Anal. Chem. 2002, 74, 547-550
Electrochemiluminescent Detection of Metal Cations Using a Ruthenium(II) Bipyridyl Complex Containing a Crown Ether Moiety Brian D. Muegge and Mark M. Richter*
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804-0089
The effects of metal ions on the electrochemiluminescence (ECL) properties of (bpy)2Ru(AZA-bpy) (bpy ) 2,2′bipyridine; AZA-bpy ) 4-(N-aza-18-crown-6-methyl-2,2′bipyridine) have been investigated. The electrochemistry, photophysics and ECL of Ru(bpy)32+ in the presence of Pb2+, Hg2+, Cu2+, and K+ are reported. The anodic oxidation of Ru(bpy)32+ produces ECL in the presence of tri-n-propylamine (TPrA) in 50:50 (v/v) CH3CN:H2O solution. Increases in ECL efficiency (photons generated per redox event) up to 20-fold that depend on both the concentration and nature of the metal ion have been observed, making this an interesting system for electrochemiluminescence metal ion sensing.
INTRODUCTION In this report, the electrochemical, photoluminescence (PL), and electrochemiluminescence (ECL) changes that occur when metal cations, such as mercury(II) and lead(II), are introduced to solutions containing (bpy)2Ru(AZA-bpy)2+ (where bpy ) 2,2′bipyridine and AZA-bpy ) 4-(N-aza-18-crown-6-methyl-2,2′-bipyridine), Figure 1, are described. A Ru(bpy)32+ core is the luminescent and redox active center in view of its well-known electrochemical and photophysical properties. A crown ether (i.e., N-aza-18crown-6) attached via linkers to a bipyridine ligand acts as a substrate binding site (e.g., a molecular recognition component) that can interact directly with metal cations in solution. Such an approach to sensing cations, anions, and changes in solution pH have been widely exploited using organic materials,1 and several studies involving the photoluminescence of coordination compounds exist;2-5 however, it is also of interest to study the changes that occur in ECL upon the addition of metal cations to solution. Electrochemiluminescence is a means of converting chemical energy into light. It involves the formation of electronically excited states by energetic electron transfer reactions of electrochemically * E-mail:
[email protected]. (1) (a) Loehrer, H.-G.; Voegtle, F. Acc. Chem. Res. 1985, 18, 65. (b) De Santis, G.; Fabbrizi, L.; Licchelli, M.; Mangano, C.; Sacchi, D. Inorg. Chem. 1995, 34, 3581. (2) (a) Yoon, D. I.; Gerg-Brennan, C. A.; Lu, H.; Hupp, J. T. Inorg. Chem. 1992, 31, 3192. (b) Beer, P. D.; Kocian, O.; Mortimer, R. J.; Ridgway, C. J. Chem. Soc. Dalt. Trans. 1991, 113, 6108. (3) Rawle, S. C.; Moore, P.; Alcock, N. W. J. Chem. Soc., Chem. Commun. 1992, 684. (4) Shen, Y.; Sullivan, B. P. Inorg. Chem. 1995, 34, 6235. (5) Shen, Y.; Sullivan, B. P. J. Chem. Educ. 1997, 74, 685. 10.1021/ac010872z CCC: $22.00 Published on Web 12/21/2001
© 2002 American Chemical Society
Figure 1. (bpy)2Ru(AZA-bpy)2+.
generated species and is a sensitive probe of electron- and energytransfer processes at electrified interfaces.6 Traditionally, ECL was generated via annihilation, in which the electron-transfer reaction is between an oxidized and reduced species, both of which are generated at an electrode by alternate pulsing of the electrode potential. ECL can also be generated in a single step by utilizing a coreactant.7,8 ECL coreactants are species that, upon electrochemical oxidation or reduction, produce species that react with other compounds to produce excited states capable of emitting light. For example, in the Ru(bpy)32+/TPrA (TPrA ) tri-npropylamine) system,8 an oxidizing potential oxidizes Ru(bpy)32+ to Ru(bpy)33+. The coreactant (TPrA) is also oxidized and decomposes to produce a strong reducing agent (presumably TPrA•) upon deprotonation of an R-carbon from one of the propyl groups. This strong reducing agent can then interact with Ru(bpy)33+ to form the excited state (i.e., *Ru(bpy)32+). ECL has also been commercially developed for the clinical diagnostic market (e.g., immunoassays and DNA probes).9 Therefore, the development of ECL sensors that can detect a wide range of metal cations, including heavy metals (e.g., Pb2+ and Hg2+) and alkali metal ions (e.g., K+), are of interest in clinical and potentially environmental applications. Therefore, the changes that occur in the photoluminescence and ECL of (bpy)2Ru(AZA-bpy)2+ (6) (a) Tokel, N.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862. (b) Glass, R. S.; Faulkner, L. R. J. Phys. Chem. 1981, 85, 160. (c) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry, Bard, A. J., Ed; Marcel Dekker: New York, 1977; Vol. 10, pp 1-95. (7) (a) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. (b) White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 6891. (8) Leland, J. K.; Powell, M. J. J. Electroanal. Chem. 1991, 137, 3127. (9) Blackburn, G. F.; Shah, H. P.; Kenten, J. H.; Leland, J.; Kamin, R. A.; Link, J.; Peterman, J.; Powell, M. J.; Shah, A.; Tulley, D. B.; Tyagi, S. K.; Wilkins, E.; Wu, T.-G.; Massey, R. J. Clin. Chem. 1991, 37, 1534.
Analytical Chemistry, Vol. 74, No. 3, February 1, 2002 547
when Pb2+, Hg2+, Cu2+, and K+ are introduced to solution are reported. EXPERIMENTAL SECTION Materials. AZA-bpy was purchased from Gateway Chemical Technologies (St. Louis, MO) and used as received. Ru(bpy)3Cl2 (98%, Strem Chemical Inc, Newbury Port, MA), acetonitrile (Burdick and Jackson spectroquality), potassium phosphate monobasic hydrate (99.0%, EM Science, Gibbstown, NJ), cisdichlorobisruthenium(II) dichloride (Strem Chemical), lead plasma emission standard, mercury plasma emission standard, copper plasma emission standard, potassium plasma emission standard (∼10 000 mg/L; Solutions Plus, Inc., Fenton, MO) and tri-npropylamine (TPrA, 98%, Avocado Research Chemicals, Ward Hill, MA) were used as received. Other materials were reagent graded and used as received. 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 (0.05M) were prepared similarly, except that it was necessary to stir them vigorously to completely dissolve the amine. The pH of these buffer solutions was adjusted to 8.0 ( 0.1 M with either 6 M H2SO4 or 6 M NaOH. Synthesis. (bpy)2Ru(AZA-bpy)(PF6)2 was synthesized by reacting 2 equiv of (bpy)2RuCl2 (1.0 mmol, 0.520 g) with AZA-bpy (0.50 mmol, 0.220 g) in 50 mL of 2:1 (v/v) ethanol:water at reflux under argon for ∼2 h. The flask was removed from the heat, and a saturated solution of NH4PF6 (75 mL) added with stirring to induce precipitation. The precipitate was collected by vacuum filtration and purified by column chromatography using a neutral alumina with 2:1 (v/v) toluene:acetonitrile eluent. The product band was the first to elute. It was collected, concentrated by rotary evaporation, precipitated by the addition to a stirred solution of diethyl ether, collected via vacuum filtration, and dried under vacuum to afford an orange solid. Typical yield, 63%. Anal. Calcd: C, 45.90; H, 4.44; N, 8.54%. Found: C, 45.73; H, 4.19; N, 8.59. UVvis (λmaxabs (nm), 50:50 (v/v) CH3CN:H2O): 286.9, 327.2 (sh), 358.6, 423.6 (sh), 454.4. Methods. Electrochemical and ECL instrumentation and experimental methods have been described elsewhere.10 All electrochemical and ECL experiments were referenced with respect to a Ag/AgCl gel electrode (0.20 V vs NHE).11 The platinum mesh (27 mm2) working electrode was cleaned prior to each experiment by repeated cycling (+2.0 to -2.0 V) in 6.0 M sulfuric acid, followed by sonication in 2 M nitric acid and rinsing in deionized water. Solutions used to obtain ECL were 0.1 mM (bpy)2Ru(AZA-bpy)(PF6)2 and 0.05 M TPrA in 50:50 (v/v) CH3CN:H2O with 0.1 M potassium phosphate as the electrolyte. Stock solutions containing Pb2+, Hg2+, Cu2+, and K+ were prepared from their respective inductively coupled plasma emission standards in 0.2 M potassium phosphate buffer solution. Photoluminescence spectra were obtained using a Shimadzu RF-5301 spectrofluorophotometer (slit widths, 5 nm). Excitation was at 454 nm for Ru(bpy)32+ and (bpy)2Ru(AZA-bpy)2+, with detection between 500 and 700 nm. ECL efficiencies (φecl ) (10) (a) Workman, S.; Richter, M. M. Anal. Chem. 2000, 72, 5556-5561. (b) McCall, J.; Alexander, C.; Richter, M. M. Anal. Chem. 1999, 71, 2523. (11) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
548 Analytical Chemistry, Vol. 74, No. 3, February 1, 2002
Figure 2. UV-vis spectra of (A) Ru(bpy)32+ and (B) (bpy)2Ru(AZAbpy)(PF6)2 (0.1 mM) in 50:50 (v/v) CH3CN:H2O (0.1 M KH2PO4).
photons generated/redox event) were obtained by the literature methods,12 using Ru(bpy)32+ (φecl ) 1) as the standard. Similarly, relative photoluminescence efficiencies followed published procedures13 using Ru(bpy)32+ (φem (H2O) ) 0.042). RESULTS AND DISCUSSION Electrochemistry. The electrochemistry, UV-vis spectra, and photoluminescence of (bpy)2Ru(AZA-bpy)2+ are similar to that of Ru(bpy)32+. (bpy)2Ru(AZA-bpy)2+ displays one reversible oxidation and three reversible reductions (ipa/ipc = 1). The oxidation at +1.28 V vs Ag/AgCl gel reference electrode is assigned to the Ru2+/3+ redox couple by comparison to Ru(bpy)32+ (E° ) +1.26 vs Ag/AgCl).14 Analogously, the reductive waves are ligand-based, with E° ) -1.39, -1.60, and -1.78 V vs Ag/AgCl for (bpy)2Ru(AZA-bpy)2+ and 1.43, 1.61, and 1.81 V vs Ag/AgCl for Ru(bpy)32+ in 50:50 (v/v) CH3CN:H2O (0.1 M KH2PO4 as electrolyte). The similarity in potentials of (bpy)2Ru(AZA-bpy)2+ and Ru(bpy)32+ also indicate that the metal and ligand excited states are not greatly affected by crown ether substitution. The peak-to-peak separation, Ep-p, is ∼0.088 V, higher than the expected 0.059 V, but comparable to ferrocene under identical experimental conditions, indicating one-electron oxidation and reduction. Absorption and Photoluminescence. UV-vis spectra of (bpy)2Ru(AZA-bpy)2+ are characterized by a series of ligand-based transitions in the UV with metal-to-ligand charge-transfer (MLCT) bands in the visible (Figure 2). In fact, the visible transitions in both (bpy)2Ru(AZA-bpy)2+ and Ru(bpy)32+ are nearly identical, with lowest energy absorption maxima at 454 nm in 50:50 (v/v) CH3CN:H2O. Interestingly, as the concentrations of metal ions (i.e., Pb2+, Hg2+, and Cu2+) are increased from 0.5 equiv to 20-fold excess, little to no change in visible absorption bands is observed for (bpy)2Ru(AZA-bpy)2+. However, increases in predominantly ligand-based transitions around 280 nm are observed with metal ion concentrations equal to or in moderate excess (5-20-fold), indicating interaction of the metal ions with the AZA-bpy ligand. (bpy)2Ru(AZA-bpy)2+ is highly photoluminescent (φem ) 0.037 ( 0.0004, λexc ) 454 nm) with an emission maxima (λem) at 603 nm and a peak width at half peak height (Wh) of 74 nm. Comparison to Ru(bpy)32+ (λem ) 598 nm, Wh ) 70) indicates that the emission is MLCT in nature (Figure 3). As expected, the direct connection of the amino substituent to the bpy nucleus (i.e., AZA(12) Richter, M. M.; Bard, A. J.; Kim, W. K.; Schmehl, R. H. Anal. Chem. 1998, 70, 310. (13) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (b) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (14) Braunstein, C. H.; Baker, A. D.; Strekas, T. C.; Gafney, H. D. Inorg. Chem. 1984, 23, 857.
Figure 3. Photoluminescent emission spectra of (A) Ru(bpy)32+ and (B) (bpy)2Ru(AZA-bpy)(PF6)2 (0.1 mM) in 50:50 (v/v) CH3CN:H2O (0.1 M KH2PO4).
Figure 4. Perturbation of the photoluminescent emission spectrum of (bpy)2Ru(AZA-bpy)(PF6)2 (0.1 mM) in 50:50 (v/v) CH3CN:H2O upon addition of Pb2+: (A) 0, (B) 0.1, (C) 1 (10-fold excess), and (D) 2 mM Pb2+ (20-fold excess). Table 1. Photoluminescence Efficiencies (Oem) for (bpy)2Ru(AZA-bpy)2+ in 50:50 (v/v) CH3CN:H2O in the Presence of Pb2+, Hg2+, and Cu2+ φem (%)a metal ion
0 mM
0.05 mM
0.5 mM
2 mM
Pb2+
4.2 (( 0.1) 4.2 (( 0.1) 4.2 (( 0.1)
4.9 (( 0.1) 4.4 (( 0.1) 4.2 (( 0.1)
5.9 (( 0.2) 5.0 (( 0.1) 4.3 (( 0.1)
6.4 (( 0.2) 5.5 (( 0.2) 4.5 (( 0.1)
Hg2+ Cu2+
aφ em are the average of at least three scans, with standard deviations reported in parentheses.
bpy) allows (bpy)2Ru(AZA-bpy)2+ to be used as a metal-ion sensor for Pb2+, Hg2+, and Cu2+ ions in solution. As shown in Figure 4 and Table 1, titration of the complex with Pb2+ in 50:50 (v/v) CH3CN:H2O leads to a slight (
