Anal. Chem. 2004, 76, 73-77
Multicolored Electrogenerated Chemiluminescence from Ortho-Metalated Iridium(III) Systems Brian D. Muegge and Mark M. Richter*
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804-0089
The electrogenerated chemilumescence (ECL) of F(Ir)pic (bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl))iridium III) and (btp)2Ir(acac) (bis[2,(2′-benzothienyl)pyridinato-N,C3′](acetlyacetonate)Ir(III) has been investigated in acetonitrile (MeCN), mixed MeCN/H2O (50:50, v/v), and aqueous solutions using the oxidative reductive coreactant tri-n-propylamine. ECL was also studied in the presence of the nonionic surfactant Triton X-100 (poly(ethylene glycol) tert-octylphenyl ether). F(Ir)pic is a blue emitter (λECL ∼ 470 nm), and (btp)2Ir(acac) emits in the red (λECL ∼ 600 nm). The ECL spectrum of each compound is identical to its photoluminescence spectrum, indicating the same metal-to-ligand (MLCT) excited states. The ECL emission spectrum of F(Ir)pic can be distinguished from Ru(bpy)32+ when both are present in the same solution, raising the possibility of using these compounds for detection of multiple analytes in the same solution. ECL intensity increased in the presence of surfactant up to 6-fold for F(Ir)pic and up to 20-fold for (btp)2Ir(acac). Oxidative current also increased for both compounds. These data support the theory of surfactant adsorption at the electrode surface, leading to greater concentrations of TPrA and Ir species near the electrode surface and higher ECL intensities. Electrogenerated chemiluminescence (often called electrochemiluminescence and abbreviated ECL) of photoluminescent tris-chelated transition metals is an area of active study.1,2 ECL is the process of generating excited states in a photoactive molecule at an electrode surface, leading to luminescence upon return to the ground state. One compound that has been extensively studied is Ru(bpy)32+ (where bpy ) 2-2′-bipyridine). Ru(bpy)32+ has been of particular interest because of its low-lying metal-to-ligand charge-transfer (MLCT) excited states,3 high emission quantum yields (e.g., φem in H2O ∼ 4.2%),4,5 and long excited-state lifetimes (τ ∼ 600 ns). In fact, the ECL of Ru(bpy)32+ and its derivatives * To whom correspondence should be addressed. E-mail: mar667f@ smsu.edu. (1) Richter, M. M. In Optical Biosensors: Present and Future; Ligler, F. S.; Rowe Taitt, C. A., Eds.; Elsevier: Amsterdam, 2002; Chapter 6. (2) Andersson, A.; Schmehl, R. H. In Optical Sensors and Switches; Molecular and Supramolecular Photochemistry; Ramamurthy, V.; Schanze, K. S., Eds.; Marcel Dekker: New York, Vol. 7, 2001; Chapter 3. (3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170. (4) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583-5590. (5) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853-4858. 10.1021/ac035038j CCC: $27.50 Published on Web 12/02/2003
© 2004 American Chemical Society
has found many analytical applications, including the detection of biological agents,1 metal cations,6-8 and DNA mutations.9 Recently, ortho-metalated iridium(III) complexes have been explored for use in both organic light-emitting diodes10-12 and ECL.13-15 These compounds often have high photoluminescent efficiencies, long excited-state lifetimes, and low-lying MLCT excited states.16 Most importantly, by changing the ligand identity or composition, it has been possible to “tune” the frequency of luminescence from blue to red.17-19 Ir(ppy)3 (where ppy ) 2-phenylpyridine) is capable of ECL in organic solutions,13,14 and in aqueous solutions in the presence of the coreactant tri-n-propylamine (TPrA).15 Coreactants are species that, upon electrochemical oxidation or reduction, produce intermediates that react with other compounds to produce excited states capable of emitting light. For example, in the case of TPrA,20 an anodic potential oxidizes TPrA to [TPrA•]+ that then decomposes (via deprotonation of an R-carbon from a propyl group) to produce a strong reducing agent (presumably TPrA•). This reducing agent can then interact with an oxidized luminophore such as Ir(ppy)3+ to form the excited state (i.e., *Ir(ppy)3). The general mechanism is
Ir f Ir+ + e• +
-
(1) •
TPrA f [TPrA ] + e f TPrA + H
+
(2)
TPrA• + Ir+ f *Ir + products
(3)
*Ir f Ir + hν
(4)
where *Ir is the light-emitting species. It should be mentioned that the mechanism proposed above for TPrA is probably an (6) Lai, R. Y.; Chiba, M.; Kitamura, N.; Bard, A. J. Anal. Chem. 2002, 74, 551. (7) Muegge, B. D.; Richter, M. M. Anal. Chem. 2002, 74, 547-550. (8) Bruce, D.; Richter, M. M. Analyst 2002, 127, 1492-1494. (9) Dennany, L.; Forster, R. J.; Rusling, J. F. J. Am. Chem. Soc. 2003, 125, 5213-5218. (10) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4-6. (11) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. Appl. Phys. Lett. 2001, 78, 1622-1624. (12) Chen, X.; Liao, J.; Liang, Y.; Ahmed, M. O.; Tseng, H.; Chen, S. J. Am. Chem. Soc. 2003, 125, 636-637. (13) Nishimura, K.; Hamada, Y.; Tsujioka, T.; Shibata, K.; Fuyuki, T. Jpn. J. Appl. Phys. 2001, 40, 945-947. (14) Gross, E. M.; Armstrong, N. R.; Wightman, R. M. J. Electrochem. Soc. 2002, 149, E137-E142. (15) Bruce, D.; Richter, M. M. Anal. Chem. 2002, 74, 1340-1342. (16) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985, 107, 1431.
Analytical Chemistry, Vol. 76, No. 1, January 1, 2004 73
used as received. Ru(bpy)3Cl2‚6H2O was obtained from Strem Chemicals (Newbury Port, MA). F(Ir)pic and (btp)2Ir(acac) were purchased from H. W. Sands (Jupiter, FL). Spectroquality MeCN was obtained from Burdick and Jackson (Muskegon, MI). Tetrabutylammonium hexafluorophosphate (Bu4NPF6), tri-n-propylamine (TPrA), and Triton X-100 were purchased from Aldrich (Milwaukee, WI). A 0.20 M potassium phosphate monobasic buffer solution (potassium phosphate monobasic hydrate, 99% EM Science, Gibbstown, NJ) was made with deionized water that had been passed through a Barnstead/Thermolyne filtration system. A 1:1 (v:v) solution of MeCN:potassium phosphate buffer was also made. pH adjustments were made using 6.0 M H2SO4 or 6.0 M NaOH. Aqueous and mixed-solvent solutions were run at a pH of 8.0 ( 0.1 unless otherwise indicated. Figure 1. Structures of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium(III), [F(Ir)pic]; bis[2,(2′-benzothienyl)-pyridinatoN,C3′](acetlyacetonate)Ir(III), [(btp)2Ir(acac)]; and (poly(ethylene glycol) tert-octylphenyl ether) [Triton X-100].
oversimplification, and recent studies have indicated that other oxidation products, such as [TPrA•]+, may take part in the reaction sequence.21 Another way to increase emission is with the use of surfactants. For example, Ir(ppy)3 ECL intensity increased g10-fold in aqueous nonionic surfactant solution.22 Surfactants have also been studied as enhancers for ECL systems using Ru(bpy)32+ and its derivatives23-26 and an osmium phosphine system.27 The surfactant, in this case Triton X-100 (poly(ethylene glycol) tert-octylphenyl ether), probably creates a hydrophobic region around the surface of the working electrode, leading to higher concentrations of luminophore and coreactant and allowing for more redox events.23 The electrochemical and ECL properties of F(Ir)pic [bis(3,5difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)) iridium (III)] and (btp)2Ir(acac) [bis[2,(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)Ir(III) (Figure 1) are reported in aqueous, nonaqueous (MeCN), mixed solvents (50:50 v:v, MeCN/H2O), and also in the presence of surfactant. These complexes were selected since they emit at wavelengths different from Ir(ppy)3. F(Ir)pic is a blue emitter, (btp)2Ir(acac) is a red emitter, and Ir(ppy)3 emits in the green region.28 Having a collection of several compounds that emit over a broad color range might be useful in studies that require an internal standard or identification of several species in a single sample solution. EXPERIMENTAL SECTION Materials. Tris(2-phenylpyridine)iridium(III) was prepared and characterized in a previous study.15 All other materials were (17) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 1494-1495. (18) Lamansky, S.; Djurovich, P. I.; Abdel-Razzaq, F.; Garon, S.; Murphy, D. L.; Thompson, M. E. J. Appl. Phys. 2002, 92, 1570-1575. (19) Nazeeruddin, Md. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790-8797. (20) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127-3131. (21) Miao, W.; Choi, J.-P.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 14478. (22) Cole, C.; Muegge, B. D.; Richter, M. M. Anal. Chem. 2003, 75, 601-604. (23) Zu, Y.; Bard, A. J. Anal. Chem. 2001, 73, 3960. (24) Workman, S.; Richter, M. M. Anal. Chem. 2000, 72, 5556-5561. (25) Factor, B.; Muegge, B.; Workman, S.; Bolton, E.; Bos, J.; Richter, M. M. Anal. Chem. 2001, 73, 4621-4624. (26) Bruce, D.; McCall, J.; Richter, M. M. Analyst 2002, 127, 125.
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EXPERIMENTAL METHODS Absorbance and Photoluminescence A Cary 100 UV-vis spectrophotometer (Varian Inc.) was used for all absorbance studies. Photoluminescence studies were conducted with a Shimadzu RF-5301 PC spectrofluorophotometer. Luminophore concentrations ranged from 1 to 5 µM. Surfactant concentrations were from 0.05 to 1 mM. Photoluminescent efficiency was calculated using a modification of procedures previously established for Ru(bpy)32+,4,5,29
Φem ) Φref(Isample/Iref)
where I is the maximum intensity of photoluminescence for the sample and the reference obtained under the same experimental conditions, with Ru(bpy)32+ as the standard (Φref ) 0.042). Electrochemistry. Electrochemical studies used a CH instruments model 620 electrochemical analyzer (Austin, TX). Methods have been described elsewhere.24 F(Ir)pic and (btp)2Ir(acac) concentrations were between 10 and 100 µM. Electrochemiluminescence. Two instrumental systems were used to study electrogenerated chemilumescence. Studies of the aqueous surfactant systems were conducted with an Origen ECL analyzer (Igen International Inc, Gaithersburg, MD).30 Nonaqueous systems, dip tests, and time trials were studied with a conventional three-electrode system described previously.23,31 Luminophore concentrations were on the micromolar order. All solutions contained 0.05 M TPrA as a coreactant. It was usually necessary to sonicate the solution for 10 min to bring the TPrA fully into the solution. ECL efficiency (ΦECL), a measure of photons emitted per redox event, was calculated in a manner similar to photoluminescent efficiency,
ΦECL ) Φref(Isample/Iref)
where I is the maximum ECL intensity for nonaqueous or mixed (27) Walworth, J.; Brewer, K. J.; Richter, M. M. Anal. Chim. Acta, In press. (28) Kawamura, Y.; Yangida, S.; Forrest, S. R. J. Appl. Phys. 2002, 92, 87-93. (29) Richter, M. M.; Debad, J. D.; Striplin, D. R.; Crosby, G. A.; Bard, A. J. Anal. Chem. 1996, 68, 4370-4376. (30) McCord, P.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91-99. (31) McCall, J.; Alexander, C.; Richter, M. M. Anal.Chem. 1999, 71, 25232527.
Table 1. Spectroscopic and Electrochemical Data for F(Ir)Pic and Ir(acac) compd F(Ir)pic
(btp)2Ir(acac)
solvent/soln
Eaa
λabs
λem
λecl
Φemb
ΦECLc
MeCN MeCN/H2O H2O H2O w/ 0.4 mM Triton H2O w/ 0.8 mM Triton MeCN MeCN/H2O H2O H2O w/ 0.4 mM Triton H2O w/ 0.8 mM Triton
1.244 1.320 1.204 1.172 1.200 1.364 1.340 1.196 1.176 1.188
374 372 369 367 368 348 348 355 355 355
468; 490 (sh) 467; 494 (sh) 472; 497 (sh) 471; 497 (sh) 474; 496 (sh) 576 600 609 609 609
498 498 498 498 498 601 603
0.1004 0.0389 0.0048 0.0575 0.0766 0.00204 0.0015 0.0020 0.0050 0.0054
0.030 0.45 0.00010 0.00063 0.00067 0.28 0.049 0.000098 0.00020 0.00033
a E obtained vs Ag/AgCl reference electrode. b Φ 2+ c a em calculated with respect to Φref ) 0.042 for Ru(bpy)3 . ΦECL calculated with respect to Φref ) 1.00 for Ru(bpy)32+. ECL solutions contained 0.1 µM Ir complex and 0.05 M TPrA.
systems, and I is the integrated intensity for aqueous solutions studied in the Origen instrument (Φref ) 1 for Ru(bpy)32+). Dip Tests. Procedures for the dip test were similar to those used previously.22,23,25 The platinum mesh working electrode was dipped for 10 min in a solution of 1 mM Triton X-100. The electrode was then removed from solution and gently rinsed with deionized H2O for 1 min to remove any unabsorbed surfactant. The electrode was placed in another solution of 10 µM Ir derivative with 0.05 M TPrA, and ECL was performed. A complete ECL cycle consisted of sweeping from 0.0 to + 2.0 to 0.0 V multiple times using cyclic voltammetry. These results were compared to the ECL of solutions with no prior “dip” treatment of the electrode containing (a) 10 µM Ir derivative with 0.05 M TPrA and (b) 10 µM Ir derivative, 0.05 M TPrA, and 1 mM Triton X-100. RESULTS AND DISCUSSION Electrochemistry. Cyclic voltammetric data for F(Ir)pic and (btp)2Ir(acac) in all solvents are presented in Table 1. F(Ir)pic demonstrates a quasireversible peak (ia/ic ) 1.77) and a peakto-peak separation of 53 mV. (btp)2Ir(acac) undergoes irreversible oxidation under the same conditions. The quasi- to irreversible nature of these oxidation processes makes assignment of the number of electrons involved in the redox processes difficult. The redox potentials of F(Ir)pic and (btp)2Ir(acac) are nearly independent of the presence of surfactant. This is consistent with earlier studies of Ir(ppy)3,22 and the lack of a potential shift implies that there is not a stronger interaction between the oxidized Ir species and the surfactant media compared to the neutral forms of the complexes.32,33 Absorbance and Photoluminescence. UV-vis and luminescence data are presented in Table 1, and Figure 2 shows the photoluminescence spectra of F(Ir)pic, Ir(ppy)3, and Ru(bpy)32+ in MeCN and mixed solvent. Both of the Ir compounds demonstrate weak absorbances, with maximum absorbances (λabs) at ∼375 nm for F(Ir)pic and ∼350 nm for (btp)2Ir(acac). When excited at these wavelengths, F(Ir)pic gave peak emission in the blue-green region (λem ∼ 470 nm). The photoluminescent efficiency (Φem) of F(Ir)pic in MeCN (Φem ∼ 0.100) was substantially greater than Ru(bpy)32+ (Φem ∼ 0.042), and approximately equal (32) Mandal, K.; Hauenstein, B. L., Jr.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1983, 87, 328-331. (33) Dressick, W. J.; Hauenstein, B. L., Jr.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984, 88, 3337-3340.
Figure 2. Photoluminescence spectra of (A) F(Ir)pic (10 µM) in MeCN, (B) F(Ir)pic (10 µM) in mixed, (C) Ir(ppy)3 (10 µM) in mixed, (D) Ir(ppy)3 (10 µM) in MeCN, (E) Ru(bpy)32+ (10 µM) in mixed, and (F) Ru(bpy)32+ (10 µM) in MeCN solution.
in 50:50 (Φem ∼ 0.039). (btp)2Ir(acac) demonstrated very weak luminescence in the red region (λem ∼ 600 nm) in all solvents. The efficiency of this compound (Φem ∼ 0.002) was e5% of Ru(bpy)32+ in all solvents. This is surprising, since several studies have used (btp)2Ir(acac) as a high efficiency dopant in solid-state OLEDs34,35 but may be a reflection of its irreversible Ir0/Ir+ oxidative redox couple. Emission wavelength was solvent-dependent for both Ir derivatives, with red-shifts in aqueous solutions. Neither compound demonstrated a significant shift of λem in the presence of surfactants, although luminescence intensity generally increased with concentration of Triton X-100. This is consistent with earlier studies of ECL-active compounds with surfactants.32 Electrochemiluminescence. The ECL of F(Ir)pic and (btp)2Ir(acac) was studied in several solvents and also in the presence of the nonionic surfactant Triton X-100. ECL efficiencies (ΦECL) are presented in Table 1. ECL was observed in every solvent system, although the efficiency for the Ir derivatives was always substantially lower than Ru(bpy)32+ under identical conditions. (34) Chen, F.; Yang, Y.; Thompson, M. E.; Kido, J. Appl. Phys. Lett. 2002, 80, 2308-2310. (35) Chen, F.; Yang, Y.; Pei, Q. Appl. Phys. Lett. 2002, 81, 4278-4280.
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Table 2. ECL Peak Intensities and Oxidation Current of F(Ir)Pic and Ir(acac)/TPrA Systems 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,b sweep 1 complex F(Ir)pic (btp)2Ir(acac)
dip soln
ECL soln
ECL (cps × 103)
none Triton none none Triton none
Ir(pic); TPrA Ir(pic); TPrA Ir(pic); TPrA; Triton Ir(acac); TPrA Ir(acac); TPrA Ir(acac); TPrA; Triton
37.276 45.928 214.284 20.944 69.060 414.532
sweep 2
sweep 3
ia (mA)
ECL (cps × 103)
ia (mA)
ECL (cps × 103)
ia (mA)
-29.7 -36.20 -38.01 -31.55 -37.27 -39.72
3.196 3.464 52.940 3.192 4.336 11.276
-16.98 -17.71 -19.40 -20.51 -18.48 -19.74
2.884 3.524 35.724 2.864 4.076 8.232
-15.33 -16.12 -16.42 -16.85 -16.76 -16.70
a Potential scan rate of 0.1 V/s. b 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. *A full cyclic voltammetric sweep (e.g., sweep 1) was from 0.0 to +2.0 to +0.0 V vs Ag/AgCl. Concentrations of reagents: 0.2 M KH2PO4, 1 mM Triton X-100, 10 µM Ir derivative and 0.05 M TPrA
Figure 3. ECL emission spectra of (A) 100 µM F(Ir)pic and (B) 100 µM Ru(bpy)32+ in the same MeCN solution with 0.05 M TPrA.
Emission spectra of electrogenerated chemiluminescence events showed that F(Ir)pic and (btp)2Ir(acac) generate ECL at the same wavelength as photoluminescence (Figures 2 and 3). This indicates that the same metal-to-ligand charge transfer (MLCT) transition is involved in both processes. Further, the wavelength of peak emission for F(Ir)pic is blue-shifted from the peak of Ru(bpy)32+ to such an extent that separate emissions can be identified in a solution containing both compounds in MeCN (Figure 3). This could be useful in future applications that use internal standards or for multianalyte determinations in a single sample solution. The ECL spectrum of Ir(ppy)3 is also distinguishable from Ru(bpy)32+.15 However, a solution containing F(Ir)pic, Ir(ppy)3, and Ru(bpy)32+ resulted in considerable overlap of the two iridium systems (λ ∼ 468 and 517 for F(Ir)pic, Ir(ppy)3, respectively) such that only a single peak was observed. Research has shown that Ru(bpy)32+,24,25 Ir(ppy)3,22 and Os(phen)2(dppene)2+ (where phen ) 1,10-phenanthroline and dppene) bis(diphenylphosphino)ethene) ECL intensity increases in aqueous surfactant media. Therefore, the surfactant effect on the ECL of solutions containing F(Ir)pic and (btp)2Ir(acac) in aqueous buffered solution with Triton X-100 concentrations from 0 to 1 mM was measured. The results of these trials are presented in Figure 4. ECL intensities increased for both compounds in the presence of Triton X-100. F(Ir)pic demonstrated an increase >3.5fold with the introduction of 0.05 M surfactant and a less pronounced effect at higher concentrations. (btp)2Ir(acac) showed increasing intensity with surfactant concentration, with a maximum increase of 2-fold at 1 mM Triton (the upper limit of our 76 Analytical Chemistry, Vol. 76, No. 1, January 1, 2004
Figure 4. Surfactant concentration dependence of ECL of (9) 10 µM F(Ir)pic and (2) 10 µM (btp)2Ir(acac) in aqueous buffered solution containing 0.05 M TPrA. Each point is the average of at least three scans, with error bars at (5%.
experiment). The increased emission from these luminophores is useful, since the addition of surfactant should allow greater analytical sensitivity in studies using these Ir(III) derivatives. F(Ir)pic and (btp)2Ir(acac) ECL was also studied over a pH range from 3 to 10 in mixed and aqueous solvents and in the presence of surfactant. For all systems, peak emission occurred in the pH range from 8 to 9.5, which is consistent with studies of other luminophores using the TPrA coreactant.36-39 The mixed solvents show peak emission at more basic pH levels, and the aqueous systems with surfactant demonstrated stronger intensities at more acidic pH levels. This has been observed previously7,8 and is probably due to the greater solubility of TPrA in mixedorganic solutions at pHs g9.4. Bulk electrolysis with no stirring was performed on solutions of the Ir(III) derivatives in aqueous solutions with surfactant concentrations of 0, 0.4, or 0.8 mM. ECL intensity was measured over a 30-min time period. The results of those trials showed a diffusion-controlled decrease in intensity. Past studies have suggested that the increase in the ECL signal in surfactant media was a result of adsorption of surfactant at the Pt electrode surface.23 According to this mechanism, the surfactant (36) Muegge, B. D.; Brooks, S.; Richter, M. M. Anal. Chem. 2003, 1102-1105. (37) McCall, J.; Bruce, D.; Workman, S.; Cole, C.; Richter, M. M. Anal. Chem. 2001, 73, 4617-4620. (38) High, B.; Bruce, D.; Richter, M. M. Anal. Chim. Acta 2001, 449, 17-22. (39) Bruce, D.; Richter, M. M.; Brewer, K. J. Anal. Chem. 2002, 74, 31573159.
molecule is arranged with its hydrophilic head in solution and its hydrophobic tail nearer the electrode. This hydrophobic region around the electrode leads to an increase of luminophore and coreactant at the electrode surface, allowing more redox events and, thus, greater ECL intensity. To test this theory, a series of “dip tests” were conducted, as described under experimental methods. The results of this experiment are similar to past studies using dip tests.22,23,25,26 Enhanced ECL and increased electrochemical current were observed at electrodes dipped in a surfactant-containing solution (Table 2), followed by washing and placement in an ECL solution compared to a control solution with undipped electrodes. 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 electrode solutions containing only Ir/TPrA and no surfactant. Since the increased surface charge on the electrode at higher potentials will lead to desorption of the surfactants,23 this is strong evidence for adsorption of surfactant onto the platinum electrode and helps explain the increased light emission of surfactant containing solutions. Regardless of the mechanism of the surfactant effect, dramatic increases of up to 6-fold intensity for Ir(acac) and 20-fold increase for F(Ir)pic are observed in the presence of Triton X-100 (Figure 4 and Table 2). This is comparable to the previously reported 10fold increase observed for Ir(ppy)3 under similar conditions.
CONCLUSIONS F(Ir)pic and (btp)2Ir(acac) undergo ECL in aqueous and nonaqueous solvents at reasonable voltage and pH levels. The blue emission of F(Ir)pic, red emission of (btp)2Ir(acac), and previously reported green emission of Ir(ppy)3 give a wide color range of possible emitters and raise the possibility of internal standards and the analysis of multiple analytes in the same solution. Additionally, ECL efficiency is increased in the presence of surfactant, allowing signal detection at lower concentrations of luminophore. The mechanism for this increase is probably adsorption of surfactant on the electrode surface. ACKNOWLEDGMENT Acknowledgment is made to the Camille and Henry Dreyfus Foundation for support in the form of a Henry Dreyfus TeacherScholar award (M.M.R.). Acknowledgment is also made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this work and the generous support of the National Science Foundation’s Course, Curriculum and Laboratory Improvement Program under Grant DUE-0124367 and Southwest Missouri State University for the purchase of the electrochemiluminescence analyzer. Received for review September 3, 2003. Accepted October 29, 2003. AC035038J
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