Anal. Chem. 2003, 75, 601-604
Effects of Poly(ethylene glycol) tert-Octylphenyl Ether on Tris(2-phenylpyridine)iridium(III)Tripropylamine Electrochemiluminescence Christopher Cole, Brian D. Muegge, and Mark M. Richter*
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804-0089
The effects of the nonionic surfactant Triton X-100 (poly(ethylene glycol) tert-octylphenyl ether) on the properties of tris(2-phenylpyridine)iridium(III) (Ir(ppy)3, where ppy ) 2-phenylpyridine, electrochemiluminescence (ECL) have been investigated. Anodic oxidation of Ir(ppy)3 produces ECL in the presence of tri-n-propylamine (TPrA) in aqueous surfactant solution. Increases in ECL efficiency (g10-fold) and TPrA oxidation current (g2.0-fold) have been observed in surfactant media. The data support adsorption of surfactant on the electrode surface, thus facilitating TPrA and Ir(ppy)3 oxidation and leading to higher ECL efficiencies. Electrochemiluminescence (often called electrogenerated chemiluminescence, ECL) in aqueous surfactant solution is currently an area of active study.1-5 ECL involves the generation of excited states at an electrode and is a sensitive probe of electron and energy-transfer processes at charged interfaces.6 Solubilization of Ru(bpy)32+ (bpy ) 2,2′-bipyridine) and analogous compounds in aqueous nonionic surfactant solutions leads to significant, and potentially useful, changes in the ECL properties.2,3,7 For example, increases in both ECL efficiency (g8-fold) and duration of the ECL signal were observed in surfactant media upon oxidation of Ru(bpy)32+ and tri-n-propylamine (TPrA).2-4 Oxidation of the coreactant TPrA is believed to form a strong reducing agent (e.g., TPrA•) that can reduce Ru(bpy)33+ to *Ru(bpy) 32+.1,8,9 The recent demonstration of ECL from Ir(ppy)3 (ppy ) 2-phenylpyridine; Figure 1) in organic10,11 and aqueous11 solutions * Corresponding author. E-mail:
[email protected]. (1) (a) McCord, P.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91. (b) Ouyang, J.; Bard, A. J. Bull Chem. Soc. Jpn. 1988, 61. (c) Lee, S. K.; Bard, A. J. Anal. Lett. 1998, 31, 2209. (2) Workman, S.; Richter, M. M. Anal. Chem. 2000, 72, 5556. (3) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960. (4) Factor, B.; Muegge, B.; Workman, S.; Bolton, E.; Bos, J. Anal. Chem. 2001, 73, 4621-4624. (5) Richter, M. M. In Optical Biosensors: Present and Future; Ligler, F., RoweTaite, C. A., Eds.; Elsevier: Amsterdam, 2002; Chapter 6. (6) (a) Faulkner, L. R.; Bard, A. J. In Electroanaytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10, pp 1-95. (b) Faulkner, L. R.; Glass, R. S. In Chemical and Biological Generation of Excited States; Cilento, G., Adam, W., Eds.; Academic Press: New York, 1982; p 191. (7) Bruce, D.; McCall, J.; Richter, M. M. Analyst 2002, 127, 125. (8) Leland, J. K.; Powell, M. J. J. Electroanal. Chem. 1991, 318, 91. (9) Smith, P. J.; Mann, C. K. J. Org. Chem. 1969, 34, 1821. (10) Nishimura, K.; Hamada, Y.; Tsujioka, T.; Shibata, K.; Fuyuki, T. Jpn. J. Appl. Phys. 2001, 40, L945-L947. 10.1021/ac0203600 CCC: $25.00 Published on Web 01/04/2003
© 2003 American Chemical Society
Figure 1. Structures of Triton X-100 (poly(ethylene glycol) tertoctylphenyl ether; n ) 9-10, MW ∼646.86 with an average cmc ) 0.24 mM)22 and tris(2-phenylpyridine)iridum(III) (Ir(ppy)3).
opens up the possibility of using these types of compounds in diagnostic applications. Although the ECL emission quantum efficiency of Ir(ppy)3 was weaker than Ru(bpy)32+ under similar conditions,11 the green ECL emission maximum of Ir(ppy)3 and the red/orange emission of Ru(bpy)32+ are far enough removed that it is possible to distinguish both signals in a single ECL experiment. Therefore, due to the analytical importance of ECL in immunoassays and DNA probe analysis12 and the improved detection sensitivity of ruthenium polypyridyl compounds’ ECL in the presence of nonionic surfactants2,3,4,7 we decided to investigate the nature of Triton X-100/Ir(ppy)3 (Figure 1) interactions. EXPERIMENTAL SECTION Materials. Tris(2-phenylpyridine)iridium(III) was synthesized and purified by the literature method.13 All other materials were (11) Bruce, D.; Richter, M. M. Anal. Chem. 2002, 74, 1340-1342. (12) Bard, A. J.; Debad, J. D.; Leland, J. K.; Sigal, G. B.; Wilbur, J. L.; Wohlstadter, J. N. Electrochemiuminescence. In Encycopedia of Analytical Chemistry; Meyers, R. A., Ed.; Wiley: Chichester, 2000; pp 9842-9849.
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used as received: Ru(bpy)3Cl2‚6H2O (98%, Strem Chemical Inc, Newbury Port, MA), potassium phosphate monobasic hydrate (99%, EM Science, Gibbstown, NJ), tri-n-propylamine (98%, Avocado Research Chemicals, Ward Hill, MA), and Triton X-100 (Avocado). Potassium phosphate buffer solutions, 0.20 M KH2PO4‚7H2O, were prepared with deionized water that had been passed through a Barnstead/Thermolyne filtration system. Buffer solutions containing TPrA (0.05 M) were prepared similarly except that it was necessary to stir vigorously to completely dissolve the amine. The pH of these buffer solutions was adjusted with either 6 M HCl or 6 M NaOH. Methods. Electrochemical, ECL, photoluminescence, and UV-visible instrumentation and experimental methods have been described elsewhere.2,4,14 All electrochemical and ECL experiments were referenced with respect to Ag/AgCl gel electrode (0.20 V vs NHE).15 The working electrode was cleaned prior to each experiment by repeated cycling (+2.0 to -2.0 V) in 6.0 M sulfuric acid,16 followed by sonication in 2 M nitric acid and rinsing in deionized water. Solutions used for ECL intensity versus surfactant concentration studies included 10-5 M Ir(ppy)3, 0.05 M TPrA, 0.2 M phosphate buffer, and surfactant solutions that varied from 0.05 to 1.0 mM. ECL was measured by sweeping from 0 to +2.0 V versus a Ag/AgCl reference electrode at 0.1 V/s using cyclic voltammetry. For surfactant adsorption studies, the working electrode was dipped for 10 min into a solution containing 1.0 mM surfactant in 0.2 M potassium phosphate buffer. When appropriate, 0.01 mM Ir(ppy)3 and 0.05 M TPrA were also present. The electrode was rinsed gently with H2O for 1 min to remove any unadsorbed species and placed in a solution of 0.01 mM Ir(ppy)3 and 0.05 M TPrA in 0.2 M potassium phosphate buffer with no surfactant. Photoluminescence spectra were obtained with a Shimadzu RF-5301 spectrofluorophotometer (slit widths 3-5 nm). Excitation was at 452 and 383 nm for Ru(bpy)32+ and Ir(ppy)3, respectively, with detection between 450 and 700 nm. ECL efficiencies (φecl, photons generated per redox event) were obtained by the literature methods, using Ru(bpy)32+ (φecl ) 1) as the standard.17,18 Reported values are the average of at least three scans with a relative standard deviation of e(5%. RESULTS AND DISCUSSION Electrochemistry. Cyclic voltammetry (CV) was used to determine whether the presence of Triton X-100 surfactant results in a shift of the oxidation potential. The reversible to quasireversible oxidative wave (ia/ic e 1.19; E° ) 0.46) has been assigned to the Ir3+/4+ couple.13,19 The redox potentials of Ir(ppy)3 are nearly independent of the presence and concentration of Triton (13) Dedeian, K.; Djurovich, P. I.; Graces, F. O.; Carlson, G.; Watts, R. J. Inorg. Chem. 1985, 24, 318. (14) (a) Alexander, C.; Richter, M. M. Anal. Chim. Acta 1999, 402(1-2), 105112. (b) McCall, J.; Alexander, C.; Richter, M. M. Anal. Chem. 1999, 71, 2523. (c) McCall, J.; Richter, M. M. Analyst 2000, 125, 545-548. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications; Wiley: New York, 2001. (16) Woods, R. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1979; Vol. 9, p 1. (17) White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 6891-6895. (18) Richter, M. M.; Bard, A. J.; Kim, W. K.; Schmehl, R. H. Anal. Chem. 1998, 70, 310. (19) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985, 107, 1431.
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Figure 2. Absorption spectrum of Ir(ppy)3 (10 µM) in 0.2 M KH2PO4 with (A) 0, (B) 0.4, (C) 0.6, (D) 0.8, and (E) 1.0 mM Triton X-100.
X-100 surfactant in aqueous phosphate buffer solution. This is similar to Ru(bpy)32+, (bpy)2Ru(DC-bpy)3+, and (bpy)2Ru(DMbpy)3+ (DC ) 4,4′-dicarboxy-2,2′-bipyridine; DM ) 4,4′-dimethyl2,2′-bipyridine) in the presence of Triton surfactant molecules.2,4,7 The lack of an oxidative potential shift implies that there is not a stronger interaction between Ir(ppy)3+ and surfactant media compared to the reduced (i.e., Ir(ppy)3) form of this complex.20,21 Absorption and Photoluminescence. The absorption spectrum of Ir(ppy)3 is characterized by a series of ligand-based transitions in the UV with MLCT transitions in the visible.13,19 The MLCT absorption bands centered at 380 nm show no dependence on Triton X-100 concentration up to 1.0 mM surfactant. However, the intensities of the predominantly ligand-based transitions in the UV do show a dependence (Figure 2), indicating possible interactions between surfactant molecules and the ppy ligands. Excitation into the broad visible absorption bands produces roomtemperature photoluminescence for Ir(ppy)3 with MLCT wavelength maximums for emission (λem) of 507 and 517 nm in aqueous solution.13,19 As noted in a previous ECL study,11 these maximums are clearly distinct from that of Ru(bpy)32+ (λem ≈ 620 nm), making these systems of interest in applications where an internal standard or multianalyte determination is desired. The wavelengths of emission maximums for Ir(ppy)3 in aqueous solution show no dependence on Triton X-100 concentration from 0.1 to 1 mM. However, the photoluminescence emission intensities do increase at higher Triton X-100 concentrations (Figure 3). In contrast, PL and excited-state lifetime studies20,21 using Ru(bpy)32+ indicate that the interaction of Ru(bpy)32+ and Triton X-100 is van der Waals or hydrophobic in nature and little/no changes in either intensity or wavelength are observed. However, a number of factors including sensitizer/surfactant binding mode, orientation, and solvation have been shown to affect the chemistry and photophysics of surfactant-bound photosensitizers.20 Whether the increased PL indicates stronger binding of Ir(ppy)3 in surfactant micelles compared to Ru(bpy)32+ is unclear and will require detailed photochemical and photophysical studies. Electrochemiluminescence. Due to the “reversible” nature of the Ir3+/4+ anodic redox couple, TPrA was used as an “oxidative-reductive” coreactant8 to generate ECL. ECL was observed for Ir(ppy)3 in aqueous and aqueous surfactant solution containing 0.05 M TPrA at a Pt interface. The (20) Mandal, K.; Hauenstein, B. L., Jr.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1983, 87, 328. (21) Dressick, W. J.; Hauenstein, B. L., Jr.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984, 88, 3337.
Table 1. ECL Peak Intensities and Oxidation Current of the Ir(ppy)3/TPrA System for Sequential Cyclic Voltammetric Sweeps of a Pt Electrode Immersed in a Dip Solution for 10 min, Rinsed, and Placed in a Separate Solution Containing 0.2 M KH2PO4 with or without ECL Luminophore/Coreactanta sweep 1b
sweep 2b
sweep 3b
dip solutionc
ECL solutionc
IECL (cps × 104)
ia (mA)
IECL (cps × 104)
ia (mA)
IECL (cps × 104)
ia (mA)
Triton Ir(ppy)3; TPrA; KH2PO4 Triton; Ir(ppy)3; TPrA; KH2PO4
KH2PO4 Ir(ppy)3; TPrA; KH2PO4 Triton; Ir(ppy)3; TPrA; KH2PO4
d 0.858
-5.27 -5.47
d 0.184
-2.87 -2.15
d 0.186
-1.31 -1.25
2.34
-13.6
0.298
-2.99
0.214
-1.20
a Potential scan rate of 0.1 V/s. Anodic oxidation current (i ) measured at the potential where the ECL peak appeared. Standard deviation of a ECL measurements is (5%; cps, counts per second. bA full cyclic voltammetric sweep (e.g., sweep 1) was from 0.0 to +2.0 to +0.0 V vs Ag/AgCl. cConcentrations of reagents: 0.2 M KH PO , 1 mM Triton X-100, 10 µM Ir(ppy) , and 0.05 M TPrA. dNo ECL signal observed above background. 2 4 3
Figure 3. Photoluminescence spectra of 10 µM Ir(ppy)3 (0.2 M KH2PO4) with (A) 0, (B) 0.2, (C) 0.6, and (D) 0.8 mM Triton X-100. Excitation wavelength was 380 nm with slit widths of 10 nm.
Figure 4. ECL intensity (counts per second, cps × 105) vs concentration of surfactant for 10 µM Ir(ppy)3 (0.2 M KH2PO4). Each point is the average of at least three scans with error bars at e(7%.
ECL intensity peaks at a potential of ∼+0.8 V in the presence and absence of surfactants. At these potentials, oxidation of both TPrA (Ea ∼+0.5 V vs Ag/AgCl) and Ir(ppy)3 (E° ∼+0.5 V) has occurred. Figure 4 illustrates the relationship between surfactant concentration and ECL intensity for Ir(ppy)3. At concentrations well below the critical micelle concentration (cmc ) 0.24 mM for Triton X-100),22 increased ECL intensity is observed. The cmc is the concentration at which species’ equilibria favor the formation of micelles over unassociated molecules.23 Below cmc values, the surfactants can form complexes with dissolved species, such as Ir(ppy)3. When cmc values are exceeded, surfactants exist primarily as micelles, and the dissolved species may partition (22) Critical micelle concentration (cmc) value provided by Aldrich Chemical Co. (Milwaukee, WI). (23) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; Chapter 1.
between the micelle and water phases. The increased ECL intensity at concentrations well below the cmc indicates interactions are occurring between surfactant molecules, Ir(ppy)3, TPrA, or both. The general trend in Figure 4 is an increase in ECL signal until ∼0.1 mM surfactant. At this point, ECL signal changes at a much slower rate. Such dramatic increases cannot be attributed solely to solubilization of Ir(ppy)3 in micelles for several reasons. First, increases are observed well below the critical micelle concentration of 0.24 mM.22 Second, previous studies on the ECL of a series of ruthenium compounds,7 and on Ru(bpy)32+ in the presence of Triton surfactants with varying cmc’s,4 showed similar trends. Finally, a second increase in ECL is observed well above the cmc at concentrations of Triton between 0.4 and 0.6 mM. The exact nature of these interactions at higher Triton X-100 concentrations is still under study. Previous experiments have indicated that adsorption of Triton X-100 renders the electrode more hydrophobic, facilitating TPrA and luminophore oxidation and leading to higher ECL intensities.3,4,7 To confirm the effect of surfactant adsorption at the electrode surface for the iridium system, a platinum electrode was immersed in a “dip” solution for 10 min, rinsed with water, and then placed in a solution containing TPrA and Ir(ppy)3. Three dip solutions were used, the first or blank solution contained 1.0 mM Triton X-100, the second 10 µM Ir(ppy)3, 0.05 M TPrA, and no surfactant; the third was of identical composition to the second but also contained 1.0 mM Triton X-100. Cyclic voltammetric and ECL results are presented in Table 1. In the first cycle, ECL intensities and oxidation currents were larger for surfactant solutions containing Ir(ppy)3/TPrA compared to both blank (no surfactant, luminophore, and TPrA) and surfactant-only solutions. During subsequent cycles, however, both current and ECL intensities dropped dramatically. By the third cycle, ECL intensities and currents of the surfactant dipped electrodes were equivalent to those observed at bare electrodes or for electrodes containing only Ir(ppy)3/TPrA and no surfactant. Since the increased surface charge on the electrode at higher potentials will lead to desorption of the surfactants,3 this clearly indicates that the adsorption of surfactant species at the electrode/solution interface plays a role in the ECL of Ir(ppy)3 and TPrA in surfactant solution. The increase of oxidation current compared to a bare electrode, or surfactant dipped electrode containing no Ir(ppy)3/ TPrA, also indicates the formation of a surfactant adsorption layer Analytical Chemistry, Vol. 75, No. 3, February 1, 2003
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Figure 5. pH dependence of ECL using 0.05 M TPrA and 10 µM Ir(ppy)3 in aqueous buffered (0.1 M KH2PO4) solution containing 1.0 mM Triton X-100. ECL in mixed solvent has been normalized to that of Ir(ppy)3 in CH3CN. Each point is the average of at least three scans with relative standard deviations e(5%.
on the Pt surface. This layer may allow for more luminophore and TPrA to be oxidized giving rise to the higher ECL intensities in both Ir(ppy)3 and ruthenium-based1,3,4,7 systems. With the observation of increased ECL in a variety of systems (e.g., Ir(ppy)3, Cu(dmp)2+, and Ru(bpy)32+), it is becoming apparent that adsorption and preconcentration of TPrA near the electrode or other as yet unidentified factors may contribute more to the surfactant effect than luminophore/surfactant interactions. Regardless of the mechanism of the surfactant effect, dramatic increases of greater than 10-fold in ECL intensity are observed for Ir(ppy)3 in the presence of Triton X-100. For example, ECL efficiencies (φecl, photons generated per redox event) were calculated using Ru(bpy)32+ as a relative standard (taken as 1 in H2O (0.2 M KH2PO4)).24 Values of φecl ) 0.000 92, 0.0084, and 0.0186 were obtained for Ir(ppy)3 in solutions containing 0.0, 0.4, and 0.8 mM Triton X-100, respectively. (24) Richter, M. M.; Debad, J. D.; Striplin, D. R.; Crosby, G. A.; Bard, A. J. Anal. Chem. 1996, 68, 4370.
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The ECL emission is also pH dependent, with maximum intensities observed at pH ∼9 (Figure 5). Similar trends are observed in the presence and absence of surfactant for both Ir(ppy)3 and Ru(bpy)32+ using TPrA as a coreactant,4,7,8,11 and indicate that deprotonation of the TPrA radical cation (i.e., TPrA•+) is critical to the generation of ECL in both iridium and ruthenium systems. This is important for potential applications since the pH of environmental and biological systems is ∼7.4 and would require less sample preparation prior to analysis. CONCLUSIONS This work clearly shows that the surfactant effect on ECL is not limited to Ru(bpy)32+ and its derivatives. It also shows that although the surfactant does not play a direct role in generating the ECL excited state, an adsorption layer forms on the electrode surface allowing for increased luminophore or TPrA oxidation. The fact that the emission maximums of Ir(ppy)3 and Ru(bpy)32+ are far enough removed that it is possible to distinguish both signals in a single ECL experiment,11 and the similar reactivities of both in the presence of surfactants (e.g., pH and increased ECL efficiencies), improve the likelihood that both ruthenium- and iridium-based compounds can be incorporated into a single ECL assay. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This research was also supported by an award from Research Corp., and we gratefully acknowledge Southwest Missouri State University for its financial support. Received for review May 31, 2002. Accepted November 26, 2002. AC0203600
