 V, respectively. Compared with that observed at the GC electrode, more facile oxidation of the tertiary amines can be achieved at the FSO-Au electrode. Previous studies revealed that the emissions of the coreactant ECL systems are electrode-material-dependent.6,7,17,18,31,32 This mainly originates from the variation of the electrochemical behavior of the coreactants at different electrodes.6 At the GC electrode (Figure 1a), when TBuA or TPrA served as the coreactant, two ECL peaks appeared at ∼0.9 and ∼1.1 V, respectively, consistent with that reported elsewhere.6,13 The emission at higher potentials followed the conventional ECL routes, where [Ru(bpy)3]2+ and the amines were oxidized simultaneously, while the LOP ECL emission involved no oxidation of [Ru(bpy)3]2+. Although the oxidation of TBuA was more facile than that of TPrA, the ECL smission intensity of the [Ru(bpy)3]2+/TBuA system was found to be lower than that of the [Ru(bpy)3]2+/TPrA system. In the case of TEtA, a very weak LOP ECL signal was observed because of the sluggish oxidation kinetics of the coreactant. At the bare gold electrode (Figure 1b), the ECL signals were much weaker because of the inhibition of amine oxidation by the gold surface oxides.6 When the gold electrode was modified with FSO (Figure 1c), the LOP ECL signals of the [Ru(bpy)3]2+/ TPrA and TEtA systems were strongly enhanced, which could be attributed to the faster oxidation kinetics of the coreactants; however, no significant change of the LOP ECL signal of the [Ru(bpy)3]2+/TBuA system was observed, although TBuA oxidation proceeded rapidly. TBuA solutions with a wide range of concentrations (from 0.1 to 100 mM) and different pH (from 4 to 10) were used in our experiments, but no strong LOP ECL emission (intensity >0.5 au) was observed at the FSO-Au electrode. To attain a better understanding of the fluorosurfactant effects, the coverage of FSO on the electrode was altered by dipping the cleaned gold electrode in a dilute (0.05 wt %) FSO aqueous solution for different time (see Supporting Information, Figure S2). Figure 2 shows that the oxidation currents of the tertiary amines increased rapidly at first and then reached plateaus gradually as the surfactant adsorption layer became saturated. Correspondingly, the LOP ECL signals rose in varying degrees first and the maximum values were attained at ∼8, ∼12, and ∼13 min for the [Ru(bpy)3]2+/TBuA, -TPrA, and -TEtA systems, respectively. However, further increase of the FSO coverage led to a drop of the LOP ECL intensity of each ECL

21880

J. Phys. Chem. C, Vol. 113, No. 52, 2009

Chen and Zu

Figure 2. Oxidation peak current of tertiary amines (a) and LOP ECL intensity (b) versus pretreatment time of the gold electrode in 0.05 wt % FSO solution. Buffer solution was 0.15 M PBS (pH 7.5). [Ru(bpy)3]2+, 1 µM. Coreactants were 20 mM TEtA (0), 10 mM TPrA (O), and 10 mM TBuA (∆), respectively. Scan rate was 100 mV/s.

system. Buffer solutions with a wide range of coreactant concentrations (from 0.1 to 100 mM) have been used and similar behavior of the oxidation current as well as the ECL intensity as a function of electrode modification time was observed. Corresponding to the rapid rise of TPrA oxidation current, the ECL signal was strongly enhanced first. This is consistent with the well-accepted mechanism of the coreactant ECL systems where the oxidation of the coreactant plays an important role in supplying the intermediate radicals for the ECL reactions. However, the fact that the faster electro-oxidation kinetics led to a weaker ECL emission at the FSO-Au electrode with a high coverage of the fluorosurfactant was somewhat surprising. We examined the electro-oxidation of [Ru(bpy)3]2+ at the FSO-Au electrode and the photoluminescence of [Ru(bpy)3]2+ in the presence of FSO, but little effect of FSO has been observed (see Supporting Information, Figures S3 and S4). In addition, according to the LOP ECL mechanism, no oxidation of [Ru(bpy)3]2+ is involved in the LOP emission. Therefore, the significant influences of FSO on the LOP ECL signals should not result from any changes of the electrochemical or luminescent behavior of [Ru(bpy)3]2+. In our recent report,33 the contravention between the electrochemical reaction kinetics and the light emission intensity of the [Ru(bpy)3]2+/TPrA system was also observed at a GC electrode, where the preoxidized GC electrode makes TPrA oxidation more facile but suppresses the ECL signal. It is believed that the surface oxygen-containing species may facilitate the deprotonation of TPrA•+ (eq 6). As the lifetime of TPrA•+ was reduced, the products of its deprotonation, TPrA• free radicals, would be more subject to oxidative consumption on the electrode (eq 9), leading to weaker ECL signals. In the current study, the adsorbed fluorosurfactant species also significantly altered the property of the electrode surface; i.e., the fluorocarbon chains of these molecules could render the gold electrode more hydrophobic, as indicated recently by contact angle measurements.17,34 We predicted that the hydrophobic environment surrounding the electrode could probably exert substantial effects on the ECL reactions. For the LOP ECL process, the quantity and lifetime of both the TPrA cation and free radicals are of great importance. The cation radicals were the crucial and only oxidant in producing the excited state (eq 12), while the free radicals were needed to generate [Ru(bpy)3]+ (eq 7). In aqueous solutions, the cation radicals could be stabilized by H2O solvation. The deprotonation of TPrA•+ involves a geometric change of the molecule from pyramidal to planar, which enables orbital overlap for the radical to be transferred to the R-carbon.12 The reorganization energy of this reaction should be larger in an aqueous solution due to the salvation effect.12 The slow rate of reaction 6 in neutral phosphate buffer solutions leads to the long lifetime (on the

submillisecond scale)13 of the cation radicals and allows for the intense emission via the LOP ECL route. In much less polar acetonitrile, however, it has been found that no LOP ECL signal of the [Ru(bpy)3]2+/TPrA system was observed, although the oxidation of TPrA proceeded facilely.35 This may suggest the extremely short lifetime of the cation radicals in the hydrophobic solvent. When fluorosurfactant species were adsorbed at the gold electrode, the environment of the ECL reactions became less polar, which would promote the deprotonation of the cation radicals. Therefore, the modification of the gold electrode with FSO exerted two opposite effects on the ECL process. On the one hand, the electro-oxidation of the amine coreactants could be facilitated greatly on the FSO-Au electrode, leading to the significant rise of the LOP ECL signals. On the other hand, more rapid deprotonation of the amine cation radicals tended to suppress the LOP ECL emission, especially at the gold electrode with a high coverage of FSO. As a result, both the oxidation current and the LOP ECL intensity increased at the moderately modified gold electrode (Figure 2), where the former effect of the electrode hydrophobicity played a primary role; when the coverage of FSO became larger, however, the latter effect could manifest itself and resulted in the downturn of the LOP ECL, as shown in Figure 2b. Compared with that of the other tertiary amines, the oxidation of TBuA was rendered more facile by the FSO modification of the gold electrode, while the LOP ECL of the [Ru(bpy)3]2+/ TBuA system was enhanced much less significantly and the downturn of the emission intensity occurred earlier as the coverage of FSO increasing. Previous studies indicated that the lifetime of tertiary amine cation radicals may vary with the length of alkyl chains.12,13,36,37 In aqueous solutions, R3N•+ radicals of tri-n-methylamine and TPrA have been observed by ESR;13,36 these species were found to be N-centered radicals that are deprotonated in neutral aqueous solutions at rates of 35 and 3500 s-1, respectively. The results may be attributable to the difference of the reorganization energy. For the more bulky tertiary amine molecules, the lower reorganization energy could result in faster deprotonation kinetics. Therefore, the rate of TBuA•+ deprotonation should be larger than that of TPrA•+ and TEtA•+, which may account for the weaker LOP ECL of the [Ru(bpy)3]2+/TBuA system than that of the [Ru(bpy)3]2+/ TPrA system at the GC electrode and the slight LOP ECL of the [Ru(bpy)3]2+/TBuA system at the FSO-Au electrode. It is known that the modification of gold electrodes by thiol monolayers with different terminal groups allows for altering the hydrophobicity of the electrode surfaces.7 To further confirm the electrode effect, the hydrophobicity of the FSO-Au electrode was tuned by partially displacing the fluorosurfactant species by alkyl chain (C6) thiols with different terminal groups

ECL of the Ru(bpy)32+/Tertiary Amine Systems

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21881

Figure 3. (a) LOP ECL intensity (at ∼0.87 V) versus pretreatment time of the FSO-Au electrode in 2 mM HS(CH2)5CH2OH (0) or 2 mM HS(CH2)5CH3 (O) aqueous solutions, respectively. (b) CV-ECL curves obtained at the FSO-Au electrodes with pretreatment in 2 mM HS(CH2)5CH2OH solution for 10 min (green) or in 2 mM HS(CH2)5CH3 solution for 15 min (blue) or without any pretreatment (red). Electrolyte solution was 0.15 M PBS (pH 7.5) containing 1 µM [Ru(bpy)3]2+ and 10 mM TBuA. Scan rate was 100 mV/s.

(-CH2OH or -CH3), and the influences on the LOP ECL of the [Ru(bpy)3]2+/TBuA system were examined. As shown in Figure 3a, the treatment of the FSO-Au electrode with these thiols exhibited distinctly different impacts on the LOP ECL. When the adsorbed FSO species was partially displaced by mercaptohexanol, the electrode surface could be rendered relatively hydrophilic, and an increase of the LOP ECL intensity was observed. In contrast, when the electrode was treated with hexanethiol, no significant change of the LOP ECL intensity occurred. Figure 3b showed typical CV-ECL curves obtained at the gold electrodes modified with the mixed layers. Generally, both of the thiols made the oxidation potential of TBuA shift positively. The effects were dependent on the substitution proportion of the thiols. By controlling the dipping time of the FSO-Au electrode in the thiol solutions, almost identical kinetics of TBuA oxidation at the two thiol-substituted FSO-Au electrodes could be obtained, as shown in Figure 3b. Despite the similar kinetics of TBuA oxidation, the intensities of the LOP ECL emission were remarkably different in these cases. The experiments clearly demonstrated that the electrode surface hydrophobicity could have a significant impact on the LOP ECL. The above experimental results indicated that the LOP ECL emissions were not only determined by the oxidation kinetics of the amines but also strongly dependent on the lifetime of the amine cation radicals. Both of the aspects could be remarkably influenced by hydrophobicity of the environment surrounding the electrode. Because of the complicated emission processes of the LOP ECL, it is important to take both of the aspects into consideration when evaluating an LOP ECL system. Conclusions The LOP ECL emissions of the [Ru(bpy)3]2+/tertiary amine systems are strongly affected by the electrode surface hydrophobicity. More hydrophobic electrode surfaces could lead to more facile coreactant oxidation and, in most cases, generate intense LOP ECL signals. However, the hydrophobic environment surrounding the electrode might promote the deprotonation reactions of the amine cation radicals, which reduced the efficiency of the LOP ECL emission. The opposite effects of the electrode hydrophobicity may account for the variations of the LOP ECL emissions observed herein. As the LOP emission contributes significantly in the ECL-based immunoassay, the results of the current study would be helpful in the design of more efficient ECL assay systems. Acknowledgment. We thank Prof. 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 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: Additional information as noted in the text. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bard, A. J. Electrogenerated Chemiluminescence; Marcel Dekker: New York, 2004. (2) Richter, M. M. Chem. ReV. 2004, 104, 3003. (3) Miao, W. J. Chem. ReV. 2008, 108, 2506. (4) 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; Xu, X-H. N., Ed.;Wiley: New York, 2007; pp 235-267. (5) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127. (6) Zu, Y.; Bard, A. J. Anal. Chem. 2000, 72, 3223. (7) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960. (8) Kanoufi, F.; Zu, Y.; Bard, A. J. J. Phys. Chem. B 2001, 105, 210. (9) Honda, K.; Yoshimura, M.; Rao, T. N.; Fujishima, A. J. Phys. Chem. B 2003, 107, 1653. (10) Cross, E. M.; Pastore, P.; Wightman, R. M. J. Phys. Chem. B 2001, 105, 8732. (11) Zhou, M.; Heinze, J.; Borgwarth, K.; Grover, C. P. Chem. Phys. Chem. 2003, 4, 1241. (12) Wightman, R. M.; Forry, S. P.; Maus, R.; Badocco, D.; Pastore, P. J. Phys. Chem. B 2004, 108, 19119. (13) Miao, W.; Choi, J. P.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 14478. (14) Blackburn, G. F.; Shah, H. P.; Kenten, J. H.; Leland, J.; Kamin, R. A.; Link, J.; Peterman, J.; Powell, M. J.; Shah, A.; Talley, D. B. Clin. Chem. 1991, 37, 1534. (15) Erler, K. Wien. Klin. Wochenschr. 1998, 110, 5. (16) Komori, K.; Takada, K.; Hatozaki, O.; Oyama, N. Langmuir 2007, 23, 6446. (17) Li, F.; Zu, Y. Anal. Chem. 2004, 76, 1768. (18) Zu, Y.; Li, F. Anal. Chim. Acta 2005, 550, 47. (19) Zheng, H.; Zu, Y. J. Phys. Chem. B 2005, 109, 12049. (20) Zheng, H.; Zu, Y. J. Phys. Chem. B 2005, 109, 16047. (21) Chen, Z.; Zu, Y. Langmuir 2007, 23, 11387. (22) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865. (23) Brune, S. N.; Bobbitt, D. R. Anal. Chem. 1992, 64, 166. (24) Liu, X.; Shi, L.; Niu, W.; Li, H.; Xu, G. Angew. Chem., Int. Ed. 2007, 46, 421. (25) Yin, X. B.; Sha, B. B.; Zhang, X. H.; He, X. W.; Xie, H. Electroanalysis 2008, 20, 1085. (26) Lee, W. Y. Mikrochim. Acta 1997, 127, 19. (27) Gerardi, R. D.; Barnett, N. W.; Lewis, A. W. Anal. Chim. Acta 1999, 378, 1. (28) Knight, A. W. Trends Anal. Chem. 1999, 18, 47. (29) Lai, R. Y.; Bard, A. J. J. Phys. Chem. A 2003, 107, 3335.

21882

J. Phys. Chem. C, Vol. 113, No. 52, 2009

(30) Burke, L. D.; Lyons, M. E. G. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O. M., Conway, B. E., Eds.; Plenum Publishing Corp.: New York, 1986; Vol. 18, p 169. and references therein. (31) Factor, B.; Muegge, B.; Workman, S.; Bolton, E.; Bos, J.; Richter, M. M. Anal. Chem. 2001, 73, 4621. (32) Bruce, D.; McCall, J.; Richter, M. M. Analyst 2002, 127, 125. (33) Chen, Z.; Zu, Y. J. Phys. Chem. C 2008, 112, 16663.

Chen and Zu (34) Tang, Y. A.; Yan, J. W.; Zhou, X. S; Fu, Y. C.; Mao, B. W. Langmuir 2008, 24, 13245. (35) Miao, W.; Bard, A. J. Anal. Chem. 2004, 76, 5379. (36) Das, S.; von Sonntag, C. Z. Naturforsch. 1986, 41b, 505. (37) Liu, W.; Yamanaka, S.; Shiotani, M.; Michalik, J.; Lund, A. Phys. Chem. Chem. Phys. 2001, 3, 1611.

JP908072V